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.ANORTHOSITIC SILLS OF THE SOUTHERN ADIRONDACK
MOUNTAINS OF NEW YORK STATE
by
Theresa Anne,Beddoe
Thesis submitted to the Graduate Faculty of the
Virginia Polytechnic Institute and State University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
APPROVED:
D.R. Wones
M.C. Gilbert
in
Geological Sciences
b.X. Hewitt, ChairmaJl
Y.W. Isachsen
June, 1981
Blacksburg, Virginia
J .A. Spee~
J.M. McLelland
ACKNOWLEDGEMENTS
I would like to thank
and for editing the manuscript,
State Geological Survey and
for directing the study
of the New York
of Colgate University
for time spent in the field and for reviewing the manuscript, and .
of the New York State Geological Survey for the use of
sodium cobaltinitrite and other helpful contributions including
reviewing the manuscript.
I would like to thank and for the
use of their analytical facilities and for his
expertise during analysis for the completion of some of the analyses and
for reviewing the manuscript. I would also like to thank
and for
their helpful contributions. To go special thanks for
taking a deep interest in the project and for providing invaluable
insights. I would like to thank
and and
for serving on my thesis committee.
My deepest thanks go to for being my constant field
assistant; to , who, at 70 years of age, was
the best field guide one could ask for; to for her
help in the field and for zealously typing the manuscript.
Finally, my special thanks go to my family for their constant
support and understanding, and to for her continuous
encouragement and friendship, and for reviewing the manuscript.
ii
This research was supported by graduate teaching and research
assistantships from Virginia Polytechnic Institute and State University
and from NSF grant EAR77-23225 AOl to , by a grant to
of the New York State Geological Survey, the New
York State Conservation Department, and a Grant-in-Aid of Research from
Sigma Xi, The Scientific Research Society.
iii
TABLE OF CONTENTS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements • List of Figures. ...................................................
ii
v
List of Tables ..................................................... vi
List of Plates ••••••••••••••• .vii
Introduction....................................................... 1
Analytical Procedures •• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Geo 1 ogy ••• ................................................. Petrography ••••••••••
Mineral Chemistry •••.•••••
Whole Rock Chemistry •• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metamorphic Petrology •.•
Intensive Parameters ••••••••
Metamorphic Assemblages.
Igneous Petrology.
Parent Magma •••••.•...•••• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystallization Sequence.
Intrusive History • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography .•.•••• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
10
15
27
45
54
54
57
65
65
66
68
76
Appendix 1: Modes. . . . • . . . . . . . . . . • • . • . • . . . . . . . • . • . . . • • • . . . . . . • . . • . . 83
Appendix 2:
Appendix 3:
Vi ta •..•••
Abstract
Chemical Analyses .•
Mineral Analyses •.••••••••.••.•••...••••••••••••••.
94
97
. . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 50
iv
LIST OF FIGURES
FIGURE PAGE
1. Southern Adirondacks, New York State--Location Map •..••.•.••••• 3
2. Lake Pleasant Quadrangle, Northern Half--Geological Map •••••.•• 5
3. Modal Plagioclase Plot for the Speculator Sheet ..•.••••...••••• 17
4. Plagioclase Compositions ........................•.............. 29
5. Pyroxene Compositions •••••••••••.••••••••••••.•••••••.•••••.••• 31
6. Arnphibole Compositions ......................................... 33
7. Speculator Sheet Amphibole Compositional Trends ••.•.••••••••••. 37
8. Biotite Compositions •••••••••.•.•••••••••.•••..•••.•••.•••••••. 39
9. Speculator Sheet Biotite Compositional Trends •••.....•..••••••. 41
10. Garnet Compositions ••••.•••••••••••••••••.••••.••••••••..•••••. 44
11. Whole Rock Chemistry vs. Distance from the Base of the Sill •••• 48
12. Harker Variation Diagrams •..••••••••••.•••••...•.••••••••.••.•• 50
13. AFM Variation Diagram for the Sills and the Oregon Dome ••.••••• 53
14. ACFM Diagram illustrating the garnet corona-producing reaction. 60
15. AFM Variation Diagram for the Snowy Mountain and
Thirteenth Lake Dome • •••••••••••••••••••••••••••••••••••.•••••• 73
Al. Sample Map for the lS Traverse ................................. 87
A2. Sample Map for the 2S Traverse . ................................ 89
A3. Sample Map for the 3S and 4S Traverses •••••••.•••••••.•••.••.•• 91
A4. Sample Map for the Wells Traverses . ............................ 93
v
LIST OF TABLES
TABLE PAGE
1. Average Chemical Compositions ••••.••..••..•••.•••••••••.••..••• 46
2. Metamorphic Geothermometry ••..•••••.••.•.••..•••.•.••••...••... 55
vi
LIST OF PLATES
PLATE PAGE
1. Speculator Sheet Garnet with Coronas and
Spongy Tenantville Garnet •.•..••••.•...•••••.••.•••••.•••.•..•• 21
vii
INTRODUCTION
A large body of knowledge exists on the major anorthositic
intrusives of the Adirondack Grenville Province (i.e. Miller, 1918;
Balk, 1930; Buddington, 1936 and 1939; Isachsen, 1969), yet little is
known of the petrology and geochemistry of the small leucocratic
intrusives of the anorthositic series (Buddington, 1969). The petrology
of two of these small bodies, the Speculator Sheet and the Wells Sill,
is the subject of this investigation. Both of these bodies crop out in
the Lake Pleasant 15' Quadrangle of the southern Adirondacks (Figure 1)
and have intruded into the Rooster Hill/Little Moose Mountain Formations
(Figure 2). The contact relationships between the Speculator and Wells
bodies and the metasedimentary country rocks are important in light of
the current controversy concerning the relationship of the major
anorthositic massifs with the overlying metasediments.
Walton and de Waard (1963) have proposed two major orogenic cycles
for the evolution of the Adirondacks: intrusion of anorthosite,
beveling by erosion, deposition of sediments on an anorthositic
basement, then final metamorphism by the Grenville Orogeny. Their
careful mapping of the eastern and south-central Adirondacks revealed a
persistent unit of marble calc-silicate rock forming the first unit of
the metasediments in contact with the massif. Based on these data, they
suggested that the massif was a pre-Grenville basement for the
deposition of the overlying metasediments and that the subsequent
metamorphism and deformation during the Grenville orogeny was solely
responsible for the present structural relationships between the two
1
2
FIGURE 1
Southern Adirondacks, New York State
showing the three anorthositic massifs (the Oregon Dome,
the Thirteenth Lake Dome and the Snowy Mountain Dome)
and two sills (the Speculator Sheet and the Wells Sill)
studied. (Isachsen and Fisher, 1970)
Indi
an L
ake
Qua
d.
Thirt
eent
h La
ke Q
uad.
43°45
1r-~~~~-x:7-'>::'C"r.m'.'C:--~7r~,.-~~~~~~~~~~~~-,
43°3
0'
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IOkm
43°1
51 ._
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.
74°3
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51
74°0
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Ple
asan
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uad.
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arris
burg
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d.
Sou
ther
n A
diro
ndac
ks
New
Yor
k S
tate
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achs
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nd F
ishe
r, 1
970
w
FIGURE 2
Lake Pleasant Quadrangle, Northern Half
(Mclelland, in preparation)
shows the relationship of the Speculator and
Wells Sills to the Glens Falls Syncline
and gives a key to sample maps
in Appendix 1.
~----~·2:---~
...-<f'g 1
~ v
-V
-10
-10
_\
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acie
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us?)
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oost
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ill/
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tle
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se
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n fo
rmat
ions
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oman
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ount
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e M
ount
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atio
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Lake
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ant
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atio
ns
1g Sac
anda
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orm
atio
ns
0 1
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I I
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etas
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as
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51
6
units. Mapping of the anorthositic Snowy Mountain Dome of the south
central Adirondacks (de Waard and Romey, 1963 and 1969) substantiated
their hypothesis.
In contrast to these conclusions, subsequent developments indicate
that the contact between the metaigneous and metasedimentary units is a
primary intrusive contact (Buddington, 1939). Husch et al. {1975) and
Isachsen et al. (1975) cite the intrusive relationships of the Wells and
Speculator anorthosite masses, which lie stratigraphically above the the
base of Walton and de Waard's (1963) supracrustal sequence, as evidence
against an unconformity. Hills and Isachsen (1975) obtained a whole-
rock Rb-Sr isochron date of 1199 +/- 14 my with an initial Sr ratio of
0.70452 for the monzonite (mangerite) and metanorite of the Snowy
Mountain Dome, which agrees well with other dates from Adirondack
metaigneous rocks (i.e. 1200 my for the Marcy massif using a Nd-Sm whole
rock-plagioclase isochron; Ashwal et al., 1981). Based on the close
correspondence of these dates, they conclude that the Snowy Mountain
Dome is intrusive into the country rock and does not unconformably
underlie them.
It is therefore important to establish the geochemical and
petrologic fingerprints of these sills, to determine if their history is
linked to that of the major anorthositic massifs, which have been
interpreted as a basement complex by Walton and de Waard (1963). By
determining their relationship, a positive contribution may be made to
this controversy.
In addition to the intrusive history of the sills, their subsequent
7
metamorphism must be investigated to understand the changing
assemblages throughout the sills and the occurrance of fine reaction
rims of orthopyroxene and plagioclase around some of the garnets.
Such reaction rims have been discussed in the literature for almost
a century (i.e. Holland, 1896; Fermor, 1912; Eskola, 1920; Shand, 1945;
Subramaniam, 1956). Holland (1896, p. 29) first proposed that the
reaction rim represented a stage in the formation of garnet from the
primary mafic minerals, and Fermor (1912) believed that the rims formed
by the destruction of garnet because of a rapid reduction of temperature
accompanied by a gradual lowering of pressure. These points of view
have been the two most often advocated.
Some recent workers in other areas (i.e. Kornprobst, 1977; Horrocks,
1980) have interpreted these coronas to be the result of lower pressures
and/or higher temperatures. In the Adirondacks, de Waard (1965, 1967)
used them to distinguish the high pressure field from the low pressure
field of the granulite facies. De Waard (1965) states that the rims
most probably form from the reaction hbd + gar + qtz = opx + plag + H2o and thus that garnet is a disappearing phase in granulite facies
terranes in rocks that also contain hornblende and quartz. The
selective occurrences of garnet rims in the rocks of the Wells and
Speculator bodies conflict with this interpretation.
ANALYTICAL PROCEDURES
McLelland's map (in preparation, Figure 2) of the Lake Pleasant 15 1
quadrangle was used as a base map for extensive sampling of the two
sills. 81 samples were collected from the Speculator Sheet and 39 thin
sections were described. 51 samples were collected from the Wells Sill
and 33 thin sections were described. The sample locations are shown in
Appendix 1. Twelve samples were collected from the Oregon Dome from the
Siamese Pond trail in the Thirteenth Lake 15 1 quadrangle. Eight thin
sections were described.
Modes, also presented in Appendix l, were determined optically from
point counts of each thin section. Grain size ranged from less than 0.5
mm to 6 mm and the number of points counted were sufficient for the
reported values to have a statistical accuracy of better than :2% (Van
Der Plas and Tobi, 1965).
Mineral analyses were performed with an ARL-SEMQ microprobe with an
operating voltage of 15kv; mineral standards for the six and nine
element analyses were Goschener titanite (Ti), Rockport fayalite (Fe),
Marjalahti olivine (Mg), synthetic tephroite (Mn), Tiburon albite (Na,
Al, Si), Ingamells' orthoclase (K), anorthite glass (Ca), synthetic
barium apatite (Ba, Cl), Tiebaghi chromite (Cr), Sillbole humite (F),
pure anhydrite (S), Durango apatite (P), and spec pure strontianite
(Sr}. Matrix correction factors used were those of Bence and Albee
(1968}.
Rock analyses were performed using a Phillips automated wavelength-
dispersive, X-ray fluorescence spectrometer. Five samples from the 2S
8
9
traverse of the Speculator Sheet and seven samples from the lW traverse
of the Wells Sill were collected for analysis. Their locations are
shown in Appendix 1. Five samples from the Oregon Dome were collected
along the Siamese Pond Trail. Major element abundances were analyzed on
fused glass disks using the technique described by Norrish and Hutton
(1969) and later modified by Harvey et al. (1973). Trace element
abundances were determined on pressed powder pellets using the matrix
correction methods of Reynolds (1963). Rock standards for the major
elements were USGS standards PCC-1, BCR-1, GSP-1, AVG-1, and G-2.
Precisions are better than 1% of the amount present for Si02, Al 2o3, + Ti02, Fe2o3, Cao, and K2o; about 3% for MgO and Na2o; and about - 0.01%
absolute for MnO and P2o5• GSP-1 was used as the standard for the trace element analyses (Rb and Sr). Precisions are approximately 1% for Rb
and Sr.
GENERAL GEOLOGY
The Adirondack Grenville Province of northern New York State
consists of intensely deformed and faulted granulite-facies metamorphic
rocks, which are cored by a 3100 km2 body of anorthosite (Marcy massif).
The study area in the southern Adirondacks is dominated by metasediments
and a number of smaller anorthositic bodies (Figure 1).
The second largest (greater than 300 km2 - Isachsen and Fisher,
1970) anorthositic massif of the province, the Oregon Dome, covers the
southwest corner of the Thirteenth Lake 15 1 Quadrangle and the northern
tips of the Lake Pleasant and Harrisburg 15 1 Quadrangles. The dome
consists of a large outer rim of anorthositic gabbro with an inner core
of anorthosite and intrudes biotite-hornblende granitic gneiss and
leucogranitic gneiss (Krieger, 1937).
To the north, the Thirteenth Lake Dome is cored by anorthosite-
norite and surrounded by a comagmatic charnockite (Letteney, 1969). To
the northwest of the Oregon Dome, in the north-central Indian Lake 15 1
Quadrangle, the Snowy Mountain Dome also has a core of anorthosite that
grades outward to charnockite (de Waard, 1969).
The two anorthositic sills of the study intrude the syenite gneiss
of the Rooster Hill/Little Moose Mountain Formations in the northern
half of the Lake Pleasant 15 1 Quadrangle (Figure 2; sample locations are
in Appendix 1). The Oregon Dome also crops out in the extreme
northeastern corner of this quadrangle. Mclelland (in preparation;
Figure 2) has shown that the dome intrudes the Lake Durant Formation
within the quadrangle. As this formation stratigraphically underlies
10
11
the Rooster Hill/Little Moose Mountain Formations, the Oregon Dome also
stratigraphically underlies the two sills.
The Speculator Sheet crops out five kilometers south of Speculator
in the center of the Glens Falls syncline, which plunges to the east-
southeast (Mclelland and Isachsen, 1980). The sheet intruded the
country rock before the formation of the syncline; therefore, the strike
of the sheet parallels the trace of the fold and its dip is 10-15°
toward the axis of the fold. The structural relationships concerning
the southern limb of the fold are not completely clear. The whole of
the Speculator Sheet dips to the south; it may be that the rest of the
sheet is faulted away (Mclelland, personal colTU'llunication) but further
work will be needed to substantiate this. The exposed thickness of the
sheet is approximately 500 feet.
Miller (1916) produced a detailed report on the geology of the Lake
Pleasant Quadrangle. He described the Speculator Sheet as "a basic
phase of syenite, usually non-quartzose and of dioritic or gabbroic
composition." He did state that "the rock looks much like certain
garnet-bearing phases of anorthosite described by both Cushing and
Kemp." Bartholom~ (1956) likewise mapped the quadrangle as part of his
study of Hamilton County, New York. He mapped the Speculator Sheet as
gabbroic anorthosite and anorthosite intrusive into syenite gneiss.
The sheet consists of gabbroic anorthosite and anorthosite that
appear grey to white in outcrop and weather buff to white. The mafic
minerals define a slight to strong lineation. The rock becomes more
mafic at the stratigraphic base; where highly garnetiferous, the outcrop
12
is colored red. In the extreme eastern part, the country rock is cut by
several thin mafic dikes. Metasedimentary inclusions of quartzite and
fine-grained well-foliated gneiss have been observed by others
(Bartholome, 1956); intrusions of gabbroic anorthosite occur in the
underlying syenite gneiss.
The Wells Sill intrudes along the southern limb of the Glens Falls
syncline, close to the contact of the Rooster Hill/little Moose Mountain
Formations with the subjacent Tomany Mountain/Blue Mountain Lake
Formations. It extends east-southeastward to Tenantville, a total
distance of about 22 kilometers (Goldberg, 1975; and Mclelland, personal
conmunication), striking N59W and dipping to the northeast at 27-34°.
The section of the sill studied in detail is fault-bounded on the west
and east and crops out on and in the vicinity of the Silver Bells ski
hill to the southeast of the town of Wells. Bartholome (1956) mapped
the sill as gabbroic anorthosite intruding syenite gneiss. It intrudes
the Rooster Hill/Little Moose Mountain Formations, which on the north
side of the sill is a locally-charnockitic syenite gneiss, and on the
south side is charnockite (Mclelland, personal communication).
The sill is composed of three different facies: a gabbro, an
anorthositic gabbro, and a fine-grained gabbroic anorthosite named the
Tenantville facies by Mclelland (personal communication) for it crops
out extensively in the Tenantville area (Harrisburg 15' Quadrangle).
The two. leucocratic facies of the sill have a stratigraphic thickness of
about 300 feet; including the gabbroic facies, the sill has a total
thickness of about 500 feet.
13
The gabbro is the most extensive lateral unit and is medium-grained
and equigranular with diffuse modal banding in outcrop due to higher
concentrations of plagioclase. It is dark to black when fresh and
generally weathers a deep rust or black. Its abrupt contact with the
intrusive Tenantville facies is well exposed at the top of the ski hill
and also at the northwest base of the hill, where the contact has been
emphasized by weathering.
Anorthositic gabbro covers an area of about one square kilometer on
the ski hill and forms the core of the sill. It appears grey to white
in outcrop and weathers buff to white. The mafic minerals define a
distinct lineation. Pyroxene rimmed with hornblende and the occasional
rimmed garnet are most striking.
The Tenantville facies surrounds and is gradational into the
anorthositic gabbro. It is intrusive into the gabbro facies and
contains many inclusions of the gabbro. The contacts between the gabbro
and the Tenantville facies are quite sharp and well-defined; the gabbro
inclusions show no signs of assimilation by the anorthositic gabbro. At
the top of the ski hill, the Tenantville facies has a number of
metasedimentary inclusions of two different types: well-assimilated
amphibolite and a few carbonate rocks. The Wells sill is cut by a few
pegmatitic quartzofeldspathic veins, one of which is best exposed in a
stream bed on the north side of the hill. In outcrop, the Tenantville
facies is white and fine-grained. It weathers white and has no
foliation. The large spongy garnets are distinctive in outcrop.
The syenite gneiss of the Rooster Hill/Little Moose Mountain
14
Formations is white with rust-colored staining in outcrop. It is
medium-grained, with large microperthitic augen imparting a
conglomerate-like texture to the rock. The mafic minerals define a
strong lineation. (See Mclelland and Isachsen, 1980.)
PETROGRAPHY
The Speculator Sheet
The well-lineated gabbroic anorthosite of the sheet contains 50 to
99% plagioclase, a to 23% hornblende, a to 24% orthopyroxene, 0 to 24% clinopyroxene, a to 16% garnet, 0 to 9% opaque minerals, a to 3% biotite, and traces of apatite, zircon, and hematite. The modal
plagioclase content of the samples collected along the 2S, 3S, and 4S
traverses (see Appendix 1 for locations) are plotted against vertical
distance from the base of the sill in Figure 3. This section shows the
typical scatter observed in all the traverses but illustrates the
increasing leucocratic nature towards the stratigraphic top of the sill
that is typical of the body.
The equant plagioclase crystals of the gabbroic anorthosite are
generally 2 mm long, and display albite and pericline twinning, as do
all the plagioclase crystals from the anorthositic bodies studied. Some
of these crystals form mosaic aggregates that are pseudomorphs after
originally euhedral plagioclase 10 - 15 mm long and 2 - 5 mm thick. The
centers of these aggregates rarely contain a relict core crystal of
plagioclase (3-4 mm). The largest of these relicts exhibit Spry•s
(1969) mortar texture with fine (less than 0.5 mm) recrystallization
around the edges. Antiperthitic blebs are rare and minute (less than 1
micron); randomly oriented opaque needles are common. Patchy alteration
to sericite or muscovite is common.
The major mafic mineral is pyroxene. Clinopyroxene is usually
dominant over orthopyroxene though abundances of the two vary
15
16
FIGURE 3
Modal Plagioclase for Speculator Sheet Traverses
2S, 3S, and 4S presented as a function of
stratigraphic feet above the lower contact.
500 +-(.) c +-c: 400 0 (.)
'-CD 300 ~ 0
~200 0 ..a c +- 100 CD CD
'+-
17
·--y· . £__
1· •
35• \ "· .----------- ./· ·:--.. ,/ -;::•2S \ Vs " o.____.~~_._~'~_._~_.__..__..~
20 40 60 80 100 modal 0/o plagioclase
18
substantially. Pyroxene occurs in subequant to elongate crystals, 0.5 -
2 rrm in size with a few relict orthopyroxene phenocrysts reaching up to
8 rrm in length. Clinopyroxene exhibits occasional orthopyroxene
exsolution along the (100) plane, especially in the larger grains
(greater than 1 mm). Rare exsolution of opaque minerals occurs along a
plane which makes a small angle to (100). Orthopyroxene shows a fine
twinning which has, in some cases, initiated alteration to amphibole
along the twin plane. The pyroxene crystals are sometimes altered to
hornblende along crystal boundaries or in patches in the interior of the
crystal. Some have reaction rims of hornblende or are altered to
chlorite •
. The hornblende occurs in elongate crystals generally less than 0.5
mm in length; a very few larger crystals extend up to 2 rrm.
Biotite occurs in both small (less than 0.5 rrm) crystals and larger
elongate crystals up to 2 rrm by 1 mm in size. Biotite and hornblende
define the foliation. It alters to a chlorite or to an unidentified
fibrous material. Locally, small (less than 0.5 mm), lath-shaped
titanite (retrograde?) grows along its cleavages.
Zircon and apatite are ubiquitous accessories. Apatite is found in
subhedral crystals, sometimes growing to large sizes (6 mm by 1.5 mm).
Hematite occurs as staining along crystal boundaries and fractures, and
in aggregates of opaque minerals. One large (greater than 1 rrm)
euhedral crystal of titanite was observed in the anorthosite of the
Speculator Sheet. The opaque phases are ilmenite with very rare
magnetite and pyrite. They usually occur in small crystals (less than
19
0.5 nm) or occasionally form irregular aggregates less than 1.5 mm in
size. These opaque phases are commonly rimmed by metamorphic titanite
and only rarely by garnet. They are often associated closely with other
mafic phases, at times surrounding and interpenetrating them.
Deep pink garnet occurs in porphyroblasts of varying shapes up to 12
mm in diameter, generally free from inclusions but cut by parallel
fractures. The distribution of the garnet varies widely; there are
substantial fluctuations in garnet content with the more mafic rocks
being those that are more garnetiferous. As compared to those in the
gabbroic veins, the garnets in the anorthositic gabbro facies display a
striking dactylic intergrowth that surrounds the porphyroblasts in a
regular and undeformed reaction rim that grows larger as the central
garnets decrease in size. In some rocks, the central garnet has
completely disappeared. The rim consists of thin (20 micron) stringers
of orthopyroxene and rarely hornblende (Plate 1, a and b) which radiate
outward approximately normal to the face of the garnet, penetrating
surrounding plagioclase or separated by equally thin stringers of calcic
plagioclase (An70_90). Where large plagioclase grains occur adjacent to
the garnet they display a sharply defined calcic rim 20-30 microns
thick. Opaque oxides often occur around and between the orthopyroxene
stringers (Plate 1, c).
The mafic dikes that cut the eastern country rock contain 28 to 70%
hornblende, 26 to 42% plagioclase, 1 to 17% orthopyroxene, 0 to 8%
garnet, 0 to 4% clinopyroxene, and traces of biotite, opaque minerals,
apatite and hematite (see Appendix 1; 4S2, 4S3, 4S5). The gabbros of
20
PLATE 1
a) Speculator Sheet garnet with an
orthopyroxene - plagioclase corona
b) Speculator Sheet garnet corona with hornblende
c) Speculator Sheet garnet corona with opaques
d) Tenantville spongy garnet
22
the basal portion of the sheet differ from these mafic dikes because
they contain pyroxene as the dominant mafic phase and they are more
leucocratic and coarser grained (i.e. Appendix 1: 4S8, 4S9, 4Sl0).
The Wells Si 11
a) Gabbro
The medium-grained, gneissic gabbro contains 39 to 49% plagioclase,
9 to 38% hornblende, 2 to 19% orthopyroxene, 1 to 15% clinopyroxene, 0
to 11% biotite (with 1 to 3% conman), 0 to 15% quartz, 0 to 9% opaque
minerals, 0 to 3% garnet, and traces of apatite, hematite and zircon.
The quartz usually occurs in veins; hematite staining is often present
along grain boundaries and fractures.
Plagioclase occurs in small (less than 0.5 nm) granulated crystals
and exhibits pericline and albite twinning. Antiperthitic blebs are
rare. The grains are equant with little indication of preferred
orientation.
The main mafic mineral is usually hornblende which occurs in
elongate crystals up to 2 mm long. These segregate into gneissic bands
of subparallel crystals with large interstitial opaques.
Orthopyroxene and clinopyroxene occur in 0.5 to 1.0 nm subequant to
elongate crystals with a few larger relict phenocrysts (about 2 mm).
The large crystals of both pyroxenes exhibit exsolution similar to the
pyroxenes of the Speculator Sheet. The pyroxenes conmonly show
alteration to amphibole along grain boundaries and in patches within the
pyroxene.
The biotite crystals are rarely longer than 1 nm and are sometimes
23
bent. A few are broken. The opaque minerals are primarily ilmenite
with little magnetite and rare pyrite. These may form irregular
aggregates up to 2 mm long. Hematite is often present in these
aggregates. Garnet, when it does occur, is very pale pink to colorless,
and either forms small euhedral crystals 0.5 mm in size, or it forms in
contact with the opaque aggregates. It has few inclusions, which are
most often apatite or quartz.
b) Anorthositic Gabbro
The anorthositic gabbro contains 50 to 81% plagioclase, 3 to 39%
hornblende, 1 to 16% clinopyroxene, 0 to 10% orthopyroxene, 0 to 8%
biotite, 0 to 24% quartz, 0.2 to 6% opaque minerals, 0 to 2% apatite and
traces of zircon and hematite. Quartz occurs in veins. The
anorthositic gabbro is compositionally and texturally similar to the
anorthositic gabbro of the Speculator Sheet, except that: (1) Garnet is
much less common. (2) Hornblende is sometimes the dominant mafic phase.
(3) The plagioclase content of the rock does not change in any
systematic manner; nor is the base of the unit more gabbroic. (4) The
pyroxenes tend to form clots which are surrounded by hornblende.
c) The Tenantville Facies
The fine-grained Tenantville facies contains 72 to 85% plagioclase,
0 to 18% hornblende, 0 to 6% clinopyroxene, 0 to 6% orthopyroxene, 0 to
3% biotite, 0 to 3% garnet, 0 to 18% quartz, 0 to 3% opaque minerals,
and traces of apatite, zircon and hematite. The quartz occurs mainly in
veins.
The Tenantville facies of the Wells Sill is generally more
24
leucocratic and finer grained than the anorthositic gabbro, with
individual crystals rarely exceeding 0.5 ITITI in size and always less than
2 rrm.
Plagioclase occurs as small (less than 0.5 ITITI) rounded grains; some
aggregates form pseudomorphs of original plagioclase phenocrysts 10-15
mm in size. Rarely, a large (5-6 ITITI) relict plagioclase will remain
intact in the center of the aggregates. The plagioclase exhibits both
albite and pericline twinning, has rare antiperthitic blebs and is more
sericitized in this facies than in any other.
Hornblende is the most common mafic mineral, and occurs in small
elongate crystals or blocky crystals 1.5 mm in size.
Pyroxene is a minor phase and may be altogether absent (Appendix 1;
5W7). Where it does occur, it is most often clinopyroxene with minor
orthopyroxene or has been completely altered to chlorite. It forms
small (less than 0.5 111Tl) rounded to irregular crystals. It exhibits no
exsolution; alteration to hornblende is rare.
Biotite occurs in small (less than 1.5 ITITI) elongate crystals. The
growth of titanite along its cleavages is characteristic. Some biotite
crystals (less than 1.0 nm in size) are riddled with opaque inclusions;
these are most probably a secondary alteration of the opaque phase.
Light pink garnets form in small (less than 0.5 mm) euhedral
crystals which have a tendency to form aggregates of what can be termed
a 11 spongy 11 garnet up to 4 l11TI in size. These spongy garnets contain
inclusions of plagioclase, pyroxene, hornblende, biotite, and opaque
minerals and have no reaction rims (Plate l, d).
25
Apatite and zircon are corrmon accessories; hematite is rare. Opaque
minerals are ilmenite with rare magnetite and pyrite, and form
aggregates generally less than 2 nm long which contain frequent
inclusions of titanite.
The Oregon Dome
The strongly lineated anorthositic gabbro contains 60 to 83%
plagioclase, 3 to 18% clinopyroxene, 0.2 to 7% hornblende, 1 to 7%
orthopyroxene, 0.2 to 7% garnet, 0.2 to 6% opaque minerals, 0 to 2%
biotite, and traces of apatite, zircon and hematite.
This facies of the dome is similar to the anorthositic gabbro and
gabbroic anorthosite of the sills except for grain size, the nature of
the garnets and the opaques. In the Oregon Dome, grain size is larger
overall, with single crystals of minerals commonly achieving lengths of
2-4 nm and larger. The garnets are pink and have no coronas of
orthopyroxene and plagioclase; they are anhedral with many inclusions
and are sometimes closely associated with mafic phases (opaques and
hornblende). There is a consistent thin rim of plagioclase between the
mafic phase and the garnet. Magnetite is also more conman in the dome
than in the sills.
The Country Rock
The finely lineated syenite gneiss of the Rooster Hill/Little Moose
Mountain Formations in contact with the Speculator Sheet contains 60 to
70% patch perthite, 10 to 15% plagioclase, 10 to 15% hornblende, 5 to
10% quartz, 1 to 3% opaque minerals, 1 to 3% clinopyroxene, 1 to 2%
biotite, and traces of orthopyroxene, zircon and apatite.
26
The coarsely-exsolved patch perthite occurs in small to medium sized
(less than 2 mm), equant to irregular crystals. Hematite staining is
heavy along crystal and exsolution boundaries. Elongated and flattened
quartz crystals occur as large 4.0 rrm laths. Most hornblende occurs in
small (0.5 mm) laths though there are a few 2 rrm irregular fractured
crystals. The plagioclase occurs as small (less than 0.5 rrm) rounded
crystals. Biotite occurs as small (less than 0.5 rrm) laths that are
altered to a red, amorphous mineral. Opaque minerals are small equant
crystals forming irregular aggregates up to 1.5 rrm long in very mafic
areas. Equant to rectangular clinopyroxene crystals are usually less
than 0.5 rrm and always less than 2 rrm in size. Patchy alteration of the
pyroxene to hornblende is corrmon. Orthopyroxene is extremely rare,
occurring as small, isolated, equant to rectangular crystals. Euhedral
zircons and euhedral apatite laths are small (less than 0.5 rrm).
Hematite staining is pervasive, especially on perthite crystals and
biotites and other mafic phases.
MINERAL CHEMISTRY
Mineral analyses were obtained from 17 thick sections of the
Speculator Sheet, two thick sections of the mafic veins that cut the
eastern end of the Sheet (4S2, 4S5), four sections each of the three
facies of the Wells Sill, and six sections of the anorthositic gabbro
facies of the Oregon Dome. The number of analyses of each mineral
species in each slide ranged from one to ten with an average of five,
depending on the abundance of the mineral. A second set of amphibole
and biotite analyses was taken to determine fluorine and chlorine
contents. Analyses are presented in Appendix 3.
Plagioclase
Plagioclase compositions for the two sills and the Oregon Dome are
presented in Figure 4. Within the Wells Sill, the plagioclase becomes
more calcic from the gabbro (An42 _47 ) to the anorthositic gabbro
(An 50_54 ) to the Tenantville facies (An52_56 ). Plagioclase composition
for the Speculator Sheet ranges from An44 to An87 • The three samples
with high anorthite content (An70_87 ) are from rocks which have a large
volume of rimmed garnets. The composition of plagioclase in garnet
coronas ranges from An74 to An87 , except for one sodic analysis
resulting from zoning in large plagioclases adjacent to garnet with no
or very minute rims. Data on the Oregon Oome 1 s anorthositic gabbro
indicates a composition ranging from An47 to An53 • Orthoclase content
in all the plagioclases is less than 2%.
Pyroxene
Pyroxene compositions are presented in Figure 5. The tie lines
27
28
FIGURE 4
Plagioclase Compositions for the Oregon Dome,
Wells Sill, and Speculator Sheet.
Oregon Dome: small circles - anorthositic gabbro
Wells Sill: squares - gabbro facies
triangles - anorthositic gabbro
large circles - Tenantville facies
Speculator Sheet: filled triangles - gabbroic anorthosite
x - from garnet coronas
squares - gabbroic veins
U> c: 0 ·-+-U> 0 a. G> E E 0 0 (.) c
c Cl) 0 U> O> 0 G> .... 0 0 0 C'I 0 -a..
... 0
... 0
29
c
30
FIGURE 5
Pyroxene Compositions for the Oregon Dome,
Wells Sill and Speculator Sheet.
Symbols are as in Figure 4.
en c 0 ..... en 0 a. E 0 (.)
Cl> c Cl> )( 0 ... ~
Cl..
Cl> E 0 0 c: 0 C'I Q) ... 0
"'O :c
en Cl)
Q)
3:
0 v
31
-Q) Cl> .c en ... 0 -0 :J (.) Cl> 0.. en
0 v
32
between coexisting clinopyroxene and orthopyroxene are generally
subparallel, both within and between the three bodies. The
clinopyroxenes in the Speculator Sheet and in the Tenantville and
anorthositic gabbro facies of the Wells Sill range in composition from
Wo46Fs12En42 to Wo48Fs19En33 ; however, those of the Oregon Dome and the
gabbro facies of the Wells Sill tend to be more iron-rich, ·with
compositions in the Oregon Dome ranging from Wo44Fs19En 37 to
Wo46Fs22 En32 and in the gabbro facies from Wo46 Fs17En37 to Wo46Fs21 En33 •
The same relationship is seen in the orthopyroxenes, with the Speculator
Sheet and the Tenantville and anorthositic gabbro facies of the Wells
sill ranging from wo1Fs34En65 to wo1Fs47En52 , whereas the Wells gabbro
and Oregon Dome range from wo1Fs46En53 to Wo1Fs51 En48 • Na, Ti, and Mn
are present only in trace amounts (less than 0.05 moles of oxide) in
both pyroxenes. The clinopyroxenes of the Speculator Sheet contain
approximately 11% of calcium Tschermak 1 s molecule while those of the
Wells Sill contain only 8%. The clinopyroxenes of the Oregon Dome
contain 12% of the molecule. The orthopyroxenes of the Speculator Sheet
contain 4% of magnesium Tschermak 1 s molecule as do those of the Wells
Sill, whereas the Oregon Dome contains slightly over 5%.
Amphibole and Biotite
Compositional data for the amphiboles in the three bodies are given
in Figure 6 according to Leake 1 s (1978) classification for calcic
amphiboles. Amphiboles for all three bodies are ferroan pargasitic
hornblende. A few amphiboles, usually from rocks affected by garnet
corona - producing reactions, are ferroan pargasite. There is a strong
33
FIGURE 6
Amphibole Compositions for the Oregon Dome,
Wells Sill and Speculator Sheet
according to Leake (1978).
Symbols are as in Figure 4.
Cal
cic
Am
phib
ole
Com
posi
tions
Ca+
Na
> 1
.34
; N
a<
0.6
7;
Na+
K >
0.5
0;
Ti<
0.5
0
Si
7.5
7.0
6.5
6.0
- Q) LL + 1.0
~ 0
.5
- ....... Ct ~
0
i >--- - I
I I
Ede
nitic
E
deni
te
Hor
nble
nde
Ferr
o-Fe
rro
-E
deni
te
Ede
nitic
H
ornb
lend
e
I
I
Par
gasi
tic
Par
gasi
te
Hor
nble
nde •••
~~i ..
.... Fe
rroan
~· ~
Parg
a site
6
.oce
. Fe
rroa
n 1
Par
gasi
tic
Hor
nble
nde
Ferr
o-Fe
rro
-P
arga
site
P
arga
sitic
H
ornb
lend
e
I I
I
I
w
-+::>
35
negative correlation between the amounts of iron and octahedral aluminum
as there is a positive correlation between titanium and tetrahedral
aluminum, as is illustrated in Figure 7. This is a reflection of
titanium and tschermakitic substitutions: 0.48 Al atoms/formula unit
were substituted in the Y site by tschermakitic substitution, and 0.11
and 0.17 Ti atoms/formula unit were substituted by Ti+4 + 2AlIV = 2Si + Mg 2+ and Ti+4 + 2Na = Mg + 2Ca, respectively. Analyses for fluorine (see Appendix 3) indicate as much as a 7.5% (= F/(F +OH+ Cl))
substitution of fluorine for (OH) in the Speculator Sheet while the
greatest substitution in Wells is 4.5%.
Biotite compositional data for the two sills and the Oregon Dome are
illustrated in Figure 8. The biotites in all three bodies show only a
small range in composition. Titanium substitutions are reflected by a
correlation between the amounts of Ti and octahedral vacancies as in the
upper plot of Figure 9. In the lower plot of Figure 9, the x-axis
corrects for the Al tschermakitic substitution and the Y axis corrects
for the Ti vacancy substitution. The data should fall along the solid
line which indicates Ti tschermakitic substitution; however, they fall
along a parallel line because 0.09 atoms/formula unit of Ti were
substituted by a third substitution. These substitutions are: 0.12 Al
atoms/formula unit were substituted in the Y site by tschermakitic
substitution, and 0.175, 0.34 and 0.095 Ti atoms/formula unit were
substituted by Ti+4 + 2AlIV = 2Si + Mg+2, Ti+4 +a vacancy = 2Mg+2 and Ti02 = Mg(OH) 2, respectively. Analyses for fluorine indicate as much as 8.25% (= F/(F +OH+ Cl)) of fluorine has been substituted for (OH) in
36
FIGURE 7
Amphibole compositional trends for the
Speculator Sheet:
tetrahedral aluminum vs. titanium and
Fe/(Fe+Mg) vs. octahedral aluminum
(atoms/formula unit).
37
·\ • c:::t" c:::t" r lQ • • • •
• o_ ' 0 -r ::::> IQ ::::> • '+-• • '+- ........... ........... \ 0 0 -• (!) - {. (!) ~ • \ ~ ·- ~ •• r- -•
38
FIGURE 8
Biotite Compositions for the Oregon Dome,
Wells Sill and Speculator Sheet
projected on the biotite quadrangle.
Symbols as in Figure 4.
39
eastonite siderophyl lite 3.0 ------.-,---~,~-----.-, ---......
"' -c -:l 0 :l E ~
0 .... ........ 2.5 -"' E 0 -0
>
40
FIGURE 9
Biotite compositional trends for the Speculator Shee
titanium vs. octahedral vacancies, and
titanium minus octahedral vacancies vs.
tetrahedral minus octahedral aluminum
(atoms/formula unit). Solid line indicates
trend of Ti tschermakitic substitution.
-::J '+--............
0.7
.So.5
I-
41
0.3 ________ ......_ __ ,_____ 0.2 0.3 0.4 0.5
:J0.4 '+--............
0 -r;i 0.2 D I
o!lI (a/fu)
I- 0.0 ____ ......._ __ ....__ __ _..___ 2.0 2.2 2.4 2.6
Alnz::.-Alm (a/fu)
42
the Speculator Sheet biotites, whereas the maximum substitution in the
Wells Sill is 4.0%.
Garnet
The garnet compositions are summarized in Figure 10. The Mn and Ca
molecular contents for the garnets are uniform at about Sp2_3 and
Gr17_21 respectively; however, the amounts of Fe and Mg vary
considerably. Garnets which do not have coronas have Py13Alm65 to
Py19Alm60 (i.e. the Wells Sill and Oregon Dome plots, figure 10).
Those with reaction rims range from Py19Alm61 to Py33Alm45 (see
Speculator Sheet plots, Figure 10).
43
FIGURE 10
Garnet Compositions for the Oregon Dome,
Wells Sill and Speculator Sheet.
Symbols as in Figure 4.
~--
........ -
.--v....-~~~-~-'"~·,--
~ A
lm
+ Sp
10
Gar
net
Com
posi
tions
Py
Ore
gon
Dom
e 4o)
0
21 f
20
30 A
lm
10
20
+ Sp
Py
Py
Wel
ls S
ill
o} S
pecu
lato
r Sh
eet
.;::. .;::.
4
.A
AA
.a
.. 21
t ..
a§>
a
30
Alm
10
20
30
40
50
Gr
+ Sp
WHOLE ROCK CHEMISTRY
Chemical analyses and norms are presented in Appendix 2; average
compositions are presented in Table 1 for five analyses of the
Speculator Sheet gabbroic anorthosite, three analyses of the Wells
gabbro, one analysis of the Wells Tenantville facies, three analyses of
the Wells anorthositic gabbro and five analyses of the Oregon Dome
anorthositic gabbro.
The data from the Speculator Sheet and Wells Sill are presented in
Figure 11 as a function of stratigraphic distance above the lower
contact with the metasediments. Variation of major oxide data in the
Speculator Sheet is minor, and is a function of the mafic content of the
rock (see Figure 3); as the rock becomes more leucocratic towards the
top of the exposure, Si02, Al 2o3, Cao and Na2o increase while Fe203,
MnO, and MgO decrease.
Variation within the Wells Sill is slightly more complex. The mafic
zone on the upper and lower sides of the sill is defined by high MgO,
Fe203, MnO, Ti02, and P2o5 values and low Si02, Al 2o3,. Na2o, and K2o values; however, the point for the basal gabbro facies (100 feet, Figure
11) is high in silica because of a fine quartz veining in the basal
section of the sill. The three data points from the anorthositic gabbro
are rich in Si02, A1 2o3, Cao, Na2o, and K2o and poor in Fe2o3, MnO, MgO,
Ti02, and P2o5• They approximate the values for the Speculator Sheet
rocks. The one value for the Tenantville facies lies at a greater Si02 value and lower Fe2o3 and MgO values than the anorthositic gabbro, but the rest of the data are comparable to the anorthositic gabbro.
45
TABL
E l
AVER
AGE
CHEM
ICAL
COM
POSIT
IONS
SPEC
ULAT
OR
WELL
S SI
LL
0.0.
Pa
rent
Ma
gma
fade
s Ga
bAn
Gab
Tena
nt
AnGa
b An
Gab
* W
eight
%
Si02
52
.54
49,7
8 57
.43
53.4
9 49
.49
54.0
5 Ti
02
0.40
2,
03
0.61
0.
61
3.02
0.
53
Al20
3 24
.91
15 .0
1 22
,06
23.8
8 19
.34
25.4
4 Fe
203
4. 12
14
.24
3,89
4.
41
10.5
7 2.
35
MnO
0.06
0.
20
0,03
0.
03
0. 11
Mg
O 2 .
15
5.33
1.
38
l,74
2.73
1.
30
+::-
cao
10,8
5 9,
07
8.32
9,
61
9.71
10
.20
O'\
Na20
3.
72
2. 14
3.
06
4.20
2.
84
4.54
K2
0 0,
54
0.85
1.
36
1.27
0.
72
0.90
P2
05
0.08
0.
24
0.20
0.
14
0.39
0.
09
H20
0.53
0,
87
0,59
0.
81
0.89
-
i
Tota
l 99
.90
99.7
6 98
.93
l 00.
19
99.8
1 99
.58
ppm
Rb
3.6
12.2
18
. l
22.2
0.
7 Sr
52
0.5
258.
5 32
9.7
413.
8 64
3.2
* Bu
dd1 n
gton
197
2
-~-._,,-.,.,.._~ -
-~,,,.-,-
_,.,,._~-
'~<"--"--'"""'~:_-
____
",."-,_--,=-.,,_~_ •. ~
~-~---
-~----~
----=~-~
-,---,~~-~
47
FIGURE 11
Oxide weight% variation of the sills as a
function of stratigraphic feet above the lower
contact of the sill with the metasediments.
Symbols are as in Figure 4.
IC> N 0 0 "'o a. .
0
N 0 0 c ~ 0
0
N
0 N-~
0
C\I N 0 I- 0
0 ...
"' 0 z N
= 0 0 °' -(.) CD CD ,._
.s::; CJ)
0
"' 0 :l Cll
N 0 0 "'o a. ci
I _o_ ·~,
49
FIGURE 12
Harker Variation Diagrams for the major
elements (weight%) and Rb and Sr (ppm).
Symbols are as in Figure 4.
50
Harker Variation Diagrams
30 16 0 i. Al 20 3 Fe203 ~ 0 26 12
MtA 0 0 i. 0
22 0 0 8 0 i. & A i.
18 9 4 ~a 0
0 i. 14 0 0 0
i. coo 12 a r:j ~ ~{;. 0 MgO 10 A
0 a ~ 4 ·6) 0 0 6 ~ MAA 0
6 Na20 i. 0 4
CJ>(}J ~ 4 Q) Ti02 0 0 '8~ 2 0 2 0
K20* 2 0 0
ell~ 0 0 60
0 Rb 0 o0 i. Sr 600 40
0 0 i. 0 0 ~
400 6 AA ~A 20 A 0 0 o• 0 0
200 0 0 0 %i-M. 0 48 52 56 60 48 52 56 60
Si02 weight 0/o Si 02 weight 0/o
51
Figure 12 shows the same data plotted on Harker variation diagrams.
Although the scatter is large, several trends are evident. With
increasing silica, which is the result of an increase in plagioclase
content, Al 2o3, Cao, Na2o, and K2o increase, and Fe2o3, MgO, and Ti02 decrease. Once again, the values for the Tenantville facies and the
quartz-veined gabbro plot on the high silica side of the diagram, but
their oxide components are similar to the anorthositic gabbro and Wells
gabbro, respectively. The Wells gabbro has a trend that shows a much
smaller change of oxide content with Si02•
The Rb and Sr data plotted in Figure 12 show a tendency to increase
and decrease respectively with increasing Si02• Rb content, as well as
K2o content, is lower in the Oregon Dome and Speculator Sheet as
compared to the Wells Sill.
In most cases, the rocks from the Wells Sill span all the values
from the other two bodies with the gabbro samples more mafic than the
Oregon Dome compositions and the anorthositic gabbro samples being very
similar to rocks from the Speculator Sheet. Though all the rocks
studied are calcic (Peacock, 1931), the AFM ternary variation diagram
(Figure 13) indicates that the Speculator Sheet and the Wells
anorthositic gabbro and Tenantville facies have a differentiation trend
showing only moderate iron enrichment, whereas the Oregon Dome and the
Wells gabbro have a differentiation trend of more extreme iron
enrichment. Nockold's and Allen's calc-alkalic (1953) and alkalic
(1954) differentiation trends are provided for reference.
52
FIGURE 13
AFM Ternary Variation Diagram for the
Speculator Sheet, the Wells Sill, and
the Oregon Dome. A =Na2o + K20; F = Total Fe + MnO M = MgO (weight%). Symbols as in Figure 4;
filled small circle is the composition of the
hypothetical parent magma for the Adirondack
anorthosite (Buddington, 1974); solid line is
the average calc-alkalic differentiation trend
(Nockolds and Allen, 1953); dashed line is the
average alkalic differentiation trend
(Nockolds and Allen, 1954).
53
lJ....
METAMORPHIC PETROLOGY
Intensive Parameters
The intensive parameters for the metamorphism of the two sills and
the Oregon Dome are similar; significant variations were not found
between the three bodies.
Estimates of geothermometry in Table 2 indicate a maximum
metamorphic temperature of 650 - 700° C, which agrees well with the
feldspar and oxide geothermometry presented by Bohlen and Essene (1977),
whose four samples from this area yield temperatures of 690 - 700° C.
The results of the clinopyroxene - orthopyroxene geothermometry are high
because of one of two reasons. First, they could be a reflection of
igneous temperatures, with reequilibration between the two minerals upon
cooling ceasing effectively at temperatures below 850° C. Second, _they
could be a reflection of the fact that both orthopyroxene -
clinopyroxene geothermometers are designed for less Fe-rich pyroxenes at
much higher temperatures than the pyroxenes in these rocks. Bishop's
(1979) ilmenite - clinopyroxene geothermometer indicates a maximum
temperature of 600 - 700° C with a late reequilibration at 425 - 525° C,
which could have caused the late formation of titanite in and around the
ilmenite. This is consistent with the conclusions of Jaffe, Jaffe and
Ashwal (1977) who determined a period of late reequilibration at 500 -
700° C in the Marcy Quadrangle, based on low pigeonite exsolution in
host augite throughout the quadrangle.
The stability of plagioclase (Ghent, 1977) places a limiting maximum
metamorphic pressure on the sills of 13 kb. Thompson (1976b) indicates
54
55
TABLE 2
GEOTHERMOMETRY
Estimate Mineral Reference oc Pair 850t25 opx .. cpx Wood and Banno (1974) 855±25 opx .. cpx We 11 s ( 1977)
625±25 cpx-hbd Kretz and Jen (1978) 655±100 gar-bio Thompson (1976b} 693±100 gar-bio Goldman and Albee (1977} 696±100 gar-bio Ferry and Spear (1978) 683±50 cpx-gar Raheirn and Green (1974) 654±100 cpx-gar Dahl (1980) 650±50 cpx .. ilm Bishop (1979)
475±50 cpx-ilm Bishop (1979)
56
that the country rock assemblage Mg-rich biotite and sillimanite is
stable at minimum pressures of 5.8 to 6.4 kb at the temperatures of
metamorphism of the area. The presence of sillimanite in metasediments
throughout the Adirondacks in itself gives an upper limiting pressure of
6, 8, and 10 kb at temperatures of 600, 700, and 800° C respectively
(Holdaway, 1971 and Richardson et al., 1968). Determinations of
pressure in other areas of the Adirondacks have yielded pressure
estimates from 7 to 10 kb (Jaffe, Jaffe and Ashwal, 1977; De Witt and
Essene, 1975). By linear extrapolation of experimental data on garnet
assemblages to crustal conditions (Kushiro and Yoder, 1966; Green and
Ringwood, 1967 and 1972; Raheim and Green, 1974), Whitney (1978)
determined that in silica-saturated rocks in the Adirondacks, garnet of
compositions reported for the Adirondack Highlands (Mclelland and
Whitney 1977; (FeO/MgO)gar/(FeO/MgO)cpx = 6.7 - 8.2) forms at temperatures of 715° ! 25° C and pressures of 7.1!1.4 kb. Martignole's and Schrijver's (1971) analysis of garnet-quartz
symp1ectites in rocks around the Adirondack and Morin anorthosite masses
suggested a load pressure of the order of 8 - 10 kb.
It is concluded that the sills were metamorphosed at a pressure of
about 8 kb, which corresponds to a depth of roughly 25 km.
Metamorphic oxygen fugacity may be estimated in two ways. The
analysis of three magnetite-ilmenite pairs yields an estimate of log f 0 2
= -14 to -17.5 for a temperature of 650 to 700° C from Buddington and Lindsley (1964), though there may be a problem because of the
(retrograde?) formation of titanite, as was noted previously. Wanes
57
(1981) has reassessed data for the assemblage titanite - magnetite -
quartz, all three of which are minor constituents of the sills and the
Oregon Dome. His data yield an estimate of a minimum log f 0 of -16.8 2
to -18.5 for a temperature of 650 - 700° C. A log f 0 greater than the 2
Wanes (1981) estimate and less than the magnetite-ilmenite estimate is
indicated for the metamorphism of these rocks.
The biotite fluorine data was used to estimate metamorphic fluid
composition, using the experimental data of Munoz and Ludington (1974,
Figure 5). A log fH 0/fHF = 3.5 - 5 is indicated for 650 - 700° C and a 2 F/(F +OH+ Cl) = 5 - 10%~ Troll and Gilbert (1972) state that
phlogopite extracts more fluorine from a vapor under comparable
conditions and at a faster rate than does tremolite. The hornblende
data generally support these conclusions, with maximum substitution
approximately 1% less than that in biotite.
Metamorphic Assemblages
The stability of garnet within the Adirondack granulite facies has
been a matter of debate for some time (Buddington 1963 and 1966; de
Waard, 1965 and 1967b; Whitney, 1978; Whitney and Mclelland, 1973;
Mclelland and Whitney, 1977). Changes in rock composition, temperature,
and load and water pressure have been cited to explain the occurrence
and non-occurrence of garnet. A study of the garnet petrology in the
Wells and Speculator Sheets provides some valuable data for resolving
the controversy because of the lack of a systematic temperature or
pressure gradient over the area, and because both unrimmed and rimmed
garnets occur in the same rock type.
58
The five mineral assemblages found in the Wells and Speculator Sills
(in addition to quartz, plagioclase, ilmenite and magnetite) are:
(1) Hornblende-Garnet-(Biotite) H-G-(B)
(2) Orthopyroxene-Clinopyroxene-(Garnet) 0-C-(G)
(3) Hornblende-Clinopyroxene-Orthopyroxene-Garnet-(Biotite) H-C-0-
G-(B)
(4) Hornblende-Orthopyroxene-Clinopyroxene-(Garnet) H-0-C-(G)
(5) Hornblende-Biotite-Orthopyroxene-Clinopyroxene-(Garnet) H-B-0-
C-(G)
Assemblage (3) H-C-0-G-(B) occurs in both the gabbro and Tenantville
facies of the Wells Sill (2Wl, 2W6, 5W4). Microprobe data indicate that
the ratio Fe/(Fe+Mg) = XFe decreases in the order Gar-Hbd-Opx-Cpx for
assemblages (1), (2), and (3). The mineral chemistry, textural evidence
of the rimming of pyroxene by hornblende, patchy interior alteration of
pyroxene to hornblende, and the garnet coronas suggest the reaction
hbd + qtz = opx + cpx + gar + an + ab + H2o can be used to describe the stability of hornblende. Assemblage (1) H-
G-(B) is found only in the Tenantville facies of the Wells Sill (1W15,
5W7). The assemblage (2) 0-C-(G) occurs only in pockets of anorthosite
in the Speculator Sheet (2S20) where the garnet is isolated from the
system by a corona.
Assemblage (5) H-B-0-C-(G) is found in the anorthositic gabbro of
both sills and also in the Wells Sill gabbro (i.e. 1S30, 2Sl7, 1W2,
1W9). It is the most common assemblage in the sills. Microprobe data
indicate that XFe decreases in the order Hbd-Opx-Bio-Cpx. The mineral
59
FIGURE 14
ACFM Diagram with mineral compositions projected
from plagioclase, quartz and water onto the
Cao - FeO - MgO plane. The three phase
triangle sweeps to the left with
metamorphism, forming the garnet coronas.
0 c (.)
60
61
chemistry and textural evidence of the close association of biotite with
other mafic phases suggests the following reaction controls the
coexistence of hornblende and biotite,
hbd + bio + qtz = an + ab + opx + cpx + or + H2o where anorthite, albite and orthoclase form a plagioclase solid
solution. Assemblage (4) H-0-C occurs in the anorthositic gabbro of
both sills (3S9, 4S8, 1Wl2).
Geothermometry indicates no regular change of temperature for the
different assemblages; there is a non-systematic variation of 65° that
is within a reasonable uncertainty for the geothermometer and is
therefore not considered to be significant. As there is no evidence to
substantiate a temperature, pressure or f 0 gradient (though changes 2
that are within the errors of the estimations used here may occur) the
factor controlling the assemblage is believed to be the fugacity of
water. There is a significant variation from assemblages with two
hydrous mineral phases to totally anhydrous assemblages. This is
consistent with Buddington (1963) who determined that the variety of
mineral assemblages in the orthogneisses between any two isograds is
best interpreted in terms of development under a wide range of PH 0• 2 The metamorphism of these rocks, either on cooling or during the
Grenville Orogeny, not only involved recrystallization of mineral
phenocrysts such as plagioclase and pyroxene and the development of a
foliation but also involved an addition of mobile components. Late
stage magmatic water or fluid from the surrounding country rock affected
the original igneous mineral assemblages. Hornblende was produced by
62
hydration of original pyroxene by reactions similar to opx + cpx + an +
ab + H2o = hbd + qtz. Biotite resulted from a mobilization of potassium from the potassic content of the original plagioclase and possibly from
the surrounding quartz syenite. The biotite present in these rocks
probably accounts for the low potassic content of this plagioclase
(Orl.O-l.S) as compared to Ashwal~s (1978) primary plagioclase
composition of Or6, as well as accounting for the relative rarity of
antiperthite. Both biotite and hornblende have been stabilized at
granulite conditions by the substitution of fluorine (Holloway, 1977).
The calcium content of the plagioclase is affected by the formation
of garnet (becomes more albitic), or subsequent garnet coronas (becomes
less albitic). With increasing grade of metamorphism, hydrous phases
react to form garnet and pyroxene and to release water. At maximum
metamorphic conditions garnet and clinopyroxene react to form the garnet
coronas, forming the anhydrous mineral assemblage 0-C-(G).
Ashwal et al. (1981) suggest that the cooling of the Adirondack
Highlands continued 200-300 my after the magmatic event, and may have
been associated with relatively rapid uplift after isobaric cooling from
magmatic temperatures to ambient lower crustal temperatures (about 800°
C), producing the metamorphic mineral assemblages. The present data are
not inconsistent with such a history. The assemblages could be the
result of two different thermal histories: (1) Cooling from magmatic
temperatures produces hydrous mineral assemblages. A subsequent rise in
temperature accompanied by decreasing f H 0 produces garnet and 2 eventually garnet coronas. (2) Cooling from magmatic temperatures to
63
Grenville temperatures produces hydrous mineral assemblages. Uplift
then causes dehydration, producing garnet and finally the garnet
coronas.
Bartholome (1956) and also Buddington (1966), with particular
reference to the Speculator Sheet, believe that the rims formed
contemporaneously with the garnet, drawing an analogy between the
orthopyroxene - plagioclase rims in this gabbroic anorthosite with the
hornblende rims of garnets in the amphibolite of the Gore Mountain
Barton garnet mine. The observations of this study are inconsistent
with this hypothesis. Garnets are found in every stage from totally
unrimmed to spherical knots of myrmekitic orthopyroxene and plagioclase
grains suggesting complete replacement of the garnet. In contrast to
the rimmed garnets of the anorthositic gabbro, those of a derivative of
the anorthositic gabbro, the Tenantville facies, have no rims. The
composition of the garnet is the critical factor determining if it
reacts: only those garnets that have a pyrope content greater than 18
or 19% show development of coronas; calcium content is not critical.
This is illustrated by the Na-absent ACFM diagram of Figure 14, where
the mineral compositions are projected from plagioclase onto the Cao -
FeO - MgO plane of the tetrahedron. All those garnets with an
Fe/(Fe+Mg) ratio less than 0.77 will be involved in the reaction gar +
cpx + qtz = opx + an, as the three phase triangle sweeps to the left with metamorphism. The effect of ilmenite would be to decrease the
amount of ferromagnesian silicate phases and to increase the amount of
quartz needed for reaction (Mclelland and Whitney, 1977). Those garnets
64
with Fe/(Fe+Mg) greater than 0.77 will not participate in the reaction
at these metamorphic conditions. The hornblende that occurs in some
rims is a retrograde product of the original orthopyroxene, formed by
increasing f H 0 with decreasing metamorphic grade. Opaque oxides which 2 occur in the rim are an indication that ilmenite or hornblende played a
role in the formation of that garnet. The zoning of plagioclase
adjacent to the rims is a result of diffusion of calcium away from the
garnet during formation of the rim.
Therefore, in these rocks, Fe-rich garnet is not a disappearing
phase, but is part of the characteristic assemblage of the granulite
facies, whereas Mg-rich garnet is a disappearing phase. The garnet's
reaction to orthopyroxene and plagioclase is a function of the
composition of the garnet and of f H 0, and not of pressure, temperature 2 or rock composition. Those garnets without coronas simply contain
enough iron to be stable at conditions at which coronas were formed
around the more magnesian garnets.
IGNEOUS PETROLOGY
Parent Magma
There have been innumerable proposals hypothesizing a parent magma
composition for anorthosite, as outlined in Table 1 of de Waard (1969).
It was Buddington (1936) who first proposed a magma of a gabbroic
anorthosite composition, based on the concept that the anorthosite
series and the spatially associated acid rocks - syenite, monzonite,
quartz monzonite or granite (syenite, mangerite, adamellite and
charnockite; see Streckeisen, 1973) - are separate intrusions
(Buddington, 1969; Anderson, 1969; Davis, 1969 and 1971; Ashwal and
Seifert, 1980). The generation of such a magma has been discussed
extensively in the literature (Emslie, 1980; Isachsen, 1969) and
experimental data have demonstrated mechanisms of generation under both
hydrous and anhydrous conditions, though a magma formed under hydrous
conditions would immediately become undersaturated with continued
temperature rise so that no gas phase would be present after the
earliest stages of generation (Yoder, 1954 and 1965; Yoder, 1969).
Anhydrous experiments in the systems anorthite - diopside - silica
(Clark et al., 1962), plagioclase (Ab40An60) - diopside - enstatite
(Emslie, 1970) and anorthite - diopside - forsterite (Presnall et al.,
1978) all indicate a dramatic shift of the piercing point toward
anorthite or plagioclase with increasing pressure up to 20 kb. This
suggests that partial melting at high pressure of plagioclase bearing
mafic and ultramafic source materials can produce aluminous melts which
would crystallize large amounts of plagioclase upon intrusion to
65
66
shallower depths (Ashwal, 1978). The high temperatures required for
generation of aluminous melts under anhydrous conditions (approximately
1400° Cat 15 kb - Emslie, 1970 and Presnall et al., 1978} are
consistent with crystallization temperatures obtained by Ashwal (1978 -
1200-1300° C at 15-20 kb}. However, further experimental work is
necessary to completely substantiate the anhydrous mechanisms of
generation of a gabbroic anorthosite magma. Evidence for a gabbroic
anorthosite parent magma ranges from the occurrence of satellitic sheets
of gabbroic anorthosite that intrude the country rocks bordering the
main Adirondack anorthosite massif (Buddington, 1972) to trace element
data (Ashwal and Seifert, 1980}. The existence of such a magma for the
Nain complex has been demonstrated by Wiebe (1979).
Buddington (1972) postulated a chemical composition of that
hypothetical magma composed of 60% liquid of gabbroic anorthosite
composition and 40% andesine crystals (see Table 1). This composition
is very similar to that of the anorthositic gabbro facies of the
Speculator and Wells Sills (Figure 13). These facies therefore may
represent a cumulate from a magma that may have also given rise to some
of the larger anorthositic bodies.
Crystallization Sequence
Feldspar accumulation from an essentially dry gabbroic anorthosite
magma has also been demonstrated experimentally (Yoder, 1969). Ashwal
(1978) has proposed a crystallization sequence for a magma of gabbroic
anorthosite composition. Plagioclase of composition An 50Ab44or6 (based
on the spectrographic analyses of Isachsen and Moxham (1969) and the
67
reintegrative microprobe analyses of Kay (1977)) crystallized first, and
when the magma was about 65% crystallized, plagioclase was joined by
augite and pigeonite (Wo36En41Fs23 and wo11En 57Fs32 respectively,
determined by microprobe reintegration of coarsely exsolved pyroxenes).
After the magma was about 80% crystalline, hemo-ilmenite and magnetite
formed, and at 92%, apatite crystallized. Such a crystallization
sequence would drive Buddington•s (1972) parent magma composition from
lower left to upper right on the AFM diagram of Figure 13 to yield the
other rocks in the sills except for the Wells gabbro.
Though the application of any crystallization sequence to the sills
is hypothetical at best because of severe complications due to
subsequent metamorphism, the textural evidence observed for the Wells
and Speculator Sills supports Ashwal's hypothesis. The Tenantville
facies and the anorthositic gabbro of the Wells Sill and the Speculator
Sheet contain large pseudomorphs of primary plagioclase phenocrysts.
There are large crystals of both orthopyroxene and clinopyroxene in the
Speculator Sheet and the Wells anorthositic gabbro, the latter of which
exhibits exsolution of orthopyroxene, though there is no evidence of
primary pigeonite. It is most probable that the magma had been emplaced
before crystallization of pyroxene because of the lack of pyroxene
megacrysts in the Tenantville facies. Opaque minerals form irregular
interstitial aggregates; apatite forms both small and very large
euhedral crystals.
Ashwal (1978) estimated conditions of initial crystallization of 15
to 20 kbar and 1200 - 1300° C from the sodic composition of his primary
68
plagioclase and his primary pyroxene compositions. At the level of
final emplacement (about 25 - 30 km), metamorphic reconstitution of the
primary mineral assemblages produced abundant garnet, hornblende and
minor biotite.
IntY'Usive History
Comparison of the mineral chemistry of the two sills and the Oregon
Dome reveals relationships between the various intrusive facies that are
most clearly illustrated by the whole rack data as presented in the
Harker diagrams of Figure 12. Though metamorphism has affected the
mineral chemistry, comparisons may still be made. Plagioclase
compositions (Figure 4) for the Wells anorthositic gabbro and
Tenantville facies and the Speculator sheet are calcic (An50_70) except
for those from garnet coronas (more calcic) and those from a few very
mafic rocks (less calcic). Wells gabbro and Oregon Dome compositions
are less calcic than these (An43_50). The ferromagnesian minerals of
the Wells gabbro and Oregon Dome are more iron-rich than those of the
anorthositic gabbro facies of the two sills. Clinopyroxene and
orthopyroxene (Figure 5) average compositions are Wo47Fs15En38 and
Wo1Fs40En 59 for the gabbroic anorthosites and wo45Fs21En34 and
wo1Fs49En 50 for the Wells gabbro and the Oregon Dome. The metamorphic
mafic minerals also show the same relationships: hornblende (Figur~ 6),
biotite (Figure 8), and, most clearly, garnet (Figure 10) of the Wells
gabbro and the Oregon Dome are more iron-rich than the metamorphic
minerals in the Speculator Sheet and Wells anorthositic gabbro.
However, in the Wells Tenantville facies, the mafic metamorphic minerals
69
tend to be slightly more iron enriched than those of the Wells
anorthositic gabbro. These data indicate that the minerals of the Wells
gabbro and the Oregon Dome coexisted with magmas that were very similar
in composition. Likewise, the minerals of the Speculator Sheet and
Wells anorthositic gabbro seem to have coexisted with magmas of similar
composition, but which were markedly different from those of the Wells
gabbro and Oregon Dome.
The mineral chemistry dictates the major element chemistry, and
similar relationships may be ascertained from Figure 12. The
anorthositic gabbro of the Wells Sill and the gabbroic anorthosite of
the Speculator Sheet have very similar analyses and trends for most of
the major elements. One analysis of anorthosite from the Speculator
Sheet shows either higher or lower values for the oxides than the
remaining four, which is expected because of the lack of mafic minerals
and the preponderance of plagioclase. The one Tenantville analysis
occurs at high Si02 because of the significant amounts of vein quartz
present. The other oxides occur in similar abundances as those for the
anorthositic gabbro and gabbroic anorthosite of the two sills (Figure
13), but the point does not lie on the trend of the Wells anorthositic
gabbro. Such a correlation is also evident in Figure 11, where the data
are plotted against stratigraphic distance from the base of the sill.
The gabbro from the Wells Sill and the Oregon Dome may also be
compositionally compared. They have similar Si02 content, except for
the one sample of the Wells gabbro which is finely veined with quartz.
The Wells gabbro is more mafic than any other rock in this study. It
70
has more MgO and Fe2o3 and less Al 2o3, Cao, and Na2o than any other facies. This mafic enrichment of the gabbro is what gives the arcuate
shape to the Wells Sill profiles in Figure 11. The Oregon Dome shows a
more mafic composition than the anorthositic facies of the two sills:
it is enriched in iron and titanium and depleted in Al 2o3 and Na2o.
However, the Wells gabbro has a different trend than the Oregon Dome on
the Harker variation diagram, and its genesis cannot be directly linked
to that of the Oregon Dome. The Oregon Dome has a trend which parallels
that of the Speculator and Wells anorthositic gabbro which is dictated
by the amount of cumulous plagioclase in the rock. The Wells gabbro has
a shallower trend, but a trend based on two points (the high silica
point should be disregarded) is hypothetical at best.
The similarities in mineral chemistry between the Speculator Sheet
and the anorthositic gabbro and Tenantville facies of the Wells Sill are
emphasized in the AFM diagram of Figure 13 that shows that they have a
differentiation trend showing only moderate iron enrichment. The only
points that deviate from this trend are the three analyses for the Wells
gabbro facies, which, along with the Oregon Dome, have a trend of more
extreme iron enrichment, contrary to the rest of the sills. The
composition of the Wells gabbro on the AFM diagram more closely
approximates that of the Oregon Dome than the compositions of the rest
of the Wells Sill and the Speculator Sheet. While these differences are
emphasized on the AFM diagram, it must be remembered that both the Wells
and Speculator bodies are calcic (Peacock, 1931).
Goldberg (1975, 1977) analyzed the rare-earth element abundances of
71
five small anorthositic bodies, including the Wells-Tenantville Sill,
and discovered large variations in rare-earth element content and in the
extent of the europium anomaly. Based on these data he could draw no
conclusions concerning the pre-Grenville history of the rocks of the
anorthositic series.
However, the geochemical evidence from this study suggests two
compositional episodes of anorthositic intrusive activity in the
southern Adirondacks, one slightly earlier than the other characterized
by iron enrichment on an AFM diagram, and the other showing more
moderate iron enrichment. Data from the Snowy Mountain Dome (de Waard
and Romey, 1969b), presented in Figure 15, indicate that this body has
an iron enrichment trend; whereas the Thirteenth Lake Series (Letteney,
1969) has a trend similar to that of the Speculator Sheet and Wells
anorthositic gabbro.
Ashwal et al. (1981) produced an age determination of about 1200 my
with an initial Nd ratio of 0.51053 for the Marcy massif using
neodymium-samarium isotopes. This agrees well with the date of 1199 +
14 my for the mangerite rocks of the Snowy Mountain Dome obtained by
Hills and Isachsen (1975). This indicates that the intrusive activity
characterized by iron enrichment trends in the southern Adirondacks
occurred concurrently with the activity in the Highlands, but does not
discount the possibility that the intrusive activity characterized by
more moderate iron enrichment is a slightly younger phenomenon. This is
indicated by the crosscutting of the gabbro facies of the Wells Sill
(which shows indication of iron enrichment in the mineral chemistry) by
72
FIGURE 15
AFM Ternary Variation Diagram for
the Snowy Mountain Dome - open circles
and the Thirteenth Lake Dome - closed circles
(de Waard and Romey, 1969b; Letteney, 1969).
A= Na2o + K20; F =Total Fe+ MnO; M = MgO (weight%). Lines are as in Figure 13.
73
• • • • •
•
•
74
the Tenantville facies.
In the southern Adirondacks, the data indicate that the first
intrusive episode led to the development of two different masses, the
Oregon Dome and the Snowy Mountain Dome. As this magma solidified and
cooled, it developed a mafic residual liquid that was expelled into the
country rock along a plane of weakness, perhaps as a result of stresses
caused by the start of the second period of intrusion. This liquid
solidified quickly, leaving the fine-grained slightly gneissic gabbro
exposed in the Wells-Tenantville Sill.
The second period of magmatic activity formed the Thirteenth Lake
Dome and at the same time sent two intrusions of gabbroic anorthosite
into the superjacent country rock. The first of the satellitic
intrusions formed the Speculator Sheet. As this thick sill of
stationary magma cooled slowly against a country rock already warmed by
its proximity to the still cooling Oregon Dome, some settling of mafic
phases caused a mo~e gabbroic rock at the base of the unit. Poorly
cemented aggregates of large plagioclase crystals caused the formation ' of more leucocratic pockets; some of these aggregates cause anomalous
occurrences of more leucocratic material at the base of the sill.
The second of the satellitic intrusions reached the zone of weakness
initially penetrated by the mafic differentiate of the Oregon Dome/Snowy
Mountain Dome magma. At the exposure in Wells, the anorthositic gabbro
magma penetrated the gabbro and formed a core. The Tenantville facies
formed as a chill margin against the gabbro (which is more than twice
the distance from the Oregon Dome as is the Speculator Sheet), as
75
suggested by the number of suspended plagioclase crystals and the
absence of other large crystals such as orthopyroxene. In Tenantville,
the mass of magma that penetrated the gabbro was too small to form a
slowly-cooled core of anorthositic gabbro; the chilled facies is the
only leucocratic facies present. The small positive europium anomaly
for the Tenantville facies found by Goldberg (1975, 1977) would be
expected from a liquid with suspended plagioclase phenocrysts.
These intrusive relationships do not support the theory of de Waard
and Romey (1963 and 1969a) that the Snowy Mountain Dome, and therefore
also the Oregon Dome, formed a basement upon which the overlying
sediments were deposited, for the geochemical history of the sills is
intimately linked with that o~ the three massifs in the southern
Adirondacks. It is therefore concluded that, at least in this area of
the Adirondacks, the contacts between metaigneous and metasedimentary
bodies are primary intrusive contacts, and are not the product of
sedimentation on an anorthosite basement.
76
BIBLIOGRAPHY
Anderson, A.T., Jr. (1969} Massif-type anorthosite: a widespread Precambrian igneous rock IN Isachsen, Y.W., ed. (1969) Origin of Anorthosite and Related Rocks, New York State Museum and Science Service Memoir 18: 47-56.
Ashwal, L.D. (1978) Petrogenesis of massif-type anorthosites; crystallization history and liquid line of descent of the Adirondack and Morin complexes: unpublished Ph.D. thesis, Princeton University: 136 p.
Ashwal, L.D., Seifert, K.E. (1980) Rare-earth-element geochemistry of anorthosite and related rocks from the Adirondacks,