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I. ABSTRACT V. TECTONIC IMPLICATIONS
VII. REFERENCES
VI. CONCLUSIONS
III. Field Relationships
II. INTRODUCTION
IV. GEOCHEMISTRY
The western Vermont Appalachians expose rocks of the Proterozoic Laurentian basement, a Neoproterozoic rift and drift sequence, and Early Paleozoic passive margin sediments in the Champlain Valley and Taconic sequences. In central and eastern Vermont, terranes accreted to the Laurentian margin are exposed. The Moretown Terrane comprises the Moretown and Cram Hill formations. The Moretown Formation comprises mostly schist, granofels (quartzite), and greenstone (Walsh et al., 2010). The Cram HIll Formation consists mostly of phyllite and greenstone. Detrital zircons extracted from quartz-rich units of the Moretown by Ryan-Davis (2012) yielded age distributions that indicate their provenance is peri-Gondwanan (Ganderian) rather than Laurentian, as had previously been assumed. Furthermore, the maximum age, based on the youngest zircons, is 514 Ma (Macdonald et al., 2014). The Shelburne Falls arc was established on this peri-Gondwanan Moretown Terrane perhaps as early as 500 Ma but with a locus of magmatic activity at ~475 Ma ( Macdonald et al., 2014). By 460 - 470 Ma, the Shelburne Falls arc had collided with Laurentian and basins associated with it may have been receiving detritus from Laurentia as well as peri-Gondwanan terranes. Ma�c rocks (greenstones) of uncertain age occur in both the Moretown and Cram Hill formations as dikes and possibly sills or �ows. Here, we use geochemistry to show that the ma�c rocks formed in supra-subduction regions, perhaps in response to two lithospheric delamination events in the tectonic history of the Vermont Appalachians.
0 500 1000 1500 2000 2500 3000
Age
Pro
babi
lity
Age (Ma)
Ganderian Belts, NB & ME(Fy e at al., 2009: n = 335)
VT-MA Moretown(n = 764)
Laurentian Margin, NL(Cawood and Nemchin., 2001: n = 342)
VT-MA Rift-Drift(n = 578)
The geochemistry of ma�c rocks from the Moretown and Cram Hill formations indicates their origin from depleted mantle in a supra-subduction zone environment. Chemical di�erences between the two formations may be explained by derivation from slightly di�erent mantle sources. The chemistry of the ma�c rocks from the Moretown especially is similar to ma�c rocks from the Ordovician Mount Norris Intrusive Suite and the Silurian Comerford Intrusive Suite.
VIII. ACKNOWLEDGEMENTS
Folded greenstone near Craftsbury, VermontConcordant contact of greenstone with phyllite in Hubbard Park, Montpelier
A few metama�c rocks are clearly folded by F1 generation folds; others cut across S2 foliations and exhibit chilled margins; many contacts simply parallel foliations with-out showing diagnostic relationships. So in the absence of radiometric age dates, we conclude that there are at least two generations of ma�c rocks. Some are clearly dikes whereas others could have been sills, �ows or dikes. All samples are metamorphosed to greenschist facies. Igneous texture is preserved in a few samples only.
Ma�c dike cutting the dominant foliation (S2) at Putnamville, Vermont
Thin section illustrating rarely preserved igneous texture in greenschist mineralogy: albite, chlorite, epidote, actinolite and titanite.
greenstone
phyllitic granofels
F1fold
F1fold
greenstone
phyllitewith coticule
metadiabase
granofels
0
1
2
3
4
TiO
2 (w
t. %
)
10
15
20
Al2O
3 (w
t. %
)
2
4
6
8
10
12
70 60 50 40 30 20
CaO
(wt.
%)
Mg #
0.0
0.2
0.4
0.6
0.8
1.0
P 2O
5 (w
t. %
)
4
6
8
10
12
14
16
Fe2O
3 (w
t. %
)
0
2
4
6
8
70 60 50 40 30 20
MgO
(wt.
%)
Mg #
1
10
100
200
Sam
ple/
Cho
ndrit
e
La Ce Pr Nd
Sm Eu Gd Tb Dy
Ho Er Tm Yb Lu
1
10
100
Sam
ple/
Cho
ndrit
e
Mariana TroughBack-Arc Ocean Crust
Mariana Arc Rift
Cram Hill Formation
Moretown Formation
Th Nb La Ce Pr
Nd
Sm Ti Gd
Tb Dy Y Er
Tm Yb
0.1
1
10
100
Sam
ple/
MO
RB
Cram Hill Formation
Th Nb La Ce Pr
Nd
Sm Ti Gd
Tb Dy Y Er
Tm Yb
0.1
1
10
100
Sam
ple/
MO
RB
Moretown Formation
Th Nb La Ce Pr
Nd
Sm Ti Gd
Tb Dy Y Er
Tm Yb
0.1
1
10
100
Sam
ple/
MO
RB
Mount Norris Intrusive Suite(MNIS)
Th Nb La Ce Pr
Nd
Sm Ti Gd
Tb Dy Y Er
Tm Yb
0.1
1
10
100
Sam
ple/
MO
RB
Comerford Intrusive Suite (CIS)420 Ma
0.01
0.1
1
10
100
0.1 1 10
Nb 9
.0 (p
pm)
Yb9.0 (ppm)
5%
5%
152540
FMM
MNIS
Classi�cation of ma�c rocks as mostly basalts with the exception of three samples that plot as andesite or dacite.
Rare earth element patterns are similar to those from basalts formed during arc and back-arc rifting in the western Paci�c.
Trace element contents in ma�c rocks from Moretown and Cram Hill formations are consistent with formation in extensional environments near subduction zones.
Calculated Nb and Yb values for primitive magmas (at 9% MgO) show that the Moretown ma�c rocks formed by ~10% partial melting of fertile MORB mantle (Pearce & Par-kinson, 1993) whereas Cram Hill magmas may have formed from a slightly more depleted MORB mantle. Initial εNd values of +3 to +5 are consistent with this interpretation.
Oxide variations with Mg number are consistent with fractionation of olivine, pyroxene & plagioclase.
Extended element plots. All suites show negative Nb anomalies, characterisitc of subduction-in�uenced mantle. Ma�c rocks of Moretown are similar to the Ordovician MNIS and Silurian CIS ma�c rocks. Ma�c rocks from the Cram Hill Formation have less enrichment in light rare earth elements.
0.001
0.01
0.1
1
5
0.01 0.1 1 10
Zr/T
iO2
Nb/Y
SubalkalineBasalt
Bas/AndAlkali Basalt
Andesite
Dacite
Rhyolite
Basa
nite
Phonolite
Trachyte
Trachy Andesite
Comendite Pantellerite
Cram Hill Forma onMoretown Forma onMF (SiO 2 > 53 %)
Oceanic arc
Alkaline arcContinental
arc
Major ocean ridge Ocean island
0.1
1
3
0.4 1 10
Nb'
/La
La/Yb0.02
0.1
1
5
1 10 80
Th/N
b'
La/Yb
Major ocean ridge
Ocean island
Oceanic arc Continental arc Alkaline arc
Y/15
La/10 Nb/8
1A calc-alkali basalts1C arc tholeiites2A continental basalts2B back-arc basalts3A rift alkali basalts3B,3C E-type MORB3D N-type MORB
3D
1C
1B
1A2A 3A
3B
3C
Back -Arc Basalts
Diagrams afterHollocher et al. (2012)
Cawood, P. A., and Nemchin, A. A., 2001, Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians: Geological Society of America Bulletin, v. 113, p. 1234-1246.
Coish, R., Ryan-Davis, J., Amidon, W., Kim, J., and Dietsch, C., 2013, Origin of an Early Paleozoic arc-related sedimentary basin in the northern Vermont Appalachians: a detrital zircon study, AGU Fall Meeting Abstracts, p. 2404.
Coish, R. A., 2010, Magmatism in the Vermont Appalachians, in Tollo, R., Bartholomew, M. J., Hibbard, J. P., and Karabinos, P., editors, From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region, Geological Society of America Memoir 206, p. 91-110.
Fy�e, L. R., Barr, S. M., Johnson, S. C., McLeod, M. J., McNicoll, V. J., Valverde-Vaquero, P., van Staal, C. R., and White, C. E., 2009, Detrital zircon ages from Neoproterozoic and Early Paleozoic conglomerate and sandstone units of New Brunswick and coastal Maine: implications for the tectonic evolution of Ganderia: Atlantic Geology, v. 45, p. Pages 110-144.
Hibbard, J. P., van Staal, C. R., Rankin, D. W., and Williams, H., 2006, Lithotectonic map of the Appalachian Orogen: Geological Survey of Canada Map 2096A, scale 1:1,500,000.
Hollocher, K., Robinson, P., Walsh, E., and Roberts, D., 2012, Geochemistry of amphibolite-facies volcanics and gabbros of the Støren Nappe in extensions west and southwest of Trondheim, western gneiss region, Norway: A key to correlations and paleotectonic settings: American Journal of Science, v. 312, p. 357-416.
Karabinos, P., Samson, S. D., Hepburn, J. C., and Stoll, H. M., 1998, Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc: Geology, v. 26, p. 215-218.
Kim, J., Coish, R., Evans, M., and Dick, G., 2003, Supra-subduction zone extensional magmatism in Vermont and adjacent Quebec; implications for early Paleozoic Appalachian tectonics: Geological Society of America Bulletin, v. 115, p. 1552-1569.
Kim, J., Gale, M., King, S. M., Orsi, C. M., and Pascale, L., 2003, Bedrock Geology of the Montpelier Quadrangle: Vermont Geological Survey Open File Map VG03-1, scale 1:24,000.
Macdonald, F. A., Ryan-Davis, J., Coish, R. A., Crowley, J. L., and Karabinos, P., 2014, A newly identi�ed Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean: Geology, v. 42, p. 539-542.
Moench, R. H., and Aleiniko�, J. N., 2002, Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England: Physics and Chemistry of the Earth, v. 27, p. 47-95.
Rankin, D. W., Coish, R. A., Tucker, R. D., Peng, Z. X., Wilson, S. A., and Rou�, A. A., 2007, Silurian extension in the upper Connecticut Valley, United States and the origin of middle Paleozoic basins in the Quebec Embayment: American Journal of Science, v. 307, p. 216-264.
Ratcli�e, N. M., Stanley, R. S., Gale, M. H., Thompson, P. J., and Walsh, G. J., 2011, Bedrock Geologic Map of Vermont: U.S. Geological Survey Scienti�c Investigations Map 3184, scale 1:100,000.
Ryan-Davis, J., 2013, Origins of the Moretown Formation, northern Vermont: A detrital zircon study: B.A. thesis, Middlebury College, 74 p.
Ryan-Davis, J., Coish, R., and Amidon, W. H., 2013, Origins of the Moretown Formation, Vermont: a detrital zircon study: Geological Society of America Abstracts with Programs, v. 45, no. 1, p. 95.
Twelker, E., 2004, Geochemistry and �eld relations of greenstones in the Moretown and Cram Hill Formations, Montpelier Quadrangle, Vermont: B.A., Middlebury College, 66 p.
van Staal, C. R., Whalen, J. B., McNicoll, V. J., Pehrsson, S., Lissenberg, C. J., Zagorevski, A., van Breemen, O., and Jenner, G. A., 2007, The Notre Dame arc and the Taconic orogeny in Newfoundland, in Hatcher Jr., R. D., Carlson, M. P., McBride, J. H., and Martínez Catalán, J. R., editors, 4-D Framework of Continental Crust, Geological Society of America Memoir 200, p. 511-552.
Walsh, G. J., Kim, J., Gale, M. H., and King, S. M., 2010, Bedrock geologic map of the Montpelier and Barre West quadrangles, Washington and Orange Counties, Vermont: U.S. Geological Survey Scienti�c Investigations Map 3111, scale 1:24,000.
We thank Evan Twelker and Scott Zolkos for their contributions to the early stages of this project. We thank Greg Walsh for discussion of aspects of the geology of the Montpelier- West Barre Quadrangle. We are grateful to Middlebury College for continuing support of student-faculty research and its commitment to maintaining �rst-class analytical facilities.
B. SF arc
SSZ ophio
lite
~480 Ma
upwellingasthenospheric
C. Ma�c Dikes
Ma�c Dikes
~470 Ma
upwelling asthenosphere
CMT
CIS
CVGT
Ganderia
~420 Ma
E.
CMTOldSF arc
LateBH arc
Laurentia Ganderia
~460 Ma
OldBH arc
Slab retreat
D. accretionarycomplex
Laurentia
amalgamatedLaurentia/Ganderia AvaloniaF.
~410 MaEarly Devoniangranites
Late Devoniangranites
~375 Ma
G. AvaloniaLaurentia/Ganderia
LaurentiaZone 2Zone 1 Zone 3,4A.
~550 Ma
Moretown Fm(Gondwanan) A sequence of sketches
showing the tectonic evolution of the New England Appalachians from Coish (2010), modi�ed after van Staal et al. (2007), Karabinos et al. (1998), and Moench and Aleiniko� (2002).
Ma�c rocks of this study may have formed at two di�erent times: 1) during the mid-Ordovician as a result of lithospheric delamination following collision of the Shelburne Falls arc with Laurentia, and 2) during the Silurian following collision of greater Ganderia with amalgamated Laurentia.
Meta-sandstones of the Moretown terrane in western New England contain detrital zircons that reveal an a�nity with the Gondwanan (eastern) rather than the Laurentian (western) side of the Cambrian-Ordovician Iapetus Ocean. In the Vermont Appalachians, metamorphosed ma�c rocks, in the form of dikes and sills, cut this Moretown terrane. The meta-ma�c rocks have geochemical characteristics similar to modern-day supra-subduction volcanics. Geochemistry suggests that the meta-ma�c rocks geochemically were formed as mostly basalts or basaltic andesites. They have moderate TiO2 contents (1- 2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. εNd values for two samples are +3 and +5. The chemistry, taken together, suggests protoliths of the meta-ma�c rocks may have formed in an extensional marginal basin(s), perhaps near a volcanic arc(s). The meta-ma�c rocks of this study are similar in chemistry to the pre-Silurian Mount Norris Intrusive Suite (MNIS) of north-central Vermont, and also to the Silurian-aged Comerford Intrusive Suite (CIS) of northeastern Vermont. If the meta-ma�c rocks of the Moretown are correlated with the MNIS, then the supra-subduction extensional environment could have formed in response to lithospheric delamination, following collision of the Gondwanan Moretown terrane with either a Laurentian microcontinent or Laurentia itself in the Ordovician. If , on the other hand, they are correlated with the CIS, then the extensional environment may have formed by lithospheric delamination following collision of a Ganderia fragment with Laurentia in the Silurian. In either case, it is likely that the magmatism occurred after the peri-Gondwanan Moretown terrane became part of the Laurentian plate.
Supra-subduction magmatism in the Moretown Terrane, a fragment of Gondwana in the western AppalachiansCOISH, Raymond1; KIM, Jonathan2; RYAN-DAVIS, Juliet1; PIERCE, Natashia3; DIETSCH, Craig3
1Geology Department, Middlebury College, Middlebury, VT 05753, [email protected]; 2Vermont Geological Survey, 1 National Drive, Davis 2, Montpelier, VT 05620-3902; 3University of Cincinnati, Cincinnati, OH 45221
72°30'W
72°30'W44
°30'
N
44°3
0'N
44°1
5'N
44°1
5'N
0 2 4 6 8 101
kilometers
Putnamville
Omq Dg
Dgr
DSu
DSu
Dg
Ochu
Omp
Dg
OchuOchp
Ochuc
Craftsbury
Omc
Ochu
Omc
Rift - Drift Sequence Moretown
Terrane
Silurian-DevonianSequence
Montpelier
RMC
RMC
Dumplin
g Hill
Belt
WrightsvilleBelt
Shad
y Ri
ll Fa
ult
Red
Indi
an L
ine
!.
#0Cram Hill FormationMoretown Formation
Omc
Dg
Gile Mountain Formation
Devonian granitoids
Waits River, Northfield &Shaw Mt. formationsundifferentiated
Cram Hill Formation
Moretown Formation
phyllite member
pin-striped granofels member
quartzite/phyllite membercarbonaceous schist member
Umbrella Hill Conglomerate
undivided member
D
Ochuc
gr
OchpOchu
Omp
Omq
Explanation
(( Thrust Fault
Sample Locations
Geology of theMontpelier - Craftsbury area
northern VermontGeology after Ratcli�e et al. (2011),Walsh et al. 2010, & Kim et al. (2003)
Richardson Memorial Contact (RMC)
DSu
Burlington
Craftsbury
Middlebury
Montpelier
73°W
73°W
72°W
72°W
45°N
44°N 44°N
43°N 43°N
45°N
²
0 10 20 30 40 505
kilometers
Mesozoic Intrusives
Devonian Intrusives
Silurian Intrusives
Connecticut Valley Trough
Bronson Hill Belt
Moretown Accreted Terrane
The Taconic Allochthon
Champlain Valley Belt
Rift-Drift Terrane
Laurentian Basement
Laurentia
Peri-Laurentian Units
Peri-Gondwanan Arcs
Ganderia
Piedmont Accretionary
Piedmont
Avalonia
Carolinia
Meguma
0 1,000500
Kilometers
Lithotectonic Units in theAppalachians
(after Hibbard et al., 2006)
Generalized geologic map of Vermont (adapted from Ratcli�e et al., 2011). The Moretown Terrane includes the Moretown and Cram Hill Formations, which are the focus of this poster.
Detailed map of the study area, showing sample locations of ma�c rocks collected from the Montpelier Quadrangle and Craftsbury Township. Thick red line represents the postulated Red Indian Line in Vermont, as suggested by Ryan-Davis et al. (2013) and Macdonald et al., (2014).
Detrital zircon data for the Moretown Formation metasedimentary rocks (Ryan-Davis, 2012; Ryan-Davis et al., 2012; Coish et al., 2013; Macdonald et al., 2014). Peaks at 600 - 800 Ma clearly indicate provenance of the Moretown is Gondwanan rather than Laurentian.
1. Metamorphosed ma�c rocks in the Cram Hill and Moretown formations of north-central Vermont formed as dikes (and probably lava �ows) in Early Ordovician to Silurian times.
2. Their geochemistry suggests they were mostly tholeiitic basalt or basaltic-andesite.
3. Rare earth element patterns and negative Nb anomalies (relative to Th and La) are consistent with derivation of primitive magmas in suprasubduction zone extensional environments.
4. The suites may have been derived by ~10 to 15% partial melting of fertile MORB mantle that had been variably enriched in selected elements mobilized from a subducting plate. The mantle source for the Cram Hill suite was less enriched than the source for the Moretown suite.
5. The magmas of the Cram Hill and some of the Moretown meta-ma�c rocks may have formed after collision of the Shelburne Falls arc with Laurentia in the Middle Ordovician, perhaps as melts associated with slab break-o� of an easterly-directed subducting plate, as postulated for the Mount Norris Intrusive Suite (Kim and others, 2003). Other meta-ma�c rocks from the Moretown may have formed from delamination in the Late Silurian, by analogy with the Comerford Intrusive Complex (Rankin and others, 2007). Unfortunately, the chemistry of the ma�c rocks cannot be used to distinguish di�erent magmatic ages.