Evolution of magmatic AFM The hornblende * melt : biotite … · 2007-08-28 · Evolution of magmatic AFM The hornblende * Libertv Hill INrnonucrroN Many of the late Paleozoic granitoid

Evolution of magmatic AFMThe hornblende *

Libertv Hill

INrnonucrroN

Many of the late Paleozoic granitoid plutons of thesouthern Appalachians are composite bodies with coarse-grained amphibole + biotite and biotite varietal facies(Speer et al., 1980). From field relations, these facies ap-pear closely related, and multiple intrusion or differentia-tion comes to mind as an explanation for the relationship.In the Liberty Hill pluton, the coarse-grained central am-phibole + biotite granitoids grade imperceptibly intomarginal biotite granitoids, seemingly ruling out multipleintrusions. The compositions of the marginal biotite fa-

* Present address: Department of Marine, Earth and Atmo-spheric Sciences, Box 8208, North Carolina State University,Raleigh, North Carolina 27695-8208, U.S.A.

0003404x/87/09 I 0-0863$02.00

Americ an Mi ne r al o gist, Volume 72, pages 863-878, 1987

mineral assemblages in granitoid rocks:melt : biotite reaction in thepluton, South Carolina

cies overlap those ofthe central granitoids, arguing againstsignificant differentiation (Speer et al., unpub. ms.). Themarginal facies do show minor wall rock-magma inter-action that affected some trace-element and oxygen-iso-tope compositions (Wenner and Speer, 1986). An alter-native explanation for the mineralogical relation betweenthe central and marginal facies is suggested by the wide-spread texture of biotite replacing amphibole.

Understanding the mineral assemblages and composi-tions in igneous rocks requires linking them to the evolv-ing conditions and compositions of the interstitial meltduring crystallization. This is a well-developed area ofinvestigation for gabbroic rocks, but much less so forgranitoid rocks. Recently, Abbott and Clarke (1979) andAbbott (1981, 1985) have given the sequence ofAFM

863

J. Ar,nxaNDER SPEER*Orogenic Studies Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University,

Blacksburg, Virginia 24061, U.S.A.

AssrRAcr

The Liberty Hill pluton comprises a central, coarse-grained edenite + biotite granitoidfacies grading to a biotite, pargasite + biotite, or muscovite * biotite granitoid marginalfacies. The replacement of amphibole by biotite, mineral compositions, and "liquid" vari-ation diagrams for the Liberty Hill help determine the AFM mineral-crystallization re-actions in the Liberty Hill pluton. These reactions evolved from liquid : amphibole toliquid : biotite with an intervening liquid + amphibole : biotite reaction resulting fromeither (l) movement along the amphibole-biotite liquidus boundary from the even, liq-uid: amphibole * biotite, to the odd, liquid + amphibole : biotite, crystallizationreaction with decreasing temperatures or (2) movement of the liquid composition fromthe amphibole field into the biotite field. The liquid + amphibole : biotite reactioninvolved additional phases; the full reaction may have taken the form liquid I + amphi-bole* i lmeni te:b iot i te+qtrar lz+anorth i te+K-fe ldspar+t i tani te+magnet i te+liquid 2, where the liquid is the interstitial melt. The extended crystallization history tolower temperatures and higher water activities at the margins of the pluton allowed thereaction to exhaust the amphibole before the liquid; the subsequent liquid : biotite crys-tallization reaction produced the biotite granitoid facies. Locally, muscovite crystallizedaccording to the reaction liquid : muscovite + biotite, producing the muscovite + biotitefacies. The pluton is intruded by a finer-grained biotite granitoid believed to be derivedfrom interstitial melt of the coarser-grained granitoids. This derivation explains why bio-tite is the only AFM mineral in the finer-grained granitoids and why biotites from allLiberty Hill granitoids are generally similar in composition.

The average Ko value l(XBJe/XB;)/(W;/Xf"b')l is 1.04 for coexisting Hbl and Bt. This K"is higher than the 0.69-0.93 normally observed in igneous rocks, but equal to values forspecific instances of biotite replacing amphibole in the Peruvian Coastal batholith. Com-bined with the results obtained from the contact aureole, the central granitoids preserveconditions of 725"C, P,oor E 4.5 kbar, Po",o E 0.5 P,"o,, andfo, - 10-15, slightly moreoxidizing than the NNO solid buffer. Marginal granitoids yield conditions of 647"C,Po,,o r P,.^,, and fo, I 10-16, suggesting a more prolonged reaction history.

864 SPEER: EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS

crystallization reactions in granitoids in more detail thanBowen's (1928) reaction series. These have been used ininterpreting the crystallization reactions in strongly per-aluminous granitoids, such as the cordierite-bearingClouds Creek pluton, South Carolina (Speer, 198 1b). Theiruse is extended here to the metaluminous Liberty Hillpluton, South Carolina, in order to understand the evo-lution of amphibole + biotite, biotite, and muscovite +biotite granitoids.

Gnolocrc sETTTNG

The Liberty Hill pluton is roughly elliptical, covering360 km'zin Kershaw, l,ancaster, and Fairfield Counties,north-central South Carolina. The Liberty Hill plutoncomprises several texturally and mineralogically distinctfacies (Fig. 1). The predominant facies is a coarse-grainedferro-edenite + biotite granitoid occupying the center ofthe pluton. Coarse-grained biotite, ferroan pargasite +biotite, and muscovite * biotite granitoids occur alongthe northern and eastern borders ofthe pluton and havegradational contacts with the central ferro-edenite * bio-tite granitoids. This reverse zoning of the mineral faciesis believed to result from accumulation of magma with agreater percentage of evolved, interstitial melt near themargin as a result of the pluton's emplacement as well asa minor amount of contamination from the wall rocks(Wenner and Speer, 1986). There are several younger fa-cies in the Liberty Hill pluton that intrude the coarse-grained granitoids as dikes or plugs up to I km2 in area.The most prominent are fine- to medium-grained biotitegranitoids in the west-central part ofthe pluton (Fig. 1),which are believed to have differentiated from the coarse-grained facies.

The contact aureole of the pluton was described bySpeer (198 I a). The pelitic assemblage in the country rock,muscovite + chlorite + albite * epidote + quartz +magnetite + ilmenite, changes in response to progrademetamorphism on approaching the pluton. The followingisograds are encountered: porphyroblastic magnetite in,epidote + albite out, biotite + cordierite in, chlorite out,K-feldspar in, and muscovite or magnetite out. Severaladditional metamorphic minerals appear in the xenoliths:garnet, orthopyroxene, andalusite, sillimanite, and fi-brolite. It was concluded that the pluton was emplacedat a depth corresponding to 4.5-kbar pressure. The mag-ma was relatively dry, and the xenoliths yield estimatedtemperatures of about 725C for the central granitoidsand 650'C for the maryinal granitoids.

MrNnnar,ocv

Compositions of the minerals in polished thin sections weredetermined with a nine-spectrometer, automated,qtr-snr're elec-tron microprobe using silicates and oxides as standards and theanalytical scheme qar-r described by Solberg and Speer (1982).These are reported in Appendix l,' and selected analyses are

rTo obtain a copy of Appendix 1, order Document AM-87-355 from the Business Office, Mineralogical Society of America,1625 I Street, N.W., Suite 414, Washington, D.C. 20006, U.S.A.Please remit $5.00 in advance for the microfiche.

given in Tables 1 to 3. Mineral formulas and components werecalculated using the computer program suPERrncer- ofRucklidge(1971). The program also calculated an assumed stoichiometricamount of water and added it to the oxide weight percentages

for the hydrous minerals. Mineral abbreviations in the text, ta-bles, and figures are those recommended by The American Min-eralogist (ketz, 1983).

Amphibole

Amphibole occurs as euhedral to subhedral prismatic crystalsup to 0.5 cm long. They are pleochroic dark yellow green tomoderate blue green and are locally twinned on (100). Micro-probe analyses (App. 1, selected analyses in Table l) show thatthe amphiboles in the central granitoids are ferro-edenites andferro-edenitic hornblendes (Fig.2a), hereafter referred to as fer-ro-edenites. These ferro-edenites have a narrow compositionalrange with an Fe/(Fe + Mg) near 0.6 and a Si content in thehalf-unit-cell formula of 6.56 to 6.85 atoms per 24 anions. TheF and Cl contents range between 0.08 and 0.22 molo/o F/(OH +F + Cl) and between 0.005 and 0.03 mo10/o CV(OH + F + CD,respectively. Amphiboles in granitoids of the northeast cornerof the pluton have thin, discontinuous aluminous rims of ferroanpargasite (Fig. 2B). These aluminous rims are less than 0.01 mmthick and are only locally visible, where there is a color difer-ence. According to graphical representations of Doolan et al.(1978) and Thompson (1981), the substitution in the aluminousrims is primarily a tschermakite [AlrMg 'Si-'] + edenite

[NaAlSi-,] substitution to form pargasite.Many amphiboles are intergrown with biotite, but generally

the order of crystallization is uncertain. Where textures are un-ambiguous, the amphiboles are partially (Fig. 3a) or completely(Fig. 3b) replaced by biotite. In what is interpreted as a laterreaction texture, amphiboles can be replaced by chlorite + epi-dote + plagioclase. Scattered throughout the pluton are amphi-boles with clinopyroxene inclusions (Fig. 3c). The amphiboleadjacent to these pyroxenes is pale green actinolite that is dis-continuously zoned to ferro-edenite at the rim, comparable incomposition to amphiboles elsewhere in the pluton (Fig. 2b).

Biotite

Greenish-black biotite is a ubiquitous mafic phase in the Lib-erty Hill pluton, forming up to 5 modal percent of the rock.Biotite occurs interstitially to feldspars and quartz, replacing am-phibole, and as inclusions in leucocratic minerals. Individualflakes are up to 5 mm across. Biotite is pleochroic grayish orangeto greenish black in thin section.

Microprobe analyses (App. l, selected analyses in Table 2)show that biotite compositions from the granitoids fall approx-imately midway between phlogopite and annite with an inter-mediate Al content. The range of molar Fel(Fe + Mg) is smallwith an average of approximately 0.59. The amount of tetrahe-dral Al is systematically related to the coexisting minerals (Fig.4). Biotites coexisting with ferro-edenite have tetrahedral Al con-tents between 2.27 and 2.35 atoms per 24 anions. The tetrahe-dral Al contents of biotites coexisting with the pargasite-rimmedhornblendes are decidedly more aluminous, in the range 2.29 to2.75 atoms per 24 anions. Biotites in the fine-grained and coarse-grained biotite granitoids and enclaves have tetrahedral AI con-tents that overlap the biotites coexisting with ferro-edenite. Thebiotites of the finer-grained rocks can be more iron rich, with anaverage Fe/(Fe + Mg) : 0.64; in biotites from the aplites, Fe/(Fe + Mg) is as high as 0.75. The biotites in coarse-grainedmuscovite + biotite granitoids are aluminous and comparablein composition to biotites coexisting with the pargasite-rimmed

Fig. 1. Geologic and sample maps of the Liberty Hill pluton, South Carolina.

866

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omphiboles (No * K)a > 0.5te l rohedrol s i l icon

700 600

q edeni l ic

.- ferroonporgosrtrchor

* : fe r ro -eden i l i cferro- edenite hornblende

Fig.2. Compositions of the Liberty Hill amphiboles accord-ing to the IMA nomenclature (Leake, 1978). The majority of theamphiboles in the granitoids are ferro-edenite and ferro-edenitichornblendes (a), whereas the amphiboles adjacent to the clino-pyroxene cores are actinolite (b), and the zoned rims on theamphiboles of granitoids along the northeastern margin of thepluton (56-8, -87, -90, -132) are ferroan pargasite (b).

amphiboles. The biotite octahedral Al and total Al contents vary,like the tetrahedral Al content, with the coexisting AFM mineralassemblage.

Except for F, the remaining elemental compositions for thebiotites are similar from diferent occurences. The F contentsof biotites in the enclaves and edenite-bearing granitoids havean average F/(OH + F) of 0. I I . The complete range of F/(OH +

SPEER: EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS

ol!+

o

00

oL

+o ^ _> ( J 5

olr

F) is 0.06 to 0.20. Biotites in the coarse-gained biotite granitoids

are more F rich with an average F/(OH + F) of 0.17. The biotitesin muscovite + biotite and fine-grained biotite granitoids areless F rich with average values of F/(OH + D of 0.05 and 0.09,respectively.

Clinopyroxene

Pyroxene in the Liberty Hill pluton occurs as very pale green,

irregular or skeletal cores in amphiboles (Fig. 3c) and in the morecalcic plagioclases. Microprobe analyses (App. l, selected anal-yses in Table 3) show the pyroxene to be salite with an averagecomposition Woou orEn' u,Fsro rrRdn, or.

Coexisting AFM minerals

Figure 5 summarizes rock and mineral compositional rela-tions from the Liberty Hill pluton. Rock compositions are moreiron rich than the ferromagnesian silicate assemblage because ofthe presence ofiron oxides.

A notable aspect of Figure 5 is the differing distribution coef-ficients for coexisting amphiboles + biotite, evident by the slopesof tie lines, and differing Al contents. The distribution coefr-cient, defined as Ko : (X',;JX";)/(W;/XF9, depends on theoverall mineral assemblage and bulk composition. In the en-claves and enclave-bearing rocks, assemblages with clinopyrox-ene have an average Ko : 0.92 with a range of 0'86-0.99 (Fig.

6a). The central edenite + biotite granitoids have Ko : 1.64with a range of 0.96-1.17 (Fig. 6b), whereas the marginal par-gasite + biotite granitoids have Ko : 1.22 with a range of I . 1 3-1.30 (Fig. 6c). The finer-grained biotite granitoids and apliteshave more iron-rich biotites than the other gtanitoids, approach-ing the Fe/(Fe + Mg) values of the granitoids themselves (Fig.

6d). The most aluminous biotites coexist with muscovite or am-phiboles having pargasite rims; biotites coexisting with edeniteor in a single AFM phase assemblage are less aluminous.

Feldspars

The alkali feldspar in coarse-grained granitoids is macro- andmicroperthite, exhibiting both primary Carlsbad growth twinsand Albite-Pericline inversion twins. I-arge'atea microprobe

analyses (App. 1) of alkali feldspars give estimated original bulkalkali feldspar compositions of OrrrnuAbrunrAnort (56-56) and

0 0

TABLE 1. Selected amphibole compositions for the Liberty Hill pluton, South Carolina

oct i nol i le

( N o + K L . < O 5

te t rohedro l s i l i con

( N o + K L ) o 5

Amphiboles from thecentral granitoidsZoned amphiboles

C p x - T r - E d - H b l Ed-Hbl - Prg

K3-1303s6-86 56-86mid. rim 56-56 F6-24

s6-s3 s6-53core nm

s6-86core

sio,Tio,Al,03FeOMnOMgoCaOBaONaroKrOFclHro.-O=F + C l

Total

52.750 .162.05

16.630.61

13.851 1.850.040.750.300.890.021 .630.38

1 0 1 . 1 5

45 381 .457.69

21.190.589.39

1 1 .400.081 .861 .05o.770.221 .580.37

't02.27

43.941.687.56

22.300.808.17

10.580.011.851.020.520.121.62o.29

99.90

42.411.099.28

23.05o.757.84

10.790.051.841.270.690.071.590.31

100.41

40.131 .53

13.O219.920.737 .18

1 0 . 1 6

1 .351 .000.51

1.670.21

96 99

43.591 .828.59

20.260.718.73

10.69

1 .681 .020.47

42.301.379.41

21 .680.707.92

11 .33

1 .611 . 1 40.59

1.650.25

99.45

1.730.20

99.09

44.20 43.061.57 ',1.46

7.93 8.7321.56 21.28o.74 0.858.35 8.20

1 1 .53 10.34o.22

1 .49 1.701 .01 1 .13o.44 0.41

0.151 7 5 1 . 6 90.19 0.21

100.38 99.01

. Cafculated on the basis of (OH + F + CD : 2124 anions.

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ssel seJoc qly( 'crpos eJoru eJe sprolruer8 elrlorq peure:3-sugeql uI soselcor?el4 ',{ys,r4cedset 'o/oVl'O pue o/ol'l u?ql ssel er"eselcor8eld eqlJo slueluoc uC pu€ JO 'Iuu elrql? snonuquocstp ?o1 eSueqc leuorlrsoduroc ldruqe u? eêq eurlcoJcrlu ur pepnlcurpue ss?upunor8 eq1 ur pelecol soselcor8el4 'ot-rtoyro sruu pueo€-stuv Jo ssroc r{lt.tr Suruoz ,{.rolepcso-lelruou e.,leq sprolruerSpeurerS-esreoc eql w seselcol8eld leql,noqs (1 'ddy) senbtuqceloqoJdoJclu pue leurrou o eq1 iq suolleurrurelep puorlrsoduro3

'suotue lalz: (lc + I + Ho) lo stsPq oql uo pelelnclec *

Lt o0l6 r '0OL'E

tt'0rt '690'0

z0'069'Z0t'099'ZZIL'LIee'e9e 9e

16'66r.9'0eIe

69' rrt'860'0

60'091.'899'069'eZ99't rrt'ee0'9e

8?'00 r09'00z'8

et ' I2 6 880'0

r.0'0r6'88r'0eL'vz6e'e r€9'e69'9t

Lt'00rer'00t't

t0 ' tz r '6z0'0

z0'090'6t9'0zg'ezLT'EIe/'e99'96

69'00 tee'0e9'e60'0ez0e969 r '0re'0I r '0e r '669'0w'ez96'e t68'Z0t'9e

9t 66r' '0WE

86'0zr'690'0

90'00r.'6er'099'ZZ90't I6L'Zr/ 9e

0z'666e'0rt'e

z6'0t L 890.0

8t'0e6'8et'09L'ZZeg'el90'tLL'98

z r'668t'0re'e

v t 'Lr6'890'0

90'0ae'69r'0ez'ez0e'e r9Z'e89'9e

r,8'66 6e' r0 rre'o Lt 0eg'e /e'e90'0 60'0z[o Lo'rt9 8 e9'660'0 9 r'0z0'0 08'00 r'0 t0'0e9'8 Z0'0 t8t'0 9e'0Ee'ez 96'zzet '9 r 6z 'er6e'e zg'tz0'9e te'98

lelol.tc + l=o-

-o"HtcI

otyo"eNoe8oP3ooY!ouy!oal

"o"lv'oll-'o!s

89-9S t9-9S ZtsgS eoer-e){ 9z-9r tz-gJ 99-9S98-9S €9-9S9Zr-9Splol!uer618 + slrlpeur:6-asrPoc

sprollue:6 19 pau;elb-esreog splolluerb lg + lqH peute:O-esreog

eurlorEC qtnos 'uolnld lllH Iuoqn eql lol suotltsodtroc olllolq pelcelas 'Z 3l€vl

S)JOU CIOJINVUC NI SADV'ItrIÎSSSV WCV CIJVI{CYW CO NOIJN'IOAS :UAAdS 898

laete 2-Continued

Fine-grained Bt granitoids Aplite

J O - O / s6-99 56-101 K3-72 K3-130 s666 s677

869

The reddish-gray ilmenite grains contain networks and blebs ofwhite-gray hematite and indicate exsolution of an original iI-menite-hematite solid solution. The ilmenites contain on aver-age 12molo/opyrophanite, in the range of 6.4 to 18.4 molo/o (App.1). Calculation of the temperature and oxygen fugacity from mi-croprobe point analyses of exsolved oxides in a ferro-edenite +biotite granitoid (56-56) by the method of Stormer (1983) showssubsolidus re-equilibration, below the range ofthe calibrations,consistent with the observed textures.

In the coarse-grained biotite and muscovite + biotite ganit-oids and in the fine-brained biotite granitoids, the magnetites donot contain exsolved phases and only rarely does the ilmenitehave exsolved hematite. This indicates that the composition ofthe original igneous oxides in these granitoids must have beencloser to the end-member compositions than were the oxides inthe ferro-edenite + biotite granitoids.

Two sulfide assemblages have been observed in the granitoids;an inclusion assemblage in the magmatic minerals and a matrixassemblage. Pyrrhotite + chalcopyrite + pyrite is the inclusion

, + ' h t n l t l a 1 J

rocK onolyses

; ! f iner -g ro ined b io t i te g ron i lo ids

O op l i tes


36.45 35.46 36.782.19 3.55 2.54

16.12 14.96 16.1 124.39 23.78 21.470.37 0.45 0.577.31 8.34 8.760.05 0.16 0.040.25 0.930 09 0.09 002

10 02 8.88 9.450.91 0 68 0.720.05 0123 45 3.51 3 570 39 0 31 0.30

101 .26 100.60 99.73

37.49 36.532.47 2.63

16.29 15.2724.04 25.09o.22 0.367.32 6.820 09 0.06

0.230.03 0 .109.37 9.58

0.650.07

3.94 3.540.29

101 26 100.64

37 19 36.343.72 3.14

14.77 13.3125.97 24.820.50 0.714.94 8.520.06 0.060.07 0.070.o7 0.139.24 9.310.76 0.820.02 0.083.49 3.43o.32 0.36

100.48 100.38

- o l

- o 2

- o 3

-04 o clinopy?oxene

o l

oN

(f

zI

N)<

I

oOIrO

o_JU

oE

o o

- o l

- 0 3

- o 4

amphibole

e d e n i l e / h o r n b l e n d e

morginol , coorse-gro ined groni io ids. omph ibo le + b io t i ie

t r b io t i le

A b io t i te + muscov i te

o 5 o 7 U ) o 6

mol . MgO / (MgO + FeO)Fig. 6. Summary AFM (A : Al,O3-I(rO-NarO-CaO, F : FeO, M : MeO) diagrams projected from quartz, alkali feldspar,

plagioclase, and water for the rocks and minerals ofthe Liberty Hill pluton, South Carolina, differentiated by occurrence: (a) enclavesand enclave-bearing granitoid, (b) central, coarse-grained ferro-edenite + biotite granitoid, (c) maryinal, coarse-grained biotite,ferroan pargasite + biotite, and muscovite + biotite granitoid, and (d) finer-grained biotite granitoid and aplite facies.

o 7o 4U 5o 6o 4o 3

b//////////////

rocK onoryses

omphibole

e d e n i t e / h o r n b l e n d e

cenlro l coorse-gro inedgroni to ids

870

assemblage in magnetite and less commonly in silicates such astitanite and zircon. No sulfide inclusions have been observed inilmenite. The matrix assemblage is pyrite * chalcopyrite andcoexists with magnetite and ilmenite. From the contrasting oc-currence ofthese two sulfide assemblages, it appears the pyrrho-tite + chalcopyrite + pyrite is the initial sulfide assemblage thatcrystallized with magnetite and the early silicates. Pyrite + chal-copyrite is the latter sulfide assemblage that crystallized in equi-librium with both magnetite and ilmenite.

Other accessory minerals in the Liberty Hill pluton are allan-ite, apatite, bastnesite (?), cofrnite, fluorite, galena, molybdenite,monazite, pyrophanite, sphalerite, thorite, titanite, uraninite, andzircon. The molybdenite occurrence in the Liberty Hill plutonhas been discussed by Speer (1978). The U- and Th-bearingminerals have been discussed by Speer et al. (l 98 l). The apatitesare fluor-apatites, with only minor OH substitution and less than0.06 wto/o Cl. Fluorite is a rare accessory mineral in the finer-grained granitoids, occurring as disseminated anhedral grains.Allanite is the ubiquitous REE primary phase in all granitoidsexcept for the coarse-grained muscovite + biotite granitoid whereits role is assumed by monazite. Titanite is the most abundantaccessory mineral in the coarse-grained granitoids. It forms sub-hedral to euhedral, wedge-shaped grains up to 3 mm in length.Titanite has a small variation in composition with substitutionsof generally less than I 50/o Al and 7o/o Fe3+ for Ti. F substitutionin the O, site is less than l0o/o (App. 1).

Late minerals

The late minerals in the Liberty Hill ganitoids include albite,chlorite, titanite, epidote, muscovite, marcasite, hematite, cal-cite, laumontite, prehnite, alkali feldspar, rutile, hematite, andhyalite.

Many of the biotites and amphiboles are intergrown with chlo-rite + rutile, which are believed to be subsolidus replacementproducts. The chlorites have Fe/(Fe + Mg) = 0.6 (App. l), sim-ilar to the original biotites and amphiboles, and are ripidolitesand brunsvigites according to the terminology of Hey (1954).Mn contents are less than 0.10 atoms per l8 anions. Fluoriteoccurs in lenticular masses in altered biotites, an occurrence thatis believed to be secondary with the F derived from the altera-tion of the F-bearing biotite. Lenticules of prehnite or alkalifeldspar also occur in biotite.

Secondary titanite foms anhedral, poikilitic grains rimmingmagnetite, ilmenite, and biotite. Its major-element compositionoverlaps that of primary titanite, but can differ in trace-elementcomposition (Speer et al., 1981).

Epidote occurs as skeletal intergrowths with biotite and am-phibole, as epitaxial rims on allanite, and as a saussuritizationproduct in plagioclase cores. Microprobe analyses (App. 1), showthat the epidotes composing the saussurite are more aluminous,with less than 16 molo/o pistacite, than matrix epidotes, whichcontain between 25 and 34 molo/o pistacite. Mn content is lessthan 0.10 molo/o piemontite in the saussurite epidotes and be-tweenO.22 and l.l3 molo/o in the matrix epidotes.

Secondary white mica has two occurrences. The first is as dis-crete poikilitic or skeletal grains of white mica intergrown withbiotite. The second is as fine-grained masses in the cores of pla-gioclases resulting from saussuritization. As noted above, thesemuscovites are interpreted as being secondary and generally havethe same composition as the primary muscovites except for theirlower Ti contents.

In weathered granite samples, supergene alteration ofthe sul-fides is common. Pyrite and chalcopyrite have rims of hematiteand hydrous iron oxides. Some pyrite is altered to marcasite.


DrscussroN

Conditions of crystallization

The probable conditions during crystallization of theLiberty Hill granitoids can be deduced from contactmetamorphic assemblages of the aureole and xenolithsand, to a lesser extent, from compositions of the igneousminerals. Speer (198 la) found that the depth of emplace-ment corresponds to a pressure of 4.5 kbar on the basisof Fe-Mg partitioning in the assemblage cordierite + gat'

net * orthopyroxene + qvartz. This is comparable to theestimated pressures of 3. I to 4.7 kbat calculated from theA1 content of the ferro-edenites using the geobarometerof Hammarstrom and Znn (1986). Water pressure wassuggested to be half the total pressure in the central gran-itoids on the basis ofthe occurrence ofdehydration meta-morphic reactions in xenoliths at temperatures muchlower than they would occur under conditions of waterpressure equal to total pressure. Xenoliths in the centralgranitoids were concluded to have equilibrated at 725 +

102'C and the border granitoids aI 647 + 2C C on thebasis of Fe-Mg partitioning between garnet, biotite, andcordierite. Although not quantified, /o, was concluded todiffer between the central and marginal granitoids.

For the assemblage biotite + magnetite + alkali feld-spar, the equilibrium annite + VzO., : alkali feldspar +magnetite + HrO can be written. For this equilibriumWones (1972) determined that logfn o:7409/T + 4.25 +t/z Log f6, + 3 log Xr,*' * 2log Xot - log 4.a15;r6, - IogaF"3o4, where I is absolute temperature, X."*2 is the frac-tion ofFe+2 in all the octahedral sites ofbiotite and thusnot Fe/(Fe + Mg), Xo" is the fraction of OH in biotite,and a is the activity of the KAlSi3O, component in alkalifeldspar and Fe.Oo component in magnetite. This rela-tionship provides a means to relate several physical andcompositional variables. Because many of the physicalparameters for the Liberty Hill are unknown, the calcu-lations for the compositions of the Liberty Hill mineralswere perfbfined at a variety of temperatures,f.rn, and'fo,(Fig. 7). Stability curves for the Liberty Hill biotites werecalculated for both absolute values of/o, and conditionsof appropriate solid f, bufers. The stability curves arebased on the following assumptions: (1) the activity ofKAlSi,O* is 0.8. This is obtained from the activity-com-positional relations model of Ghiorso (1984) using theestimated original magmatic feldspar compositions in theLiberty Hill pluton. (2) The activity of FerOo in magnetiteis 1.0. (3) The average biotite X."*2 is 0.5 and the X"" is0.9. (4) The oxygen-fugacity values for the solid bufersare from Chou (1978) for MH, from Huebner and Sato(1980) for NNO, from Hewitt (1978) for QFM, and fromWones (1981) for TMQHIL. TMQHIL is the titanite *magnetite + qtartz + hedenbergite * ilmenite buffergiven by log.fo,: -30939/T + 14.98 + 0.142 (P - l)/Z, where 7is in kelvins and P is in bars. Additional limitson the probable conditions were Pn"d : P,.o, from Burn-ham et al. (1969), the stability of muscovite + qoarlzfrom Chatterjee and Johannes (1974), and the minimum-

SPEER: EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS 8 7 1

melting reaction in the granite system from Johannes(1984).

The stability calculations for the igneous biotites of theLiberty Hill pluton and conditions derived from the con-tact aureole show that water and oxygen fugacities in-creased from the central granitoids to the marginal gran-itoids whereas temperature decreased. In Figure 7, theconditions of the marginal granitoids are constrained tolie in the region below the curve Pn"ia : P,o,",, above thegranite solidus, and in the stability field of muscovite +qaartz. This small area, perhaps fortuitously, also in-cludes the temperature estimated for the border granit-oids. Estimated water fugacity would be 2.75 kbar, cor-responding nearly to conditions of &",0 : P,.,, (Burnhamet al., 1969), andforwould be approximately l0-15 8. Theconditions for the central granitoids are less constrainedand lie below Po,,o : P,o,or, above the granite solidus, andto the low-temperature side of the QFM solid buffer. Ifthe estimated temperature of 725C and water pressureof Po-u : 0.5P,o,", corresponding to.fnro: 1.7 kbar arecorrect for the xenoliths, thef, was about l0-'5, slightlymore oxidizing than the conditions of the TMQHIL andNNO solid buffers (Fig. 7).

The X."*' and Xo" values of biotites from the amphi-bole * biotite granitoid core facies and the biotite gran-itoid rim facies are nearly identical. Originally, this sug-gested the possibility of subsolidus re-equilibration atuniform conditions throughout the pluton. However, thesecalculations of Liberty Hill biotite stability from the es-timated conditions of the different facies derived fromthe xenoliths predict uniform magmatic biotite compo-sitions. It is a field demonstration of the results of Wintsch(1980) who found that the biotite composition in a bio-tite + K feldspar + magnetite assemblage is more or lessindifferent to the fluid composition at unspecified P, Z,f^ro, a\d for. However, the interpretation that conditionsin the Liberty Hill pluton's central and marginal granit-oids differed is plausible because the intensive parametershave been determined from independent sources.

The differences in oxygen fugacity and temperature be-tween the central and marginal facies ought to be reflectedin the oxide mineral compositions. The particular oxygenfugacities and temperatures obtained for the central andmarginal facies lie along the same ilmenite compositionalisopleth; however, the ulv6spinel component in the mag-netites of the central facies should be about 20 molo/ohigher than the amount of ulvdspinel in the magnetitesfrom the marginal facies (Spencer and Lindsley, l98l).The compositions of the ilmenites throughout the plutonare comparable in the amount of exsolved hematite; how-ever, the magnetites of the border granites contain onlytraces of exsolved Ti oxides whereas the magnetites ofthe central granitoids contain up to 25 volo/o exsolvedilmenite.

Evolution of the rnineral assemblages

There are five AFM varietal mineral assemblages inthe Liberty Hill pluton: clinopyroxene - actinolite - fer-

500 600 700 800 900temperoture, "C

Fig.l. The biotite + magnetite + alkali feldspar assemblagestability in the Liberty Hill pluton corresponding to the equilib-rium annite + t/z Or: alkali feldspar + magnetite * HrO overa range of temperatures and /rro for both absolute values of/o,and conditions of the MH (: magnetite + hematite), TMQHIL(: titanite + magnetite + qrafiz + hedenbergite + ilmenite),NNO (: nickel + nickel oxide), and QFM (: quartz + fayal-ite + magnetite) solid buffers. Other calculated curves in thediagram are Po",o : P,.*, (Burnham et al., 1969), stability of mus-covite + quartz (Chatterjee and Johannes, 1974), and beginningof melting in the granite system (Johannes, 1984). The temper-ature and pressure estimates for the Liberty Hill granitoids arefrom Speer (1981a).

ro-edenite, ferro-edenite + biotite, biotite, muscovite +biotite, and ferro-edenite - ferroan pargasite + biotite.The clinopyroxene-bearing assemblage is restricted to en-claves or their immediate vicinity. Most of the pluton isdominated by the central, coarse-gtained ferro-edenite +biotite granitoids that show abundant evidence for re-placement of ferro-edenite by biotite. The central granit-oids grade imperceptibly into coarse-grained biotite,muscovite + biotite, or ferro-edenite - ferroan pargas-ite + biotite granitoids at the pluton margins. Thesecoarse-grained facies are intruded by the later and finer-grained biotite granitoids. The mineralogical evolutionamong these varietal assemblages is discussed in this sec-tron.

Clinopyroxene - actinolite - ferro-edenite. The oc-cwrence of clinopryoxene in the Liberty Hill pluton couldrepresent either crystallization from the melt, xenocrysts,or restite minerals in equilibrium with the melt. Oncepresent, it reacted with the melt, first producing actino-lite, then ferro-edenite. The sharp, discontinuous natureof the zoning in the amphiboles may indicate either in-complete reaction because of slow diffusion, a miscibilitygap, or a discontinuity in crystallization conditions. Thecompositional gap is similar to though wider than, that

Ptoto, = 4.5 kbor

o

o

E


R3-924* - l 4i l q u r o - \ K 3 _

bro l i le

B1

e.F

TRANSITIONTO THE MARGINAL

, BIOTITE- I, . MUSCOVITE + BIOTITE

GRANITOIDS

l l q u i d\ .< )Dro l lTe

om ph i bo le

L iq = 31

CENTRALFERRO-EDENITE + BIOTITE

GRANITOIDS

EARLYCRYSTALLIZATION

REACTIONS

nb lende

L iq = Hbl + Bf

500 600 700 800Temperoture, "C

Fig. 8. Regions of distinct AFM liquidus topologies in P-Z space and pertinent schematic phase relationships among hornblende,biotite, cordierite, garnet, aluminum silicate, and muscovite in AFM (A : ALO3-KrO-Na'O-CaO, F : FeO, M : MgO) liquidusprojections from quartz, alkali feldspar, plagioclase, and water for granitoid magmas (Abbott and Clark, 1979; Abbott, 1981, 1985).The surrounding AFM diagrams illustrate the evolution of crystallization reactions in the Liberty Hill pluton: (a) The Liq + Cpx :

Act : Hbl reaction observed in the enclaves and enclave-bearing granitoids. (b) The probable Liq : Hbl * Bt reaction in the

L iq+ f l [ 1 =31

clrnopyr0xene

L iq+6p*+Ac t+Hb lLiq = 1161

found by Czamanske and Wones (1973), Mason (1978),and Chivas (1981) for other igneous occurrences. At theelevated pressures, temperatures, and Fe contents oftheamphiboles in the Liberty Hill, the compositional gapshould be small or nonexistent. Oba (1980) found no mis-cibility gap between tremolite and pargasite at magmatictemperatures above I kbar and found the gap decreasedwith increasing Fe content at lower pressures (Oba, 1986).Based on thermochemical data, Graham and Navrotsky(1986) have suggested that disequilibrium rather than amiscibility gap is the more likely interpretation for suchcompositional gaps. The Si content of the actinolite being>7.6 atoms per 24 anions also is evidence that it is nota magmatic phase; Chivas (1981) concluded that Si : 7.3is the limit of hypersolidus crystallization on the basis ofLeake's (1971) slggestions about the compositional lim-its of igneous amphiboles.

In the absence of a miscibility gap, the actinolite rimson clinopyroxene must be incomplete reaction productsof xenocryst + melt or restite + melt, and the actinolitedid not crystallize from the Liberty Hill magma. If theactinolite is not magmatic, it is unlikely that the clino-pyroxene is either. However, clinopyroxene was presentin the melt; forming inclusions in plagioclase and reactingwith the melt to produce a more magesian ferro-edeniteand giving amphibole-biotite KD values between 0.86 and0.99 (Fig. 6a). The reaction sequence clinopyroxene ractinolite - ferro-edenite could have occurred either be-fore or after biotite began to crystallize. Because the bio-tite compositions appear unafected by the presence ofthe clinopyroxene, the Liq + Cpx (xenocryst, restite) -actinolite ' ferro-edenite sequence must have occurredbefore the appearance ofbiotite, suggesting that the earlycrystallization reaction in the Liberty Hill magma wasLiq: Hbl (Fig. 8b).

Ferro-edenite + biotite. The ferro-edenite + biotitegranitoids are the dominant crystallization products ofthe Liberty Hill magma. The widespread texture of bio-tite replacing ferro-edenite is evidence that crystallizationconditions and compositions at the level of emplacementwere those of the reaction Liq + Hbl : Bt described byAbbott (1981).

In its simplest form, the Liq + Hbl : Bt reaction woulduse the melt as both source and sink of all the necessarycomponents to complete the reaction and thus is better

873

written as Liq I + Hbl : Bt + Liq 2. Because the gran-itoids were emplaced as mixtures of melt * crystals, LiqI and Liq 2 would be the interstitial melt. The finer-grained biotite granitoids intruding the pluton are be-lieved to be derived from the interstitial liquids of thecrystallizing coarse-grained granitoids and thus can beused as estimates of these Liquid compositions. A con-straint on Liq I and Liq 2 in the reaction should be thecompositional evolution of the Liberty Hill magma asshown by "liquid" variation diagrams. On the basis ofthe variation diagrams for the Liberty Hill pluton (Speeret al., unpub. ms.), Liq 2 should differ from Liq I bydecreased Si, Al, Ti, Fe, Mg, and Ca, yet have generallythe same K and Na contents. As shown below, this liquidvariation could only be accomplished with the partici-pation of other phases in the reaction. Indeed, the reac-tion may be the reason that K and Na remain relativelyconstant, an atypical trend in a crystallizing melt.

The least-squares petrologic-mixing-model programcENMrx (Le Maitre, l98l) was used to balance the an-hydrous components of the various Hbl + Liq : Bt mod-el reactions. A finer-grained granitoid (K3-72) was usedas an estimate of Liq I, and a more differentiated, finer-grained biotite granitoid (K3-924) was used as an esti-mate of the product, Liq 2. Mineral compositions fromK3-1303.8, in the same drillhole as K3-72, were used asestimates of the phases that may have participated in thereaction (Tables l, 2, and 3). The feldspars and quarlzwere assumed to have end-member and ideal composi-tions, respectively. All Fe was calculated as FeO, andanalyses were recalculated to l00o/o on an anhydrous ba-sis. Magnetite was assumed to be 93.090/o FeO.

Model I (Table 4) is the essential Liq I + Hbl : Liq2 + Bt reaction. The differences indicate insufficient Si,Ti, Fe, Mg, and Na and excess Al and Ca to balance thereaction. Ilmenite is ideal as a source for Ti. Anorthiteor titanite can serve as Ca sinks; Al, K, and Na sinks arealbite and K-feldspar. Si and Fe can be balanced by useof magnetite and quartz. Model 2 (Table 4) is the reactionincorporating these remaining minerals. The reaction canbe acceptably balanced, as would be expected, by addingrelatively simple minerals and more phases than com-ponents. Excellence in balancing the reaction does notprove that it occurred but does show that biotite may beproduced from amphibole while allowing the melt to con-


ts

magma. (c-'e) There are three possible reasons for the Liq + Hbl : Bt reaction ofthe central, coarse-grained ferro-edenite + biotitegranitoid: (c) the low-temperature end of the Hbl-Bt liquidus boundary is odd, (d) the low-temperature end of the Hbl-Bt liquidusboundary is odd but with different geometry from (c) because the amphibole is pargasite, or (e) Liq has moved from the Hbl intothe Bt field. (f+) With completion of the Liq + Hbl : Bt reaction by the consumption of Hbl in the marginal, coarse-grainedgranitoids, the crystallization reaction is (f) Liq : Bt or, more rarely, (g) Liq : Bt + Ms. (h) the finer-grained biotite granitoids areconcluded to be interstital melt separated from the coarse-grained granitoids in which only Bt crystallizes by the reaction Liq : Bt.The estimated conditions of the central granitoids are725"C, P.u E 4.5 kbar, Pu",o = 0.5P,"o,, andfo, = 10-t5 above to the NNOand TMQHIL solid buffers. The maryinal granitoids yield conditions of 647'C, Pnuia = Poar, and fo, = 10-16, relatively moreoxidizing.


TneLe 4. Coefficients (N, wt%) of least-squares mixing models for progressively more inclusive magmatic Liquid 1 + Hbl : Bt +Uquid 2 and subsolidus Hbl : Bt reactions in the Liberty Hill pluton, South Carolina

sio, Tio, Alro3 FeO MnO Mgo CaO Naro KrO

ReactantsLiquid 1tHbt-

ProductsLiquid 2fBt-

DifferenceResidual sums of souares: 0.7297

Model 1

1 .9521 .99

0.040.88

1.04 0.0424.33 0.61-o.22 -0.03

Model 2

1.95 0.0421.99 0.8846.17 5.26

3.08 5.371 . 7 6 1 . 1 7

3.58 5.17o.17 9.90

-0.34 -0.03

99.950.05

96.533.47

<0.00

95.144.86

84.305.433.693.931.92o.25o.47

49.5242.625.042.82

5.0526.1352.646.593.69o.250.79

1.0424.33

2.0093.090.00

Model 3

21.99

93.0946.17

24.3334.5712.40

2.00

0.00

0.418.48

0.209.49

-0.24

0.418.480.31

0.209.49

o.23

0.00

8.48

0.31

9.4913.770.22

0.23

0.00

1.5510.69

1 . 1 80.170.43

1.5510.690.23

1 . 1 80.17

20.16

27.38

0.00

10.6920.16

0.23

o.170.18

22.89

27.3856.030.00

3.081.760.12

3.580.17

o.14

0.00

1.76

0.12

0.170 . 1 10.10

11.82

0.14

0.00

5.371 . 1 70.10

5.179.90

16.920.09

0.00

1 . 1 7

0.10

9.900.100.10

0.09

0.00

72.5244.51

74.4637.83-0 .17

72.5244.5'l0.08

74.4637.83

100.0043.1964.7631 .18

0.00

44.5143.19

0.08

37.8329.2339.7968.74

100.0031 .18

0.00

0.35 14.741.51 9.02

0 .15 14 .183.00 14.510.06 0.54

0.35 14.741.51 9.02

47.64 0.08

Residual sums of squares: 0.0001

ReactantsLiquid 1tHbt-llmt

ProductsLiquid 2fBt-QtzAnKfSTtn.Mag

Difference

ReactantsHbt-AnMagllm'

ProductsBt.cht-Ep'AbQtzTtn-Cal

Difference

0 . 1 5 1 4 . 1 83.00 14.51

36.5518.83

36.83 1.83

0.00 0.00

1.51 9.O236.65

47.64 0.08

3.00 14.510.16 20.870.17 23.81

19.44

36.83 1.83

0.00 0.00

0.040.61

0.31

0.00

0.88

5.26

0.611.010.52

0.31

0.00Residual sums of squares: 0.0000

t Composition of a less-evolved dike of fine-grained biotite granitoid K3-72.+ Composition of a more evolved dike of fine-grained biotite granitoid K3-924.- Mineral analysis from the coarse{rained amphibole + biotite granitoid K3-1303.8 (Tables 1J).

tinue its magmatic trend by involving other minerals inthe reaction in amounts commensurate with their modalabundances. Biotite appears to be replacing a subequalamount ofhornblende. Textural evidence for the reactionis the replacement of ferro-edenite by biotite (Figs. 3a,3b) and rims of secondary titanite on ilmenite. The pro-duction of anorthite component could show up in a morecalcic zone in plagioclase. However, the calcic zone wouldbe minor because of the small modal abundance of theproducts. Plagioclases do have oscillatory zoning, but itis not clear how a particular calcic zone can be correlatedwith the Liq + Hbl : Bt reaction.

Albite has been omitted from these model crystalliza-tion reactions. Addition of NaAlSi.O, as a phase changesthe reaction to the dominant ganitoid crystallization re-action Liq : An * Ab + Kfs + Qtz, and the Liq +Hbl: Bt reaction is subordinated to less than lolo of the

overall reaction, as would be expected. In order to betterexamine the Liq + Hbl : Bt reaction, albite was notincluded.

The average Ko value for the coexisting ferro-edenite +biotite of the Liberty Hill is 1.04, with a range of 0.96-1.17 (Fig. 6b). Although many coexisting metamorphichornblendes and biotites have K" values near l.0, Mason(1985) has noted that in igneous rocks, hornblendes aremore magnesian than the coexisting biotites (Kanisawa,1972). Mason (1985) found that, for the coexisting horn-blendes and biotites in the Peruvian Coastal batholith,the Ko is about 0.63. The only exceptions in the Peruviangranitoids are samples in which biotite has formed byreplacement of hornblende. In these instances, Ko valuesare near 1.0. and Mason concluded that the distributioncoefficient can be used to distinguish biotites that crys-tallized directly from a melt from those formed by re-

placement of hornblende. Such a compositional distinc-tion is consistent with the apparent formation of biotitesby the replacement of ferro-edenite in the Liberty Hillpluton. The absence of more "igneous" Ko values mayreflect the difrculty of re-equilibrating the amphibole.

The Liq + Hbl : Bt reaction in the Liberty Hill plutoncould have taken place as a result of either equilibriumor disequilibrium crystallization. Abbott (1981) has con-cluded that the low-temperature end of the Bt-Hbl liqui-dus boundary is odd; thus, the crystallization path couldencounter a higher-temperature even reaction, Liq :Hbl + Bt (Fig. 8B), after the supposed initial Liq : Hblreaction (Fig. 8a). With continued equilibrium crystalli-zalion, liquids following the boundary would crystallizeby the odd reaction, Liq + Hbl : Bt, observed through-out the pluton. It is possible that the melt encounteredthe Hbl-Bt liquidus boundary where the equilibrium Liq +Hbl + Bt is odd; this would eliminate a need for theintervening even reaction. Abbott (1981) has drawn afairly gentle liquidus boundary; however, the composi-tions of the Liberty Hill amphiboles and biotites requirea sharper change in direction in the AFM diagram (Fig.8c) for an odd reaction. Alternatively, the sharp changein liquidus direction could be in the larger compositionalspace rather than in the simple AFM projection or couldbe made less sharp by interpreting the pargasite rims asigneous minerals (Fig. 8d).

With disequilibrium crystallization, the interstitial liq-uid could move from a position in the Hbl field (Fig. 8a)to one within the Bt field (Fig. 8e). Such an abrupt move-ment could result from either a change in the positionsof the AFM field boundaries or a change in melt com-position because of crystallization or contamination. Themovement of the AFM field boundaries would be causedby a change in physical conditions, most likely P.*, orPo",o. Increasing &,ia would increase the size of the liqui-dus field for biotite (Abbort, l98l).

It is possible that the reaction ofbiotite replacing horn-blende is a subsolidus one. Using what are interpreted asthe subsolidus minerals intergrown with hornblende-chlorite, epidote, calcite, and quartz-and their compo-sitions from sample K3-1303.8 (Table 3), a reaction toproduce biotite from amphibole without involvement ofa melt can be written (Model 3, Table 4). Although fea-sible, this reaction is considered unlikely to have causedthe replacement of amphibole by biotite because the pro-portions of the replacement products are not consistentwith their modal abundances. Biotite or chlorite, ratherthan epidote, are the most abundant replacement prod-ucts. Locally, biotite replacement ofamphibole occurredin the absence ofchlorite, epidote, albite, or calcite, andthe replacement of amphibole by chlorite, epidote, albite,or calcite occurred in the absence ofbiotite. In addition,late-stage subsolidus or deuteric reactions in granitoidsproduce magnesian actinolite (Mason, 1978; Lanier et al.,1978; Chivas, 1981), not a ferro-edenite. For these rea-sons, biotite replacement of amphibole in the Liberty Hillpluton is interpreted as a magmatic process.

875

Marginal facies. The possible origins of the three va-rietal AFM mineral assemblages in the coarse-grainedbiotite, muscovite + biotite, and ferroan pargasite + bio-tite granitoids along the margins of the Liberty Hill plu-ton have been examined previously by Wenner and Speer(1986). They concluded that the marginal facies compo-sitionally overlap the central granitoids, which arguesagainst significant differentiation. The marginal facies doshow minor amounts of wall rock-magma interaction thataffected some trace-element and oxygen-isotope compo-sitions.

The varietal biotite assemblage could be produced eas-ily from the central granitoids if Hbl is exhausted beforeLiq in the reaction Liq + Hbl : Bt. Estimations of con-ditions for the marginal facies yield lower temperaturesand higher water pressures than those for the centralgranitoids; the interpretation is that the marginal-faciesconditions are conditions of final re-equilibration. Be-cause it might be assumed that at some earlier instancethe conditions in the Liberty Hill magma were relativelyhomogeneous, an extended reaction or re-equilibrationhistory for the marginal rocks is implied, allowing thecomplete reaction of Hbl. Once the amphibole has dis-appeared, the reaction equation would be Liq : Bt (Fig.8I). The extended reaction history of the maryinal rocksmay have been feasible because of a minor influx of waterfrom the wall rocks (Wenner and Speer, 1986). The in-creased arro could also shift the AFM liquidus bound-aries if the Liq + Hbl : Bt reaction resulted from dis-equilibrium crystallization.

The origin of the biotite + muscovite granitoids in thenorthern margin of the pluton is probably the extremeconsequence of events that formed the marginal biotitegranitoids. Decreasing temperature and increasing waterpressure would allow the appearance of the muscovitefield in the AFM topology (Fig. 8g) and the crystallizationreaction Liq : Bt + Ms (Abbott, 1985). The absence ofintervening assemblages containing cordierite and alu-minum silicate more than likely results from their re-stricted occurrence in P-T space or from subsequent re-actlons.

The occurrence of amphiboles with aluminous ferroanpargasite rims coexisting with aluminous biotites alongthe border of the pluton is not readily predicted. Theirregular zoning of the amphiboles from ferro-edenite toferroan pargasite is essentially a manifestation of a rapidincrease in Al with only a small increase in Fe/(Fe + Mg)(Fig. 2b). The occurrence as thin rims is evidence of alate feature; however, the more aluminous and magne-sian nature ofthe coexisting biotites (Figs. 4, 6c) indicatethat the biotites equilibrated with the Al-rich amphibolenms.

The occurrence of aluminous amphiboles in metamor-phic rocks has been attributed to extremely aluminousrock compositions, differing phase assemblages, differingtemperatures (Bunch and Okrusch,1973), increased pres-sure (Leake, 1965), and increased oxygen fugacity (Mor-teani, 1978). The granitoids along the margin of the plu-

SPEER.: EVOLUTION OF MAGMATIC AFM ASSEMBLAGES IN GRANITOID ROCKS


ton have rock compositions and silicate and oxide mineralassemblages comparable to other granitoids in the pluton.Hammarstrom and Zen (1986) quantified the relation ofAl content of amphiboles in granitoid rocks with pres-sure. Using their geobarometer, the Al contents of theLiberty Hill pargasites correspond to estimated pressuresof 3.4 to 6.3 kbar, compared to estimated pressures of3.1 to 4.7 kbar for the ferro-edenites. This simple expla-nation is attractive, but there is no evidence that the gran-itoids in the northeastern section ofthe pluton are deeperor preserve a late crystallization at deeper levels. Thecompositional trend of aluminous amphibole rims is op-posite to those found by Czamanske and Wones (1973),Mason ( 1978), and Chivas ( I 98 I ), whose rocks have alsorecorded an increase in fo, and Si activity with crystalli-zation to form tremolite rims. The zoning to aluminousamphiboles with crystallization in the Liberty Hill plutonis also at odds with the expected compositional trendfrom the tremolite + albite : edenite + quartz reactionqualitatively determined by Graham and Navrotsky(1986). Chivas (1981) has noted that amphiboles crystal-lizing from magmas that did not evolve a fluid (nonmin-eralized trends) should have a limited compositionalrange. In a magma that does evolve a fluid, magmaticamphiboles would re-equilibrate or grow rims in the sub-solidus and have variable compositional trends depend-ing on the conditions and coexisting phase assemblage.

The most reasonable conclusion is that the ferroan par-gasite rims are late magmatic or subsolidus features. Theaverage K" value of 1.22, with a range of l.l3-1.30, forthe coexisting ferroan pargasite + biotites may be indic-ative of such nonmagmatic origin and explain Ko valuesof > 1.0 found in other marginal Liberty Hill granitoidsnot studied in such detail (Fig. 8c). The higher water pres-sures and oxygen fugacities of the marginal facies mayfavor such reactions and explain the restriction of suchferroan pargasite + biotite assemblages to the margin ofthe pluton.

Finer-grained biotite granitoids. The flner-grained bio-tite granitoids are interpreted as having crystallized frominterstitial melts, extracted from the coarse-grained gran-itoids. The multiple compositions of these rocks (melts)indicates that they separated either at several differenttimes or from different volumes with differing degrees ofcrystallization. Interstitial melts of the coarse-grainedgranitoids were producing biotite by the Hbl + Liq I :Bt + Liq 2 andLiq: Bt crystallization reactions. If in-terstitial melts separated to produce the finer-grainedrocks, these crystallization reactions would explain theessential similarity in compositions between biotites inthe coarse-grained rocks and biotites crystallized from theextracted melts.

Pyrrhotite - pyrite. The inclusion assemblage pyrrho-tite + chalcopyrite + pyrite is the initial sulfide assem-blage that crystallized with an early magnetite + silicateassemblage. However, this sulfide inclusion assemblageis a re-equilibrated one. At temperatures between 400and 600"C, pyrrhotite and chalcopyrite react to form py-

rite + intermediate Cu-Fe solid solution + vapor(Vaughan and Craig, 1978). The high-temperature inclu-sion assemblage, therefore, was pyrrhotite + pyrite +intermediate Cu-Fe solid solution + magnetite, whichhas a thermal stability at higher temperatures than thegranite solidus. The pyrite * chalcopyrite matrix assem-blage is the later sulfide assemblage that crystallized inequilibrium with the magnetite + ilmenite * silicate as-semblage. This sequence of sulfides indicates that the ac-tivities of oxygen and sulfur appear to have increasedduring crystallization of the Liberty Hill magma.

CoNcr,usroNs

Enclaves and a traverse from core to rim ofthe LibertyHill pluton represent a small portion of the cooling his-tory of the pluton and indicate the nature of the Liq +Hbl : Bt crystallization reaction and reactions before andafter it (Fig. 8). The prevalence of replacement texturesand the rarity of Ko : 6Bfl/XB$/(W;/XF!) < 1 indicatethat little biotite was present before the Liq + Hbl : Btreaction. It is envisioned that the Liberty Hill magmainitially began crystallizing with the Liq : Hbl reaction,concurrent with a Liq + Cpx - Act - Hbl reaction wherethe Cpx crystals were xenocrysts or restites (Fig. 8a). Ifthe magma solidified as a result of equilibrium crystalli-zation, the Hbl-Bt liquidus boundary would have beenreached where either (1) the Liq : Hbl + Bt even reactionwas encountered before the Liq + Hbl : Bt odd reaction(Fie. 8b) or (2) conditions of the Liq + Hbl : Bt oddreaction were directly encountered (Figs. 8c, 8d). If themagma solidified as a result of disequilibrium conditions,the interstitial liquid would move from a position in theHbl field (Fig. 8a) to one within the Bt field (Fig. 8e).Such an abrupt change in the positions of the AFM fieldboundaries, melt compositions, or both would result inthe Liq + Hbl : Bt reaction. Such changes could bebrought about by movement of magma, adding or re-moving melt, or an influx of water from the country rocks.Perhaps significantly, the finer-grained biotite granitoidmagmas are believed to represent the interstitial melt thatseparated from the coarse-grained rocks at this stage. Theinterstitial melt of the coarse-grained granitoid was crys-tallizirlg Bt by either Hbl + Liq : Bt or Liq : Bt and,upon separation, the Liq : Bt reaction would be expectedto be the separated melt's crystallization reaction (Fig.8h).

The Liq solidification of the central granitoids wascompleted before the consumption of Hbl, preserving theHbl + Bt assemblage. Locally, the Hbl may have beensufrciently isolated from the Liq to prevent further re-action. The extended reaction history of the marginalcoarse-grained granitoids allowed the Hbl to be exhaust-ed before Liq in the reaction Liq + Hbl : Bt, whereuponthe melt completed its solidification by the Liq : Bt re-action (Fig. 8f) and, locally, by the Liq : Bt + Ms reac-tion where there was sufrcient water pressure (Fig. 8g).Thus, difering coarse-grained granitoid mineral facies inthe Liberty Hill pluton have not resulted from multiple

intrusion or differentiation, but rather from extended re-action histories of the marginal portions of the pluton.The extended reaction history may be caused by the in-creased availability of water at the margins, possibly fromthe country rocks.

AcrNowlnncMENTS

This study is part of an investigation of low-temperature, geothermalenergy resources supported by the U.S. Department of Energy (contractnumbers EV-76-S-05-5103 and ET-78-C-05-5648 to J. K. Cosrain, L.Glover III, and A. K. Sinha; and contract number DE-AC05-78ET27001to J. K Costain and L. Glover III), with supplemental support from theU.S. Geological Survey (contract l4-08-0001-G-685 to L. Glover III andJ. K. Costain). Assistance in this research by S. W. Becker, S. S. Farrar,and C. W. Russell and comments on the manuscript by J. Hogan, RichardN Abbott, and James F. Luhr are appreciated.

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Evolution of magmatic AFM The hornblende * melt : biotite … · 2007-08-28 · Evolution of magmatic AFM The hornblende * Libertv Hill INrnonucrroN Many of the late Paleozoic granitoid