THE AMERICAN MINERALOGIST, VOL. 55, JANUARY-FEBRUARY, 1970 Mg/Fe PARTITIONING IN GARNET AND BIOTITE FROM SOME GRANITIC, PELITIC, AND CALCIC ROCKS J. B. LvoNs,Dartmouth College, Honoaer, Neu Hampshi.re03755 AND S. A. Monsr, Franklin and, Marshall College, Lancaster,P enns^'tloanialT 604. O"ao^"" Ko Mg/Fe for garnet-biotite pairs in calcium-rich metamorphic rocks from New Ilampshire of the garnet to kyanite zones is signifcantly lower (0.10*0.01) than that in isogradic pelitic schists (0.16+0.02). Sillimanite zone pelitic rocks show a wide range of KD (0.16+.03), but much lower values than those in the enclosed Kinsman Quartz Mon- zonite (Kp: Q.28+ .03), which has been variously described as a magmatic, granitized, or metamorphic rock. The Kinsman and the enclosing pelitic rocks have closely similar chem- ical compositions and mineral assemblages, and formed under high load pressure ()5 kbar), low to intermediate oxygen fugacity, and (probably) similar HzO fugacity. Thus the higher Ko of the Kinsman rocks is taken as evidence of higher temperature of formation, and of probable magmatic origin. In these rocks CaO and MnO in garnet is not directly related to metamorphic grade, as several investigators have proposed in other areas. INrnooucrrorr Several attempts have been made to establish an interrelationship between the partitioning of Mg and Fe in coexisting garnet and biotite, and the known or inferred geologic temperature and pressure.The pres- sure effect should be relatively inconsequentialfor reactions of the type: Fe garnet{Mg biotite-Mg garnetf Fe biotite, and if these minerals behaveas ideal solidsolutions, it follows (Kretz,1961, p. 367) that: _AG" InKp: (1) RT AGo: change in standard free energy for the reaction R : gas constant ? :"Kelvin (Mg/Fe) garnet Ko: distribution coefficient: atomi. (MglFe) biotite The literature on Mg/Fe partitioning in biotite and garnet has been surveyed by Albee (1965a),who concludedon the basis of 70 pairs of analysesthat: (1) Ko in Mn-free garnetsranges from 0.2 in garnet-zone rocks to 0.37 in sillimanite-orthoclasezone rocks, but that (2) Kp is re- duced in proportion to the atom percent of [Mn/(Mnf FefMg)] in 23r 232 T. B. LYONS AND S. A. MORSE garnet. Frost (1962) had previously shown an increase in Ko with in- ferred temperature increase, but found it necessary to introduce an arbitrary correction for both Mn and Ca in garnet in order to introduce some regularity into his data. Sen and Chakraborty (1968) using 87 pairs of analyses concluded that Kp is a systematic function of both metamorphic grade and of Ca/Mn ratio in garnets' This result is not quite what might have been predicted from earlier literature surveys by Sturt (1962) and Nandi (1967) which show that a plot of weight percent (CaO*MnO) against (FeO*MgO) in garnet is nearly linear, (as it should be becauseof diadochy) and that the latter two oxides system- atically increase at the expense of the former with increasing meta- morphism. This generalization with respect to temperature probably is valid only for nearly isochemicalrocks. Mn zoning in garnet is nearly ubiquitous (Hollister, 1962; Atherton, 1968; Drake, 1968), with Mn usually decreasingand Fe usually in- creasingfrom core toward rim. Even though Mg/Fe in the garnet rim is somewhat higher than in the core,the effect on Kn in individual samples is generally not large (Albee, 1968), and is often masked by other variables such as differencesin bulk rock composition, or differencesin mineral phaseassemblages. Chinner (1960) has established a relationship between rock oxidation ratio (definedas mol.2FezOaX100/2FezOtlFeO) and suites of opaque oxides, showing that as oxidation ratios rise above 43, there is a major increasein biotite MgO/FeO ratio, and in garnet MnO/FeO ratio. This implies the possibility of non-ideality of Fe-Mg substitution in one or both phases,and that at a given grade of metamorphism Ko might be decreased in rocks of high oxidation. Because his garnet analyses are incomplete, it is not possible to confirm this deduction from Chinner's data, but Hounslow and Moore (1967) have shown in coexisting biotite- garnet-staurolite assemblagesthat strong variations in FeO:MgO ratios in these minerals do correlate with rock oxidation ratios, as do variations in the opaque mineral suites. It is interesting, however, that their Ko for Mg/Fe in garnet-biotite pairs is fairly consistent (see Table 1), and independent of rock oxidation ratios. Table 1 is a compilation of Ko Mg/Fe in garnet-biotite pairs published since Albee's (1965a) review of 70 older analyzed pairs' Not shown in Table 1 are 40 additional new analysesby Albee (1968) which have been presented graphically, and for which the calculation of Ko has not been made. From Table 1 and from earlier literature surveys' the conclusion is in- escapablethat Ko Mg/Fe for garnet-biotite assemblagesincreases with metamorphic grade. Perturbations, however, are severe and may be PARTITIONING IN GARNET AND BIOTITE 233 'ferrn 1. Soun RrcBNr Dntpnulrrlrrows o1 Kp[Mc/(Fn+rfFs+a)] rr,vG.lr:vrr-Brorrm Parns Approximate Grade Ko Reference Garnet 0.187,.t73, .077,.089, .094, .097 Atherton, 1968 Av.:0.130 .112,.ll5 Hietanen, 1969 (n:10) .t60,.200 Senand Chakraborty, 1968 0.150,.137, .150, .114, .127, .141 Atherton, 1968 Srauroliteand kyanite .132,.163, .215, .217 , .lgg Gorbatschev, 1968 .132,.134, .t55 Hietanen, 1969 Av.:0.151 .125,.115, .133, .139, .127, .739, .I55 Hounslow and Moore, (n:33) 1967 .095,.156, .131, .tI4, .152,.208 Saxena, 1968a .173,.143, .1?9, .177 Saxena and Hollander, 1969 .190,.190 Sen and Chakraborty, 1968 Sillimanite and 0.196,.359, .194 Gobatschev, 1968 sillimanite-orthoclase .224,.227 Hietanen, 1969 Av.:.274 .329,.259,.230, .338 Saxena,1968b (n:23) .360,.400, .400, .250,. 180, .320, .330, Sen and .250,.280,.300, .200, .t90, .200, .240 Chakraborty, 1968 causedby factors such as disequilibrium, zoning in garnet, analytical and mineral separation errors, differing load pressure, and significant dif- ferencesother than Fe/Mg in the composition of garnet and/or biotite (cf . Albee, 1965b, p. 277-278). Many of the variables may be estimated or circumscribed, and it is with this in mind that the determination of partitioning coemcientsis applied here to (1) the estimation of temper- ature differencesbetween the Kinsman Quartz Monzonite and its wall- rocks, and (2) the determination of differencesin Ko in isogradic pelitic and calcareousschists. GBor.ocrc RnrarroNs oF rHE KrNsuaN Quanrz Mouzor.rrrB The Kinsman Quartz Monzonite forms several stocks and batholiths in western New Hampshire (Billings, 1956, Plate 1) and in everywhere in contact with pelitic wallrocks. The formation is generally gneissicand is characterizedby striking megacrysts of potash feldspar up to 12 cm in length. In most exposuresthe Kinsman is of quartz monzonitic compo- sition, consisting of approximately 30/o potash feldspar, 30/e quartz, 25/p oligoclase-andesine,I2/s biotite, and 3/s muscovite. The rock is 234 I. B, LYONS AND S. A, MORSE "two-textured," and has a generally coarsegrained aspect, but all of the major minerals are strained and deformed, and the groundmassis a fine- grained mosaic-textured aggregateof the same mineralogy as that of the main rock, except that myrmekite and micrographic intergrowths are extraordinarily well developed. Garnet is locally abundant, sillimanite is not uncommon, and cordierite is often noted, particularly near contacts. Although most investigators have concluded that the rock is of magmatic origin, Chapman (1952) has argued that it is a granitized pelite, and Barker (1961) has describedit as a metamorphosed plutonic rock which has undergone a reaction such as: (1) biotite*muscovite:cordierite * garnet* microcline or (2) biotite* sillimanite : cordieritef garnetf potash feldspar. Barker concludes that the Kinsman is of higher meta- morphic grade than the adjacent schist, either because of a higher temperature, or a lower HrO fugacity (thus explaining its lower biotite content). PptnocnNBuc PARAMETERS The Kinsman Quartz Monzonite was formed in the late stagesof the Acadian orogeny (Billings, 1956) and its areal distribution is restricted to the high-temperature side of the sillimanite isogradic surface in west- central New England (Thompsonet a|.,1968, p. 216).Most of the samples whose analysesare reported here for the first time (Tables 2, 3, 4) Iie on the high-pressure side of an areal boundary separating kyanite-sil- limanite from andalusite-sillimanite assemblages(Thompson and Nor- ton, 1968, Fig. 1). This suggeststhat the Kinsman Quartz Monzonite and the enclosing pelitic schists formed at a load pressure generally in excessof 5 kbar (Richardson et ol., 1969).There is no geologicindication that degree of deformation or inferred intensity of shearing stress is related to grade of metamorphism. Whether Pnro+Piroia: Proad is uncertain. Avaliable analyses of middle and high-grade pelitic metamorphic rocks in New Hampshire (Billings and Wilson, 1964) show that these have slightly higher H2O contents than the Kinsman. On the other hand, potash feldspars in the pelitic schists are generally less "triclinic" than those in the Kinsman, and the higher degree of annealing in,the latter may reflect higher HzO fugacities. Oxidation ratios in 9 analyzed sillimanite-zone schists from areas near the Kinsman plutons (Billings and Wilson, 1964) range from 2 to 45 (one sample having an anomalously high value of 61). Eight available analyses of Kinsman have oxidation ratios between 9 and 42, and like most