Origin of the Baltimore Gneiss Migmatites at Piney Creek, Maryland

Origin of the Baltimore Gneiss migmatites at Piney Creek, Maryland

SAKIKO N. OLSEN Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland, 21218

ABSTRACT layers (neosomes1) in the migmatite with the initial melt compositions in experimental studies (see Brown, 1967; Härme, 1959; The net compositions of neosome plus mafic selvage in two bioti- Hedge, 1972; King, 1965; Lowman, 1965; Misch, 1968; von Pla- tic layered migmatites from the Baltimore Gneiss at Piney Creek, ten, 1965); and (3) textural and mineralogical criteria, such as re- Maryland, closely approximate the paleosome compositions: the lict mineral orientations (see Goodspeed, 1948; Hopson, 1964; migmatization must have occurred in rocks closed to all except Loberg, 1963), replacement textures (see Bellière, 1960; King, possibly volatile components. Anatexis or metamorphic dif- 1965; Ljunggren, 1957; Misch, 1968), comparison of plagioclase ferentiation is indicated as the migmatization mechanism. Parallel compositions (see, Loberg, 1963; Misch, 1968; Tobschall, 1971), tie lines between the neosome, selvage, and paleosome in a modal metamorphic mineral assemblages (see Kretz, 1966; Ramberg, biotite-microcline plot suggest a subsolidus metamorphic dif- 1956), or distributions of major and minor elements (see Hedge, ferentiation mechanism in which microcline replaces biotite in the 1972; White, 1966) in paleosome and neosome. neosome by a reaction such as biotite + 6H+ —•> microcline + 3 In all but a few areas of regional migmatization, these criteria ++ ++ (Fe , Mg ) + 4H20; the Fe and Mg released by the reaction dif- failed to establish conclusively the mechanism of migmatization. fuse from the neosome to the selvage; biotite replaces microcline in Two factors are perhaps most responsible for the failures; (1) in the selvage by the reverse of the reaction. Anatexis (simple partial fusion) in the Piney Creek rocks is also indicated because (1) the 1 The definitions of terms for migmatites compiled by Mehnent (1968) after Dietrich closer a paleosome composition is to the granite minimum the and Mehnert (1961) are followed in this paper (see Appendix 1). more extensive is the migmatization in the rock; and (2) the neosomes are closer to the granite minimum than the paleosomes. It is postulated that the Piney Creek migmatites formed by metamorphic differentiation induced by anatexis. Anatexis decreases fHM locally because much water must be dissolved in the first melt, thus initiating the reaction by which biotite breaks down to microcline. The fm0 gradient between the neosome and paleosome maintained by the presence of melt in the neosome drives the metamorphic differentiation and should also create an asm gradient leading to the observed quartz migration from the selvage to the neosome.

INTRODUCTION

The Precambrian Baltimore Gneiss, exposed in the cores of mantled gneiss domes around Baltimore, Maryland, is a typical example of a migmatitic basement complex (Fig. 1). Sederholm (1923, 1926), Wegmann (1935), Holmquist (1920), Dietrich (1960), Mehnert (1968), Misch (1968), and others have advanced various theories for the cause of migmatization and for the process of migmatite formation. Although there has been no general agreement on the origin of migmatites, the models postulated can be grouped into four categories (see also White, 1966; Misch, 1968): (1) lit- par-lit injection of a granitic magma along the foliation planes, (2) anatexis (simple partial fusion) with segregation of the initial melt, (3) metasomatism with introduction of postassium, or less commonly sodium, from external source(s), and (4) metamorphic dif- Figure Map showing some of Baltimore Gneiss domes, after Doe and ferentiation (subsolidus) within a locally closed system by mechan- others (1965), and study area (A). Domes: C = Chatallanee; P = Phoenix; ical and (or) chemical processes. T = Towson; Tx = Texas; W = Woodstock. B is sample location for The most commonly used criteria for determining the Hartley Augen Gneiss. Circle pattern = Cretaceous coastal plain sedimntary rocks; random-dash pattern = upper Precambrian and mechanism of migmatization are (1) field correlation between a Paleozoic meta-igneous rocks, mainly Baltimore Gabbro; Pzcw = upper migmatite and its premigmatization equivalent and the differences Precambrian to lower Paleozoic Glenarm Series Cockeysville and Wissahic- in their bulk compositions caused by migmatization (see Berth- kon Formations (marble and schists); black areas = Setters Formation, elsen, 1960; Brown, 1967; Cheng, 1944; Engel and Engel, 1958); Glenarm Series; pCb = Precambrian Baltimore Gneiss. Isograds around (2) comparison of the composition of the leucocratic veins and northeast end of Phoenix dome are from South wick (1969).

Geological Society of America Bulletin, v. 88, p. 1089-1101, 11 figs., August 1977, Doc. no. 70805.

1089

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most areas, regional migmatites cannot be traced back to strati- 1) and fibrolite found in and near the domes (Hopson, 1964, p. 76; graphically equivalent but unaltered rocks. Chemical changes that South wick, 1969) indicate that the temperature of the gneiss during may have occurred during the formation of these migmatites can- Paleozoic metamorphism has been slightly above the univariant line not be determined directly. (2) Critical textural or compositional kyanite = sillimanite. The minimum temperature indicated is about evidence for the primary process of migmatization may be 670 °C. Anatexis is possible under these pressure-temperature con- obscured by recrystallization or later metamorphic processes. The ditions provided PHl0 is nearly equal to Pt0tal (Luth and others, two following additional criteria have been useful in this study of 1964). The 500-m.y.-old Gunpowder Granite (Davis and others, the Baltimore Gneiss migmatites: (1) Mass balance within the 1965) intruding both the Baltimore Gneiss (in the Towson Dome) migmatite (see, Mehnert, 1968, p. 250; Kretz, 1966, for slightly and overlying Glenarm Series rocks has been interpreted by Hop- different approaches with the same criterion). If the compositions son (1964) as a rheomorphic offshoot of the Baltimore Gneiss. A of neosome and its mafic selvages combined equals that of the large-scale anatexis of the gneiss seems to have occurred during paleosome (host rock), the rock must have been closed to all except Paleozoic metamorphism, at least at a deeper level than the present possibly volatile components during migmatization. In such a case, exposure. anatexis or metamorphic differentiation must have been the cause of its migmatization. Conversely, if some material must be added to PINEY CREEK MIGMATITES or subtracted from the net neosome pi JS selvage composition to match the paleosome composition, the rock was open to external Piney Creek cuts the Phoenix dome near Verona, Maryland (Fig. components. Metasomatism or igneous injection is most likely the 1), exposing about 100 m of migmatitic Baltimore Gneiss beneath dominant or sole mechanism for such migmatization. (2) Deducing the Setters Formation, the lowest Glenarm unit (Fig. 2). Four rock the presence of chemical potential gradients. If chemical potential types are present in the Piney Creek migmatites: (1) biotitic layered gradients are found to be present between the zones in a migmatite migmatites: quartz-plagioclase-microcline layers (neosomes), one and the gradients match the observed migration of constituents, to several centimetres thick, occur in homogeneous or weakly metamorphic differentiation by diffusion was most likely responsi- veined biotite-quartz-plagioclase-microcline gneiss (paleosome); ble for the migmatization (see, Fisher, 1970a; Olsen and Fisher, (2) biotitic layered migmatites with microcline-rich neosomes: 1974). Using these cirteria, the four models for the mechanism of migmatization were tested on some Baltimore Gneiss migmatites from a 150-m outcrop at Piney Creek near Baltimore. At least two of these migmatites clearly formed in a closed system. These rocks contain evidence for both subsolidus metamorphic differentiation and anatexis. I propose that the closed system and most other migmatites at Piney Creek have formed by metamorphic differentiation induced by anatexis.

GEOLOGIC SETTING OF BALTIMORE GNEISS

The Baltimore Gneiss is exposed in the cores of seven mantled gneiss domes, located at the arc of the Appalachians in the Mary- land Piedmont (Fig. 1). Metasedimentary rocks of the upper Pre- cambrian to lower Paleozoic Glenarm Series surround and mantle the gneissic cores. The gneiss consists of layered migmatite, veined gneiss, augen gneiss, amphibolite, and granitic gneiss, all of the amphibolite grade. Hopson (1964) concluded that the gneiss was Figure 2. Map of the Precambrian basement reactivated by migmatization during Piney Creek locality (A in early Paleozoic time and deformed with the overlying Glenarm Fig. 1), after Hopson strata into anticlines. The metamorphic grade of the Glenarm rocks (1964, Fig. 18). increases toward the domes, and. the almandine, staurolite, and kyanite isograds closely parallel the general outlines of the domes (Fig. 1). The Rb-Sr ages of the Baltimore Gneiss are 1,050 ± 100 m.y. for the whole-rock samples and 300 m.y. for the biotite and plagioclase (Wetherill and others, 1968), but the lead-lead ages of some zircons are as much as 1,300 m.y. (Tilton and others, 1970). The radiometric ages show that there have been at least two periods of major isotope equilibration, one each in the Precambrian and Paleozoic, and that the Precambrian episode has been either a plutonism or metamorphism of higher grade than that of the Paleozoic episode. The migmatization of the gneiss as seen today could have occurred during either episode. No andalusite has been found in the Baltimore area, so Punal during the Paleozoic metamorphism must have been at least 6 kb (see Richardson and others, 196 92). The kyanite isograd around the gneiss domes (Fig. EXPLANATION -gS Strike and dip of bedding jf Vertical bedding 2 Holdway {1971) showed the aluminosilicate triple point at 501 °C and 3.76 kb, J* Overturned bedding much below the 622 °C and 5.5 kb values of Richatdson and others. Because Hold- Strike and dip of foliation way's main objection to the data of Richardson and others is that their material con- Ve r t i c a I f o I i a t i on tains fibrolite, and because "sillimanite" in the Baltimore area is mainly fibrolite, the Lithologie contacts data of Richardson and others seem more appropriate as the reference. Inferred structural trends

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same as type 1, except that the neosome is pegmatitic and contains 2, above) which compose perhaps 10% to 20% of all layered neo- abundant prophyroblasts of pink microcline; (3) biotite- somes. Neosomes in the veined gneiss vary greatly in both composi- hornblende layered migmatites: same as type 1, except that tion and texture: two common types are those similar to the neo- hornblende occurs as an additional dark phase; and (4) veined somes in the layered migmatite and those that are almost gneisses: the neosomes occur as irregular and discontinuous bodies monomineralic, coarse-grained mosaics of feldspar. All of the neo- rather than layers. Types 1 and 4 are the most common at this out- somes, in both layered and veined migmatites, are granoblastic in crop. The four rock types occur in units as much as several metres thin section and contain no obviously igneous texture. thick, with the transitions between them varying from gradual to Selvages bordering many of the neosomes are 2 to 6 mm thick sharp and discontinuous. This major layering probably reflects and enriched in the mafic and accessory minerals of the host paleo- premetamorphic compositional variations. The minor layering of some. The contact between the neosome and selvage is very sharp alternate mafic and felsic layers appears to be due mainly to mig- and commonly marked by a quartz vein less than 1 mm thick. The matization. Both major and minor layering parallel the main folia- contact between the selvage and paleosome is recognized mainly by tion except for minor deviations. Most neosome layers are remark- the difference in the amount of mafic minerals, but the selvage is ably straight and parallel sided, but locally pinch-and-swell struc- also slightly more coarse grained than the paleosome. The textural tures and ptygmatic folds are present. The volume of neosome differences in the three zones are summarized in Table 1. For more ranges from about 5% to more than 70% of the rock and is gener- detailed descriptions, see Olsen (1972). ally smaller in the veined gneiss than in the layered migmatite. The major dark phases in all zones are either dark brown biotite The neosomes in the layered migmatites are white to buff and or the biotite plus blue-green hornblende. No difference in optical sugary textured, except for the microcline-rich neosomes (see type properties of mafic minerals in the three zones was detected. The

TABLE 1. PROPERTIES OF MINERALS IN PINEY CREEK MIGMATITES

Neosome Selvage Paleosome Layered Microcline-rich Veined migmatite layer gneiss

Plagioclase Zoning Diffuse and gradual; re- Diffuse and gradual; Diffuse and gradual; Diffuse and gradual; Diffuse and gradual; nor- verse, normal, oscil- normal, oscillatory; normal?; may be normal; occurs in mal; occurs in lh to % latory (normal-re- not common, (difficult common or rare less than Vi of grains of grains verse); occurs in about to tell because of al- Vi of grains terations) Twinning Carlsbad-albite, peri- Albite, pericline Albite, pericline See layered migmatite See layered migmatite cline-albite, albite, pericline Size (mm) 0.1 to 1.0 0.1 to 1.0 0.1 to 2.5 0.3 to 2 0.2 to 1 Mode of As large, irregular As dusty, irregular Varied; as coarse mosa- Medium-size ovoids; Medium-grained, granular occurrence ovoids; as fine-grained ovoids; enclosed in ics; fine-grained ag- subhedral grains aggregates with quartz microcline gregates with quartz and microcline and microcline Alteration Minor Moderate to extensive Minor to very extensive Minor Minor Albitic rim Present Very common Present Rare Present but very narrow Myrmekite Present Common Present Rare Rare Epidote Present Present Common Rare Present inclusion

Microcline Grid twin Present Well developed Present Not common Not well developed, but common Perthite None None Present None None Size (mm) 0.1 to 0.8 0.3 to 2.0 0.2 to 2.5 Avg, 0.2 to 0.3 Avg, 0.4 to 0.5 Mode of Lobate to amoebalike; Serrated; as rinds on Interstitial; as extremely Interstitial patches; ir- Granular with straight occurrence serrated plagioclase serrated porphy- regular grains boundaries roblasts

Biotite Subidioblastic, fine to Moderately to ex- Subidioblastic, when Subidioblastic, medi- Subidioblastic, unaltered medium flakes; may tremely chloritized present; unaltered um to coarse flakes; or slightly chloritized be slightly chloritized unaltered

Hornblende Rounded or spindle- Not present As large, serrated por- Subidioblastic; medi- Subidioblastic; medium shaped; as much as 4 phyroblasts in some um size size mm in size when present

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TABLE 2. MICROPROBE ANALYSES OF BIOTITES, BIOTIT1C LAYERED MIGMATITES, PINEY CREEK

PC1-10 PC1-12 Neosomt Selvage Paleosome Neosome Selvage Paleosome

Weight percent, with standard deviation

Si02 36.3 ± 3.5 36.3 ± 0.3 36.2 ± 0.3 36.8 ± 0.1 37.0 ± 0.3 37.2 ± 0.4 AI2O3 16.0 ± 3.3 16.0 ± 0.3 15.9 ± 0.2 15.8 ± 0.2 16.1 ± 0.2 16.1 ± 0.1 TiOz 2.9 ± 3.4 2.4 ± 0.1 2.6 ± 0.03 2.3 ± 0.01 2.2 ± 0.1 2.3 ± 0.1 FeO 21.8 1 21.8") 21.2") 19.3 1 19.3") 19.9") ± 3.2 F ±0.5 F +0.3 > ±0.4 > ±0.6 > ±0.4 Fe203* 3.6 J 3.6 J 3.5 J 3.2 J 3.2 J 3.3 J MgO 6.3 ± 0.1 6.6 ± 0.1 6.4 + 0.1 8.4 ± 0.2 8.6 ± 0.1 8.4 ± 0.1 MnO 0.4 ± 3.1 0.4 ± 0.1 0.4 ± 0.1 0.4 ± 0.1 0.3 ± 0.1 0.3 ± 0.04 CaO

NazO Tr. Tr. Tr. Tr. Tr. Tr. K2O 9.6 ± 0.1 9.5 ±0.1 9.4 ± 0.1 9.3 ± 0.1 9.4 ± 0.1 9.3 ± 0.1 H2O* 3.5 3.4 3.4 3.5 3.5 3.5 Total 100.4 100.0 99.0 99.0 99.6 100.3

No. of analyses 4 11 9 3 15 10

No. of ions on basis of 24 (O, OH) Si 5.62 5.64 5.66 5.70 5.67 5.67 A1 2.38 2.36 2.34 2.30 2.33 2.33 8.00 8.00 8.00 8.00 8.00 8.00

A1 0.53 0.56 0.58 0.59 0.57 0.57 Ti 0.34 0.28 0.31 0.26 0.25 0.27 +2 Fe ) 2.92 FE+3 J 3.24 3.24 3.18 2.86 2.83 Mg 1.45 1.53 1.50 1.93 1.97 1.91 Mn 0.05 0.05 0.05 0.05 0.04 0.04 5.61 5.66 5.62 5.69 5.66 5.71

K 1.89 1.88 1.86 1.83 1.84 1.81 OH 3.56 3.56 3.56 3.56 3.56 3.56

Note: Tr. = trace. *• Calculated using Fe+2/Fe+3, OH/total O of biotite from Hartley Augen Gneiss (see Appendix 2).

slight variations in their compositions between the zones may or may not be real, because they are within the limits of statistical variations for the probe analyses (see Table 2 for the biotite compo- 3 sitions; see Table 6 for the other mafi: compositions). The com- Paleosome mon accessory minerals are sphene, epidote (in all except the muscovite-bearing gneisses), allanite (as the core of epidote), apa- Figure 3. Biotitic tite, magnetite, and pyrrhotite. Most pyrrhotite (and all of that in layered migmatite, PC1- Neosome £ contact with biotite) is rimmed by magnetite. Less common acces- 10, at Piney Creek. Rock consists of three zones: sories are muscovite, ilmenite (invariably rimmed by sphene), cal- lepidoblastic paleosome cite, and traces of zircon, rutile, graphite, and fluorite. Very fine, of fairly homogeneous scattered needles of what seems to be sillimanite occur in some of biotite gneiss; biotite-rich Selvages: the feldspars. selvage; and granoblastic, felsic neosome. CLOSED-SYSTEM MIGMATITES

Biotitic Layered Migmatites 3 cm Mafic selvages commonly border the neosomes in the biotitic graphic mixing code of Bryan and others (1969; see also Appendix layered migmatites at Piney Creek. Planimetric analyses and modal 2) show excellent agreement with the observed paleosome compo- compositions of two biotitic layered migmatites with particularly sitions (Table 5A). The relative proportions of neosome to selvage, well-developed and clearly defined selvages (Fig. 3; Table 3) yield estimated by planimetric analyses and the petrographic mixing net compositions, neosome plus selvage, that closely approximate code, agree within 10% (Table 5B). These results indicate that the the paleosome compositions (Table 4: see also Appendix 2 for neosome and selvage formed from the paleosome by simple segre- method). The paleosome compositions estimated by the petro- gation, without addition or subtraction of material except possibly volatiles. (The standard deviations of the coefficients for the vola- 3 Table 6, GSA supplementary material 77-4, maybe ordered from Documents Sec- tiles in Table 5B are too large for these coefficients to be mean- retary, Geological Society of America 3300 Penrose Place, Boulder, Colorado 80301. ingful.) Therefore, these neosomes must have formed by either

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TABLE 3. MODAL COMPOSITIONS OF BIOTITIC LAYERED MIGMATITES WITH SELVAGES, PINEY CREEK

PC1-10 PC1-12 Neosome Selvage Paleosome Neosome Selvage Paleosome

Volume percent Quartz 34.9 25.7 32.2 32.4 11.2 22.1 Microcline 29.7 15.6 26.2 23.1 3.6 12.6 Plagi oclase 33.0 27.0 28.9 41.5 45.1 41.4 Biotite (+ chlorite) 1.9 28.3 11.0 2.5 33.1 19.8 Sphene .. 1.1 0.5 0.2 3.3 1.4 Epidote 0.1 0.9 0.6 0.2 1.2 1.3 Allanite Tr. 0.6 0.2 0.1 0.4 0.5 Apatite Tr. 0.5 0.3 0.1 2.0 0.5 Magnetite 0.1 Tr. Tr. Tr. Tr. 0.1 Pyrrhotite 0.2 Tr. 0.1 Tr. Tr. 0.2 Calcite __ 0.1 Tr. Tr. Total 99.9 99.8 100.0 100.1 99.9 99.9

Total points counted 5,365 3,762 3,278 2,711 2,953 1,780 Total area (mm2) 1,361 940.5 819.5 678 738 890

Plagi oclase An19 An20 An,8_2, An20 An20 An195 composition Or, Or, Or, Or, Or, Or, Microcline

compsotion Or925 Or95 Or935

Note: Tr. = trace.

metamorphic differentiation or anatexis, and not by injection or TABLE 4. NET COMPOSITIONS IN BIOTITIC LAYERED metasomatism. These migmatites contain evidence for both sub- MIGMATITES WITH SELVAGES, PINEY CREEK, solidus metamorphic differentiation and anatexis. COMPARED TO PALEOSOME COMPOSITIONS

Evidence for Metamorphic Differentiation PC1-10 PC1-12 Neosome Neosome 3. Metamorphic differentiation may occur by either mechanical or + selvage* Paleosome + selvage Paleosome chemical differentiation. Mechanical differentiation seems unlikely Quartz 31.7 32.2 20.9 22.1 for the Piney Creek migmatites because the development of the Microcline 24.4 26.2 12.6 12.6 neosomes involved the segregation not only of felsic and mafic Plagioclase 30.7 28.9 43.5 41.4 minerals but also of microcline from plagioclase. Because the two Biotite 11.9 11.0 19.1 19.8 feldspars are structurally similar, it is unlikely that their mechanical Sphene 0.4 0.5 1.9 1.4 behavior would differ enough to permit this separation. Epidote 0.4 0.6 0.7 1.3 Chemical differentiation takes place by diffusion of constituents, Allanite 0.3 0.2 0.3 0.5 probably as ions or associated species through an intergranular Apatite 0.2 0.3 1.1 0.5 phase (see Carmichael, 1969; Eugster, 1970; Fisher, 1970a). The Pyrrhotite- magn etite 0.2 0.1 0.3 chemical potential gradients driving the diffusion may be created, Total (vol. %) 100.2 100.0 100.1 99.9 for instance, by the difference in mineral assemblages (Fisher, 1970a), a pressure gradient (see, for example, Elliott, 1973), or a * Calculated using volume proportions, obtained from planimetrie temperature gradient (see, for example, Katchalsky and Curran, analysis, as follows: PC1-10: neosome = 70.5%, selvage = 29.5%, 1967, chap. 13). The largest chemical variations for the zones of PC1-12: neosome = 45.7%, selvage = 54.3%. the two closed-system migmatites, PC1-10 and PC1-12, are in the amounts of Si02, FeO, MgO, and CaO (Fig. 4). The main modal differences are in their biotite, microcline, and quartz contents (Ta- alumina stays fairly constant in the three zones of these migmatites ble 3). The following is the model for a subsolidus metamorphic (Fig. 4). The annite-phlogopite solid solution and pure microcline differentiation process that can account for the observed variations are used above in order to simplify this part of the discussion. The in Fe-Mg and biotite-microcline contents. The variations in Si02, actual reactions in these rocks must have been much more complex CaO, and quartz are discussed below. and will be discussed below. The ionic equilibrium between biotite and microcline, control- Equilibrium 1 is a combination of two equilibria such as

ling the activities of Fe and Mg, may be written as + ++ annite + 6H = microcline + 3Fe + 4HaO (2) + ++ ++ biotite + 6H = microcline + 3(Fe , Mg ) + 4H20. (1) and Equilibrium 1 is written conserving alumina in the solid because phlogopite + 3Fe++ = annite + 3Mg++, (3)

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a ksp ' (flKe-+)3 • (ÎhmY f o- If some independent factor could lower f or raise a ~ selec- (K ) : Ht Hl0 H 2 ; tively in one part of the rock, the biotite there would break down to aS ' K+)6 microcline, releasing Fe and Mg ions. The resultant increase in ac- a SiS. ' (a ++ )3 (K»), Hlî tivities of Fe and Mg forces both ions to diffuse outward. This zone 3 + a £¡8 ' (flr,++) of low /„jo or high aH becomes enriched in microcline and de- Therefore, activities of Fe and Mg must increase with decreasing pleted in biotite to form a neosome. Increased activities of Fe and

/nao at constant aH+ and decrease with increasing an+ at constant Mg in the adjacent paleosome would cause biotite to replace mi-

TABLE 5A. PALEOSOME COMPOSITIONS ESTIMATED BY TABLE 5B. RELATIVE PROPORTIONS OF ZONES IN PETROGRAPHIC MIXING CODE FOR CLOSED-SYSTEM CLOSED-SYSTEM MIGMATITES ESTIMATED BY MIGMATITES, PINEY CREEK PETROGRAPHIC MIXING CODE

PC1-10 PC1-12 Coefficient Standard Density Vol % Y estimated Y observed Y estimated Y observed (in weight) deviation (g/cm3) Estimated Observed*

Si02 71.2 71.2 63.7 63.7 PC1-10 13.5 13.4 15.6 15.4 AI2O3 Neosome 0.660 0.013 2.64 67.4 70.5 Ti0 0.5 0.6 1.3 1.1 2 Selvage 0.337 0.015 2.79 32.6 29.6 FeO 2.8 2.8 4.2 4.7 H2O 0.000 0.001 Fe O 0.5 0.6 0.9 1.1 2 a S -0.001 0.001 MgO 0.8 0.8 1.9 1.9 2 p o 0.001 0.001 MnO 0.1 0.1 0.1 0.1 2 5 Paleosome -1.000 2.69 CaO 1.7 1.8 3.4 2.9 Na 0 3.1 2.9 4.1 3.8 2 PC1-12 K2O 5.0 5.2 3.9 4.0 H2O 0.5 0.5 0.8 0.8 Neosome 0.438 0.020 2.65 45.4 45.8 0.567 0.027 2.85 54.6 54.3 s2 0.1 0.1 0.2 0.2 Selvage 0.000 0.003 P2O5 0.2 0.2 0.2 0.2 H2O Total 100.0 100.2 100.3 99.9 S2 0.002 0.003 p o -0.003 0.003 Sum of squares 2 5 Paleosome -1.000 2.77 of residuals 0.116 0.672 * Obtained using planimetrie analyses (see App. 2). Note: Values in weight percent.

Figure 4. Variation in major oxides for zones of the two closed-system migmatites. Length of horizontal axis between zones is inversely pro- portional to their observed volume percentages (see Table 4).

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crocline because of equilibrium 1, forming a mafic selvage along There is a volume decrease of about 20% as reaction 4 proceeds to the neosome. the right. Reaction 5 is written assuming that Si02 has migrated The modal amounts of biotite and microcline in the zones of a from the selvage (where a volume increase occurs) to the neosome migmatite formed in this manner are related by (where a volume decrease occurs). This migration would enable the reaction to proceed at constant volume. Such segregation of quartz M Vm «MC = w°Mc + " ( m°B1 - mB1 ) , into pressure shadows is well known and is thought to be driven by »«V, \ ) the chemical potential gradient created by the pressure gradient (see, for example, Elliott, 1973). The common occurrence of a thin in which mMc and mBi are the final amounts of modal microcline quartz vein between the neosome and selvage in these rocks could and biotite, w°Mc and are the initial amounts of modal micro- indicate such migration: cline and biotite, n M is the number of moles of microcline formed from wB moles of biotite, and VM and VB are the molar volumes of 1% biotite + 0.228% quartz^ 1.024% K-feldspar (Or65Ab35) microcline and biotite. The volume of the rock is assumed to stay 0.716% microcline (Or93Ab7) + 0.284% plagioclase (Ab100). (6) constant. In a modal biotite-microcline plot, this relationship pro- duces parallel tie lines between the neosome and selvage through The potassium feldspar composition (Or65Ab35) has been taken different paleosome compositions (Fig. 5). In general, because there from Barth's curve (Deer and others, 1963, vol. 4, Fig. 23) and is will be a volume change in forming the neosome, or selvage, from approximately that in equilibrium with a plagioclase of composi- the paleosome, the calculated mode of the neosome, or selvage, will tion An20 (see Table 3) at about 670 °C, the minimum temperature have to be normalized to 100%. This normalization causes the tie estimated for the migmatization in these rocks. Reaction 6 is a lines to deviate slightly from the parallel orientation. two-step reaction in which the present microcline (Or93Ab7) and Figure 5 shows the tie lines predicted by the model above, and albite are assumed to have exsolved from such feldspar (Or65Ab35). Figure 6 shows the observed tie lines for the Piney Creek rocks. The Albite would be incorporated into the existing plagioclase: slope of the predicted tie lines depends greatly on the mineral com- 1% biotite 1.024% K-feldspar (Or65 Ab35) positions used and other factors. This is demonstrated by the four -> 0.716% microcline (Or93 Ab,) + 0.284% plagioclase (Ab10„). (7) sets of tie lines in Figure 5, representing four different reactions. All four constructions use the same biotite formula, with 2.90 A1 and Reaction 7 is reaction 6 at constant volume, assuming migration of 5.66 Si for 24 (O, OH), approximating the biotite compositions in quartz from the selvage to the neosome (see reaction 5). these rocks (see Table 2). The slope of predicted tie lines would There is excellent agreement between the slopes of observed tie change with the biotite composition. All components except lines and predicted tie lines in Figure 5,b'. More important, how- alumina and silica are assumed to have diffused freely between the ever, there is a general parallelism of observed tie lines in both Fig- zones. ure 5 and Figure 6. These strongly suggest that the segregation of The four reactions for the tie lines in Figure 5 written in volume biotite and microcline occurred as outlined above: by the coupled percentage are biotite-microcline reactions in the neosome and selvage with diffusion of components between the zones. If the biotite was concen- 1% biotite + 0.229% quartz ^ 1.024% microcline (Or93Ab7). (4) trated in the selvage by simple subtraction of an anatectic melt, the The microcline composition is the present composition of micro- tie lines in Figures 5 and 6 should converge toward a very narrow cline in these rocks: range of microcline content (expected for such melts) at zero biotite

1% biotite + 0.024% quartz ^ 1.024% microcline (Or93Ab7). (5) content, and the neosome should have a smaller range and the sel-

Figure 5. Modal biotite-microcline plot for biotitic layered migmatites. Migmatites formed by metamorphic differentiation, with reactions biotite —> microcline in neosomes and microcline —» biotite in selvage, must have paleosome, neosome, and selvage lying on same predicted tie line (solid line). Tie lines in parts a, a', b, and b' have been constructed using four different biotite-microcline reactions (reactions 4, 5, 6 and 7; see text). Predicted tie lines in part a, for instance, show that a paleosome with 40% modal biotite and no microcline will become a neosome with 44% modal microcline and no biotite if all its biotite breaks down to microcline by reaction 4. Dashed lines are observed tie lines between selvages (solid squares), paleosomes (solid circles), and neosomes (open triangles) of the two closed-system migmatites.

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vage a larger range in microcline content than the paleosome. aqueous solution decreases with decreasing activity of water. On However, the biotite-microcline reactions alone will not result in the other hand, they found that the activity of hydrogen had little

the observed neosome and selvage compositions. This is best illus- effect. The activity of Si02 in quartz would remain as one through- trated by using the granite ternary plo :s as below. out the rock, but the activity of Si02 in the intergranular fluid For the closed-system migmatites, PC1-10 and PC1-12, the tie would be higher in the selvage than in the neosome. Such a gradient

lines in the granite ternary can be constructed (Fig. 7). The pre- could have led to a migration of Si02 through the fluid phase from dicted compositions of the neosomes arid selvages have been calcu- the selvage to the neosome. Although this mechanism does explain lated using reactions 4, 5, 6, and 7 (except for using the biotite the migration of quartz, the closeness of both neosome compo- compositions of each zone, which are very slightly different from sitions to the granite minimum seems to be more than a coinci- that used for these reactions). The calculations assume that (1) the dence. It suggests that anatexis may have been involved in the for- observed modal biotite in the paleosome minus that in the neosome mation of these neosomes. has broken down to microcline, and (2/ the observed biotite in the selvage minus that in the paleosome has formed by replacing mi- Evidence for Anatexis crocline in the paleosome. The observed and predicted compositions do not agree even for reaction 7. However, they would agree The pressure and temperature conditions estimated for the mig- very well if there had been a migration of quartz from the selvage to matization of Baltimore Gneiss indicate a possibility of anatexis in the neosome in addition to that assumed for reaction 7. Figure 4 these rocks (see Geologic Setting of Baltimore Gneiss). Two fea- indicates that there has been such migration. tures suggest that anatexis did occur in the two closed-system mig- The additional migration of quartz could have occurred because matites and most or all of the Piney Creek migmatites. The first is a

the fHzo gradient, higher in the selvage than in the neosome, created correlation between the paleosome composition and the degree of a corresponding gradient in the chemical potential of silica. Ander- migmatization. The paleosomes in the layered migmatites at Piney son and Burnham (1967) showed that the solubility of quartz in an Creek plot closer to the granite minimum than those in the veined gneisses (Fig. 8). In other words, the closer the paleosome compositions are to the granite minimum, the more voluminous the neosomes become, or the rock is more thoroughly migmatized. Of the two closed-system migmatites, PC1-10 has a paleosome composi- 50- < tion closer to the minimum (Fig. 7) and a larger neosome/selvage ratio than PC1-12 (Table 4, footnote). It is not too likely that the relationship can result from a subsolidus diffusion. Because the dif- Figure 6. Observed biotite-microcline tie lines fusion coefficients are strongly temperature dependent (Jost, 1960, 40- between nonpegmatitic p. 135), the efficiency of any diffusion mechanism should be similar neosome and its mafic for the rocks at the same temperature except in the cases of very host rock (solid triangles), different rock types. On the other hand, the relationship must fol- which may be either sel- low if the migmatites are anatectic in origin, because the tempera- vage (solid squares) or ture of anatexis and the amount of melt produced are strongly de- paleosome (solid circles), pendent on the rock composition. in layered and veined migmatites at Piney Creek containing biotite as only Qz major dark phase. Note general parallelism of tie lines. See caption of Fig- ure 5 for explanation.

10 20 30 X Modal Microcline >

Figure 7. Tie lines in granite ternary for the two closed-system migmatites. Dashes = observed tie lines between selvage (S), paleosome (P), and neosome (N). Light solid lines connect paleosome with calculated selvage (solid squares) and neosome (open triangles) compositions predicted by metamorphic differentiation model; a, a', b, and b' correspond to reactions 4, 5, 6, and 7 used in calculating compositions (see caption of Fig. 5 and text). Heavy solid line, marked 1 kb and 5 kb, is granite minimum from Luth and others (1964). All data obtained in this study are plotted in granite ternary as modal percent, although granite minimum is in weight percent. This is adequate because maximum difference between volume and weight percentages in these plots is about 1% and should be hardly notice- able.

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Qz maximum difference in the composition of the feldspars between the zones in the Piney Creek migmatites is about 2% albite content (Table 3), the same order of magnitude as the error in the analytical 50/™ methods (see Appendix 2). The difference, even if real, is far smaller than predicted by the experimental data of Yoder and others (1957). However, the feldspar could have re-equilibrated by Ab8°Mc diffusion between the zones after crystallization of the melt. Any zoning in plagioclase found in these rocks is very slight and diffuse, the compositional range being 2% to 4% An content, and therefore does not rule out an extensive equilibration. The CaO gradient be- Figure 8. Relationship between paleosome composition and degree of tween the three zones (Fig. 4) may simply indicate that Ca was less migmatization in Piney Creek migmatites. Note that paleosomes of more mobile than Na. extensively migmatized layered migmatites (bounded by dotted line) are closer to granite minimum (heavy solid line) from Luth and others (1964) Metamorphic Differentiation Induced by Anatexis than paleosomes of less extensively migmatized veined gneisses (bounded by light solid line). For details of plotting in granite ternary, see caption of The evidence given above for both subsolidus metamorphic dif- Figure 7. ferentiation and anatexis in the two closed-system migmatites suggests that these migmatites formed by a combination of the two The second indication of anatexis in the Piney Creek rocks is that processes. As discussed earlier, the metamorphic differentiation by their neosomes plot closer to the granite minimum than the paleo- coupled biotite-microcline reactions can be initiated by a decrease somes do (Fig. 9, b). The neosome compositions, however, show a •n /HJO 'n the neosome. One possible cause for such decrease is scatter toward microcline from the granite minimum. Most of the anatexis. A model of migmatization by metamorphic differentia- shift is not likely to be due to the Ca content in these rocks. Von tion induced by anatexis is shown in Figure 10. Platen's (1965) study at 2 kb and James and Hamilton's (1969) It is known that the initial melts formed in deep-seated rocks study at 1 kb PHM show that as the CaO content of a rock in- must contain a large amount of water (Luth and others, 1964; creases, the initial melt composition shifts from the granite mini- Burnham, 1967). Therefore, as such melt forms, it must absorb all mum toward the Or-Q sideline. The Piney Creek rocks contain 2% of the available water in the intergranular fluid in its vicinity.4 As- to 6% CaO. However, James and Hamilton concluded that the ini- suming that the initial melts form at scattered sites in a rock (see tial melts in Ca-bearing rocks probably shift toward the Ab corner below for discussion of this assumption), this causes activity of with increasing PHz0, as in the case of the Ca-free system (see also, water in the immediate vicinity of the melt to fall below that Winkler and others, 1975). In such a case the shift toward the elsewhere. The decrease in fni0 initiates the biotite breakdown reac- K-rich liquid at a high PHl0 should appear to be very slight. Possible tion, forming a neosome. The water released by the reaction com- projected trends of initial melts in Ca-bearing rocks are shown in bines with quartz, albite, and microcline to form still more melt. Figure 9, a as closely paralleling the granite minimum. A more The Fe and Mg released by the same reaction diffuse into the adja- likely explanation for the scatter of the neosome compositions in Figure 9, b is given below. 4 A H20-C02 mixture, rather than pure HaO, is considered to be the more likely If the neosome in a migmatite has been composed of melt, it metamorphic fluid (see, for example Burnham, 1967). The discussion to follow should should have contained feldspars more sodic than those of the apply equally well to the case of anatexis in the presence of such fluid, because H20 is paleosome and selvage at the time of its crystallization. The strongly partitioned into the melt (see, for example, Mysen and others, 1975).

>°

Figure 9. a. Granite minimum trend (solid line) of Luth and others (1964) and possible projected trends (dashed lines) of granitic initial melts with Ab/An ratios of 10.4 and 4.0. Crosses with 10.4 or 4.0 are 1-kb compositions for these melts taken from James and Hamilton (1969). b. tt Modal compositions of Piney Creek migmatites. Note that neosomes (open triangles) plot closer • 1 * to granite minimum than paleosomes (solid circles; question marks by circles indicate large A uncertainty). For details of plotting in granite .¿?kb A A ternary, see caption of Figure 7. A

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cent paleosome, where biotite replaces inicrocline and a selvage is gradient across the foliation plane, and this may be another reason formed. The water needed for the selvage reaction is brought in why the neosomes and selvages are developed parallel to the folia- either from farther out in the paleosome or from some source out- tion. side the migmatite. As long as melting continues in the neosome, The external source of water necessary for this metamorphic some gradient in fHs0 would be maintained between the neosome differentiation—anatexis mechanism is up for speculation. There is and the paleosome, and this gradient continues driving the no clear-cut evidence for a dehydration reaction in the Piney Creek metamorphic differentiation. This mechanism will create the paral- migmatites. A possible source of water outside the migmatites is in- lel tie lines of Figures 5 and 6. At the same time, it should contrib- terstitial or structural water of the minerals in the overlying ute to the scatter of neosome compositions to the microcline-rich Glenarm sedimentary rocks. The isograd reactions in the Glenarm side of the granite minimum as observed in these rocks (Fig. 9, b): if Series have not been worked out, but they are probably dehydra- the neosomes have been minimum melt, microcline forming from tion reactions. The dehydration due to migmatization in the domes biotite would modify their minimum composition to those inter- may have caused the isograds to closely parallel the outlines of the mediate between the minimum and the microcline apex. domes (Fig. 1; see also Fisher, 1970b, p. 313). This model is based on at least two assumptions: (1) initial melting starts at scattered sites in a rock, and further melting occurs OTHER MIGMATITES AT PINEY CREEK either at these sites or even at only some of these sites; and (2) the rate of diffusion of water from the external source is slower than The results of the tests for the closed system applied to the re- the rate of formation of melt. The first assumption seems valid be- mainder of the layered migmatites at Piney Creek have not been cause it is unlikely that a rock is so homogeneous that initial melt- conclusive. One migmatite may have formed in an open system (see ing would occur simultaneously throughout a rock. It may be also Olsen, 1972). The test has failed mainly because these rocks lack that there is some structural control over the sites for the first melts either well-developed selvages or host rocks that can be identified to form, because most neosomes are developed parallel to the folia- with confidence as the unaltered paleosome. However, these rocks tion. Perhaps a slight shearing movement favors melting along the probably also formed by metamorphic differentiation plus anatexis shear planes. The mechanism as postulated is a dynamic and dis- (excepting the one that may have formed in an open system), be- equilibrium process about which almost nothing is known for the cause they contain some of the evidence for both mechanisms deep-seated environment. However, once scattered melts form, it found in the closed-system migmatites. The evidence, briefly sum- seems likely that the chemical and mechanical inhomogeneities marized, is (1) the parallel tie lines in the biotite-microcline plot created would become concentrated into localized zones rather (Fig. 6), (2) the positive correlation between the degree of migmati- than distributed uniformly throughout a rock. This may be likened zation and the closeness of the paleosome composition to the gran- to the formation of faults or cleavages. The controlling factor ite minimum (Fig. 8), and (3) the near-granite minimum composi- should be the scarcity of water, which restricts melting only to the tion of the neosomes (Fig. 9, b). Furthermore, these rocks are simi- favored sites. The second assumption concerning the diffusion rate lar to the closed-system migmatites in migmatitic structure and tex- seems valid also. Again, the scarcity of water makes it likely that ture. The patterns of major-element variation in the zones are also the diffusion of water from the external source must be almost in- similar: depletion of CaO, FeO, and MgO and enrichment of Na20 stantaneous to prevent a fHlD gradient from developing. Such a fast and Si02 in the neosome compared to the paleosome, and the re- rate seems unlikely, especially for the diffusion across the foliation verse for the selvage (see Fig. 4). plane, although the diffusion along the foliation plane may be very The neosomes with pegmatitic textures in the layered migmatites fast. The nonequal rates of diffusion would create a much steeper at Piney Creek show a much larger shift toward a microcline-rich composition than the normal, nonpegmatitic neosomes (Fig. 11). The slope of their tie lines in the modal biotite-microcline plot is smaller than that for more granitic neosomes. These indicate an abnormal enrichment of these neosomes in microcline, possibly by the introduction of potassium from external sources. A pegmatitic texture is commonly interpreted as an indication of the presence of H2O Fe+1\Mg+* HjO

Figure 10. Migmatization by metamorphic differentiation induced by anatexis: /H,O gradient has been set up between paleosome and neosome when initial melt formed in what was to become neosome. Biotite in neo- ++ ++ some breaks down to microcline, releasing H20, Fe , and Mg (see equilibrium 1 in text). H20 combines with albite, microcline, and quartz to form more melt. Fe++ and Mg++ diffuse out to selvage to form biotite from Modal Microcliim + microcline. HaO for this reaction is brought in from "outside." H is re- Figure 11. Modal biotite-microcline plot for observed tie lines (dashes) leased and diffuses into neosome, where it i:> consumed in biotite break- between microcline-rich pegmatitic neosomes (ooen trianeles) and their down reaction. As long as melt is present in neosome,/H2O gradient is main- paleosomes (solid triangles). Note that they have smaller slope than tie lines tained and reactions proceed, causing neosome to replace selvage and sel- for granitic neosomes (solid, taken from Fig. 6). See also caption of vage to replace paleosome. It is not inferred here that/„20 gradient is linear. Figure 5.

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abundant fluid (see, for example, Jahns and Burnham, 1969). The fully acknowledged. H. P. Eugster, C. A. Hopson, G. W. Fisher, extensive recrystallization of these microcline-rich neosomes is in- and D. A. Hewitt critically read earlier versions of the manuscript dicated by (see also Table 1) (1) their boundaries, which tend to be and made numerous helpful suggestions. In particular, Figure 10 is more diffuse and irregular than those of the granitic neosomes, (2) after a suggestion by H. P. Eugster. L. Finger and D. Elliott also abundance of secondary, clayey material on the feldspars, and (3) gave some helpful advice. I thank B. O. Mysen for his patient abundant albitic rims (Tuttle, 1952) and myrmekites (Shelly, 1964) coaching with the probe analyses, and the Geophysical Laboratory on the plagioclase. If there had been any potassium metasomatism for use of the microprobe. W. B. Bryan kindly made available the in these rocks, the presence of abundant fluid along certain neo- petrographic mixing code. somes would have made them preferred passages for the introduction of potassium, leading to potassium enrichment of these neo- APPENDIX 1. NOMENCLATURE FOR MIGMATITES somes. Unlike the layered migmatites, the veined gneisses at Piney Creek Layered migmatite: a rock in which "the neosomes form light and dark contain neosomes with great variations in composition and texture layers in the paleosome generally parallel to the plane of schistosity ... As a that cannot be explained at present by the metamorphic rule, the neosomatic layers are not entirely even, but thicken and thin out irregularly, or they may be folded and even contorted" (Mehnert, 1968, p. differentiation—anatexis mechanism. Further investigations are 18) needed to determine the origin of these veins. Migmatite: "megascopically composite rock consisting of two or more petrographically different parts, one is the country rock in a more or less DISCUSSION metamorphic stage, the other is of pegmatitic, aplitic, granitic, or generally plutonic appearance" (Mehnert, 1968, p. 355) This study of the Baltimore Gneiss migmatites has demonstrated Neosome: "newly formed part of a migmatite; cf. paleosome" (Mehnert, that at least two of these migmatites formed in locally closed sys- 1968, p. 356) tems and that most of these migmatites have formed by segregation Paleosome: "parent rock of a migmatite; cf. neosome" (Mehnert, 1968, of the constituents by diffusion, which could have been initiated p. 356) and driven by either an or an a gradient. The model of mig- Veined gneiss: a rock in which "the paleosome is irregularly traversed by H vein-like neosomes, so that the resulting structure has the rough appearance matization presented here postulates that the cause of this segrega- of the vein system of a human body" (Mehnert, 1968, p. 17) tion has been an fH^ gradient created by anatexis. The model is necessarily simplified, most of all because so little is known about the processes occurring in the deep-seated environments. Admit- APPENDIX 2. METHODS OF STUDY tedly, also, the model is based on the study of a small number of samples from one locality. However, the Piney Creek migmatites Test for a Closed System are similar, both texturally and chemically, to the other migmatites in the Baltimore Gneiss and to many described in the literature. Two methods are used to calculate mass balance within a migmatite. In Many regional migmatites are observed to have the following gen- the first, relative proportions of neosome to selvage and the compositions of eral characteristics: (1) they occur in the geologic settings where these zones are used to calculate the net neosome plus selvage composition. anatexis is expected, based on the results of experimental petrol- The relative proportions are obtained by measuring the areas of these zones ogy; (2) in spite of this, the neosomes in these migmatites do not in by planimetric analyses on slabs and thin sections. The second method uses general have the granite minimum compositions, although they the least-squares analysis code of Bryan and others (1969) for petrographic tend to be closer to the minimum than the paleosomes; and (3) the mixing. A linear equation is written in matrix form, XB = Y, in which X is neosomes have metamorphic textures. The Baltimore Gneiss is a a matrix of independent vectors, x, to x„, B is a vector of regression coefficients for the xs and Y is the dependent vector. The code calculates typical case of such regional migmatites. Hopson (1964, p. 38—41) those numerical values of B that reduce the sum of squares of difference concluded that the Baltimore Gneiss migmatites were formed by between observed Y and estimated Y to the minimum. The compositions of metasomatism or by the solid-state transfer of the potassium neosome, selvage, and volatiles have been used as x, to x„ and paleosome feldspar component from the paleosome into the neosome, and he composition as Y. It is concluded that the rock has been closed except to ruled out anatexis. His main chemical evidence against anatexis is volatiles if (1) the analysis gives the result neosome + selvage (± vapor) = the neosome compositions, which are too potassic to be minimum paleosome (± vapor); (2) the volume fractions of the zones calculated from melt. The model presented here predicts that the addition of micro- the values of B agree closely with the estimates from planimetric analyses or cline formed by the breakdown of biotite could modify a granite visual inspection; and (3) the sum of squares of deviations is small. minimum neosome to one intermediate between the minimum and the microcline apex. Many of Hopson's neosomes plot in that area Bulk Compositions (Hopson, 1964, Fig. 16). He also cited much textural evidence, especially that of microcline for migmatization in the solid state. A FORTRAN program (see Olsen, 1972) has calculated all chemical The mechanism postulated in this study does not conflict with his compositions from the modal data and mineral compositions. The mineral compositions have been obtained as follows: plagioclase by the refractive evidence if only part of the neosome was molten and some, if not index on cleavage flake (Morse, 1968), using temperature-controlled stage most, of the microcline was formed by the breakdown of biotite. and refractometer, and by U-stage (Slemmons, 1962); and albite content of The conclusions reached in this study, therefore, may be applicable microcline by Orville's A28 method for the homogenized samples, using his to many other regional migmatites similar to those in the Baltimore curve for microcline (1967). The compositions of biotite, hornblende, epi- Gneiss and should be tested on a more regional scale. dote, and sphene were determined by electron microprobe, with an auto- mated system (Finger and Hadidiacos, 1971), using the Bence-Albee correction (Bence and Albee, 1968; Albee and Ray, 1970). The Fe 0 and H O ACKNOWLEDGMENTS 2 :) z contents in biotite have been estimated using the Fe203/Fe0 and H20/total O ratios in the chemical analysis of the biotite from the Hartley Augen Gneiss This study is based on part of a Ph.D. dissertation submitted to (Olsen, 1972). The augen gneiss is a phase of the Baltimore Gneiss in the Tow- Johns Hopkins University. The helpful direction of G. W. Fisher, son dome and has the same mineral assemblage as the Piney Creek migma- support from the university, and a fund from the Maryland tites (see Fig. 1). The feldspar compositions were also checked by the micro- Geological Survey during the work for the dissertation are grate- probe, and they agreed with those determined by the other methods (see

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above) within 1% to 2% albite content. Thin sections for the modal tains, N.Y.: Geol. Soc. America Bull., v. 69, p. 1369-1413. analyses have been stained for potassium feldspar (Chayes, 1952). Thick- Eugster, H. P., 1970, Thermal and ionic equilibria among muscovite, ness of most neosomes and selvages in the Piriey Creek rocks are contained potash-feldspar and aluminosilicate assemblages: Fortschr. Min- within a standard thin section. Therefore, point-counting the thin section eralogie, v. 47, p. 106-123. should give a representative analysis of these zones. Analysis of one or two Finger, L. W., and Hadidiacos, C. G., 1971, Aspects of computer automa- thin sections each is considered to be representative of a paleosome in these tion of an electron microprobe: Carnegie Inst. Washington Year Book rocks because the paleosomes are fairly homogeneous. The traverse inter- 70, p. 269-275. vals, at 0.5 to 1.0 mm, are greater than the average grain size (about 0.35 Fisher, G. W., 1970a, The application of ionic equilibria to metamorphic mm), except in porphyroblastic and pegmatitic samples, which, however, differentiation: An example: Contr. Mineralogy and Petrology, v. 29, are rare. The analytical error estimated is 2% for a major mineral by p. 91-103. Chayes' method (1956, Fig. 12), and about 2.5% for a mineral with 30 vol 1970b, The metamorphosed sedimentary rocks along the Potomac % by van der Plas and Tobi's table (1965). The only real uncertainty in River near Washington, D. C., in Fisher, G. W., Pettijohn, F. J., Reed, applying the results of Chayes and of van der Plas and Tobi is the effect of J. C., and Weaver, K. N., eds., Studies of Appalachian geology: Cen- inhomogeneity in grain size in some of these rocks. The error resulting from tral and southern: New York, Interscience Pubs., p. 299-316. this source cannot be estimated but should not be very large. The error in Goodspeed, G. E., 1948, Xenoliths and skialiths: Am. Jour. Sci., v. 246, the calculated chemical composition is estimated to be on the order of 2 wt p. 515-525. % for Si02 and 1 wt % for other major oxides (compare Engel and Engel, Härme, M., 1959, Examples of the granitization of gneisses: Finlande 1958, p. 1380-1382). Comm. Geol. Bull., v. 184, p. 41-58. Hedge, C. E., 1972, Source of leucosomes of migmatites in the Front Range, REFERENCES CITED Colorado: Geol. Soc. America Mem. 135, p. 65-72. Holdway, M. J., 1971, Stability of andalusite and the aluminum silicate Albee, A. L., and Ray, L., 1970, Correction factors for electron probe mi- phase diagram: Am. Jour. Sci., v. 271, p. 97—131. croanalysis of silicates, oxides, carbonates, phosphates, and sulfates: Holmquist. P. J., 1920, Om pegmatitpalingenes och ptygmatisk veckning: Anal. Chemistry, v. 42, p. 1408-1414. Geol. Foren. Stockholm Förh., v. 42, p. 191-213. Anderson, G. M., and Burnham, C. W., 1967, Reactions of quartz and Hopson, C. A., 1964, The crystalline rocks of Howard and Montgomery corundum with aqueous chloride and hydroxide solutions at high Counties, in Geology of Howard and Montgomery Counties: Mary- temperatures and pressures: Am. Jour. Sci., v. 265, p. 12—27. land Geol. Survey, p. 27-215. Belliere, J., 1960, Signification des structures de corrosion dans les roches Jahns, R. H., and Burnham, C. W., 1969, Experimental studies of pegma- migmatitiques: Internat. Geol. Cong., 21st, Copenhagen 1960, rept. tite genesis: I. A model for the derivation and crystallization of granitic session, Norden, pt. 14, p. 30—36. pegmatites: Econ. Geology, v. 64, p. 843-864. Bence, A. E., and Albee, A. L., 1968, Empirical correction factors for the James. R. S., and Hamilton, D. L., 1969, Phase relations in the system

electron microanalysis of silicates and oxides: Jour. Geology, v. 76, p. NaAlSi30g-KAlSi308-CaAl2Si208-Si02 at 1 kilobar water vapor pres- 382-403. sure: Contr. Mineralogy and Petrology, v. 21, p. 111-141. Berthelsen, A., 1960, An example of structural approach to the migmatite Jost, W., 1960, Diffusion in solids, liquids, and gases: New York, Academic problem: Internat. Geol. Cong., 21st, Copenhagen 1960, rept. session, Press, 652 p. Norden, pt. 14, p. 149-157. Katchalsky, A., and Curran, P. F., 1967, Nonequilibrium thermodynamics Brown, P. E., 1967, Major element composition of the Loch Coire migma- in biophysics: Cambridge, Mass., Harvard Univ. Press, 248 p. tite complex, Sutherland, Scotland: Contr. Mineralogy and Petrology, King, B. C., 1965, The nature and origin of migmatites: Metasomatism or v. 14, p. 1-26. anatexis, in Pitcher, W. S., and Flinn, G. W., eds., Controls of Bryan, W. B., Finger, L. W., and Chayes, F., 1969, Estimating proportions metamorphism: New York, John Wiley 8c Sons, p. 219-234. in petrographic mixing equations by least squares approximation: Sci- Kretz, R., 1966, Metamorphic differentiation at Einasleigh, N. Queens- ence, v. 163, p. 926-927. land: Geol. Soc. Australia Jour., v. 13, p. 561-582. Burnham, C. W., 1967, Hydrothermal fluids at the magmatic stage, in Ljunggren, P., 1957, Banded gneisses from Gothenburg and their transfor- Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits: New mations: Geol. Foren. Stockholm Förh., v. 79, p. 113 — 132. York, Holt, Rinehart and Winston, p. 34—76. Loberg, B., 1963, The formation of a flecky gneiss and similar phenomena Carmichael, D. M., 1969, On the mechanism of prograde metamorphic in relation to the migmatite and vein gneiss problem: Geol. Foren. reactions in quartz-bearing pelitic rocks: Contr. Mineralogy and Pe- Stockholm Förh., v. 85, p. 3-109. trology, v. 20, p. 244-267. Lowman, P. D., 1965, Non-anatectic migmatites in Gilpin County, Col- Chayes, F., 1952, Notes on the staining of potash feldspar with sodium orado: Geol. Soc. America Bull., v. 76, p. 1061-1064. cobaltinitrite in thin section: Am. Mineralogist, v. 37, p. 337-340. Luth, W. C., Jahns, R. H., and Tuttle, O. F., 1964, The granite system at 1956, Petrographic modal analysis: New York, John Wiley &c Sons, pressures of 4 to 10 kb: Jour. Geophys. Research, v. 69, p. 759-773. 113 p. Mehnert, K. R., 1968, Migmatites and the origin of granitic rocks: New Cheng, Y. C., 1944, The migmatite area around Bettyhill, Sutherland: Geol. York, Elsevier, 393 p. Soc. London Quart. Jour., v. 99, p. 107-154. Misch, P., 1968, Plagioclase compositions and non-anatectic origin of Davis, G. L., Tilton, G. R., Aldrich, L. T., Hart, S. R., Steiger, R. G., and migmatitic gneisses in N. Cascade Mountains of Washington State: Hopson, C. A., 1965, The minimum age of the Glenarm Series, Balti- Contr. Mineralogy and Petrology, v. 17, p. 1-70. more, Maryland: Carnegie Inst. Washington Year Book 1964—65, p. Morse, S. A., 1968, Revised dispersion method for low plagioclase: Am. 174-177. Mineralogist, v. 53, p. 105-115. Deer, W. A., Howie, R. A., and Zussman, J., 1963, Rock-forming minerals, Mysen, B. O., Arculus, R. J., and Eggler, D., 1975, Solubility of carbon Vol. 4: New York, John Wiley & Sons, 435 p. dioxide in melts of andesite, tholeiite, and olivine nephelinite composi- Dietrich, R. V., 1960, Banded gneisses: Jour. Petrology, v. 1, p. 99-120. tion to 30 kbar pressure: Contr. Mineralogy and Petrology, v. 53, p. Dietrich, R. V., and Mehnert, K. R., 1961, Proposal for the nomenclature 227-239. of migmatites and associated rocks: Internat. Geol. Cong., 21st, Olsen, S. N., 1972, Petrology of the Baltimore Gneiss [Ph.D. dissert.]: Bal- Copenhagen 1960, Norden, pt. 26, sec. 1-21, suppl. vol., p. 56-67. timore, Johns Hopkins Univ., 295 p. Doe, B. R., Tilton, G. R., and Hopson, C. A., 1965, Lead isotopes in Olsen, S. N., and Fisher, G. W., 1974, Origin of metamorphic segregations feldspars from selected granitic rocks associated with regional in the Baltimore Gneiss: Geol. Soc. America Abs. with Programs, v. 6, metamorphism: Jour. Geophys. Research, v. 70, p. 1947-1968. p. 60. Elliott, D., 1973, Diffusion flow laws in metamorphic rocks: Geol. Soc. Orville, P. M., 1967, Unit-cell parameters of the microcline-low albite and America Bull., v. 84, p. 2645-2664. the sanidine—high albite solid solution series: Am. Mineralogist, v. 52, Engel, A.E.J., and Engel, C. G., 1958, Progressive metamorphism and p. 55-86. granitization of the major paragneiss, northwest Adirondack Moun- Ramberg, H., 1956, Pegmatites in West Greenland: Geol. Soc. America

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/8/1089/3433760/i0016-7606-88-8-1089.pdf by guest on 25 September 2021 BALTIMORE GNEISS MIGMATITES AT PINEY CREEK 1101

Bull., v. 67, p. 185-213. Tuttle, O. F., 1952, Origin of the contrasting mineralogy of extrusive and Richardson, S. W., Gilbert, M. C., and Bell, P. M., 1969, Experimental de- plutonic salic rocks: Jour. Geology, v. 60, p. 107-124. termination of kyanite-andalusite and andalusite-sillimanite equilib- van der Plas, L., and Tobi, A. C., 1965, A chart for judging the reliability of ria; the aluminum silicate triple point: Am. Jour. Sci., v. 267, p. point counting results: Am. Jour. Sci., v. 263, p. 87-90. 259-272. von Platen, H., 1965, Experimental anatexis and genesis of migmatites, in Sederholm, J. J., 1923, On migmatites and associated Precambrian rocks of Pitcher, W. S., and Flinn, G. W., eds., Controls of metamorphism: southwestern Finland: Pt. I: Finlande Comm. Géol. Bull., v. 58, p. New York, John Wiley & Sons, p. 203-218. 1-153. Wegmann, C. E., 1935, Zur Deutung der Migmatite: Geol. Rundschau, v. 1926, On migmatites and associated Precambrian rocks of southwest- 26, p. 305-350. ern Finland: Part II: Finlande Comm. Géol. Bull., v. 77, p. 1-143. Wetherill, G. W., Davis, G. L., and Lee-Hu, C., 1968, Rb-Sr measurements Shelley, D., 1964, On myrmekite: Am. Mineralogist, v. 49, p. 41-52. on whole rocks and separated minerals from the Baltimore Gneiss, Slemmons, D. B., 1962, Determination of volcanic and plutonic plagio- Maryland: Geol. Soc. America Bull., v. 79, p. 757-762. clases using a three- or four-axis universal-stage: Geol. Soc. America White, A.J.R., 1966, Genesis of migmatites from the Palmer region of Spec. Paper 69, 64 p. South Australia: Chem. Geology, v. 1, p. 165-200. Southwick, D. L., 1969, Crystalline rocks of Harford County, in Geology Winkler, H.G.F., Boese, M., and Marcopoulos, T., 1975, Low temperature of Harford County, Maryland: Maryland Geol. Survey, p. 1-76. granitic melts: Neues Jahrb. Mineralogie Monatsch., v. 6, p. 245- Tilton, G. R., Doe, B. R., and Hopson, C. A., 1970, Zircon age mea- 268. surements in the Maryland Piedmont, with special reference to Balti- Yoder, H. S., Stewart, D. B., and Smith, J. R., 1957, Ternary feldspars: more Gneiss problems, in Fisher, G. W., Pettijohn, F. J., Reed, J. C., Carnegie Inst. Washington, Year Book 56, p. 206-214. and Weaver, K. N., eds., Studies of Appalachian geology: Central and southern: New York, Interscience Pubs., p. 429-434. Tobschall, H. J., 1971, Zur genese der migmatite des Beaume-Tales MANUSCRIPT RECEIVED BY THE SOCIETY JULY 3, 1975 (Mittlere Cévennen, Dép. Ardéche): Contr. Mineralogy and Petrology, REVISED MANUSCRIPT RECEIVED JUNE 7, 1976 v. 32, p. 93-111. MANUSCRIPT ACCEPTED SEPTEMBER 15, 1976

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