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 compo- sitions 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 neo- somes 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 de- creases 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 paleo- some maintained by the presence of melt in the neosome drives the metamorphic differentiation and should also create an asm gra- dient leading to the observed quartz migration from the selvage to the neosome. INTRODUCTION The Precambrian Baltimore Gneiss, exposed in the cores of man- tled gneiss domes around Baltimore, Maryland, is a typical exam- ple 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 mig- matite 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 com- monly 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 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 1090 S. N. OLSEN 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 dif- ferentiation 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 plagio- clase (Wetherill and others, 1968), but the lead-lead ages of some zircons are as much