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6 Metamorphism Modern metamorphic petrology has evolved largely from re- search in the New England Appalachians and in the Alps. The metamorphic histories of terranes in some cases can now be re- lated to large scale tectonic processes, such as the thin-skinned thrusting thought to have occurred in the southern Appalachians. The proposed deep research drill hole offers an exciting opportu- nity to apply new techniques in metamorphic petrology, and to integrate the results of it with those of geochronologic and de- formational investigations. In addition, sampling at depth with the drill will circumvent the problem of deep weathering of many rock units, particularly in the terranes southeast of the Brevard fault zone. Above all, research core drilling will provide a truly vertical section through a major metamorphic terrane that cannot be obtained from surface outcrops. METAMORPHIC ISOGRADS Metamorphic petrologists have traditionally deciphered the thermal structure of continental crust in orogenic belts by mapping isograds based on the appearance and disappearance of certain minerals produced by chemical reactions in rocks during meta- morphism. The shape of isogradic surfaces at depth is almost 24
25 completely unknown. Attempts to project these surfaces down- ward are largely conjectural because the positions of isograds at depth are controlled by processes of heat and fluid flow that do not necessarily follow simple geometric rules. Only in a few cases have isogradic surfaces been mapped over vertical distances of more than 1 km and never over more than approximately 2 km. Yet the geometry of isogradic surfaces is a powerful probe into the thermal structure of the crust during orogeny and offers a key to the nature of the heat source for metamorphism. Correlation of isograds mapped at the surface with those established in core sam- ples from the southern Appalachian site would provide points on isogradic surfaces over vertical distances approaching 6 to 8 km. This unprecedented increase in scale of observation could make possible pioneering investigations of the thermal structure of the crust before the main overthrust event, and investigations of heat sources and the controls of heat flow that ultimately determine the types of metamorphic rocks that are developed deep in the continental crust. PRESSURE-TEMPERATURE PATHS Petrologic study of metamorphic rocks provides unique ways of characterizing the emplacement of large thrust sheets. New de- velopments in metamorphic petrology permit determination of pressure-temperature (P-T) paths that individual samples fol- lowed during their crystallization. The P-T paths are based on studies of mineral assemblages, compositional zoning in miner- als, and mineral and fluid inclusions in crystals (Hollister, 1979; Spear et ai, 1984). The estimated P-T paths provide fundamen- tal insights into a rock's tectonic history because rates of thermal equilibration of large rock masses generally are much slower than tectonic motions. For example, rocks from a relatively hot sheet, thrust over cooler rocks, would record a P-T path of cooling and decompression. Rocks from the lower plate would record increas- ing temperatures and pressures with final values that converge with the final conditions preserved in the samples from the up- per plate. Furthermore, P-T paths determined for rocks can be quantitatively modeled to estimate thrust sheet thickness, initial pre-thrusting temperatures, and details of post-thrusting uplift, erosion, and heat flow (England and Thompson, 1984). An exciting prospect for the southern Appalachian site would
26 be determination of P-T paths followed by rocks in the Blue Ridge terrane above the main thrust surface as they cooled and were up- lifted, and by the platform sedimentary rocks below as they were deformed, metamorphosed, and finally equilibrated in a steady- state geothermal gradient. Such a study would provide indepen- dent confirmation of the thin-skinned thrusting hypothesis. Be- sides offering verification of the large thrust sheet model, the P-T paths could characterize the initial conditions of the thrusting event and help to determine the timing of earlier thrusts in the upper plate. Temperature-time paths deduced from mathemati- cal models of the P-T paths would complement thermal histories obtained by geochronological techniques. POLYMETAMORPHISM Many rocks of the southern Appalachians are polymetamor- phic, having been subjected to as many as three major episodes of regional metamorphism corresponding to the Taconic, Aca- dian, and Alleghenian orogenies. Local polymetamorphism may have occurred near faults associated with emplacement of thrust sheets, as described above. The effects of polymetamorphism are manifest in a number of ways, such as textural relations among minerals (e.g., psuedomorphs, reaction rims), solid inclusions in refractory minerals (such as garnet) that are not in chemical equi- librium with the rock's matrix, and anomalous chemical zoning in minerals. Systematics of mineral textural relations, microstruc- tures, and geochronologic data allow polymetamorphism to be distinguished from sequential metamorphic reactions that take place when rocks are buried and heated, and then uplifted and cooled. Specific metamorphic episodes probably can be correlated with major tectonic events that define the various orogenies, even in the complex polymetamorphic terranes of the southern Ap- palachians, through coordinated petrologic, microstructural, and geochronologic analysis of drill-core samples. HEAT AND MASS TRANSFER DURING METAMOSPHISM Petrologic studies of metamorphic rocks can provide informa- tion on the mechanism and patterns of heat and mass transfer dur- ing metamorphism. Analysis of reaction progress in metamorphic rocks, particularly when reactions involve volatile gain or loss, can
27 record the time-integrated flux of fluid through rocks associated with a particular interval of their history as well as the amount of heat added to or extracted from them (Ferry, 1983). Water/rock ratios, for example, can be estimated and compared with stable isotope data from the rocks. Compositions of fluids that coexisted with observed mineral assemblages can be calculated from ther- mochemical data and compared with fluid inclusions. Knowledge of these parameters can help to constrain models of deformational processes and thermal history that are based on other types of observations. Because of the fine grain size of cuttings from drilling of crys- talline rocks and the variable response anticipated from different rock compositions, core is essential for research in metamorphic petrology at the southern Appalachian site. As anyone who has seen outcrops in metamorphic terranes can appreciate, rock type commonly changes on a centimeter scale. To ensure success in deciphering P-T paths and understanding reactions, continuous coring over intervals of at least 10 m (Tables 1 and 2), and per- haps as much as 50 m, is required because of the heterogeneity of metamorphic rock that will be encountered at depth in the hole. In addition, useful rock types such as pelitic schists often consti- tute less than 1 to 5 percent of an outcrop. Continuous coring will be needed both to assure representative sampling of all rock types present and of less common rocks that are of unusually great petrologic significance.
