Petrology and Tectonic Significance of the Polly Wright Cove Pluton, Blue Ridge Province, Virginia: An Extension-Related, Multiply-Intrusive Magmatic System of Anorogenic Origin

Abstract

The Late Proterozoic Polly Wright Cove Pluton (PWCP) intrudes Grenville-age (1.0-1.1 Ga) basement rocks in the core of the Blue Ridge anticlinorium in central Virginia. The PWCP is part of a regional Late Proterozoic (760-700 Ma) anorogenic supersuite which consists of numerous A-type granite bodies exposed throughout Lauretian terrain in Virginia and North Carolina. The pluton includes four major lithologic phases (from oldest to youngest): (1) fine-grained, biotite granite (present only as xenoliths), (2) medium-grained biotite granite (the most areally extensive lithologic unit), (3) medium-grained biotite leucogranite, and (4) fine- to medium-grained biotite granite (occurs only as dikes). These units are distinguishable in both hand sample and thin section, as well as by their geochemical characteristics. The PWCP granites are metaluminous in composition and have geochemical signatures characteristic of an anorogenic origin including: (1) high Ga/Al and FeO/MgO, (2) marked enrichment of Y, Nb, Zr, Ce and Ga, and (3) low Ba and Sr. The broad range in SiO₂ values (67.1 — 76.5 wt. %), FeO/MgO (12.6 — 41.6), and Ba/Rb (0.34 — 3.50) indicate that fractionation of major minerals was responsible for developing significant compositional variation within the pluton. The late-stage granites (leucogranites and dikes) of the PWCP show a marked depletion in Ba and Sr, as well as an increase in Ga/Al, relative to the older units suggesting that fractionation of alkali feldspar and plagioclase was a major factor controlling differentiation of the PWCP magmatic system. Removal of ferromagnesium silicate minerals is suggested by the range in FeO/MgO and by development of late-stage leucogranite. Additionally, a marked decrease in Ce/Nb relative to Y/Nb in the late stage granites suggests fractionation of allanite. U-Pb isotopic data for zircons from the medium-grained biotite granite yield an upper intercept age of 706 ± 4 Ma, indicating that most of the PWCP was emplaced near the end of the period of regional anorogenic magmatism. The pluton is similar in age to metaluminous granitoids that constitute the southern part of the Robertson River batholith, located 40 km to the north, and it appears to have intruded at the same time as rift-related peralkaline rocks of the Robertson River. A ²⁰⁷Pb/²⁰⁶Pb age of 972 Ma from a single xenocrystic zircon preserves isotopic evidence of an inherited component of possible Middle Proterozoic, post-Grenville age. Whereas the supersuite has been indirectly associated with extensional faulting in this region, other studies have linked the PWCP directly to rift-related faulting in the central Virginia Blue Ridge. For this reason, the age of the PWCP has important implications for the timing of extensional tectonics in the region.