Early Paleozoic alkalic and calc-alkalic plutonism and associated contact metamorphism, central Virginia Piedmont

Early Paleozoic plutonism in the central Virginia Piedmont emplaced the igneous Lahore Complex and the younger Ellisville pluton into deformed metavolcanic rocks of the Chopawamsic Formation as well as into greenschist-facies metasedimentary rocks of the Mine Run Complex.The Lahore Complex consists of a small, altered pluton of serpentine and (or) actinolite-diopside metapyroxenite at its north end that has been intruded by the shoshonitic, potassic-alkalic Lahore pluton.This pluton is composed of a narrow band of pyroxene monzonite on its east margin that grades into the more abundant amphibole monzonite on its west side.The pyroxene monzonite, composed chiefly of andesitic plagioclase and lesser amounts of perthitic microcline, has augite and greenish-brown biotite as less abundant constituents.Accessory epidote occurs as large euhedral magmatic grains as well as granular saussuritic alteration of plagioclase.The Lahore pluton is cut by dikes, some of which may have been derived from the Ellisville pluton.Cale-alkaline, peraluminous granodiorite characterizes the younger Ellisville pluton.Andesitic plagioclase is the characterizing major mineral, and quartz is generally more abundant than microcline and the less common orthoclase that constitute the remaining major felsic minerals.Brown-to-green biotite is the ubiquitous dimensionally aligned accessory that defines foliation.Myrmekite, amphibole, titanite, and apatite are accessory.Subhedral magmatic epidote that locally encloses allanite is an important and characterizing accessory.Opaques are rare and are probably mostly ilmenite.The granitoids of both the Lahore and Ellisville plutons have a flow foliation.Emplacement of the Ellisville warped the regional tectonic foliation of the country rocks around its margin.Gravity data suggest that the Ellisville may have an appreciable subsurface extension to the northeast from its surface exposure.The emplacement of the Lahore and Ellisville plutons into deformed, greenschist-facies country rocks has elevated their metamorphic grade.Muscovite porphyroblasts enclosing fibrolite developed in country rock near the pluton contacts may have formed through late-stage metasomatic alteration by fluids emanating from the plutons.Kyanite, staurolite, and chloritoid, in places closely associated with each other within the contact-metamorphosed rocks of the Chopawamsic Formation and Mine Run Complex, are of prograde metamorphic origin.This thermal metamorphism locally may have contributed to upgrading the original volcanogenic sulfide deposits within the Chopawamsic Formation into the ore deposits that comprise the Mineral district of Virginia.Based on geobarometric (Al-in-hornblende) estimates and order-of-magnitude Manuscript approved for publication April 22, 1992 geothermometric calculations (amphibole-plagioclase geothermometer), a general depth of emplacement of about 18-13 km (6-4.5 kb) is estimated for the Lahore and Ellisville plutons at a temperature of emplacement of about 760°C.A combination of U/Pb (zircon), Rb/Sr (whole-rock isochron), and 40Ar/39Ar (age spectra and isochrons of amphiboles) geochronologic data supports an age of about 450 Ma for the Lahore pluton and a younger age of about 440 Ma for the Ellisville.^Ar/^Ar plateau and Ar/Ar isochron ages for amphibole also indicate that heat from the Ellisville pluton reset some of the amphiboles within the adjacent part of the Lahore pluton and some of the amphiboles within mafic olistoliths in the Mine Run Complex; these amphiboles have 40Ar/99Ar plateau and Ar/Ar isochron ages analytically indistinguishable from the age of the Ellisville pluton.The earliest Silurian MArP°Ar plateau ages of the magmatic amphiboles within the Ellisville pluton contrast with the Alleghanian (Carboniferous) metamorphic ages of amphiboles to the east of the Long Branch thrust fault.Juxtaposition of these two terranes by faulting post-date Alleghanian amphibolite-grade metamorphism.Thrusting did not appreciably thermally affect the greenschist-facies rocks west of the Long Branch thrust.The Lahore pluton is a monzonitic, K-alkalic pluton as characterized on AFM and Si02-alkalic diagrams and by its strongly fractionated REE patterns.Its high content of Sr (1027-1540 parts per million [ppm]) and Ba (1644-2060 ppm) and overall enrichment in LILE and other critical chemical features strongly support its shoshonitic character.The Ellisville pluton, however, is primarily of granodioritic composition with characteristics like those generally found in erogenic granodiorites formed along continental margins.Both the shoshonitic monzonites of the Lahore pluton and the granodiorites of the Ellisville pluton could have formed along the ancestral North American (Laurentia) continental margin after accretion of deformed and metamorphosed back-arc and island-arc terranes along the Mountain Run fault zone.The Lahore has an initial 87Sr/86Sr of 0.7046 and could have been derived from a mantle source whereas the Ellisville with an initial ^Sr/^Sr of 0.7062 could have formed within deep crustal levels.An alternative scenario for the origin of the Lahore is that it formed as a distal, continentward facies from a shoshonitic magma after accretion.The high K-magma could have been supplied from a mantle wedge above a still active subduction zone that had formed the island arc to the east of the Lahore pluton.The report was penetratingly and helpfully reviewed by our colleagues David B. Stewart and Jane M.