TWO PERIODS OF GARNET LEUCOGRANITE FORMATION IN THE GOOCHLAND TERRANE, VA

Tectonics of the Goochland Terrane begins in the collisional Grenville event with intrusion by the bio-hbd State Farm granite, and ends after intrusion of bio-hbd Petersburg and related granites of the collisional Alleghanian event. The areal extent of the Goochland Terrane was defined by Farrar (1984) as the terrane of granulite facies rocks bearing opx in intermediate to mafic gneisses and bearing silli-Kspar in migmatitic pelitic gneisses. The anatexis of the pelitic gneisses produced what is now silli-gar leucogranite occurring in strongly foliated lenses up to several m thick, found between South Hill, VA and the North Anna River. (Very similar leucogranites commonly occur in other medium pressure granulite terranes, including the Adirondacks of NY.) This Grenville anatexis was followed by exhumation to shallow levels with intrusion of hypersolvus A-type to peralkaline, probably rift-associated, granites ca. 600-550 Ma between Raleigh, NC and Richmond, VA.

Later, Paleozoic metamorphism, ending in the Alleghanian, re-metamorphosed the Goochland terrane at upper greenschist to amphibolite facies over most of the terrane, reaching ky-Kspar, high pressure granulite facies north of Richmond. Along the North Anna and South Anna Rivers, strongly foliated, pelitic ky-Kspar migmatite contains lenses of ky-Kspar-gar leucogranite which could be re-equilibrated from Grenville silli-Kspar-gar leucogranite, but it is cut by a post-foliation, gar leucogranite dike with ky-Kspar pseudomorphic after muscovite. This anatexis and high-pressure granulite metamorphism must have immediately preceded the second exhumation that accompanied right-lateral translation of the Goochland terrane on bounding mylonite zones in latest Alleghanian. Thus the Goochland Terrane was metamorphosed in two time-separated and distinct metamorphic events, one medium pressure granulite facies event in the Grenville, and one high pressure upper greenschist to granulite facies event ending in the late Paleozoic, both events producing potentially datable garnet leucogranites.