THE GRENVILLIAN GOOCHLAND TERRANE: THRUST SLICES OF THE LATE NEOPROTEROZOIC LAURENTIAN MARGIN IN THE SOUTHERN APPALACHIANS

The Grenvillian granulite-facies Goochland terrane extends from Fredericksburg, VA to near Raleigh, NC. Its area is comparable to the Blue Ridge Grenvillian massifs farther west. Unlike other southern Appalachian Grenvillian terranes, the Goochland is entirely fault-bounded with no defined unconformable sedimentary or volcanic cover. Goochland lithologies comprise tonalitic to granitic gneisses, amphibolite, pelitic gneisses, and minor quartzite, calcsilicate gneiss, and marble. It has been intruded by Grenvillian granitoids and anorthosite, Late Neoproterozoic A-type (some peralkaline) granites, Late Paleozoic (Alleghanian) granites, and Mesozoic diabase and felsic dikes.

The Goochland terrane was attenuated by normal faulting and intruded by A-type granites during the second (successful)stage of Neoproterozoic rifting (600-550 Ma), leaving the Goochland terrane, and the Blue Ridge massifs, as the thinned edge of the Laurentian continent. Paleozoic compressional events clipped the Blue Ridge and Goochland off the edge and thrust them farther onto the continent, followed by Carolina and Chopawamsic volcanic arcs which pushed the Blue Ridge farther west but rode over the Goochland terrane. (No evidence has been found that Carolina or Chopawamsic intruded through Goochland.) The culminating Alleghanian collision with Africa produced an orogenic welt with intrusion of granite, metamorphism, upward bulging of the Raleigh belt, and reactivation of faults as strike-slip zones, altogether producing a huge structural window exposing the Goochland terrane through the overlying Carolina and Chopawamsic terranes. Mesozoic attenuation reactivated the fault zones as normal faults, further uplifting the Goochland terrane as a horst between graben to the east and west.

Contrasting lithologies of the Goochland and Blue Ridge Grenville, and lack of identified earlier Neoproterozoic granites in the Goochland suggest that Goochland and Blue Ridge aren't chips off the same block, but the nearly identical and unique chemistry of the Blue Ridge Roseland and Goochland Montpelier alkalic anorthosites suggests a close spatial association in the Grenville. In the southern Appalachians, the Goochland is the easternmost Grenville terrane and potentially the closest to the rifted margin of Laurentia.