The Carolina terrane in northwestern South Carolina, U.S.A.: Late Precambrian‐Cambrian deformation and metamorphism in a peri‐Gondwanan oceanic arc

Abstract

The Carolina terrane comprises an exotic volcanic island arc in the hinterland of the Southern Appalachian orogen. The western portions of the Carolina terrane consist of zoned mafic‐ultramafic plutonic complexes intruded into a pile of basalts and basaltic andesites. This sequence of rocks has been interpreted to represent an episode of intra‐arc rifting prior to regional metamorphism and deformation. New U‐Pb zircon ages from the Mean Crossroads complex in northwestern South Carolina along the central Piedmont suture confirm relative ages inferred from detailed mapping. Two foliated metadiorites yield U‐Pb dates of 579±4 and 571±16 Ma, interpreted to be crystallization ages. A foliated metaquartz diorite yields a U‐Pb date of 538±5 Ma interpreted to be a crystallization age. An undeformed, unmetamorpbosed diorite intruding these metamorphosed zoned complex intrusive rocks yields an age of ≈ 535 Ma. Hence we believe that intra‐arc rifting and regional metamorphism and foliation development both occurred between =580 and 535 Ma. While petrographic and Ar‐Ar studies support subsequent middle to late Paleozoic regional metamorphic overprint(s), or at least static recrystallization and/or uplift through hornblende‐biotite‐muscovite blocking temperatures for Ar, the 535 Ma, undeformed, unmetamorphosed intrusion suggests late Precambrian‐Early Cambrian regional metamorphism and deformation was the event responsible for the regional metamorphic fabric in this area of the Piedmont. These observations contradict the idea that this metamorphism and fabric development are related to presumed early Paleozoic accretion of the Carolina arc to Laurentia as well as correlations with Middle Ordovician fabric elements in the eastern Piedmont. Instead, this fabric is interpreted to record a changing plate kinematics (e.g., migration of triple junction or change in relative motion vectors) during semicontinuous, diachronous development of an island arc on the Gondwanide margin.