Deep in the heart of Dixie: Pre-Alleghanian eclogite and HP granulite metamorphism in the Carolina terrane, South Carolina, USA

摘要

The central part of the Carolina terrane in western South Carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the Carolina slate belt (to the SE) and amphibolite facies metavolcanic and metaplutonic rocks of the Charlotte belt (to the NW). This region, termed the Silverstreet domain, is characterized by penetratively deformed felsic gneisses, granitic gneisses, and amphibolites. Mineral assemblages and textures suggest that these rocks formed under high-pressure metamorphic conditions, ranging from eclogite facies through high-P granulite to upper amphibolite facies. Mafic rocks occur as amphibolite dykes, as metre-scale blocks of coarse-grained garnet-clinopyroxene amphibolite in felsic gneiss, and as residual boulders in deeply weathered felsic gneiss. Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies conditions. All samples have been partially or wholly retrograded to amphibolite assemblages. We infer the following P-T-t history: (1) eclogite facies P-T conditions at greater than or equal to 1.4 GPa, 650-730 degreesC (2) high-P granulite facies P-T conditions at 1.2-1.5 GPa, 700-800 degreesC (3) retrograde amphibolite facies P-T conditions at 0.9-1.2 GPa and 720-660 degreesC. This metamorphic evolution must predate intrusion of the 415 Ma Newberry granite and must postdate formation of the Charlotte belt and Slate belt arcs (620 to 550 Ma). Comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis. Eclogite and high-P granulite facies metamorphism in the Silverstreet domain may coincide with a approximate to570-535 Ma event documented in the western Charlotte belt or to a late Ordovician-early Silurian event. The occurrence of these high-P assemblages within the Carolina terrane implies that, prior to this event, the western Carolina terrane (Charlotte belt) and the eastern Carolina terrane (Carolina Slate belt) formed separate terranes. The collisional event represented by these high-pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to Laurentia. [References: 83]