AN EARLY CAMBRIAN AGE OF RIFTING OF CAROLINIA FROM GONDWANA: GEOCHEMICAL CONSTRAINTS FROM THE STONY MOUNTAIN GABBRO, NORTH CAROLINA

Carolinia comprises a collection of Neoproterozoic-early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In North Carolina, mafic rocks of the Stony Mountain gabbro (SMG) intrude sub-aqueous volcanic and sedimentary rocks of the Albemarle Group. The age of the SMG is constrained to the Early Cambrian-Late Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic-earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia.

Rocks of the SMG are characterized by sub-alkaline basaltic compositions with low TiO2 and HFSE contents and low to moderate Ti/V and Zr/Y ratios. Mg# values range from 70-50 and imply that the rocks span a? range from primitive to more evolved. Primitive mantle-normalized REE patterns are characterized by variable LREE enrichment with moderate sloping extended REE patterns. Zr, Nb and Yb systematics imply derivation from mantle sources that are incompatible element-depleted to moderately enriched. The degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of tholeiitic basalts in modern island-arc, subduction related lavas. The SMG is interpreted to reflect partial melting of depleted mantle that was contaminated by subduction-related fluids in the mantle wedge above a subduction zone. A subset of samples have lower TiO2 and very high Cr, Ni, Co and MgO contents and Mg# values >70, attributes that are similar to modern low-Ti (LOTI) tholeiitic lavas.

Modern geochemical analogues to the LOTI-rocks occur in a variety of supra-subduction zone settings and are low-pressure melts of refractory peridotite mantle that form from an influx of hydrous fluids derived from the subducting oceanic slab. These are largely restricted to forearc environments and extensional tectonic regimes and are associated with the initial stages of arc rifting and the beginning of seafloor spreading. The presence of LOTI-rocks in Carolinia is interpreted to represent the Early Cambrian commencement of arc rifting thereby providing a mechanism for the separation of Carolinia from Gondwana.