EVIDENCE FOR POST-GRENVILLE RIFTING AT ~960 MA IN THE SAMS CREEK VOLCANICS/WAKEFIELD CARBONATES COMPLEX, WESTERN PIEDMONT, MD

New zircon dates by laser ablation ICP-MS(MC) reveal a 950-970 Ma post-Grenville Rodinian rift sequence in the Westminster Terrane of Maryland. Dates for the Sams Creek volcanics/Wakefield carbonates were acquired from three units: greenstone layers interpreted as a basaltic floor for the carbonate buildup, a quartz rich sandstone channel within the carbonate sequence, and phyllite, interpreted as an andesitic ash layer also within the carbonate sequence. The metabasalt preserved Grenville and pre-Grenville xenocrysts (n=6) formed at 1036-1269 Ma. The andesitic ash hosted two zircon populations, Grenville and pre-Grenville plutonic sourced xenocrysts (n=15) of 1016-1321 Ma, and an eruptive age of 950-970 Ma (n=3). The sandstone channel had a prismatic and rounded zircon population (n=132) that yielded approximately the same ages and distribution as the andesitic ash. This population is restricted to Grenville aged rocks, and contemporaneous volcanics no younger than ~960 Ma.

At 960 Ma, it is feasible that Rodinia was breaking up or collapsing, allowing for a rift basin with an oceanic outlet. Field studies indicate a contemporaneous origin for the volcanic and carbonate units indicated by metabasalt bombs at the base of the carbonate buildup, and interlayers of basaltic and andesitic ash, now phyllite, throughout the sequence. This sequence is characterized by rift geometry and lithology; a long, narrow outcropping extent (~75x10 km) with many hundreds of meters of mixed limestone and dolomitic carbonates interbedded with basaltic through andesitic greenstones, ash derived phyllites, and quartz sandstone channels, all of greenschist rank.

A rift hypothesis is also supported by the geochemistry of meta-volcanic rocks. The trace element composition of greenstones resembles within-plate through plate-boundary (EMORB) basalt, possibly tracking the opening of a rift basin. Phyllitic layers are enriched in rare earth elements relative to average shale composition, indicating that they are ash derived. A comparison the Sams Creek meta-volcanics to known rift volcanics having similar SiO₂ content indicates a very strong similarity to rift volcanism.