IMPLICATIONS OF NEW ~960 MA DATES FOR THE SAMS CREEK VOLCANICS/WAKEFIELD CARBONATE COMPLEX, WESTMINSTER TERRANE, MD

The Sams Creek volcanics/Wakefield carbonates complex in the Westminster Terrane of Maryland is preliminarily interpreted as a 950-970 Ma post-Grenville, Rodinian breakup rift sequence. New zircon dates by laser ablation ICP-MS(MC) were acquired for three units, an andesitic layer within the Wakefield carbonate complex, a quartz rich sand channel within the carbonate complex, and basaltic layers at the lower contact of the main carbonate buildup. The andesitic ash provided zircon dates (n=15) compatible with the Grenville orogeny mixed with pre-Grenville plutonic sources ranging in age from 1016-1321 Ma, all interpreted as xenocrysts, and a fraction (n=3) of post-Grenville dates interpreted as the eruptive age in the range of 950-970. The sandstone channel within the Wakefield carbonate complex had a prismatic and rounded zircon population (n=132) with the same approximate age and distributions as the andesitic ash. The <980 Ma prismatic population (n=6) provides an age of 952-955 Ma. The basaltic sequence, which is mapped as the traditional Sams Creek metabasalt, contained only Grenville and pre-Grenville xenocrysts (n=6) of 1036-1269 Ma.

A post-Grenville Rodinian breakup rift is proposed for the Sams Creek/Wakefield complex because (1) At approximately 960 Ma, it is feasible that the Rodinian breakup would have been underway as rifting with an oceanic outlet; (2) The outcropped extent indicates rift geometry, as a long, narrow, 75 x10 km basin. Facies include basaltic though andesitic greenstones, ash derived phyllites, quartz sandstone channels, many hundreds of meters of mixed limestone and dolomitic shallow water carbonates, and phyllite-rich deep water limestones (Silver Run Fm.), all of greenschist rank; (3) A rift setting is geochemically compatible with, and would provide a source of basaltic to andesitic volcanic rocks, including ashes; (4) The carbonate buildup contains volcanic bombs and detritus at the Wakefield/Sams Creek boundary, with interlayers of basaltic and andesitic ash throughout, indicating a contemporaneous age for the carbonates and volcanics; (5) The sandstone channel within the Wakefield carbonates contains a zircon population restricted to ages of Grenvillian rocks and contemporaneous volcanics no younger than ~960 Ma.