THE ORDOVICIAN GREEN SPRINGS PLUTON OF CENTRAL VA: POSSIBLE SOURCE OF GLACIOGENIC COBBLES IN THE DEVONIAN ROCKWELL FORMATION OF PA, MD, AND WV?

TIMS dating of zircons from the main diorite phase of the Green Springs Pluton yield a concordant age of 453.7 ± 1.1 Ma on a sample containing 0.629% TiO₂.  Mineralogically, it contained approximately 37% plagioclase, 30% amphibole, 26% K-spar, 8% biotite, and 0.4% quartz as determined by X-ray powder diffraction (J. H. Barnes, pers. comm., 10/9/2016), and trace visible epidote, sulfides, and apatite.

Linear plots of SiO₂ vs. MgO, CaO, TiO₂, MnO, P₂O₅, Co, Ge, Sc, Y, and exclusively trivalent lanthanides for 7 samples suggest a single differentiation series for the Green Springs Pluton and adjacent Poore Creek intrusives. Together, they range from basaltic to rhyolitic with the more felsic samples being adakitic. The more mafic samples are consistent with back arc basin basalts. On a Ta-Hf/3/Th diagram they have active margin compositions, with most near the arc end of the field.

Zircons from two glaciogenic cobbles from the Upper Devonian Rockwell Formation of south central Pennsylvania were dated at 453.9 ± 2.1 Ma and 452.7 ± 0.6 Ma (TIMS dating by Jahan Ramezani in Smith and Skema, 2012). The glaciogenic cobbles are volcanic and hypabyssal and range in composition from picrobasalt to silicified rhyolites, but the majority are dacites. Synchronous crystallization ages, the 0.560% and 0.628% TiO₂ content of the dated cobbles, and the thermal aureole around the Green Springs Pluton made it an attractive potential source of the glaciogenic cobbles. However, the glaciogenic cobbles are not adakitic and are lower in the mobile alkalis. Using immobile, incompatible elements as discrimination tools, the Green Springs Pluton does not appear to be an obvious source for the volcanic rock preserved as glaciogenic cobbles in the Rockwell Formation, but is coeval with widespread arc plutonism in the Late Ordovician.

Two diorite samples of the heterogeneous, undated Rowlandsville Granodiorite of Maryland, which remains a source candidate for the cobbles, contain 0.558% and 0.634% TiO₂, and match for many elements and geochemical trends. Crystallization ages of ca. 453 Ma for the upper Ammonoosuc volcanics and some of the Bronson Arc domes in New England also make them appealing targets.