Age and petrogenesis of the Striped Rock granite pluton: Blue Ridge province, southwestern Virginia
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The Striped Rock granite pluton is an epizonal A-type intrusion emplaced into Grenville age gneisses at 748 Ma. Isotopic, petrographic, and textural evidence indicate that the facies comprising this pluton are differentiates of a homogeneous magrna and not the result of multiple intrusions. Strontium isotopic data preclude the Striped Rock granite from being a partial melt of basement gneisses similar to those presently exposed in the Blue Ridge province. Isotopic data also suggest that it is unlikely that the Striped Rock granite fonned by simple differentiation of a mantle derived melt. Of the many models proposed for the petrogenesis of A-type granites the models that are most consistent with the physical and chemical characteristics of Striped Rock granite are those employing interaction between a basaltic magma and an evolved crustal source. Of particular interest, here, is the Inodel proposed by Barker et al. (1975) for the formation of the Pikes Peak batholith. The environment of formation, intensive variables (as interpreted from petrographic studies), mineralogy, and chemistry of the fayalite free granite of Pikes Peak and the Striped Rock granite closely parallel one another, suggesting a similar process for their formation. The "reaction melting" process outlined by Barker et al. (1975) is consistent with both isotopic and limited trace element data from Striped Rock granite.
The Striped Rock granite is part of the Late Precambdan Magmatic Province (LPMP). A detailed study of Striped Rock granite has allowed this pluton to be compared and contrasted with other LPMP rocks. This comparison has lead to the recognition of two distinctly different categories of late Precanlbrian felsic intrusives; (i) Crossnore-Lansing type (Crossnore and Lansing granites) and (ii) Striped Rock type (Striped Rock, Beech, and Stewartsville(?) granites). Crossnore-Lansing type granites are characterized by small volumes (as inferred from areal distribution, ~lkm2), high Sr initial ratio, sodic amphibole, and multiple zircon populations. Striped Rock type granites are characterized by relatively large volumes (>65km2), intermediate Sr initial ratio, a single zircon population, and hornblende, where amphibole is present. Recent radiometric age determinations support a temporal relationship between Striped Rock type granites.
The age of formation for the Striped Rock granite, determined here, strongly supports the emerging model that late Precambrian magmatism associated with extensional tectonism occurred in at least two pulses. The earlier pulse of magmatism was dominated by felsic intrusives and lasted for 60 My between 760 Ma and 700 Ma. The Second pulse of magmatism was dominated by basaltic magmatism and occurred approximately at 570 Ma. This later magmatic event was probably related to the opening of the Iapetus ocean.
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