THE STRIPEDROCK PLUTON, A REVERSELY ZONED, A-TYPE, NEOPROTEROZOIC GRANITE IN THE BLUE RIDGE OF SOUTHWESTERN VIRGINIA

The Striped Rock Pluton (SRP) is an elongate body (N30 – 40E) some 10mi long by 2.5mi wide emplaced into variably deformed granulite and amphibolite grade schists and gneisses of Grenvillian age. Dated at 681 ± 10 Ma (Rb-Sr whole rock) the SRP is located within Crossnore Complex of the western Blue Ridge Thrust sheet in southwestern Virginia. Described by Stose and The pluton was mapped by Stose and Stose (1957) who considered the body composed of three lithologic units based on field observations. This work is a reinvestigation of the body and presents the first geochemical analyses of the pluton. The bulk of the pluton is a two feldspar, gray granite ranging in SiO₂ from 65.6 wt. % to 77.5 wt%, the Striped Rock granite (Srg) of Stose & Stose, (1957). Bordering the Srg to the north and west is the Carsonville granite (Cvg) (Stose & Stose, 1957), a two feldspar, pink granite that ranges in SiO₂ from 72.8 wt % to 77.1 wt. %. Plotted on the N+K/A vs A/CKN diagram these units range from peraluminous to metaluminous with a few samples in the peralkaline field. Major, minor and trace element geochemical data indicates a systematic change from the core to the rim of the pluton consistent with reverse zoning. Utilizing trace element plots such as RB vs Y + Nb, Y vs Nb, Nb vs 10000 Ga/Al, and Agpaitic Index vs 10000 Ga/Al show SRP samples plot outside of the accepted fields for I, S & M type granites and have 'within plate' signatures. This, together with the high Ga/Al ratio for SRP rocks, indicates that the SRP belongs to the A-type family of granitoids. A possible origin of the reverse zoning in the SRP may be explained through the use of a two-stage model. In this model, a lower horizontally stratified magma chamber develops and slowly leaks material through its roof into an upper magma chamber. Successive tapping of deeper levels of the lower chamber leads to emplacement of more mafic magma into the center of the upper chamber forming a reversely zoned pluton. Finally, it may be possible to explain the formation of the red Carsonville granite through the development of a late-stage hydrothermal system in the upper portions of the pluton where it is in contact with the country rock - Grayson granodiorite gneiss.