GNEISS DOME EMPLACEMENT: AN ALTERNATIVE MODEL FOR METAMORPHIC CORE COMPLEX DEVELOPMENT IN EASTERN NEVADA

The Northern Snake Range (NSR) in eastern Nevada has long exemplified a classic metamorphic core complex. The McCoy Creek Group, which comprises part of the lower plate in the NSR, is also exposed as lower plate rocks in the Schell Creek Range (SCR), and the Southern Snake Range (SSR). The intensity of deformation and metamorphism varies widely between these mountain ranges. The highest metamorphic grades are recorded by metapelites in the NSR where 600°C and 0.8 Gpa conditions are indicated, corresponding to 30 km paleodepths. These rocks occur within 1 km of the NSR detachment, inferred to represent the 10-12 km deep brittle-ductile transition. Inclusions in garnet and staurolite porphyroblasts preserve an S1 fabric (compressional) which is overprinted by extensional fabrics (S2). Microstructures suggest that extension initiated at 30 km depths. Retrograde mineral assemblages and textures suggest a nearly adiabatic retrograde PT path accompanying active extension. In the SSR, lower plate rocks were metamorphosed to greenschist facies conditions. Original sedimentary structures are well preserved. S1 is reflected by crenulations in phyllitic partings in the Prospect Mountain Quartzite (PMQ) and by a slaty cleavage in lower pelitic units. S2 is best developed in the upper part of the PMQ where top to the east extension is indicated. In the SSR, the PMQ is 3000 feet thick. Contrast this to the 300 feet thickness of this unit in the NSR, reflecting at least 1000% extension in the NSR. In both the NSR and the SSR, fabrics indicate eastward directed extension. Lower plate rocks in the SCR display intermediate metamorphic and deformational characteristics as compared to the NSR and SSR. Here, pelitic units range from greenschist facies phyllites near the top of he sequence, to amphibolite facies schist in lower-most exposures. Closely spaced slaty cleavage records east directed thrusting (S1) which is cut by west directed extensional fabrics (S2). These deformational and metamorphic characteristics record compression and possible thrust stacking, followed by diapiric emplacement of a gneiss dome at depth, with accompanying ductile flow of material from the flanks of the dome. The dome is centered under the NSR where the deformation is greatest. Less intense deformation and metamorphism displayed in the SSR and SCR reflect less intense motion on the flanks of the dome.