EVIDENCE FOR GNEISS DOME EMPLACEMENT IN THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, EASTERN NEVADA

The Northern Snake Range (NSR) in eastern Nevada has long exemplified a classic metamorphic core complex. This, and associated mountain ranges, are cored by deformed lower plate rocks metamorphosed to varying grades. Lower plate rocks in the NSR and the Schell Creek Range (SCR) to the west are dominated by stratified sequences of quartzites and metapelites. Original sedimentary bedding (S0) is preserved in the the SCR, but is obliterated or transposed in the intensely deformed lower plate of the NSR. Metamorphic grade also varies. The highest metamorphic grades are recorded by stratigraphicaly low metapelites in the NSR where 600°C and 0.8 GPa conditions are recorded, corresponding to 30 km paleodepths. These rocks occur within 1 km of the NSR detachment which is inferred to represent the 10-12 km deep brittle-ductile transition. Two periods of metamorphism are evident. Aligned inclusions in M1 garnet, staurolite and biotite porphyroblasts preserve an S1 (compressional?) fabric. Outside of M1 porphyroblasts, S1 completely overprinted extensional fabrics (S2) which defines the primary foliation in these rocks. A second generation of small staurolite porphyroblasts (M2) contained within S2 fabrics suggest that extension initiated at 30 km depths. At higher stratigraphic levels, lower plate rocks in the NSR are retrograded to greenschist facies, but often preserve relicts of higher grade metamorphic conditions. The presence of albite porphyroblasts suggest a steep, nearly adiabatic retrograde PT path. Thermobarometric calculations using M1 and M2 biotite suggest a thermal pulse associated with the initiation of extension.

The growth of retrograde porphyroblasts and evidence for a thermal pulse suggest gneiss dome emplacement. This is consistent with observations from the SCR . Here, closely spaced slatey cleavage records east verging thrusting (S1) which is cut by down to the west extensional fabrics (S2). These deformational and metamorphic characteristics can be accounted for through the diapiric emplacement of a gneiss dome at depth, with accompanying flow of ductile material from the flanks of the dome. The dome is centered under the NSR where the deformation and uplift are greatest. Less intense deformation and metamorphism displayed in the SCR reflects its position on the flanks of the dome.