2-STAGE CONTAMINATION DURING CRUSTAL ASSIMILATION - ISOTOPIC EVIDENCE FROM VOLCANIC-ROCKS IN EASTERN NEVADA

Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest volcanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclase-rich andesites and dacites have greater Sr and Sr-87/Sr-86, but less Nd and lower epsilon(Nd) and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two "bites". The separation of the crustal source into two elementally distinct contaminants causes divergent trends with respect to the variation of Sr-87/Sr-86 verSUS epsilon(Nd) that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contaminants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.