WIDESPREAD CRUSTAL CONTAMINATION OF EARLY PROTEROZOIC MAGMAS: AN EXAMPLE FROM THE FENNER GNEISS, PIUTE AND OLD WOMAN MOUNTAINS, SAN BERNARDINO COUNTY, CALIFORNIA

The Fenner Gneiss comprises a variably metamorphosed and deformed sub-alkaline granitoid batholith, which has a U/Pb crystallization (zircon) age of 1685 +/- 0.15 Ma, which is cut by ubiquitous mafic dikes of the similar age. It is a highly distinctive rock, with the common presence of 2 to 5 cm phenocrysts of K-feldspar. Compositionally, the suite is highly variable, ranging from a rather mafic diorite (50 wt.% SiO₂, 2.5 wt.% K₂O, 11% wt.% FeO*) to granite (70 wt. % SiO₂, 2 wt. % K₂O, 2.5 wt.% FeO*). All rocks, even the most mafic, are highly enriched in incompatible elements. REE are unusually high in the more mafic rocks, (La = 113, Lu = 0.78 ppm) but drop off rapidly in felsic rocks (to La = 40, Lu = 0.42 ppm). The primary mineral assemblage in addition to quartz and feldspars includes biotite +/- hornblende and abundant accessories. Course relicts and pseudomorphs of garnet occur locally, possibly as xenocrysts. The Fenner batholith intruded older metamorphic rocks (high-Al metasedimentary rock, 1694 and 1720 Ma) that were already at high grade. Subsequently it was metamorphosed during the Proterozoic and locally again during the Mesozoic. Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source. Trace element models (AFC) suggest that the Fenner could have been largely derived by a fairly mafic melt with up to approximately 18% assimilation of the surrounding country rock. Based on measured Pb/Pb isotopes, it would seem that the assimilant would have to have elevated 206/204, 207/204 and 208/204, which is consistent with it being older crustal material (and also based on available Nd isotope data). Similar geochemical signatures observed from ubiquitous coeval plutonic bodies throughout the lower Colorado River trough, suggest that this was a common mode of magma generation and emplacement during the Early Proterozoic.