EXPERIMENTAL CONSTRAINTS ON THE PROTOLITH CONTRIBUTIONS TO THE GRANITOIDS OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA

Metamorphic and igneous rocks of the Sierra Nevada Mountains, CA, constitute deeper regions of continental crust from and through which subsequent granitoids of the batholith were derived and intruded. Four samples represent the most abundant country rocks and thus the most likely to have been involved in the generation of batholithic magmas: hydrous gabbro cumulate, mafic metavolcanic rock, psammitic schist, and quartz diorite gneiss. Magmatism occurred in two distinct episodes – a more westerly, predominantly tonalitic Jurassic pulse, and a more easterly, predominantly granitic Cretaceous pulse, indicating variations in mantle vs. crustal input as a function of longitude and time.

Thirty-three piston-cylinder experiments were conducted at 10 kbar, 800-1000°C, wet & dry, for 24-166 hours to determine whether different proportions of the assumed crustal protoliths could generate the range of granitoid magmas seen in the Sierras. Even though the separate end-members are fairly distinct compositionally, the starting compositions of the mixed powders are relatively homogeneous. In contrast, the partial melts produced from these mixtures have a wide compositional range: SiO2 from 49 to 74; Al2O3 from 11.5 to 18; FeO from 0.8 to 11.4; MgO from 0.6 to 3.7; CaO from 1.2 to 7.8; Na2O from 1.5 to 4; K2O from 0.6 to 2.9 wt.%. Melts that were generated were predominantly tonalitic and granodioritic, with only one clearly granitic composition. Assimilation of the more easterly Sierran country rocks by intruding mantle-derived mafic magmas can produce metaluminous granites similar to those found in the high Sierras. However, much of the data are not as clear-cut in that a presumed east-west difference in country rock composition resulted in a wide range of overlapping melt compositions; two of the six most silicic/alkalic melts were derived from sources rich in mafic metavolcanics, while two of the top three had no schist input. In these latter two 900°C runs, wet melting (8% added H2O) versus dehydration melting had little effect on the melt composition or melt fraction (30-35 vol.%), although in other runs dehydration melting produced very little melt. Delineation of protolith components by Sr isotopes may indicate deep crustal or mantle metasomatism rather than involvement of distinctly different crustal source rocks.