Deciphering the source and contamination history of peraluminous magmas using delta(18)0 of accessory minerals: examples from garnet-bearing plutons of the Sierra Nevada batholith

Peraluminous granitoids provide critical insight as to the amount and kinds of supracrustal material recycled in the central Sierra Nevada batholith, California. Major element concentrations indicate Sierran peraluminous granitoids are high-SiO2 (68.9-76.9) and slightly peraluminous (average molar Al2O3/(CaO + Na2O + K2O)=1.06). Both major and trace element trends mimic those of other high-silica Sierran plutons. Garnet (Grt) in the peraluminous plutons is almandine-spessartine-rich and of magmatic origin. Low grossular contents are consistent with shallow (< 4 kbar) depths of garnet crystallization. Metasediments of the Kings Sequence commonly occur as wallrocks associated with the plutons, including biotite schists that are highly peraluminous (A/CNK=2.25) and have high whole rock (WR) delta O-18 values (9.6-21.8 parts per thousand, average=14.5 +/- 2.9 parts per thousand, n=26). Ultramafic wallrocks of the Kings-Kaweah ophiolite have lower average delta O-18 (7.1 +/- 1.3\parts per thousand, n=9). The delta O-18(WR) of the Kings Sequence is variable from west to east. Higher delta O-18 values occur in the west, where quartz in schists is derived from marine chert; values decrease eastward as the proportion of quartz from igneous and metamorphic sources increases. Peraluminous plutons have high delta O-18(WR) values (9.5-13 parts per thousand) consistent with supracrustal enrichment of their sources. However, relatively low initial Sr-87/Sr-86 values (0.705-0.708) indicate that the supracrustal component in the source of peraluminous magmas was dominantly altered ocean crust and/or greywacke. Also, plutons lack or have very low abundances (< 1% of grains) of inherited zircon (Zrc) cores. Average delta O-18(Zrc) is 7.9 parts per thousand in peraluminous plutons, a higher value than in coeval metaluminous plutons (6-7 parts per thousand). Diorites associated with peraluminous plutons also have high delta O-18(Zrc), 7.4-8.3 parts per thousand, which is consistent with the diorites being derived from a similar source. Magmatic garnet has variable delta O-18 (6.6-10.5 parts per thousand, avg.=7.9 parts per thousand) due to complex contamination and crystallization histories, evidenced by multiple garnet populations in some rocks. Comparison of delta O-18(Zrc) and delta O-18(Grt) commonly reveals disequilibrium, which documents evolving magma composition. Minor (5-7%) contamination by high delta O-18 wallrocks occurred in the middle and upper crust in some cases, although low delta O-18 wallrock may have been a contaminant in one case. Overall, oxygen isotope analysis of minerals having slow oxygen diffusion and different times of crystallization (e.g., zircon and garnet), together with detailed textural analysis, can be used to monitor assimilation in peraluminous magmas. Moreover, oxygen isotope studies are a valuable way to identify magmatic versus xenocrystic minerals in igneous rocks.