MAFIC ROCKS OF HALL MOUNTAIN, CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA

Small bodies of mafic rocks found throughout the Sierra Nevada Batholith (CA) represent one of the least studied aspects of its magmatic history. Some mafic bodies are unequivocally late injections into granitic magmas; other mafic units may represent the heat source for anatexis, or the primitive parent magma of granitic plutons. At Hall Mountain, seven 0.25km² gabbro bodies are surrounded and cross cut by the Cretaceous Shaver Lake Intrusive Suite (SLIS). On average, the gabbros contain 49.7 ± 4.7 wt % SiO₂, 5.3 ± 0.9 wt % MgO and 12.2 ± 3.0 wt % Fe₂O₃. These bodies are high-K, alkaline rocks characteristically depleted in Ta, Zr, Nb, and Hf compared to MORB. Incompatible trace element ratios suggest that the Hall Mountain gabbros are not related to felsic members of the SLIS or the source of abundant mafic enclaves found within the adjacent Dinkey Creek granodiorite. The Hall Mountain gabbros are LREE enriched (10-60x chondrite), have no Eu-anomalies or HREE depletion, and have an average La/Yb(n) = 4-6. Major element geochemistry is not consistent with primary arc magmas, and suggests these bodies may have formed by melting of mafic cumulates in the lower crust. Whole rock oxygen isotope ratios (7.2-8.3 ‰, n=10) are higher than mantle values, indicating a supracrustal component in the source region. Variation between the seven bodies can be modeled by assimilation fractional-crystallization in the mid-crust (2-4 kbar deeper than final emplacement of the SLIS). The geochemistry of the Hall Mountain gabbros implies that these bodies may represent evolved remnants of the late Jurassic arc.