Oxygen isotopic study of late mesozoic cooling of the mount Barcroft Area, central White Mountains, Eastern California

The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulkrock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average 6180 value of 7.4+/-0.6parts per thousand (all values+/-1sigma). Four Barcroft specimens average epsilon(Nd)=-3.6+/-1.8, Sr-87/Sr-86=0.707+/-0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite-aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average delta(18)O of 8.6+/-05parts per thousand. Four McAfee-type samples average epsilon(Nd) = -7.8+/-1.71, Sr-87/Sr-86=0.711+/-0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting similar to5-10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low delta(18)O in the sequence quartz > plagioclase > K-feldspar much greater than amphibolegreater than or equal tobiotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in delta(18)O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk O-18/O-16 ratios of Barcroft border rocks (and constituent plagioclase + subsolidus tremolite-actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz-amphibole and six quartz-biotite pairs indicate apparent subsolidus temperatures averaging 519 +/- 49 degreesC. Quartz-plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 degreesC. A quartz-biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 degreesC, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz-plagioclase, quartz-amphibole, and quartz-biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion similar to70 million years later by the McAfee Creek Granite had no discernable effect on delta(18)O values of Barcroft minerals and bulk rocks.