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. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average δ¹⁸O value of 7.4±0.6‰ (all values±1σ). Four Barcroft specimens average ε_Nd=?3.6±1.8, ⁸⁷Sr/⁸⁶Sr=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 δ¹⁸O of 8.6±0.5‰. Four McAfee-type samples average ε_Nd=?7.8±1.7, ⁸⁷Sr/⁸⁶Sr=0.711±0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5–10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low δ¹⁸O in the sequence quartz>plagioclase>K-feldspar>>amphibole≥biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in δ¹⁸O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk ¹⁸O/¹⁶O 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 °C. Quartz–plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 °C. A quartz–biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 °C, 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 ~70 million years later by the McAfee Creek Granite had no discernable effect on δ¹⁸O values of Barcroft minerals and bulk rocks.