Petrochemistry of granitic rocks in the Mount Barcroft area; implications for arc evolution, central White Mountains, easternmost California.

Abstract

The north-northwest-trending White-Inyo Range locally defines the western edge of the Great Basin. The northeast-trending Barcroft structural break lies astride the province boundary. Along this preintrusive, high-angle reverse fault, middle Mesozoic White Mountain Peak alkaline volcanic and intercalated volcaniclastic rocks on the north are separated from uppermost Proterozoic--Lower Cambrian miogeoclinal quartzite and carbonate strata on the south by the 165 + or - 1 Ma (SHRIMP-RG, [sensitive, high-resolution ion microprobe--reverse geometry], U-Pb zircon) Barcroft pluton. Although locally faulted, the subparallel southeast and northwest borders of the body display intrusive contacts. Finer-grained comagmatic metadiorite occurs as early-stage dikes in the wall rocks. Eastward, the 100 Ma McAfee Creek granite intrudes the Barcroft pluton. Tertiary diabase dikes crosscut the section. The two granitic series, reflecting local evolution of the Mesozoic arc system, are described in this paper: (1) mafic granitoid rocks of the calc-alkaline Barcroft series, including chemically intergradational granodiorite, gabbro/diorite, metadiorite, and rare alaskite-aplite-all rich in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and (2) granite of the felsic McAfee Creek series, which has even greater enrichment of LILEs and greater depletion in HFSEs than the Barcroft pluton. Rocks rich in hornblende+ or -clinopyroxene and belonging to the Ca-rich, metaluminous Barcroft series exhibit a broad range of chemical and mineral compositions and represent products of both mixing between high- and low-silica members of the series to generate intermediate compositions and fractional crystallization to generate the most mafic (cumulate) rocks. The younger, more homogeneous, K-rich McAfee Creek-type muscovite granite possesses mildly peraluminous, minimum-melt compositions. Field evidence for magma mixing and isotopic data for the Barcroft series (epsilon (sub Nd(t)) = -1.53 to -5.50, (super 87) Sr/ (super 86) Sr (sub (i)) = 0.7053-0.7063) and the McAfee Creek granite (epsilon (sub Nd(t)) = -5.64 to -9.76, (super 87) Sr/ (super 86) Sr (sub (i)) = 0.7062-0.7116) require open-system processes and involvement of preexisting crustal rocks in their genesis. The Barcroft series is one of several shallow-level magma systems now recognized in the Sierran arc that include significant amounts of reworked, preexisting crustal material. These magma series are distinct from those that represent demonstrably deeper levels of Mesozoic magma systems and lack isotopic evidence for significant involvement of older crust. Despite these differences in magma sources and evolution, both magma series evolved along indistinguishable petrologic and chemical trends. Igneous rocks in the White Mountains record post-Paleozoic growth of this part of the Californian margin: (1) Jurassic and younger subduction, partial fusion of the overlying mantle wedge, and/or deep-seated mafic crust, ascent of mildly alkaline and later calc-alkaline magmas attending Andean-style volcanic-plutonic arc production, with significant involvement of preexisting crustal rocks; (2) Late Cretaceous thickening, heating, and crustal contamination, followed by rise of peraluminous granite-minimum melts; and (3) Neogene Basin and Range lithospheric transtension, tapping of upper-mantle diabasic melt, and dike emplacement.

37 samples from this reference