Source variation for Mesozoic granitoid plutons in the White-Inyo Range, California, and implications for changes in the lithospheric structure

Abstract:

The White–Inyo Range (WIR) of eastern California lies to the east of Owen’s Valley and the Sierra Nevada batholith (SNB). Over twenty recognized Mesozoic, granitic to gabbroic plutons intruded into Precambrian and Cambrian sedimentary units that were previously metamorphosed to the greenschist facies. These plutons represent intrusive equivalents of volcanics erupted during two magmatic events at ca. 180–167 Ma and 102–86 Ma (Coleman et al, 2003). The motivation of this study is to understand changes in magma sources over this time period.

We propose there is a time-transgressive change in source material in the WIR. Preliminary trace element data suggest that Jurassic plutons were sourced from deep crustal material, whereas later Cretaceous plutons may have had a shallower origin. This is supported by high Sr/Y ratios in the Jurassic plutons (up to 300) and relatively lower ratios in Cretaceous plutons (down to 14). Chiaradia (2015) suggests elevated Sr/Y ratios are directly linked to increasing crustal thickness, therefore the WIR plutons may record the period of crustal thinning in this region or maturation of the crust that allowed shallower melting in the Cretaceous. The highest values also suggest possible adakitic source material. REE patterns suggest that the youngest plutons (e.g. Sage Hen Flat) are more similar to samples collected from the eastern Sierra Nevada batholith than older plutons with higher La/Yb ratios. Relatively flat patterns REE patterns of Tertiary basalts and a late diabase dike in the WIR are also similar to SNB samples.

Another goal of this study is to determine the degree of mixing as a function of time during the emplacement of plutons in the range. Field and petrographic analyses suggest that older plutons represent more mafic materials and younger plutons are more granitic with mafic enclaves. We propose either (1) older plutons represent initial lower lithosphere melting, and with time, there is an increase of incorporation of shallower lithospheric material, or (2) two source materials are present at all times with varying degrees of mixing; older plutons represent a greater degree of mixing whereas younger plutons are primarily unmixed. Whole rock isotopic data and amphibole, pyroxene, feldspar compositional data will be used to explore these hypotheses.