LITHOSPHERIC STRUCTURE CONTROL ON CHANGES IN MAGMA SOURCE OVER TWO MAGMATIC PULSES IN THE WHITE–INYO RANGE, CA

Plutons of the White–Inyo Range (WIR) represent the easternmost extent of Mesozoic subduction magmatism in California. Over twenty recognized granitic to gabbroic plutons intruded during two magmatic events at ca. 180–167 Ma and 102–86 Ma into greenschist facies Neoproterozoic and Cambrian sedimentary units. This study utilized Sr, Nd, and Pb isotope systems to understand changes in magma source and to interpret the tectonic evolution of the underlying lithosphere.

Jurassic plutons of the WIR are characteristically amphibole-bearing gabbros to granites. Whole rock ƐNd_i values of Jurassic plutons reflect a relatively juvenile source with values ranging from –0.15 to –8.32, ⁸⁷Sr/⁸⁶Sr_i ranging from 0.7045 to 0.7074, and ²⁰⁶Pb/²⁰⁴Pb_i ranging from 18.55 to 19.26. Comparatively, Cretaceous plutons are dominantly aluminous, two-mica granitoids. Isotopic ratios for the Cretaceous plutons fall into two ranges, and the plutons are spatially distributed. In the southern WIR, the ƐNd_i values range from –18.6 to –20.1, ⁸⁷Sr/⁸⁶Sr_i from 0.7110 to 0.7122, and ²⁰⁶Pb/²⁰⁴Pb_i from 17.59 to 17.70. These ratios are atypical of the region in that they reflect an older, Paleozoic to Proterozoic crustal source. Contrastingly, Cretaceous plutons of the northern WIR have ƐNd_i values that range from –5.76 to –9.19, ⁸⁷Sr/⁸⁶Sr_i from 0.7071 to 0.7080, and ²⁰⁶Pb/²⁰⁴Pb_i from 18.52 to 19.11, reflecting a source like that for the Jurassic plutons.

We propose that the lower lithosphere below the WIR during the Jurassic was juvenile, but inhomogeneous in major and trace elements, perhaps due to underplating of compositionally heterogeneous material to the lithosphere. In contrast, the isotopic ratios of southern Cretaceous plutons suggest melting of upper-crustal sedimentary units with Proterozoic provenance. The sedimentary units in the WIR have ƐNd_i values of –18.0 to –20.9 that match those of the plutons. The transition to a sedimentary source for some plutons in the WIR between the Jurassic and Cretaceous eras suggests heating of the crust, perhaps due to thickening during tectonic compression.