SPATIAL AND TEMPORAL CHANGES IN SOURCES OF MESOZOIC ARC PLUTONS IN THE WHITE-INYO RANGE, CALIFORNIA, USA

The White-Inyo Range (WIR) comprises the White Mountains in the north and the Inyo Range in the south. The WIR is a Mesozoic magmatic arc in east-central California. Calc-alkaline plutons in the WIR were emplaced into metamorphosed Proterozoic and Paleozoic sedimentary units during two major pulses, first in the Jurassic and second in the Cretaceous. These pulses overlap with the two latest pulses of magmatism in the adjacent Sierra Nevada batholith (SNB). This study utilized Sr, Nd, and Pb isotope systems to understand changes in WIR magma source and to interpret the tectonic evolution of the underlying lithosphere.

Whole rock ƐNd_i values of Jurassic plutons range from –0.15 to –8.32, ⁸⁷Sr/⁸⁶Sr_i range from 0.7045 to 0.7074, and ²⁰⁶Pb/²⁰⁴Pb_i range from 18.55 to 19.26. These values reflect a relatively juvenile lithosphere source with respect the the North American continent. The Sr, Nd, and Pb isotopic data of Cretaceous plutons fall into two ranges of values, and the plutons represented by these values are spatially separated. Cretaceous plutons of the Inyo Range have ƐNd_i values ranging from –18.6 to –18.8, ⁸⁷Sr/⁸⁶Sr_i from 0.7116 to 0.7122, and ²⁰⁶Pb/²⁰⁴Pb_i from 17.59 to 17.70. The Sr, Nd, and Pb isotopic values of these plutons are atypical of the SNB region, in that they reflect an older, Paleozoic, crustal source. Contrastingly, Cretaceous plutons of the White Mountains have ƐNd_i values that range from –5.76 to –9.19 and ²⁰⁶Pb/²⁰⁴Pb_i that range from 18.52 to 19.11, reflecting a similar source to the Jurassic plutons throughout the WIR. Differences in ƐNd_i values of the Cretaceous plutons are interpreted to reflect a spatial variation in source rather than temporal because Cretaceous magmatism throughout the WIR occurred over a relatively short time from 97 to 82 Ma with respect to Mesozoic magmatism in the WIR and surrounding North American margin.

We propose the Jurassic plutons and Cretaceous plutons of the White Mountains represent a juvenile lower lithospheric source, similar to that of the SNB. Cretaceous plutons of the Inyo Range, however, resulted from melting of upper crustal sedimentary units, as supported by ƐNd_i values of –18.0 to –20.9 of those sediments. Upper crustal melting may be due to lithospheric thinning below the southern WIR between the Jurassic and Cretaceous magma pulses.