Abstract

Monzonitic rocks are found in two plutons in the Deep Springs Valley area of the White-Inyo Range, California. These intrusive rocks, among the oldest in the Sierra Nevada region (170 m.y.), are characterized mineralogically by their abundance of potassium feldspar and paucity of quartz, and chemically by exceptionally high concentrations of alkalis (K2O + Na2O generally greater than 8% by weight) and strontium (greater than 1,000 ppm) and low silica (generally less than 60%). Both quartz-bearing and feldspathoidal varieties are present.

Modal and chemical data clearly show that these alkalic intrusive rocks are not petrogenetically compatible with the far more abundant quartz-rich, calc-alkalic rocks that characterize the Sierra Nevada batholith. Instead, the monzonitic rocks constitute part of a discrete petrogenetic unit that is in part contemporaneous but certainly not comagmatic with the Sierra Nevada batholith.

The available data are compatible with an origin for the Inyo monzonites during an early Mesozoic magmatic episode related to an arc-trench–subduction system that tapped an upper-mantle source beneath the western edge of the North American plate.

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