The petrology and geochemistry of Precaldera Magmas, Long Valley Caldera, Eastern California

Abstract

Precaldera volcanism between 3.2-2.6 M.a. produced a basalt -trachybasalt -trachyandesite -quartz latite suite peripheral to the present Long Valley caldera from a heterogeneous, interactive, deep crustal magmatic -system. The suite consists of ( 1) widespread, predominately porphyritic olivine-augite basalt / trachybasalt / trachyandesite flow sequences (> 24 km³), (2) local orthopyroxene -phyric silicic trachyandesite flows (> I km³), and (3) sparsely -phyric orthopyroxene -hornblende -plagioclase quartz latite dome-flows and coarsely -phyric biotite -hornblende -plagioclase quartz latite dome-flows ( > 4 km³) erupted in that general sequence. Field, petrographic, and major-, minor-, and trace-element, as well as Sr isotopic studies of representative precaldera lavas on the northwest periphery of the caldera suggest that: (I) the basaltic magmas were generated from a lherzolite partial melt modified by minor crystal fractionation (limited fractionation due to their high incompatible element content) and contamination by older sialic rocks or their derivatives (represented by granitic inclusions, quartz xenocrysts, and progressively higher ⁸⁷Sr/⁸⁶Sr, 0.7062 to 0.7067), (2) the silicic trachyandesite was probably the result of intimate mixing of basaltic and quartz latite magmas (reflected in compositional gaps in progressively more silicic bulk compositional trends and the similarity of the quartz latite and silicic trachyandesite initial ⁸⁷Sr/⁸⁶Sr ratios, 0.7070-0.7074), and (3) the quartz latite was derived by crustal melting at different depths (as reflected in the variable ⁸⁷Sr/⁸⁶Sr, 0.7072-0.7095) and underwent radically changing crystallization conditions and contamination by trachyandesite (represented by heterogeneous mineral assemblages, chemistry, and textures indicating changing equilibrium conditions most evident in the trachyandesite enclave-rich quartz latite). The basaltic magmas provided the heat and mass to the crust promoting partial melting and generation of quartz latitic magmas. Synchronous basaltic intrusion and generation of crustal melts interacted and hybridized to yield trachyandesite. The isolated occurrence of trachyandesite enclaves in the youngest quartz latite dome-flows, suggests the disruption of a quartz latite-trachyandesite interface during late stages of the eruptive drawdown of a small volume magmatic system. Heat from continued basaltic input and coalesence of initially separate quartz latite bodies could possibly have resulted in development of the larger silicic magma chamber from which the younger rhyolitic (Glass Mountain-Bishop Tuft) magmas erupted.