DEATH THROES OF A SILICIC SYSTEM: FINAL RE-ACTIVATION OF GRANITIC CRYSTAL MUSH AND TRANSITION TO ANDESITIC VOLCANISM, HIGHLAND RANGE, SOUTHERN NEVADA

The Miocene volcanic section of the southern Highland Range (HRV) is interpreted to include erupted units derived from nearby Searchlight pluton (SLP). From 16.6 to 16.0 Ma, first dacite lavas and then rhyolite tuffs and lavas, similar in age and composition to SLP, erupted, followed by andesite lavas. The transition from rhyolite to andesite (termination of silicic magmatism) is marked by mingled intrusive and effusive rocks emplaced at ~16.0 Ma. Mingled rocks include (1) a porphyritic rhyolite plug (71% SiO₂); (2) an andesite dike (58% SiO₂) cross-cutting the silicic sequence and partly mixed and mingled with a smaller porphyritic body (72% SiO₂); and (3) a 25-50 m-thick mingled and partially mixed rhyolite + andesite lava (60-65% SiO₂). Andesite lava (58-61% SiO₂) that locally underlies the mingled lava is similar to the thick overlying andesite sequence.

Field and textural evidence and geochemical data suggest that the mixed rhyolite-andesite lava is likely a mixture of magmas that formed the silicic porphyries and andesite dike and lavas: sampled mafic enclaves from the lava geochemically resemble the andesite dike as well as capping andesite lava (58% SiO₂); large (to 1 cm), abundant sanidine, qtz, and plag phenocrysts within the lava are identical to those in the rhyolite plug, and sphene is present in both. Mixing variable proportions of crystal-rich rhyolite with andesite would generate the compositional range of the lava, and all units are essentially identical in age at 16.0 Ma (Colombini, 2009; Faulds et al, 2002).

Abundant large phenocrysts in the porphyries, portions of the andesite dike, and the capping mingled lava likely were products of prolonged crystallization of highly evolved magma within SLP during its waning stages. This crystal-rich leucogranite mush was then reactivated by intrusion of hot andesite magma. Andesite had not previously erupted as lava, but had been entrained as small enclaves in earlier rhyolite (Kelly et al, this meeting). The solidifying SLP, which had previously formed a low-density, low-strength cap, was probably dense and rigid enough to permit intrusion by andesite at the stage marked by the mingled porphyry and lava. Thus, this rhyolite-to-andesite transition represents the final episode of HRV-SLP magmatism - the death rattle of a silicic system.