T14 Different Personalities of Granites and Rhyolites: Silicic Magmas from the Lower Crust to the Surface I—A Session in Honor of Calvin Miller, Recipient of the MGPV Distinguished Geological Career Award

Sunday, 4 November 2018
8:00 AM - 12:00 PM
Room 240-241

TEMPORAL RECORD OF VOLCANISM AND PLUTONISM IN THE MIOCENE SEARCHLIGHT MAGMATIC SYSTEM (NEVADA, USA)

Paper number: T14-4
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

Eruptions of high-silica rhyolite are explosive, violent, and potentially hazardous events. However, questions remain about the processes and timescales responsible for generating and storing these magmas. One popular model for their generation is separation of high-silica melt from interstices in crystal-rich upper-crustal magmatic systems. This model suggests a link between high-silica rhyolite (volcanic) and granitoid (plutonic) rocks, with the latter representing the uneruptible, crystal-rich magma from which the rhyolite was extracted. An essential test of this model is to link erupted rhyolite with a fossil magma reservoir (i.e,. pluton). This has been done in areas where exposure of plutonic rocks is limited. However, localities where an entire magmatic system is exposed are rare.

We present preliminary chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology from the Miocene Searchlight Magmatic System (Nevada, USA). Tilting during regional extension has exposed a 13 km crustal section containing plutonic and volcanic rocks. These rocks are thought to represent a single magmatic system and they have previously been used to model differentiation, melt extraction, and eruption. Our geochronology builds on existing geochemical data and detailed geologic maps to produce a high-resolution temporal record of magmatism in this system. In contrast to models that have treated the Searchlight Pluton as a single, differentiated magma reservoir, our results suggest that it was built as a series of sills with variable thickness over a duration >1 Myr. Individual sills have zircon dispersion of ca. 100-200 kyr, which we interpret to represent in situ crystallization. The largest sill (~2.5 km in thickness) appears to be geochemically stratified with high-silica granite near its roof. However, despite this apparent differentiation, preliminary U-Pb zircon geochronology from the overlying rhyolites has not identified an eruption that temporally corresponds to this intrusive unit. We use our full dataset to discuss the relationship between volcanic and intrusive rocks in the Searchlight Magmatic System and the role, if any, that upper crustal differentiation played in generating high-silica rhyolite.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-324975

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