T115 Source to Surface Magma Thermodynamics and Transport: Interdisciplinary Approaches to Documenting the How, Where, and When of Magma Generation and Evolution during Ascent, Storage, and Eruption II

Monday, 10 October 2022
1:30 PM - 5:30 PM
502

INVESTIGATION OF AZTEC WASH PLUTON: USING 87SR/86SR AS A TOOL TO UNDERSTAND MUSH ZONE DIFFERENTIATION

Paper number: T115-7
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

How do magmas differentiate in the upper crust? Geochronologic evidence suggests that silicic plutonic rocks cool rapidly from 850 to 550°C but then reside at 550°C to 330°C for 10+ Myrs [1], far below the haplogranite solidus. The lack of time above 550 but extended time below 550 is consistent with proposals that significant differentiation may occur by reaction between pre-existing mafic rocks and low-temperature melt (LTM) [2]. This work showed that a Na-silicate liquid with 38wt% H2O coexisted with quartz and albite from 550°C to 330°C. If intruding sills continuously underplate previous ones, the needed heat and LTM for this mush zone reaction process would exist [3].

The Aztec Wash pluton (AWP), located in Southern Nevada, is a part of the Colorado River extensional corridor. The AWP contains two predominant zones, an overlying homogeneous granite composed primarily of a single composition granite and a heterogeneous section consisting of repeated sequences of monzodiorite to monzonite. Here we examine sequences of monzodiorite to monzonite to seek evidence for this melt-rock reaction process; namely, do isotopic signatures of minerals in the monzonite suggest its pre-existing life as monzodiorite?

This study will present whole-rock (WR) solution-based and plagioclase laser-ablation (LA) MCICPMS analyses of 87Sr/86Sr along a 10-meter transect of monzodiorite and monzonite rocks within a well characterized petrographic and x-ray mapping analysis of the contact. WR 87Sr/86Sr values show an abrupt change in isotopic signatures across the contact: monzodiorite ranges 0.7089 – 0.7091, while monzonite 0.7099-0.7100. These isotopic differences would traditionally suggest two-distinct magma sources. However, micro-analysis of LA plagioclase and micro-drilled plagioclases in the monzonite have 87Sr/86Sr plagioclase values of 0.7096-0.7089, far below the WR value, with some plagioclase matching the whole values of the monzodiorite. These data are consistent with the hypothesis that the monzonite formed from a precursor monzodiorite by the proposed melt-rock reaction process.

[1] Davis et al, 2011

[2] Lundstrom, JGR: Solid Earth, 125, 2020

[3] Lundstrom, IGR 58, 371, 2016

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379598

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