30 Crystal Windows into Igneous Processes II

Thursday, 16 May 2019
1:30 PM - 3:30 PM
Room C124

A TALE OF CRYSTAL-MELT MIXING AND LARGE, PROLONGED MAGMA MUSH BODIES TOLD BY PLAGIOCLASE AND K-FELDSPAR MEGACRYSTS FROM THE INNER UNITS OF THE TUOLUMNE INTRUSIVE COMPLEX

Paper number: 30-5
Program: Theme Sessions
Time: 2:50 PM – 3:20 PM

Authors

Abstract

Large silicic plutonic systems such as the 1,100 km2, 95-85 Ma Tuolumne intrusive complex (TIC) typically have broad whole rock element and isotope signatures that overlap between units. This suggests similar magma sources and common magma histories, but the large data scatter also hints at the complexities preserved at the mineral scale. This study explores the presence of multiple feldspar populations and the degree to which populations are recycled between inner TIC units to determine degrees of hybridization and magma body size at the emplacement level.

K-feldspar megacrysts (Kfsm; <12cm) and groundmass Kfs and plagioclase (plag) from the equigranular and porphyritic Half Dome (eHD, pHD) and Cathedral Peak (CP) granodiorites/granites were analyzed for trace element concentrations (Ba, Sr, Ti, Rb, Ga, Ce, etc.). While euhedral plagioclase (plag) is a liquidus phase preserving an earlier magmatic history, euhedral to anhedral Kfs display rims that extend into the groundmass, providing information to the solidus. Three chemically distinct, oscillatory zoned plag populations occur in all three units. In the pHD, plag cores near the eHD have eHD signature and pHD and CP compositions near the CP boundary. All rim compositions converge to evolved CP. PHD groundmass Kfs indicate mostly mixing of eHD and CP melts. The larger pHD Kfsm show eHD, CP and mixed (pHD) compositions and are characterized by asymmetric oscillatory zoning with constant element maxima (e.g. Ba, Sr) from core to near the rims where elements drop off. The number of oscillations is proportional to the size of the crystal. CA-ID-TIMS U-Pb zircon geochronology from the core and rim of one Kfsm indicates pHD core ages and CP rim ages surrounded by an even younger groundmass.

These data suggest that 1) Kfsm are longer lived than groundmass Kfs and their longevity is proportional to their size, 2) the element oscillations likely relate to magma replenishment/mixing events, and 3) pHD phenocrysts were recycled into CP magma. Element distributions in Kfsm and plag suggest 1) the pHD unit is likely a mixture of fractionated eHD with primitive CP magma, and 2) plag and Kfsm were recycled from older into younger magmas. This implies the inner TIC units formed an active magma mush body undergoing replenishment, magma erosion, mineral mixing, and fractional crystallization.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329861

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.