ZONING IN FELDSPARS AS TRACERS OF LENGTH SCALES OF MAGMA MIXING AND MAGMA CHAMBER SIZE IN THE TUOLUMNE INTRUSIVE COMPLEX, YOSEMITE NATIONAL PARK, CA

The size and longevity of magma chambers and degree of interaction between magma batches in long-lived, composite intrusions is poorly understood. This study focuses on characterizing K-feldspar and plagioclase populations from the equigranular and porphyritic Half Dome (eHD and pHD) and Cathedral Peak (CP) granodiorites from the SE section of the Tuolumne Intrusive Complex (TIC). The goal is to determine the degree of interunit mixing, amount of crystallinity in each unit during such mixing, and extent of the magma mush bodies during different stages of TIC evolution.

Feldspar zoning morphologies were identified using cathodoluminescence on thin sections from transects across major unit contacts, resulting in the ID of regionally four K-feldspar and at least three plagioclase populations, with some samples containing multiple populations. Selected K-feldspars (≤2 cm) and plagioclase (≤0.5 cm) were analyzed for major elements with an electron microprobe and trace elements (e.g. Ba, Sr, Ga, Y, etc.) with LA-ICP-MS, along core-to-rim transects.

Our results include the following: 1) eHD K-feldspars are often ≤1 cm and exhibit simple, normal trace element zoning; 2) K-feldspars in pHD and CP exhibit oscillatory trace element zoning and increase in number of high-Ba spikes with increased grain size. Furthermore, pHD K-feldspars are more inclusion-rich and ≤4 cm long, while CP K-feldspars are ≤12 cm and typically inclusion-poor with some inclusion-rich cores. 3) K-feldspar from along the eHD-pHD contact exhibit characteristic grains of both units, plus groundmass grains lower in trace elements. 4) pHD and CP both have one type of K-feldspar with similar trace element concentrations, but each also contain at least two different K-feldspar populations. 5) Plagioclase in all units are normally-zoned with minor oscillations in An content (An₆₆ to An₁₂) and Sr. 7) Higher concentrations of compatible elements in some larger plagioclase (e.g. Sr) and K-feldspar (e.g. Ba) phenocrysts compared to smaller feldspars suggest that mineral growth occurred during extended times of in situ magma differentiation.

In sum, inter- and intra-unit mineral recycling is common in the pHD and CP implying that at least the pHD was still “mushy” during CP emplacement and formed an extensive interconnected magma mush body at the interface of the two units.