Publication Date

Fall 2016

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Jonathan Miller

Keywords

Hafnium Isotopes, Mount Givens Pluton, Oxygen Isotopes, Sierra Nevada Batholith, Trace Elements, Zircon

Subject Areas

Geology; Geochemistry; Petrology

Abstract

The Late Cretaceous Mount Givens pluton, located in the central Sierra Nevada batholith, is noteworthy for its large size (~1400 km2) and relative compositional and textural homogeneity. The pluton is characterized by a 30 km wide, ellipse-shaped northern lobe (ca. 98-91 Ma) that connects with a long mass (ca. 95-91 Ma) about 15 km wide and extends 50 km SE. Zircon trace elements and d18O and eHf from a sample suite representative of the observable petrologic diversity of the pluton document significant heterogeneities. All zircons have high Ti-in-zircon model temperatures (850-1000 °C), pronounced negative Eu anomalies, and concave down MREE and HREE patterns, indicating initial melts were undersaturated in zircon and that zircon growth occurred during ascent or emplacement of Mount Givens magmas. Zircon trace elements record appreciable variation, and suggest that crystal transfer was ongoing throughout pluton growth. Significant variations in O and Hf isotopes require mixing of multiple, isotopically distinct sources. Longitudinal variation in both systems suggests increasing mantle contributions in the eastern portion of the pluton. A clear decrease in isotopic variability at all scales in post-95 Ma samples indicates that later magmas were more thoroughly mixed prior to emplacement. This increase in zircon isotopic homogeneity paired with the narrow timespan of intrusion hints that the inner portion of the northern lobe may have existed as a large melt reservoir.

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