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Geology

Magma Mixing in the Lake Edison Granodiorite, Sierra Nevada, California

Beeler, Katherine ('10);  Lackey, Jade Star

The Lake Edison Granodiorite is one of several Cretaceous granitoid plutons making up the John Muir Intrusive Suite in the central Sierra Nevada. This study explores variations within the Lake Edison pluton to gain insight into magma mixing processes. Previous work indicates that this pluton may represent a transitional mixture between the older Lamarck Granodiorite, which gives relatively low oxygen isotope (δ18O) values, and the younger, higher-δ18O Round Valley Peak Granodiorite (Lackey, et al., 2008). A total of thirteen new samples have been collected from both the margins and the core of the Lake Edison pluton. Observed mingling textures indicate a heterogenous magma system and help clarify relationships between the Lake Edison pluton and the surrounding rock units. Pending δ18O analyses for these new samples will allow testing of the mixing hypothesis and provide significant insight into the tempo and mode of magma system switching in convergent margin arcs.
Funding provided by: Pomona College SURP

Reexamining the Shaver Intrusive Suite of the Central Sierra Nevada

Frazer, Ryan ('09);  Lackey, Jade Star

The Shaver Intrusive Suite of the Central Sierra Nevada (CSN) includes a group of plutons collectively referred to as the granites of Dinkey Dome (Bateman, 1992). Bateman assigned this group to the lower Cretaceous with an approximate age of 102 million years (Ma). Recent U-Pb dating of zircon yields an approximate age of 120 Ma for one pluton in this suite (Lackey, unpub.). This new date opens the possibility that the ages of this entire suite may have formed during the onset of magmatism in the CSN, which would yield fresh insight into conditions in the nascent Sierran magmatic arc. New fieldwork was performed to examine this suite of plutons and their context within the CSN. Mapping of field relations between plutons revealed that few contacts show definite age relations within this suite;  to unambiguously determine these plutons’ ages, laser ablation U-Pb zircon dating of 10 plutons will be performed.
Funding provided by: Pomona College SURP

Determine Magma Sourcing and Assimilation Variability Within the Bass Lake Tonalite in the Western Sierra Foothills

Windham, Cameron ('10);  Lackey, Jade Star

Our study is comprised of 29 samples from the Bass Lake Tonalite (Kbl), a member of the Fine Gold Intrusive suite of early Cretaceous age (104-128 Ma) in the Western Sierra Foothills, a field area measuring roughly 2,200 square kilometers. Field relations show considerable textural and mineralogical variation throughout the pluton, especially in an east-to-west direction. Relationships between Magnetite (Fe3O4) and Hematite (Fe2O3) pertain to the oxidization conditions of coeval melts. Ultimately, this study seeks to refine our understanding of the mode of emplacement of large-scale intrusive bodies. Oxygen fractionation processes are recorded with great fidelity in magmatic zircon due to retention of δ18O originally sourced from the melt (Valley, 2003). A comparison between the values of δ18O recorded in these zircons and the δ18O values obtained through whole-rock geochemical analysis will provide meaningful insight into the sourcing and assimilation history of the intrusives with respect to their country rock, as a portion of the ongoing research.
Funding provided by: Pomona College SURP

Research at Pomona