V41D-04
Zircon Geochemical and Isotopic Constraints on the Evolution of the Mount Givens Pluton, Central Sierra Nevada Batholith
Thursday, 17 December 2015: 08:45
308 (Moscone South)
Callie Sendek1, Jade Star Lackey2, Jonathan S Miller1, Gareth R Davies3, John W Valley4 and Kouki Kitajima5, (1)San Jose State University, San Jose, CA, United States, (2)Pomona College, Claremont, CA, United States, (3)Free University of Amsterdam, Amsterdam, Netherlands, (4)University of Wisconsin Madison, WiscSIMS, Dept. of Geoscience, Madison, WI, United States, (5)University of Wisconsin, Madison, WI, United States
Abstract:
The Late Cretaceous Mt. Givens pluton (central Sierra Nevada batholith, CA) is noteworthy for its large size (≈1400 km2) and relative compositional and textural homogeneity. It has been proposed as a plutonic analog for "monotonous intermediate" ignimbrites. The pluton is characterized by a 30 km wide ellipse shaped northern lobe that connects with a long mass about 15 km wide and extending 50 km SE. The northern lobe was constructed over 7 m.y. (from 98 to 91 Ma) with progressively younger ages toward the interior. This inward younging is accompanied by transitions to more felsic compositions and from equigranular to K-spar porphyritic textures. The large elongated mass extending to the SE (ca. 95-91 Ma) is more homogeneous, mostly equigranular granodiorite with subordinate K-spar-phyric granodiorite. Small diorite intrusions (10's to 100's m2) are also present and locally mingle and hybridize with the host granodiorite. Unlike other Late Cretaceous zoned intrusions (the Sierra Crest intrusions), the equigranular, and K-spar porphyritic phases of the Givens have similar trace element characteristics. All zircons have high Ti-in-zircon model temperatures (850-1000 °C), pronounced negative Eu anomalies, and curved MREE and HREE patterns. These characteristics indicate that zircon grew early and that initial magmas were likely undersaturated in zircon. Significant within sample variations in δ18O (up to 1.5‰) and eHf (up to 8 units) require mixing of isotopically distinct magmas in the Givens magma system after they had begun crystallizing zircon, but well before solidification. O and Hf isotopic variation within the granodiorites shows distinct geographic variation, with higher δ18O and more negative eHf values along the western margin of the pluton. This trend is consistent with earlier work suggesting that the Givens intruded across the Panthalassan-North American lithospheric boundary.
