IGNITING THE SIERRAN ARC: EVOLVING OXYGEN AND HAFNIUM ISOTOPE RATIOS IN THE BASS LAKE TONALITE

Much recent insight into the construction of Cordilleran batholiths comes from detailed studies of the emplacement histories of individual plutons in the eastern Sierra Nevada Batholith (SNB), yet there is growing recognition of the importance of long-term orogenic cyclicity as a control of magmatism (DeCelles et al. 2009, Nat. Geosci.). Thus, greater whole-arc perspective is needed to evaluate the interplay of long-term variations in magma source and short-term controls of plutonic emplacement. To help bridge these viewpoints, by improving our understanding of magmatism in the nascent Sierran arc, we have conducted a regional study of the ~2000 km² Bass Lake tonalite in the west-central SNB.

Zircon (Zrc) from the BLT has been analyzed for δ¹⁸O by laser fluorination of bulk zircon concentrates. Single crystal U-Pb age and Hf isotope ratios are measured by MC-LA-ICP-MS. BLT zircons show magmatic zoning in CL, and U-Pb ages (103-121 Ma; n = 17) are largely concordant with minimal inheritance. Older (>112 Ma) rocks are found west of the Oakhurst roof pendant and in a domain near Yosemite; younger (103–110) sections of the BLT extend east and southeast of the Oakhurst pendant, substantially expanding the Oakhurst “pluton” of Bateman (1992, USGS Prof. Pap.). The temporal and spatial pattern of U-Pb ages demonstrates two major episodes of incremental emplacement of the BLT.

Values of δ¹⁸O(Zrc) range from 5.7–8.1‰ (n=30), spanning nearly the entire range of values in the SNB (Lackey et al. 2008 J. Pet.); yet, values of δ¹⁸O are less variable over time, and lower (<6.5‰) values are absent in younger rocks. Values of εHf range nearly 11 units (–6.9 to 3.9, n=16), and decrease and become less variability with time. Together, δ¹⁸O and εHf indicate binary mixing of young (Phanerozoic) altered ocean crust or arc sediments with relatively old mantle in older BLT; younger rocks, including intrusions adjacent to the older BLT, show higher crustal input into sources. Thus, magmatic sources transitioned from those with primitive, heterogeneous compositions to homogeneous supracrustal values after 110 Ma. These results suggest that the re-constitution of the arc crustal column occurred quickly compared to the overall lifespan of the arc, although subsequent tectonic modification of the arc may have rejuvenated magma sources.