2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 13
Presentation Time: 11:15 AM


LACKEY, Jade Star1, VALLEY, John W.1 and SALEEBY, Jason2, (1)Department of Geology and Geophysics, Univ of Wisconsin, Madison, WI 53706, (2)California Institute Technology, Pasadena, CA 91125-0001, jadestar@geology.wisc.edu

In the southern Sierra Nevada batholith (SNB), exposures of high pressure (5-8 kbar) igneous and metamorphic rocks allow detailed study of magmatic processes that occurred deep within the Cretaceous Sierran magmatic arc. We present d18O data for zircons (Zrc) previously analyzed for U-Pb age from the Tehachapi Mts, and Lake Isabella areas in the southern SNB. Additional samples from representative plutonic rocks in the north and west foothills, and the central SNB were analyzed for comparison. Zrc is highly retentive of magmatic d18O, even in rocks that underwent intense metamorphism or alteration.

Values of d18O(Zrc) in the southern SNB are elevated (7.8±0.8‰, n=46) relative to the rest of the batholith (6.1±0.8‰, n=27), and are considerably higher than mantle d18O (Zrc) values (5.3±0.3‰). Comparison of d18O(Zrc) to d18O(WR) shows that some of the d18O(WR) values were changed by hydrothermal alteration.

Both the >103 Ma Tehachapi gneiss complex (TGC) and the <103 Ma Bear Valley Springs (BVS) intrusive suite were previously reported to have mostly mantle-like Sri (< 0.705) and eNd (>0) values (Pickett and Saleeby, CMP, 118, 1994). Average d18O(Zrc) values are nearly identical in TGC (7.4±0.6) and BVS rocks (7.5±0.4‰).

The combination of the high-d18O with low Sri and high eNd values, especially in BVS rocks is best explained by contamination of mantle derived magmas with hydrothermally altered, high-d18O ophiolitic or volcanic rock, or by fluids that exchanged with such rocks. Kings Sequence metasedimentary rocks in the area tend to have high Sri values (>0.710) and low eNd values (< –6), thus BVS rocks appear to have been contaminated by a high-d18O material before emplacement at their current crustal levels. Some BVS rocks, e.g., the granodiorite of Claraville, have elevated Sri suggesting local contamination of BVS magmas (up to 19 vol. %) by Kings Sequence metasediments.

In sum, high-d18O material was added to BVS magmas at depth. This is consistent with xenoliths studied in the central SNB that confirm high-d18O supracrustal rocks at depth, (Ducea, Ph.D., 1998). The size of the BVS suite (>1000km 2) also indicates that large amounts of this material were incorporated. Collectively, these data indicate that large amounts of high-d18O material were added to magmas at depth throughout the Sierran arc.