EVIDENCE FROM ZIRCON FOR HIGH-d18O CONTAMINATION OF MAGMAS IN THE DEEP SIERRA NEVADA BATHOLITH, CALIFORNIA

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 Sr_i (< 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 Sr_i 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 Sr_i 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 Sr_i 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.