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