GEOCHEMICAL HETEROGENEITIES ALONG A PALEO-CONTINENTAL MARGIN: AN EXAMPLE FROM LATE CRETACEOUS INTRUSIONS, SANTA ROSA RANGE, NORTHWESTERN NEVADA

The abrupt juxtaposition of accreted oceanic terranes and the Precambrian North American continent is well-documented in western Idaho and eastern Oregon. However, immediately to the south, in present-day Nevada, this transition becomes more diffuse. New geochronologic and geochemical investigations of isolated granitoid intrusions in the Santa Rosa Range (SRR) of NW Nevada reveal that two pulses of magmatism in close spatial association were generated from geochemically distinct reservoirs during Late Cretaceous time. New U-Pb zircon geochronology via LA-ICP-MS yielded two groupings: the Santa Rosa/Andorno group (SRA) (Santa Rosa - 103.3 +/- 3.4Ma; Andorno - 102.1+/- 1.1Ma) and the Granite Peak/Sawtooth group (GPS) (Granite Peak - 94.7 +/- 1.4Ma; Sawtooth Stock - 94.5 +/- 2.2Ma). Petrographic, geochemical, and isotopic observations also indicate that two distinct granitoid compositional/textural groups exist within close spatial association within the SRR. Whereas, the SRA group is geochemically reminiscent of the Sierra Nevada Batholith, the younger GPS pulse is more reminiscent of magmatism in the Idaho Batholith and the in-land belt of peraluminous granites. Our data suggests that the SRR plutons shared similar magmatic histories to the larger batholithic systems and provide further evidence for the continuity of the Mesozoic magmatic arc through NW Nevada. Additionally, the elevated Sr_(i), low εNd_(t), and abundant inherited zircon cores with U-Pb ages >1Ga observed within the GPS group is consistent with the presence of ancient crustal materials/reservoirs, not previously documented, beneath the SRR region.

New whole-rock Pb isotope data also suggest complexities in the geochemical reservoirs underlying the SRR. The SRR granitoids have Pb isotope compositions that are more consistent with young subduction associated volcanic/sedimentary rocks (Pb 206/204 - 18.6 -19.6; 207/204 - 15.60 - 15.68; 208/204- 38.5 -38.9). The Granite Peak stock, however, yields a Pb isotope signature similar to Precambrian basement exposed to the east. Collectively, these observations emphasize the importance of the isolated intrusions in NW Nevada and their contribution to understanding the tectonomagmatic development of the Mesozoic continental margin.