HF-ISOTOPIC AND TRACE ELEMENT ANALYSIS OF ZIRCON FROM THE DEVONIAN BOWMAN LAKE BATHOLITH, NORTHERN SIERRA TERRANE, CA: IMPLICATIONS FOR SOURCE COMPOSITIONS

The Late Devonian Bowman Lake batholith (BLB) intrudes the lower three allochthons of the lower to middle Paleozoic, deep-marine Shoo Fly Complex (SFC) in the Northern Sierra terrane, CA; cogenetic hypabyssal rocks intrude the upper parts of the SFC and are inferred to be feeders to submarine island-arc rhyolitic to andesitic volcanic rocks of the Sierra Buttes Formation, which unconformably overlies the SFC. The entire package has been bulk rotated to the east. Ages for these units provide a context for construction of the Shoo Fly Complex and middle Paleozoic tectonism along the Laurentian margin. We measured Lu-Hf isotopes and trace element concentrations from different units of the composite BLB to investigate the likely tectonic influences on magma generation and as a possible source of Devonian to Carboniferous grains present in many Triassic and younger detrital sedimentary strata to the south along the Cordilleran margin.

Igneous facies in the BLB include trondhjemite (ca. 362 – 365 Ma, εHf avg. -12; -13), granite (ca. 365 - 367 Ma, εHf avg. -10; -20), granodiorite (ca. 367 Ma, εHf avg. -23), and tonalitic to gabbroic enclaves (367 Ma, εHf avg. -11). The εHf values indicate a moderate to strong contribution of continental materials to the source magma. Trace elements that can be used to assess continental- vs. oceanic-domain contributions include Yb, Nb, Ti, Hf, Th, and U and ratios of those elements. Zircon from the BLB has concentrations and ratios of U/Yb vs. Nb/Yb and U/Yb vs. Hf that suggest continental/fractionated source regions well out of MOR/OI realms. Th vs. U shows a broad range, however, including values close to those in OI settings. In comparison to other, more evolved settings these ratios show that BLB magmas had a mix of chemical characteristics in the source.

Our results support the hypothesis that although the BLB intruded in an overall oceanic-island setting, the thick (~15 km) SFC, which contains abundant continentally derived sandstone, contributed strongly to magma chemistry. As a deep-marine unit, however, the crust on which it was deposited was likely primitive, and this input contributed to tectonic indicators that more closely approach oceanic island settings. This contrasts with trace element values from similar-age zircon grains in Triassic strata to the south, which uniformly have highly evolved-magma signatures.