TIMING AND CRUSTAL INFLUENCES OF THE LAKE COMBIE VOLCANIC COMPLEX, NORTHWESTERN SIERRA NEVADA FOOTHILLS

The Central Belt of the Sierra Nevada foothills is a Triassic- to Jurassic-aged, fault-bounded belt formed by collision, arc accretion, and subduction. The polarity of subduction is debated due to the lack of age and isotopic data at critical locations. Some reconstructions suggest an ocean basin closure resulting from one long-lived or numerous short-lived west-dipping subduction zones, while others point to sparse evidence for west-dipping subduction in the Central Belt. The Lake Combie Volcanic Complex in the northern Central Belt is a crucial target for resolving controversy due to its position immediately east of the late Jurassic Smartville arc, south of the Late Triassic – Early Jurassic Slate Creek arc, and west of the Calaveras Complex mélange and respective correlations with these terranes.

The Lake Combie Complex hosts abundant mafic and intermediate volcanic flows, volcaniclastic breccias, chert-rich epiclastic rocks, and intrusive bodies of varying composition and is unlike rocks of the Smartville Complex to the west. U-Pb and Lu-Hf isotope analyses of igneous and detrital zircon on a range of rock types by Laser Ablation Split-Stream will provide insight to address the conflicting correlations and models.

The variety of rock types in the Lake Combie Complex provide ample avenues to address the various tectonic reconstructions of the western margin of north America. A recent detrital zircon study in the Smartville complex shows two Mesozoic sub-populations: ~159 – 185 Ma, and ~198 – 217 Ma, with the younger population having a wide range of Hf isotope values, from very juvenile to evolved. The diverse εHf_(i) values over the 158-185 age range may reflect the existence of more than one coeval arc providing source material to local basins or variable crustal contribution to magmas in the same system. Zircons from the Lake Combie Complex could prove to be a contributor to these populations or may record distinct ages of isolated arc volcanism. Zircon Lu-Hf isotope data from the Lake Combie Complex will help determine the source of potentially inherited zircon cores and test whether the LCC was erupted through evolved continental crust in its current position or primitive oceanic crust as an island arc related to an ancient subduction zone.