TECTONIC SETTING OF THE SMARTVILLE AND SLATE CREEK COMPLEXES, NORTHERN SIERRA NEVADA, CALIFORNIA: EVIDENCE FROM ZIRCON GEOCHRONOLOGY AND COMMON PB STUDIES

A key question in the Jurassic tectonic evolution of western North America is whether arc rocks, such as the Smartville and Slate Creek Complexes in the northern Sierra Nevada, were transported to the continental margin tectonically or were added through in situ igneous processes. Data from U-Pb zircon geochronology indicate:(1) The Slate Creek Complex was part of a larger magmatic arc built upon older, disrupted ocean crust between ca. 207 Ma, the age of the oldest plutons we have dated, and ca. 171 Ma, the age of the Lexington Hill volcanics, the uppermost volcanic unit in the complex. (2) The Smartville Complex was built by arc volcanism and plutonism between ca. 164 and 152 Ma. The presence within the Smartville of the deformed, 179±4 Ma tonalite of Deer Creek , whose age is similar to the youngest Slate Creek rocks and which is intruded by an undeformed ca. 159 Ma leucotonalitic dike, suggests that the Smartville was built upon older arc crust. (3) Whereas older Slate Creek plutons (ca. 207-200 Ma) show only slight evidence of inheritance of older components, a younger Slate Creek pluton (ca. 191 Ma), a Middle Jurassic (ca. 166 Ma) pluton intrusive into Slate Creek rocks, and plutons in the eastern part of the Smartville Complex all have zircon populations that indicate significant inheritance of a Precambrian component.Indeed, one ca.106 Ma dike intruding a Smartville pluton yielded rounded zircons with a 207Pb/206Pb age of 2153±1 Ma. The isotopic composition of common Pb in feldspars from both Slate Creek and Smartville rocks indicates that the source of the old zircon component either contained mantle-like Pb or did not contribute much Pb to the magmas.(4)The 157±3 Ma Yuba Rivers pluton, which also shows clear evidence of incorporation of Precambrian zircon components, intrudes the eastern edge of the Smartville Complex and the western edge of the Slate Creek basement, indicating that the plutonic phase of Smartville magmatism took place in situ We conclude that the Slate Creek and Smartville complexes were built over a period of about 65 Ma proximal to the continental margin. The presence of inherited Precambrian zircon components suggests that either Precambrian crystalline crust or, more likely, sedimentary accumulations such as the Early Paleozoic Shoo Fly formation, were present above or within the zone of magma formation and ascent.