TIMING AND CRUSTAL INFLUENCES OF EARLY AND LATE JURASSIC ARC MAGMATISM IN THE LAKE COMBIE COMPLEX, SIERRA NEVADA METAMORPHIC PROVINCE, CALIFORNIA

The Triassic-Jurassic-aged Central Belt within the Sierra Nevada metamorphic province includes various volcanic complexes, sedimentary assemblages, and plutonic suites. Several tectonic reconstruction models have been proposed that correlate the Central Belt magmatic complexes with proposed Triassic- Jurassic subduction at the margin of the Central Belt . Temporal and crustal linkages between several known arcs in the Central Belt have not been determined, and the polarity of subduction-related arc accretion remains uncertain. We provide new geochronologic and isotopic data from the Lake Combie Complex, an igneous complex in the northern Central Belt, to address the duration of arc magmatism, the proximity of the arcs to continental North America, and to refine existing tectonic models.

U-Pb zircon and Lu-Hf isotopic analyses of igneous and detrital zircon from the Lake Combie Complex were conducted using Laser-Ablation Plasma Mass Spectrometry. A 160 Ma pluton within the Lake Combie Complex has positive ε_Hf values ranging from 2.3 to 19.8, and a 198 Ma pluton has positive ε_Hf values clustering at about 15. Detrital data from coarse epiclastic rocks capping mafic volcanic rock show a maximum depositional age of 205 Ma with 83% Precambrian zircon grains.

The new data presented here indicate that the Lake Combie Complex preserves two distinct episodes of Jurassic magmatism with differing ε_Hf signatures that relate to proximal arc volcanic and plutonic rocks of both the Central Belt and the Western Belt of the metamorphic province. Detrital and igneous zircon results do not support continuous magmatism during the Jurassic. Jurassic igneous activity occurred proximal to the North American continent, as indicated by the influence of Precambrian zircon grains in detrital samples, but intruded juvenile, oceanic crust. These findings reduce the probability of reconstruction models that suggest a far-traveled origin of Mesozoic arc terranes in the Sierra Nevada metamorphic province and provide constraints to models with contrasting subduction polarity.