ICP-MS DATING OF ZIRCON IN THE IRON MOUNTAIN PENDANT: IMPLICATIONS FOR SIERRAN TECTONICS

The Mesozoic Sierran arc and Phanerozoic host rocks in California have undergone a number of tectonic events that have been preserved in metamorphic host rock pendants throughout the region. Members of the USC 2009 undergraduate team research program have studied the Iron Mountain pendant (SW of the Tuolumne Batholith) to establish ages of the different rock units and to clarify which tectonic blocks are preserved in the pendant. The Iron Mountain pendant is composed of a mix of metavolcanic, volcaniclastic, metasedimentary, and hypabyssal plutonic rocks. ICP-MS, U/Pb zircon dating has established that an overlying meta-andesitic to rhyolitic units are approximately 96-103 m.y., one package of marine metatsedimentary phyllites, schists, and quartzites have a minimum age peak of about 140 m.y. (thus the units are likely Jurassic), and an older package of mature quartzites, and phyllites with Precambrian zircon populations with peaks at 1100, 1400, 1770, and 2700 m.y. that closely resemble miogeoclinal rocks elsewhere. We interpret these data to indicate that this pendant exposes part of the miogeoclinal Snow Lake block and a Jurassic marine overlap sequence, both of which are overlain by Cretaceous continental margin arc volcanics. Ductile shear zones occur in the area and may separate the Jurassic overlap from the miogeoclinal rocks. These observations add support to hypotheses that the Snow Lake Block extends throughout the central and southern Sierra Nevada and that a Jurassic marine overlap sequence is also present throughout the central and southern Sierras as well. To our knowledge the overlying Cretaceous volcanics in this pendant, part of a belt of Cretaceous volcanics locally preserved along the central axis of the Sierra, represent the westernmost exposure yet recognized to date. The coarse clast size in some volcanic layers, map pattern, and intrusion by a hypabyssal pluton suggests that these may be near or remnants of a Cretaceous caldera complex.