Structural, uranium-thorium-lead geochronologic, thermochronologic, thermobarometric, and geochemical studies of the Chinese Altai Ranges: Implications for the phanerozoic evolution of the Central Asia Orogenic System.

摘要

The Central Asia Orogenic System (CAOS) is a vast accretionary province that composes a major fraction of the continental crust of Asia. The tectonic processes responsible for concentrating the materials in this once-oceanic, juvenile province may reveal new paradigms about how our continental crust is generated and assembled. I explored these tectonic processes by conducting field mapping, 40Ar/39Ar thermochronology, thermobarometry, and Th/Pb monazite dating across the southern segment of the Chinese Altai Range. In this region, the tectonic framework is defined by three northeast-dipping thrusts. The southernmost thrust (Ertix thrust) places high-grade gneiss over Carboniferous volcaniclastic strata and its hanging-wall records Permian metamorphism and subsequent cooling. To the north, the Kuerti thrust exposes an orthogneiss unit and records late Permian to Triassic cooling in its hanging-wall. Farthest north, schists in the Barils hanging-wall record lower amphibolite facies metamorphic conditions (∼4.2 kbar and 500°C). In-situ monazite Th/Pb geochronology indicates this occurred in the Triassic (∼230 Ma), and 40Ar/39Ar data for the same hanging-wall reveals Jurassic biotite ages of ∼180 Ma. The spatial distribution of these data suggests that the timing of thrust-related deformation and exhumation is younger farther north within the range and indicates out-of-sequence thrusting. Additionally, U-Pb zircon geochronology reveals a rich magmatic history during the assembly of central Asia. South of the Ertix thrust fault, granites exposed along the Junggar basin have Carboniferous ages (∼320 Ma and ∼365 Ma). North of the Ertix thrust, a Paleozoic to Mesozoic magmatic history is recorded in Ordovician (ca. 450 Ma) gneisses, Devonian (ca. 415--380 Ma) foliated plutons, and relatively undeformed Permian (∼280 Ma) plutons. In addition, two large plutons in the northeastern-most portion of the field area record complicated zircon populations with components as young as Triassic and are interpreted as the latest stage of magmatic activity associated with the tectonic development of central Asia. Geochemical results are consistent with interpreting these granitoids as subduction-related magmatism. The relative age of deformed and undeformed granitoids indicates most ductile deformation occurred before the Permian, but the spatial variation in the data confirms a northward migration of deformation throughout the late Paleozoic to Mesozoic.