THE CENTRAL ASIAN OROGENIC SYSTEM: TIMING OF MAGMATISM, GEOCHEMISTRY, AND CONTINENTAL CRUSTAL GROWTH DURING THE PHANEROZOIC

While the Alpine orogen is responsible for the accretion of material to Eurasia during the Cenozoic, the Central Asian Orogenic System (CAOS) is the primary locus of material accreted to Eurasia during the Paleozoic. The CAOS is composed primarily of oceanic arc-derived materials, with granitoids typically displaying a positive ε_Nd. Because of the vast size (~8000 x 6000 km) and possible juvenile nature of the CAOS, its Phanerozoic construction may represent a significant addition to the continental crust and impact the suitability of global continental crustal growth models. Three end-member models aimed at describing the evolution of the CAOS differ in whether the bulk of the material separating Siberia and North China consists of: (1) a single, huge early Paleozoic island arc, (2) multiple Paleozoic island arcs and back-arc basin systems or (3) several fragments of Precambrian crust rifted from Gondwana. Various workers have also cited the influence of “anorogenic” magmatism in the region, typically considered Permian in age. Distinguishing between these models and understanding the origin of magmatic suites in the region has important implications for the rate of addition of material to the continental crust. We've undertaken a survey of U-Pb ion microprobe zircon ages for gneisses, foliated plutons, and unfoliated plutons in the Altai Mountains of NW China, the southernmost portion of the CAOS. Results from 10 samples are summarized below as the weighted mean ²⁰⁶Pb/ ²³⁸U ages from 4 to 12 zircon spot analyses, generally from separate grains. Four gneiss bodies have ages of 451 ± 14 Ma, 451 ± 23 Ma, 420 ± 17 Ma and 392 ± 17 Ma. Three foliated granites have ages of 402 ± 18 Ma, 448 ± 30 Ma, and 400 ± 12 Ma. Two unfoliated plutons and an unfoliated dike have ages of 278 ± 7 Ma, 286 ± 12 Ma and 278 ± 9 Ma. When combined with other data from the region, our results indicate two or three periods of magmatism: Ordovician (~450 Ma), Devonian (400-390 Ma) and Permian (~280 Ma). Major and trace element geochemistry indicates that plutons and/or gneisses from each of these time periods in the region are arc-related; the youngest, Permian plutons do not have typical “anorogenic” signatures such as high SiO₂ or Ga. The magmatic component of crustal growth in NW China appears to be pulsed or episodic, and dominantly younger than Ordovician in age.