Origin of Permian andesites from Xi Ujimqin, the Hinggan Mountains: Contributions of lower crust North China Carton

Abstract:

Andesite magmas at convergent margins are enriched in silica compared to magmas erupting at mid-ocean ridges and intra-plate volcanoes. Determining the cause (s) of silica enrichment is fundamental for models of continental crust formation, arc growth rates and across-arc mass balances (Plank and Langmuir, 1993; Rudnick, 1995; White et al., 2006).The Xi-Ujimqin is located the eastern segment of the Central Asian Orogenic Belt , the CAOB is composed of mainly subduction-accretion complexes, intruded by vast plutons of mainly magmatic arc origin and covered in places by their volcanic derivatives (Sengor et al. 1993; Sengor and Natalin 1996). As the most important site for Phanerozoic crustal growth in the world, the subduction-accretion complexes added ~ 5.3 million km2of material to Asia, half of which may be of juvenile origin (Sengor et al. 1993).The andesitic lavas of Daotenuoer Fms. at Xi-Ujimqin have different trace and Sr-Nd-Pb isotopic characteristic from them, these rocks span a SiO₂ range of 56.83 ~ 59.17% and MgO of range of 1.21~2.91%, characterized by obviously LREE/HREE fractionation (Ce/Yb_CN = 11.6 ~ 16.8), strong LILE enrichment and variable Nb-Ta and Sr depletion. They have more radiogenic Sr and less radiogenic Nd and Pb isotopic composition (⁸⁷Sr/⁸⁶Sr(i) = 0.7063 ~ 0.7066, e_Nd(t) = -10.5 ~ -6.3,T_{DM =} 1.22 ~ 1.51Ga, ²⁰⁶Pb / ²⁰⁴Pb(i) = 17.29 ~ 17.33, D7/4 = 3.7 ~ 6.9, D 8/4 = 94 ~ 104) than the contemporaneous intermediate-felsic volcanic lavas. Zircon U-Pb dating results give an emplacement age of ~253 Ma for the these rocks by LA-ICP-MS, corresponding to the Late Permian. The features of major, trace and Sr-Nd-Pb isotope of These rocks imply its sources are of attributes of lower crust North China Carton (NCC) during Paleozoic subduction and collision related to the closure of the Chinese segment of the Paleo-Asian Ocean between the NCC and south Mongolia Block (Sengor et al., 1993).