Geochemistry Of The Ordovician Volcanic Rocks In Aershan,Inner Mongolia And Its Tectonic Significance

Toudaoqiao-Xinlin-Tayuan ophiolite belt locates between the Ergun block andXingan block. There are outcrops of Xinlin ophiolite， Huanyu ophiolite and Toudaoqiaoblue schist along the belt. According to the recent studies，it is considered as the suturezone of the two blocks. However the timing and model of the collision are still subject tocontroversy. Duobaoshan Ordovician island arc locates to the southeast of the ophiolitebelt. Its genesis and property can provide basis for the regional tectonic evolution. Due tothe lack of systematic chronological and geochemical study on the Ordovician volcanicrocks from the southwest part of the island arc，its genesis and property remains unknown.Therefore this paper conducts systematic petrological，chronological and geochemicalstudy on the representative Ordovician volcanic rocks from the southwest part of theisland arc，whick is Aershan area，aiming to provide evidence for the regional tectonicevolution.The volcanic rocks of Duobaoshan formation in Aershan area consist of metabasalts，meta basalt-andesites and meta dacites. The meta basalt-andesites and metadacites yield LA-ICP-MS zircon U-Pb age of469±6Ma and475±4Ma，respectively.These rocks are high field-strength element enriched（e.g.，Th，U，Zr，Hf） and Nb，Ta depleted，showing a geochemical characteristics of transitional basalt from arc volcanicrocks to Oceanic island basalt. The meta basalts of Jiagedan formation in Aershan areayields LA-ICP-MS zircon U-Pb age of464±7Ma，which can be classified as two groups.The first group of High-titanium basalts are characterized by srongly incompatible elements enriched（e.g.，Rb，Ba，Th，U，K），positive anomaly of Nb，Ta and aregeochemical similar to the Oceanic island basalts. The second group of low-titaniumbasalts are geochemical similar to the volcanic rocks of Duobaoshan formation in Aershanarea. These characteristics indicate that both the deep upwelling mantle of OIB and thefluid released by the subducted oceanic crust contribute to the magma source of volcanicrocks of Duobaoshan formation and low-titanium basalts of Jiagedan formation，while themagma source of the high-titanium basalts of Jiagedan formation is mainly the deepupwelling mantle of OIB. Combined with the tectonic location of Aershan area，weconcluded that the Ordovician volcanic rocks in Aershan area are formed during the earlystage of back-arc basin.The early Paleozoic tectonic evolution of Toudaoqiao-Xinlin-Tayuan ophiolite beltdeveloped as follow：at late Cambrain，Ergun block collaged with oceanic island arc or theXingan block，leading to the form of Toudaoqiao blue schists；at early-middle Ordovician，Duobaoshan volcanic arc is formed by the northward subduction of the Paleo-AsianOcean；at the same time，a rock association of transitional basalts and OIB are formed atthe back arc basin，while the typical arc volcanic rocks should be found at the southeast ofAershan area.