Paleozoic To Mesozoic Magmatic Evolution And Its Tectonic And Metallogenic Significance Of The Erenhot-Dongwuqi Area

The Paleo-Asian Ocean underwent a protracted closure history during Late Paleozoic.Here we investigate the magmatic evolution during this process based on a detailed study in the Baiyinwula region along the Uliastai continental margin.The major rock types in this area are Late Carboniferous-Early Permian volcanic sequences and coeval intrusions.We identified five stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages.The first stage is represented byandesites with a zircon 206Pb/238 U age ofca.326±12 Ma.These rocksare metaluminous to weakly peraluminous,high-K calc-alkaline,and possess high Na2O/K2 O ratios in the range of 1.23 to 2.45.They also displayenrichment of large ion lithophile elements(LILE)and depletion of high field strength elements(HFSE),with markedly positive zircon? Hf(t)varying from 8.1 to 15.6.The geochemical features of these andesites are similar to those of typical arc volcanic rocks.The second stage includes granodiorites emplaced at 318.6+1.8 Ma.The rocks are high-K calc-alkaline with A/CNK values ranging from 0.95 to 1.06,and show enrichment in LILE and depletion in HFSE.They show geochemical affinities to adakites,with high Sr and low Y and Yb contents,indicating magma derivation from thickened lower crust.Zircon grains from these rocks display positive initial ? Hf(t)values ranging from 11.1 to 14.6 with corresponding two-stage Hf model ages(TDM 2)of 394-622 Ma.The third stage consists of syenogranitetogether with a volcanic suite ranging in composition from rhyolite todacite,which formed during 303.4±1.2 to 285.1±2.2Ma.They possess elevated silica and alkali contents,high Fe Ot/ Mg O and Ga/Al ratios,low Al2O3,Mg O and Ca O contents,and high Rb,Y,Nb,Ce,Zr,Y,and Ga contents,strong negative Ba,Sr and Eu anomalies,showing I-to A-type granitic affinities.Zircons in these rocks showelevated Hf isotopic compositions(? Hf(t)=9.9 to 14.6)with TDM2 varying from 324 to 673 Ma.The fourthmagmatic pulse is represented by K-feldspar granitewith zircon U-Pb ages from 283.2±1.9Ma to 280.0 ± 1.4 Ma,and typical alkalic A-type granite geochemistry.These rockspossesspositive? Hf(t)values in the range of9.7 to15.2,and a restricted range of Hf model age from 327 to 684 Ma.The fifth stage is a set of acid volcanic rocks.The zircon U-Pb age of rhyolite is 141.9 + 3.2 Ma.The rhyolite is a high potassium calc alkaline series,showing the characteristics of A type granite after orogeny.The magmatic rocks from the four stages show comparable ?Hf(t)and T2 DM,suggesting that the magmas were derived from the same evolving mantle-derived source.We propose a tectonic model linking the evolution of the magmatism with the closure of the Paleo-Asian Ocean that involved the following stages.The andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle.Stage 2 adakite-like rocks were derived from subduction-induced thickened crust.Subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the I-and A-type syenogranite,rhyolite anddacite suite,finally followed by the A-type K-feldspar granite.The Mesozoic Early Cretaceous igneous rocks in the study area are mostly characterized by post orogenic or post orogenic granites.Combined with regional data,we believe that they formed in post orogenic stage of paleo-Pacific evolution.The research area is located on the southern margin of the Siberia craton,the late Paleozoic continental margin belt.Since the early Paleozoic,it has undergone two major structural evolution,namely,the Paleo-Asian Ocean and the Paleo-Pacific.The mineralization time was concentrated in the late Paleozoic(320-250Ma),the early Mesozoic(237Ma)and the late Mesozoic(141-128 Ma).According to the spatial and temporal distribution characteristics of magmatic rocks in the study area,the three metallogenic stages correspond to the period of more intense magmatism in the study area.The first metallogenic period corresponds to the subduction stage of the evolution of the paleo-Asian Ocean.Second stage of mineralization corresponding to the paleo-Asian Ocean orogenic and post orogenic extensional transition stage and the third stages of mineralization corresponding to the paleo Asian Ocean was closed,Erenhot Dongwuqi area came into the Paleo Pacific post-orogenic evolution background.