The Study Of Granite-Greenstone Belt In Xihongshan Area, Inner Mongolia

The study area is located in Yinshan landmass ,west of North China Plate. It is the most northern edge of Precambrian basement, a part of GuYang-Alashan granite-greenstone terrane. As an important part of Craton basement, the Early PreCambrian strata very developed.The granite-greenstone terrane is an ideal area for study of Precambrian ,experiencing complex metamorphism and magmatic activities, and saving a lot of geological information. WuChuan high grade gneiss terrain in the south of Xihongshan granite-greenstone belt also creates an opportunity for the discuss of the relationship between granite-greenstone belt and high grade gneiss terrain.Currently,There are still controversy in the protolith of granite-greenstone,relationship between granite-greenstone and metamorphosed plutonitic rocks and isotope chronology. The formation mechanism of metamorphic plutonitic rocks and the tectonic setting of greenstone are alse not very clear, both of them need further study. So this paper will discuss the evolution models and important events by study of the geochemistry and ages,providing information of the crust formation and evolution of North-China Craton in Neoarchean.1.Geochemistry and tectonic setting of greenstoneMetavolcanic rocks can be divided into two types according to distribution of REE: REE-flat and LREE-enriched. The type of LREE-enriched depleted in HFSE(Nb, Ta, P, Hf, Zr, Ti),having the characteristics of Island-arc basalt. In the tectonic diagrams, the analysed samples fall into Intra-ocean island arc area. In addition, this type samples have a relative high Mg concentration and low Al,Ti concentrations, having the geochemeistry characteristics of komatiites(komatiite basalt) ; The REE-flat tholeiite have a relatively flat distribution in REE and trace. In the tectonic diagrams, the analysed samples fall into MORB basalt area.2. Geochemistry and tectonic setting of metamorphism plutonitic rocksMetamorphism quartz diorite and granite represented the main regional magmatic activity in Late Archean. The quartz diorite has I-type granite characteristics. SiO2=61.05 ~ 64.50%, Na2O> 4.92%, w (K2O) / w (Na2O) <0.58., A / CNK <1.1. In addition, the rock is rich in Si and Mg, with relative high Al2O3 Mg#, depleted in HFSE(Nb, Ta, P, Ti), the rock has features as evident fractionation of LREE and HREE, no obvious Eu anomalies, (La/Yb)n=9.19~25.43, enriching of Sr, dep;eting of Y and Yb, al these indicating that the roch belonged to Archean TTG or adakites, it falls into the adakite region in both (Y-Sr / Y ) diagram and (Yb-La / Yb) n diagram. Present research shows that high-alumina TTG has similar features with the adakite, which is the result of the partial melting of oceanic crustal basaltic rocks in the subduction system.The major composition of the metamorphic granite in the area are quartz , plagioclase, perthite and biotite. The content of sio2 ranged from 73.34% ~ 75.59%, its value of A/CNK is between 1.01~1.17, and the corundum(CIPW)is often more than 1%. Besides, the rock is low in calcium (CaO between 0.55~2.97%), TFeO/(TFeO+MgO) is between 0.62~0.70(lower than 0.8), this is corresponding with the CPG of the classification scheme by Barbarin(1999), indicating the granite is basically or completely crust origined. However, no matter which classification scheme is apllied, they all suggest that the granite in the area has the features of Al-rich granite ,in the tectonic diagrams, the analysed samples fall into syn-collision area. Therefore, we can conclute that the fomation of the metamorphic granite is related with the melting of crustal rock with high maturity in the syn-collision tectonic setting.3. formation of granite - greenstone belt and tectonic evolutionIn researching of Crustal growth in Archean, Timothy M. Kusky & Ali Polat pointed that form Neoarchean, continental crustal growth was finished through accretionary at convergent margins. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. It shows that during 2500Ma the inner area were existing Continental collision though the Existence of the peraluminous granite in the area and the Existence of the white eye-socket texture structure of the granulite in WuChuan high grade terrain in the south studying area.Based on the above studies, this article propose the formation of granite - greenstone belt and the regional tectonic evolution history: (1) Early stage of formation of intra-oceanic island arc and back-arc extensional basin.Calc-alkaline basalt in Intra-ocean island arc and MORB basalt in back arc basin formed in this stage; (2) The continent-arc collision stage. Oceanic island arc basalt and fragments of oceanic crust in back arc basin splice at plate convergent margin. Subsequently, oceanic crust subducted to the south, and TTG magma formed by ehydration melting in this process. About 2.53 Ga , TTG magma which arised and intrusived into convergent margins accretionary wedge formed by above-mentioned greenstone, and greenstone together formed the early granite–greenstone terranes at convergent margins. But in the deep Subduction zone, after oceanic crust dehydration, the fluid Entered the mantle wedge. Then it lead to partial melting of mantle wedge. The result is that High-Mg diorite's magma were formed. And all these things also explained reasonably how the Sanukite along the south-west of the study area formed. (3) The Continental collision stage. About 2.5Ga collisional orogenesis started. It transformed north marginof craton, then the Granulite facies belt appeared. During this time, the emplacement of the Charnockite emerged widely. While, the situation in the area was the continental material which subducted to the deep crust emerged anatexis and Emplacement.then the large area Calc-alkaline peraluminous granite had been forming.