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Characteristics Of Late Palaeozoic Granitoids And Their Geological Significances In The Middle Segment Of North Margin Of North China Plate

Posted on:2012-04-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q W ZhangFull Text:PDF
GTID:1110330368978713Subject:Mineralogy, petrology of ore deposits
Abstract/Summary:PDF Full Text Request
Granitic rocks are commonly developed extensively within orogenic belt,so their forming ages,origins and geochemical features of source area have some important implications on the evolution,interactions between the mantle and the crust in orogenic belt.The study area is located in Urad zhongqi-Bayan obo-Guyang area of Inner Mongolia,which lies in the orogenic belt of Mid-Proterozoic continental margins in the middle segment of north margin of north China plate,with 80km-100km distances to northward suture line between the Siberian plate and the North China Plate.Granitoids are well developed in the study area,and their rock associations are composed of diorites,granodoirites,two feldspar ganites, and garnet granites.On the bases of data investigations of the former researchers,a field geological survey was carried out,then petrography,petro-geochemistry,and SHRIMP U-Pb dating of zirons from the typical granitoids were performed,through which Late Palaeozoic granitoids exposed in the study area was subdivided into four episodes of intrsions,finally the geneses,source features of granitoids were deduced and the subduction process of paleo-Asian Ocean and tectonic evolution history of north margin of north China plate were discussed.The results of the paper are as follows:(1) Early Devonian granitoids are a association of quartz monzonites,hornblende monzonites,and quartz diorites.Their geochemistry shows that they are of the I-type adakitic rocks,deficient in K and rich in Na,attributing to subluminous,belonging to high-K calc-alkaline series;enriching large ionic lithophile elements(LIL) such as Rb,Ba and Sr,and depleting high field strength elements(HFS) such as Na,Ta and Ti;having a distinct fractionations between the light and the heavy rare earth elements(REE),with a highly LREE-enriched pattern with slight positive Eu anomaly.The intrusions of granitoids were formed by direct melting of the underplated basaltic lower crust in the settings of arc-continent collisions.(2)The carboniferous intrusions of granitoids are consisted of granodiorites with varied fabrics.Their geochemistry indicates that they are of the adakitic granodiorites, belonging to high-K calc-alkaline series of the metaluminous to peraluminous,enriched in LIL such as Rb,Ba and Sr and HFS such as Th,U,Nd and Zr and depleted in Nb and Ti;with a LREE-enriched pattern but without the pronounced Eu anomaly.This kind of intrusions,with the dual features possessed by granites derived from active continental margin and interpolate,were formed pre-collision to syn-collision tectonic environment, resulted from the crust-mantle mixing.SHRIMP U-Pb datings of zircons from two granodiorite samples yield a weighted mean age of (322.1±3.4)Ma and (323.4±3.4)Ma, respectively,which are assigned to the early Carboniferous.(3)The early Permian granitoids were made up of biotite granites and monzonitic granites.The major elements geochemical analysis indicates that they overall belongs to metaluminous to peraluminous,and calc-alkaline to high-K calc-alkaline series.The trace elements analysis are characterized by high concentrations of LIL such as Rb,Sr and Ba and HFS such as Th and U,and depletion of Nd,Ta,Ti,Tb and Y.REE distribution patterns are of the strong LREE enrichment,and both LREE and HREE are fractionated,but LREE more pronounced,with slight negative anomaly to no anomaly.The intrusions of this episode were formed in the tectonic settings.before collision of active continental margin plate or the syn-collision,resulted from the crust-mantle mixing.SHRIMP U-Pb dating of zircons from biotite granites in Wulan of Urad zhongqi yield a weighted mean age of (279±3)Ma,which are assigned to the early Permian.(4)The middle-late Permian granitoids are composed of biotite granites,garnet- bearing two mica granites,and two feldspar granites,which are characterized by high Si,Al and and per-alkalis,belonging to peraluminous and high-K calc-alkaline series. Analysis of trace elements shows that LIL such as Rb, Ba,Th,U,Nb and Ta are enriched, and HSF such as Sr,Ti and Y are depleted,with a strong LREE enrichment patterns and slight to highly negative Eu anomaly.The granitoids of the episodes are mainly resulted from the remelting of the crust substances,formed in syn-collision tectonic settings,with an evolution tendency to within plate and post-collision to post-collision,and their source rocks were derived from greywacke of upper continental crust.SHRIMP U-Pb datings of zircons from biotite granites and garnet-bearing granites yield the weighted mean age of (266±3)Ma and (256.4±2.2)Ma, respectively.(5)Finally,it is concluded from petrological,geochemical and chronological comparis- ons among the granitoids of different episodes that the tectonic evolutionary regimes of the middle segment of north margin of the north China plate in late Palaeozoic were from ocean crust subduction,through arc-continent collosions and collosions between the Siberian plate and the North China Plate to post-collision orogen.
Keywords/Search Tags:Granitoids rocks, Geochemistry, Zircon U-Pb dating, Tectonic settings, North margin of North China plate
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