Petrogeochemistry And Zircon U-Pb Geochronology Of Yanshanian Granite In Kangbao Coumty,North Heibei Province

The granitic pluton of Lanchengzi, Kangbao mainly exposed in the wind farm2km away from the south of Lanchengzi village, Kangbao coumty. It is a little stockwith only4square kilometers, include2facies belt:marginal facies and transientfacies. Its lithology is medium to close granied porphyritic monzogranite. Phenocrystcontent is about25to35precent, consist of smoky gry quartz(5to8precent),K-feldspar(10to23precent), Plagioclase(4to6precent) and Biotite(about2precent).Matrix content is about65to75precent, include quartz(25to30precent),K-feldspar(10to15precent), Plagioclase(22to30precent), Biotite(about1precent)and so on.The characteristics of granites’ major element are rich in SiO2content(74.33precent to77.25precent, Average is75.31precent), Alkali content (Na2O+K2O are8.05precent to9.03precent, Average is8.60precent) and Kalium content(K2O/Na2Oare1.161to1.533,Average is1.349). Their Rittmann indexs(σ) are1.89to2.61,belong to calc-alkali(σ=1.8to3.3). Their A/CNK are0.998to1.09, Average is1.044,belong to Weakly peraluminous. Corundum content are less than1precent(0.08precent to1.21precent, only2data are greater than1precent, Averageis0.64precent). n（Fe3+）/n（Fe3++Fe2+） are0.179to0.765, only1data is greaterthan0.7, Average is0.534. All the characterstics above accord with the charactersticsof I-type granite. The differentiation index of the granites are93.08to95.61, Themajor element content Al2O3、TiO2、CaO、MnO、MgO、Fe2O3*、P2O5, trace elementcontent Sr、Ba、P、Ti and the trace element ratio Ba/Sr、K/Rb、Ba/Rb are all negative correlation with the SiO2content, Which reflect the high differentiation of themagma. The (La/Yb)N、(Ce/Yb)Nand (La/Lu)Nwhich can reflect the fractionation ofLREE and HREE are higher, also reflect the high differentiation of the magma. TheEu negative anomaly are obvious, and posotive correlation with CaO, show that thegranites experienced the fractional crystallization of Plagioclase. On the chart ofthe10000Ga/Al-（Zr+Nb+Ce+Y） and （K2O+Na2O）/CaO-（Zr+Nb+Ce+Y）,Allpoints drop into the fractionated granites area, On the chart of Rb/Ba-（Zr+Ce+Y）,Most points drop into the high fractionated I-type granites, further instructions thatthe granites are I-type granites experienced high differentiation.The Nb/Ta and Sm/Nd of the rock mass reflect its feature of crust source,Eu/Sm ratio indicate that the granites probablely get contaminated from mantlemateril. On the chart of the (La/Yb)N-δEu, All points drop into the crust source area,but into the sedimentary rocks and calcilutite area on the La/Yb-ΣREE chart, showthe feature of coming from partial melting of sedimentary rocks. As we know theLanchengzi granites belong to I-type granite, So the characteristics above show thatthe granites come from partial melting of neutral-basic igneous rock in lower crust,probablely get contaminated from mantle materil, experienced high fractionated andmay well get contaminated from upper crust.On Maniar and Piccoli＇major element ω（TFeO）/[ω(TFeO)+ω(MgO)]-ω（SiO2）、ω（Al2O3）-ω（SiO2）、ω(TFeO)-ω(MgO) and [ω(TFeO)+ω(MgO)]-ω(CaO)tectonic environment discrimination diagrams, almost all points drop into thepost-orogenic granite area(POG). Furthermore, the points of Rb-(Y+Nb) tectonicenvironment discrimination diagram of granite also show that they come fromtectonic environment of post-collision, well consistent with post-orogenic of majorelement. All the features above indicate that local granites are the products ofstretched tectonic environment of post-orogenic.Zicron SHRIMP U-Pb geochronology manifest that the granites formed at150±2.2Ma, belong to late Jurassic epoch, Yanshanian, combine with the tectonicbackground back then, We can consider the Lanchengzi granites as products ofPacific plate dived towards North China plate, formed similar Andes orogenic belts,leaded to crustal thickening, then, for dealmination and convection thinning, causedthe upwelling of asthenosphere material, finally, made lower crustal heated andpartial melting.