Geological Characteristics And Significance Of Late Paleozoic Adakitic Granitic Rocks At Guyang Area Of Inner Mongolia

The field geological characteristics,petrology,geochemistry,and zircon U-Pb geochronology of Liubugou and Xiyingzi pluton which is locating in Guyang area,Inner Mongolia,and geological belong to north margin Paleozoic accretion zone of north margin of North China plate are reported in this thesis.The main points are as follows:(1) The geochemical characteristics of Liubugou pluton:SiO₂,56.84-68.54wt.%,all＞56wt.%;Al₂O₃,16.54-18.56wt.%,all＞15wt.%;MgO,0.32-2.41wt.%,all＜3%;Na₂O, 3.71-6.78wt.%,the average is 5.04wt.%;K₂O,2.30-4.76%,the average is 3.69%;the Na₂O/K₂O ratio is among 1.0-1.85,the average is1.42,implicated to be Sodium series rocks;High Sr (657-2255μg/g),all＞400μg/g;low Y,4.52-20.45μg/g,the average is 13.15μg/g,low than 18μg/g;Sr/Y,43.08-269.69,higer than 20-40;Yb,0.63-1.90,all≤1.90;(La/Yb)_N(N is shorted for Normalized to chondrite),8.69-17.91(the average is 12),most＞10;rich in LREE,depleted in HREE,weakly positive Eu anomaly,no obvious Eu negative anomaly;geochemically belongs to high-k calc-alkaline series;be lower in MgO,Mg^#(the average is 0.35),Cr(the average is 6.85μg/g),Ni(the average is 7.25μg/g) than typical adakite formed by melting of subducted ocean plate,and have no imprint of metasomatic of adakitic magma by mantle peridotite,and so the granitic magma can not origined by melting of subducted plate;be similar to Yanshanian granitic rocks of East China(C-type adakite) and Cordilera Blanca pluton(the average Na₂O/K₂O ratio is 1.3) which is not subducted plate origined,maybe the product of melting of methickened lower crust triggered by heating of underplated basaltic magma or upwelling aesthenosphere mantle in extensional setting of post-orogenic stageThe LA-ICP-MS zircon U-Pb ages of medium-fine grained amphibole monzonite and porphyraceous quartz monzonite are 401.9±1.1Ma(MSWD = 0.26) and 398.9±1.4Ma(MSWD= 1.12),respectively,which are approved to be formed in Devonian.The Devonian adakitic granitic rocks of Guyang area can be the kay identification of crust thickening event happening after the close of Peo-Asian Ocean.(2) The Xiyingzi pluton is belongs to calc-alkaline series,with the characteristics of island arc geochemical affinity,and similar to typical adakite in petrochemistry.Geochemical of those rocks are:SiO₂＞56wt.%(57.28-70.59wt.%);Al₂O₃,14.78-18.04wt.%(only one sample is lower than 15wt.%);MgO,0.98-3.47wt.%(two sample higher than 3wt.%),the average is 2.25wt.%; Na₂O,3.02-4.1wt.%,the average is 3.59wt.%;rich in Na,and poor in K,Na₂O/K₂O ratio is 1.06-2.31,implicated to be Sodium series rocks;low Y(5.61-18.39μg/g,the average is 12.02μg/g,lower than 18μg/g),and Yb(0.42-2.06μg/g,only one sample higher than 1.9μg/g),high Sr(446-582μg/g),all samples are higher than400μg/g,the Sr/Y ratio is 24.69-103.74,higher than 20-40,the(La/Yb)_N(N is shorted for Normalized to chondrite) ratios are 6.13-36.42(the average is 16.93,most higer than 10,rich in LREE,and depleted in HREE,with weakly positive Eu anomaly,and no obvious Eu negative anomaly;the rare earth elements are showed strength differentiation and LREE riched curve on the chrondrite normalized REE pattern diagram,and Sr is showed the obvious positive anomaly on the spider diagram of trace element.The above geochemical characteristics of Guyang granitic rocks are implying the magma is related to subduction of ocean plate,the melting of subducted ocean are the most possible mechanism.The LA-ICP-MS zircon U-Pb ages of medium-fine grained quartz diorite and medium-fine grained tonalite are 281.9±3.1Ma(MSWD=3.3) and 277.8±2.8Ma(MSWD=1.5),respectively, and considering the early Permian is an important stage of subduction of Peo-Asian Ocean.(3) The discovery of late Paleozoic adakites from Guyang area can provide constraints on the close time of Paleo-Asian ocean,and also get very important signification for late Paleozoic structural evolution of Central Asian orogenic belt,geodynamic character and mineralization research of this area,and maybe solve the long lasting arguement about issues on Inner Mongolia and Paleo-Asian ocean.(4) Implication for mineralization:the kay factor which decided the favor of mineralization of adakite is dehydration aroused by phase change from amphibolitic facies to eclogite facies adakitic rocks in favor of mineralization,other ones including high temperature and pressure,rich in volatile,high oxygen fugacity and fast ascend characteristics of adakitic magma.The adakitic magma which formed by melting of young subducted ocean plate with rich in water and sulfur, high oxygen fugacity charater can leach Cu,Au and other ore-forming elements from mantle or basaltic rocks,and ascend fast to the shallow crust.The ore-forming elements can be transported by magma hydrothermal and deposited at favorable location under the suitable physical and chemical condition.The discovery of adakitic rocks provided new clues of prospecting for adakite-related Cu-Au ore deposits.