The Geochemistry And Its Tectonic Significance Of Granitic Rocks In Danghenanshan Region,South Qilian Mt.

Danghenanshan is located in the Southern Qilian Mountains tectonic zone,on the east of the Yangtze plate,west of Altun fault and north of Qilian tectonic belt,to the south of the North Qaidam high pressure metamorphic belt,it is an important part of the Qilian orogenic.Its tectonic position is very unique,and have experienced several tectonic movements.Since Proterozoic Danghenanshan has experienced strong,especially in the early Paleozoic which hanppend a large area of granite emplacement,and these magmatic activity are closely related to mineralization.This paper analyzed geological features,petrology,geochemistry and isotopic dating of the four main rocks to explore the area granitic magma evolution and tectonic evolution.This is useful to understand the tectonic evolution of the South Qilian and exploration of mineral resources.This papers obtained the following results:1)Danghenanshan mainly emplaced Early Paleozoic granite,the lithology is pyroxene diorite,granodiorite,quartz monzonite,monzonite granite,quartz diorite.In addition to granodiorite is batholith,mostt other rock output with strains and dyke.This article was first obtained these LA-ICP-MS zircon U-Pb age of rock,namely:Jijiaogou quartz monzonite(455.6± 5.6Ma)and pyroxene diorite(476.2 ± 6.17Ma),Jiagongtai quartz monzonite(457.8 ± 6.3Ma),fine-grained Zhazigou adamellite(450±12Ma)and Zhazigou porphyritic monzonite granite(420±11Ma).Zircons aspect ratio of 1.5 to 2,most of the features of magmatic zircon have shock belts,narrow belts;zircon shapes,the larger the size difference,reflecting the multi-stage magmatic event characteristics;individual rock black zircon,may be related to U,Th content;section zircon particles with irregular residual nucleus,with white spots on some of zircon,and has black edges.Meanwhile,there are two rooster crows concordia age 760.8±9.35Ma and 1228.2Ma,may represent the source of zircon ages ditch pyroxene diorite rock.2)According to the zircon dating results,combined with the proceeds of the age and previous geological background Qilian granite magmatism in the area is divided into four of the times:the first phase(510Ma?)belongs to the Late Cambrian,including Zhazigou granodiorite and plagioclase granite outcrops in the west ofDanghenanshan,the former has type O adakite characteristics of the product of the early oceanic crust,which has M-type granite features,belong area ophiolite endmember composition-Ocean plagiogranite;phase II(476?450Ma)is in-late Ordovician magmatic activity in the largest and most widely distributed range,forming a quartz II gabbro-adamellite-positive diorite-pyroxene diorite of acid rock mix;III(420Ma?)belong in-late Silurian,the formation of two types of granite,one for with adakite features Jiagongtai plagiogranite,a class of high rare earth content Zhazigou porphyritic monzonite granite.Phase IV(age unknown)formed East Dongdonggou quartz diorite and lamprophyre,less exposed in the region,resulting in the extension environment,marking the end of Qilian Mountain Ocean subduction process,and then into the continental evolution stage.3)Geochemical studies show that the first phase of plagiogranite with high TFe,MgO,metaluminous to peraluminous,no Eu anomaly,LREE fractionation weak.Geochemical characteristics of granitoids of the second phase is divided into three phases:the early stages of pyroxene diorite to shoshonitic series,with a low silicon,Yb,high K2O and TiO2,geochemical characteristics ?REE the interim stage quartz diorite-syenite diorite-quartz monzonite to shoshonitic series,metaluminous-peraluminous,total rare earth of earlier stage pyroxene diorite low potassium;third stage adamellite low iron,MgO,potassium.Phase two types,Jiagongtai plagiogranites adakite features high Sr and low Y and Yb,displayed a weak negative to positive Eu anomalies,Zhazigou porphyritic monzonite granite high K2O,with potassium mysterious quality of the series.Fourth of lamprophyre and quartz diorite,peraluminous strongly depleted in high field strength elements.4)Sr-Nd isotopic of Danghennahsan rock results show Zhazigou granodiorite Sr initial ratio of 0.70643,indicating mantle source;Jiagongtai plagiogranite Sr initial ratio of 0.706467,Nd initial ratio is 0.512134,?Nd(t)is close to 0,the second stage model ages(T2DM)is 1.10?1.17Ga,its source rock formed from basalt mantle source region,primarily during the Mesoproterozoic Jixian differentiation chondrite not original the mantle of stars;second rock Sr initial ratio of 0.709399,Nd initial ratio of 0.511761,sNd(t)-4.79?-6.31,the former is able to save oceanic crust dehydration makes the bottom pad mafic remelting magma rising process of crustal contamination of a large number of substances,two-stage model ages(T2DM)is 1.58?1.74Ga,its mafic rocks of Proterozoic source mantle materials Wall period;third Zhazigou porphyritic ? adamellite Sr initial ratio of 0.713830,Nd initial ratio of 0.511805,?Nd(t)is-5.70,indicating a source area contribution to continental crust based,two-stage model ages(T2DM)is 1.43?1.62Ga,zircon saturation temperature higher partial melting may granitic basement rocks from the original formation.Fourth magmatic initial Sr initial ratio of 0.709787 and Nd ratio of 0.511748,Mg#with an average of 52,?Nd(t)1.66?1.87,showing its characteristics enriched mantle source region,the source rocks formed in the Early Proterozoic Mesoproterozoic mantle material may be formed in extensional environment.5)The form of the granitoids in the study area is controlled by evolution of dynamic tectonic background of Northern Qaidam and North China Block.The relationship between granite causes and tectonic evolution in Danghenanshan suggests:Early Cambrian,Danghenanshan-Lajishan ocean existed to the north of South Qilian Mountains;Middle Cambrian(about 510Ma)the ocean began to dive,formed Zhazigou island arc granite;Late Ordovician(476?450Ma)it subduct to northward,formed large subduction environment island arc granite;Late Silurian(420?Ma),Danghenanshan-Lajishan ocean closed,continental collision,formed thecollision granite and adakite,and later the formation of some lamprophyre neutral and small shallow porphyry rock.