| The Qilian orogenic belt,as an important part of the Qinling-Qilian-Kunlun Orgenic Belt in China,is held between the Alashan block and the North Qaidam block-west Qinling belt,and also surrounded by Tarim craton to the west and cut by the Altun fault.In recent years,an early Paleozoic arc belt in the south of Central Qilian has been discovered.The arc belt is large in scale and exposed completely,which records a series of complex geological processes,including magma origin,evolution,storage and emplacement.In this paper,the Wulanyaodong pluton,from the Yanchiwan area,western Central Qilian belt,is selected as the object.Based on detailed field geological survey and petrography,combined with whole rock geochemical data,zircon U-Pb dating,Lu-Hf isotope and calibration of the geothermobarometers,we have made clear the ages,petrogenesis and formation conditions of the samples.We also discussed the mechanism of crust-mantle interaction related to formation of the pluton and its significance in the genesis of arc magma.The main conclusions obtained are as follows:1.The Wulanyaodong pluton is mainly composed of granodiorite,diorite,monzogranite and syenogranite,and there also developed dioritic enclaves and multi-stage veins,such as granite,diabase,andesite porphyrite veins.The resuls of U-Pb zircon dating indicate the pluton grew episodically,the first phase(475Ma ? 460Ma),the second phase(455Ma),and the third phase(445Ma).Considering the recent advances of regional geological evolution,the first stage of magmatic activity was formed in the rear-arc settings,and the subsequent magmatic events may represent the extension process of back-arc basin.2.The Lu-Hf isotopic analysis of zircon shows that ?Hf(t)values are all negative,indicating that the source of different samples is mainly crustal materials.It is revealed by high MgO,Cr,Ni characteristics of most rocks,the mantle-derived magma played an important role,which shows that there is obvious crust-mantle interaction in the area.With the U-Pb age of zircons getting younger,the enrichment degree of ?Hf(t)value decreases,reflecting the increase of mantle-derived components,which may be related to the back-arc extension process.3.The rocks formed about 475 Ma have various types and characteristics,which may represent the primitive magma.Subducted slab-derived melts and sediments mixed with mantle-derived magma formed sodium-rich adakitic granodiorite and crystallized in a magma reservoir at 7.21 ? 8.99 km.The quartz diorite crystallized at 12.98 ? 20.35 km.About 467 ?460 Ma,the mafic magmatism,represented by dark enclaves,induced the crystallization and differentiation of the early stage magma reservoir,and formed cumulates and highly differentiated granite,which led to the re-growth of the pluton.The emplacement depth of diorite enclaves ranged from 8.78 km to 10.94 km,and the emplacement depth of highly differentiated monzonitic granite varied from 2.24 km to 4.94 km.The first stage of magmatism represent the history of emplacement,differentiation from deep to shallow for the pluton;the second and third stages of magmatic events are mainly a series of dioritic veins,reflecting the characteristics of late deepper magma source.There are still some problems to be solved in the study of wulanyaodong plutonic complex,the lack of Sr-Nd isotope analysis made it impossible to further explore the magma source area,which needs further work. |
PDF Full Text Request