The Composition,Deformation-metamorphic Characteristics And Metallogenic Response Of The Dome Geological Bodies On The South Margin Of Songpan-garze Block

A domal deformation-metamorphic geological body,formed in different evolutionary periods,tectonic settings and regimes,is a link between the surface and deep process of the earth.It can reveal the geological structure of the continental orogenic belt as well as its formation and rules.The domal bodies(Taka,Changqiang and Jianglang)on the south margin of Songpan-Garze Block are taken as objects of study thoughgeochemistry,zircon genetic mineralogy,chronology and mineralization based on the field geology in this paper.The domal geological bodies,including Taka,Changqiang and Jianglang,can be divided into a core,a mantle and cover.The core formation of the dome is Liwu Group Complex or granite,and the mantle is Silurian-Permian strata(including Metabasic dikes),and the cover is Xikang Group.The granitic plutons exposure in the core of the Jianglang dome,it is distinctly different from Changqiang and Taka dome in terms of composition.The Liwu Group Complex in the core of dome is mainly composed of meta-clastic rocks and ortho-metamorphic rocks,while the lamprophyre dikes intruded into the Liwu Group Complex in the later stage.Most of the meta-clastic rocks in Liwu Group Complex are composed of quartz schist and quartzite with mica schist.The protolith is mainly composed of sandstone,siltstone,and highly siliceous sandstone.According to the U-Pb chronology of the zircon in the meta-clastic rocks,there exist eight major peaks including ca.460Ma,500 Ma,540Ma,600Ma,800Ma,900Ma,1000 Ma and 2500 Ma,revealing an age basically the same as that of the western margin of Yangtze Block and Kangdian Oldland,and indicating that the materials came from a near source deposited in passive continental margin tectonic environment and have a strong genetic relationship with the Yangtze Block.The protolith of the actinolite schist or actinolite rocks in Liwu Group Complex is metabasite(basanitoid)and its U-Pb isotopic chronology indicates that age of the zircon corresponds to ca.254.9±4.9 Ma and ca.191.5±1.9-223.7±1.3 Ma.The primitive mantle normalized distribution pattern of the rare earth elements in these samples shows a slightly right deviation,indicating an obvious fractionation between the light and heavy rare-earth elements.? Eu is 0.88-1.03 and ?Ce is 0.83-1.00.It is resembled with the primitive mantle normalized distribution pattern of OIB.Thus,the age of the Liwu Group Complex is limited to ca.254-466Ma.According to the U-Pb chronology of the detrital zircon in Liwu Group Complex,there is a strong genetic relationship between the formation of protolith and the Yangtze Block,and the features of the stratigraphic sedimentary environment and provenance in the Tethys-Himalayan Ordovician system.It is confirmed the fact that the meta-clastic rocks of the Liwu Group Complex formed in the Ordovician period.The Wulaxi granite is composed of syengranite and monzonite,While the Xinhuoshan granite is composed mainly of monzonite in the core of Jiang dome..The geochemical characteristics indicate that they are peraluminous highly fractionated granite,calc-alkalic to alkaline series and an A-type granite.The U-Pb age of zircon was ca.163.1±1.3 Ma?165.88±0.78 Ma and ca.147.3±2.3 Ma?154.5±2.4 Ma.The LA-ICP-MS U-Pb age of the monazite in Wulaxi granite is ca.154.06±0.71 Ma,and Five groups of LA-ICP-MS U-Pb age(ca.164.0±3.0 Ma,172.5±1.9 Ma,182.7±2.5 Ma,193.3±3.6 Ma and 203.6±2.9 Ma)have been obtained from Xinhuoshan granite.The LA-ICP-MS U-Pb age of apatite in Wulaxi granite is ca.152.1±4.6 Ma,155.0±4.8 Ma,163.3±1.7 Ma and 168.0±4.6 Ma.The above age data indicate that Xinhuoshan and Wualxi granite may had undergone multiple stages of magmatism between the late Indosinian period and the early Yanshanian period.There are many metabasic dikes and metabasalt of Dashibao Group in the mantle of the dome on the south margin of the Songpan-Garze Block,and the protolith is basalt and alkaline basalt.These samples are of light REE enrichment and the 8Eu is 0.92-1.06(average 1.01).The normalized distribution curve of rare-earth elements in the primitive mantle is tilting to the right,with the geochemical characteristics of the Permian basalt of Dashibao Group in the Baoxing and Danba area,which is basically consistent with the normalized distribution pattern of the OIB primitive mantle.The U-Pb age of zircon is divided into five Groups,of which ca.200-230 Ma represents the metamorphic event that occurred in the Indosinian period of the Songpan-Garze Blocks,and ca.258.8±5.4 Ma is basically consistent with the forming age(ca.254.9±4.9 Ma)of the basic dikes in Liwu Group Complex.The metamorphic mafic dikes in the Liwu Group Complex,metamorphic mafic dikes and basalt of Dashibao Group in the mantle of dome has basically the same age,was related to the response of Emeishan Large Igneous Province to this region.The Changqiang and Jianglang area formed in the Permian period during the transition tectonic setting of continental rift to the small ocean basin.In Jianglang,Taka and Changqiang dome,tectonic association of along-bedding slip bands,dome structure and strike-slip structure are pervasively developed.Regionally penetrating bedding foliations(S1,S2),bedding folds,ductile shear zones,stretching lineation,mineral lineation and crenulation lineation was morphologically formed,associated with the formation of syndeformed secretory crystalline veins.Evidence from these above indicate that thrust compression developed in the section at the earlier stage while a decollement fault came into being at the later stage.U-Pb dating of the apatite in metamorphic clastic rocks and 39Ar-40Ar dating of biotite and hornblende in Jianglang,Changqiang and Taka dome,All the dating results indicate that the domal geological bodies on the south margin of the Songpan-Garze Block underwent four stages(ca.197.09±2.61 Ma?191.39±2.30 Ma,170.75±16.89 Ma?171.34±1.80 Ma,147.2±3.7 Ma?147.77±5.15 Ma,123.0±5.7 Ma?129.83±2.65 Ma)of Indosinian-Yanshanian metamorphic and deformation events,in correspond to low greenschist facies,high greenschist facies to low amphibolitic facies,low greenschist facie and low greenschist facie.Eight groups age(ca.4 Ma?6.3-9.7 Ma,11.1-19.8 Ma,27.2 Ma,37.7-44.7 Ma,49.2-52.9 Ma,55-60.8 Ma and 64-70 Ma)were determined by the ZFT and AFT,and it indicates that the domal geological bodies on the south margin of the Songpan-Garze Block had gone through the same uplift-related exhumations since ca.10 Ma,and it has something to do with the uplift of mountains caused by the escape of materials from the southeastern or eastern margin of the Qinghai-Tibet Plateau.The P-T-t-D path on the south margin of the Songpan-Garze Block appears to be a clockwise path of isothermal decompression.It reveals that the metamorphism of the study area had something to do with the subduction in the Garze-Litang junctional zone as well as the tension and closure system of the Yajiang Basin,and with the sideway extrusion caused by the uplift of the Qinghai-Tibet Plateau at the later stage.It is believed that the dome formation in the Jianglang dome began in the Indosinian period(ca.210-180 Ma),coincides with the orogenic period of the Songpan-Ganze block,and then formed a dome with the West Yangtze rigid plate at the edge.To the stage of the ca.140-160 Ma,lithosphere was thickened and remelted to form the Wulaxi and Xinhuoshan granite,and the dome was basically finalized.Compared with Changqiang and Taka dome only existed mineral occurrence,Liwu type copper deposits are widely distributed in Jianglang dome.The isochronal age of Re-Os from the chalcopyrite in the 4 ores samples of Wajingou and Liwu Copper Deposit is 183.5±7.1 Ma(MSWD=141),and the initial 187Re/188Os is 1.13±0.25.In addition,the 39Ar-40Ar plateau age of the biotite was closely associated with the chalcopyrite in Wajingou is 138.9±1.2 Ma,representing the final mineralization age of the Wajingou area.The Re-Os isotopic age of the molybdenite in the Daniuchang Deposit is ca.160.3±1.6Ma,171.4±1.7Ma,164.5±1.3 Ma and 168.5±1.2 Ma,in accord with the U-Pb isotopic age of the zircon and apatite in the Wulaxi granitic pluton.According to above mentioned,it indicates that the metallogenic events was related to the multiple phases of magmatism in ca.160-180 Ma.The copper,zinc,tungsten and tin ores in the domal geological bodies on the south margin of the Songpan-Garze Block were formed under the combined action of volcanism,sedimentation,magmatism,metamorphism,deformation and uplift-related cooling,in coincides with collisional orogeny,thermal uplift-related extension,thrust extrusion and intracontinental orogeny.Thus,the Liwu Cu-Zn Ores deposit underwent two phases of mineralization,i.e.,the syngenetic sedimentary and epigenetic transformation mineralization,and the Daniuchang tungsten-molybdenum deposit is closely related to the Wulaxi granite.For the target region of the Liwu Copper-Zinc Deposit,it can be extended from Changqiang,Jianglang and Taka to the junctional area with the western margin of Yangtze Block in the north-east direction.We also suggest exploring tungsten and molybdenum deposits in the surrounding areas of the Xinhuoshan,Tiechanghe,and Qiaopengzi granite in Jiulong area.