Geochemical Composition,Chronology And Tectonic Significances Of Sanyanlong-Galazi Granites In The Southern Songpan-Garzeorogenic Belt

Many important geological information is covered by Triassic flysch sediments in Songpan-Garze orogenic belt,which is known as many unsolved mysteries of the“China Geological Bermuda”.Granites have achieved great significance in discussing the formation and evolution of continental crust as a probe to study deep geological processes because of its mature research methods.In order to reveal the property and evolution process of the basement in the southern Songpan-Garze orogenic belt,material origin,tectonic background and forming mechanism of the granitoid was discussed in the thesis based on the research of petrology,geochemistry and chronology.The thesis mainly gained the following understandings:?1?The field geological survey and petrography show that the Sanyanlong-Galazi granitoid have a series of quartz diorite–granodiorite–monzonitic granite,which emplaced into Triassic Xikang Group Formation,and that its wall rock alteration mainly consists of hornstone and contact schist.?2?Zircon U-Pb dating result of the monzonitic granite and granodiorite were211.5±1.2Ma,215.6±1.1Ma by LA-ICP-MS.It was close to the age?210²30Ma?of granitic magmation which is widely distributed regionally.The zircon Hf isotopic analysis results showed that ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.282347 to 0.282640,and?Hf?t?range from 10.38 to 0.50.They yield Hf model ages?TDM?of 1173 to 1896Ma.It indicates that the substance origins of granitoid were a new crustal material formed in Mesoproterozoic and there may be a Precambrian basement in Songpan-Ganze orogenic belt.?3?Geochemical analysis shows that the content of SiO₂ in quartz diorite ranges from 54.08 to 63.37 wt.%.The content of K₂O ranges from 2.18 to 4.52 wt.%.The content of total alkali?Na₂O+K₂O?ranges from 4.83 to 5.92 wt.%.The total content of iron?TFe?ranges from 4.30⁷.73 wt.%.The value of A/CNK ranges from 0.73to 0.99,and the value of composite index???ranges from 1.57 to 3.21.It proves that quartz diorite is quasi-alumina calc-alkali series with a diversity index of 39.75 to50.86 in these geochemical characteristics.The content of SiO₂ ranges from 58.03 to 69.89 wt.%in granodiorites.The content of K₂O ranges from 1.32 to 6.95 wt.%,and the content of total alkali?Na₂O+K₂O?ranges from 4.40 to 8.99 wt.%.The content of total iron?TFe?ranges from 1.46 to 6.11 wt.%.The value of A/CNK ranges from 0.77 to 1.02,and the value of composition index???ranges from 0.70 to 3.12.The granodiorites is quasi-alumina-weak peraluminous calc-alkali series with a diversity index of 54.17to 88.08.The content of SiO₂ in monzonitic granite ranges from 68.46 to 77.26 wt.%.The content of K₂O ranges from 3.48 to 5.37 wt.%,and the contents of total alkali?Na₂O+K₂O?ranges from 6.09 to 8.34 wt.%.The content of total iron?TFe?mostly ranges from 1.14 to 4.08 wt.%.The value of A/CNK ranges from 0.83 to 1.0,and the value of the composition index???ranges from 1.57 to 2.33.The monzonitic granite is quasi-alumina-weak peraluminous calc-alkali series with a diversity index of61.29 to 91.98.The granitoid have high silicon contents,high alkali contents,high potassium contents and low iron contents.The contents of K₂O+Na₂O have increased with the increase of acidity,while the contents of FeO,MgO,MnO and P₂O₅ have decreased,and Al₂O₃ changes little.According to those characteristics,it excludes the possibility that the magma come directly from the mantle.The trace elements of the three types of granitoid are very similar.The spider diagram shows that it enriched large ion lithophile elements?Rb,Sr,Th?,and depleted high field-strength elements?Nb,Ta,Y?,and its primitive mantle-normalized trace element diagramsis typical of"M"type.The total REE contents of granites range from 183.49×10^-6 to 404.2×10^-6,and the values of LREE/HREE range from0.80 to 20.83,and its distribution pattern is steep right-dipping distribution curves by enrichment in light REE.?Eu ranges from 0.60 to 0.91,Eu negative anomaly.Based on those researches,Sanyanlong-Galazi Granites belong to I type granite derived from the same magma source.?4?By comparing with the granite data of the same period,such as dingtianzhu,sanyanlong-fangmaping granite adjacent to the region,and the Nb vs Y,Ta vs Yb,Rb vs Yb+Ta,Rb vs Y+Nb tectonic environment discrimination diagrams indicated that the Sanyanlong-Galazi granite was formed in the post-collision environment.Those indicated that the Ganze-Litang oceanic crust was closed before the late Triassic?215Ma?.Thus,the granite was formed in conditions of the thickened crust and gravity destabilized,which gave rise to the lithosphere dissolved and the upwelling of the asthenosphere,and partially melted the lower crust under the effect of Songpan-Ganze orogeny.