| The North Qaidam is located in the northeast margin of Qinghai-Tibet plateau,north of the Qaidam block and south of the Qilian orogenic belt.North Qaidam ultrahigh pressure collision zone has been identified as a typical continental deep subduction collision orogenic belt.This paper elucidates the forming age,geochemical characteristics,petrogenesis and tectonic setting of the Paleozoic granitoids from the North Qaidam,based on detailed studies on their petrography,geochronology,petrogeochemistry and zircon Hf isotope analysis.The Paleozoic granitoids from the North Qaidam are mainly monzonitic granite,granodiorite and quartz diorite.Combined the data in this study and the chronological results from the literatures,the granitoid rocks can be divided into four periods: the first stage monzogranite?500450 Ma?;the second stage granodiorite?450430 Ma?;the third stage monzogranite?420 380 Ma?;the fourth stage quartz diorite?375 350 Ma?.The SiO2 content of the first stage monzogranite is 72.53% 76.00%,total alkali content?Na2O+ K2O?is 6.36% 7.67%,belong to high potassium calc-alkaline series,low Mg# value?18 29?and A/CNK is 0.87 1.05.The rock type analysis is shown as high differentiation of I-type granite,total rare earth??REE?relatively high(151.11×10-6447.20×10-6).The samples exhibit LREEs enrichment,HREEs depletion [?La/Yb?N= 2.71 5.24),with Eu negative anomaly??Eu =0.49 0.80?.Besides,the samples enriched in large ion lithophile elements?LILEs?,depleted in high field strength elements?HFSEs?.Zircon ?Hf?t?values is 7.4 1.3 and corresponding two-stage model ages TDM2 is 13591689 Ma.Magma originated from the partial melting of the Mesoproterozoic crust,which corresponds to the oceanic subduction stage of the North Qaidam orogenic belt.The SiO2 content of the second stage granodiorite is 62.98% 63.86%,Al2O3 content is higher?17.60% 18.70%?,A/CNK is 0.88 1.06.The total rare earth(?REE =46.68 × 10-6104.46 × 10-6)is relatively low,the samples exhibit LREEs enrichment,HREEs depletion [?La/Yb?N = 15.00 21.28],the ?Eu is 1.10 1.24.Besides,the granodiorite shows enrichment of large ion lithophile elements?LILE?Rb,Th,K,and Sr,depletion in high field strength elements?HFSEs?Nb,Ta and Ti.Combined with the characteristics of high Sr(705.00×10-6865.00×10-6)low Y(4.55×10-69.23 × 10-6)and high Sr/Y ratio?76.38178.68?,it is suggested that the granodiorite is I-type granite with the characteristics of adakite.The magma is derived from partial melting of basic materials in the thickened lower crust.It corresponds to the collision orogeny in the North Qaidam.The SiO2 content of the third stage monzogranite is 68.12% 74.86%,and total alkali content?Na2O + K2O?is 7.10% 9.48%,belong to high potassium calc-alkaline series.Mg# value of monzoranite is 0.32 0.46 and A/CNK is 0.99 1.08.The total rare earth(?REE=57.76×10-6165.45×10-6)is relatively low,all of which are characterized by enrichment of light rare earth elements and deficit of heavy rare earth elements [?La/Yb?N= 2.53-22.27],with Eu negative anomaly??Eu=0.08-0.76?.The samples are enriched in large ion lithophile elements?LILE?,such as Rb,Th and U,depleted in high field elements?HFSE?.The ?Hf?t?values are ranging from-3.3 + 3.3,and corresponding two-stage model ages(TDM2)is 1083 1411 Ma.The magma origined from the Mesoproterozoic crust remelting,caused by the mantle-derived magma underplating,and mixed with the mantle materials.These granitoids formed during the slab break off-exhumation stage,belong to the co-collision to post-collision granite.The SiO2 content of the fourth stage quartz diorite is 54.12% 56.06%,and total alkali content?K2O+ Na2O?is 4.49% 6.74%,The Al2O3 content of quartz diorite is16.58% 22.37%,A/CNK is 0.74 0.96,high Mg O content?3.00% 6.56%?and Mg# value?58 67?.The total rare earth(?REE = 42.11×10-673.01×10-6)is relatively low,all of which are characterized by enrichment of light rare earth elements and deficit of heavy rare earth elements[?La/Yb?N=4.468.92)].The samples are enriched in large ion lithophile elements?LILEs?,such as Rb,Th and K,depletion in high field strength elements?HFSEs?Nb,Ta.The ?Hf?t?values is 4.6 13.4,it is derived from the partial melting of lithospheric mantle and mixing with the crust,which corresponds to the extension and collapse stage of the North Qaidam orogenic belt,and belongs to post-collision granite. |
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