Geological And Geochemical Characteristics And Petrogenesis Of Qingshanbao Pluton In Longshou Moutains, Gansu

The Qingshanbao pluton,which is located in the central part of Longshou Mountain in Gansu,next to Jiling pluton.The geotectonic location is located on the southern margin of the Alxa arcuate structure margin of the North China Plate.In this paper,15 samples of K-feldspar granite,granodiorite,quartz monzonite,fine-grained diorite,monzodiorite and aplite granite were selected in Qingshanbao area.The microscopic identification,elemental geochemical analysis,zircon U-Pb dating and zircon Hf isotope test were carried out respectively.Combined with the previous research results,the magmatic activity period in the area was divided,and the relationship between rock genesis and tectonic evolution was further explored.The samples can be roughly divided into three categories:high Sr/Y type granitic?K-Feldspar Granite,granodiorite,quartz monzonite?,Island arc type-I diorite?fine granodiorite,monzodiorite?and A-type granite?aplite granite?.The geochemical characteristics:?1?High Sr/Y type granites?441.6±4.2⁴32.8±4.2Ma?,with ASI less than 1.1,enriched with large ion lithophile elements and light rare earth elements,showing typical I-type granite characteristics,while also meeting the characteristics of typical Adakite rocks;The results of Hf isotope show that the values of?Hf?t?are negative?-14.32^-3.19?,and the values of TDM2 are between 1641 and 2346 Ma.It is revealed that the source of rock material is mainly from ancient crust and may contain a small amount of mantle-derived material.?2?Islandarc type-I diorite?438.6±4.7⁴34.3±9.2Ma?,rich in aluminum,low of silicon,strong in LREE fractionation,enriched with large ion lithophile elements,loss of high field strength elements,showing island arc rock characteristics;The results of Hf isotope show that epsilon?Hf?t?is predominantly positive?-5.65⁺7.27?and has a younger T_DM2?957-1772 Ma?.It reveals that the rocks are mainly formed by partial melting of new crustal materials,and also contain a large amount of mantle-derived materials.?3?Type A-granite?430.1±5.6Ma?,the rare earth distribution curve has a strong negative?Eu anomaly,and the whole presents a"Seagull"type distribution;The results of Hf isotope show that the value of?Hf?t?is?-4.05⁺1.69?and the value of T_DM2 is between 1351 and 1666 Ma,which reveals that the source of rocks may be mainly crustal,with a small amount of mantle-derived materials involved.Petrogenesis and Tectonic Evolutionary relationships indicate:?1?The early formation of high Sr/Y type granites may be related to the collision orogeny of the Alxa massif and the North Qilian Island Arc after the closure of the North Qilian Ocean.?2?The island arc I-type diorite may have formed during the transitional period from compression to extensional environment during land-land collision in the Alxa massif.?3?Late A-type granite is the product of intraplate extensional movement after collisional orogeny.Therefore,the granitoid rocks in Qingshanbao area record the tectonic evolution of the Alxa massif and the North Qilian island arc from collisional orogeny to intraplate extensional movement after the closure of the North Qilian oceanic crust.