Geochronology And Geochemistry Of Late Paleozoic Granites In The Danfeng Area,North Qinling Belt

The North Qinling Terrane,one of the important tectonic units in the Qinling Orogenic belt,locates between Luonan-Luanchuan fault and Shangdan suture belt.Paleozoic granites can provide important information about the evolution of the Shangdan ocean.Previous studies indicate that the Paleozoic granites in the North Qinling can be subdivided into three stages:507-470Ma?oceanic subduction?,460-422Ma?syn-collision?and 415-400Ma?post-collision?.However,the genesis Late Paleozoic granite after the collision of Shangdan ocean?400Ma?still remains unknown,petrogenesis of these granites has potential significance for the nature of the basement of the Northern Qinling and the crust melting mechanism after the collision of Shangdan ocean.This paper focus on the Late Paleozoic Dafanggou pluton,Liangchahe pluton,Yaogou pluton and Tieyupu pluton to the north of the Shangdan Suture,the major and trace elements,Sr-Nd isotopic compositions,mineral chemistry,zircon LA-ICP MS U-Pb dating and Lu-Hf isotope have been reported in this thesis,determine their formation age,genetic types,source region and geodynamic implication.According to the above results,the following conclusions is got:Zircon U-Pb chronology reveals Early Silurian and Early Devonian granite magmatism in the Danfeng area,the early-period magmatism occurred around 440 Ma,which include the Tieyupu pluton?438±1.8Ma?,while the second stage granite magmatism occurred around 400 Ma,which include the Dafanggou pluton?404.1±2.7Ma?,Liangchahe pluton?405.8±2.8Ma?and Yaogou pluton?403± 2Ma?.The Tieyupu biotite granodiorite,has SiO₂= 69.1%?70.1%,Na2O = 4.8%?5.51%,Al2O3 = 15.0%?15.7%,Na20/K2O=1.9?2.26,A/CNK =1.04?1.10,belongs to the meta-aluminous-peraluminous,low total REE??REE=55.4?60.8 ppm?,8Eu= 1.04?1.10,high Sr/Y ratios and low Nb/Ta?7.44?8.62?ratios.The granite display evolved Sr-Nd isotopic compositions?sNd?t?=-0.6?-0.3?and depleted zircon Lu-Hf isotopic compositions?sHf?t?=2.65?12.89?,the zircon saturation temperature is medium?732?739??.The Tieyupu granodiorite was likely the product derived from the dehydration partial melting of the juvenile plagioclase-rich basic lower crust,and the evolved Sr-Nd isotope may be caused by the contamination of a small amount of enriched components.The Dafanggou pluton is mainly composed by biotite monzogranite,they have SiO₂=68.4%?71.7%,Na2O=4.18%?5.59%,Al2O3=15.6%?17.9%,Na2O/K2O=1.07?2.74,their evolved Sr-Nd isotopic?sNd?t?=-10.4?compositions and low total REE??REE=52.6?90.4ppm?suggesting a sediment source region.They have insignificant Eu anomalies of 0.73 to 1.23,negative ?Hf?t?values of-9.35 to-0.25,in combination with their low melting temperature?662-721??,it can be inferred that the Dafanggou monzogranite derived from muscovite dehydration melting of Proterozoic sediment rocks under low temperature.The Yaogou pluton is mainly composed by biotite monzonitic,they have SiO₂=67.0%-68.7%,Na2O=3.80%?4.42%,Al2O3 =15.2%?17.1%,Na2O/K2O= 0.81?1.00 and A/CNK ratios of 1.04?1.08,enriched in K,belongs to high-K calc-alkaline series;The granite has higher total REE??REE=105?191 ppm?,enriched in P,K and Ba,?Eu =0.71-0.95;they have juvenile Sr-Nd isotopic compositions??Nd?t?=0.1?and positive zircon Hf isotopic ratio??Hf?t?=1.24?12.22?,the zircon saturation temperature is 781-833? reveals that high melting temperature.The comprehensive analysis shows that Yaogou monzonitic granite was likely derived from the partial melting of the Neoproterozoic biotite plagioclase gneiss in the lower crust caused by the biotite dehydration at the temperature of 800? to 850?.The Liangchahe pluton is mainly composed by gneissic biotite monzogranite,they have SiO₂ = 66.9%?68.0%,Na2O=4.85%?5.19%,Al2O3=17.4%?16.2%,Na2O/K2O=2.16?3.11,A/CNK ratios of 1.02?1.08,low total REE??REE=56.3-134 ppm?contents,?Eu =0.83-1.24,high Sr/Y and extremely low Nb/Ta?0.21-0.57?,similar to Tieyupu pluton.Besides,they have evolved Sr-Nd isotopic compositions?sNd?t?=-2?and variable zircon Hf isotopic composition?sHf?t?=-14.97?9.80?,these feature suggesting heterogeneous source region.They have medium zircon saturation temperature?758 to 771??.The Liangchahe monzogranite may derived from the dehydration melting of the plagioclase-rich basic lower crust.In combination with regional geology and previous works,the four granite plutons along the Shangdan suture reveal two crustal melting events after the collision between the Qinling micro-terrane and North China Block.The first stage occurred in Early Silurian??440Ma?,which referred to the reworking of Danfeng Group to form the Na-rich granites?Tieyupu pluton?,formed in the syn_collisional setting;the second stage occurs in Early Devonian,including the reworking of Danfeng Group to form the Na-rich granites?Yaogou pluton?,the melting of heterogeneous Danfeng and Qinling groups,formed the Na-rich granites that have variable Sr-Nd and zircon Lu-Hf isotopic compositions?Liangchahe pluton?and emplacement of the Na-rich granites into middle crust will induce the melting of sediment-rich rock to form the Dafanggou pluton,this crustal melting events occurred in post-collisional setting,which was formed in the transitional environment of compressional orogeny and extensional structure after the the collision between the Qinling micro-terrane and North China Block.