Petrogenesis And Magmatic-hydrothermal Evolution Of A-type Granite In The Yanshan And Kuiqi Areas, East China

Being one of the peculiar non-orogenic igneous rock types, A-type granite commonly occurs in post-orogenic or intraplate extensional settings. It can provide important information on continental dynamics and record the history of continental crust reworking and regional tectonic evolution. The intensive and widely distributed A-type granites of the Early Cretaceous in the eastern part of the North China Craton have been research focus for decades. Although numerous research works have been done, controversies still exist in regarding the source(s) and tectonic settings. The concept of the A-type granite was put forward in the late 1970 s, however, after nearly fifty years extensive investigations, the behaviors of characteristic elements(Th, U, Hf, Y, P, LREE etc.) and volatile components during the magmatic-hydrothermal evolution are poorly documented.The main objective of this dissertation is to delineate trace elements and volatile behavior during magmatic-hydrothermal evolution through case study of A-type granites in the Yanshan area(Xiangshan arfvedsonite granite, Yansehu arfvedsonite granite and Xiwanzi magnesio-arfvedsonite granite) and Kuiqi miarolite. Systematic petrological, element geochemical, isotopic geochemical and zircon U-Pb geochronological analyses were carried out on the basis of detailed field investigation in order to constrain the petrogenesis, source, and geodynamic setting of the A-type granite in the Yanshan area. Inclusion and zircon formed in different stage were examined to trace the evolution of the A-type granitic magma and discuss the geochemical characteristic of the primary magma and the behavior of the element and volatile during magmatic-hydrothermal evolution. Our results demonstrate that:(1) The Xiangshan and Yansehu arfvedsonite granite and the Xiwanzi magnesio-arfvedsonite granite have high SiO2, Na2O+K2O, and HFSE contents, high FeOT/(FeOT+MgO) ratios and zircon saturated temperature, which are typical for A-type magmatism. The Xiangshan and Yansehu arfvedsonite granite were probably generated by dehydration melting of the biotite- or hornblende-bearing granitic crustal source rocks triggered by the upwelling mantle magma, and mingling and/or mixing might be involved during the ascending and emplacement of the granites.The relative high εNd(t)(-7.6 ~-7.1) and εHf(t)(-8.0 ~-2.4) values of the Xiwanzi magnesio-arfvedsonite granite suggest the input of depleted mantle. The eastward young of the Early Cretaceous A-type granites in the Yanshan area was possibly related to the change of subduction direction of the Paleo-Pacific oceanic plate changed.(2) The Xiangshan and Yansehu arfvedsonite granites were formed at 116.4 Ma and 115.4 Ma, respectively. The zircon morphology, texture and composition of the two complexes are similar. Zircon grains form both granites can be divided into three types: Type-1A zircon grains are euhedral crystals, multiple oscillatory growth zones and transparent and have abundant apatite inclusions. They have low U and Hf contents, indicating they crystallized from hypersolvus granitic stages with high temperature and volatile-undersaturated environment. The Type-1B zircon grains are euhedral crystals, oscillatory growth zones and cloudy and red-brown in transmitted light and contain plenty of thorite, Fe-xoide and xenotime inclusions. They have obviously high U, Hf, La, Pr and Nd contents, which suggest that they also crystallized from hypersolvus granitic stage and reacted with the F-rich fluid in a volatile-saturated environment. The Type-2 zircon grains are pyramidal, occasionally oscillatory growth zones. Their transparencies are between Type-1A and Type-1B zircon grains. They occur in the graphic texture with quartz and alkali feldspar, and sometimes coexist with fluid inclusions. The Type-2 zircon grains have the highest Hf, REE, Y, Th and P contents, indicating that they crystallized from subsolvus granitic system at relatively low temperature in a volatile-supersaturated environment.(3) δ18O values of the three types of zircon from the Xiangshan arfvedsonite granite decrease gradually consisting with the transition from magma to hydrotherm. We suggest that the decreasing of the δ18O values resulted from the Rayleigh fraction.(4) The Xiwanzi magnesio-arfvedsonite granite was formed at 132.6 ~ 130.3 Ma. Zircon from the magnesio-arfvedsonite granite can be divided into two types. Type-1 zircon grains are euhedral crystals, multiple oscillatory growth zones and transparent and have less inclusions. Zircon grains of this type coexist with intergrowth quartz and K-feldspar. They have slightly positive Ce and negative Eu anomalies, high Hf contents and Zr/Hf ratios. The Type-1 zircon grains crystallized from the hypersolvus granitic stage at high temperature under a volatile-saturated environment. The Type-2 zircon grains are pyramidal, multiple oscillatory growth zones and transparent and have abundant inclusions. They have the highest Hf contents and low Zr/Hf ratios, indicating that they crystallized from subsolvus granitic system with relatively low temperature in volatile-supersaturated environment.(5) The Kuiqi miarolite has two types of zircon. Type-1A zircon grains are euhedral crystals, multiple oscillatory growth zones and transparent and rarely have inclusions. They have obviously positive Ce and negative Eu anomalies, indicating they crystallized from the hypersolvus granitic stage at temperature and under a volatile-undersaturated environment. The Type-1B zircon grains are subhedral or anhedral crystals and have slightly oscillatory growth zones and little inclusions. This type of zircon also crystallized from the hypersolvus granitic stage but later than that of the Type-1A zircon. Some Type-1B zircon grains are commonly murky with cracks, pores and mineral inclusions. They have high content of LREE, flat LREE distribution patterns and slightly positive Ce and obviously negative Eu anomalies. Maybe zircon grains interact with the fluid in the volatile-supersaturated stage.(6) The occurrence of different zircon types with different morphology and texture in the A-type granites can be used to trace the evolution of A-type granitic magma. During the evolution from hypersolvus to subsolvus, incompatible elements such as Th, U, Hf, Y, P and REE can be enriched in the system.(7) Quartz crystals from the Kuiqi miarolite contain abundant inclusions, which have preserved a complete record of the magmatic-hydrothermal evolution. The M-type inclusions represent the melt entrapped during crystallization, in addition, the occurrence of euhedral arfvedsonites prismatic crystals in the melt inclusions indicates that the primary magma was probably volatile-rich. Both ML-type inclusions and the coexisting M- and V-type inclusions reflect the appearance of magmatic fluids circulation(towards the end of crystallization) and the magma-volatile immiscibility, characterizing the transition to the hydrothermal stage. And the initial magmatic fluid was supercritical fluid.