Evolution Mechanism Of Silicic-rich Magma Reservoir

As an important part of continental crust,silicic igneous rock（SiO₂>60%）contain important information about continental crust differentiation,crust-mantle magmatism and volcanic eruption mechanism in their genesis,which is of great significance for exploring magma evolution mechanism and continental crust evolution history.The evolution process of silicic magma reservoir is an important process to control the differentiation mechanism of silicic igneous rock and the diversity of its rock character and geochemical composition.Understanding its evolution process is of great significance for exploring the petrogenesis mechanism of silicic igneous rock and the petrogenesis relationship between different igneous rocks.The late Mesozoic granitic rocks with large area and multiple stages developed in South China,and their petrogenesis has always been controversial.Among them,the Gan-Hang Belt experienced strong tectonic-magmatic activities in the early Cretaceous,and developed a large area of silicic igneous rock,mainly characterized by A-type granite and volcanic intrusive complex.Previous researchers have done a lot of research on the space-time distribution,material sources and regional tectonic background of these rocks.However,there are still great problems about the mechanism of magmatic differentiation and evolution and the genetic relationship between different lithologies.Therefore,this paper chooses the Honggong,Caiyuan,Zhangkengkou and Shanghekou-Sucun-Huangkang-Lingkeng pluton in the middle part of Gan-Hang Belt as the research objects for petrography,whole rock major and trace element,zircon U-Pb geochronology,zircon trace element and zircon Hf isotope analysis.Petrographic analysis shows that quartz syenite,monzoporphyry is developed in Caiyuan pluton,granite porphyry and silicic lava are developed in Honggong pluton,quartz syenite porphyry and granite porphyry are developed in Zhangkengkou pluton,and alkali feldspar granite is developed in Shanghekou-Sucun--Huangkang-Lingkeng pluton.Zircon U-Pb chronology indicates that these rocks were formed between 125 133 Ma and were products of early Cretaceous magmatic activity.These rocks generally have geochemical characteristics of alkaline-rich and low Mg O and Fe O.The characteristics of petrology,geochemistry and zircon Hf isotope indicate that the primitive magma of these rocks mainly came from the partial melting of Proterozoic metamorphic basement rocks,and experienced the mantle-derived magma mixing process.The Caiyuan monzoporphyry,Zhangkengkou quartz syenite porphyry,Shanghekou--Sucun-Huangkang-Lingkeng alkaline feldspar granite,and Zhangkengkou granite porphyry shows a similar trend in major and trace elements,and have similar Hf isotope composition,zircon structural characteristics,and zircon trace element composition,indicating that these rocks originated from the same magma reservoir.The deep magma reservoir with high crystallinity have undergone the injection of basic magma and the activation process of magma reservoirs,and monzoporphyry represents the residual accumulation of crystals in deep magma reservoirs after melt extraction.After the activation of the deep magma reservoir,the extracted silica-rich melt accumulates in the shallow crust to form the shallow magma reservoir,and undergoes crystal-melt separation.The high silica melt is extracted into the shallow crust to form Zhangkengkou granite porphyry and alkaline feldspar granite with A-type granite characteristics,while the residual crystal accumulation after the melt extraction forms Zhangkengkou quartz syenite porphyry.Honggong quartz syenite and Honggong granite porphyry represent a high Zr magmatic evolution series,and the large variation of zircon Hf isotope indicates that the magma reservoir has experienced more intense mantle-derived magma mixing.The injection of high-temperature mantle-derived magma into the crustal magma reservoir,on the one hand,activates the crustal magma reservoir and causes crystal-melt separation;On the other hand,the injection of high-temperature magma leads to Zr unsaturation in the magma system,and most of the early crystallized zircons are melted.Zr are transferred from the crystal to the melt,and Zr is enriched in the melt.And driven by crystal-melt separation,the Zr-rich melt was extracted into the shallow crust to form Honggong granite porphyry,and the early cumulate crystals remaining at the bottom of the magma reservoir and the residual Zr-rich melt formed Honggong quartz syenite.