Self-mixing Of Late Cretaceous Siliceous Magma And It's Implacationg In Eastern Zhejiang

Magma mixing is interactions that occur between two or more different homogenous or heterogeneous magmas which has different compositions,viscosities,densities,or temperatures,in a way of mechanical mixing or chemical mixing.Magma mixing is one of the main causes of igneous rock diversity,is also the important mechanism inducing volcanic eruptions.However,in recent years,the mixing effect between siliceous magma has received more and more attention and research.Most of the magma mixing processes related to volcanism occur in the shallow magma chamber of the earth's crust,often accompanied by the magma replenishment process,is also an important mechanism for inducing volcanic eruptions.The Late Mesozoic volcanic activity in the southeastern coastal areas of China is intense,and the magma mixing phenomenon is widely recorded.However,previous studies have mostly focused on the mixing between the basic and acidic end-mass magma in the plutonic rocks,with the close association of plutonic intrusive rocks in space and time,closely linked to the causes of volcanic rock,less reported related to magmatic mixing phenomenon.In This paper,we studied the mixing phenomenon between the trachytic magma and rhyolite magma in a secondary crater of the Xiaoxiong volcano in eastern Zhejiang.Through detailed field investigation,mineralogy,petrography and geochemical research,the Genetic relationships between the two siliceous magma and the formation mechanism are revealed.At the same time,we discussed the structure of the acidic volcanic magma chamber ?the magma evolution and Eruption process of the Late Mesozoic eruption in the southeastern coast of China.Field investigations and petrographic studies have shown that trachytic magma is mostly strips of different sizes and lentoid distribution in rhyolite magma,which is locally mixed.The phenocrysts in trachytic magma rocks are basically orthoclases,which strongly corroded and developing macular structure.reaction side structures are often seen around the secondary mineral polycrystals(consisting of titanium magnetite + apatite + zircon).The phenocrysts in rhyolite are main feldspar,sanidine and quartz,the crystal size is small,and the erosion phenomenon is not developed.The characteristics of major?trace element of rock samples,and other geological evidences show that the two siliceous magma are mainly mechanically mixed.The LA–ICP–MS U–Pb geochronology study of zircons in uniform mixing rock samples with trachytic magma and rhyolite magma shows that the age of the volcanic rock is 88.88±0.46 Ma,which is the same as that of other subvolcanic rocks reported in xiaoxiong basin.those subvolcanic rocks have always been the product of the late Cretaceous acid volcanic activity in the region.Good covariation between the major and trace elements of the trachyte and rhyolite,and the near-parallel distribution of the trajectory of the rare earth element chondrite?the concentrated distribution of Hf(t)values of zircons in mixed rocks and other geological evidences indicate that they are products of crystallization crystallization of homologous magma.The differences in mineral composition and structure between trachyte magma and rhyolite magma indicate that they are derived from different parts of the same layered magma chamber,where the trachyte magma should represent the porphyrite-rich portion of the bottom and edge of the magma chamber rich in crystal and lean(orthoclases + magnetite + zircon + Apatite);the rhyolite magma with higher degree of differentiation gathers in the upper part of the magma chamber to form a rich and crystal-depleted part.Cathodoluminescence images(CL images)and electron probes were selected from apatite particles in trachyte and rhyolite mixed rocks.Apatite can be classified into two types according to structure and composition,namely ZA with nuclear structure and HA type apatite with uniform structure,the cores of ZA type apatite and the mutation of Si,REE and other trace elements between the core,attributed to the change of physical and chemical composition of magma.The process of mixing trachyte grain with rhyolite magma was recorded;while HA-D type and HA-B type apatite with uniform structure and composition were crystallized separately in rhyolite magma and trachyte magma.The particles are basically unaffected during the mixing process.By studying the apatite particles of different structures and compositions,the mixing effect between the coarse magma and the rhyolite magma is further proved.The mixing of trachyte magma and rhyolite magma is the result of self-mixing in the shallow magma chamber of the crust.This process may be controlled by the underplating of the basic magma at the bottom of the magma chamber.In the case of more basic magma underplating,the coarse magma in the porridge area at the bottom of the magma chamber is rapidly warmed up and activated,thereby moving upward and self-mixing with the upper rich melt-rich crystal rhyolite magma.The process leads to an increase in pressure in the magma chamber,which is likely to be the main cause of volcanic eruptions.This finding provides an important reference for understanding the large-scale acid volcanic eruptions and magma evolution mechanisms and magma chamber structures in the southeastern coastal areas of China.It provides a reliable illustration for understanding the self-mixing effect between magma in the shallow magma chamber.