Magma Mixing Of Basaltic Trachyandesite In Shangshu Formation,Jiangshan,Zhejiang

Magmatic activity is widely developed in the plate assembly zone,which is also an important place for magmatic mixing.The Jiangshan-Shaoxing fault zone was formed in the Neoproterozoic associated with basic and acid magmatic activity and magmatic mixing.Most petrographic and geochemical investigations on the Jiangshan-Shaoxing fault zone’s magmatic mixing are done macroscopically,however,it has not been properly investigated if diagenetic minerals in igneous rock can reveal the process of magmatic mixing.In this paper,basaltic trachyandesite from the Shangshu Formation in Jiangshan is considered the subject of research.On the basis of detailed observation of geological phenomena,the magmatic mixing,petrogenesis and dynamic process of basaltic trachyandesite of Shangshu Formation were systematically researched by means of EPMA,LA-ICP-MS mineral geochemical composition and whole rock geochemical analysis.This paper mainly obtains the following understandings:(1)According to petrographic analysis,the basaltic trachyandesites have the core-mantle-edge zone texture of clinopyroxene and the core-edge texture of plagioclase.Many mineral inclusions,including ilmenite,clinopyroxene,and plagioclase,are present at the contact boundary between the mantle and the clinopyroxene edge and the core and the plagioclase edge,indicating a complicated magma mixing process during the development of phenocrysts.(2)The petrochemical characteristics show that the basaltic trachyte belongs to the calc-alkaline series with high-potassium,calc-alkaline to dolerite;the degree of depletion and enrichment of trace elements is different,like Nb,Ta,Ba,P,and Ta,Nb and Ti show a negative“TNT”negative;the rare earth elements show a positive Eu anomaly and no Eu anomaly,respectively,suggesting the inhomogeneity of the magmatic composition.The chemical characteristics of clinopyroxene show that the core and mantle of phenocrysts are crystallized under high pressure,and the edge and matrix of phenocrysts are crystallized under low pressure.The phenocryst Mg~#([100*Mg/(Mg+Fe)]increases from the core to the mantle,and decreases to the edge.The core and the mantle have similar an Al content and higher than the edge,with the characteristics of oscillatory zoning.The plagioclase core has An([100*Ca/(Ca+Na)])=44～52,shows a corroded tabular,belonging to labradorite–andesine;while the edge has An=54～66,belonging to labradorite,with anti-zonal features.Overall,it is supposed that the magmatic chemical mixture occurred during the crystallization of the basaltic trachyte in the Shangshu Formation.(3)The results show that the partial basic magma in the deep magma chamber crystallizes the clinopyroxene phenocrysts’core and mantle,then the magma carried the early crystallized clinopyroxene phenocrysts up and injected into the shallow acidic magma chamber.Due to the addition of basic magma,the plagioclase that is crystallizing in the shallow acidic magma chamber finally forms the plagioclase with anti-zonal structure,while the clinopyroxene phenocryst continues to crystallize to form the edge and matrix clinopyroxene.(4)The magma source for basalt trachyandesite in the Shangshu Formation is an enriched mantle with a heterogeneous composition.The magma was formed by the mixing of crust and mantle,which was formed in the tectonic setting of the active continental margin arc.Magmatic chemical mixing occurred during the formation.Different from the mechanical mixture of magma in the eastern part of the Jiangshan-Shaoxing fault area,the chemical mixing of basaltic trachyandesite in Jiangshan Shangshu Formation suggests that Jiangshan has a unique magmatic dynamic background in the Neoproterozoic,which may represent the turning point in the regional structure of Jiangshan-Shaoxing fault zone.The magma mixing may be related to the supercontinent Rodinia breakup.