The magmatic and post-magmatic processes of granitic magma:Constraints from in situ element and isotope compositions of minerals
As an important conponent of continental crust,granitoid together with its magmatic processes have been hot issues in earth science study for a long time.The petrogenesis and evolution processes of granitoids were traditionally constrained by the whole-rock geochemical and isotopic data which become less precise at present.This paper systematically summarizes the in situ analyses data of minerals from granitoids reported in recent years which record not only the intra-but also the inter-mineral variations in elemental and isotopic compositions that are not evident from whole-rock geochemistry,and thus greatly advance the understanding of the magmatic and post-magmatic processes of granitoids.Firstly,in situ mineral composition is an indicator of source nature and mixing process of granitic magma.Significant Hf-isotope variations in magmatic zircons of granitoid may indicate a disequilibrium and preferential melting of zircon at the source,rather than a crust-mantle mixing origin,which extends the idea of"zircon effect"to crustal anataxis.The rare earth element(REE)patterns,Eu anomalies,Sr contents and Sr-Nd isotopic compositions of apatites from a single granitoid sample can be markedly different,indicating that some of the grains are captured from wall rocks during magma generation and ascent,and are products of small-scale crustal assimilation.The compositional zoning in the titanites records various magma-mixing processes and also reveals changes in melt composition,oxygen fugacity and temperature.Zircon Hf-O and apatite Sr-Nd isotopic compositions of granitoids and the enclaves/inclusions hosted in them,as well as the related wall rocks can record the magma mixing and assimilation processes during the formation of these rocks.Secondly,in situ mineral composition reflects the fractional crystallization process of granitic magma.Distinct REE patterns of magmatic apatites suggest the influence of fractional crystallization of other minerals,such as epidote-group,titanite,hornblende,and plagioclase.The regular change of Sm/Nd ratios in magmatic monazites from the granitic pegmatite system reveals the varying degrees of magma fractionation.Element contents and their co-variations of titanite may be controlled by fractionation process and oxygen fugacity of the granitic melt.Oscillatory zoning in magmatic epidote indicates that the granitic magma is depleted in Fe3+during the late stage of epidote crystallization,and there was no complete miscibility between allanite and epidote growth during the crystallization of magma.Thus,the allanite core and epidote rim are compositionally discontinuous.The composition of hornblende analyzed by Electron Probe Micro Analyzer can calculate the temperature,pressure and fo2at which the granitic magma crystallized,and the depth of the magma during generation can be inferred from the above parameters.Thirdly,in situ mineral composition records post-magmatic tectonothermal events and mineralization processes of granitoids.Apatites from granitoids altered by late stage metamorphic or metasomatic processes are characterized by low light REE contents and large variations in Nd isotopic compositions,and the apatite gNd(t)and zircon εHf(t)for most of the granitoid samples exhibit significant decoupling.δ18 O values of apatite and titanite are prone to be affected by metamorphism and/or fluid circulation,resulting in decoupling of δ18O in accessory minerals.The major and trace element compositions and U-Th-Pb isotopic systems of alterated monazites suggest that coupled substitutions and common Pb contamination and/or Pb loss may have occurred during fluid-aided modification.The compositional zoning patterns of hydrothermal epidote-group minerals indicate that the hydrothermal fluid evolves continuously under oxidizing conditions.The temperature of mineralization and pH condition of the ore fluid can be calculated by the composition of hydrothermal epidote using mineral-fluid equilibria modeling.Research shows that adequate quantities of Ca2+ions released by hydrothermal epidote into the fluid can promote the sulfide mineralization efficiently.To summarize,the improvements on analytical techniques of in situ compositions of minerals have advanced our understanding of the magmatic and post-magmatic processes of granitoids.In the future research,how to make good use of the above datasets and offer their complementary strengths will be an important direction in this field.
GranitoidMagmatic and post-magmatic processesIn situ mineral compositionSource natureFractional crystallization processMetamorphism and metasomatism
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维普
