Source,Magmatic Evolution And Hydrothermal Fluid Activity Of Cretaceous Granites In The Coastal Region Of SE China:Records From Apatite Geochemistry

The existence of granite in the Earth is unique,which distinguishes the Earth from other planets in the solar system.As a main component of the continental crust,granite can provide important constraints on the formation and evolution of the continental crust and crust-mantle interaction through the Earth's history.Large-scale magmatic activity developed in the southeastern coast of China during late Mesozoic.However,the petrogenesis of these granitic rocks remains controversial.In order to investigate the source,magmatic evolution and post-crystallization hydrothermal activity of the Cretaceous granite,we conducted in-situ geochemical(major-,trace-element and Nd and O isotope compositions)analyses on apatite,together with whole-rock geochemistry from gabbro and granitic rocks in Pingtan and Zhangzhou intrusive complex.The major conclusions are summarized below:(1)Apatite textures and geochemical features reveal the magma source,magma evolution and hydrothermal activity of the Pingtan intrusive complex.Regardless of limited range in Nd isotopes in both bulk rock[?_Nd(t)=-2.0 to-0.4]and associated apatite[?_Nd(t)=-3.8 to-0.4]from the Pingtan igneous complex,the apatite shows wide compositional and textural variations from gabbro to granite.Apatite from the gabbro displays a zoning structure characterized by increasing F and Sr but decreasing Cl and LREE from the core to rim.The increase of Sr from the core to rim is attributed to plagioclase accumulation,and the decreases of LREE and Cl contents from the core to rim are caused by post-crystallization hydrothermal activity.The high Cl content in apatite from the gabbro further suggests derivation of the mafic magma from a mantle wedge metasomatized by Cl-rich sediment.In contrast,apatite from the granitic rocks have low Cl and Sr but high F and Yb contents.Among them,apatite from granodiorite shows a negative correlation between Sr content and La/Yb ratio,indicating that crystallization of apatite is co-precipitated with fractionation of hornblende and plagioclase.Apatite from granite shows a positive correlation between Sr content and Eu/Eu*,indicating that granitic magma experienced a feldspar-dominated fractionation.The negative correlations between Sr and U content as well as F/Cl ratio in some apatite from granite indicate highly fractionated I-type granite was influenced by hydrothermal fluid.Besides,the discontinuous compositional variations of apatite from the three rock types suggest the gabbro and granitic rocks of the Pingtan intrusive complex were,respectively,derived from the mantle and crustal sources with similar?_Nd(t)values.Therefore,a new hypothesis is put forward to decipher the origin of‘equal-isotope'bimodal igneous rocks in SE China and a new explanation is proposed for the geochemical similarity of apatite from highly differentiated I-type granite and S-type granite.(2)Apatite textures and geochemical features record magma devolatilization,self-mixing and hydrothermal activity of Zhangzhou intrusive complex.Based on texture and composition,apatite form granodiorite can be divided into two groups of magmatic apatite and hydrothermally altered apatite.Magmatic apatite is dominated by euhedral crystals and shows positive correlations between Sr and Eu/Eu*and La/Yb ratio.These apatites have a limited range in O isotopes(?¹⁸O=5.6-6.7‰)and their elemental features fall within the"I-type granite".These features indicate that apatite crystallized in a granitic magma derived from newly accreted arc crust and co-precipitated with plagioclase and titanite.Hydrothermally altered apatite is mainly anhedral crystals,often in the form of apatite overgrowth rims.This type of apatite has high F/Cl ratio and low Th/U ratio but low?REE content and low?¹⁸O value(2.9-6.2‰),and shows a negative correlation between Sr content and Eu/Eu*.These geochemical features reflect high F/Cl and oxidized hydrothermal fluid activity.Apatite from monzogranites occurs as an early crystallizing phase forming inclusions in biotite,plagioclase and titanite,and is characterized by core-rim zonation textures with a dark core and bright rims in back-scattered images.Both apatite core and rim domains show geochemical compositions similar to that from typical I-type granite and also have mantle-like O isotope compositions(the core has??¹⁸O=5.3-6.8‰and the rim has??¹⁸O=5.2-6.4‰,respectively),indicating crystallization from granitic melts derived from newly accreted crust.In contrast,the core domains have remarkably higher SO₃,Na₂O,and Li concentrations but lower SiO₂,REE,Y,and Zr concentrations than the rim domains.These features suggest mixing between two cogenetic magma batches,with variable degrees of subsequent differentiation.Partial melting of the newly accreted sulfur-rich arc crust is likely to form high S granitic magma with apatite core crystallization.The apatite rim is formed in granitic magma,which experienced devolatilization with low Cl,Li and S contents.(3)Our study,therefore,demonstrates that apatite geochemistry has many advantages over whole-rock chemistry in petrogenetic discussion.In SE China,hydrothermal fluid activity may lead to isotopic heterogeneity of granite,and mixing of different batches of cogenetic magma is an important mechanism for chemical diversity of granite.