开放体系下岩浆的注入-混合作用:闽西南晚中生代油心地辉长岩的成因

福建武平县油心地辉长岩的年代学、矿物学以及全岩和矿物化学的研究成果表明,油心地辉长岩形成年龄为(87±2)Ma,全岩地球化学显示为大陆边缘弧特征,类似闽东南晚中生代的钙碱性角闪辉长岩,高Mg含量(MgO=8.35%～9.33%).以辉石为主.辉长岩的主要组成矿物为单斜辉石和斜长石,根据两种矿物的环带类型和成分变化,将岩...

Magma recharge and mixing processes in open system:Petrogenesis of Late-Mesozoic Youxindi gabbros from southwestern Fujian

We report geochronological,mineralogical,whole-rock and mineral geochemical data for the Youxindi gabbros from Wuping,Fujian Province.SHRIMP U-Pb dating for zircons indicates that it was crystallized at 87±2 Ma.The geochemistry characteristics of Youxindi gabbros are similar to those of the Late Mesozoic calc-alkaline hornblende gabbros from the southeastern Fujian Province formed in typical active continental margins.However,comparing to those hornblende gabbros,it has higher MgO content(MgO=8.35%-9.33%) and more pyroxene modal.More importantly,electron microprobe analyses and back-scattered electron(BSE) images show that textures and compositions of clinopyroxenes and plagioclases recorded open-system processes of magma recharge and mixing.The zonation types and compositional variations of clinopyroxenes and plagioclases represent three phases of magma evolution,i.e.,early phase,clinopyroxenes with moderate Mg# values,low Ti,Na(Mg#≈70-74,TiO2<0.3%,Na2O<0.3%) contents and plagioclases with An≈70 mol%;middle phase,clinopyroxenes with high Mg# values,moderate Ti,Na(Mg#≈74-80,TiO2≈0.3%-0.8%,Na2O≈0.3%-0.5%) contents and plagioclases with An≈80 mol%;and the final phase,cocrystallized clinopyroxenes with low Mg# values,high Ti,Na(Mg#≈63-70,TiO2>1.7%,Na2O>2%) contents and plagioclases An≈60-10 mol%.Minerals with reverse zonation in the early and middle phases are characterized by cal-alkaline,high Mg#,Ca minerals but crystallized at different physical conditions,such as variable temperature and water pressure.The large variation of mineral compositions from the final phase suggests that relatively high-T alkaline magma recharged and mixed,and the low Mg#,alkaline minerals rapidly crystallized on the preexisting crystal.The whole-rock and mineral geochemistry from the Youxindi gabbros indicates that they were crystallized via cal-alkaline magma recharged and mixed with alkaline magma,implying that an influence of Paleo-Pacific plate subduction underneath the southern China during the Late Cretaceous was weakening due to slab rollback,meanwhile lithospheric extension led to alkaline magma gradually replacing calalkaline magma.This study is of special significance for Mesozoic magma mixing of South China and provides a possible scenario of magma mixing between the same kind of magmas.