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

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

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,将岩浆演化分为三个阶段:早期,中等Mg#值、低Ti、Na含量辉石(Mg#≈70～74、TiO2<0.3%、Na2O<0.3%)和～An70斜长石;中期,高Mg#值、中等Ti、Na含量辉石(Mg#≈74～80、TiO2≈0.3%～0.8%、Na2O≈0.3%～0.5%)和～An80斜长石;晚期,低Mg#值、高Ti、Na含量辉石(Mg#≈63～70、TiO2>1.7%、Na2O>2%)和An60-10斜长石。具反环带结构的早、中期矿物均代表了钙碱性高Mg#值和高Ca含量的岩浆,形成时温度和水压等物理条件有变化;晚期矿物成分变化范围大,指示有高温中性偏碱性岩浆的注入和混合,在已有晶体上快速结晶低Mg#值偏碱性矿物。研究结果表明,辉长岩在钙碱性岩浆与碱性岩浆双重作用下形成,暗示早期钙碱性岩浆的形成与晚白垩世古太平洋板块的影响有关,随着研究区岩石圈伸展增强,碱性岩浆代替了大陆边缘型的钙碱性岩浆;同时也证明岩浆混合作用可以存在于酸性和基性端元岩浆之间,也可以存在于成分大致相近的岩浆中。

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 gabhros, it has higher Mg# content （Mg# = 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#≈70～74d、TiO2〈0.3%、NaO〈0.3%） contents and plagioclases with An=70 mol%; middle phase, clinopyroxenes with high Mg# values, moderate Ti, Na （Mg#≈7d～80、TiO2≈0.3%～08%、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〉17%、Na2O〉2%） contents and plagioclases An^60 10 tool%. 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 cry stallized 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 cal-alkaline 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.