Origin of mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Zhejiang Province, Southeast China: Implications for magma mixing and crust–mantle interaction
Origin of mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Zhejiang Province, Southeast China: Implications for magma mixing and crust–mantle interaction
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DOI：
10.1016/j.lithos.2012.12.005
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发表时间：
2013-02
期刊：
影响因子：
3.5
通讯作者：
Liang Liu;J. Qiu;Zhen Li
中科院分区：
文献类型：
--
作者：
Liang Liu;J. Qiu;Zhen Li
花岗岩类深成岩体中的镁铁质包体的起源一直是一个有争议的问题。本文通过详细的岩石学、矿物学、地球化学性质和SrNdHf同位素资料、UPB锆石定年、野外和岩石学观察，对中国东南部沐臣岩体中的镁铁质微颗粒包体及其寄主石英二长岩进行了详细的岩石学、矿物学、地球化和锶锶汞同位素数据的研究，以评价其成因。锆石LA-ICPMS UPB定年获得MME和寄主石英二长岩的结晶年龄分别为112.4±1.2 Ma和112.1±1.0 Ma，表明它们形成于早白垩世晚期。野外和岩石学观察，如球状、背脉、双包体、捕虏晶、针状磷灰石和斜长石中具有反复再吸收表面的振荡分带，表明MME是注入并与寄主长英质岩浆混合的更镁铁质岩浆的球状。在地球化学上，寄主二长岩为中酸性、偏铝质、碱性和富K。寄主二长岩富Rb、Th、U，亏损Sr、P、Nb、Ta、Ti，具中-重度Eu亏损(δEu=0.12~0.6 0)。这些矿物学和地球化学特征将石英二长岩归类为I型花岗岩类。MME具有与寄主二长岩基本相似的微量元素特征，但不同之处在于相对富含Sr和P，亏损更多的Zr和Hf，以及弱至中等的Eu亏损(δEu=0.43-0.93)。MME和寄主石英二长岩的主量元素和痕量元素数据与同位素数据相比，产生了协变阵列，这是岩浆在其形成过程中混合的结果。MME和寄主石英二长岩的初始~(87)Sr/~86Sr比值(ISR)分别为0.7058~0.7070和0.7062~0.7065，且都具有较高的εND(T)值(MME为−2.6~+0.6；石英二长岩为−3.2~−2.4)。然而，来自MME的锆石的εHf(T)值(−0.4~+6.2)与寄主石英二长岩(−1.0~+1.8)不同，表明MME和寄主花岗岩类主要来自不同的岩浆结晶，为镁铁质-长英质岩浆混合过程提供了直接证据。锆石饱和地温计和角闪铝地质压力计表明，木臣岩体的结晶温度为797-851℃，深度为6-7公里。综合的岩石学、元素和同位素组成表明，MMES和寄主石英二长岩是在伸展环境下由亏损的地幔来源的镁铁质岩浆和地壳物质部分熔融产生的长英质岩浆混合而成的。
The origin of mafic enclaves in granitoid plutons has long been a matter of debate. In this paper, we present detailed petrographic, mineralogical, geochemical, and SrNdHf isotopic data, UPb zircon dates, and field and petrological observations, for mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Southeast China to evaluate their origins. LA–ICP–MS UPb dating of zircon yields crystallisation ages of 112.4±1.2Ma and 112.1±1.0Ma for the MMEs and host quartz monzonites, respectively, indicating their coeval formation during the late Early Cretaceous. Field and petrological observations, such as spheroidal shapes, back-veining, double enclaves, xenocrysts, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. Geochemically, the host monzonites are intermediate-acidic, metaluminous, alkaline, and K-rich. In contrast, the MMEs are relatively poor in Si and K. The host monzonites are enriched in Rb, Th and U, and depleted in Sr, P, Nb, Ta and Ti, and show moderate to strong europium depletions (δEu=0.12–0.60). These mineralogical and geochemical features classify the quartz monzonites as belonging to the I-type granitoids. The MMEs have broadly similar trace element signatures to those of the host monzonite, but are distinct in having relatively enriched Sr and P, more depleted Zr and Hf, and weak to moderate Eu depletions (δEu=0.43–0.93). Major and trace element data plotted versus isotopic data for the MMEs and the host quartz monzonites yield covariant arrays that result from magma mixing during their petrogenesis. The MMEs and the host quartz monzonites have similar initial87Sr/86Sr ratios (ISr) of 0.7058–0.7070 and 0.7062–0.7065, respectively, and both have high εNd(t) values (−2.6 to +0.6 for MMEs; −3.2 to −2.4 for quartz monzonites). However, zircons from the MMEs have different εHf(t) values (−0.4 to +6.2) than the host quartz monzonites (−1.0 to +1.8), indicating that the MMEs and host granitoids largely crystallised from different magmas, providing direct evidence for mafic–felsic magma mixing processes. The zircon saturation geothermometer and Al-in-hornblende geobarometer show that the Muchen pluton crystallised at temperatures of 797–851°C and depths of 6–7km. The integrated petrology, and elemental and isotopic compositions suggest that the MMEs and host quartz monzonites were generated by mixing of depleted, mantle-derived, mafic magmas and felsic magmas produced by partial melting of crustal materials in an extensional setting.