Geochronological, geochemical, and Sr–Nd–Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks
Geochronological, geochemical, and Sr–Nd–Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks
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DOI：
10.1016/j.lithos.2014.03.010
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发表时间：
2014-05
期刊：
影响因子：
3.5
通讯作者：
Liang Liu;J. Qiu;Jiao Zhao;Zewei Yang
中科院分区：
文献类型：
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作者：
Liang Liu;J. Qiu;Jiao Zhao;Zewei Yang
本文介绍了浙西中国东南部马头和达赖两个深成岩体的岩石学、地球化学和锶-钕-氢同位素资料，旨在约束二长岩的成因，并对中国东南部壳幔岩浆相互作用的深部过程提供新的认识。马头岩体由石英二长岩组成，达赖岩体由石英二长闪长岩组成。激光烧蚀-电感耦合等离子体质谱获得的锆石U-Pb年龄表明，两个深成岩的侵位年龄均为99-101 Ma。两个深成岩的岩石成分都是中等到硅质、偏铝到弱过铝、亚碱性和富K。样品富含大离子亲石元素(如Rb、K、Pb)和轻稀土元素，亏损高场强元素(如Nb、Ta、Ti)，具有较小的负Eu异常或无Eu异常。此外，这些岩石具有高的Mg#值(高达53.9)，高的锆石εHf(T)值(高达−的1.4)，以及低的Nb/U和Ta/U比值。地球化学证据表明，亏损的软流圈成分和交代富集的地幔成分参与了这些二长岩的形成。马头石英二长岩中存在古元古代时代的继承锆石和εHf(T)值极低(−=12.9)的锆石，表明古地壳物质也参与了其成岩作用。结合大量球状到椭球状的镁铁质微颗粒包体(MME)和包体中的捕虏晶，以及微量元素模拟结果，我们认为马头石英二长岩是由地幔来源的镁铁质岩浆和地壳来源的硅质岩浆混合而成。达赖岩体相对均匀，比马头岩体含有更少的MME。达赖岩体中的锆石没有继承性成分，说明地壳物质在石英二长闪长岩的形成中起到了有限的作用。与马头石英二长闪长岩相比，达赖石英二长闪长岩的SiO_2含量较低，Mg#值较高，铬、钴、镍含量较高且变化较大。达赖岩体的锆石Hf同位素组成相对均匀(εHf(T)=−5.2~−3.2)。综合岩石学、地球化学和同位素特征表明，达赖二长闪长岩是由玄武岩岩浆中以橄榄石和辉石为主的分离结晶作用形成的，而玄武岩岩浆又是由亏损的软流圈熔体和俯冲富集地幔混合而成。我们的解释表明，中国东南部晚中生代二长岩需要大量地幔熔体的输入，其中一些可能仅由玄武岩岩浆的分馏作用产生。该成岩模式同样适用于中国东南部的其他二长花岗岩，也适用于世界范围内类似的构造环境和二长岩岩浆生成场所。
We present comprehensive petrological, geochemical, and Sr–Nd–Hf isotopic data for the Matou and Dalai plutons in western Zhejiang Province, Southeast China, with the aim of constraining the petrogenesis of monzonites and to offer new insights into the deep processes of interaction between crustal- and mantle-derived magmas beneath SE China. The Matou pluton comprises quartz monzonite, whereas the Dalai pluton consists of quartz monzodiorite. Zircon U–Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that both plutons were emplaced at 99–101 Ma. Rocks of both plutons are intermediate to silicic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. Samples of the plutons are enriched in large ion lithophile (e.g., Rb, K, and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and have small negative or no Eu anomalies. In addition, the rocks have high Mg# values (up to 53.9), high zircon εHf(t) values (up to − 1.4), and low Nb/U and Ta/U ratios. Geochemical evidence suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of these monzonitic rocks. The presence of inherited zircons with Palaeoproterozoic ages and zircons with unusually low εHf(t) values (− 12.9) in the Matou quartz monzonites indicates that ancient crustal materials were also involved in their petrogenesis. In combination with the presence of abundant mafic microgranular enclaves (MMEs) with spheroidal to ellipsoidal–ovoidal shapes and xenocrysts within the more diffused enclaves, and the results of trace element modelling, we suggest that the Matou quartz monzonites were generated by mixing between mantle-derived mafic magmas and crustally derived silicic magmas. The Dalai pluton is relatively homogeneous and contains fewer MMEs than the Matou pluton. Zircons from the Dalai pluton show no inherited components, indicating that crustal materials have played a limited role in the petrogenesis of the quartz monzodiorites. The Dalai quartz monzodiorites have lower SiO2contents, higher Mg# values, and considerably higher and variable Cr, Co, and Ni concentrations than the Matou quartz monzonites. Zircon Hf isotopic compositions of the Dalai pluton are relatively homogeneous (εHf(t) = − 5.2 to − 3.2). The combined petrological, geochemical, and isotopic features indicate that the Dalai monzodiorites were generated by olivine- and pyroxene-dominated fractional crystallisation from basaltic magmas, which were in turn produced by mixing between melts from depleted asthenosphere and subduction-enriched mantle. Our interpretation implies that Late Mesozoic monzonitic rocks in Southeast China require a significant input of mantle melts, and some may have been generated solely by fractionation of basaltic magmas. This petrogenetic model may be applicable to other monzonitic rocks in Southeast China, and to similar tectonic settings and sites of monzonitic magma generation worldwide.