Early Cretaceous adakitic granites in the Northern Dabie Complex, central China: Implications for partial melting and delamination of thickened lower crust
Early Cretaceous adakitic granites in the Northern Dabie Complex, central China: Implications for partial melting and delamination of thickened lower crust
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DOI：
10.1016/j.gca.2007.03.008
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发表时间：
2007-05
影响因子：
5
通讯作者：
Qiang Wang;D. Wyman;Ji‐Feng Xu;P. Jian;Zhenhua Zhao;Chaofeng Li;Wei Xu;Jinlong Ma;B. He
中科院分区：
文献类型：
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作者：
Qiang Wang;D. Wyman;Ji‐Feng Xu;P. Jian;Zhenhua Zhao;Chaofeng Li;Wei Xu;Jinlong Ma;B. He
迄今为止，很少有埃达质岩石与当代大陆内伸展构造直接相关的报道，这使人们对涉及下地壳部分熔融的成因模型产生了怀疑。本研究发现早白垩世（143 ~ 129ma，新的灵敏高分辨率离子探针（SHRIMP）锆石U-Pb年龄）阿达质花岗岩，与当代变质核杂岩（即大别地区北大别杂岩）直接相关。这些花岗岩具有较高的Sr含量，Eu和Sr正异常可以忽略不计，La/Yb和Sr/Y比值较高，但Yb和Y含量非常低，与俯冲洋壳源埃达岩相似。它们的MgO或mg#、Ni值非常低，Nd-Sr同位素组成(εNd（t)= - 14.6 ~ - 19.4, (87Sr/86Sr)i=0.7067 ~ 0.7087）与大别-苏鲁造山带俯冲的三叠系陆源榴辉岩相似。此外，阿达基岩体中晚期花岗岩脉Sr含量低，Eu和Sr负异常明显，La/Yb和Sr/Y比值低，但Yb和Y含量相对较高，与大别杂岩北部118-105Ma花岗岩相似。根据新元古代角闪岩和正长岩、三叠纪高压-超高压变质岩和早白垩世基性-超基性侵入岩的组成和年代学资料，以及闪长岩、变质玄武岩和榴辉岩的实验熔融数据约束，认为阿达基花岗岩极有可能是三叠纪-中侏罗世俯冲和逆冲作用下加厚的角闪孔或含金红石的榴辉岩下地壳部分熔融形成的。晚期岩脉可能起源于含斜长石的中间麻粒岩。此外，我们认为晚中生代增厚的榴辉质下地壳的拆沉作用也可能是大别地区早白垩世基性-超基性侵入岩形成的一种更有可能的机制，这种机制可能涉及由榴辉质下地壳组成的混合源的部分熔融，该混合源由上层岩石圈或软流圈地幔橄榄岩拆沉而成。碰撞后岩石圈地幔剥离引起的软流圈上升流可能为白垩纪岩浆活动提供了热源。
To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143–129Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg#and Ni values, and Nd–Sr isotope compositions (εNd(t)=−14.6 to −19.4 and (87Sr/86Sr)i=0.7067–0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie–Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118–105Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic–ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic–Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic–ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.