Tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean: Geochronological and geochemical constraints from metamorphic rocks in the Hong'an orogen
Tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean: Geochronological and geochemical constraints from metamorphic rocks in the Hong'an orogen
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古特提斯洋闭合过程中从大洋俯冲到大陆碰撞的构造演化：红安造山带变质岩的年代学和地球化学约束
DOI：
10.1016/j.gr.2014.03.009
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发表时间：
2015-08
影响因子：
6.1
通讯作者：
Yueheng Yang
中科院分区：
文献类型：
--
作者：
Yong-Fei Zheng;Ren-Xu Chen;Zhaochu Hu;Yueheng Yang
对红安造山带高压（HP）和超高压（UHP）榴辉岩相变质岩进行了全岩主微量元素和Sr-Nd同位素以及锆石U-Pb年龄、微量元素和Lu-Hf同位素的联合研究。研究结果揭示了古特提斯洋在南北陆块闭合期间从大洋俯冲到大陆碰撞的构造演化过程。根据红安造山带全岩地球化学特征，将红安造山带榴辉岩分为陆相型和洋相型。陆相榴辉岩广泛赋存于整个造山带，表现为LILE和LREE富集，而HFSE和HREE富集。锆石U-Pb测年结果显示原岩年龄约为750 ~ 1200 Ma，与华南地块具有构造亲缘关系。全岩和锆石的εNd(t)和εHf(t)值均为正、负，表明它们的原岩既来自古壳岩，也来自古壳岩。海洋型榴辉岩仅分布在造山带西北缘，呈现洋中脊玄武岩（MORB）型扁平稀土元素模式和弧型微量元素模式。锆石U-Pb定年结果表明，岩浆成因的残余锆石原岩年龄为~ 420 Ma。它们具有可变的εNd(t)和正的εHf(t)值和稍高的（87Sr/86Sr）辐照度。这些地球化学特征表明它们的原岩相当于大陆边缘的弧后盆地玄武岩。两类榴辉岩的相变质年龄和品位也不同。石炭系海洋型榴辉岩仅在高压条件下变质，而三叠纪陆相榴辉岩则主要在高压-特高压条件下变质。因此，这两类榴辉岩记录了古特提斯洋在华南和华北地块之间闭合期间由大洋俯冲到大陆碰撞的构造转变。可能是早古生代在南中国海北缘发育了弧后盆地，并在石炭世成为一个新的俯冲带，弧后盆地玄武岩及其上覆陆源沉积物被带至一定深度，发生HP榴辉岩相变质作用。之后大陆地壳开始向北俯冲，最终导致三叠纪的高压到超高压榴辉岩相变质作用。因此，红安造山带是一个复合造山带，既与西秦岭—桐柏造山带有构造上的不同，又与东大别—苏鲁造山带有构造上的不同。红安造山带洋型榴辉岩和陆型榴辉岩的同时出现，表明在同一俯冲带洋壳的俯冲作用下，陆壳的俯冲将受到重力的牵引，HP榴辉岩相变质岩melànge将沿同一俯冲通道一起被挖走。
A combined study of whole-rock major-trace elements and Sr–Nd isotopes as well as zircon U–Pb ages, trace elements and Lu–Hf isotopes was carried out for high-pressure (HP) and ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Hong'an orogen, China. The results provide insights into the tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. Based on the whole-rock geochemistry, eclogites in the Hong'an orogen are categorized into continental- and oceanic-type, respectively. The continental-type eclogites widely occur in the whole orogen, exhibiting general enrichments of LILE and LREE but depletion of HFSE and HREE. Zircon U–Pb dating yields protolith ages of about 750 to 1200 Ma, demonstrating a tectonic affinity to the South China Block. They have both positive and negative εNd(t) and εHf(t) values for whole-rock and zircons, suggesting that their protoliths were originated from both juvenile and ancient crustal rocks. The oceanic-type eclogites only occur in the northwestern edge of the orogen, exhibiting mid-ocean ridge basalt (MORB)-like flat REE patterns and arc-like trace element patterns. Zircon U–Pb dating on the relict zircon cores of magmatic origin yields protolith ages of ~ 420 Ma. They have variable εNd(t) and positive εHf(t) values and slightly high (87Sr/86Sr)iratios. These geochemical features suggest that their protoliths are equivalent to backarc basin basalts (BABB) in a continental margin.The eclogite-facies metamorphic ages and grade are also different for these two types of eclogites. The oceanic-type eclogites were only metamorphosed under HP conditions in the Carboniferous whereas the continental-type eclogites were mostly metamorphosed under HP to UHP conditions in the Triassic. Thus, the two types of eclogites record the tectonic transition from oceanic subduction to continental collision during the closure of the Paleotethyan ocean between the South and North China Blocks. It is possible that the backarc basins were developed in the northern margin of the SCB during the Early Paleozoic, and then became a new subduction zone in which both backarc basin basalts and overlying terrigenous sediments were carried to a depth in the Carboniferous for the HP eclogite-facies metamorphism. Afterwards the continental crust would start to subduct northwards, eventually leading to the HP to UHP eclogite-facies metamorphism in the Triassic. Therefore, the Hong'an orogen is a composite one that is tectonically different not only from the Qinling–Tongbai orogens to the west but also from the Dabie–Sulu orogens to the east. The occurrence of both oceanic-type and continental-type eclogites in the Hong'an orogen suggests that the subduction of continental crust would be gravitationally pulled by the subduction of oceanic crust at the same subduction zone and the HP eclogite-facies metamorphic melànge would be exhumed together along the same subduction channel.
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