Age and nature of eclogites in the Huwan shear zone, and the multi-stage evolution of the Qinling-Dabie-Sulu orogen, central China
Age and nature of eclogites in the Huwan shear zone, and the multi-stage evolution of the Qinling-Dabie-Sulu orogen, central China
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DOI：
10.1016/j.epsl.2008.10.031
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发表时间：
2009-01
影响因子：
5.3
通讯作者：
Y. Wu;J. Hanchar;Shan Gao;P. Sylvester;M. Tubrett;Huan-Ning Qiu;J. Wijbrans;F. Brouwer;
中科院分区：
文献类型：
--
作者：
Y. Wu;J. Hanchar;Shan Gao;P. Sylvester;M. Tubrett;Huan-Ning Qiu;J. Wijbrans;F. Brouwer;
锆石原位LA-ICPMS U-Pb、微量元素和Hf同位素数据表明，大别山湖湾剪切带志留系—泥盆系原岩中榴辉岩相变质作用的时代为石炭世。这一时代与秦岭-大别-苏鲁造山带高压-超高压变质作用更为盛行的三叠纪时代形成对比。苏家河2个榴辉岩变质锆石具有Th/U比值低、Eu负异常小、ree模式平坦、176lu /177Hf比值低的特征。这些地球化学特征表明，锆石在有石榴石存在和没有斜长石存在的情况下结晶。此外，根据锆石的Ti含量计算，~655℃和~638℃的温度与它们在榴辉岩相变质作用期间的形成一致。该锆石的加权平均206pb /238U年龄为309±4 Ma (2δ)，与以往湖湾剪切带榴辉岩相变质年龄估计（420 ~ 220 Ma）一致。虎家湾榴辉岩的变质锆石年龄估计为312±11 Ma，极有可能形成于进变质期。3个样品的岩浆锆石岩心测定了榴辉岩的岩浆原岩年龄在420±7 ~ 406±5 Ma之间，确定了中古生代华北地区与秦岭地区碰撞的年代。3种榴辉岩样品的锆石晶体εHf(t)值在−4.9 ~ 21.3之间变化较大。变质锆石生长体的εHf(t)值范围与继承的岩浆晶体内部相同。这表明变质锆石的过度生长可能是由岩浆锆石的溶蚀-再沉淀形成的，从而保留了其原始的Hf同位素组成。较高的εHf(t)值表明原岩来自枯竭的地幔源，极有可能是古特提斯洋壳；而较低的εHf(t)值则与地壳污染有关。湖湾剪切带的榴辉岩具有明显的大陆地壳特征，极有可能源自扬子克拉通。湖湾剪切带古生代洋壳与新元古代陆壳共存，且变质时代相似，说明古生代古特提斯洋壳是在北扬子克拉通边缘盆地内形成的。古特提斯洋的开闭比晚古生代秦岭-大别-苏鲁造山带华北克拉通与秦岭地体碰撞的时间稍晚。古特提斯洋壳的俯冲作用及其相关的大陆基底导致湖湾剪切带发生309±2 Ma （2σ）榴辉岩相变质作用。随后的三叠纪大陆-大陆碰撞导致了扬子和中朝克拉通的最终合并。因此，扬子和华北克拉通的合并是一个多阶段的过程，至少持续了200 Ma。
In situ LA-ICPMS U-Pb, trace element, and Hf isotope data in zircon demonstrate a Carboniferous age for eclogite-facies metamorphism in Siluro-Devonian protoliths in the Huwan shear zone, Dabie Mountains, Central China. This age contrasts with the more prevailing Triassic age for high- to ultrahigh pressure (HP to UHP) metamorphism in the Qinling-Dabie-Sulu orogen. Metamorphic zircon in two eclogite samples from Sujiahe is characterized by low Th/U ratios, small negative Eu anomalies, flat HREE patterns, and low176Lu/177Hf ratios. These geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar. Furthermore, temperatures of ~655 and ~638 °C, calculated using the Ti content of zircon, are consistent with their formation during eclogite-facies metamorphism. The weighted mean206Pb/238U age of 309±4 Ma (2δ) for this zircon improves previous age estimates for eclogite-facies metamorphism in the Huwan shear zone, ranging from 420 to 220 Ma. Metamorphic zircon from one eclogite sample from Hujiawan, most likely formed during prograde metamorphism, yields an equivalent age estimate of 312±11 Ma. Magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420±7 to 406±5 Ma, and post-dating the middle Paleozoic collision of the North China and the Qinling terrain. The zircon crystals in the three eclogite samples display a large variation of εHf(t) values of −4.9 to 21.3. The metamorphic zircon overgrowths show the same range of εHf(t) values as those of the inherited magmatic crystal interiors. This suggests that the metamorphic zircon overgrowths may have formed by dissolution-reprecipitation of pre-existing magmatic zircon thereby preserving their original Hf isotopic composition. The high εHf(t) values suggest that the protoliths were derived from depleted mantle sources, most likely Paleotethyan oceanic crust; while the low εHf(t) values are attributed to crustal contamination. Some eclogites in the Huwan shear zone have a distinctive signature of continental crust most probably derived from the Yangtze Craton. The coexistence of Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamorphic ages in the Huwan shear zone implies that Paleozoic Paleotethyan oceanic crust was produced within a marginal basin of the northern Yangtze Craton. The opening of the Paleo-Tethyan ocean was slightly younger than the collision of the North China Craton and the Qinling terrain during the Late Paleozoic in the Qinling-Dabie-Sulu orogen. Subduction of the Paleo-Tethyan oceanic crust and associated continental basement resulted in the 309±2 Ma (2σ) eclogite-facies metamorphism in the Huwan shear zone. The subsequent Triassic continent-continent collision led to the final coalescence of the Yangtze and Sino-Korean cratons. Amalgamation of the Yangtze and North China cratons was, therefore, a multistage process extending over at least 200 Ma.