Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope
Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope
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DOI：
10.1016/s0024-4937(03)00104-x
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发表时间：
2003-10
期刊：
影响因子：
3.5
通讯作者：
Yong‐Fei Zheng;B. Gong;Zi‐Fu Zhao;B. Fu;Yiliang Li
中科院分区：
文献类型：
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作者：
Yong‐Fei Zheng;B. Gong;Zi‐Fu Zhao;B. Fu;Yiliang Li
与大别山双河超高压榴辉岩密切共生的片麻岩有黑云母副片麻岩和花岗质正片麻岩两类。利用EA-MS在线技术测定了磷灰石和寄主片麻岩中的体碳含量和同位素组成。通过XRD和FTIR技术检测了磷灰石中的结构碳酸盐。在磷灰石中观察到显著的13 C亏损，δ 13 C值为−28.6‰至−22.3‰，碳浓度为0.70-4.98 wt.%尽管这些片麻岩的δ 18 O变化很大，从−4.3‰到+10.6‰。在几十米尺度上，片麻岩的δ 13 C和δ 18 O均具有明显的不均一性，表明超高压变质作用后存在次生作用。在某些片麻岩中也有大量的碳酸盐碳，这些片麻岩最初也是亏损13 C的，但在超高压变质作用后受到富含13 C的含CO2流体的叠加。片麻岩中的13 C贫化碳被解释为继承自板块俯冲前遭受大气热液蚀变的前体。低δ 13 C值和磷灰石中的结构碳酸盐表明超高压变质流体中存在贫13 C的CO2。贫13 C的CO2无疑是超高压变质作用中地下流体中有机质氧化的产物。两个花岗质正片麻岩样品的锆石δ 18 O值较低，为-4.1 ‰ ~-1.1 ‰，表明其原岩在岩浆结晶前明显亏损18 O。亏损锆石U-Pb不一致测年结果表明，该花岗质正片麻岩的新元古代原岩年龄为724-768 Ma，与大别-苏鲁造山带其它地区榴辉岩和正片麻岩的原岩年龄一致。因此，该蚀变作用的直接年代定为新元古代中期，并可能与Rodinia超大陆裂解和雪球地球事件有关。据此推断，在新元古代中期，花岗质正片麻岩和部分榴辉岩的火成岩原岩侵入扬子板块北缘黑云母副片麻岩和部分榴辉岩的沉积原岩组成的较老序列，并驱动了当地的大气-热液循环系统，其中13 C-和18 O-亏损流体与大别山现今出露的超高压岩石的原岩相互作用。
There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant13C-depletion was observed in the apatite with δ13C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO2despite a large variation in δ18O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in13C primarily, but subjected to overprint of13C-rich CO2-bearing fluid after the UHP metamorphism. The13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of13C-poor CO2in the UHP metamorphic fluid. The13C-poor CO2is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ18O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in18O prior to magma crystallization. U–Pb discordia datings for the18O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both13C- and18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.