班公湖MOR型蛇绿岩Re-Os同位素特征对班公湖—怒江特提斯洋裂解时间的制约

摘要

The MOR-type Bangong Co ophiolite in northwest Tibet is a tectonic melange mainly consisting of brec-ciated peridotites and MORB-affinity basalts. The mantle peridotites are composed of clinopyroxene-bearing harzburgites (cpx-harzburgites) and minor clinopyroxene-free harzburgites classified in terms of petrography,mineralogy and geochemistry. Geochemical data suggest that the cpx-harzburgites were formed via a refertiliza-tion process, which occurred at low melt/peridotite ratios (1:9 to 1: 4) and under the interaction between basaltic melts and clinopyroxene-free harzburgites. The authors performed a Re-Os isotopic study of 9 cpx-harzburgites, which display a relatively narrow range in Os concentrations and Os isotopes, with Os ranging from 2.91×10-9 to 5.40 × 10-9 and 187Os/188Os ranging from 0.12113 ± 0.000 44 (2σ) to 0.128 53 ± 0.000 36 (2σ). Measured Re concentrations range from 0.19 × 10-9 to 1.49 × 10-9 and 187Re/188Os range from 0.169 ± 0.009 (2σ) to 1.833 ± 0.183 (2σ). The positive correlation defined by cpx-harzburgites (five solid circle points) in the isochron diagram (187Os/188Os versus 187Re/188Os) yields an apparent age of 254 ± 28 Ma. The peridotite-melt interaction at the low melt/rock ratios could result in an obvious increase in peridotite Re abundances and Re/Os ratio, and then the decay of 187Re to 187Os produced a modification of the 187Os/188Os ratios. Serpentinization seemed to have had little effect on the Re-Os system, which is considered to have been a closed system during the low temperature process. Therefore, this pseudo-isochron age may reflect the melt/rock reaction between cpx-free harzburgites and basaltic melts. In addition, this pseudo-isochron age is consistent with the opening age of Neo-Tethys defined earlier, and hence provides evidence for the initiation of the Bangong-Nu-jiang Tethys Ocean in Late Perimian-Early Triassic period.%西藏北部班公湖MOR型蛇绿岩主要由角砾状的地幔橄榄岩和玄武岩组成,其中地幔橄榄岩主要是低Cr#尖晶石相含单斜辉石(Cpx)方辉橄榄岩和少量不含Cpx的方辉橄榄岩,玄武岩具有MORB地球化学特点.岩石地球化学特征和二元混合模拟计算表明,含Cpx方辉橄榄岩是由较为亏损的方辉橄榄岩与玄武质熔体发生反应再富集形成的,玄武质熔体和方辉橄榄岩的混合比例为1∶9至1∶4.9个含Cpx方辉橄榄岩样品(含5个重复测试样)的Re和Os含量分别为0.19×10-9～1.49×10-9和2.91×10-9～5.40×10-9,187Re/188Os变化范围为0.169±0.009(2σ)～1.833±0.183(2σ),187Os/188Os变化范围相对较小,介于0.121 13±0.000 44～0.128 53±0.000 36(2σ)之间.含Cpx方辉橄榄岩的Re-Os参考等时线年龄为254±28 Ma.由于不同比例熔体的加入造成橄榄岩具有不同的Re/Os比值,因而不同含Cpx方辉橄榄岩样品具有不同187Os/188Os比值.样品的Re含量与烧失量中的H2O没有相关性,说明蛇纹石化对样品Re-Os体系的影响可以忽略,Re-Os同位素体系在低温地质作用下能够保持相对封闭.参考等时线年龄可能代表亏损方辉橄榄岩与玄武质熔体发生反应的时间,即含Cpx方辉橄榄岩的形成年龄,它表明在该时期特提斯洋经历了一次构造热事件.这一构造岩浆热事件的时间与早期定义的班公湖-怒江特提斯洋的裂解时间晚二叠世至早三叠世较为一致,推测本文MOR型蛇绿岩地幔橄榄岩的Re-Os同位素年龄可能代表班公湖-怒江特提斯洋开始裂解的时间.