滇东南马关巨晶单斜辉石原位Sr同位素特征及其含水性

对滇东南马关火山爆砾岩筒以及玄武岩中的单斜辉石巨晶进行了主量元素、微量元素、原位Sr同位素以及含水性研究,结果显示,单斜辉石巨晶均为铝质普通辉石,Mg#与Al、Fe、Ti呈负相关,与Ca呈正相关;稀土元素总量不高,HREE及LREE亏损,而MREE相对富集,大离子亲石元素均出现明显亏损,高场强元素Nb、Zr出现亏损而Hf略富集,Nb、Ta与Zr、Hf分馏明显,Rb、Ba的变化范围较大。原位Sr同位素87Sr/86Sr值为0. 703 92～0. 705 06,巨晶单斜辉石来源于上地幔,形成于同一母岩浆,与寄主玄武岩同源,为源岩浆在高压下的结晶产物。运用显微红外-傅里叶变换红外光谱仪(Micro-FTIR)分析测试了单斜辉石巨晶的水含量,结果显示OH吸收峰位于3 700～2 800cm-1,含水量赋值范围为318×10-6～693×10-6,水含量稍富于女山及汉诺坝地区的单斜辉石巨晶,也高于马关地区幔源橄榄岩包体中的单斜辉石结构水含量,暗示该区上地幔岩石圈具高含水性,这同时证实了俯冲洋壳在研究区的活动。新近纪到达地幔过渡带的西太平洋洋壳俯冲发生转向东撤,此时新特提斯洋洋壳正在NE向俯冲,是马关地区深部地幔富水的主因,到达软流圈(或地幔过渡带)的新特提斯洋俯冲洋壳导致该区地幔熔融,岩浆随之而生。

In situ Sr isotopes and water content of clinopyroxene megacryst from Maguan, southeastern Yunnan Province

In this paper, the authors studied the major elements,trace elements, ⁸⁷Sr/⁸⁶Sr isotopes and water content of clinopyroxene megacrystals hosted by Maguan alkali basalt in southeastern Yunnan Province. All clinopyroxene megacrystals are aluminum augite, and their Mg^# are negatively correlated with Al, Fe, Ti, but positively correlated with Ca. MREE is relatively more enriched than HREE and LREE. The LILE (large ion lithophile elements) are obviously deficient. The HFSE (high field strength elements) Nb and Zr are depleted, but Hf is slightly enriched. Nb, Ta and Zr, Hf fractionation is obvious respectively, and Rb and Zr change dramatically. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.703 92 to 0.705 06. All these data suggest that the clinopyroxene megacryst originated from the same parent magma as the hosted alkali basalt. The water content of clinopyroxene megacrystals is 318×10^-6~693×10^-6as measured by Micro-FTIR, which is richer than that of Nushan and Hanoba as well as the peridotite xenolith in Maguan. It indicates that the upper mantle is rich in water in the study area, suggesting oceanic subduction in Cenozoic. The western Pacific Ocean crust which reached the transitional zone retreated eastward in Neogene, whereas the Neo-Tethys ocean crust subducted northeastward. These factors were probably responsible for the abundant water in Maguan mantle. Subsequently, mgama was formed in the asthenosphere (or mantle transition zone) by the Neo-Tethys ocean crust subduction.