摘要
沿澜沧江构造带东侧,出露有许多镁铁—超镁铁杂岩体,由北向南较为著名、规模较大的岩体依次为德钦燕门-飞来寺岩体群、维西吉岔岩体、景谷茂密河岩体、景谷葳里岩体、景谷半坡岩体、景洪南联山岩体、景洪怕冷岩体等。许多专家、学者曾将这些岩体看作是澜沧江洋盆俯冲消减后的残迹,即澜沧江蛇绿混杂岩的主要组成部分；而张魁武、张旗等则称之为澜沧江镁铁—超镁铁质岩带,并将它们与阿拉斯加型岩体进行了对比研究；并认为属岛弧岩浆作用的产物。
笔者等在参与云南1：25万澜沧县、勐海县、勐腊县、景洪市等4幅区调修测的工作中,于半坡岩体和南联山岩体之间新发现一个规模较大的类似岩体——雅口岩体。并对其进行了岩石学、岩石地球化学、年代学的研究。研究表明,雅口岩体的岩石类型主要有辉长岩、辉橄岩、角闪岩、闪长岩等,其中的部分辉橄岩、辉长岩中发育有良好的火成堆积层理；镁铁质岩石化学成分表现为明显的富铝、富钙特征,类似于岛弧地区高铝玄武岩的特点；岩石的Na2O+K2O较高,经硅—碱图解、A-F-M图解判别为钙碱性玄武岩系列；化学成分变异趋势与相邻的怒江-昌宁-孟连构造带上的铜厂街蛇绿混杂岩有较大差异,早期表现出明显的富钙趋势。雅口岩体的镁铁质岩类的稀土元素相对于超镁铁质岩类较高,二者的稀土配分曲线均较为平坦型,LREE轻度富集,HREE分布曲线大致平行。在微量元素比值蛛网图中,雅口镁铁质岩类的曲线分布类似于大洋岛屿玄武岩,但显著富集Sr、K、Rb、Ba大离子亲石元素和亏损Nb、Zr、 Hf等高场强元素,这与典型岛弧环境或活动陆缘环境(钙碱性玄武岩系列)的配分曲线很相似。综合研究认为,雅口岩体与同一构造带上的维西吉岔岩体、景谷半坡岩体、景洪南联山岩体、景洪怕冷岩体等具有相似的岩石组合、岩石地球化学特征及侵位时代,它们不属于蛇绿混杂岩的组成部分,而是岛弧或陆缘弧岩浆活动的产物；结合区域地质情况分析,可将澜沧江镁铁—超镁铁岩带视为昌宁—孟连洋盆向东俯冲消减作用开始时岩浆弧(挤压环境)与弧后盆地(拉张环境)之间的过渡带上岩浆活动的产物。采用锆石激光剥蚀法(LA-ICPMS)获得了雅口岩体中堆晶辉长岩的锆石结晶年龄为296.3Ma,与前人在维西吉岔岩体的闪长岩中获得的年龄值基本一致,从而也精确限定了怒江—昌宁—孟连洋盆开始向东俯冲消减的时代不晚于早二叠世阿瑟尔期(299—294.6Ma)。
Along the Lancangjiang fault zone on the eastern side, exposed many mafic, ultramafic intrusive complex. From north to the south, there are famous large rock in Deqin yanmen-feilai rock, Weixi jicha rock, jinggu maomi river rock, jinggu waili rock, jinggu banpo rock, jinghong nanlian mountain rock, paleng rock and so on. south of rock mass. Zhang Kuiwu, Zhang Qi, called it Lancang magnesium iron ultramafic rock belt, then compared them with the Alaska rock mass and made the studies.
In the1:250000four areas survey work of Lan Cangxian, Menghai County, Mengla County, Jinghong City,we found a larger similar rock--YaKou rock among the rock and south mountain rock. Then we made the studies from the angles of petrology, geochemistry, geochronology of. Rock types are mainly pyroxene peridotite, gabbro, diorite, amphibolite, part of the pyroxene peridotite, gabbro are well developed in igneous cumulate layering. The studies showed that YaKou magnesium iron rock composition variation trend and Ophiolitic Melange has many differences, apparently showed the trend of rich calcium. The chemical composition of Mafic rock showed obvious aluminum rich features, similar to the high alumina basalt rock characteristics of island area, higher Na2O+K2O, belongs to Calc alkaline series. The rare earth element of YaKou mafic rocks are relatively high compared with the ultramafic rocks. Its REE curve is flat, LREE mild enrichment, HREE distribution curves are roughly parallel. In the web diagram of trace element ratio, although YaKou mafic rocks curve distribution similar to that of oceanic island basalts, but it is significant enrichment of Sr, K, Rb, Ba large ion lithophile elements and loss of Nb, Zr, Hf high strength element. It is similar with the the distribution curves of typical island arc environment or active continental margin environment (calcium alkaline series). Comprehensive research shows that, YaKou rock has the similar combinations of rock, rock geochemistry characteristics and emplacement age with the Weixi Kyrgyzstan bifurcated rock, Jinggu the south of Jinghong mountain rock which in the same structural belt. They showed the features of island arc or continental margin arc tectonic magmatic type. So we could consider
the Lancang River magnesium iron ultramafic rock band as the magma activity on a transitional zone between magmatic arc (extrusion environment) and back arc basin (extension) when Chang-Menglian eastward subduction of oceanic role begins. By using the CZ laser ablation method (LA-ICPMS), we obtained YaKou rock cumulate gabbro zircon U-Pb age296.3Ma. It is basically the same in age value with the former in Kyrgyzstan bifurcated rock diorite, thereby accurately defined the era of eastward subduction of beginning Nu River-Changning-Menglian ocean as no later than the early Permian Arthur period (299-294.6Ma).
引文
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