西藏唐加地区石炭纪洋岛型岩石组合及其构造意义

Ocean Island Rock Assembly and Its Tectonic Significance in Tangga-Sumdo Area, Tibet

西藏松多(超)高压变质带对认识古特提斯洋的演化具有十分重要的作用,然而目前关于该带代表的洋盆早期演化记录发现较少,制约了对松多古特提斯洋盆演化的理解.对唐加地区的洋岛型岩石组合进行了野外地质特征、岩浆岩全岩地球化学和锆石U-Pb定年研究.唐加地区洋岛型岩石组合的野外地质特征具有典型的“二元结构”,下层为玄武岩基底,上层为灰岩(大理岩)盖层和塌积砾岩,在上部盖层中还可见变质玄武岩夹层和大理岩与绿片岩互层的现象,与“佛得角型”洋岛类似.两件玄武岩和一件辉绿玢岩脉的锆石U-P定年结果分别为330 Ma、310 Ma和307 Ma,为早石炭世晚期-晚石炭世.玄武岩和辉绿玢岩均具有较高的TiO_(2)、P_(2)O_(5)和(Na_(2)O+K_(2)O)含量,稀土元素和微量元素组成与OIB类似,显示明显的Nb、Ta富集,在判别图解中落在OIB和板内玄武岩区域,其岩浆源区可能为尖晶石-石榴子石二辉橄榄岩地幔,熔融深度较小.结合前人研究,初步认为松多古特提斯洋早石炭世晚期可能已经发育了初始洋盆,在早石炭世晚期到晚石炭世为慢/超慢速扩张,形成了具有“佛得角型”洋岛特征的唐加地区洋岛型岩石组合.

The Sumdo(Ultra)high Pressure Metamorphic Zone plays an important role in understanding the evolution of the Paleo Tethys Ocean in Tibet. However, there are few records about the early evolution of the ocean basin on behalf of this zone, which restricts the understanding of the evolution of the Paleo Tethys Ocean. In this paper, we have studied the field geological characteristics, magmatic whole rock geochemistry and zircon U-Pb chronology of the ocean island rock assemblages in Tangga area. The field geological characteristics of the ocean island rock assemblages in Tangga area show the typical "double layer structure", with basalt basement in the lower layer, limestone(marble) cover layer and colluvial conglomerate in the upper cover layer, and metamorphic basalt interbedded with marble and tuff in the upper cover layer, which is similar to the "Cape Verde" ocean island. Zircon U-Pb geochronology of two basalts and a diabase porphyrite dike is 330 Ma, 310 Ma and 307 Ma,respectively, indicating the least Early Carboniferous to Late Carboniferous. Both basalt and diabase porphyrite have high content of TiO_(2), P_(2)O_(5)and(Na_(2)O+K_(2)O). The rare earth elements(REE) and trace element compositions are similar to those of OIB,showing significant enrichment of Nb and Ta, which fall in OIB and intraplate basalt regions in the discriminant diagram. The REE partition curve and the trace element spider diagram are similar to those of OIB, showing obvious Nb and Ta enrichment. The magma source area of basalt and diabase porphyrite may be the spinel-garnet peridotite mantle, with a small melting depth,indicating that the overlying lithosphere may be a new-born ocean crust with a small thickness. Combined with previous studies,we preliminarily consider that the Sumdo Paleo Tethys Ocean developed an initial ocean basin in the least Early Carboniferous,and it expanded slowly/ultra-slowly from the least Early Carboniferous to the Late Carboniferous, forming the ocean island rock assemblages with the characteristics of "Cape Verde" in Tangga area.