藏北羌塘中部早古生代蛇绿岩堆晶岩中斜长花岗岩的地球化学特征  

Geochemical characteristics of Early Palaeozoic plagioclase granite from ophiolitic cumulate in central Qiangtang, northern Tibet, China

作　　者：胡培远[1] 李才[1] 李林庆 解超明[1] 吴彦旺[1] 

机构地区：[1]吉林大学地球科学学院,吉林长春130061 [2]河北省区域地质调查研究所,河北廊坊000650

出　　处：《地质通报》2009年第9期1297-1308,共12页Geological Bulletin of China

基　　金：国家自然科学基金项目(编号:40872146;40672187);中国地质调查局项目(编号:i212010818115)资助

摘　　要：桃形湖蛇绿岩是龙木错-双湖缝合带近期的重要发现。这一发现回答了龙木错-双湖缝合带中不存在完整蛇绿岩剖面的质疑,是该缝合带存在的重要证据,也是古特提斯洋早期裂解时间确定的主要依据之一。其中斜长花岗岩的岩石地球化学特征反映出洋脊花岗岩的属性,包括Al2O3含量较高,准铝质至过铝质(12.56%～16.19%),富Na2O,贫K2O(Na2O/K2O>3),主量元素、微量元素和稀土元素含量较低等,但是其稀土元素配分曲线、微量元素蛛网图又与标准大洋斜长花岗岩有所区别,表现为轻重稀土元素分异很明显,具强烈的Eu正异常(Eu/Eu*=1.92～9.19,均值为5.18),初步推测原因是岩浆迅速上升过程中Ca分离不完全和样品中石榴子石分布不均匀。结合斜长花岗岩的野外产状及以往的研究成果,初步结论是:桃形湖蛇绿岩中的斜长花岗岩形成于近洋脊或准洋脊环境,岩浆源自地幔,与桃形湖蛇绿岩中的变质橄榄岩、堆晶辉长岩、基性熔岩的岩浆同源,是基性—超基性岩浆分异的残余,应属桃形湖蛇绿岩中的浅色岩组分,是蛇绿岩的端元岩石。Taoxinghu ophiolite was an important discovery of Longmu Co-Shuanghu suture zone in recent years. This discovery has given an answer to the doubt that there was not a complete ophiolite section in Longmu Co-Shuanghu suture zone, and it is an important evidence of the existence of this suture zone and at the same time it is one of the main foundations to confirm the time of the decomposition of Paleo-tethys Ocean. The plagiogranite′s petro-geochemistry characteristics have indicated the attribute of oceanic ridge granite, including high Al2O3 content, from medium-aluminous to super-aluminous （12.56%16.19%）, high Na2O, low K2O （Na2O/K2O〉3）, low content of major elements, trace elements and rare-earth elements. However, the REE distribution patterns and trace element patterns of plagiogranite were different from the standard oceanic ridge plagiogranite, which displays obvious differentia- tion of LREE and HREE and intensely Eu positive anomaly （Eu/Eu＊=1.929.19, mean value is 5.18）. Preliminary estimation is that this is due to complete Ca separation during the rapid rising process of magma and non-uniform distribution of garnet in the samples. According to former research results, the preliminary conclusion was that the magma of plagiogranite came from mantle,which is the same as metamorphosed peridotite, cumulate gabbro and basic lava of Taoxinghu ophiolite. It is the remains of basic magmatic differ- entiation and a component unit of ophiolites.

关 键 词：青藏高原 羌塘中部 蛇绿岩 早古生代 斜长花岗岩 地球化学 

分 类 号：P588.121[天文地球—岩石学] P595[天文地球—地质学]