内蒙古月牙山蛇绿岩特征及形成的构造背景:地球化学和Sr-Nd同位素制约  

Characteristics of the Yueyashan ophiolite from western Nei Mongol and its tectonic setting: Geochemistyy and Sm-Nd isotopic constraints

作　　者：周国庆[1] 赵建新 李献华[3] 

机构地区：[1]南京大学地球科学系,江苏南京210093 [2]DepartmentofEarthSciences,TheUniversityofQueensland,Brisbane Qld4072 [3]中国科学院广州地球化学研究所,广东广州510640

出　　处：《地球化学》2000年第2期108-119,共12页Geochimica

基　　金：国家自然科学基金!49672102;澳大利亚科学基金会!ARC

摘　　要：内蒙古白云山－月牙山－洗肠井蛇绿岩（简称月牙山蛇绿岩）发育较完整，推覆构造也很发育。其变质橄榄 岩、方辉橄榄岩强烈亏损ＴｉＯ_２，辉绿岩ＴｉＯ_２也较低，而稀土元素分布模式为平缓型。斜长花岗岩类亏损高场强元素 （ＨＦＳＥ）和重稀土元素，富集大离子亲石元素（ＬＩＬＥ），一个石英问长岩样品（Ｙ１０）具有高Ｓｉ、高Ｍｇ、低Ｔｉ的玻镁安山 岩特征。Ｎｄ同位素特征表明，辉绿岩和石英闪长岩具有高而均一的正ε_（Ｎｄ）（ ｔ）值（＋ ６． ４～＋ ７． ９），说明辉绿岩和石英 问长岩来自同一亏损地幔所产生的岩浆，没有遭受明显的陆壳混染，但受过由俯冲洋壳释放出的流体的交代，花岗 质岩石是其衍生物。参考等时线年龄（４７０ Ｍａ）与地层古生物证据相符。蛇绿岩的（^（８７）Ｓｒ／^（８６）Ｓｒ）；值变化较大，具海水蚀变特征。岩石、地层、地球化学和Ｓｒ－Ｎｄ同位素综合制约表明，该蛇绿岩形成于塔里木北缘多岛弧弧间盆地的初期扩张阶段，属Ｐｅａｒｃｅ提出的ＳＳＺ型蛇绿岩。Baiyunshan-Yueyashan-Xichangjing Ophiolite (BYXO) shows a good sequence, but some nappes break partly down the sequence. The metaperidotite is strongly depleted in TiO_2. The dolerite has low TiO_2 and some of the granitoids on the FAM diagram show a Skaergaard liquid trend. The dolerite is depleted in HFSE and HREE, and enriched in LILE. One sample (Y10) is characterized by relatively high SiO_2, MgO, Cr and low TiO_2 and is similar to the boninite. Based on those above, BYXO is assigned to Pearce's SSZ type. The isotopic features indicate that the dolerites and granitoids have high homogeneous and positive ε_(Nd)(t) values of (+6. 4 -+ 7. 9), suggesting that they come out of a same depleted magma source that derived from mantle, and that they are not evidently contaminated by the continental crust. The Sm-Nd reference isochron age of 470 Ma of the ophiolite is consistent with stratigrahic and fossil evidences. The ophiolite samples have a relatively wide range of initial (^(87)Sr/ ^(86)Sr)_i, because of the sea-water alteration. The constraints of petrology, stratigraphy, geochemistry and Sr-Nd isotopes indicate BYXO was developed in an inter-arc basin setting of multiple-arc that was located on the north margin of Tarim Plate at the initial stage of the spreading of the basin.

关 键 词：蛇绿岩 微量元素 内蒙古自治区 锶-钕同位素 

分 类 号：P588.1[天文地球—岩石学] P597[天文地球—地质学]