北阿尔金地区米兰红柳沟蛇绿岩的岩石学特征和SHRIMP定年  

Petrology and SHRIMP age of the Hongliugou ophiolite at Milan,north AItun,at the northern margin of the Tibetan plateau.

作　　者：杨经绥[1,2] 史仁灯[1,2] 吴才来[1,2] 苏德辰[1,2] 陈松永[1,2] 王希斌[1,2] WOODEN Joseph 

机构地区：[1]中国地质科学院地质研究所,国土资源部大陆动力学重点实验室,北京100037 [2]中国地质科学院青藏高原大陆动力学研究中心,北京100037 [3]Stanford University,Stanford,CA,USA

出　　处：《岩石学报》2008年第7期1567-1584,共18页Acta Petrologica Sinica

基　　金：中国地质调查(1212010610107,1212010610105);国家自然科学基金重大国际合作项目(40610098)资助

摘　　要：米兰红柳沟蛇绿岩是北阿尔金蛇绿岩带中发育和保留最好的蛇绿岩，主要由地幔橄榄岩、镁铁-超镁铁质堆晶杂岩、岩墙群和基性熔岩等组成。它们以规模不等的构造岩块产出，大者长十余km，宽近1km，组成一条近100km长的蛇绿混杂岩带。地幔橄榄岩以方辉橄榄岩为主，有少量纯橄岩，主要由橄榄石（Fo=91．2～92．7），斜方辉石（En=93～98）和少量单斜辉石（En=46）组成；副矿物尖晶石Cr^#为43～69（平均55），Mg^#为43～64（平均58），表现出深海橄榄岩（Abyssal peridotite）和俯冲带环境（SSZ）橄榄岩成分特点。深成堆晶岩主要由异剥橄榄岩-橄榄二辉石岩-（橄榄）辉石岩-辉长岩-斜长岩，该组合的堆晶岩通常被认为是SSZ构造背景的产物。席状岩墙群的岩石成分与熔岩一致，其TiO2（1％～1．5％）和低含量的K2O〈0．3％和P2O5表明具有MORB型的岩石特征，并得到了不相容元素和LREE平坦型和亏损型的球粒陨石标准化模型等证据的支持。该地区另存在一套高Ti的洋岛型拉斑玄武岩。两类熔岩的存在，以及地幔橄榄岩和堆晶岩的不同特征，表明米兰红柳沟蛇绿岩组合可能来自不同构造背景。带中与洋壳俯冲有关的蓝片岩和榴辉岩组成的高压变质带的存在，以及与俯冲碰撞有关的不同类型花岗岩类的产出，表明米兰红柳沟蛇绿混杂岩带代表了一个复杂的板块缝合带。蛇绿岩中辉长岩的锆石SHRIMP年代为479±8Ma，这是获得的第一个北阿尔金蛇绿岩的锆石SHRIMP U—Pb同位素年龄，认为代表蛇绿岩的形成时代。因此。北阿尔金缝合带无论在年龄和特征等方面，均可以与阿尔金断裂带东部的北祁连缝合带对比，证实两个带曾经是一个带，被阿尔金断裂左旋错断了约400km。The Hongliugou ophiolite in Milan is the best-developed and best-preserved body in the North Ahun ophiolite belt at the northern edge of the Tibetan plateau. The ophiolite consists mainly of mantle peridotites, mafic-uhramafic cumulates, sheeted dikes and basaltic lavas, which crop out as tectonic blocks in a melange zone about 100km long. Individual blocks are variable in size, with the largest over 10km long and 1km wide. The mantle peridotites consist chiefly of harzburgite accompanied by minor dunite. The harzburgite consists mainly of olivine （ Fo91.2-92.7 ） , orthopyroxene （ En93 -98 ）, and sparse clinopyroxene （ En46 ）. Spinels in these rocks have Cr^# （ 100Cr/（Cr ＋ Al） ） of 43 to 69 （ average 55） and Mg^# （ 100Mg/（ Mg ＋ Fe） ） of 43 to 64 （ average 58） , typical of both abyssal and suprasubduction zone （SSZ） peridotites. The cumulates are composed of wehrlite, Ol-websterite, （ Ol ） -clinopyroxenite, gabbro and anorthosite, an assemblage similar those formed in SSZ settings. Both the sheeted dikes and basaltic lavas have relatively uniform, MORB-like compositions with 1% -1.5wt% TiO2 and very low contents of K2O （ 〈0.3 wt% ） and P2O5. Their MORB character is confirmed by their incompatible element compositions and their flat, LREE-depleted, chondrite-normalized REE patterns. A few ocean island basalts （OIB） have also been identified in the ophiolite. The two types of basalt suggest that some of the ophiolitic units in the melange may have originated in different tectonic settings. The presence of blueschists and eclogites in the melange, coupled with the occurrence of various subduction-related granitic rocks in the region, suggest that the melange zone represents a complex plate suture. Zircon from an ophiolitic gabbro yielded a SHRIMP U-Pb age of 479.4 ± 8.5Ma, which represents the formation age of the ophiolite. Thus, the North Altun suture zone is comparable in age and character to the North Qilian suture on the east side of the Altyn Tagh fault

关 键 词：蛇绿岩 造山带 红柳沟 阿尔金 青藏高原 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]