滇中南中元古代撮科蛇绿混杂岩地质特征及构造背景

Geological characteristics and tectonic background of the Mesoproterozoic ophiolite mélange in central and southern Yunnan

撮科蛇绿混杂岩出露于元江县撮科大寨及周边地区,受到后期构造作用影响发生一定的变质和变形。蛇岩性较为复杂,岩块与基质之间为断层接触。基质以硅质岩、熔结凝灰岩和花岗质砾岩为主,岩块的类型主要有辉石岩、辉长岩、辉绿岩、闪长岩、玄武岩、斜长花岗岩等。其中,撮科蛇绿混杂岩中熔结凝灰岩的SHRIMP锆石U-Pb龄为(1170±7)Ma(MSWD=0.54,n=12),花岗质砾岩基质具有单一的年龄,为(1151±5)Ma(MSWD=1.3,n=17);辉长岩岩块的SHRIMP锆石U-Pb年龄为(1168±8)Ma(MSWD=0.72,n=17),斜长花岗岩岩块的SHRIMP锆石U-Pb年龄为(1163±9)Ma(MSWD=1.83,n=15),花岗斑岩岩块的SHRIMP锆石U-Pb年龄为(1186±6)Ma(MSWD=0.92,n=17)。这些年龄指示该蛇绿混杂岩形成于中元古代晚期。撮科蛇绿混杂岩中辉长岩REE标准化曲线整体表现较为平坦的右倾趋势,具有与E-MORB相似的稀土元素特征。在MORB标准化图解中Nb、Ta没有出现明显的负异常,极低的Zr/Nb(8.65)、La/Nb(1.05)、Ba/Nb(10.72)、Th/Nb(0.10)、Th/La(0.09)、Ba/La(10.56)、Zr/Hf(43.4)平均值,且微量元素判别投点主要落入板内玄武岩范围,指示撮科蛇绿混杂岩应属于MORB型蛇绿岩,可能形成于板块伸展的构造环境。熔结凝灰岩锆石的ε_(Hf)(t)值为-8.7~-4.7,平均值为-6.6,所有ε_(Hf)(t)均为负,Hf同位素模式年龄(t_(DMC))为2534~2296 Ma,平均为2408 Ma,指示熔结凝灰岩来自古元古代结晶基底的重熔。撮科蛇绿混杂岩的厘定,对重新认识原大红山群老厂河组的属性,研究扬子陆块西南缘中—新元古代岩浆和构造事件,划分扬子陆块基底大地构造单元具有重要意义。

Outcrops of ophiolitic mélange are found in the Cuoke Dazhai area of Yuanjiang County.The Cuoke ophiolitic mélange has undergone metamorphism and deformation due to late tectonism.The lithology of the ophiolitic mélange is complex and fault contact exists between the block and matrix.The bedrock is mainly composed of siliceous rock,fused tuff and granitic conglomerate.The main types of rock blocks are pyroxenite,gabbro,diabase,basalt,granite,etc.The zircon SHRIMP U-Pb age for tuff in the Cuoke ophiolitic mélange was(1170±7)Ma(MSWD=0.54,n=12);for granite conglomerate matrix(a single age)(1151±5)Ma(MSWD=1.3,n=17),gabbro block(1168±8)Ma(MSWD=0.72,n=17),plagiogranite block(1163±9)Ma(MSWD=1.83,n=15),and granite porphyry block(1186±6)Ma(MSWD=0.92,n=17).These ages indicate the ophiolitic mélange is formed in the Late Mesoproterozoic.The chondrite-normalized REE patterns for gabbro of the Cuoke ophiolitic mélange showed a relatively flat right dipping trend similar to that of E-MORB.In the MORB standardized diagram,no obvious negative NB-TA anomalies were observed.The extremely low average Zr/Nb(8.65),La/Nb(1.05),Ba/Nb(10.72),Th/Nb(0.10),Th/La(0.09),Ba/La(10.56)ratios,extremely low Zr/Hf(43.4)ratio,along with the observation that trace element discrimination points mainly fell inside or near the range of intraplate basalt,suggest the Cuoke ophiolite belongs to MOR-type ophiolite and may be formed in a tectonic environment of plate extension.Theε_(Hf)(t)values for zircon from fused tuff ranged,all negative,from-8.7 to-4.7,or averaged at-6.6.The Hf isotopic model age(t_(DMC))was 2534-2296 Ma,averaging at 2408 Ma,indicating the tuff is derived from the remelting of the Paleoproterozoic crystalline basement.Characterization of the Cuoke ophiolitic mélange is of great significance for recognizing the attributes of the Laochanghe Formation of the Dahongshan Group and for understanding the Mesozoic and Neoproterozoic magmatic and tectonic events in the southwestern margin of the Yangtze block,as well as for delineating the tectonic units of the basement of the Yangtze block.