扬子陆块西缘中元古代菜子园蛇绿混杂岩的厘定及其地质意义

对扬子陆块西缘会理关河—通安地区菜子园蛇绿混杂岩进行了厘定。菜子园蛇绿混杂岩主要以强烈剪切变形的基质和洋板构造岩块混杂堆积为特征。基质主要有变质粉砂岩、板岩、硅质板岩、片岩、千枚岩等。洋板构造岩块由蛇纹岩、辉长岩、玄武岩、硅质岩、大理岩等组成,部分地区玄武岩保留枕状构造,各岩块之间为构造接触关系。菜子园蛇绿混杂岩中桃树湾辉长岩和玄武岩具有轻稀土元素亏损、类似N-MORB(正常洋中脊玄武岩)的稀土元素配分模式,相对于N-MORB富集大离子亲石元素,亏损Nb、Ta高场强元素,极低的Nb/U(9.74)、Nb/Th(3.02)和V/Ti(0.1)平均值,具典型MORB-like玄武岩(前弧玄武岩-FAB)地球化学特征,认为可能形成于洋内弧环境。桃树湾辉长岩LA-ICP-MS锆石U-Pb同位素定年表明,辉长岩的年龄为1375±7Ma(MSWD=1.2,n=21),可能代表菜子园-通安洋洋壳初始俯冲的时间。菜子园蛇绿混杂岩的厘定,对重新认识通安群(通安组)的属性,研究扬子陆块西缘古—中元古代地层系统、地质演化、岩浆作用、构造定位,重新划分扬子陆块基底大地构造单元,以及探索全球Columbia超大陆裂解到Rodinia超大陆汇聚的演化过程等都具有重要意义。

Ascertainment of the Mesoproterozic Caiziyuan ophiolitic mélange on the western margin of the Yangtze Block and its geological significance

The Caiziyuan ophiolitic mélange along the Guanhe-Tongan area on the western margin of the Yangtze Block have been ascertained by the authors. The Caiziyuan ophiolitic mélange is characterized by the strong shearing deformation of the matrix and the oceanic tectonic rocks. The substrate is mainly composed of metamorphic siltstone, slate, siliceous slate, schist and phyllite, whereas the oceanic tectonic rock consists of serpentine, gabbro, basalt, siliceous rock and marble, with some basalts retaining the pillow structure. The relationship between the tectonic rocks is tectonic contact. The gabbro and basalt from the Caiziyuan ophiolitic mélange exhibit LREE loss, similar to the N-MORB rare earth element distribution model, relative to the N-MORB enrichment of large ion lithophile elements (LILE), and depletion of Nb and Ta, extremely low Nb/U ratio (averagely 9.74), Nb/Th ratio (averagely 3.02) and V/Ti ratio (averagely 0.1), showing typical MORB-like geochemical features of basalt and suggesting probable formation in an intra-oceanic arc environment. LA-ICP-MS zircon U-Pb dating shows that the age of the Taoshuwan gabbro is 1375±7Ma (MSWD=1.2, n=21), which may represent the time of the initial subduction of the Caiziyuan-Tongan oceanic crust. The ascertainment of the Caiziyuan ophiolitic mélange has the great scientific significance for further understanding of the properties of the Tongan Group (or Tongan Formation), studying the Paleo-Mesoproterozoic stratigraphic system, geological evolution, magmatism, and tectonic orientation of the western Yangtze Block, re-dividing the basement tectonic unit and exploring the evolution from the global Columbia supercontinent breakup to the Rodinia supercontinent convergence.