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.