Restoration of the Devonian-Carboniferous limited ocean basin and deep structure of ophiolitic melange in the Hongshishan area of Gansu Province

Based on the comprehensive studies of tectonic lithofacies, restorations of tectonic paleogeographic units, AMT, and high-precision ground magnetic survey, the Devonian-Carboniferous limited ocean basin in the Hongshishan area of Gansu province was reconstructed.Furthermore, the depth structures of ophiolitic melange were detected by the means of AMT and tectonic lithofacies.It is classified into five tectospheres, namely, Pre-Variscan tectosphere, Variscan tectosphere, Indosinian tectosphere, Yanshanian tectosphere and Himalayan tectosphere.The northern tectonic belt is the Siding Heishan-Quershan Late Paleozoic magmatic arc facies, the central tectonic belt is the Hongshishan ophiolite melange belt which is restored to the Devonian-Carboniferous limited oceanic basin facies + oceanic crust remnant facies, and the southern tectonic belt is the Baishan-Langwashan Late Paleozoic island arc facies.The Hongshishan ophiolitic units from the lower to the top are the Devonian-Carniferous metamorphized ultramafic complex→cumulate ultramafic to mafic complex→homogeneize gabbro and diabase dikes→cherts and carbonic fine-clastic rocks formed from the deep water environment in the Early Carniferous Saozishan period(the limited oceanic facies).The north and south sides of the continental margin marine island arc terrain are Early Carboniferous Saozishanian and Baishanian shallow marine volcanic island arc belts.As a result of southward subduction of the Kazakhstan plate, the Hongshishan SSZ-type ophiolitic melange might be formed on the accretionary wedge on the northern margin of the Dunhuang terrain.Hongshishan ophiolitic melange belt and its peripheral terrain are defined by different tectonic petrography boundaries, which obviously show four tectonic petrography zones from north to south.The center is ultramafic rocks characterized by gabbro and serpentine schist mylonite.The ophiolite melange belt is generally a large rootless lens.The Hongshishan tectonic-ophiolite melangite belt involved the geological terrains on its both sides to form a complex mosaic structural lithofacies conducive to the formation of large-super-large porphyry Au, Cu, Mo deposits, epithermal gold deposits and orogenic gold deposits.