The Rock Assemblage And Deformation History Of The Western Lanping Basin, Western Yunnan, China

The Sanjiang composite orogenic and metallogenic belt, located in southeastern part of the Tibetan Plateau, has gotten a lot of attention form geologists worldwide for a long time due to its special tectonic settings, a long and complicated geologic evolution history, the unique geological structure and richness in mineral resources. The Lanping Basin, the centre of the Sanjiang orogenic belt, is not only an important production base of Pb-Zn metals, but is also a key region to study the Cenozoic tectonic evolution of the Tibetan Plateau. Many controversies remain considering its regional geology in spite of the fact that numerous geologists in different aspects have studied this basin in quite detail. As such, this dissertation choose the rock assemblage and structures along the Yingpan-Lajing transaction in western part of the Lanping Basin as the major topic, in order to reveal the sedimentary, volcanic, and structural characteristics of the Lanping basin. On the basis of detailed field observations and measurement, sampling and/then geochronological and geochemical analyses, we try to discuss tectonic evolution of the Lanping basin.Field study, checked by thin-section examination, help us to identify four rock suites along the Yingpan-Lajing transaction (in upward):(1) basaltic to andesitic volcanoclastics and lava;(2) purple to dark-purple volcanic breccia-bearing elastics (including multiple conglomerate, middle to thick sandstone, and fine-grained elastics);(3) volcanics-absent elastics and bioclastics; and (4) the Late Cretaceous Nanxing Formation and the Eocene Yunlong Formation terrestrial elastics. The (2) and (3) exhibit a successive sedimentary assemblage, which, along with the volcanics (suite (1)), forms the hanging wall of a reversal fault. On the other hand, the Cretaceous and Eocene sedimentary rocks are autochthonous.Zircon LA-ICP-MS U/Pb results demonstrate that the volcanism took place during the middle Triassic (ca.238to229Ma) rather than during the Carboniferous or Late Triassic. Bulk geochemistry suggests that the andesites in lower part of the volcanic sequence are generally arc-like volcanics, while the basalts of upper portion are enriched-middle-oceanic-ridge-basalt (E-MORB) affinity. Such a temporal variation in geochemistry implies that the later basalt likely have resulted from partial melting of mantle wedge induced by breakoff of the subducted oceanic slab. Synthesizing available geochronological and geochemical data, we suggest here that these volcanic rocks along the western margin of the Lanping basin are part of the Permian to middle Triassic volcanic arc across the entire Sanjiang orogenic belt, which developed along the western margin of the Yangtze plate due to eastward subduction of the Paleotethyan oceanic crust under the Yangtze continent.Structural data of the Lanping basin are generally rare. Our detailed structural analysis revealed two stages deformation recorded in the rocks along the western margin of the Lanping basin, which display distinctive structural pattern:(1) the structures in the middle Triassic strata are featured by tight fold and tightly spaced axial cleavage. Several anticlines and synclines can be identified according to the distribution of the mark stratum and the spatial variation in the cleavage-bedding relationship. And (2) the Cretaceous and Eocene strata are deformed giving rise to open folds and reversal faults. Between the Triassic and Cretaceous strata is a large scale reversal fault, which has duplicated the Triassic rock over the Cretaceous and Eocene rocks. Outcrop observations and stereo projection of the structural elements suggest that the tight folds are broadly cylindrical with NNE-SSW-trending hinge traces, which likely resulted from ESE-WNW (in present coordinates) directed compressional tectonism. Taking regional geology into account, we suggest that the ESE-WNW direction compression was caused by a late Triassic continent-continent collision following the Permian to middle Triassic ocean-continent subduction. On the other hand, the second stage deformation likely was caused by the India-Eurasia collision.In summary, the tectonic evolution of the Lanping basin may be synthesized as below:(1) The Paleotethys subducted eastward under the Yangtze continent during the Early Permian to Middle Triassic. At the latter stage of the subduction, breakoff of the subducted slab occurred, leading to formation of the E-MORB like basalts.(2) Immediately after the subduction, continent-continent collision started at the Late Triassic deforming the arc-like volcaniclastics which have given rise tight folds of the volcaniclastic rocks.(3) The Jurassic to Eocene history has not been well-constrained due to absence in geological record, but it is quite sure that the Lanping basin region was in an intra-continent environment then.(4) Since the middle Eocene (ca.35Ma), the deformation due to the India-Eurasia collision expanded to the Lanping basin region which caused folding and thrust of the Mesozoic to Cenozoic strata in the region.