On the Characters of Sedimentary Formations In the Luhuo Rift,Western Sichuan

Key words: 
• rift /
• sedimentary formation /
• geological evolution

Abstract: The Luhuo rift occurs in the internal parts of the Songpan-Ganzi massif,western Sichuan,which was bounded by the deep-seated Jiajilong-Quesuo fault and Seda fault. The boundary faults dipped northeastward and southwestward respectively, which constituted the garben-like type sea basin. Teh Luhuo rift is character- istically different from the neighboring parts in that there are basic volcanic rocks and calcareous breccias.Through the classificaiton and correlation of strata and formation analysIs,it is determined that there are turbidity formation,(volcanic) melange formation and bassyal sea carbon-silicon-mud formation in the Luhuo rift. The turbidite formation is the chief formation in the Triassic system. The Bouma sequence,various bedding structures are existed in the rock series. The granulometric analysis result as well as the lower maturities of composition and texture suggests the origin of turbidity fluid. Compared with the chemical composition of major elements and rare earth elements for graywack and slate in different distinct tectonic environments, it can be revealed that the continental island-arc source for the provenance of the Triassic turbidite in the rift.The melange formation and/or volcanic melange formation was olistostrome formed by short-distance transportation and deposition,which occur as stratified between age-determined marine strata. The bassyal sea carbon-silicon-mud formation consists of purple radiolaria-bearing silicon mudstone,gray,gray-purple and purple sponge-spiculte-bearing micrite, and silty mudstone. There are horizontal bedding and quasi-boxwork in mi-crite,which shows less hydrodynamic condition and starved calm bassyal environment. Combined with other data related to tectonic activity,vilcanoic and magmatic intrusive,the evolution from beginning,development to closing of the Luhuo rift may be divided into four stages as follows. 1)Primary rifting(in late-Permian) in which carbonate rock formation, clastic formation and tholeiite formtation were developed, accompanied with stratified basic-ultrabasic intrusive bodes,2)Deposition depression stage(from early-Triassic to late stage of late-Triassic) during which bassyal sea carbon-silicon-mud formation,melange formation nad turbidite formation were developed,3)Intense rifting stage (in the middle stage of late-Triassic)in which the volcanic melange formation was developed, accompanied with the intrusion of basic-ultrbasic dykes, and 4 ) Withering stage(in the late stage of late-Triassic) in which the turbidite formation was formed.