Research On Metamorphism And Deformation Characteristics Of Maoxian Group In The Shuimo Area Of North Longmenshan

The Longmen Mountain is located between eastern Tibetan Plateau and theSichuan Basin. Southeast is connected with the northwest margin of the YangtzeBlock, northwest is adjacent to Songpan-Ganzi orogenic belt. It is NE-SE elongatedand sandwiched between many plots, which is a convergent and combining zone ofdifferent blocks and kinds of orogenic belt and shows complex tectonic styles andmatching history. The Longmen Mountain also plays an important role in the tectonicframework of southwest China.Combining comparative analysis of information on field geological survey withprevious studies, this paper studies comprehensively on metamorphism anddeformation characteristics, metamorphic and deformation sequence of MaoxianGroup in the Shuimo area and achieves the following results and preliminaryconclusions by using the methods of petrology, geochemistry:1.The main lithological characteristics of the Maoxian Group in the Shuimo areaare yellow gray-silver-gray quartz-sericite slate, phyllite, and few metamorphicfine-siltstone and biological debris crystalline limestone. Deformation fabrics are alsowell developed. The lithology can be divided into two stratigraphic units. They areHuangping Group （Shp） slate, phyllite and Huatianpo group （Sh） slightly lowermetamorphic grade phyllite slate, metamorphic fine-siltstone, crystalline limestone.The two stratigraphic units are located in the Lishuba nappe and the Chenjiashannappe. According to the characteristics of rocks and minerals and single-mineralelectron microprobe analysis, the main minerals are phengite, ironchlorite-magnesium chlorite, quartz, ankerite （magnesian siderite）.2.Maoxian Group metamorphic rocks are metasediments. The Original rockswere muddy-silty flysch and carbonate build-up, which were formed at the edge of the sea or the epicontinental sea. It is the ancient continental margin of the Yangtzeextends to the Paleo-Tethys shelf shallow water-bathyal semi-stability environment.3.The Maoxian Group rocks are characterized by solid-state rheological structureand Multi-stage tectonic schistosity. According to the superimposition andreplacement features of deformation fabrics, they can be divided into at least fourdeformation periods: D1formed in the mid-late of the Indosinian Movement (T_2-3)extrusion nappe environment and was mainly characterized by nappe and regionalpenetrative foliation S1. D2formed during the end of the Indosinian Movement to theearly stage of Yanshan Movement （T₃-J₁） extrusion and partial stretch slippageenvironment and mainly characterized by regional penetrative foliation S1andparalleled to S1schistosity Quartz veins. D1～D2was the main metamorphic stage ofMaoxian Group rocks, which generally suffered from regional low temperaturedynamic metamorphism and then generated metamorphic minerals of sericite, chlorite,quartz, ankerite （magnesite ore）. D3formed in the mid-late of the Yanshan Movement（T₃³ï½žK₁） and dominated by lifting movement in the pressure-shear dynamicenvironment. It mainly formed slip （pleats） cleavage S2，the rootless hook quartz veinsand S-C fabric. With dynamic metamorphism in the main fault zone, D3generatedductile shear zone，protomylonites and dynamic m，etamorphic rocks. D4, which wascharacterized by the widely developed X joints, formed in the Himalayan Movementstrike-slip shear&extrusion environment. Dynamic metamorphism formed bsrittlecataclastic rocks, etc.4.According to the characteristics of metamorphic mineral assemblages, thechlorite component thermometer and muscovite-chlorite P-T stability curve,temperature and pressure in the peak period of metamorphic was, respectively,330～400℃,0.255～0.309Gpa. It was a regional low temperature dynamicmetamorphism of low-greenschist facies with lower metamorphic rocks chlorite zonein a low-temperature low-pressure environment. The main metamorphism types weremetamorphic crystallization, recrystallization, metamorphic differentiation,deformation and fragmentation.5. Combining with previous data on comparative analysis of the regional tectonicevolution history, this paper classified comprehensive logical events in seven stages:Jinningian and the double basement was formed beforeâ†'Chengjiang uplift periodformed large-scale magmatic activity and deposited of marine volcaniclasticrocksâ†'Caledonian tensile period formed a passive continental margin marinedepositsâ†'Hercynian tensile uplift period missing depositionâ†'Indosinian extrusion period folded orogenic and suffered regional low temperature dynamicmetamorphismâ†'Yanshan uplift period denudedâ†'Himalayan extrusion shear periodnappe orogeny.