Research On Composition And Chronology Of The Niujingshan Ophiolitic Melange In The Changning-Menglian Suture Zone

Ophiolite is a fragment of ancient oceanic crust or its related tectonic units preserved in orogenic belts,,which can provide important information for the study of orogenic evolution reconstruction,deformation and deep-seated mineralization.The Niujingshan ophiolite melange is located in the middle of the Changning-Menglian Suture Zone and has been studied little.Based on the study of the geological characteristics,petrology,chronology and geochemistry of the niujingshan ophiolite melange,the material composition of the structural units of the melange is determined,and the evolution of changning-meng lianyang is discussed with previous research results.The following understandings are achieved:(1)There are two sets of ophiolite melange in the early Paleozoic and late Paleozoic respectively.The early Paleozoic ophiolitic mélange is exposed in the area of Xiaomangguan village on the northwest side of Niujing Mountain.It is mainly composed of metamorphic ultrabasic-basic-ophiolite blocks such as amphibole schist,plagioclase amphibole schist,epidote-amphibole schist,epidote-actinolite schist,oceanic island-seamount rock block composed of OIB properties slate-limestone,mixed matrix of radioactive siliceou(slate)and quartz sandstones on the continental margin.The late Palaeozoic ophiolitic melange is exposed in the area of xiaoheijiang-mengyong in the northeast of the central part of Niujing mountain.It is mainly composed of serpentinization peridotite,altered gabbro,amphibolite,altered basalt,ophiolite block,limestone block and mica quartz schist.Ductile fault contacts were found between blocks of different tectonic units and between blocks and mixed matrix.The original sequence of ophiolite in early Paleozoic has been destroyed to varying degrees.Ophiolite blocks are now all dismembered ophiolites.(2)The U-Pb weighted average ages of zircon from the ophiolite amphibole schist and epidote-amphibole schist of early Paleozoic were 448.0±2.5Ma(MSWD=0.64,n=25)and 434.6±6.5Ma(MSWD=1.3,n=8).Amphibolite schist and epidote-amphibole schist has certain mid-ocean ridge basalt(N-MORB)geochemical characteristics and has high field strength elements Nb,Ta,Zr and Hf,loss of IAT characteristics,such as rare earth elements distribution figure shows similar quad effect is not obvious type "M",indicating source area is likely to be affected by subduction fluid,the ophiolite may is SZZ type ophiolite,may be formed in the back-arc basin environment,indicates that cows probably in early ordovician in the well region-has been formed in late ordovician back-arc basin.(3)The U-Pb weighted average age of the amphibolite of late Paleozoic ophiolite is 272±1.7Ma(MSWD=1.1,n=21),with N-MORB geochemical characteristics,indicating that the ophiolite may be of MOR type.Combined with previous studies,it indicates that the dilatation and subduction of the changning-menglian yangyang basin occurred simultaneously in the Permian.(4)The early Paleozoic ophiolite and late Paleozoic ophiolite are intermixed with the Devonian-Permian proliferative complex through the ocean-continental and land-continental collision orogeny after permian.