Geological Studies Of Gabbro In Ophiolitic Mélange Belt In West Ujimqin Banner, Inner Mongolia

Three ophiolite tectonic melange belts are distributed in the West Ujimqin BannerInner Mongolia, which are, in order, Meitelaowula ophiolite tectonic mélangezone,Baiyinbulage ophiolite tectonic mélange zone and Naolaiketu ophiolite tectonicmélange zone from north to south．In the three ophiolite tectonic mélange belts, gabbrowidely development,mainly consist of layered gabbro, massive gabbro and mylonitizegabbro which by late deformed.The gabbros in the three ophiolite tectonic mélange belts, are typical comulatesbelonging to the accumulational complex of ophiolite．They are distributed rootless inblock mass on metamorphic peridotites and are in tectonic contact with the other units．Thegabbros have obvions sequence features: at the bottom of gabbro is layede gabbro, at thetop of gabbro is homogeneous gabbro, missing the critical zone from the mantle peridotiteunit to the accumulational complex of ophiolite,some parts of the top the gabbrodevelopment anorthosite (Baiyinbalage).The gabbros consists of45.53~53.76%of SiO2, high Al203and low TiO2and P2O5,belonging to mafic cumulate of ophiolite．For the gabbro located in this area, ΣREE is4.65×10-6~9.54×10-6, LaN/YbN is0.15~1.20δEu is1.08~3.49, which indicates positiveanomaly．In the primitive mantle-normalized trace element spider diagram, the large-ionlithophile elements (LILE) such as Ba, Ta and Sr, are rich, Rb, Th, U and Nb are relativelydepleted, the Dy-Lu section shows a slight left-leaning flat pattern．In addition, differentspecimens have similar trace element spider diagram and rare earth element (REE)distribution patterns． Base on the research on the geological and geochemicalcharacteristics of the gabbros in Xiwuqi area, it is concluded that the gabbros in theophiolites of Xiwuqi are the product of active continental margins or island arcenvironments and that the formation environment of the ophiolite in this area takes on theSSZ-type.According to the characteristics of the zircon and the date of the isotopic dating of thethree samples of the gabbro: the age of the gabrros in the three ophiolite tectonic mélangebelts are308.5Ma,310Ma and320Ma from north to sorth, determine the gabbro wereformed in Middle Devonian. Integrated the characteristics of geological and geochemical gabbro in the WestUjimqin Banner Inner Mongolia ophiolite show that gabbro were formed in a monocyclicor polycycle and closed system in a small magma chamber, its scale of oceanization isrelatively small,and it is in a rapid open and close environment.The gabbros of the ophiolite in the West Ujimqin Banner Inner Mongolia exist themassive sulfide deposit and Ni,Cr,Co,Cu mineralization associated with the ophiolite, Itwill benefits the the prospecting andassessments of Ni,Cr, Co,Cu.