Geochemical Characteristics And Petrogenesis Of Tarenben Basalt In The Middle Segment Of Bangongco-Nujiang Suture Zone

This dissertation conducts detailed research about the Tarenben basalts in the middle segment of the Bangongco-Nujiang Suture Zone.By using the methods of large-scale geological mapping,measuring stratigraphic sections,geochemistry and many other means,combining with historical achievements of predecessors,this paper illustrates the rock types,the characteristics of lithofacies assemblages and sedimentary sequences of Tarenben basalts,as well as discusses the tectonic setting,petrogenesis,magma source characteristics of Tarenben basalts,and its structural significance.Tarenben basalt area is constituted by two volcano-sedimentary rhythms: the base part often comprises pillow-shaped basalts where the breccia basalts are also exists;the middle part is composed by breccia basalts,tufaceous volcaniclastic rock,tuff rock and other rock types with limestone and radiolarian siliceous rocks interlayers;the upper part mainly includes bioclastic limestone with silty mudstone and tufaceous volcaniclastic rock interlayers;the rocks type of top part is the conglomerate which characterized by its simple component,also the boulderstone can be seen in some areas.The upper-top parts plays role as a boundary layer lying in the volcanic-sedimentary cyclothems.The lithofacies assemblages of Tarenben basalt respectively are overflow facies,deep-sea sedimentary facies,eruption-sediment facies,marginal sedimentary facies,bioherm facies and pelagic sedimentary facies.This paper divides the evolutionary stages of Tarenben basalts into the construction stage and the cover stage.Tarenben basalt is characterized by the double-layers structure in the form of oceanic island which means that it has the volcanic rocks of construction stage at the basement and the carbonate rocks of cover stage at the upper part.The peripheral clastic sedimentary and collapse deposit of Tarenben basalt also have the feature of “petticoat”.The rock type of Tarenben basalts shows combination of sodic alkaline basalttrachyte-mugearite,which belongs to the Hawaii rock type.The primitive mantle-normalized trace element spider diagram shows intensive enrichment of LILE,and without negative Nb and Ta anomalies,as well as in the Rb-Y segment,the curve presents the unimodal uplift form of the incompatible elements enrichment;Meanwhile,the REE distribution patterns of Tarenben basalts are characterized by right-inclined type with fairly great gradient,and LREE are concentrated evidently and HREE are deficient,as well as without negative Eu anomalies.Both of trace element spider diagram and REE distribution patterns can be compared with the typical OIB-type basalt.Tarenben oceanic island basalts are OIB-type which formed within oceanicplate,and its origin is associated with mantle plume source regions.Meanwhile,Tarenben basalts are partially melted(9%-11%)from enriched mantle sources which mainly composed of garnet peridotite.Based on the field geological evidences that Tarenben basalts have an unconformable contact with overlying volcanic-clastic rocks of Qushenla Formation in some areas,in this dissertation,we assumes that Bangongco-Nujiang suture zone should be existing as transitional forms between residual oceanic basin-continental volcanic basin at middle-late period of Early Cretaceous in Tarenben areas.