Petrogenesis And Tectonic Implications Of The Late Jurassic Volcanic Rocks In The Kongnongla Area Of Baigoin County,Xizang

Mesozoic magmatic rocks are widely distributed in the Bangong-Nujiang junction zone in Tibet,which is of great significance for inverting the tectonic evolution of the Bangong-Nujiang Tethys Ocean.In this paper,through field geological survey,detailed petrological,chronological,whole-rock geochemical and isotope geochemical studies of the Kongnongla volcanic rocks have been carried out,and the following knowledge is obtained:Kongnongla andesites occur in the Beila-Lanong ophiolites and are in conformable contact with the high niobium basalts.The andesites are porphyrictic and massive,the phenocrysts are dominated by elongated plagioclase,with barely melanocratic minerals phenocrysts.The stroma is hyalopilitic and composed of plagioclase microcrystalline and glassy aphanite.Besides,calcite minerals and veins are found under the microscope.The basalts develop intergranular cryptic structure,pillow structure,and vesicular structure.The phenocrysts are mainly plagioclase,clinopyroxene and olivine.The Stroma is mainly cryptocrystalline plagioclase and amphibole.The Kongnongla andesites are high K calc-alkaline rocks,which are characterized by moderate SiO₂ contents(59.34?61.68%),high Mg O contents(3.74?4.69%),high Mg^#values(61.3-69.0),and enriched in compatible elements,such as Cr(110?150 ppm)and Ni(24.7?36.5 ppm).These geochemical characteristics indicate that the andesite is typical sanukite-type Magnesium andesite.The Kongnongla magnesian andesites are riched in large ion lithophile elements(LILEs)(such as U,Th,etc.)and depleted in large ion lithophile elements(HFSEs)(such as Nb,Ta,Ti,etc.),light REEs are more abundant than heavy REEs,and show negative Eu anomalies(Eu/Eu*=0.60-0.75).The basalt samples have low SiO₂ contents(45.69?48.17%),high Ti O₂ contents(2.75?2.9%),high total alkali contents of Na₂O+K₂O(3.9?4.9%),high Nb contents(24.8?25.9 ppm),and are enriched in light rare earth elements relative to heavy rare earth elements,with positive Nb-Ta-Ti anomalies and no obvious negative Eu anomalies.Those geochemical characteristics show that the basalt is high Nb basalt.The Kongnongla andesites were emlaced at ca.158Ma,indicating they were formed in the late Jurassic.The age of the andesites is similar to the Peng Tso high magnesian andesites/rhyolites and Daru Tso high magnesian andesites,indicating that these magnesia-rich rocks may be the product of magmatic activity of the same age.Kongnongla magnesia andesites have low(⁸⁷Sr/⁸⁶Sr)_ivalues(0.704491?0.704857),(¹⁴³Nd/¹⁴⁴Nd)_i values(0.512121?0.512237),and negtive?_Nd(t)values(-3.85?-6.12)and?_Hf(t)values(-10.8?-15.7).The basalt(⁸⁷Sr/⁸⁶Sr)_ivalues are0.704749?0.704878,(¹⁴³Nd/¹⁴⁴Nd)_i values are 0.512616?0.512736,?_Nd(t)values are3.54?5.88.Based on the geochemical and isotopic characteristics of Sr-Nd-Hf of the Kongnongla volcanic rocks,we suggest that the andesites were formed by interation between slab fliuds,subducting sediments and overlying mantle materials under the abnormal geothermal gradient of high temperature and low pressure in fore-arc setting.The basalts were formed by magma mixing between highly enriched component(OIB-like)and relative depleted component(MORB-like)in the garnet stable field.Based on the regional geological background and the research results of this thesis,it is considered that the Konnongla volcanic rocks were probably the product of northward initial subduction of the southern branch oceanic basin of Bangong-Nujiang Tethys Ocean under the Dongkaco micro-continent.