Geochemical Characteristics And Geological Significance Of The Early-Middle Jurassic Gabbros From Numa-Dongga In The Southern Lhasa Terrane

The Lhasa terrane is located at the southernmost part of Eurasia.It has undergone the process of Neo-Tethys ocean subduction and India-Eurasia collision orogeny,and is a hot area for global geoscience research.Previous studies mainly focused on the magmatism and tectonic evolution of Cenozoic and Cretaceous magmatic rocks,and rarely on the Early Mesozoic tectono-magmatism in the southern Lhasa terrane.Therefore,based on the geological survey project“the research and summary of regional metallogenic law of Gangdese metallogenic belt”and the Chengdu University of Technology Project Everest Science Project“The Evolution Process and Metallogenesis of the Tethys Ocean in Central Tibet”,carried out field geological survey,geochronology,petrography,mineralogy,and geochemistry of the Numa-Dongga gabbro.Then,we determined the diagenetic age and discussed petrogenesis.Meanwhile,the geological tectonic environment and crustal growth and evolution process are discussed,and the early Mesozoic magmatism and mineralization of the southern Lhasa terrane are summarized.And the following knowledge is obtained:（1）The zircon U-Pb dating results show that the formation age of the Numa hornblende gabbro and Dongga cumulate hornblende gabbro are 163.7±2.2 Ma and182.8±2.2 Ma,respectively.（2）The Numa hornblende gabbro has low SiO₂ content（46.23～48.23%）,TiO₂（0.48～0.53%）,P₂O₅（0.04%）and K₂O contents（0.32～0.60%）,high Al₂O₃（16.46～18.20%）,Mg O（8.19～9.82%）and Na₂O contents（0.94～1.80%）.And the sample have higherε_Hf（t）（+11.4～+17.2）,ε_Nd（t）（+3.6～+4.3）,and lower Sr((⁸⁷Sr/⁸⁶Sr)_i=0.703913–0.704102)and Pb((²⁰⁶Pb/²⁰⁴Pb)_i=18.2186–18.4389,(²⁰⁷Pb/²⁰⁴Pb)_i=15.5766–15.6529,(²⁰⁸Pb/²⁰⁴Pb)_i=38.2187–38.6398)isotope compositions,similar to the isotopic characteristics of Neo-Tethys ocean ophiolite.The geochemical characteristics of the Numa hornblende gabbro suggested that it generated by partial melting of depleted mantle wedge which was metasomatized by slab-derived fluid,and experienced fractional crystallization of pyroxene and olivine.（3）The Dongga cumulate hornblende gabbro has low contents of SiO₂（42.22～46.35%）,TiO₂（0.69～1.39%）,P₂O₅（0.01～0.49%）and K₂O（0.15～0.25%）,high contents of Al₂O₃（17.03～21.02%）,Mg O（7.44～11.50%）and Na₂O（1.12～1.33%）.And the samples have higherε_Hf（t）（+11.1～+17.3）,ε_Nd（t）（+5.3～+6.0）,lower Sr((⁸⁷Sr/⁸⁶Sr)_i=0.703874～0.704035)and Pb((²⁰⁶Pb/²⁰⁴Pb)_i=18.3810～18.5305,(²⁰⁷Pb/²⁰⁴Pb)_i=15.5848～15.6574,(²⁰⁸Pb/²⁰⁴Pb)_i=38.2721～38.6668)isotopic compositions.Mineralogical analysis shows that the gabbro crystallizes at high temperature（～900℃）,pressure（～8.5 kbar）and oxygen fugacity.These geochemical characteristics indicate that it generated by partial melting of depleted mantle wedge which was metasomatized by subduction fluid,followed by fractional crystallization of mantle-derived magma and accumulation at the bottom in the magma chamber.（4）The Numa-Dongga gabbros are generally depleted in high field strength elements（e.g.,Nb,Ta）and enriched in large ion lithophile elements（e.g.,Rb,Ba,Sr）with typical arc magmatic characteristics.The gabbro shows the characteristics of high Al,low P and Ti,which are like those formed in the island arc are like the characteristics of the magmatic rocks formed in the subduction zone.Combined with previous studies,we considered that the Numa-Dongga gabbro was formed in the continental margin arc environment related to the northward subduction of the Neo-Tethys Ocean.（5）The ratios Sr/Y,La _N/Yb _N and Ce/Y of the early Mesozoic magmatic rocks in the southern Lhasa terrane show that the crustal thickness gradually increased during the Early-Middle Jurassic,and the underplating of mantle derived mafic magma and its induced crust-mantle magma mixing played an significant role in the crustal thickening during this period.（6）Based on the results of this paper and previous studies,it is shown that the early Mesozoic plutons related to Neo-Tethys subduction reported in the southern Lhasa terrane have the characteristics of depleted Hf-Nd isotope,high oxygen fugacity and water content,which are favorable for the formation of porphyry deposits.The distribution area of volcanic sedimentary rock caprock in the southern Lhasa terrane can be used as a prospecting area for early Mesozoic subduction related porphyry deposits.Meanwhile,the juvenile mafic lower crust formed by the underplating of Jurassic mantle derived magma may have provided abundant metal sources for the Miocene porphyry deposits,leading to large-scale magma-hydrothermal mineralization.