Geological Characteristics And Tectonic Significance Of The Mafic Rocks From South Rongma Area In The Middle Part Of The Bangong-nujiang Suture Zone,Tibet

The study area is located on the southern edge of south Qiangtang area and the northern edge of the Bangong-Nujiang suture zone,which is of great significance for the evolution of Tethys ocean in the middle part of the Bangong-Nujiang suture zone.The study of the mafic rocks in southern Rongma is nearly gap.Therefore,this study can provide important information for the middle section of the Bangong-Nujiang suture zone.This study used petrological,geochemical and chronological methods on the mafic rocks from the south of Rongma and the north of Ejiu,and combined the geological features-chronology-geochemistry of the mafic rock blocks in the melange to study the geochemical characteristics and tectonic evolution process of the mafic rock blocks in the tectonic ophiolite melange in the south area of Rongma.According to the field occurrence and geochemical characteristics,mafic rocks can be divided into three types,OIB-type basalt,OIB-type gabbro and N-MORB-type gabbro.Moreover,OIB-type basalt and OIB-type gabbro have the characteristics of magma homology,which is obviously different from N-MORB gabbro.OIB-type basalt and OIB-type gabbro have the characteristics of alkaline basalt,enriched with Na,Mg,Al,Fe elements,and loss in K elements;enriched with high field strength elements Nb,Ta,Ti,without Th,U,Nd,P obvious loss,showing the characteristics of high Nb and high Ti;enrichment of light rare earth elements,loss of heavy rare earth elements,obvious differentiation of light and heavy rare earth elements,no Eu abnormality.N-MORB-type gabbro has the characteristics of subalkaline basalt,enriched with Na,Mg,Al,Fe,K elements,weakly loss high field strength elements of Nb,Ta,without obvious anomalies of Ti,Th,U,Nd,P.Light rare earth elements are relatively deficient,heavy rare earth elements are relatively enriched,and there is no Eu abnormality.OIB mafic rock magma originated from the mixing of EM? and DM.It was produced by partial fusion of 5-10% of garnet diorite and a small amount of spinel-garnet diorite,which is the Inner Ocean Island of the mantle plume.The NMORB-type gabbro magma mainly originated from the MORB source region,which resembled the Indian Ocean.It was produced by the partial melting of spinel diorite at the ratio of 10%,and showed the depletion characteristic,which represented the normal mid-ocean ridge.Both mafic rocks have DUPAL anomalies.The N-MORB-type gabbro has a relatively consistent concordant zircon ages of 174.6 ± 2.5Ma.The dispersed zircon ages of OIB-type gabbro is obtained,and its sphene ages are concordant at 200.2 ± 3.2 Ma.The mafic tuff formation age is 270.3 ± 2.4Ma,which can represent the diagenetic age of OIB-type basalt.Based on the geological information of this study,we believes that the study area is affected by the mantle plume.The Bangong-Nujiang Ocean has been opened before 270 Ma,and the initial ocean basin has appeared around 270 Ma.At the 200 Ma,the Bangong-Nujiang Ocean continued to develop,with the ocean islands are mature,and gabbro intruded into the slopes of the previous ocean islands.The ocean islands and the trenches collided between 200-174 Ma,which formed the ocean islands slices.At the 174 Ma,the mid-ocean ridge collided with the pre-collaged ocean island slices.Due to the blocking effect of the ocean islands,the broken ocean ridge ophiolite pieces remained.