Geological Characteristics And Significance Of The Middle Jurassic Zhela Formation In Rongbu Area,Southern Tibet

Rongbu area,located in the south of Tibet Autonomous Region,belongs to the middle part of Tethys Himalayan orogenic belt.Based on the study of sedimentary petrology,magmatic petrology and petrochemistry,combined with the previous research data,this paper report the field geology of the shales and basalts from the Middle-Jurassic Zhela Formation,measured stratigraphic profile and sample data analysis.The study shows that the strata of Zhela formation in Rongbu area change in an orderly way,which shows the change of argillaceous rock to carbonate rock?or terrigenous clastic rock+volcanic rock?to argillaceous rock from the first to the second member.Through the study of shales in the first member of Zhela formation,it is found that shale samples have a high ratio of total rare earth elements??REE?and light to heavy rare earth elements,which are much higher than the standard value of shales in North America,indicating that the light to heavy rare earth fractionation of shales from the Zhela formation is relatively obvious,and shale samples have a negative anomaly of?Eu,while?Ce does not show an anomaly,suggesting that the deposition of Zhela formation is in a reducing environment.In addition,there are a large set of relatively pure grayish black argillaceous rock in the second member of the Zhela formation.It is speculated that the sedimentary environment of the Zhela formation belongs to the continental shelf and slope environment.And the ammonite fossils Haplophyloceras sp.,which are common in the Middle Jurassic,are found in the shales of the Zhela formation.Through the study of basalts in Zhela formation,it is found that the lithology of basalts are mainly porphyritic basalt and aphanitic basalt,and most of them occur in the first member of Zhela formation.The major element features show that the basalt samples have the characteristics of high Ti,high Fe,high P and low Mg;the trace element shows that the large ion lithophile elements Rb,Th and Sr of the basalts samples are relatively deficient,except for Rb,K,Ba,Sr and other elements,which are obviously similar to OIB.The Zr/Nb ratio of the basalts are 5.10-7.64,and the Hf/Th ratio is 2.20-3.94,similar to the intraplate basalt?Hf/Th<8?.The REE characteristics show that the normalized values of REE chondrites decrease from La to Lu,and there is no Eu anomaly,which is roughly the same as the OIB type REE distribution trend;the isotopic characteristics show that the Zhela formation basalts have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd.The major,trace,rare earth and isotopic characteristics of the Zhela formation basalts are consistent with those of the Sangxiu formation basalts,indicating that the formation of the Zhela basalts are related to hot spots or mantle plumes.According to the characteristics of lithologic assemblage and petrochemistry of the Zhela formation,it is concluded that the Zhela formation was deposited in continental shelf and slope environment,and there was a large-scale interaction between mantle plume and hot spot during the deposition of the Zhela formation,which also provided conditions for the evolution of the Yarlung Zangbo River ocean basin.