A Comparative Study Of The Tectonic Setting And Minerality Between Santaishan Ultrabasic Rocks And Yingjiang Ultrabasic Rocks In Yunnan Province

Through the study of rock geochemistry,zircon U-Pb chronology,Lu-Hf isotope and Sr-Nd isotope,this paper defines the origin and tectonic setting of ultrabasic rocks in the Santaishan and Yingjiang areas in the western Yunnan area.Besides,it discusses the Characteristics of metallogenic elements and the evolution of Neo-Tethys.The following conclusions were reached:(1)As for the petrology,the ultrabasic rocks in Santaishan are serpentinized pyroxene peridotites.The main mineral serpentine is metamorphic mineral of olivine in the primary rock,accounting for 70% of the total rock.The ultrabasic rocks in Yingjiang is fine-medium spine hornblende ehrwaldite,and its main mineral is monoclino pyroxene trapezite which accounts for 60% in the total rock.(2)As for the major elements,high content of Mg O and low content of Si O2,Al2O3,Ca O and Ti O2 exist in Santaishan ultrabasic rocks,however,relatively low content of Mg O and high content of Si O2,Al2O3,Ca O and Ti O2 exist in Yingjiang ultrabasic rocks,whichare in accordance with the characteristics of major elements in SSZ and MORB ophiolite in Bangonghu-Nujiang suture zone.(3)As for the tombarthite and microelement,there are high content of LILE like Rb,Ba and Th and low content of HFSE like Ti,P,U and Nb in Santaishan and Yingjiang ultrabasic rocks.?REE is low in Santaishan,but is high in Yingjiang,which are in accordance with the characteristics that?REE in SSZ-type is lower than in MORB-type in Bangonghu-Nujiang suture zone.(4)The zircon chronology shows that the age of Santaishan Ultrabasic rocks is 190.5±1.3Ma(MSWD=0.57),while Yingjiang is 183.7±1.8Ma(MSWD=0.7).(5)Lu-Hf isotopes in the zircon microregion indicates that the?Hf(t)in Santaishan ultrabasic rocks is 11.2~13.8,while is 3.5~6.5 in Yingjiang ultrabasic rocks.In?Hf(t)-T diagram,both of them are distributed between the evolution line of depleted mantle and Hf isotopic of chondrite.Moreover,the melting degree of Santaishan ultrabasic rocks is higher than Yingjiang ultrabasic rocks.(6)The Sr-Nd isotope of whole rocks shows that the value of(87Sr/86Sr)i is abnormally high in Santaishan ultrabasic rocks,which is-1.1~9.4,and all specimens are basically distributed in depleted mantle,of which 2 are in primitive mantle indicat that they may be contaminated by crustal material.(7)Compared with the continental crust in eastern China,the Santaishan ultrabasic rocks are significantly enriched with Ni,Cr and Co elements and relatively enriched with Pt elements.While the Yingjiang ultrabasic rocks are significantly enriched with Ni,Cr and Co elements,and relatively rich in Pt,Pb,Au,Cu,V and Y elements.(8)The subduction time of Bangonghu-Nujiang ocean on Tengchong massif should start from 190.5Ma,Santaishan is located in the Continental margin arc environment,affected by the subduction fluid,SSZ-type pyroxene peridodite is formed.The back-arc basin is formed in Yingjiang,and P-MORB-type ehrwaldite is formed at 183.7Ma.