Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust:New U-Pb Geochronological and Oxygen Isotopic Data  

作　　者：Sergey G.Skublov Natalia A.Rumyantseva Qiuli Li Boris G.Vanshtein Dmitriy I.Rezvukhin Xianhua Li 

机构地区：[1]Institute of Precambrian Geology and Geochronology of Russian Academy of Sciences,St.Petersburg 199034,Russia [2]Saint Petersburg Mining University,St.Petersburg 199106,Russia [3]Institute for Geology and Mineral Resources of the World Ocean(VNIIOkeangeologia),St.Petersburg 190121,Russia [4]State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China [5]Sobolev Institute of Geology and Mineralogy,Siberian Branch of the Russian Academy of Sciences,Novosibirsk 630090,Russia

出　　处：《Journal of Earth Science》2022年第1期5-16,共12页地球科学学刊（英文版）

基　　金：funded by the National Key R & D Program of China(No.2016YFE0203000);the National Natural Science Foundation of China(No.41673018);conducted under state contract(Institute of Precambrian Geology and Geochronology of Russian Academy of Sciences)(No.FMNU-2022-0005);supported by the state assignment project of IGM SB RAS。

摘　　要：Over the past two decades, a significant number of discoveries of ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for the presence of ancient continental crust within young oceanic lithosphere. Almost all finds of ancient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, however, similar data are very limited. This report presents the first age determinations(U-Pb, SHRIMPII) and isotope-geochemical data(oxygen, trace and rare earth elements) for zircon xenocrysts from gabbro-diorites of the Shaka Ridge, in the vicinity of the Shaka fracture zone, near the western end of the Southwest Indian Ridge. The work is based on a study of bottom rock material dredged during expeditionary research on the R/V"Akademik Fedorov"(Russia) in 2016. The U-Pb isotope system of the zircon xenocrysts recorded a crystallization age of ~2.8 Ga at an upper discordia intercept and an age of ~600 Ma interpreted as the timing of a superimposed thermal event at a lower discordia intercept.The zircon xenocrysts show geochemical signatures of magmatic origin, i.e., fractionated REE distribution spectra with an increase in chondrite-normalized values from light to heavy REE, positive Ce anomalies and negative Eu anomalies, and high Th/U ratios(0.59-7.77). In discrimination diagrams based on a series of inter-element relationships, zircon compositions fall into the fields of zircons from rocks of continental crust, mostly granitoids. The Li content of the zircons is high(1.8 ppm-50 ppm),adding further evidence to their derivation from rocks of continental crust. During their residence within young oceanic crust, the zircon xenocrysts experienced alterations under the influence of submarine high-temperature hydrothermal fluids, which selectively affected the distribution of trace elements in the zircons and reduced the δ;O values to 1.75‰-3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrat

关 键 词：ZIRCON U-Pb geochronology oxygen isotopes trace elements rare earth elements Shaka Ridge South Atlantic Southwest Indian Ridge 

分 类 号：P597.3[天文地球—地球化学]