Over the past two decades, a significant number of discoveries of ancient zircon xeno- crysts 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 an- cient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, how- ever, similar data are very limited. This report presents the first age determinations (U-Pb, SHRIMP- Ⅱ) 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 ex- peditionary 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 distribu- tion 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 with- in 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 δ18O values to 1.75‰-3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrates the presence of ancient continental fragments and their re- cycling into the mantle at the western end of the Southwest Indian Ridge.

Sergey G.Skublov;Natalia A.Rumyantseva;Qiuli Li;Boris G.Vanshtein;Dmitriy I.Rezvukhin;Xianhua Li

Institute of Precambrian Geology and Geochronology of Russian Academy of Sciences,St.Petersburg 199034,Russia;Saint Petersburg Mining University,St.Petersburg 199106,Russia;Institute for Geology and Mineral Resources of the World Ocean(VNⅡOkeangeologia),St.Petersburg 190121,Russia;State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;Sobolev Institute of Geology and Mineralogy,Siberian Branch of the Russian Academy of Sciences,Novosibirsk 630090,Russia

地球科学学刊（英文版）

[1]Sergey G.Skublov;Natalia A.Rumyantseva;Qiuli Li;Boris G.Vanshtein;Dmitriy I.Rezvukhin;Xianhua Li-.Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust:New U-Pb Geochronological and Oxygen Isotopic Data)[J].地球科学学刊（英文版）,2022(01):5-16

Xenocrysts,Shaka,crysts,diorites,dredged,peditionary,Akademik,discordia

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