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

Abstract Over the past two decades, a significant number of discoveries ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for presence continental crust within young oceanic lithosphere. Almost all finds are from Mid-Atlantic Ridge. For other localities, however, similar data very limited. This report presents first age determinations (U-Pb, SHRIMP-II) and isotope-geochemical (oxygen, trace rare earth elements) gabbro-diorites Shaka Ridge, vicinity fracture zone, near western end Southwest Indian The work is based on study bottom rock material dredged during expeditionary research R/V “Akademik Fedorov” (Russia) 2016. U-Pb isotope system recorded crystallization ?2.8 Ga at an upper discordia intercept ?600 Ma interpreted as timing superimposed thermal event lower intercept. show geochemical signatures origin, i.e., fractionated REE distribution spectra with increase chondrite-normalized values light to heavy REE, positive Ce anomalies negative Eu anomalies, high Th/U ratios (0.59–7.77). In discrimination diagrams series inter-element relationships, compositions fall into fields zircons crust, mostly granitoids. Li content (1.8 ppm–50 ppm), adding further their derivation crust. During residence experienced alterations under influence submarine high-temperature hydrothermal fluids, which selectively affected elements reduced ? 18 O 1.75‰–3.15‰. obviously older clearly demonstrates fragments recycling mantle