Abstract
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, 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 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 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 recycling into the mantle at the western end of the Southwest Indian Ridge.
Highlights
The finding of ancient zircons (Paleozoic to Proterozoic) in oceanic gabbros from the Mid-Atlantic Ridge near the Kane fracture zone (Pilot et al, 1998) triggered extensive discussion that is still under way
For the Indian Ocean, the available data are scarce (Ashwal et al, 2017; Cheng et al, 2016; Torsvik et al, 2013). In this contribution we report the first age determina‐ tions and isotope-geochemical data for zircons from the Shaka Ridge, adjacent to the Shaka fracture zone, near the western end of the Southwest Indian Ridge
Igin of zircons that crystallized from melts in continental and oceanic settings can be revealed based on their Yb, U, Y and Hf abundances
Summary
The finding of ancient zircons (Paleozoic to Proterozoic) in oceanic gabbros from the Mid-Atlantic Ridge near the Kane fracture zone (Pilot et al, 1998) triggered extensive discussion that is still under way. The primary issue is the mechanism of Manuscript received Novenber 24, 2020. A significant number of such discoveries (about twenty publications, links below), and a wide range of laboratories in which sample preparation was performed, suggest that this phenomenon is not uncommon, and zircons should be considered as true xenocrysts (Bea et al, 2020). The results of these studies provide convincing evi‐ dence for the presence of ancient continental crust in young oceanic lithosphere (e.g., Bortnikov et al, 2019; Ashwal et al, 2017; Kamenetsky et al, 2001)
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