Geoscience Frontiers (Nov 2020)
Compositional and geochronological signatures of metamafic dykes from the Sangsang peridotites, South Tibet: Evidence for magma-starved forearc rifting during Neo-Tethyan subduction re-initiation
Abstract
In this study we present new mineral chemistry, whole-rock geochemical and zircon U–Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet (China). Modal analyses of these dykes gave averages of ~40%–65% plagioclase and ~35%–60% amphibole and small amounts of (igneous) clinopyroxene, epidote and opaque minerals. This mineral assemblage resembles that of typical orthoamphibolites. Nevertheless, due to the absence of foliation the investigated rocks are described as metamafic lithologies. These rocks have primitive mantle (PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak, negative Ti anomalies. In addition, they have initial 87Sr/86Sr ratios [(87Sr/86Sr)i] of 0.702844–0.703581, initial 143Nd/144Nd ratios [(143Nd/144Nd)i] of 0.512891–0.512959 and high εNd(t) values (+7.9 to +9.3). Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at ~119.0–118.5 Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process, from (epidote-)amphibolite facies (~470–610 °C, ~1.9–4.3 kbar) and to prehnite-pumpellyite facies (≤280 °C, ≤ 3 kbar), active within a rift-produced oceanic lithosphere. Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros. Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle (DM) source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab. The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone (YZSZ) in the literature (~129–123 Ma). We, therefore, suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction re-initiation (Aptian). Our tectonomagmatic model provides insights into the igneous accretion and post-solidification evolution of the oceanic lithosphere in South Tibet.
