• 标题：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
• 关键词：Zircon; Dyke; Ophiolite; Sangsang; Tibet; Neo-Tethys.
• 作者：Chi Xu, Hao Zheng, Argyrios Kapsiotis, Yang Xiao, Zhi-feng Wan

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 to 0.703581, initial 143Nd/144Nd ratios [(143Nd/144Nd)i] of 0.512891 to 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.