Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: a paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the Zhongba-Zhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 ± 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)N = 13–16], high initial 87Sr/86Sr ratios (0.70598–0.70765), and low εNd(t) values (–2.6 to –2.3). Zircons separated from this group of dolerites have slightly radiogenic εHf(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 ± 2.5 Ma and 124.4 ± 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)N = 0.6–0.9] with noticeable negative Nb and Th (±Ti) anomalies, and have high 87Sr/86Sr(i) (0.70295–0.70618) and high εNd(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic εHf(t) values (+3.5 to +17.0). Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5%–10% polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORBtype mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous.