Fate of an oceanic plate in the Neo-Tethys intra-oceanic subduction system: Evidence from elemental and Rb/Sr – Sm/Nd isotopic systematics

Abstract

We report the coexistence of normal mid-ocean ridge basalt (NMORB)-type and alkaline ocean island basalt (OIB)-type mafic volcanics from the Zildat Ophiolitic Mélange (ZOM), Indus Suture Zone (ISZ), eastern Ladakh Himalaya. Trace element modeling portrays chemically heterogeneous mantle sources, wherein blueschists and sub-alkaline mafic volcanics depict almost flat REE-patterns [(La/Sm)N = 1.04–2.9] and respectively <20 % and >20 % degrees of partial melting from a depleted spinel-lherzolite mantle source. In contrast, alkaline mafic volcanics depict enriched LREE-patterns [(La/Yb)N = 35–48] with <5 % degrees of partial melting from a transitional garnet-spinel lherzolite enriched mantle source. Since the prevalent hypothesis considers seamounts and island arc rocks of the subducting oceanic plate within the Neo-Tethys Ocean to be the protolith for the contemporaneous Shergol Ophiolitic Melange (SOM) blueschists. In contrast, ZOM blueschists depict different protolith signatures. The εNd(t = 140 Ma) values for ZOM blueschists (+3.7 to +7.4) and sub-alkaline mafic volcanics (+9.2 to +9.3) are analogous to present-day Indian Ocean NMORB, while for OIB-type mafic volcanics (+3.7 to +4.3) are comparatively lower and analogous to Hawaii and Kerguelen OIB. Therefore, the observed protolith decoupling in the contemporaneous blueschists suggest the involvement of different protolith contenders, which were partially off-scraped from of the subducting plate during prolonged deep subduction followed by exhumation along the ISZ.