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EPSL: Subduction re-initiation at dying ridge of Neo-Tethys: Insights from mafic and metamafic rocks in Lhaze ophiolitic mélange, Yarlung-Tsangbo Suture Zone
ABSTRACT
Ophiolitic mélanges provide critical information on the history of subduction and metamorphism and, thus, on the evolution of ancient oceans. Blocks of mafic rocks, garnet-clinopyroxenites and amphibolites have recently been discovered in the Lhaze ophiolitic mélange of the Yarlung-Tsangbo Suture Zone, Southern Tibet. These blocks represent different origins from the Neo-Tethys. The unmetamorphosed MORB-type mafic rocks are considered as the oceanic crusts formed during the spreading of the dying mid-ocean-ridges. The garnet-clinopyroxenites and amphibolites are interpreted to be parts of dismembered metamorphic soles beneath the lowest part of the ophiolites. In spite of different mineral assemblages and metamorphic facies, both clinopyroxenites and amphibolites show strong affinities
to the oceanic crust rocks, as evidenced by their geochemical and isotopic data. In addition, the inherited igneous zircon U–Pb results indicate that the protolith ages of these metamorphic rocks are 124–125 Ma, which are coeval with the formation ages of oceanic crusts and the metamorphic ages of amphibolites within the YTSZ. Geothermobarometry calculations indicate that these rocks went through metamorphism to peak conditions of granulite-facies in excess of 900℃ at minimum pressure of 9 kbar for garnet-clinopyroxenites and of high-amphibolite facies of 650℃ at 7.5 kbar for amphibolites, corresponding to a quite high geothermal gradient (25–30℃/km). Such a high geotherm implies that the lithospheric mantle and the subducted oceanic crusts were nascent and still hot when the mantle obducted onto the ocean crust of the Neo-Tethys. A scenario of subduction re-initiation at the dying mid-ocean-ridge close to the Eurasian continental margin is proposed to reveal the evolution of the NeoTethys and to provide new insights into the mechanism of formation of ophiolites and metamorphic soles worldwide.
to the oceanic crust rocks, as evidenced by their geochemical and isotopic data. In addition, the inherited igneous zircon U–Pb results indicate that the protolith ages of these metamorphic rocks are 124–125 Ma, which are coeval with the formation ages of oceanic crusts and the metamorphic ages of amphibolites within the YTSZ. Geothermobarometry calculations indicate that these rocks went through metamorphism to peak conditions of granulite-facies in excess of 900℃ at minimum pressure of 9 kbar for garnet-clinopyroxenites and of high-amphibolite facies of 650℃ at 7.5 kbar for amphibolites, corresponding to a quite high geothermal gradient (25–30℃/km). Such a high geotherm implies that the lithospheric mantle and the subducted oceanic crusts were nascent and still hot when the mantle obducted onto the ocean crust of the Neo-Tethys. A scenario of subduction re-initiation at the dying mid-ocean-ridge close to the Eurasian continental margin is proposed to reveal the evolution of the NeoTethys and to provide new insights into the mechanism of formation of ophiolites and metamorphic soles worldwide.
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