Recognizing different types of ocean plate stratigraphy in the southeastern South Qilian Accretionary Belt, northeastern Tibet Plateau

Dissection of relicts of ocean plate stratigraphy (OPS) in orogens could provide important insights into oceanic evolution, orogenic formation and continental growth. Here we describe a study of petrology, geochronology, geochemistry and Sr-Nd isotopes for OPS in the South Qilian Accretionary Belt with the aim of providing new constraints on the evolution of the Proto-Tethys Ocean (PTO). Three types of OPS units were recognized by their differing petrological, geochemical and isotopic features. Type 1 OPS, representing relicts of ocean islands or seamounts, comprises ocean island basalt and gabbro (t = 525 +/- 4 Ma, (Sr-87/Sr-86)(i) = 0.7035-0.7039 and epsilon(Nd)(t) = +3.63 to +4.16) and (siliceous) carbonate rocks. Type 2 OPS, representing fragments of normal oceanic crust, consists of normal mid-ocean ridge basalt (N-MORB) and basaltic andesite ((Sr-87/Sr-86)(i) = 0.7059-0.7063 and epsilon(Nd)(t) = +6.33 to +7.44), chert and siliceous mudstone. Type 3 OPS could be oceanic plateau-derived and comprises enriched (E)-MORB and dolerite ((Sr-87/Sr-86)(i) = 0.7051-0.7064 and epsilon(Nd)(t) = +4.81 to +6.24), carbonate-siliceous mudstone and carbonate rock. These OPS units witnessed evolution of the PTO. Using available studies on regional arc-type magmatism and U-Pb geochronological data for detrital zircon in this study, a model of 'trench jam accompanying subduction flip induced by accretion of the OPS' is put forward.