Probing intra-oceanic subduction infancy in ancient orogenic belts: Example from Chinese South Tianshan

Subduction infancy corresponds to the first few million years of the start of subduction following heat transfer from the incipient mantle wedge towards the slab-top, as witnessed by metamorphic soles which represent slivers of oceanic crust metamorphosed up to granulite facies conditions welded beneath obducted ophiolites. In this study, integrated petrological, geochemical, mineralogical, geochronological, and thermodynamic studies were carried out on samples from the Yushugou high-temperature metamorphic ophiolitic complex (YHTM) in the South Tianshan Accretionary Complex (STAC), where a massive exposure of coherent granulite accompanied by a thick peridotite body is preserved. Bulk-rock compositions and Sr–Nd–Hf isotopes demonstrate the petrogenesis of meta-basalts with oceanic island basalt (OIB)-like and mid-ocean ridge basalt (MORB)-like affinities, with little infiltration by subduction-derived melts and/or fluids (e.g., no negative Nb–Ta anomalies). Thermodynamic modelling and U–Pb chronology reveal that the YHTM meta-basalts experienced granulite facies metamorphism of ~840–940°C and ~0.92–1.02 GPa at c. 392 Ma and then possibly reheating and zircon alteration in the Carboniferous. In addition, detrital zircons in sedimentary host rocks of the YHTM show limited Precambrian records and offer maximum depositional ages of c. 410–400 Ma together with the oldest Palaeozoic cluster around c. 470–450 Ma. It is suggested that the YHTM granulites could be of Ordovician–Silurian protolith and such an age pattern significantly deviates from those of adjacent terranes (the Central Tianshan, STAC, and North Tarim Craton). Combined with the compilation of pressure–temperature–time estimates of the YHTM and ages of regional ophiolites, arc intermediate-mafic rocks, A-type granites, and deformation, a model of induced, temporarily northward, intra-oceanic subduction initiation is proposed, which probably occurred along the previously existing weak zone close to a seamount or oceanic plateau in the earliest middle Devonian during the northward subduction of the South Tianshan Ocean (STO). Anomalously high geothermal gradients could be triggered by asthenosphere upwellings, further facilitating the formation of OIB-type metamorphic soles. The YHTM, which represents the remnant of metamorphic soles and associated ophiolites, was finally emplaced to the north margin of the STAC during the relatively long-term (c. 160 million years) accretion and continuous subduction of the STO before its closure. This finding also presents a new natural example of OIB-type metamorphic soles as a snapshot of fossil intra-oceanic subduction infancy during the complex evolutionary history of the STO.