Recycling of Paleotethyan oceanic crust: Geochemical record from postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens

摘要: Plate tectonics can develop from oceanic subduction to continental collision, with disappearance of presubducted crust in zones. This is typical for collisional orogens where Tethyan was consumed during convergence, recycling the into mantle. However, it difficult trace geochemical signature previously subducted orogens. Postcollisional mafic igneous rocks Tongbai-Hong’an provide a record crust-mantle interaction Paleotethyan preceding Triassic collision. These were emplaced Cretaceous after collision and exhibit oceanic-island basalt (OIB)−like trace-element distribution patterns, enrichment large ion lithophile elements (LILEs) light rare earth (LREEs) but no depletion high field strength (HFSEs). They also show relatively depleted whole-rock Sr-Nd-Hf isotope compositions, initial 87 Sr/ 86 Sr ratios 0.7043−0.7050, e Nd ( t ) values −1.8−4.5, Hf 4.7−10.3. Such features indicate their origin juvenile mantle source that contained crust−derived material. Thus, we suggest generated by melt-peridotite reaction at slab-mantle interface channel The original peridotite would have been characterized melt-mobile incompatible radiogenic isotopes, corresponding subarc lithospheric In contrast, material LILEs LREEs HFSEs, felsic melts derived partial melting subducting sediment outside rutile stability field. As such, generate pyroxenite-rich ultramafic metasomatites lower part wedge, relative HFSEs. this model, gave rise postcollisional Therefore, orogen.