Annular crustal accretion and reworking around the Yangtze nuclei during Neoproterozoic: a link to the initiation and termination of annular subduction of oceanic crust

Abstract

Subduction zones are primary sites of crustal growth and thus subsequent continental accretion on the Earth. In the modern cases, continental crustal growth along subduction zones generally takes place at one side of continental margins. However, the Yangtze Block provides us an annular style for the crustal growth and subsequent continental crust accretion during the late Mesoproterozoic to early Neoproterozoic. A series of lithologic associations at the time, including ophiolites, arc basalts, blueschist-facies metamorphic rocks, Stype and I-type granites, and high-K volcanic rocks, occur along the margins of the Yangtze Block. Hf isotopes in zircons from Neoproterozoic magmatic rocks around the Yangtze Block suggest growth of juvenile crust at ca. 0.95-1.3 Ga, indicating an annular subduction of oceanic crust around the Yangtze nuclei. The arc terranes may be accreted to the margins of the Yangtze Block during arc-continent collision of 0.95-0.86 Ga, with some newly-formed arc-derived rocks immediately remelted to generate syn-collisional volcanic rocks and granites. A subsequent giant reworking event of juvenile and old crustal materials took place at 0.85-0.80, which may be resulted from the upwelling of deep mantle and extension of lithosphere due to the termination of annular subduction. The mafic rocks of 0.85-0.80 Ga generally show geochemical features similar to the typical arc basalts, suggesting their sources may have been metasomatized during early subduction. The 0.95-0.86 Ga arc-continent collision and associated remelting has provided an efficient way not only for growth of continental nuclei during the late Precambrian, but also for chemical differentiation of crustal composition from basaltic to granitic ones.