An early Neoproterozoic accretionary prism ophiolitic mélange from the western Jiangnan orogenic belt, South China

Abstract

The Neoproterozoic Jiangnan orogenic belt delineates the suture zone between the Cathaysia and Yangtze blocks of the South China Craton. The western part of the belt, in the Longsheng region, consists of a disrupted mafic-ultramafic assemblage of pillow basalt, gabbro, diabase, and peridotite along with siliceous marble, ophicalcite, and jasper mixed with basalt. Significant talc deposits occur on the margins of the ultramafic bodies as well as in the transition zone between marble and basalt. Primary rock relations are largely overprinted by pervasive shearing, resulting in disruption of the assemblage into series of discontinuous blocks within a phyllite matrix. West-dipping thrust faults mark the eastern contact of blocks, and the overall succession has the appearance of a tectonic mélange. U-Pb zircon age data from the gabbros and diabases yield crystallization ages of 867 ± 10, 863 ± 8, and 869 ± 9 Ma, with positive εHf(t) values. The gabbro, basalt, serpentinite, and some talc samples display minor light rare earth element–enriched patterns with obvious depletion of Nb and Ta, indicating a subduction-related setting. The tuffaceous phyllite shows similar geochemical features. A few mafic rocks and the altered ultramafic rocks display mid-ocean ridge basalt (MORB) affinity. Overall lithostratigraphic relationships, age data, and geochemical signatures suggest a forearc setting that was imbricated and disrupted within an accretionary prism environment to form an ophiolitic mélange. The pillow basalt, red jasper, and MORB-type mafic-ultramafic rocks within the mélange occur as exotic blocks derived from the subducting oceanic plate, whereas the arc-type mafic rocks occur as autochthonous blocks, which are all exposed in a matrix of sandy and tuffaceous phyllite.