Geochronology And Petrogenesis Of Triassic Low-Ti Mafic Intrusions In Funing: Implications For The Tectonic Evolution Of Nanpanjiang Basin, Sw China

The Nanpanjiang Basin, one of the largest Triassic turbidite basins in the South China Block, has long been disputed with respect to its tectonic affinity. To better understand the tectonic processes of the basin, we conducted geological mapping, geochronological, and geochemical studies on the mafic intrusions (the Anding group) in Funing emplaced within the southern Nanpanjiang Basin. The Anding group mafic intrusions in Funing consist of gabbronorite and gabbroic diorite, and they are characterized by low TiO2 (0.54-1.18 wt%) and Ti/Y ratios (162-222), low (La/Yb)(CN) ratios (3.28-4.19), flat REE patterns, pronounced negative Nb, Ta and Ti anomalies. These mafic intrusions can be further divided into two subgroups: the Group 1 at 244 +/- 3 Ma and the Group 2 at 224 +/- 2 Ma. The Group 1 exhibits negative Eu anomalies, high Th/Hf ratios (1.61-1.91), high (Sr-87/Sr-86)(i) ratios (0.708710-0.711843), and low epsilon(Nd)(t) values (-6.70 to -7.82), while the Group 2 exhibits positive Eu anomalies, lower Th/Hf ratios (1.05-1.07), lower (Sr-87/Sr-86)(i) ratios (0.708397-0.708425) and higher epsilon(Nd) (t) values (-3.28 to -3.21). Both the Group 1 and Group 2 are derived from partial melting of spinel-bearing lithospheric mantle and formed at an intracontinental back-arc setting. These results, combined with regional considerations, support a new model that the Triassic Nanpanjiang Basin is an intracontinental back-arc extensional basin triggered by the westward subduction of the Palaeo-Pacific Plate, and the Triassic magmatism in Funing was a response to the extension.