Rift- and subduction-related crustal sequences in the Jinshajiang ophiolitic mélange, SW China: Insights into the eastern Paleo-Tethys

The Paleozoic Jinshajiang ophiolitic mélange in southwest China marks an important branch ocean (i.e., the Jinshajiang Ocean) of the Paleo-Tethys. Basic-intermediate rocks are widespread features in the mélange; their formation age is well known, but the petrogenesis has not been well studied, which means that the evolutionary history of the Jinshajiang Ocean is not well constrained. To understand the nature of the mélange and the ocean, we present a set of elemental and isotopic data from two typical crustal sequences in two areas of the Jinshajiang ophiolitic mélange, Zhiyong and Baimaxueshan. The basalts in the ca. 343 Ma Zhiyong crustal sequence show mid-ocean-ridge basalt–like geochemical compositions with Nb/La ratios of 0.98–1.15 and εNd(t) values of +6.5 to +7.7, indicating that the basalts formed in the spreading ridge of the ocean. In contrast, the 283 Ma Baimaxueshan crustal sequence consists of gabbros and basaltic-andesitic lavas, which have an arc affinity with Nb/La ratios of 0.54–0.67 and εNd(t) values of +5.1 to +6.5. The geochemical differences were not caused by crustal assimilation but reflect mantle metasomatism by fluids dehydrated from the subducting slab. Therefore, we propose that the Zhiyong and Baimaxueshan crustal sequences formed in seafloor spreading and subduction settings, which were related to the opening and closure of the ocean, respectively.