Enhanced terrestrial input into Paleoproterozoic to Mesoproterozoic carbonates in the southwestern South China Block during the fragmentation of the Columbia supercontinent

Abstract

Bulk rock elemental, C, O and Sr isotopic data for carbonates of the Palaeoproterozoic to Mesoproterozoic Luoxue and Luzhijiang formations from the southwestern margin of the South China Block are studied to infer aspects of the provenance and depositional environment. Elemental tracers demonstrate that the upper Luoxue and Luzhijiang carbonates were not significantly affected by crustal contamination and diagenetic/metamorphic overprinting, thus preserving their primary chemical signature, which is also consistent with the preservation of primary sedimentary structures and textures. In contrast, C, O and Sr isotopes of the lower Luoxue carbonates appear to have been variably altered by ore-forming fluids. All the Luoxue carbonates lack enrichment of Y, corresponding to chondritic Y/Ho ratios of 25–31, in contrast to modern seawater and marine carbonates. Shale-normalized REE + Y patterns for the Luoxue carbonates show pronounced enrichment in middle REE, but lack strong elemental anomalies (La, Gd, Y). Based on petrographic and sedimentological characteristics, the Luoxue Formation was probably formed in a shallow water, epicontinental basin with restricted but variable exchange to the open ocean, and dominant terrigenous input. The Luzhijiang carbonates, on the other hand, are pure chemical sediments devoid of detrital material, and shale (PAAS) normalized REE + Y distributions exhibit positive La, Gd, and Y (Y/Ho: 39–47) anomalies, which are typical features of marine waters throughout the Archean and Proterozoic periods. 87Sr/86Sr values in both the upper Luoxue and Luzhijiang carbonates are significantly higher than coeval seawater, indicative of enhanced contribution from weathered continental sources. This increased influx from terrestrial sources very likely corresponds to the break-up of the hypothesized Columbia/Nuna supercontinent, as the Luoxue and Luzhijiang carbonates were deposited in a rift-related basin associated with supercontinental fragmentation.