Basalt-dacite assemblage in Longsheng district of the Guizhou-Guangxi border area and its geological implications

In the Guizhou-Guangxi border area, several dacite layers were identified, which occur in interlayered form within the basalt of Sanmenjie Formation in upper Neoproterozoic Danzhou Group. The basalt and dacite compose a bi-model volcanic sequence. Both the basalt and the dacite are geo-chemically calcic and peraluminous, and display REE patterns with strong differentiation between LREE and HREE. Their primitive mantle normalized spider diagram is similar to that of enriched middle oceanic ridge basalt (E-MORB), suggesting that they might have been formed in a continental rift. The identification of bi-model volcanism, along with previously published in-site zircon U-Pb results, provides positive constraints for the stratigraphic correlation of Guizhou and Guangxi and for the tectonic reconstruction of the southwestern segment of the huge Jiangnan orogenic belt.