Mantle Plume in the Sanjiang Paleo-Tethyan Lithosphere:Evidence from Mid-Ocean Ridge Basalts
“三江”古特提斯地幔热柱——洋中脊玄武岩证据

The Lancangjiang and Jinshajiang-Ailaoshan suture zones in Southwestern China are products of the subduction and closure of both southern and northern branches of the Paleo-Tethys Ocean, located in the original triangular gap between the Gondwana and Larauisa Lands. The ocean-island basalts associated with ophiolite have been investigated along the two suture zones.OIBs occurred in the region of range from 220 - 240N along the Lancangjiang suture zone and in 240-270N along the Jinshajiang-Ailaoshan suture zone, respectively, and were overlapped by shallow marine limestones. Three OIB types were determined to be erupting along both oceanic ridges based On their major, trace and REE compositions, i. e., trachytic basalt, hawaiite and picritic basalt. The trachytic basalt occurred in the cerntral segment of OIB region (e. g., 220 - 240N in the Lancangjiang suture zone ), and closely associated with picritic basalt, whereas hawaiite distributed at the margins of the OIB region, and overlain on the top of MORBs and radiolarian chens. These OIBs show enrichment in the incompatible (LILE and HFSE) and LRE elements and can be characterized as typical hot-spot basalts. The quantitative calculation for incompatible element abundance and ratios in the mantle source suggests that the source that produced picritic basalt highly enriched in incompatible elements comparing with MORB source, and sources for trachytic basalt and hawaiite probably are mixing sources of an end-member, characteristic of a picritic basalt source with an end-member of normal depleted MORB source.Based on the spatial-tempral distribution, rock association and geochemical characteristics of OIBs in the region, a model is suggested, in which two ridge-centered mantle plumes with diameter of about 200-300km occurred in the Lancangjiang and Jinshajiang oceanic basins in DevonianCarboniferous period, respectively. Both the mantle Plumes have chemical-thermal structure being different from surrounding mantle. The melting of narrow trial region and large head in which the plume mixed with surrounding mantle can explain the development of picritic basalt, trachytic basalt and hawaiite as well as geochemical variation seen along the Jinshajiang and Lancangjiang suture zones.