The Rb-Sr age determinations of the “bimodal” volcanic rocks in the Luchun-Hongponiuchang superimposed rift basin, Deqen, Yunnan

The Luchun-Hongponiuchang superimposed rift basin, Deqen, Yunnan is situated in the active marginal volcanic zones intermediate between the Jinshajiang suture zone and Qamdo landmass, and interpreted as an important Mesozoic host basin along the Jinshajiang tectonic zone. The whole-rock Rb-Sr isotopic ages are determined for the basalt and rhyolite from the rift basin. The average age for the bimodal volcanic rocks equals to 224 Ma, suggesting that they were formed during early Late Triassic time. The initial Sr⁸⁷/Sr⁸⁶ values for the basalt range from 0.7065 to 0.7194, whereas those for the rhyolite from 0.7074 to 0.7199. This implies that both the basalt and rhyolite were derived from the same magmatic source, i. e. the basic magmas in the early stage and the acidic magmas in the late stage, and that they were emplaced in the crustal extensional settings following the Jinshajiang arc-continent collision.