Geochemistry of the volcanic rock association from Lianghe area in Mianluesuture zone, southern Qinling and its tectonic significance.

The volcanic rock slab in Lianghe area is located in the boundary tectonic zone: Bashan arcuate tectonic melange between Qinling microplate and Yangtz e plate. The rocks consisted of subalkaline tholeiitic basalt and calc-alka line dacitic rhyolite. Both of the basalt and dacitic rhyolite exhibit the typical geochemical features of high content of element Bar low content of elements Th and U. remarkable depletion of element Nb, Ta and Ti. Especiall y, the Th/Yb-Ta/Yb and Ti/Zr-Ti/Y features of the basalt indicate that the volcanic rock association in the studied area developed in a special tecton ic setting which belongs to a rift valley within an island-arc. The volcani c rock association in the studied area was the result of magmatism that due to the ancient oceanic crust subduction and rift valley development within the island-arc during the evolutionary process of Mianlue ancient ocean ba sin at Middle Paleozoic-Early Mesozoic era. This volcanic rock block should be an important evidence which indicates that the Mianxian-Lueyang suture zone had extended to the Bashan arcuate area.