Magmatism of Bamao caldera, Zhejiang province: Composite evolution of poly-mechanism in open system.

The Bamao volcanic complex is characterized by wide ranges in the compositi on including basaltic, trachy basaltic, trachyte andesitic, trachy dacitic, rhyodacitic, rhyolitic and high-silica rhyolitic rocks. They did not evolv ed from single parental magma, and also did not extrude from single magma c hamber, as came from poly-types magma chamber, had undergone the processes of complex evolution, being composite evolution of poly-mechanics in open s ystem. The basic element of bimodal volcanism in early stage was formed by partical melt of upper mantle, while the acid element was the product of cr ust material immiscibility melt. The trachy dacitic rocks which are compose d of the major part of the complex were formed by mixing of basaltic magma and rhyodacitic magma in high level major chamber. Trachyte andesitic magma was formed from basaltic magma by crystallization differentiation in deep magma chamber. The rhyodacitic dome and high-silica rhyolite-porphyry dike extrusion and intrusion in late stage represent major composition of high l evel proximate grade and its upper silica cap.