HIGH-VELOCITY DEFORMATION AND SYNCHRONOUS MAFIC MELT INTRUSION

The Tastau volcanoplutonic complex of the Chara transpressional belt, Eastern Kazakhstan, contains the mafic microintrusions (1-70 cm) in the form of strings of globules and other bodies that resemble stretched and boudinaged dykes in low-grade sedimentary rock. All mafic bodies have got chilled margins and are located exclusively in the linear zone of tectonic brecciation of metagreywacke host rock. In outcrop, the mafic bodies thus often look like beads that connected by a bead string. These structures are result of primary magma emplacement rather than postconsolidation stretching. The formation of these mafic microintrusions was controlled by compressive shear deformations. Progressive stretching during a high-speed decompression deformation of metasedimentary rock was accompanied by intrusion of mafic melt. Magma fragmentation due to the significant decrease in matrix viscosity took place during high-velocity shearing and cataclastic flow. The synkinematic model of fragmentation and mingling between magma and deforming rock on the hypabyssal level of crust is offered.

In the Tastau area the melt fragmentation structures and tectonites are evidence of dramatic changes in the tectonic regime from compression to extension and again to compression. Two main factors determined such inversion: (1) regional tectonics and (2) magmatic fluid pressure, which could operate simultaneously.

The model assumes a relative long preservation of regional compressive conditions. Impenetrable areas impeded the magmatic melt motion and produced a fluid overpressure. At excess of limiting pressure a deviator stress relaxation and a hydrofracturing were realized which appeared in the tectonic crushing. The disintegration of host rock was accompanied by a mafic melt filling of fractures.

Softening of the crushed matrix, a loss of mechanical solidity and viscosity decrease of matrix (up to acquisition of quasi-liquid property) took place in ruptured zones with the participation of melt fluid. Consequently stress relaxation was realized with cataclastic flow and melt injection in northwest linear local fault zones in the compression conditions.