Uncoupling: a new type mingling-like structures
Abstract
Uncoupling is a term describing the processes of magma fragmentation during it emplacement at a progressive developing shear zone. In contrast to the well-known magma mingling and peperite structures, uncoupling structures occur as fragmented both extrusive and intrusive blocks emplaced within a metamorphic sequence which has been undergone a voluminous shearing motion realized at a late orogenic stage. We stress that the processes of magma fragmentation of a such kind, i.e. the uncoupling, due to a significant decreasing of a matrix viscosity that progressively extends during a high-velocity shearing. Consequently, the matrix viscosity becomes comparable with a melt viscosity leading to a development of mingling- and peperite-like structures. We present an example of the uncoupling process occurred in the Chara shear zone, Altaid orogen, Eastern Kasakhstan. The uncoupling structure demonstrates an outcrop- to meso-scale picture of numerous pillow-like and drop-like fragments of felsic volcanics plunged into the Early Carboniferous low-grade metapelites. The felsic volcanic fragments are clearly related with dike and subvolcanic conduits which form the Triassic Tastaut volcano-plutonic ring complex. The complex is cut by the regional-scale Chara shear zone in which the uncoupling processes of felsic magma fragmentation occur. These display as small (smaller 30 sm), closely spaced, mainly interconnected, globular and amoeboid bodies enclosed by metapelites. It is important that chilled margins were preserved throughout all of the observed felsic fragments. The boundaries with host rocks are scalloped, drop-shaped and flame-shaped. The field observations of relationships between the Tastaut felsic rocks and the Chara sheared metapelites suggest that both of uncoupling, mingling or peperite processes proceed from intimate hydrodynamic conditions evolved the mechanical magma fragmentation occurring in host/matrix rocks with low-viscosity characteristics. In contrast with the mingling or peperite system displaying a mechanical magma interaction with water saturated surroundings, the magma/sheared metamorphic rock uncoupling occurs in relatively dry hosting rocks significant viscosity decreasing of whose can be explained by a fast ductile shearing processes.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA......268V