Magmatic and meteoric fluid flow in the Bitterroot extensional detachment shear zone (MT, USA) from ductile to brittle conditions

作者：Quilichini Antoine; Siebenaller Luc; Teyssier Christian; Vennemann Torsten W

来源：Journal of Geodynamics, 2016, 101: 109-128.

DOI：10.1016/j.jog.2016.05.006

The Bitterroot shear zone developed as a rolling-hinge detachment system where a syntectonic granodiorite in the footwall was progressive exhumed beneath a detachment shear zone, providing a record of deformation and fluid-rock interaction during progressive exhumation and cooling. The shear zone displays a high strain gradient over similar to 1 km of structural section from the relatively undeformed footwall, where the granodiorite contains a magmatic foliation and lineation, through a mylonite sequence that culminates upward in the fine interlayering of ultramylonite layers and the development of a breccia zone. We measured the stable isotope composition of quartz, muscovite, biotite, chlorite, and epidote across the shear zone and estimated equilibrium temperatures using oxygen isotope thermometry based on mineral pairs. We also measured the hydrogen isotope ratios of hydrous minerals and of quartz fluid inclusions. The main results are: (1) The relatively undeformed footwall granodiorite interacted with magmatic fluids at 500-600 degrees C; (2) the mylonitic fabric defined by muscovite, biotite, and chlorite developed between 500 and 300 degrees C and interacted with a fluid system that was connected to the Earth's surface (meteoric fluids), as indicated by the low delta D values of hydrous mineral phases, including muscovite; (3) the fluxes of surface fluids were not sufficient to shift the delta O-18 values of muscovite significantly, but were sufficient to control the delta O-18 composition of biotite and chlorite during deformation-induced recrystallization and chloritization, and (4) the isotopic composition of fluid inclusions in quartz as well as the BD values of late quartz veins track the mixing of fluid sources between the magmatic and meteoric reservoirs. The distribution of stable isotope compositions in the various tectonites of the granodioritic Bitterroot shear zone, from ductile to brittle, provides a rich spatial and temporal record of the interaction between deformation and fluid flow in a crustal-scale detachment.

• 出版日期2016-11