T281 Metamorphic, Metasomatic, and Igneous Processes in the Mid- and Deep Continental Crust: Mechanisms and Processes That Impact Mass Transport and Rheology

Wednesday, 4 November 2015
8:00 AM - 12:00 PM
Room 317

COUPLED REACTION DRIVEN DEFORMATION, STRAIN SOFTENING AND CO2 METASOMATISM IN PERIDOTITES FROM THE REINFJORD ULTRAMAFIC COMPLEX, NORTHERN NORWAY

Paper number: T281-7
Program: Technical Sessions
Time: 9:35 AM – 9:50 AM

Authors

Abstract

The Seiland Igneous Province (SIP) consists of large volumes (>50,000 km3) of mafic, ultramafic, silicic and alkaline melts intruded into the lower continental crust (25-30 km) at 570-560 Ma under an extensional regime. Understanding the tectonics of the region during and post emplacement of ultramafic melts is vital in order to understand the formation, evolution and rheological behavior of dense continental crust below a large igneous province.

The ultramafic rocks SIP is cut locally by mm-cm thick shearzones, containing extremely fine-grained material with a distinct shape-preferred orientation, which offsets dykes across numerous micro-faults in areas close to the major fault zone cutting through the area. Combined optical, SEM and EBSD studies reveals mineralogical changes in relation to the shear zones, with olivine + clinopyroxene replaced by orthopyroxene + dolomite according to the reaction:

Olivine + Clinopyroxene + CO2 = Dolomite + Orthopyroxene

As evidenced by coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shearzones. In addition large olivine grains proximal to the shearzones show a conspicuous microstructure with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. With clinopyroxene at least hundreds of microns away this suggested at least some material transport within the shearzone.

The shearzones thus gives a unique view into CO2-metasomatism of the lower crust and provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localised type of deformation (Rowettta and Blasic, 1987) as demonstrated by the fractures extending into large olivine grains proximal to the shearzones.

To further constrain variations in mineral assemblages and the tectonic implications of the microfaults and shearzones more detailed fieldwork, utilising the dyke offsets as a proxy for tectonic movements and microstructural and mineralogical work should be carried out. In addition the CO2-source should be studied using carbon isotopes.

Rowettta and Blasic (1987): JGR, VOL. 92, NO. B12, pages 12,902-12,910