Serpentinization and fracture formation in peridotites on Otrøy, Western Gneiss Region, Norway: Late stage PT-conditions and implications for tectonic decompression

Abstract

Hydration reactions in mantle peridotites substituting olivine with serpentine causes a volume change of 20-55% in the rocks. This volume change is accompanied by fracturing at several scales. Fractures form sets according to a hierarchical fracture pattern or a kernel pattern, both characterized by fracture sets at 90o angles to each other. The main difference between both patterns is in the onset of fracturing. This is either caused by stress differences in the rock, which forms one set at first and other sets later on (hierarchical fracturing); or by a different degree of serpentinization within so-called kernels (blocks in the rock), which form fractures in three directions at the same time (kernel pattern). To see whether other – larger scale – processes, like tectonic stresses, are also involved in fracture formation, two peridotite bodies on the island of Otrøy, WGR, Norway, are studied. The abundance of fractures and the mode of fracturing will be determined, as well as the pressure and temperature conditions of their formation, in order to place the hydration and fracturing of the peridotites in the larger scale tectonic history of the Western Gneiss Region. Spatial analyses are performed determining the fracture form and orientation, on both large and small scale. A clear dominance of a set of early fractures in one direction would give implications for extensional tectonic stresses perpendicular to that, and similarity of fracture normals with orogennormal extensional features in the surrounding rocks would show the influence of tectonic stress on fracture formation. Both peridotite bodies show a fracture pattern with fracture sets having approximately 90o angles, but no relative timing between sets can be observed in the field. Stereographic projections of the normals to the fracture planes form a range between (W)NW-(E)SE, which is the same direction of the orogen-normal extension and exhumation of the peridotite bodies. One set clearly plots near the direction of the WNW-oriented orogen-normal lineations in the surrounding gneiss. This implies that the orientation of at least one set of fractures was caused by tectonic extension during exhumation. Fracture orientation with respect to the compositional banding would be caused by anisotropy between the compositional layers and subsequent differentiation in volume change. Results from stereographic rotation of fracture poles to equal compositional bandings throughout the bodies show only minor dependence. However, it can explain the scatter between poles within sets, as well as the slight difference in orientation of the first formed fracture set between both bodies: Ugelvik has more NW directed poles in this set and Raudhaugene has more WNW directed poles. Determination of fracture filling – e.g. serpentine polymorphs and their growth forms – and the mode of fracturing are used to quantify pressure and temperature conditions of serpentinization and fracturing in the latest stages of the tectonic history. This is done using electron microprobe and microscope observations. Two generations of mesh formation are present; lizardite formed first and is later (partly) replaced by chrysotile. Mesh formation went on during fracturing and fracture filling in the rock. At least four generations of (ongoing) fracture filling are observed: a first filling by banded/kinked chrysotile changes to triangular lizardite, ribbon antigorite and micro-granular lizardite, followed by some cross-fractures of fibrous and banded/kinked chrysotile. Calcite is present in many fractures as a latest stage, but is probably not linked to the first fracture filling phase. Early hydration occurs at about 530oC and 0.4-0.5 GPa through reaction of olivine (± orthopyroxene) + talc (± chlorite) + water to serpentine (lizardite). Later serpentine polymorphs are formed by ongoing reaction of mainly olivine and orthopyroxene with water, and by recrystallization of earlier forms of serpentine. Calcite forms after serpentinization, below 180oC and below 0.3 GPa, through infiltration of a CO2-rich fluid in the system, which binds free Ca that has probably provided by Carich garnet and amphibole in the rock.