Differential stress induced mineral replacement around strong clasts in a weak biotite matrix

During progressive deformation, a strong inclusion in a weaker matrix causes a heterogeneous differential stress field, which not only results in strain localisation nucleating in the compressive quadrants of the clast but potentially also causes local pressure variations. Numerical modelling indicates that especially in a polyphase rock with a clast-in-matrix structure, pressures can locally vary by up to 1 GPa. So far, it is not clear whether and how local tectonically induced pressure changes are reflected in the mineral paragenesis of metamorphic rocks. Here, we present an example of stress-induced mineral replacement from the north Norwegian Caledonides (Finnmark) which is consistent with a local variation in mineral paragenesis due to pressure variations around strong inclusions.

A subvertical metadolerite dyke was rotated to align with the penetrative regional foliation during the emplacement of the overlying nappe. The metadolerite, now reduced to a thickness of approximately 1.4 cm is sandwiched between two quartzite layers and has undergone alteration to a schist comprising biotite, titanite, epidote, garnet, quartz and accessory apatite. The garnets are subhedral and frequently exhibit two growth zones, with inclusions of predominantly titanite and rare amphibole. The surrounding metasedimentary schists contain staurolite, suggesting mid-amphibolite-facies metamorphic conditions (~550 °C and 6 kbar). During later deformation of the altered metadolerite (i.e., biotite schist), some garnets were pushed into the adjacent quartzite, forming prominent ultramylonitic quartz tectoglyphs, while garnets remaining within the biotite schist were rotated to form delta-type structures. In contrast to garnets, epidote and apatite clasts are characterised by a lower aspect ratio and locally appear to have aligned in a stable orientation within the strongly foliated biotite matrix. Such stable clasts show a thin layer (< 25 µm) of phengitic white mica accompanied by nanocrystals of quartz in their compressive quadrants. The phengitic nature of the white mica suggests a pressure value deviating from the accepted regional mid-amphibolite facies conditions (~550 °C and 6 kbar), potentially indicating a local tectonic overpressure around the strong clasts in the weak biotite matrix. This hypothesis, however, still needs to be validated by further quantification of the local variations in pressure and temperature conditions.