The inter-relationships between long-lived metamorphism, pluton emplacement and changes in the direction of bulk shortening during orogenesis

Abstract

Foliation intersection/inflexion axes combined with pseudosections and garnet-core isopleths reveal only 1.5 kbar variation in P–T conditions while plutons were emplaced regionally and deformation and metamorphism continued during orogenesis lasting 70 Myr. Tectonism ended with slight decompression into the cordierite stability field. Garnet growth was always overstepped by up to 100 °C occurring at conditions that staurolite growth was also possible. Episodic start, stop, start growth behaviour of both of these phases throughout this period did not result from the effects of bulk composition on their stability fields. Different porphyroblast growth patterns in same bulk composition and outcrop samples reveals reaction start/stop behaviour was controlled by the manner in which deformation partitioned through an outcrop. The regional isograds were established during the first period of bulk shortening near orthogonal to the orogen trend. They did not migrate across lower grade rocks during each of the subsequent periods of metamorphism in spite of dramatic changes in the direction of bulk shortening; rather they contracted slightly. During the youngest periods of orogenesis directed at a high angle to the current orogen trend the isograds were folded about axial planes parallel to the fold belt. The regional distribution of these isograds directly reflects the oldest period of pluton emplacement, with both controlled by orogen-scale partitioning of bulk shortening at a high angle to the current orogen trend relative to intervening zones of transform-like shear.