A review of criteria for the identification of magmatic and tectonic foliations in granitoids

Abstract

Foliations in granitoids can form by magmatic flow, ‘submagmatic flow’, high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. A review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, emplacement during regional deformation, or regional deformation post-dating emplacement. However, a magmatic origin is favoured for foliations defined by the alignment of igneous, commonly euhedral minerals, particularly where the foliation is parallel to internal or external pluton contacts. Foliations formed during expansion or ‘ballooning’ of diapirs may be strictly magmatic in origin, although some studies suggest that solid-state deformation also may occur. If so, we would hope to find evidence of deformation of crystal-melt systems, and that the solid-state deformation occurred at high temperatures. The inference of syntectonic foliations is most convincing where magmatic and high-temperature solid-state foliations are subparallel, these foliations are continuous with regionally developed foliations in the wall rocks, synkinematic porphyroblasts are present in the wallrocks, and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage. A strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals, no alignment of igneous minerals occurs, the foliation is locally at high angles to pluton-wallrock contacts, and the foliation is continuous with a regionally developed cleavage.