Variation in technical properties of granitic rocks with metamorphic conditions

Granitic rocks constitute a global raw material key asset for aggregate production. Their technical performance is, however, highly variable and knowledge on the aggregate functionality of granites in different geological settings is incomplete. This study investigates systematic variations in resistance to fragmentation (Los Angeles value) and wear (Micro-Deval value) for granitic rocks along a 150 km long east–west trending metamorphic gradient across the Eastern Segment of the Sveconorwegian Province in Scandinavia. In essence, the metamorphic gradient represents a transition from pristine granite to stromatic migmatitic orthogneiss and granulite. Along this profile, the aggregate functionality is governed by the behaviour of quartz and feldspar during metamorphic recrystallisation. Two critical metamorphic variables were identified: temperature and hydrous fluids. Recrystallisation at comparably low metamorphic temperatures, ≤ 600 °C, result in irregular grain shapes and fine-grained quartz aggregates that in turn result in low Los Angeles and Micro-Deval values, i.e., high-quality materials for road construction. At higher metamorphic temperatures, > 600 °C, grain coarsening and smoothening of grain boundaries result in significantly higher Los Angeles and Micro-Deval values. At high temperatures, high availability of hydrous fluids promoted extensive partial melting and post-deformational crystallisation which resulted in remarkably poor aggregate performance, with Los Angeles and Micro-Deval values consistently above 40% and 12%, respectively. In contrast, deformation at high temperature but low availability of hydrous fluids resulted in inequigranular textures with high proportions of very fine-sized crystals, interlocking textures, and aggregates with Los Angeles and Micro-Deval values down to 19% and 5%. The consistent variation in metamorphic microtextures correlate with macro-fabrics at the outcrop scale, allowing assessment of aggregate functionality already in the field. For migmatitic gneisses, the proportion and strain state of leucosome are the most critical parameters.