Low-temperature phase stability in calc-alkaline granitic systems and significance for cold granites

Abstract

Granite formation and evolution are influenced by various physical and chemical processes in magmatic systems, resulting in diverse textures and compositions. While the generation of granitic rock through dehydration melting of hydrous minerals, which requires higher temperatures, is widely accepted, the formation and evolution of cold granite remains a topic of considerable debate. Our experiments aim to understand how small changes in granitic magma composition influence late-stage crystallization, providing insights into the chemical and textural evolution of granitic rocks and determining the significance of these conditions in shaping the diversity observed in nature. We conducted crystallization experiments at 1 and 2 kbar, between 680 and 815 °C, using two granitic compositions: a natural I-type granitoid (MA) and a slightly more mafic, synthetic analogue (FC). In these relatively low-pressure and low-temperature experiments, orthopyroxene remains stable at 1 kbar regardless of the starting material. At 2 kbar, its stability is limited to the more mafic sample FC when containing 3.7 wt% water. Hornblende only crystallizes in the more mafic rock FC at 2 kbar across all temperatures tested when water concentration exceeds 4 wt%. Plagioclase spans a broad temperature range (700–815 °C) at both 1 and 2 kbar in the FC material, whereas in the MA material at 2 kbar, it appears only at temperatures ≤725 °C. At 2 kbar few crystals were observed in the MA run at 750 °C, indicating that the liquidus temperature was nearly reached, contrasting with the FC starting material. Additionally, temperature cycling proved more efficient in promoting crystal growth in FC samples. Under low-temperature conditions, only local equilibrium was achieved, highlighting the significant role of late-stage crystallization processes and kinetic limitations in shaping the diversity observed in cold granites. Our experiments demonstrate that even small changes in CaO and FeO content can markedly alter the solidus/liquidus temperature of melts, significantly influence phase stability, melt production, and texture in granitic systems. This study surmises the need to further pursue systematic investigations of the influence of major elements on crystallization sequences in granitic systems, and we compare our findings to previous observations made on the Gentio metagranitoids of the Mineiro belt, Brazil.