Microchemical and Sr isotopic investigation of zoned K-feldspar megacrysts: Insights into the petrogenesis of a granitic system and disequilibrium crystal growth (original) (raw)

K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/ Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87 Sr/ 86 Sr ratios ( I Sr ) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high I Sr -Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of I Sr , but a larger range of Rb/Sr. Lower I Sr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high I Sr silicic melts (as seen in the inner cores) and later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.