Interaction between basaltic melts and their wallrock in dykes and sills in Åland, southwestern Finland

Abstract

Abstract Middle Proterozoic dykes and sills of mostly continental tholeiitic composition occur along the contact between the Middle Proterozoic Aland rapakivi batholith and the Early Proterozoic Svecofennian country rock of southwestern Finland. These dykes and sills have interacted with their granitic wallrocks in different ways. The dykes in two studied localities formed breccias in which the xenoliths are partly disaggregated due to melting along the grain contacts. The xenoliths are also depleted in feldspars and relatively enriched in quartz as compared with the wallrock, and are mantled by thin amphibole rims. In contrast, a basaltic sill melted the wallrock and mixing occurred between the basaltic magma and the palingenic granitic melts. This resulted in the formation of porphyry-like hybrid rocks and mafic enclaves. The difference between the dykes and the sill as regards interaction with their wallrocks shows that the heat flow from the sill was high enough to produce local palingenic melts, wher...