Interactions between basaltic and granitic magmas in a Svecofennian postorogenic granitoid intrusion, Åland, southwest Finland

Abstract

The Lemland intrusion is one of four postorogenic (1815-1770 Ma) granitoid intrusions occurring in southwestern Finland. The intrusions are dominated by porphyritic granite and basic-intermediate rocks. They show features of a bimodal magmatism, causing net-veining and formation of mixed rocks. The current investigation of the southern part of the Lemland intrusion reveals that the basic-intermediate rocks form a ring-like structure in which the rocks can be divided in three tentative units: (1) fine-grained mafic pillows in porphyritic granite; (2) mixed rocks with mafic inclusions in a hybrid matrix; and (3) medium-grained mafic rock. It is suggested that the ring-like structure with the different units was emplaced from a disrupted, zoned magma chamber, and the different rock units were created due to differences in chemical and physical properties of the magmas. The process started with the intrusion of basic magma into overlying acid magma, creating basic pillows within it. During the upward movement, the basic magma left behind an area where the basic and the acid magma were intimately commingled and mixed. This mixed magma intruded into the area inside the mafic pillows, which formed an isolated barrier against the cooler acid magma. The medium-grained mafic rock in the middle of the ring-structure is regarded as the same rock as the fine-grained mafic rock, the coarser grain size depending on the longer time for crystallization. This rock was back-veined and brecciated by the mixed magma, creating a mega breccia. When this process ended the acid magma was still active and formed cutting dykes in the mafic and mixed rocks.