Bulletin of the Geological Society of Finland (Dec 2002)
Mafic-silicic magma interaction in the layered 1.87 Ga Soukkio Complex in Mäntsälä, southern Finland
Abstract
The Svecofennian layered Soukkio Complex (1.87 Ga) in Mäntsälä, southern Finland, consists of layered tholeiitic gabbro and porphyritic calc-alkaline monzonite, quartz monzonite and granite, mingled together. The gabbro belongs to a group of ten mafic-ultramafic intrusions of Mäntsälä, part of the 150 km long and 20 km wide, linear, E-W trending Hyvinkää–Mäntsälä Gabbroic Belt (HMGB), representing syn-collisional magmatism. Structures and textures related to magma mingling and mixing occur in a 1– 2 km wide zone around Lake Kilpijärvi, located at the center of the Soukkio Complex. The complex is compositionally stratified and consists of four zones: its base, found at the Western Zone, is a dynamically layered gabbro. The following tonalite is probably a result of magma mixing. Felsic amoeboid layers and pipes, alternating with or cutting the fine-grained gabbro in the Central- Western Zone, resemble those of mafic-silicic layered intrusions in general. Mafic magmatic enclaves (MMEs) and pillows form the South-Central Zone and disrupted synplutonic mafic dykes or sheets intruded the granite in the Eastern Zone. The MMEs and disrupted synplutonic mafic dykes or sheets show cuspate and chilled margins against the felsic host, quartz ocelli, corroded K-feldspar xenocrysts with or without plagioclase mantles, and acicular apatite, all typical features of magma mingling and mixing. Mixing is suggested by intermediate composition of MMEs between granitoid and gabbro, as well as by their partly linear trends in some Harker diagrams. REE composition of the MMEs is similar to that of the Soukkio Gabbro, as expected for granite hosted MMEs. The model proposed for evolution of the Soukkio Complex involves intrusion of mafic magma into the crust, causing its partial melting. This generated granitic magma above the mafic chamber. Injections of mafic magma invaded the felsic chamber and those magmas interacted mainly by intermingling. Mingling and mixing of contrasting magmas is also found elsewhere in the Mäntsälä region and HMGB, suggesting a widespread synorogenic zone of coeval mafic-silicic plutonism and crust-mantle interaction during the Svecofennian Orogeny in Finland.
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