Bulletin of the Geological Society of Finland (Jun 1992)
The 1.88 Ga old Mäntylä Complex, central Finland: emplacement and deformation of mafic to felsic plutonic rocks and associated Mo mineralization
Abstract
The Mäntylä complex is part of the extensive Central Finland Granitoid Complex. The intrusive sequence was established from contact relations as well as from structural and petrographic criteria. Metagabbros, occurring as narrow strips, are the oldest plutonic rocks. Quartz diorite varies from a fairly homogeneous rock to a migmatitic one with abundant xenoliths of supracrustal rocks. Porphyritic monzodiorite contains two small bodies of gabbro pegmatoid. Hypersthene granites, occurring as five small bodies, contain molybdenite in garnet-orthoclasite pockets and veins as well as in quartz veins. Biotite granite and monzogranite, the predominant rock type of the complex, are the most felsic rock types. The complex is surrounded by coarse-porphyritic granitoids except for the northern contact where high-grade gneisses prevail. The supracrustal rocks were metamorphosed and deformed at least once before the emplacement of the plutonic rocks. A local Da-Dd deformational sequence was established in the complex. Penetrative Sa foliation can be discerned in one of the hypesthene granite bodies. The prevailing, NW-WNW striking Sb foliation occurs as a penetrative or spaced foliation in the plutonic rocks. Sc spaced foliation and younger shear zones (Dd) are encountered in restricted areas. The emplacement of the Mäntylä complex started before Da with the intrusion of the metagabbros. The rest of the complex intruded during a short time interval, about 1886–1880 Ma ago. The quartz diorite and the monzodiorite were emplaced fairly contemporaneously. The hypersthene granites intruded after the quartz diorite, during and after Da deformation. The monzogranite and the coarse-porphyritic granitoids intruded during Db. Possibly the forceful emplacement of the monzogranite pushed apart the previously closely arranged hypersthene granite bodies. This mechanism explains the differences in molybdenum mineralization between the hypersthene granite bodies at the present erosional level. The garnet orthoclasite was formed 1880 Ma ago, during Dc deformation. A small body of orbicular quartz diorite is slightly younger (1876 Ma) than the other rock types. The petrography of the Mäntylä complex and the surrounding coarse-porphyritic granitoids indicates a relatively deep source area with dry conditions. Geochemical characteristics indicate that the rock types represent different source materials. Only the most evolved rocks, i.e. the hypersthene granites, the biotite granite and the monzogranite are possibly comagmatic. The geochemical diversity and the short time interval of emplacement indicates rapid crust-forming and differentiation processes in the early Proterozoic.
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