Geochemical characterisation of a polyphase deformed, altered, and high grade metamorphosed volcanic terrane: implications for the tectonic setting of the Svecofennides, south-central Finland

Abstract

Abstract The Virtasalmi metavolcanic belt forms part of the Early Proterozoic Svecofennide Terrane in central Finland. The preserved supracrustal lithologies (40% of the assemblage) have undergone extensive synvolcanic hydrothermal alteration, polyphase regional ductile deformation, low pressure regional polyphase metamorphism to granulite facies with subsequent amphibolitisation, and extensive emplacement of a synkinematic I-Type plutonic suite. Despite these overprinting events, three chemically distinct suites of metatholeiites and one suite of coeval coarser-grained intrusive lithologies are recognized, and their petrogenesis resolved. Primary igneous geochemistry is preserved mainly by the HFSEs and Cr and Ni. Variable major and LIL element mobility is demonstrated. Significant differences in geochemistry are reflected in different metamorphic assemblages between suites. Preservation of vesicular pillow lavas indicate subaqueous extrusion of the three metatholeiitic suites. Trace element geochemistry indicates that Suite I compositions are controlled by low pressure fractional crystallization. In contrast, Suite III amphibolite compositions are controlled by batch partial melting, with limited modification by open system fractional crystallization. These differences in fractionation history point to the evolution of these two main suites within distinct upper crustal magmatic plumbing systems. Also, differences in more-HYG/less-HYG element and other incompatible element ratios, point to derivation of Suites I and III by different degrees of partial melting from different, heterogeneous mantle source regions. Coeval eruption of these suites indicates that variations in source geochemistry and melting processes reflect spatial rather than temporal differences. The volumetrically smaller Suite II comprises lava flows and pillowed units that are interlayered with, but have a more depleted chemistry than the other suites. Marked variation in Cr and Ni contents within a narrow range of Zr contents in individual flows in Subgroups IIa/1 and IIa/2, and marked TiO 2 enrichment in Subgroup IIb, suggests that the more depleted character of this suite is due to fractional crystallisation and crystal settling in high level magma chambers. Suite IV amphibolites comprise a number of small intrusive bodies that are emplaced within the volcanic pile. They are coarser grained than the enveloping amphibolites, and are interpreted as tectonised relics of high level magma chambers and/or sills emplaced into the (coeval) metavolcanic assemblage. They have a depleted geochemistry and there is textural and geochemical evidence to suggest a partial cumulate origin. Similarities in fractionation histories suggest that Suite IV intrusions are cogenetic with Suites I and II. Enrichment in incompatible elements, particularly Nb, demonstrates that the Virtasalmi metatholeiites are not related to supra-subduction zone magmatism. All the amphibolites are Within Plate Basalts or E-Type MORBs, with similar compositions known today from a range of tectonic settings. However, the interlayering of E-MORB metavolcanic rocks with marbles and voluminous fine grained clastic metasediments is more likely to indicate an intracratonic rift (passive margin?) geotectonic setting. Subsequent deformation and metamorphism of the volcanics and syn-kinematic calc-alkaline intrusives are interpreted as accretion at a destructive margin. The latter was most probably a continental calc-alkaline arc, various crustal levels of which are now exposed in the study area.