Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology

Abstract

The Palaeoproterozoic Lapland granulite belt was juxtaposed between Archaean and Proterozoic terrains in the NE part of the Fennoscandian Shield concurrently with the accretion of Svecofennian arc complexes at ~1.9 Ga. The belt consists mainly of aluminous migmatitic metagreywackes. Abundant noritic to enderbitic magmas were intruded concordantly into the metasediments and were probably an important heat source for metamorphism, which took place during the crystallization of the magmas. This is supported by structural and contact relations of metasediments and igneous rocks, and by the lack progressive metamorphic reaction textures in the igneous rock series. The peak of metamorphism took place above the dehydration melting temperature of the biotite-sillimanite-plagioclase-quartz assemblage at 750−850°C and 5−8.5 kbar which lead to formation of a restitic palaeosome and peraluminous granitic melt in metapelites. Subsequently, the rocks were decompressed and cooled below the wet melting temperature of pelitic rocks (650°C) under the stability fi eld of andalusite coexisting with potassium feldspar (2−3 kbar). Cooling was accompanied by the crystallization of the neosomes, often carrying aluminium-rich phases. Postmetamorphic duplexing of the LGB is clearly seen in the distribution of calculated PT conditions.

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part II: Isotopic dating

“…1.94 Ga (Tables 1 and 2). As shown above and in part I (Tuisku et al, 2006), norites and enderbites intruded into the metasediments ca. 1.92 − 1.91 Ga ago in relatively high pressure.…”

“…The geological setting of the LGB is described in more detail in the first part of this paper (Tuisku et al, 2006). The LGB consists of metasedimentary metasedimentary granulites, ranging in composition from psammitic to pelitic and, noritic to enderbitic igneous rocks.…”

“…This rock also has exceptionally graphite-rich portions, which were used for carbon isotope measurements (Korja et al, 1996). Although the banded type prevails in the outcrop, the schlieren type was sampled because mesosomes, melanosomes and leucosomes were easier to separate (compare Figs .4 and 6 in Part I, Tuisku et al, 2006) Sample I1.3 (Nordsim number n273) was separated from foliated mesosomes predominantly consisting of fine-grained oligoclase-quartz matrix with garnet porphyroblasts. Garnet contains inclusions of quartz, sillimanite and rutile and is partially replaced by biotite, sillimanite and plagioclase at the margins.…”

“…Its petrology has been a subject of many studies during the last 50 years (Eskola, 1952;Meriläinen, 1976;Hörman et al, 1980;Raith & Raase, 1986;Barbey & Raith, 1990;Korja et al, 1996;Leibinger, 1996;Korja et al, 1996;Leibinger, 1996;Leibinger, 1996;Perchuck et al, 2000) and has been recently revised ) and has been recently revised by Tuisku et al (2006) who demonstrated a regional scale, clockwise PT evolution. Isotopic dating has confirmed that the LGB is Palaeoproterozoic in age (Meriläinen, 1976;Bernard-Griffiths et al, 1984;Huhma & Meriläinen, 1991;Bibikova et al, 1993;Sorjonen-Ward et al, 1994;Tuisku & Huhma, 1998Daly et al, 2001) and geochemical data suggest that the igneous rocks of the belt were formed in arc setting (Barbey et al, 1986;Bibikova et al, 1993).…”

“…The aim of this study was to connect the petrologic and tectonic evolution of the migmatitic metapelites of the LGB and their isotopic ages using SIMS U-Pb zircon dating and other radiogenic isotope methods (Tuisku et al, 2006). U-Pb zircon geochronology of high-grade, partly melted rocks may give a wealth of information on their evolution, because zircon may partly inherit the old crystallization history and may partly be overgrown or recrystallized during younger high-temperature events (Pidgeon & Aftalion, 1972).…”