Matti Vaasjoki

New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthosite-diabase dyke association of southeastern Finland

Abstract New U-Pb data on zircons, monazites, and baddeleyite suggest that the Wiborg rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640±5 and 1630±5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km3 of rock material over a period of 50 Ma. Within southern Finland, the rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.

1700 Ma Shachang Complex, Northeast China; Proterozoic rapakivi granite not associated with Paleoproterozoic orogenic crust

Proterozoic rapakivi granites are found on all continents and are characteristically associated with Paleoproterozoic crustal domains that predate the granites by

U-Pb ages of plutonic and volcanic rocks in the Svecofennian Bothnian Basin, central Sweden, and their implications for the Palaeoproterozoic evolution of the Basin

Abstract New U-Pb age data on two early orogenic granodiorites (ca. 1930 Ma) and one felsic metavolcanite (1870 Ma) from the Svecofennian Bothnian Basin area of central Sweden are presented. Combined with literature data they suggest an extended, at least ca. 80 m.y. long, evolution of this part of the Bothnian Basin. Sedimentation started before 1950 Ma, and continued up to at least 1870 Ma. Early-orogenic plutonic activity started at ca. 1950 Ma, perhaps even earlier, and continued up to ca. 1850-1840 Ma. Detrital zircons found in low-grade greywackes of the region range in age from ca. 1880 to 2020 Ma. According to the tentative model suggested here, these zircons may emanate from granitoids intruded into the greywacke sequence of the Bothnian Basin. They were subsequently exposed to erosion and included in upper parts of the sedimentary column. These upper parts could (statistically) be of a lower metamorphic grade than older greywackes, which are generally migmatized.

Age differences between RbSr whole rock and UPb zircon ages of syn- and postorogenic Svecokarelian granitoids in Sottunga, SW Finland

Abstract The UPb age on pooled data of zircnos from the postorogenic Svecokarelian Mosshaga granite is 1788 ± 11 Ma (2 σ). The corresponding RbSr whole rock age is 1742 ± 34 Ma (2 σ). The Mosshaga granite intrudes a synorogenic granodiorite, which has a UPb zircon age of 1889 ± 10 Ma and a RbSr whole rock age of 1850 ± 28 Ma. The age differences between the UPb zircon ages and RbSr whole rock ages are concluded to be geologically significant. It is shown that the Mosshaga pluton cooled rapidly and as a consequence the RbSr isotopic system closed at a time very close to the crystallization of the zircons. A commonly observed feldspar and biotite alteration in Svecokarelian granitoids indicates migration of Rb and Sr isotopes in the whole rock system which may lead to apparently younger RbSr ages.

The age of mid-proterozoic phosphatic metasediments in Finland as indicated by radiometric UPb dates

Abstract Two uranium-bearing phosphatic metasedimentary occurrences at Lampinsaari, Vihanti and Temo, Nilsia, in Finland have been dated by isotopic UPb analyses of whole rock samples to be 1876±2 Ma old. The dates reflect the time of the Svecokarelian regional metamorphism in high amphibolite facies. An analysis of a uranium thucolite from the deposit at Nuottijarvi, Paltamo, that was metamorphosed in intermediate greenschist facies gives a date of 1897±7 Ma, suggesting that the phosphatic sedimentation had occurred before that time. An upper time limit for the sedimentation is set by a previous date of 2080±45 Ma, which was obtained from a banded iron one formation. The absence of thorium in the deposits suggests a marine environment of deposition.

Gold in bedrock and glacial deposits in the Ivalojoki area, Finnish Lapland

Abstract Three types of Au mineralization are known in the Ivalojoki area, in the southern part of the granulite complex of Lapland. These are epigenetic quartz-carbonate and quartz-hematite veins, the likely source of placer gold, and an arsenopyrite-bearing shear zone at Harrioja. The average Au content of the quartz-carbonate veins is 0.2 ppm, varying between 0.002 and 3.0 ppm; 2–6 ppm Au has been detected earlier in the Makararova quartz-hematite vein. The maximum Au value in the Harrioja shear zone, where no placer gold has been found, is 0.4 ppm. Arsenic and Sb are both correlative with Au in veins and glacial till, as are Au and vein quartz in the till. The correlation between panned gold and instrumentally analyzed Au is low, as in most cases are the correlations between Au and other trace metals, till lithology and heavy minerals. The isotopic composition of trace lead from placer gold samples suggests the age of granulite metamorphism, but this may be spurious due to restricted sample material.

Zircon UPb versus RbSr whole-rock age data from eastern finland: A critical comment on the papers of Barbey & Martin and Martin, Precambrian research, vol. 35, 1987

Discussion et reponse sur les articles «The role of komatiites in plate tectonics. Evidence from the Archean and early Proterozoic crust in the eastern Baltic Shield Precambrien» par Barbey P. et Martin H., 1987, 35, p. 1-14 et «Evolution and composition of granitic rocks controlled by time-dependent changes in petrogenetic processes: examples from the Archean of eastern Finland», par Martin H, 1987, 35, p. 257-276

Regional implications of an age determination of a gneissose granitoid south of the Skellefte district, northern Sweden

Abstract A gneissose granitoid, the Siktrask dome, situated c. 20 km south of the central parts of the Skellefte District has been dated at 1859±3 Ma. This age excludes the possibility that this gneiss dome is a basement to the Svecofennian metamorphosed supracrustral rocks surrounding the intrusive. The age further suggests that this dome and other similar deformed granitoids south of the Skellefte District could be interpreted as synkinematic Jorn-type intrusives, but situated in an amphibolite facies environment, contrary to the undeformed Jorn granitoids which are situated in a greenschist facies environment.

Archaean–Proterozoic geochronology of the Vallen-Alhamn area, northern Sweden

Abstract A minor massif of approximately 2700 Ma Archaean granodiorite and tonalite is exposed in a Svecofennian environment at Vallen-Alhamn, near to the Archaean–Proterozoic Palaeoboundary in northern Sweden. Close to this massif occur conglomerate-like rocks (in part interpreted as a magmatic, hydraulic breccia) with fragments of Palaeoproterozoic metavolcanites and granitoids as well as of the Archaean granitoids. The present work comprises dating of the granitoids of the massif, c. 2700 and 1900 Ma granitoid fragments in two occurrences of conglomerate-like rocks, and the volcanogenic matrix of one of the conglomerate-like rocks. The age of igneous-looking zircons in the matrix (1871±2 Ma) is interpreted to be close to the time of formation of the conglomerate-like rock, and points to an Upper Svecofennian stratigraphic position. The bedrock of the Vallen-Alhamn area is thus a unique example of Archaean rocks in close geographic association with Upper Svecofennian supracrustal or shallow-level intrusive rocks.