Karin Appelquist

Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden

The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with 207Pb/206Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.

Age, tectonic setting and petrogenesis of the Habo Volcanic Suite: Evidence for an active continental margin setting for the Transscandinavian Igneous Belt

Abstract The Habo Volcanic Suite is situated in the southwestern part of the Smaland-Varmland batholiths of the Transscandinavian Igneous Belt (TIB), Sweden. It comprises mainly intermediate to mafic pyroclastic and redeposited volcanic rocks and is distinct from other, more felsic, volcanic rocks of the Smaland-Varmland batholiths. The Habo Volcanic Suite has been intruded by a TIB-granite, here dated at 1660±9 Ma. U-Pb ages of zircon from the Habo Volcanic Suite fall into two groups; 1795±14 Ma for domains with Th/U >0.2 and 1694±7 Ma for rims with Th/U <0.2. The 1795±14 Ma age is interpreted as the crystallization age of the Habo Volcanic Suite, linking it to other Smaland volcanic rocks, whereas the 1694±7 Ma rims are interpreted to reflect a thermal event related to intrusions of younger TIB rocks. These interpretations are supported by LA-ICPMS REE analyses which show flatter REE patterns and Th/U <0.2 for metamorphic zircon; and positive Ce anomalies for magmatic zircon. Two geochemically different...

The Palaeoproterozoic Malmbäck Formation in S Sweden: age, composition and tectonic setting

The Palaeoproterozoic Malmbäck Formation in southern Sweden comprises mafic to felsic volcanic rocks and syn-eruptive volcaniclastic and volcanogenic sedimentary deposits. Field observations indicate terrestrial to fluvial deposits – predominantly with massive to highly vesicular lavas, but also with ignimbrites, ashes, redeposited volcanic rocks and conglomerates. Three groups of volcanic rocks can be distinguished geochemically: (1) primitive basalts to basaltic andesites, (2) dacites-andesites and (3) rhyolites. The groups constitute a calc-alkaline to shoshonitic series. Large ion lithophile elements and light rare earth elements enrichment, combined with depletion in heavy rare earth elements and high field strength elements, particularly in Nb (and Ta), are characteristic of typical arc rocks. Discriminant diagrams suggest emplacement along a one-time active continental margin, which is now marked by the Transscandinavian Igneous Belt (TIB): an array of ∼1.85–1.65 Ga granitic to gabbroic plutonic rocks and associated volcanic rocks more than 1000 km long. In the TIB, the Malmbäck Formation forms mega-xenoliths and other inclusions. U–Pb secondary ion mass spectrometry dating of zircon from a rhyolite of the Malmbäck Formation yielded a concordia age of 1796 ± 7 Ma, which is interpreted as the crystallisation age of the rhyolite and the age of deposition of the Malmbäck Formation, during a relatively early part of the formation period of the TIB. Altogether, the setting, composition and age of the Malmbäck Formation and the TIB confirm the existence of an Andean-type active continental margin in the present southern part of the TIB at c.1.8 Ga.

Nd isotope systematics of 1.8 Ga volcanic rocks within the Transscandinavian Igneous Belt, south-central Sweden

Mafic to felsic volcanic rock suites dated at c. 1.8 Ga within the southern part of the Transscandinavian Igneous Belt have been analysed for Sm and Nd isotopes. Primitive, basaltic parts are interpreted to be derived from mildly depleted mantle (DM)-derived magmas with ϵNd values around +2, similar to those for other mafic intrusive rocks of the same age. Felsic-intermediate parts, with somewhat lower initial ϵNd values, probably resulted from a combination of fractional crystallisation of the basaltic magmas and mixing with two different crustal components, one consisting of juvenile felsic material and another of intermediate composition containing somewhat older crustal material. These components were probably of regional importance and may have been involved in the formation of other extrusive and intrusive TIB-1 rocks in southern and central Sweden as well.