Tectonic implications of ca. 1.45 Ga granitoid magmatism at the southwestern margin of the East European Craton

Between ca 1.53 and 1.40 Ga, the southwestern margin of the East European Craton was subjected to extensive magmatism and deformation. While various suites of anorthositic, mangeritic and charnockitic-granitic rocks were emplaced between ca. 1.53 and 1.50 Ga, a major event of A-type granitic magmatism took place around 1.45 Ga. During that event, numerous voluminous plutons were intruded in a wide region around the southern Baltic Sea (“the SBS region”). Petrologically, the various SBS granitoids are rather similar to each other. Like many A-type granites worldwide, they are enriched in silica, high field strength elements (HFSE) and rare earth elements (REE), and have high Fe/Mg and K/Na ratios. The most common ferromagnesian silicate minerals are biotite and amphibole, clinopyroxene occurring occasionally. Another feature characteristic of the SBS plutons is their formation by the emplacement of multiple pulses of melt. Such pulses were occasionally responsible for separate suites of rocks and appear to have originated from slightly different sources. In general, however, the melt sources of the SBS granitoids were relatively juvenile and rich in aluminum and potassium as well as in HFSE:s and REE:s. The isotopic characteristics of the rocks may also suggest some interaction between crustal and mantle materials. During the ca.1.45-Ga event, the Blekinge-Bornholm region experienced notable regional compression and ENE-WSW shortening. That compression caused syn- and post-magmatic deformation of the involved granitoids as well as deformation and metamorphism of the host rocks. Due to its activity, also EW-striking shear zones were either developed or reactivated and apparently controlled the emplacement of the SBS granitoids. As different from the traditional concept of a liaison between A-type granitic magmatism and anorogenic extension of the crust, the present study thus strongly evidences that the SBS granitoids were intruded during compressional tectonic processes. Causally, they are interpreted to have been related to the Mesoproterozoic Danopolonian orogeny which may have led to the collision of the East European Craton with another proto-continent, possibly Proto-Amazonia (Bogdanova, 2001)

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Emplacement and deformation of the ca. 1.45 Ga Karlshamn granitoid pluton, southeastern Sweden, during ENE-WSW Danopolonian shortening

Anisotropy of magnetic susceptibility and structural geology of the ca. 1.45 Ga Karlshamn pluton (southern Sweden) are used to study its emplacement and structural evolution. The Karlshamn pluton is one of the largest metaluminous A-type granitoid intrusions in southern Sweden. It is a multiphase body made up of two suites that differ in composition but which have similar crystallization ages. The magmatic foliation, ductile shear zones and granite-pegmatite filled fractures were mapped as well as the metamorphic foliation and extension lineation in the metamorphic host rocks. The anisotropy of magnetic susceptibility was used to map the magnetite petrofabric of the pluton, providing a larger data set for both the magmatic foliations and lineations, which could not be mapped in the field. The fabrics within the pluton are continuous with the metamorphic fabrics in the country rocks. Both the pluton and the country rock fabrics were folded during ENE-WSW compression, while the pluton was still a magma mush. The stress field orientation during cooling of the pluton is determined on the basis of magmatic, ductile and brittle structures in the Karlshamn pluton that formed successively as the pluton cooled. The compressional event is referred to as the Danopolonian orogeny and therefore the Karlshamn granitoids, and other plutons of similar composition and age in central and southern Sweden, on the Danish Island of Bornholm, and in Lithuania, may be considered as syntectonic intrusions and not as anorogenic, as was previously thought

A revised geochronology for the Blekinge Province, southern Sweden

Zircon grains from nine samples of metavolcanic rocks, gneisses, granitoids and migmatites from the Blekinge Province in southeasternmost Sweden have been dated by U-Pb using ion microprobe. The results suggest that most of the Blekinge bedrock was formed within a narrow time interval of 1.77-1.75 Ga, including the Vastana supracrustal formation and 'coastal gneisses' previously dated to c. 1.70 Ga. One sample, the 1.81 Ga Nattraby gneissic granite, appears to represent a sliver of slightly older proto-crust. An age of c. 1.75 Ga for an aplitic granite crosscutting the deformed Tving granitoids in eastern Blekinge brackets their deformation to between 1.77 and 1.75 Ga, whereas zircon grains of similar age from a migmatite neosome at Lindo may be inherited, making the result inconclusive. Thin metamorphic zircon overgrowths and resetting of the U-Pb system in titanite indicate a regional tectonothermal event at 1.45 to 1.40 Ga, accompanying the intrusion of the Karlshamn-type granites. The crust of the Blekinge Province thus was formed at 1.77-1.75 Ga from relatively juvenile sources in a subduction-related environment along the southern edge of Fennoscandia. It was deformed and partly uplifted relative to the undeformed TIB-1 granitoids of the Smaland block further north, prior to the intrusion of the Karlshamn-type granites at 1.45 Ga

The Zemaiciu Naumiestis granitoids: new evidences for Mesoproterozoic magmatism in western Lithuania

We report new occurrences of ca. 1460 Ma magmatism in western Lithuania, where large volumes of granitoid melts were intruded into host supracrustal and charnockitoid rocks, and formed the Zemaiciu Naumiestis pluton. Numerous granitoids of that age have previously been described from southern Sweden and from the Danish island of Bornholm, but none have been reported from the eastern side of the Baltic Sea, where the crystalline basement is covered by Phanerozoic sediments. Petrological studies demonstrate that the Zemaiciu Naumiestis intrusion consists of quartz monzodiorites, monzogranites and syenogranites. The dominant ferromagnesian mineral is biotite. Rare clinopyroxene grains occur in the quartz monzodiorites. The rocks are fine to coarse-grained, often porphyritic. Chemically, the studied granitoids are dominantly alkali-calcic and shoshonitic, metaluminous to peraluminous, and ferroan to magnesian. The geochemical data indicate that the pluton consists of two rock suites, one representing monzodiorites and monzogranites, and other mostly syenogranites. The suites originated from slightly different sources. The rocks within the Zemaiciu Naumiestis pluton are variably deformed and locally cataclased. Some are, however, rather massive. Two samples of monzogranite yielded ID-TIMS U-Pb zircon ages of 1462 +/- 8 (MSWD = 1.09) and 1459 +/- 3 Ma (MSWD = 0.28)

The Stenshuvud and Taghusa granitoids: new representatives of Mesoproterozoic magmatism in southern Sweden

New sites of Mesoproterozoic granitoid magmatism in southern Sweden have been discovered by recent fieldwork and U-P zircon age determination. In the area around Stenshuvud, in eastern Skane, granitic melts were intruded into country-rock gneisses of unknown age at c. 1450 Ma. The 1458 6 Ma Stenshuvud intrusion is composed of several rock varieties including quartz monzonite, tonalite, monzogranite, and late aplites. The Stenshuvud granitoids proper have glomeroporphyritic textures defined by monomineralic aggregations of feldspar or quartz and polymineralic aggregations of amphibole, biotite and magnetite. At 1442+/-9 Ma, the Taghusa granites were intruded along the contact between the Stenshuvud granitoids and the country-rock gneisses. These granites have streaky appearances, which are due to the presence of short, sub-parallel mafic mineral aggregations. The subsequent intrusion of leucogranites as cross-cutting veins and small bodies was the last phase of the entire magmatic event. All these granitoids appear to be co-genetic and belong to a metaluminous to marginally peraluminous, ferro-potassic, high-K calc-alkaline to shoshonitic sequence. Trace elements indicate similar source materials for both principal intrusions. An epsilon(Nd)-value of -0.6 and a T-DM model age of 1.85 Ga indicate involvement of older crustal materials in the generation of the melt(s). The studied granitoids feature both I- and A-type characteristics but are not typical of either type. While the Stenshuvud granitoids were intruded during NE-SW compressional stress that caused shearing an folding, the Taghusa granites are post-compressional and show no solid-state deformation

Danopolonian migmatization of Mesoproterozoic sedimentary rocks in southernmost Sweden: a SIMS zircon study

To assess the age and origin of the metasedimentary migmatites in southernmost Sweden and their relationships with the Mesoproterozoic granitoid magmatism in the area, we have dated migmatite zircon using the secondary ion mass spectrometry U-Pb method. The studied metasedimentary migmatites, here called the Noteboda migmatites, occur along the southwestern boundary of the 1442 Ma Taghusa granitoid intrusion in southeastern Skane. They contain the mineral assemblage garnet + biotite +/- muscovite + cordierite + sillimanite + quartz + plagioclase + K-feldspar and were formed during a retrograde evolution from c. 750-720 degrees C and 6 kbar (peak conditions) to c. 675 degrees C and 4 kbar. Zircon is characterized by detrital cores surrounded by U-rich rims and overgrowths. Separate rounded metamorphic grains also exist. The age probability-density distribution peaks for detrital zircon are at c. 1700, 1670, 1650, 1610, 1570 and 1530 Ma. These ages suggest Gothian orogenic rocks in the present west as the most probable principal source. Sedimentation occurred after c. 1530 Ma, the age of the youngest detrital zircon, indicating the existence of a previously unknown period of Mesoproterozoic sedimentation in southernmost Sweden. A homogeneous zircon overgrowth yielded a concordant Pb-207/Pb-206 age of 1439 +/- 5 Ma, which dates the migmatization and is close to the age of the Taghusa intrusion. We conclude that the burial of the sediments down to c. 20 km, their metamorphism and progressive migmatization took place concurrently with granitic magmatism, NE-SW compression, folding and shearing of the crust between 1460 and 1440Ma during the Danopolonian orogeny