Showing papers by "Geological Survey of Sweden published in 1986"

40Ar/39Ar mineral dates from retrogressed eclogites within the Baltoscandian miogeocline: Implications for a polyphase Caledonian orogenic evolution

Abstract: Late Proterozoic, rift-facies dolerite dikes within Baltoscandian rocks of the Seve Nappe Complex locally underwent eclogite metamorphism during Caledonian orogenesis. Hornblende from retrograde amphibolite selvages developed around two eclogite boudins exposed at Grapesvare, Norrbotten County, Sweden, record identical 40Ar/39Ar plateau dates of 491 ± 8 Ma. Phengitic muscovite from host schists records plateau dates of 447 ± 7 Ma and 436 ± 7 Ma. Coexisting biotite yields plateau dates of 594 ± 10 Ma and 808 ± 13 Ma. The biotite dates are interpreted to reflect the presence of extraneous argon components. The hornblende and phengitic muscovite ages are interpreted to date times of postmetamorphic cooling through argon retention temperatures. Together with previous 40Ar/39Ar mineral ages from Jamtland, Sweden, they confirm that a significant pre-Scandian tectonothermal event is recorded regionally in allochthonous sequences which originated within the Baltoscandian miogeocline. The eclogite assemblages are interpreted to have formed during westerly subduction of distal portions of the miogeocline with attendant development of an accretionary wedge. The latter was subsequently uplifted and eroded, providing a source for Middle Ordovician through Lower Silurian clastic successions which accumulated in both eastern and western basins. These, together with previously metamorphosed older portions of the miogeocline, were imbricated, folded, and variably metamorphosed during Late Silurian to Early Devonian transport onto the Baltoscandian Platform.

Chemical and Physical Properties of Paleozoic Potassium Bentonites from Kinnekulle, Sweden

Abstract: The < 1-μm fraction of 17 bentonite samples from Kinnekulle, southwest Sweden, were studied by chemical analysis, X-ray powder diffraction, and cation-exchange capacity. The bentonites are interbedded with undeformed, flat-laying Ordovician and Silurian sediments and were formed by the transformation of volcanic ash (dated at about 450 Ma) into smectite, which later converted to mixed-layer illite/smectite (I/S). The reaction, possibly driven by heat from an overlaying diabase intrusion (about 300 Ma), stopped at different stages of conversion, as evidenced by the I/S which ranges in composition from 60 to 10% smectite layers. A 2-m-thick bed shows zonation, with decreasing smectite proportions towards the upper contact. The zonation is not symmetrical towards the lower contact. In thin beds the illite proportion is higher and the regularity of ordering is inversely proportional to the thickness of the bed. K:Sr and K:Rb ratios follow the illite pattern; the ratios are highest at the contact and in thin beds. The inhibiting effect of Ca and Mg on the smectite-to-illite conversion probably was the cause of less-reacted smectite in the center of the thick bed.

Opabinia Anomalocaris, unique Cambrian 'arthropods'

Abstract: The Cambrian Opabinia Anomalocaris are odd animals known mainly from the Middle Cambrian Burgess Shale. Opabinia has usually been regarded as an arthropod, e.g. as a branchiopod crustacean. Parts of Anomalocaris have been referred to three or four different phyla. Recent redescriptions have clarified much of their morphology resulted in their removal from the arthropods. Additional observations considerations indicate that the two genera have important similarities, including scale-like structures arranged segmentally in transverse dorsal sets, which are separated by transverse tergal plates. Although external views are rare, traces of segmented appendages are identified in Anomalocaris. The animals are therefore again considered as arthropods, although they do not seem to be related to any of the other arthropod phyla.

Magma sources for some mid-Proterozoic granitoids in SE Sweden: Geochemical and isotopic constraints

Abstract: Three granitoid suites from near Linkoping, SE Sweden, belong to a major belt of mid-Proterozoic (post-Svecokarelian) magmatic rocks which displays Andinotype geological characteristics. The suites have sub-alkaline major element geochemistry and ‘within plate’ trace element (high Y, Nb) characteristics. Meta-aluminous chemistry and low Ga contents suggest dominantly igneous sources. Assuming an intrusion age of around 1.7 Ga, Sm-Nd isotope systematics indicate an average crustal residence period of about 0.2 Ga. One interpretation of these data is that these suites have developed in an Andinotype environment, not necessarily as subduction generated marginal suites but possibly in an ensialic rift environment on the continental side of a hypothetical calc-alkaline belt. An alternative interpretation is that the ‘within-plate’ trace element characteristics are inherited from a Proterozoic crustal precursor.

Maximum age of the synmetamorphic Svecokarelian fold phases in south central Sweden

Abstract: At St. Vika in south central Sweden, a pegmatite has conformably intruded an early Proterozoic, Svecokarelian limestone. Isoclinal folds F1 along N-S axes and subsequent crossfolds F2 caused the present structures. Zircons separated from the pegmatite have an age of 1854 ± 5 Ma (2sd), which is considered to reflect the time of intrusion of the pegmatite and consequently the maximum age of the deformation. The Rb-Sr whole rock age of the pegmatite is 1777 ± 24 Ma (2 sd). The closure of the Rb-Sr whole rock system reflects the cessation of the Svecokarelian late-kinematic actitvity and a time when the bedrock had cooled below the threshold for 87Sr migration.

Metazoan evolution–a new model

Abstract: Within their limits of resolution, different methods to reveal biochemical evolution appear to give reliable results where they can be controlled, as in the case of the Vertebrata. Compounds such as cytochrome c, SS rRNA and globins yield closely comparable results, which further strengthens the reliability. When the methods are applied to biochemical data from as many metazoan phyla as possible, the result is a phylogenetic tree which contrasts in certain respects with phylogenetic trees based on comparative zoology data. Important conclusions seem to include the following: (1) many metazoan phyla appear to have branched off from a shared, very conservative spiralian/protostome ancestral stock through the adoption of basically new feeding and locomotory strategies; (2) this origination makes it almost impossible to use a comparative zoology approach to solve problems of interphylum (in contrast to intraphylum) affinities, as similarities tend to be due only to shared derived Characters and similar life strategies, while dissimilarities are due to different basic life strategies; (3) protostome characters constitute a synapomorphy for triploblastic metazoans, and where absent they are secondarily lost, which means that protostome and deuterostome characters are irregularly distributed and mixed; (4) true deuterostomes form a sister group of the Mollusca, and they are only a top branch of the evolutionary tree.