4-dimensional geological modelling of the Skellefte district, Sweden
The Skellefte mining District is located in northern Sweden and comprises Palaeoproterozoic, metamorphosed, supracrustal rocks and associated intrusive rocks, which are all host to several ore deposits. Modelling was performed in the GoCAD software platform, using structural geology and petrology as well as magnetic, gravity, seismic, electric and electromagnetic geophysical data, in order to produce robust 3-dimensional models for the upper 3000 m of the Earth's crust in selected areas within the Skellefte District. Further interpretations from the regional structures and tectonics are providing a base for restoring the geological history of the district in time, the 4 dimension. * Corresponding author
Relationship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield
Abstract The three main intrusive suites: early calc-alkaline, late I/A-type, and late S-type intrusive rocks in relation to the Svecokarelian orogeny (1.9–1.8 Ga) have been dated at the Archaean craton margin in the Palaeoproterozoic Skellefte district and surrounding areas in northern Sweden. In addition, new SIMS data have been obtained on a calc-alkaline intrusion for which unusually young TIMS ages existed, compared to similar calc-alkaline intrusions elsewhere in the region. Titanite and zircon from a subvolcanic intrusion affected by a major N-S trending shear zone have also been dated to constrain the last ductile deformation in the area. The 1895+14 −12 Ma zircon age for a calc-alkaline intrusion is interpreted as the crystallisation age and is significantly older than the existing 1825 Ma age on titanite from a pyroxene skarn in a marble horizon close to the contact. The latter age is instead interpreted as the age of peak metamorphism in this area. The 1798plusmn;4 Ma age for the S-type granite confirms that the S-type magmatism is largely coeval with the I/A type magmatism previously dated at 1803plusmn;6 Ma. At a larger scale, a zoned belt over 2000 km long with A/I-type magmatism in the west and S-type magmatism in the east can be inferred. Either mafic underplating or Cordilleran type settings can explain the magmatic belt, which trends oblique to the roughly NE-directed subduction that led to the accretion of volcanic arcs onto the older craton between 1.95 and 1.87 Ga. An intimate temporal relationship between the extensive 1.80 Ga magmatism and regional N-S-trending shear zones in the area is confirmed by the titanite age of c. 1.80 Ga from one such shear zone. Kinematics on this shear zone suggest E-W shortening. SIMS data from a calc-alkaline intrusion at Sikträsk indicate that the previously obtained conventional zircon ages of 1.85–1.86 Ga are actually mixed ages of 1.88 Ga magmatic zircons, and c. 1.80 to 1.82 Ga metamorphic overgrowths. This shows that the 1.80 Ga event was not only constrained to shear zones. It is argued that both the 1.80 to 1.82 Ga deformation and metamorphism discussed here is related to E-W shortening and the voluminous magmatism at 1.82–1.80 Ga. This is in contrast to the older c. 1.88 Ga deformation identified to the north and east within the Karelian craton that was related to Svecokarelian accretionary processes.
Ion-microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland
We present ion microprobe UPb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve the discrepancy between SmNd and PbPb isochron ages for trondhjemite and hornblendite components. Whole-rock samples imply ca. 2.8 Ga. ages for both components (Whitehouse et al., 1996), but separated minerals have yielded ca. 2.4–2.5 Ga for trondhjemite and 3.3 Ga for hornblendite (Burton et. al., 1994). Zircons separated from three representative lithologies at Gruinard Bay (trondhjemite, hornblendite, and banded tonalite) yield concordant or near concordant UPb data with 207Pb206Pb ages mostly between ca. 2.85 Ga and ca. 2.75 Ga, and a few outliers at ca. 2.65 Ga. Backscattered electron images of zircons from the trondhjemite allow differentiation between ca. 2.85 Ga cores and ca. 2.75 Ga overgrowths; the latter are interpreted as igneous on the basis of field relationships of the trondhjemite to the other samples. Morphology of the hornblendite zircons is more consistent with crystallisation under metamorphic rather than igneous conditions at ca. 2.8 Ga, although the previous arguments of Whitehouse et al. (1996) for a protolith age not greatly in excess of this remain valid. The new zircon data suggest that magmatism at Gruinard Bay occurred over a relatively short time interval, in accord with the previous observations from whole-rock isotopic data. There is no evidence from the analysed zircons for ca. 2.4–2.5 Ga or 3.3 Ga magmatic ages in the trondhjemite and hornblendite, respectively. The mineral isochron ages for the trondhjemite probably represent metamorphic resetting related to the regional ca. 2.5 Ga high-grade event recorded in the central region. For the hornblendite, the mineral SmNd ages are confirmed as spurious, resulting from post-crystallisation open-system isotope behaviour on the mineral scale. Regional comparison of ages within the Lewisian Complex suggests that the ≤5 ca. 2.85 Ga Gruinard Bay TTG gneisses might have been amalgamated with ≥ 2.96 Ga gneisses of the central region before or during a ca. 2.76 Ga event and subsequently have experienced a similar geological evolution.