Abstract Abstract The core of the Tømmerås Window is made up of Precambrian rhyolites with dykes and sheets of basic rocks. The Precambrian basement rocks are covered by a thin horizon of metasediments, the Bjørndalen Formation, which corresponds to the lower part of the Jämtland Supergroup in the eastern thrust front. The Bjørndalen Formation is in turn covered by a nappe composed of the same lithologies as the core of the window. The rocks of the Bjørndalen Formation were unmetamorphoscd prior to the Caledonian orogeny. Together with available isotopic mineral cooling ages and ages from fossiliferous strata, metamorphic data are used to determine the physical conditions and time constraints for the Scandian overthrusting in the Tømmerås area. It is concluded that the Scandian thrusting occured later than 435 Ma in the Tømmerås area. The thickness of the overthrusted nappe stack was about 30 km. Erosion down to a thickness of about 8 km at 415 Ma gives an erosion rate of 0.5–2.5 mm yr-1. Such rapid erosion is inferred to have been accomplished by tectonic activity. Key Words: Scandinavian CaledortidesTømmerås WindowoverthrustingJämtland Supergroupmetamorphismerosion rates
<p>Southern Sweden (Scania region) is located in the peripherical parts of the East European Craton (EEC). In the Mesozoic, up to three pulses of volcanic activity took place between 191 and 110 Ma (Bergelin et al., 2006, IJES; Tappe <em>et al.,</em> 2016, GCA). Some of the alkali basaltoids carry ultramafic, mafic and felsic xenoliths (Rehfeldt <em>et al</em>, 2007, IJES). In this study, we focused on the evolution of the lithospheric mantle sampled by anhydrous, spinel-facies lherzolites, harzburgites, and subordinate dunites.</p><p>Based on the Fo content in olivine, the peridotites were classified into three groups. Group X peridotites are characterized by Ca-rich olivine (890-1470 ppm) with Fo=91.1-91.7. &#160;Enstatite has Mg#=91.5-91.9 and Al=0.16-0.22 atoms per formula unit (apfu), while the Cr-augite has Mg#= 90.8-91.2 and Al=0.21-0.28 apfu. Clinopyroxene is chemically homogenous in terms of trace elements and is LREE-enriched with positive Eu-anomaly. The Nd and Sr isotopic ratios in clinopyroxene are <sup>143</sup>Nd/<sup>144</sup>Nd=0.512548 (&#949;Nd=2.63) and&#160; <sup>87</sup>Sr/<sup>86</sup>Sr=0.704237, respectively. Olivine in group Y peridotites is Ca-poor (<951 ppm) and has Fo=89.5-91.1, enstatite has Mg#=89.7-91.7, and Al content of 0.084-0.169 apfu. The Cr-diopside has Mg#=90.8-93.5 and Al=0.118-0.232 apfu. Trace element patterns in clinopyroxene allow subdivision of this group into two subgroups: subgroup Y1 &#8211; with heterogeneous LREE-enriched clinopyroxene, and subgroup Y2 &#8211; with homogenous LREE-enriched clinopyroxene; both groups are characterized by a positive Eu anomaly, but in subgroup Y1 it is significantly more pronounced. The Nd and Sr isotopic ratios in clinopyroxene from subgroup Y1 are <sup>143</sup>Nd/<sup>144</sup>Nd=0.512624&#8211;0.512644 (&#949;Nd=4.13-4.52) and <sup>87</sup>Sr/<sup>86</sup>Sr=0.703027&#8211;0.703100, therefore significantly more depleted than group X. In group Z peridotite the Fo content in olivine is 88.1-89.1, the Mg# in enstatite is 89.1-89.5 and its Al content is 0.19-0.20 apfu. The Mg# of Cr-diopside is 88.5-89.4 and the Al content is 0.24-0.25 apfu. The trace elements contents in clinopyroxene is homogenous and the REE pattern is flat at values double that in the primitive mantle. &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</p><p>&#160;The highest equilibration temperatures were estimated for the group X xenoliths, where T<sub>WES</sub>=1101-1110 &#176;C (Witt-Eickschen and Seck, 1991, CMP) and T<sub>BK</sub>=1214-1241 &#176;C (Brey and K&#246;hler, 1990, JoP). &#160;The temperatures calculated for group Y xenoliths are T<sub>WES</sub>=875-1033 &#176;C and T<sub>BK</sub>=872-1027 &#176;C and do not significantly differ between subgroups. Temperatures recorded by the group Z sample are T<sub>WES</sub>=1040-1056 &#176;C and T<sub>BK</sub>=1065-1081 &#176;C.</p><p>The composition of group X peridotites suggests their metasomatism by a high-temperature mafic melt resembling the basaltoids from Scania. Alternatively, they may represent high-pressure cumulates, as suggested by their coarse-grained texture. The group Y peridotites record cryptic metasomatism of a significantly depleted peridotite (melt extraction ranging typically between 25 and 30%) by a carbonatitic melt. The carbonatitic metasomatic agent was fractionating chromatographically from REE-, Th- and U-rich in subgroup Y2 to -poor in those elements in subgroup Y2. The group Z peridotite possibly represents depleted peridotite which was further metasomatized by a mafic melt. The lithospheric mantle beneath the marginal part of EEC has a complex composition, which is however different from a typical cratonic mantle.</p><p>&#160;</p><p>Founded by Polish National Science Centre grant no. UMO-2016/23/B/ST10/01905 and WTZ PL 08/2018.</p>
Abstract U–Pb dating of titanite from a mafic lens in the Mylonite Zone in southwestern Sweden yields an almost concordant age of c. 920 Ma. The titanite formed during retrogression of granulite-to amphibolite-facies rocks. Sources for Ti and Ca in titanite were Fe-Ti oxides and relatively grossular-rich garnets, respectively. Deformation is contemporaneous with or post-dates the growth of titanite, which took place at temperatures above c. 550ÅC. The obtained U–Pb age of titanite is close to the Sm-Nd isochron ages of garnet-bearing granulite-facies mineral assemblages in the Varberg area just south of the Mylonite Zone. Since the blocking temperature is higher for Nd diffusion in garnet than for the Pb diffusion in titanite it is suggested that the rocks of the Mylonite Zone were undergoing amphibolite-facies deformation, uplift and cooling while the granulites of the Varberg area were still at a considerably higher temperature. Key Words: GranuliteMylonite Zoneretrogressionsouthwestern SwedenSveconorwegiantitaniteU-Pb dating
