ABSTRACT The Seve Nappe Complex in the Scandinavian Caledonides records a range of peak metamorphic conditions and timings. To better understand pressure-temperature-deformation-time differences throughout the complex and possible tectonic scenarios, metamorphosed mafic rocks within the Tarfala Valley of the Kebnekaise Massif (Sweden) were investigated using integrated petrologic and geochronologic techniques. Thermodynamic modeling of two samples using domainal and whole-rock compositions integrated with mineral chemistry, mineral textures, and titanite and zircon U-Pb geochronology constrained a portion of the pressure-temperature ( P-T ) path. Peak metamorphic conditions of 590–660 °C and 9.7–10.5 kbar were followed by near-isothermal decompression or a subsolidus clockwise P-T path. Amphibolite units in the valley record retrograde conditions at 450–550 °C at less than 7.5 kbar, although mineral modes and textures are most consistent with pressures <4 kbar. The majority of titanite growth occurred due to the introduction of hydrous fluids during cooling and following exhumation to midcrustal levels. U-Pb ages of retrograde titanite define a spread from ca. 480 to 449 Ma, and the oldest age is interpreted to constrain the timing of retrogression following exhumation. This interpretation is supported by a U-Pb zircon crystallization age of 481 ± 7 Ma for a metamorphosed intermediate to felsic synkinematic dike hosted in one of the amphibolite units. These results indicate that the Kebnekaise region records Early Ordovician deformation and metamorphism that was of lower grade compared to other Seve Nappe Complex locations to the south. The tectonic history of these rocks includes metamorphism and exhumation during the Cambrian–Ordovician pre-Scandian event, followed by thrusting of the Seve Nappe Complex and neighboring rocks onto Baltica during the Silurian Scandian orogeny.

Geochronological investigations of early Archean gneiss complexes by U-Pb dating methods are complicated by the presence of zircon populations that contain multiple ages of magmatic and metamorphic growth, and inherited components from premagmatic crust formation processes. In an early ion-microprobe study in the Saglek block, Nain Province, northern Labrador, identical ages for magmatism of 3.73 Ga were reported for both amphibolite- and granulite-grade orthogneisses, based on the oldest 207 Pb/ 206 Pb ages in two and three grains, respectively ( Schiøtte et al., 1989 ). Here we present zircon ages for the same rock units as in the previous study of 3348 ± 7 Ma and 3634 ± 31 Ma, respectively, obtained by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) methods, and based on the best quality grains from the dominant population of each rock. Zircon cores in the amphibolite-facies orthogneiss were separated from igneous overgrowths using a brief HF acid treatment and give ages ranging from 3700 to 3760 Ma, the age reported for magmatism in Schiøtte et al. (1989) . The time of major metamorphism in the amphibolite-facies orthogneiss is recorded by both Pb loss in the magmatic zircons and by new zircon growth within remnants of cross-cutting metadiabase at 2702 ± 2 Ma. The age for the early stage of granulite-facies metamorphism was reported by Schiøtte et al. (1989) as 2766 ± 17 Ma compared to the age reported here at 2740 ± 4 Ma. The results of this comparative study are instructive for the interpretation of sequential inherited, magmatic, and metamorphic zircon components in a complex population and for devising sampling strategies based on simple geological relations. Our new results demonstrate the way different ages can be obtained for the same rock unit due to contrasting approaches to dating geological events, and it is suggested that the earlier published data require reinterpretation. The new data reported here add to the database on igneous activity in the region so that the formation of crustal components is documented at 3850–3800, 3750–3700, ca. 3630, 3348, and 3200 Ma. This crustal instability may reflect the lack of an early mantle keel that was not fully established until the 2740–2702 Ma episode of metamorphism and ductile flow. A thin-skinned protocrust with multiple ages of intrusion and metamorphism, possibly driven by plume-related heat from below, i.e., multistage crustal recycling rather than competent plate interactions, is suggested for the formation of these Archean gneisses.

Pumpellyite associated with actinolite, epidote, prehnite, chlorite, albite, white mica, titanite, smectite, and calcite is found in tholeiitic dolerites of Late Triassic (Liassic?) age, where it fills veinlets, replaces primary plagioclase and orthopyroxene, and appears in the groundmass. The dolerites, metamorphosed during the Eo-Alpine event, crop out as tectonic blocks several hundred meters in size, covering an approximate area of 0.1 km 2 next to the town of Archidona, Málaga province, southern Spain. A 4.0-mm-thick vein filled with compact bundles of needlelike pumpellyite was selected for analysis. Very minor amounts of calcite and smectite appear as a thin wall coating in this vein. Separates of pure pumpellyite were analyzed by X-ray diffraction, transmission electron microscopy–analytical electron microscopy, inductively coupled plasma, and electron probe microanalysis (XRD, TEM-AEM, ICP, and EPMA). The chemical analyses show a close agreement between the ICP analysis and the mean value of 60 probe measurements. The average of six semiquantitative AEM analyses plot close to the points above. The X Fe 3+ values (= 100Fe 3+ /Fe 3+ + Al tot ) are 20.7% for the EPMA mean (all Fe as Fe 3+ ) and 18% for the ICP analysis (with Fe 3+ and Fe 2+ analyzed separately). Cation distribution in the Z, Y, X, and W positions agree closely with the ideal stoichiometry (6, 4, 2, and 4, respectively), whereas the cation totals are 15.91 for the EPMA mean and 15.99 for the ICP analysis. The Archidona pumpellyite is strongly impoverished in rare earth elements (REE) as a whole and shows a smooth U-shaped REE pattern. A maximum enrichment of about four to five times chondrites is shown by the heaviest rare earth elements Yb and Lu, respectively. The unit-cell parameters of the pumpellyite were determined as follows: a = 8.814 ± 0.002, b = 5.925 ± 0.001, c = 19.125 ± 0.003 A, V = 990.307 ± 0.228 A 3 , and β = 97° ± 0.8′. Pumpellyite needles in the vein were also examined by selected-area electron diffraction (SAED) and TEM imaging. From [010] zone axis electron diffraction patterns with no streaks along c * and images showing only some twin faults parallel to (001), it was concluded that pumpellyite was not significantly intergrown with epidote, lawsonite or sursassite. This absence of microdefects indicates a remarkable structural homogeneity of this pumpellyite sample.