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.

Abstract The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic ɛHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic ɛHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana. Supplementary material: Figs S1 and S2 are available at http://www.geolsoc.org.uk/SUP18625

Sapphirine + quartz-bearing pelitic gneisses from Wilson Lake, in the Grenville Province of central Labrador, and from other granulite-facies terranes, are well known for their reaction rims and nonequilibrium textures. However, some corundum-bearing gneisses from the Red Wine Mountains massif in the Wilson Lake area have a low variance assemblage that appears to record equilibrium conditions of regional metamorphism. The silica-undersaturated assemblage sapphirine (Spr) + ortho-pyroxene (Opx) + sillimanite (Sil) + garnet (Grt) + spinel (Spl) + corundum (Crn) + magnetite + titanhematite (+ plagioclase + biotite) approaches invariance in the six-component system FeO-MgO-Al 2 O 3 -SiO 2 -Fe 2 O 3 -TiO 2 . The resulting cordierite [Crd]-absent invariant point appears to be stable near 900 °C and 1000 MPa, and at an oxygen fugacity (fO 2 ) defined by coexisting pure magnetite and titanhematite (after integration of exsolved ferrian ilmenite). Further evidence of high fO 2 is exsolved titanhematite (X Fe 2 O 3 = 0.73) in orthopyroxene, exsolved hematite in sillimanite, and high dissolved Fe 3+ in sillimanite and corundum. Systematic partitioning of Fe 3+ in the coexisting silicate and oxide phases documents their mutual equilibrium. The P-T- fO 2 conditions represented by the [Crd]-absent invariant point in corundum-bearing rocks are consistent with the stability of spinel + quartz. This would not be the case if X Mg (Grt) < X Mg (Spl). The phase relationships presented here and the absence of garnet + cordierite assemblages in this area further suggests that the [Crd]-absent invariant point is stable in the field of sapphirine + quartz. The partitioning sequence X Fe 3+ (Spr) > X Fe 3+ (Spl) > X Fe 3+ (Opx) > X Fe 3+ (Grt) we estimate from microprobe analyses extends the stabilities of sapphirine and spinel relative to orthopyroxene and garnet, shifting the [Crd]-absent invariant point to a lower temperature.