Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the M?rma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian-Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the M & aring;rma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U-Pb zircon geochronology yielded crystallization ages of 835 +/- 8 Ma for a mylonitic orthogneiss, 864 +/- 3 Ma for the Vistas Granite and 840 +/- 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U-Pb zircon crystallization ages of 856 +/- 3 Ma and 850 +/- 1 Ma, respectively. U-Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 +/- 7 Ma and a lower intercept age of 443 +/- 20 Ma. Pressure-temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5-11.75 kbar and 550-610 degrees C and 7.4-8.1 kbar at 615-675 degrees C, respectively. Metamorphic pressure-temperature estimates for the Vistas Granite yield 6.5-7.5 kbar at 600-625 degrees C. Whole-rock chemistry coupled with U-Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma.