Field relationships and geochemistry of the Ostereide Dykes, western Norway: implications for caledonian tectonomorphic evolution

Structural, geochemical and isotopic investigations have been car ri ed out a cross the contact zone between the Precambrian rocks of the Lindås Nappe (Lindås Complex) and Palaeozoic rocks (Major Bergen Are Zone) at Ostereide in the Caledonides of western Norway. T he Lindås Complex structurally overlies the Totland Unit of the Major Bergen Are Zone. The Totland Unit consists of amphibole-garnet-mica-schists, and garnet-biotite geothermometry from this unit yield metamorphic temperatures in the range 600-645 °C. This corresponds to upper amphibolite facies, whereas underlying units show peak metamorphism no higher than lower amphibolite facies. Syntectonic granitoid dykes - the Ostereide Dykes - intruded in to the Lindås Complex and into the Totland Unit dose to the base of the overlying unit. The dykes are characterized by high contents of Sr and Ba, 87Sr86Sr420Ma of 0.7046-0.7057 and ?of -0.7 to -1.2. Zircons from one of the dykes analysed by ion microprobe data indicate an U-Ph age of 410-440 Ma for the latest p hase of crystal overgrowth interpreted as the age of intrusion. Zircon cores yield U-Ph ages in therange 920 to 1478 Ma, and are evidence of widespread Proterozoic inheritance. The geochemical and isotopic compositions indicate that the granitoid intrusions may have formed by anatexis of metasediment similar to the garnet-amphibole mica schists from the Totland Unit in the presence of a water-rich fluid phase. T he heat source for the high temperatures is suggested to be dissipative heating from the contact zone generated during the juxtaposition of the Lindås Complex with the Totland Unit.

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