Early Ordovician to Silurian evolution of exotic terranes in the Scandinavian Caledonides of the Ofoten–Troms area – terrane characterization and correlation based on new U–Pb zircon ages and Lu–Hf isotopic data

Abstract

AbstractIsotope dilution thermal ionization mass spectrometry U–Pb dating and coupled Lu–Hf solution inductively coupled plasma mass spectrometry analyses of zircon were acquired from magmatic rocks along two transects across the Scandinavian Caledonides in the Troms–Ofoten region of Norway to explore possible correlations and gain insight into the evolution of far-travelled nappes within the Upper and Uppermost Allochthons. One pulse of magmatic activity was recorded at c. 489 Ma in the Tromsø Nappe. In the underlying Nakkedal Nappe, a magmatic pulse was recorded at c. 450 Ma, being contemporaneous with eclogite facies metamorphism in the area. Tonalites in the structurally underlying Lyngen and Gratangseidet ophiolitic complexes, both forming the substratum to carbonate–schist–quartzite sequences (Balsfjord and Evenes groups, respectively), yielded ages of 481 and 474 Ma. Obtained εHf(t) values are, however, distinctly different and indicate a juvenile origin for the Gratangseidet tonalite (εHf(474)=+9.57) and the presence of Palaeoproterozoic source material for the Lyngen tonalite (εHf(481)=−16.8 to −2.3). The 474 Ma age of the Snaufjellet granite intruding the Bogen Group structurally above the Evenes Group requires a thrust between the two units. An age of 435 Ma on the Heia gabbro of the Nordmannvik Nappe is comparable to that of the Råna norite (437 Ma) in the Narvik area, supporting a correlation with the Narvik Nappe Complex. The youngest intrusion dated to 425 Ma is a leucogranite that intruded the Balsfjord Group. The Lu–Hf data indicate a common source for several of the magmatic rocks in the Uppermost Allochthon, as well as Laurentian arc granitoids from East Greenland and possibly from Newfoundland and related areas.