CONSTRUCTION OF A 477-440 MA CALC-ALKALINE ARC ON AN OLDER, OBDUCTED ARC AND OPHIOLITE IN THE UPPER ALLOCHTHON, MID NORWAY CALEDONIDES: GEOCHEMICAL HINTS OF A CLOSED WESTERN IAPETUS?

The Upper Allochthon is primarily composed of older (500-481 Ma) oceanic arc and back-arc ophiolitic rocks, and the epicontinental Gula Complex onto which the ophiolites were obducted at 480-475 Ma, and a younger, broadly Taconian (477-440 Ma), calc-alkaline plutonic complex and coeval volcanic-sedimentary sequence. The plutonic complex, near the coast, is divided by the brittle-ductile Hoybakken detachment fault into rocks having little (NW) and strong (SE) Devonian (Scandian) deformation. Here we discuss 153 plutonic samples in both age groups, from SE of the fault.

The plutonic rocks are in NE-SW belts, an older belt to the SE, based on one 482 Ma zircon age, and a younger belt to the NW, based on five ages 477-444 Ma. In the SE belt are mafic volcanics, intruded by mafic to tonalitic plutons. The mafic rocks have mostly LREE-depleted to flat REE patterns. Felsic rocks (including the 482 Ma sample) have patterns that are flat to slightly LREE-enriched, many with high HREE contents like ocean-ridge “plagiogranites”. Many are depleted in water-mobile elements (K, Pb, Sr, Li), indicating hydrothermally altered sources. Based on geochemistry and one age, all these placed in the older group with the ophiolites.

The younger plutons to the NW span a composition continuum from ultramafic to granite. Mafic rocks are LREE-enriched, commonly with saddle-shaped HREE patterns, typical of arcs. The felsic rocks include abundant adakites, indicated by LREE-enrichment, HREE-depletion, high Sr/Y ratios, etc. More typical felsic arc-type rocks are separated from adakites on graphs by composition gaps for several elements, indicating distinct sources.

Upper Allochthon plutonic complexes of the younger age elsewhere in Norway also have abundant LREE-enriched mafic rocks like those in the NW belt. Adakites are also abundant, as in our data set. In the Uppermost Allochthon, considered to be from the Laurentian margin, the same is true. Adakitic rocks are relatively rare in Phanerozoic arcs. Their abundance in both the Upper and Uppermost Allochthons suggests that the presently mapped Upper and Uppermost Allochthons were already together near or on the Laurentian margin during pluton emplacement, a conclusion also supported by 445 Ma ‘stitching’ plutons across ‘Taconian’ NW-directed thrusts in SW Nordland.