ORDOVICIAN MAGMATISM AND DEFORMATION IN THE UPPERMOST ALLOCHTHON, CENTRAL NORWAY: AN ORPHAN OF THE TACONIC OROGENY?

In the Caledonides of central Norway, the Uppermost Allochthon (UmA) includes the Helgeland Nappe Complex (HNC) intruded by the ~481-430 Ma Bindal Batholith (BB). The HNC can be divided into 4 nappes juxtaposed along moderately to steeply east-dipping shear zones (lowest to highest nappes): 1) Upright, Ordovician ophiolite slivers and unconformably overlying (east dipping), kyanite-grade metasedimentary rocks intruded by the peraluminous, crustally derived Vega batholith (477±4 Ma [weighted mean age], 22 analyses, MSWD=1.6 & 466±5 Ma [weighted mean age], 12 analyses, MWSD=12, SHRIMP U/Pb zirc); 2) Proterozoic diatexitic to stromatic migmatitic gneiss, calc-silicate, marble, a large 2 mica granite (~481 Ma), and a suite of 447-448 Ma dioritic to quartzmonzonitic plutons with porphyritic granitic (PG) intrusions located along the margins of the diorite plutons; 3) same as Nappe 1 without kyanite or intrusive rocks; and 4) Migmatitic gneiss, calc-silicate, and marble of probable Proterozoic age intruded by dioritic to granodioritic, and local PG rocks. Amalgamation of the HCN occurred prior to emplacement of the 448 Ma, 600 MPa (Al-in-hnb) Andalshatten pluton that truncates nappe-bounding shear zones. SHRIMP U/Pb analyses of two zircon populations from one sample of PG yield ages of 468±5 Ma (5 analyses, MSWD=0.6) and 447±5 Ma (6 analyses, MSWD=1.2) whereas a second sample displays two closely spaced populations on a relative probability plot yielding a weighted mean age (21 analyses) of 480 Ma (MSWD=2.2). We infer that the older zircon ages represent a regional metamorphic and magmatic episode that culminated during intrusion of a suite of peraluminous granitoids (e.g., Vega batholith) between 480 and 470 Ma. The younger age is consistent with the interpretation of a genetic relationship between PG and dioritic plutons and a local migmatite-producing event at ~447 Ma.Existing regional syntheses propose that the UmA contains an Early Paleozoic lithologic and structural evolution that is distinct from Baltica. Based on the age of magmatism, structural asymmetry (Ordovician-aged west-directed thrusting, folding, cleavage development), and lithologic similarities, we propose that the UmA contains assemblages that are consistent with Taconic-aged orogenic events in NE US, Canada and east Greenland.