MIGMATIZATION ASSOCIATED WITH EMPLACEMENT OF MAFIC PLUTONS, HELGELAND NAPPE COMPLEX, NORWEGIAN CALEDONIDES

Migmatitic rocks constitute and important part of the Helgeland Nappe Complex (HNC), the structurally highest part of the Uppermost Allochthon of the Norwegian Caledonides. Many of the imbricate, fault-bounded units of the HNC are predominantly migmatitic but these are juxtaposed against units with non-migmatitic rocks of the same bulk compositions. Thus, the timing, thermal history, and consequent melt productivity of these units provide important constraints on Caledonian magmatism. In the Velfjord area, dioritic to monzonitic plutons were emplaced into a terrane that consists of migmatitic metapelite and metasandstone plus intercalated marble. The nature of the pelitic migmatites changes with proximity to the plutons, from distal stromatic migmatite with local, late-stage, leucocratic dikes--to intercalated diatexitic and stromatic migmatite--to completely diatexitic migmatite. Locally, the contact zone is intruded by feldspar-phyric granitic to tonalitic rocks that contain accessory garnet and sillimanite (“contact granites”). The P-T-t history of these rocks suggests emplacement of the plutons at about 7 kbar (local kyanite stability). Pluton emplacement was apparently associated with regional migmatization. Exhumation was synchronous with, or immediately followed pluton emplacement, such that some late-stage leucosomes crystallized cordierite. The broad types of leucosome identified are: (1) early? felsic layers in stromatic migmatites, (2) contact granites, (3) leucocratic tonalitic to granitic segregations in diatexites, and (4) latest stage leucocratic tonalitic veins and dikes. Locally, contact granites and late-stage tonalitic dikes show mutual intrusive relations. When compared to experimental results, leucosomes (1) and (2) are typical of dehydration melting of graywacke-like compositions. In contrast, leucosomes (3) and (4) are typical of H2O-saturated melting of semipelites. These data suggest that the early leucosomes formed during peak metamorphic conditions and represent relatively efficient extraction from a diatexitic host. The later leucosomes probably formed during exhumation, as melt trapped in the diatexites began to crystallize and released H2O. This excess H2O led to local, water-saturated remelting and the formation of leucotonalitic magmas.