New geochemistry and geochronology data from the Prøven Igneous Complex, North-West Greenland

The Paleoproterozoic Prøven Intrusive Complex in North-West Greenland is the largest magmatic intrusion in Greenland. It is dominated by orthopyroxene-bearing granitic to granodioritic rocks, collectively known as charnockites, with subordinate mafic and ultramafic components. Detailed mapping of the complex was undertaken by GGU in the 1960 and 1970’s but was only much later followed by modern geochemical and isotopic investigations that established a Paleoproterozoic emplacement age. Despite the recognised significance of the PIC to understanding the Rinkian Orogen, a precise geo-tectonic association has remained enigmatic. The PIC was variably ascribed to anorogenic, rift-related, or syn-collisional settings. To finally address this uncertainty, we report a comprehensive geochemical and geochronological dataset based on old and newly collected samples. In tandem with this, a structural analysis of the PIC was conducted based on interpretation of archive maps and supported by new reconnaissance fieldwork and mapping. This has showed that the PIC was emplaced as sheet-like bodies into metasedimentary strata of the Karrat Group near the contact to the underlying Archean basement, and that it later underwent a prolonged phase of ductile deformation and partial migmatisation. The 1:100 000 scale mapping has identified two main charnockitic units: (1) a massive upper part constituted by massive coarse-grained granitic charnockite dominated by magmatic state fabrics with a weak to moderate overprinting, and (2) a heterogeneous, pervasively migmatised charnockite unit that represents the floor of the complex and that contains mafic enclaves and inclusions of wall rock lithologies and intrusive leucogranite sheets. The ductile deformation led to flattening and transposition of the inclusions and enclaves along the basal contact, supplying unique guidance for the mapping. The roof of the complex is not preserved. Geochemically the PIC forms a calc-alkalic series with metaluminous to mildly peraluminous compositions encompassing both magnesian and ferroan granitoid types. The magnesian group has a typical I-type geochemical signature, while the ferroan group has some similarities to A2-type (post-orogenic) granites. The new geochronology data expands the recorded time span for PIC magmatism and shows three intrusive phases: an early phase at ca. 1900 Ma (largely obliterated), a main phase at ca. 1875 Ma, and a late phase at ca. 1850 Ma. The two older phases are exclusively magnesian and dominate the central and southern part of the complex. The youngest phase mostly includes ferroan granitoid types and occurs mainly in the northern part. The regional peak metamorphism reached granulite facies during the collisional stage of the Rinkian orogen at ca 1830 Ma which caused widespread migmatisation as witnessed by injection of leucogranite sheets of this age. The new geochemical and geochronological data indicate that the PIC was emplaced in a subduction setting over a 50 Myr period leading up to the Rinkian orogen, in a generally NW-ward retreating arc system.