Kjell Petter Skjerlie

Generation of anorthositic magma by H2O-fluxed anatexis of silica-undersaturated gabbro: an example from the north Norwegian Caledonides

The Skattora migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4% SiO 2 , 0.2% TiO 2 , 23% Al 2 O 3 , 1.8% FeO t , 0.7% MgO, 6.3% CaO, 7.8% Na 2 O, 0.2% K 2 O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An 20–50 ) is the dominant mineral (85–100%) in the dykes and the leucosome, but 0–15% amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H 2 O-fluxed anatexis of the gabbroic host rock.

Volatile phase fluxed anatexis of metasediments during late Caledonian ophiolite obduction: evidence from the Sogneskollen Granitic Complex, west Norway

In the west Norwegian Caledonides minor granitic sheets were intruded during obduction of the c. 443 Ma Solund–Stavfjord Ophiolite Complex. The Sogneskollen Granitic Complex is the largest intrusive complex and consists of several minor lenses and one larger lens of granodiorite. The Sogneskollen Granitic Complex samples generally have high Ba, Sr and Na2O, very low Y values (<7 ppm), negative εNd420 Ma values (c. −7 to −10.1) and 87Sr/86Sr420 Ma ratios of 0.70437–0.70705. The REE patterns are strongly HREE depleted and La/Ybn varies from 0.3 to 36. Most samples display small positive Eu anomalies, but those that are most fractionated have small negative anomalies. The main part of the Sogneskollen Granitic Complex is a white composite granodiorite which is intruded by a suite of grey biotite-granite dykes. It is suggested that this, and other similar granitic sheets in the area, most probably formed by melting of sediments in the presence of a H2O-bearing volatile phase during obduction of the ophiolite. The quartzofeldspathic source sediments most probably contained detritus from long-term Rb-depleted lower continental crust and from an island arc. The sediments were deposited in large amounts in the marginal basin now represented by the Solund–Stavfjord Ophiolite Complex. During obduction the sediments were buried beneath the hot overriding Ophiolite Complex and the associated island-arc, and yielded granitic magmas during partial melting.

Contrasting tonalite genesis in the lyngen magmatic complex, north norwegian caledonides

Abstract The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km2 large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K 2 O Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive eND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K 2 O Rb ratios (average 0.025), and highly variable contents of Na2O, K2O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable eND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H2O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (eND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.

Geochemistry and petrogenesis of extrusive rocks, dykes and high-level plutonic rocks on the island of Oldra, Solund-Stavfjord Ophiolite Complex, western Norway

The island of Oldra, part of the Late Ordovician Solund-Stavfjord Ophiolite Complex of the western Norwegian Caledonides, comprises extrusive rocks, a sheeted dyke complex and high-level gabbros. The metabasalts are of N-MORB affinity, and their Nd isotopic composition ( k Nd = +7.8 to +8.4) indicates generation from a relatively homogeneous, but strongly depleted mantle. Eighty percent of the metabasalts classify as FeTi-basalts (FeO t /MgO >1.75, TiO 2 >2 wt%), but pseudostratigraphically their distribution varies significantly (51% of the extrusive rocks and 92% of the sheeted dykes). The contents of FeO t , MgO, Al 2 O 3 , Cr, Ni and Zr indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase and clinopyroxene at different levels in the mantle and crust (<8 kbar) and magma mixing affected the composition of the rocks. The MgO-Zr relations of the established chemostratigraphy of the extrusive rocks indicate that magma mixing caused significant scatter...

Petrology, tectonics, and hydrothermal alteration of a fossil backarc oceanic crust: Solund-Stavfjord ophiolite complex of the western Norwegian Caledonides- A review

The Late Ordovician Solund-Stavfjord ophiolite complex in southwest Norway developed in a backarc basin through at least two episodes of seafloor spreading. The youngest phase is a structural domain (domain 1) in oceaniccrust that formed along a backarc rift system that opened at an intermediate spreading rate and propagated north-northeastward into preexisting oceanic crust (domain 2). These two domains are separated by a 1-km-wide zone of anomalous oceanic crust, with fracture-zone affinity (domain 3), that is composed of strongly sheared gabbros and metabasalt dikes hosting abundant serpentinite bodies. Domains 1 and 2 contain high-level gabbro, sheeted dikes, a transition zone of dikes and volcanic rocks, and a volcanic suite consisting of various proportions of pillow lavas, massive lava flows, and hyaloclastite breccias. The metabasalts of both domains have MORB (mid-oceanic-ridge basalt) composition, with minor evidence of an island-arc component, and those of domain 1 are dominantly Fe-Ti basalts. Their contents of FeO t (total Fe as FeO), MgO, Al 2 O 3 , Cr, Ni, and Zr indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase, and clinopyroxene at different levels in the mantle and crust (<8 kbar), and magma mixing controlled the composition of the metabasalts. The high proportion of massive lava flows, the lack of pervasive seafloor extensional tectonic deformation, and the hydrothermal history of the fossil oceanic crust in domain 1 are best comparable to the features of young oceanic crust of the Costa Rica Rift. The Solund-Stavfjord ophiolite complex is conformably overlain by (1) a heterogeneous suite (Stavenes Group) of quartz-dominated metasediments hosting pillow lavas, volcaniclastic rocks, and minor intrusions of island-arc tholeiite-like composition with a clear subduction signature (the Heggoy Formation) and (2) the supposedly later calc-alkalic and alkalic rocks of the Hersvik and Smelyaer units, respectively. This progressive change in chemical composition of the igneous rocks stratigraphically upward within the sedimentary cover indicates the development, through time, of a mature arc in the subduction-zone environment represented by the Solund-Stavfjord ophiolite complex. The mode and nature of magmatic, tectonic, and hydrothermal processes that operated during the evolution of the complex in a backarc basin are similar to those observed from modern mid-ocean ridge settings, particularly at intermediate-spreading ridge axes. The geologic features of the sedimentary cover of the Solund-Stavfjord ophiolite complex and the underlying Sunnfjord melange suggest that the inferred Caledonian backarc basin evolved in proximity to a continental landmass, reminiscent of the tectonics of the modern Andaman Sea region.