Geochemistry and petrology of the ferropicrite dikes and associated rocks of Vestfjella, western Dronning Maud Land, Antarctica

This study provides insights into the composition and origin of ferropicrite dikes (FeOtot = 13 17 wt. %; MgO = 13 19 wt. %) and associated meimechite, picrite, picrobasalt, and basalt dikes found at Vestfjella, western Dronning Maud Land, Antarctica. The dikes crosscut Jurassic Karoo continental flood basalts (CFB) that were emplaced during the early stages of the breakup of the Gondwana supercontinent ~180 Ma ago. Selected samples (31 overall from at least eleven dikes) were analyzed for their mineral chemical, major element, trace element, and Sr, Nd, Pb, and Os isotopic compositions. The studied samples can be divided into two geochemically distinct types: (1) The depleted type (24 samples from at least nine dikes) is relatively depleted in the most incompatible elements and exhibits isotopic characteristics (e.g., initial εNd of +4.8 to +8.3 and initial 187Os/188Os of 0.1256 0.1277 at 180 Ma) similar to those of mid-ocean ridge basalts (MORB); (2) The enriched type (7 samples from at least two dikes) exhibits relatively enriched incompatible element and isotopic characteristics (e.g., initial εNd of +1.8 to +3.6 and initial 187Os/188Os of 0.1401 0.1425 at 180 Ma) similar to those of oceanic island basalts. Both magma types have escaped significant contamination by the continental crust. The depleted type is related to the main phase of Karoo magmatism and originated as highly magnesian (MgO up to 25 wt. %) partial melts at high temperatures (mantle potential temperature >1600 °C) and pressures (~5 6 GPa) from a sublithospheric, water-bearing, depleted peridotite mantle source. The enriched type sampled pyroxene-bearing heterogeneities that can be traced down to either recycled oceanic crust or melt-metasomatized portions of the sublithospheric or lithospheric mantle. The source of the depleted type represents a sublithospheric end-member source for many Karoo lavas and has subsequently been sampled by the MORBs of the Indian Ocean. These observations, together with the purported high temperatures, indicate that the Karoo CFBs were formed in an extensive melting episode caused mainly by internal heating of the upper mantle beneath the Gondwana supercontinent. My research supports the view that ferropicritic melts can be generated in several ways: the relative Fe-enrichment of mantle partial melts is most readily achieved by (1) relatively low degree of partial melting, (2) high pressure of partial melting, and (3) melting of enriched source components (e.g., pyroxenite and metasomatized peridotite). Ferropicritic whole-rock compositions could also result from accumulation, secondary alteration, and fractional crystallization, however, and caution is required when addressing the parental magma composition.

Suomussalmen varhaisproterotsooisten diabaasien mineralogia tektonisessa tulkinnassa

The object of this research is to study the mineralogy of the diabase dykes in Suomussalmi and the relevance of the mineralogy to tectonic events, specifically large block movements in the Archaean crust. Sharp tectonic lines separate two anomalies in the dyke swarms, shown on a geomagnetic map as positive anomalies. In one of these areas, the Toravaara anomaly, the diabases seem to contain pyroxenes as a main component. Outside the Toravaara anomaly hornblende is the main ferromagnesian mineral in diabases. The aim of this paper is to research the differences in the diabases inside and outside the anomalies and interpret the processes that formed the anomalies. The data for this sudy consist of field observations, 120 thin sections, 334 electron microprobe analyses, 19 whole-rock chemical analyses, a U-Pb age analysis and geomagnetic low-altitude aerial survey maps. The methods are interpretation of field observations, chemical analyses, microprobe analyses of single minerals and radiometric age determination, microscopic studies of the thin sections, geothermometers and geobarometers. On the basis of field observations and petrographic studies the diabases in the area are divided into pyroxene diabases, hornblende diabases and the Lohisärkkä porphyritic dyke swarm. Hornblende diabases are found in the entire study area, while the pyroxene diabases concentrate on the area of the Toravaara geomagnetic anomaly. The Lohisärkkä swarm transects the whole area as a thin line from east to west. The diabases are fairly homogenous both chemically and by mineral composition. The few exceptions are part of rarer older swarms or are significantly altered. The Lohisärkkä dyke swarm was dated as 2,21 Ga old, significantly older than the most common 1,98 Ga swarm in the area. The geothermometers applied showed that the diabases on the Toravaara anomaly were stabilized at a much higher temperature than the dykes outside the anomaly. The geobarometers showed the pyroxenes to have crystallized at varying depths. The research showed the Toravaara anomaly to have formed by a vertical block movement, and the fault on its west side to have a total lateral transfer of only a few kilometers. The formation of the second anomaly was also interpreted to be tectonic in nature. In addition, the results of the geothermobarometry uncovered necessary conditions for the study of diabase emplacement depth: the minerals for the study must be chosen by minimum crystallization depth, and a geobarometer capable of determining the magmatic temperature must be used. In addition, it would be more suitable to conduct this kind of study in an area where the dykes are more exposed.