Abstract The 170 Ma Straumsvola nepheline syenite complex in western Dronning Maud Land, Antarctica, is located on the eastern edge of the Penck-Jutul Trough, a major tectonic feature which may be a Palaeozoic-Mesozoic rift system. The 5 km diameter pluton consists entirely of nepheline syenite and can be divided into two volumetrically important units: a relatively structureless outer zone which is overlain by a layered zone. The latter exhibits continuous rhythmic alternations of layers containing different proportions of alkali feldspar to amphibole+Na-rich pyroxene+biotite+nepheline throughout its 350 m thickness. The mafic zone is a volumetrically minor unit which unconformably overlies the layered zone and consists almost entirely of mafic minerals and nepheline. The layered zone shows no systematic stratigraphic variation in major or trace composition or mineral chemistry which is interpreted as being due to a combination of migration of intercumulus liquid and action of deuteric fluids. The remarkably constant thickness of successive layers throughout the layered zone suggests that layering resulted from an internally self-regulating process(es). The layering is defined by changing proportions of alkali feldspar to mafic minerals+nepheline which is consistent with layering being caused by differences in nucleation rate during eutectic crystallization. Dykes associated with the complex show wide-ranging compositions and include both over-and undersaturated types. Low σ 18 O values of the nepheline syenites (mean 5.9%, n = 9) suggest that the magma from which the nepheline syenites crystallized was mantlederived. Peralkaline microgranite dykes have higher σ 18 O values (mean 7.3%, n = 4) and it is suggested that they are either undersaturated liquids contaminated by siliceous crust, or derived by partial melting of partly fenitized gneiss. Oxygen isotope ratios of the surrounding gneiss indicate that the intrusion of the nepheline syenite did not cause extensive circulation of meteoric or magmatic hydrothermal fluids.
Chlorine-rich fluid or melt activity in pelitic gneisses during near-peak metamorphism at ca. 600 Ma has been detected throughout the Sor Rondane Mountains, East Antarctica (Higashino et al., 2012). Here we report an example of a Cl-rich fluid activity during retrograde metamorphism from Brattnipane (Koyubi-one). New example was found as a vein developed in a mafic, garnet-orthopyroxene gneiss (TK2009121002c). The vein is about 1 cm thick, discordantly cutting the penetrative gneissosity in this area. Axial plane of the isoclinal to tight fold that is parallel to the penetrative gneissosity is also discordantly cut by the same vein. The vein mainly consists of garnet, amphibole, biotite, quartz without orthopyroxene. On the other hand, the host gneiss additionally contains plagioclase and orthopyroxene, and lacks quartz. Garnet is weakly zoned, Mn content is lower and Mg content is higher in the core. Development of the higher Mn, lower Mg rim is strong in the vein and gets weaker distant from the vein. Plagioclase distant from the vein shows weak zoning of rimward decrease of Ab content. This plagioclase is termed ‘core’ hereafter because nearer to the vein, replacement/overgrowth by more albitic plagioclase (termed ‘rim’) on such a plagioclase ‘core’ becomes common. This core/rim boundary is sharp, and the ‘core’ is almost absent in the plagioclase in the vein. The garnet-hornblende Fe-Mg geothermometer (Ravna, 2000) applied to the garnet-hornblende pair that coexist in the center of the vein gave 830 o C and the pair that are about 2 cm away from the vein center gave 690 o C, respectively. Coexistence of garnet, orthopyroxene and Cl-poor hornblende off the vein center is supported from the microstructural constraints, so pressure condition is estimated using the garnet-orthopyroxene geobarometer (Harley, 1984), which gave 8.5-9.5 kbar at 690-830 o C. The high-temperature result in the center of the vein and the lower-temperature obtained off the vein center could either represent a real temperature gradient, or a result of Mg-Cl avoidance rule in amphibole, because XMg of amphibole decreases towards the vein center where Cl content of the amphibole is high. If the Mg avoidance rule is not considered, the temperature condition of 690 o C could represent the closure temperature condition of the system. Therefore, this result suggests that the Clrich fluid of ca. 830 o C infiltrated when the wall rock was cooled below 690 o C. This result, combined with the mode of occurrence of the vein cutting the penetrative gneissosity, suggests that the vein was formed during relatively early stage of the retrograde metamorphism. Since biotite and amphibole near the vein have higher Cl content than those occur distant from the vein, it is evident that infiltration of Cl-bearing fluid occurred during the vein formation. The fugacity ratio of the fluid that coexisted with the Cl-rich biotite, log[fHCl/fH2O] (Selby & Nesbitt, 2000) is estimated to be -2.97 to -2.46 at 690 o C. This value is almost the same as or slightly higher than the log[fHCl/fH2O] value estimated for the pelitic gneiss that experienced the infiltration of Cl-rich fluid or melt (Higashino et al., 2012). Plagioclase zoning implies that supply of Na from the vein to the host rock also occurred, and therefore, we consider that one of the cations that was mobile with Cl was Na. Manganese, although minor, could be another element that was introduced by the Cl-rich fluid activity because garnet near the vein shows development of higher Mn rim compared to the garnet distant from the vein.
Abstract New 40 Ar/ 39 Ar data from dykes intruded into Sverdrupfjella and Ahlmanryggen, Dronning Maud Land, Antarctica, indicate that dyke emplacement commenced at c. 207 Ma and lasted until c. 178 Ma. Whereas the ages ascribed to the Karoo-age magmatism contributing to Gondwana breakup are typically inferred as being c. 182 Ma, the data indicate that ages older than c. 192 Ma in the broader Karoo Province are restricted to western Dronning Maud Land, Antarctica, indicating the locality where breakup was initiated. Limited palaeomagnetic data from c. 178–185 Ma dykes combined with published palaeomagnetic data from similar-aged dykes in Vestfjella and the Ferrar Province, suggest that Antarctica had already drifted/rifted significantly away from southern Africa from c. 207 to c. 180 Ma, earlier than previously thought. The data, if correct, require a re-evaluation of the ages ascribed to ocean-floor anomalies used to constrain reconstructions of Gondwana and may provide insight into the history of microcontinental blocks including the Falkland Islands, Haag nunataks, Ellsworth–Whitmore block and Maurice Ewing Bank. Supplementary material: Petrography, Ar isotope data from all samples and summary of ages are available at https://doi.org/10.6084/m9.figshare.c.5612838
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