Granulites and garnet–cordierite gneisses from Dronning Maud Land, Antarctica

Abstract The central sector of Mühlig-Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene-bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite-bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K-feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K-feldspar combined with interaction with an H₂O-rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high-grade K-feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re-equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.     

The Petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica

The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (SiO₂ 49?3–51?6 wt.%; MgO 5?1–6?6 wt.%),but the concentrations of certain incompatible elements (e.g.,Zr) vary by factors of approximately two or more. Most interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R₀=0?7049–0?7065) may beexplained by 7% contamination by crustal material with an R₀of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (¹⁸O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland.     

Geochemistry of basalt lavas from Vestfjella and adjacent areas,Dronning Maud Land,Antarctica

KAr ages, major- and trace-element compositions, and Sr-isotope data are presented for basalt lavas from Vestfjella, Dronning Maud Land, Antarctica. The new conventional KAr age data have yielded ages from 171 ± 2 to 695 ± 11 Ma, but the youngest (i.e. Middle Jurassic) ages are preferred. Mineralogical and chemical data show that the majority of the basalts are tholeiites. Petrographic mixing calculations, REE modelling, and the Sr isotope data suggest that they were derived by partial melting of garnet-free lherzolites with variable REE patterns, and subsequently modified by fractionation of olivine, Ca-rich pyroxene and plagioclase. Incompatible trace-element data from nearby Middle Jurassic basalt lavas (from Kirwanryggen and Heimefrontfjella) suggest a different source and REE modelling indicates generation from garnet lherzolites.     

Petrology and geochemistry of Jurassic basalt dykes from Vestfjella, Dronning Maud Land, Antarctica

Mineral and whole rocks analyses of 12 Jurassic basalt dykes from Vestfjella, Dronning Maud Land, Antarctica, are presented, and their genesis discussed. On the basis of major oxides and norms the basalts may be classified as olivine and quartz tholeiites. Plotted in the Plag---Cpx---(Opx + 4Q) and Ol---Plag---Q projections, the compositions are most compatible with fractional crystallization of olivine, clinopyroxene and plagioclase from a basalt liquid at very low pressure. The ratios between strongly incompatible elements such as Rb, Cs, Zr, Hf, Ta and Th vary considerably, and petrographic mixing calculations give poor fits with respect to Rb, Cs, Ta, Th and light REE. Initial ⁸⁷Sr/⁸⁶ Sr ratios range between 0.70347 and 0.70687, and show no correlation with Rb/Sr or any other SIE ratios. The trace element and Sr isotope data thus do not suggest any simple cogenetic petrogenetic model. It is concluded that the basalt melts most plausibly have been contaminated by, or mixed with anatectic melts of crustal material, rather than reflecting mantle heterogeneity.     

The geology and geochronology of the Annandagstoppane granite,Western Dronning Maud Land,Antarctica

The Annandagstoppane Granite is exposed at three nunataks in Western Dronning Maud Land, Antarctica. It comprises medium- to coarse-grained granite crosscut by veins of pegmatite and graphic granite and has many S-type characteristics such as containing normative corundum greater than 1.1%, molecular Al₂O₃/(CaO+K₂O+Na₂O) greater than 1.1 and very little zircon. Hydrothermal alteration in the Granite is variably developed and has affected only certain minerals in any phase. R-Sr and Pb whole rock and mineral isotopic data suggest: 1) that Sr isotopes within it were nearly homogenized on a whole rock scale about 2823 Ma ago by this hydrothermal alteration; 2) that the Pb isotopic system was also disturbed at that time, and 3) that the Granite may have been was emplaced sometime during the interval 3115 Ma to 2945 Ma ago. The Granite was probably intruded by the Annandagstoppane Gabbro about 1200 Ma ago, resetting the Rb-Sr system in biotite. The Annandagstoppane Granite may form part of a basement complex to the Proterozoic sedimentary, volcanic and mafic igneous rocks exposed to the east in the Ahlmannryggen and the Borgmassivet. Its chemical composition and geologic history appears to be unique in Antarctica and in the Kaapvaal Craton of Southern Africa, consistent with the possibility that the Annandagstoppane Granite is part of a crustal fragment that joined Antarctica relatively late in the history of that continent.     

Geochemistry of the Mesozoic regional basic dykes of western Dronning Maud Land,Antarctica

Regional dolerite dykes of Mesozoic age in western Dronning Maud Land are variable in both major and trace element composition and include picritic types (MgO>18 wt%). The range in incompatible element concentrations is considerable (e.g. Zr 40–478 ppm) and shows little correlation with MgO content. Both high-and low-Ti, Zr (HTZ and LTZ) magma types are present and there is a spread of compositions between these types. Major element oxide variations in dykes having MgO>10 wt% indicate that olivine and orthoyproxene fractionation occurred, presumably at an early high-pressure stage of magma evolution. Major element oxide variations in dykes having MgO<10 wt% indicate control by olivine and clinopyroxene. A minority of the more evolved dykes are compositionally similar to the nearby Kirwan basalts, but the majority cannot be related to the Kirwan basalts by any simple petrogenetic process as they contain higher concentrations of incompatible elements and have higher Mg-numbers. The HTZ Dronning Maud Land dolerites have incompatible trace element concentrations which are very similar to the HTZ basalt magma types of the Karoo of southern Africa with the exception of lower K and Rb in DML dolerites. The HTZ dolerites occur in the part of Dronning Maud Land which appears to have been tectonically stable since the Archaean and are not found to intrude the surrounding high-grade (about 1000 Ma) metamorphic rocks of the Sverdrup Group. These data provide qualified support for models which seek to relate spatially the HTZ Mesozoic basalt types of Gondwana to sources beneath stable Archaean cratons.     

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.     

Petrogenesis of peralkaline granite dykes of the Straumsvola complex,western Dronning Maud Land,Antarctica

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI?=?molecular Al/[Na?+?K]) of the dykes is 1.20 (n?=?29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171?±?4.4 Ma, with variable initial ⁸⁷Sr/⁸⁶Sr ratio (0.7075?±?0.0032), and have an average ε _Nd of ? 12.0. Quartz phenocrysts have δ¹⁸O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ¹⁸O values of quartz and aegirine (average Δ_{quartz?aegirine} = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε _Nd (< ? 10) and magma δ¹⁸O values averaging 8.6‰ (assuming Δ_quartz?magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ¹⁸O values and much less negative ε _Nd (average ??3.1, n?=?3). The country rock has similar δ¹⁸O values to the granite dykes (average 8.0‰, n?=?108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ¹⁸O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.     

Influence of climatic teleconnections on the temporal isotopic variability as recorded in a firn core from the coastal Dronning Maud Land,East Antarctica

Ice and firn core studies provide one of the most valuable tools for understanding the past climate change. In order to evaluate the temporal isotopic variability recorded in ice and its relevance to environmental changes, stable isotopes of oxygen and hydrogen were studied in a firn core from coastal Dronning Maud Land, East Antarctica. The annual δ ¹⁸O profile of the core shows a close relation to the El Niño Southern Oscillation (ENSO) variability. The ENSO indices show significant correlation with the surface air temperatures and δ ¹⁸O values of this region during the austral summer season and support an additional influence related to the Southern Annular Mode (SAM). The correlation between the combined ENSO-SAM index and the summer δ ¹⁸O record seems to have been caused through an atmospheric mechanism. Snow accumulation in this region illustrates a decreasing trend with opposite relationships with δ ¹⁸O data and surface air temperature prior and subsequent to the year 1997. A reorganization of the local water cycle is further indicated by the deuterium excess data showing a shift around 1997, consistent with a change in evaporation conditions. The present study thus illustrates the utility of ice-core studies in the reconstruction of past climate change and suggests possible influence of climatic teleconnections on the snow accumulation rates and isotopic profiles of snow in the coastal regions of east Antarctica.     

New aeromagnetic view of the geological fabric of southern Dronning Maud Land and Coats Land,East Antarctica

The ice shield of Antarctica, which measures several kilometers in thickness, presents a challenge when attempting to unravel the subglacial geology. Here, we report about systematic airborne magnetic surveys conducted over the last decade, which investigated a significant part of Dronning Maud Land (DML), imaging for the first time the crustal architecture of the interior of this sector of East Antarctica. High-resolution data reveal parallel, elongated magnetic anomalies in southeastern DML. These NW–SE trending anomalies can be traced farther east into sparser Russian magnetic data sets. Several high amplitude magnetic anomalies with values above 400 nT have been observed in southwestern DML and Coats Land. They differ clearly in wavelength and amplitudes from the magnetic pattern found in the east and do not show any evidence of a Pan-African orogenic belt or suture zone connecting the Shackleton Range with eastern DML, as hypothesized in several studies. This leads to the assumption of the existence of a hitherto unrecognized large tectonic province in southeastern DML. Whereas an over 100 km long magnetic lineament in the interior of the Dronning Maud Land may reflect a major shear zone akin to the Pan-African age Heimefrontfjella shear zone. Both findings bring new evidences to the still open question about the amalgation of East and West Gondwana. In addition, the magnetic data allow mapping the eastern extent of the presumable cratonic province of Coats Land, a region considered as a key piercing point for reconstructions of Rodinia. Furthermore, the Beattie Magnetic Anomaly in southern Africa is assumed to continue into East Antarctica. Two magnetic highs in western DML are identified as possible eastward continuation of this prominent anomaly.     

Neoproterozoic/Lower Palaeozoic Events in Central Dronning Maud Land (East Antarctica)

New geochronological data indicate that central Dronning Maud Land in East Antarctica underwent polyphase Neoproterozoic/Lower Palaeozoic metamorphism that can be correlated with the final amalgamation of E- and W-Gondwana. Central Dronning Maud Land most probably represents part of the southern continuation of the Mozambique Belt into E-Antarctica. The Neoproterozoic/Lower Palaeozoic metamorphism is preceded by a period of anorogenic anorthosite-charnockite magmatism at ca. 600 Ma. Polyphase metamorphism is recorded from ca. 580 to 515 Ma. Voluminous syntectonic magmatism has been documented at ca. 530 Ma, which is probably the most voluminous Neoproterozoic/Lower Palaeozoic syntectonic magmatism so far recorded in E-Antarctica. The Neoproterozoic/Lower Palaeozoic structural evolution evolved in an overall sinistral transpressional setting, and thus can be correlated with the broad tectonic setting of the Mozambique Belt in Africa.     

Reconstruction of Antarctic climate change using ice core proxy records from the coastal Dronning Maud Land,East Antarctica

Antarctic ice core records have provided unprecedented information on past climatic changes and forcing factors on decadal to millennial timescales. The glaciochemical and stable isotope records of a shallow ice core from the coastal Dronning Maud Land (East Antarctica) were used here to reconstruct the coastal Antarctic environmental variability during the past ∼470 years. Sea salt ion data indicate a significant additional contribution of chloride ions compared to sea water values, possibly through atmospheric scavenging. The nitrate (NO₃ ⁻) profile exhibit significant temporal shifts than that of the sulphate (SO₄ ²⁻), with a major shift around 1750 AD. The changes in NO₃ ⁻ record are synchronous with the proxy record of solar activity (¹⁰Be profile from a South Pole ice core), suggesting enhanced NO₃ ⁻ values during periods of reduced solar activity like the Dalton Minimum (∼1790–1830 AD) and Maunder Minimum (∼1640–1710 AD). The δ¹⁸O records reveal that the more negative δ¹⁸O values were coeval with several events of increased NO₃ ⁻ concentrations, suggesting enhanced preservation of NO₃ ⁻ during periods of reduced air temperatures. The δ¹⁸O and δD records of the core also suggest significant short-term and long-tem variability with more negative values indicating relatively lower air temperatures prior to 1715 AD. The δ¹⁸O records also revealed a significant warming of 2.7°C for the past 470 years, with a warming of ∼0.6°C per century.     

Flood Basalts of Vestfjella: Jurassic Magmatism Across an Archaean-Proterozoic Lithospheric Boundary in Dronning Maud Land, Antarctica

Continental flood basalts (CFBs) of Jurassic age make up theVestfjella mountains of western Dronning Maud Land and demonstratean Antarctic extension of the Karoo large igneous province.A detailed geochemical study of the 120-km-long Vestfjella rangeshows the CFB suite to consist mainly of three intercalatedbasaltic rock types designated CT1, CT2 and CT3 (chemical types1, 2 and 3) that exhibit different incompatible trace elementratios. CT1 and CT2 of north Vestfjella record wide ranges ofNd and Sr isotopic compositions with initial     

The glacially sculptured landscape in Dronning Maud Land, Antarctica, formed by wet-based mountain glaciation and not by the present ice sheet

The glacial landscape beneath the Maudheimvidda ice sheet in East Antarctica was most probably formed during a more temperate phase of Antarctic glaciation than the present. Overdeepened glacial cirques and U-shaped valleys are found in the Heimefrontfjella and Vestfjella mountain ranges. These glacial landforms, located beneath the ice sheet, have been mapped with radio-echo sounders. The present ice sheet covering these landforms is cold and frozen to its bed, and has a negligible erosive effect on the substrate. Ice sheet thickening during the Quaternary glacial periods is not believed to have caused any significant increase in erosion at the investigated sites. Instead, the glacial morphology was most likely formed by smaller, temperate glaciers when the Antarctic climate was warmer than at present. Datings of foraminifera and ash layers from the Transantarc-tic Mountains indicate that the present cold ice sheet was formed 2.5 Ma years ago. Other studies imply that a cold Antarctic ice sheet has lasted even longer. The glacial landforms in Maudheimvidda may thus be of a pre-Quaternary age.     

Subglacial imprints of early Gondwana break-up as identified from high resolution aerogeophysical data over western Dronning Maud Land, East Antarctica

Determining the location and geometry of possible subglacial rifts in western Dronning Maud Land is a key element to address processes leading to early Gondwana break-up. However, previous geophysical investigations did not lead to unambiguous delineation of rift structures over this region. We interpret high-resolution airborne radar and aerogravity data to image subglacial rift structures. Subglacial topography, free-air and Bouguer gravity maps, coupled with 3D inverse gravity models, image a rift–rift–rift triple junction at the intersection of the Jutulstraumen ice stream and the Pencksökket glacier. These continental rifts were associated with alkaline and tholeiitic intrusions, minor dyke swarms and flood basalts of Jurassic age, but not with huge volumes of Karoo magmatism, such as that which characterizes the southern Africa conjugate margin. The western Dronning Maud Land triple junction may be linked to the Karoo mantle plume and represents an early stage of magmatism and rifting during Gondwana break-up.     

Evidence for constitutional supercooling from orthopyroxene - pigeonite - plagioclase relations in the Borgmassivet suite of western Dronning Maud Land,Antarctica

Summary The Kullen-type sills of western Dronning Maud land are up to 400 m thick and form part of the Meso- to Neoproterozoic Borgmassivet suite. They consist of a Basal, Cumulate, Central and Upper zone. The petrography of the Cumulate zone, with special emphasis on the occurrences at Hammer Heads in the Annandagstoppane area, is described. Microstructures, such as delicate chains of orthopyroxene primocrysts and an increase in grain size of plagioclase chadacrysts from the cores to the margins of postcumulus clinopyroxene oikocrysts, indicatein situ crystallization. Composite grains with orthopyroxene at the core, which is mantled, in turn, by inverted pigeonite, a second generation orthopyroxene and, on the outer margin, clinopyroxene in a reaction relation with the orthopyroxene, are sporadically developed.The relation between the composite structures and plagioclase nucleation and growth is investigated by the study of plagioclase grain size distributions, nucleation density and An compositions of plagioclase cores, which indicate that two periods of plagioclase nucleation occurred. The first period of nucleation, represented by plagioclase chadacrysts enclosed in primocrysts of orthopyroxene, occurred during supercooling following intrusion. Release of latent heat of crystallization resulted in an increase in magma temperature and a normal sequence of fractional crystallization in which orthopyroxene crystallization was succeeded by pigeonite. The second period of plagioclase nucleation depleted the interface liquid in the zone of crystallization of calcium. The composition of the interface liquid was thus changed from the compositional field from which pigeonite crystallized to that from which orthopyroxene crystallized (cf the liquidus boundaries in the system CaMgSi₂O₆-Mg₂SiO₄SiO₂ ofKushiro, 1972). This constitutional supercooling (Chalmers, 1964;Tiller, 1991) resulted in crystallization of orthopyroxene and the reverse crystallization sequence En-rich orthopyroxene -pigeonite - En-rich orthopyroxene, prior to reaction of the last-formed orthopyroxene with the liquid to form clinopyroxene and subsequent inversion of pigeonite. A possible crystallization sequence is discussed by making use of thespeculative Fo-Di-An-SiO₂ (forsterite-diopside-anorthite-silica) system ofMorse (1980).

---

Orthopyroxen-Pigeonit-Plagioklas-Beziehungen in der Borgmassivet Suite des westlichen Dronning Maud Landes, Antarktis: Hinweise auf konstitutionelles Supercooling

Zusammenfassung Lagergänge des Kullen-Types im westlichen Dronning Maud Land sind bis zu 400m mächtig und sind Teil der meso- bis neoproterozoischen Borgmassivet-Abfolge. Sie bestehen aus einer basalen, einer Cumulat, einer zentralen und einer oberen Zone. Die Petrographie der Cumulat-Zone wird mit besonderer Betonung der Vorkommen von Hammer Heads im Annandagstoppane-Gebiet beschrieben. Mikrostrukturen, wie z.B. feinkörnige Ketten von Orthopyroxen-Primokristen und eine Zunahme in der Korngröße von Plagioklas-Chadakristen von den Kernen zu den Rändern in postcumulus Klinopyroxen-Oikokristen, weisen aufin situ Kristallisation hin. Zusammengesetzte Körner mit Orthopyroxen im Kern, der von invertiertem Pigeonit umgeben ist, einer zweiten Generation Orthopyroxen, und, am äußeren Rand, Klinopyroxen in Reaktionsbeziehung mit Orthopyroxen, kommen sporadisch vor.Die Beziehung zwischen diesen zusammengesetzten Strukturen und der Nukleation, sowie dem Wachstum von Plagioklas wurde durch das Studium der Plagioklaskorngrößen, der Nukleationsdichte und der An-Zusammensetzung von Plagioklas-Kernen untersucht. Diese weisen darauf hin, daß zwei Perioden von Plagioklas-Nukleation vorgekommen sind. Die erste Nukleationsperiode wird durch Plagioklas-Chadakristen, die in Primokristen von Orthopyroxen eingeschlossen sind repräsentiert, und fand während einer Phase von Supercooling im Anschluß an die Intrusion statt. Das Freiwerden latenter Kristallisationswärme führte zu einer Zunahme der Magmentemperatur und zu einer normalen Abfolge fraktionierter Kristallisation, in der auf die Orthopyroxen-Kristallisation Pigeonit folgte. Die zweite Phase der Plagioklasnukleation führte zu einer Verarmung der Schmelze an Kalzium in der Kristallisationszone. Die Zusammensetzung der Schmelze bewegte sich so aus dem Pigeonitfeld, in das, aus dem Orthopyroxen kristallisierte (siehe die Liquidus Grenzen im System CaMgSi2O₆-Mg₂SiO₄-SO₂ von Kushiro, 1972). Dieses konstitutionelle Supercooling (Chalmers, 1964;Tiller, 1991) führte zur Kristallisation von Orthopyroxen, und der reversen Kristallisationsabfolge En-reicher Orthopyroxen-Pigeonit-En-reicher Orthopyroxen, noch vor der Reaktion des zuletzt gebildeten Orthopyroxenes mit der Schmelze, das zur Entstehung von Klinopyroxen und zur anschließenden Inversion von Pigeonit führte. Eine mögliche Kristallisationsabfolge wird auf der Basis desspekulativen Fo-Di-An-SiO₂ (Forsterit-Diopsid-Anorthit-Quartz) Systems von Morse (1980) diskutiert.

---

---

With 12 Figures