Representative analyses of pre-tectonic metabasic rocks and metapelitic gneiss from Sverdrupfjella, East Antarctica

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic-Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U-Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar-Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P-T-t evolution than previously assumed. A c. 540?Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of Pan-African age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet-omphacite assemblage within strain-protected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T = 687-758°C, P = 9·4-11·3?kbar) were attained subsequent to decompression from P > 12·9?kbar. By analogy with limited U-Pb single zircon age data and on circumstantial textural grounds, this earlier eclogite-facies metamorphism is ascribed to subduction and accretion around 565?Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of post-orogenic granite at c. 480?Ma. The recognition of extensive Pan-African tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035?Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140?Ma volcanic arc and 1072 ± 10?Ma granite.

Tab. 1+2: Representative electron-microprobe analyses of minerals from Du Toit Nunataks

An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time. The cordierite-bearing rocks constitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite. The predominant mineral assemblage in the cordierite-bearing rocks is chemically homogeneous cordierite (XMg 0.61) and biotite (XMg 0.47), strongly zoned garnet (XMg 0.18-0.11), sillimanite, K-feldspar (Or81-94Ab5-18An0.6), plagioclase (An28), and quartz. Inclusions of sillimanite and biotite relics in both garnet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + H2O. Various garnet-biotite and garnet-cordierite geothermometers and sillimanite-quartz-plagioclase-garnet-cordierite geobarometers yield a continuous clockwise path in the P-T diagram. The P-T conditions for equilibrium between garnet core and cordierite and between garnet core and biotite during peak metamorphism and migmatization were estimated to be 690 °C at 5-6 kb. This was followed by cooling and unloading with continuously changing conditions down to 515 °C at 2-3 kb. This low-pressure re-equilibration correlates with the pressure conditions evaluated by SCHULZE (1989) for the widespread granitic gneisses of the Read Group in the Shackleton Range. The associated relict enderbitic granulites representing low-pressure type granulite (8 kb; 790 °C) are comparable to similar low-pressure granulites from the East Antarctic craton. They were either formed by under-accretion processes after collision (WELLS 1979, p. 217) or they are a product of remetamorphism at P-T conditions intermediate between granulite and amphibolite facies. A model of a multiple imbrication zone with crustal thickening (CUTHBERT et al. 1983) is discussed for the formation of the relict granulites of the central and eastern Read Mountains which show higher pressure conditions (8-12 kb, SCHULZE & OLESCH 1990), indicating a Proterozoic crustal thickness of at least 40 km.

40Ar/39Ar analysis for incremental heating experiments at DSDP Holes 77-537 and 77-538A

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the southeastern Gulf of Mexico. At Hole 538A, located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite displays internally discordant 40Ar/39Ar age spectra, suggesting initial post-metamorphic cooling at about 500 Ma followed by a mild thermal disturbance at about 200 Ma. Biotite from the gneiss yields a plateau age of 348 Ma, which is interpreted to result from incorporation of extraneous argon components when the biotite system was opened during the about 200 Ma thermal overprint. A whole-rich diabase sample from Hole 538A records a crystallization age of 190.4 ± 3.4 Ma. A lower grade phyllitic metasedimentary sequence was penetrated at Hole 537, drilled about 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display internally discordant 40Ar/39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at about 500 Ma. The age and lithologic character of the basement terrane penetrated at Holes 537 and 538A suggest that the drilled fault blocks are underlain by attenuated fragments of continental crust of "Pan-African" affinity. This supports pre-Mesozoic tectonic reconstructions that locate Yucatan in the present Gulf recess during the amalgamation of Pangea.

(Table T1) Radioelement concentration and radiogenic heat production of ODP Holes 173-1067A and 173-1068A

Mean values of radiogenic heat production are derived from measurements of radioelements on the following rock types: amphibolite (0.358 ± 0.118 µW/m**3) and tonalite gneiss (0.802 ± 0.039 µW/m**3) at Site 1067 and serpentinized peridotite (0.0108 ± 0.0003 µW/m**3) at Site 1068. The results for serpentinized peridotite and amphibolite are consistent with previous measurements on samples from Sites 897 and 900. These results suggest that the thin continental crust in this region would contribute very little to the conductive heat flow.

Magnetic viscosity and paleomagnetism of the basement complex at DSDP Hole 77-538A

The basement at Catoche Knoll consists of Paleozoic gneiss and amphibolite intruded by several generations of early Jurassic diabase dikes. Upon exposure to a 1-oersted field for 9 days, the diabase and amphibolite acquire a viscous remanent magnetization (VRM) which ranges from 42 to 2047% of their natural remanent magnetization (NRM). A magnetic field of similar intensity is observed in the paleomagnetic facility of the Glomar Challenger, and it is therefore doubtful if accurate measurements of magnetic moments in such rocks can be made on board unless the facility is magnetically shielded. The significant VRM also indicates the futility of attempting to discern magnetic lineations from an ocean floor composed of such rocks. No strong correlation exists between the Königsberger ratio, which is usually less than 1, and the tendency to acquire a VRM. The VRM decay is typical of a Richter aftereffect, but the relaxation times vary widely among the samples studied. A stable remanence is observed after alternating field demagnetization to 200 Oe. The range of magnetic inclinations in the diabase dikes is consistent with 40Ar/39Ar dates of 190 and 160 Ma. The inclinations suggest that the Catoche Knoll block tilted more than 20° to the north after the final dike intrusion.

Geochemistry and mineralogy of garnet grains from ODP Sample 183-1137A-33R-3,50-53

On Elan Bank, a southwestern promontory of the Kerguelen Plateau in the southern Indian Ocean, we cored an interval of conglomerate and minor sandstone within a thick section of Cretaceous flood basalts. Most of the detritus in these sedimentary rocks is volcanic with the exception of a small amount of conspicuous material of probable continental derivation. The anomalous clasts include several pebbles of gneiss (Nicolaysen et al., 2001, doi:10.1130/0091-7613(2001)029<0235:POPGBG>2.0.CO;2) and garnet sand grains. The presence of continental material on the plateau bears significantly on the interpretation of Indian Ocean basalts (Weis et al., 2001, doi:10.1130/0091-7613(2001)029<0147:OOCCII>2.0.CO;2). The purpose of the present study was to determine the composition of the garnets to provide additional constraints on the nature of the source area.

Geochemistry on rocks from Shackleton Range

During the GEISHA expedition (Geologische Expedition in die Shackleton Range 1987/88), the Pioneers Escarpment was visited and sampled extensively for the first time. Most of the rock types encountered represent amphibolite facies metamorphics, but evidence for granulite facies conditions was found in cores of garnet. These conditions must have been at least partly reached during the peak of metamorphism. For the Pioneers Escarpment a varicolored succession of sedimentary and bimodal volcanic origin is typical. It comprises: quartzites muscovite quartzite, sericite quartzite, fuchsite quartzite, garnet-quartz schists etc.; pelites: mica schists and plagioclase or plagioclase-microcline gneisses, aluminous schists; marls and carbonates: grey meta-limestones, carbonaceous quartzites, but also pure white, often fine-grained, saccharoidal marble, or a variety of tremolite marble, olivine (forsterite) marble, diopside-clinopyroxene-tremolite marble, etc.; basic volcanic rocks: amphibole fels, amphibolite schist, garnet amphibolite, and acidic to intermediate volcanic rocks: garnet-biotite schist, epidote-biotite-plagioclase gneiss, microcline gneiss. These rocks are considered to be a supracrustal unit, called the Pioneers Group. In the easternmost parts of the Pioneers Escarpment, e.g. at Vindberget, nonmetamorphic shales, sandstones and greywackes crop out, which are cover rocks of possibly Jurassic age. These metasediments, which represent a quartz-pelite-carbonate (QPC) association, indicate that deposition took place on a stable shelf, i.e. on the submerged rim of a craton. Marine shallow-water sedimentation including marls and aluminous clays form the protoliths. The volcanics may be part of a bimodal volcanics-arkose-conglomerate (BVAC) association. Geochemical analyses support the assumption of volcanic protoliths. This is demonstrated especially by the elevated amounts of the immobile, incompatible high-field-strength elements (HFSE) Nb, Ta, Ti, Y, and Zr encountered in some of the gneisses. Microscopic investigation suggests the existence of ortho-amphibolites. This is confirmed by the geochemistry. A bimodal volcanic association is evident. The amphibolites plot in both the tholeiite and calc-alkaline fields. The acidic volcanics are mainly rhyolitic. The sediments and volcanics were subjected to conditions of 10-11 kbar and 600°C during the peak of metamorphism, i.e. granulite facies metamorphism, which can be deduced from the Fe mole ratios of 0.71-0.73 in the garnet cores. Due to the relatively low temperatures, no anatectic melting took placc. The rims of the garnets show a Fe mole ratio of 0.84-0.86, and the coexisting mineral association garnet-biotite-staurolite-kyanite indicate amphibolite facies. The thermobarometry shows P-T conditions of 5-6 kbar and 570-580°C for this stage. The metamorphic history indicates deep burial at depths down to 35 km (subduction?) i.e. high pressure metamorphism, followed by pressure release due to uplift associated with retrograde metamorphism. This may have happened during a pre-Ross metamorphic event or orogeny. The Ross Orogeny at about 500 Ma probably just led to the weak greenschist facies overprint that is evident in the rocks of the Pioneers Group. Finally, sedimentation resumed in the area of the present Shackleton Range, or at least in the eastern part of the Pioneers Escarpment, probably when detritus from erosion of the basement (Read Group and Pioneers Group) was deposited, forming sandstones and greywackes of possibly Jurassic age. There is no indication that these sediments belong to the former Turnpike Bluff Group.

Table I. Composition of till in north Greenland

From 1950 through 1900 studies on the glacial geology of northern Greenland have been made in cooperation with the U.S. Air Force Cambridge Research Laboratories. As a result of these studies four distinct phases of the latest glaciation have been recognized. The last glaciation extended over most of the land and removed traces of previous anes. Retreat of the ice mass began some time previous to 6000 years ago. This was followed by a rtse in sea level which deposited clay-silt succeeded by karne gravels around stagnant ice lobes in the large valleys. Marine terraces, up to 129 meters above present sea level, developed as readjustment occurred in the land free of ice. About 3700 years ago an advance of glaciers down major fjords took place followed by retreat to approximately the present position of the ice. Till in Peary Land, north of Frederick E. Hyde Fjord, contains only locally derived matertals indicating that the central Greenland ice cap did not cover the area.