Mineral and chemical composititon of sediments from ODP Hole 119-740A

ODP Hole 740A is located on the inner part of the East Antarctic continental shelf in Prydz Bay, at the seaward end of a major onshore rift structure known as the Lambert Graben. Drilling at this site led to the recovery of some 65 m of continental sediments (Prydz Bay red beds) that form part of a much thicker (2-3 km) pre-continental breakup sequence, the development of which may be related to the initiation and rifting of the Lambert Graben. Palynological and paleomagnetic studies have not been able to determine the age of the sediments; they may be equivalent to the onshore late Permian Amery Group or younger. The succession consists predominantly of sandstone, siltstone, and claystone arranged in erosively based, pedogenically influenced fining-upward sequences up to 5 m thick. These were deposited by shallow, braided streams draining an extensively vegetated alluvial plain, with sufficient topographic relief to trap fine-grained sediment and inhibit rapid channel shifting. Pedogenic processes were initiated on the alluvial plain, but climatic conditions were generally unsuitable for extensive pedogenic carbonate formation and the development of mature soil profiles. The sediments were probably derived from a rapidly uplifted fault block terrain composed of upper Proterozoic and Archaean gneisses lying to the southeast of the depositional site. Uplift may have taken place along the tectonically active seaward extension of the eastern faulted margin of the Lambert Graben, which passes immediately southeast of Hole 740A. Differences in mineralogical composition between the Amery Group and the Prydz Bay red beds probably reflect differences in rock composition in the source area. The age of the Prydz Bay red beds has still to be resolved.

Geochemistry and age determination of zircons in Cretaceous to Quaternary sediments

In central Antarctica, drainage today and earlier back to the Paleozoic radiates from the Gamburtsev Subglacial Mountains (GSM). Proximal to the GSM past the Permian-Triassic fluvial sandstones in the Prince Charles Mountains (PCM) are Cretaceous, Eocene, and Pleistocene sediment in Prydz Bay (ODP741, 1166, and 1167) and pre-Holocene sediment in AM04 beneath the Amery Ice Shelf. We analysed detrital zircons for U-Pb ages, Hf-isotope compositions, and trace elements to determine the age, rock type, source of the host magma, and "crustal" model age (T(C)DM). These samples, together with others downslope from the GSM and the Vostok Subglacial Highlands (VSH), define major clusters of detrital zircons interpreted as coming from (1) 700 to 460 Ma mafic granitoids and alkaline rock, epsilon-Hf 9 to -28, signifying derivation 2.5 to 1.3 Ga from fertile and recycled crust, and (2) 1200-900 Ma mafic granitoids and alkaline rock, epsilon-Hf 11 to -28, signifying derivation 1.8 to 1.3 Ga from fertile and recycled crust. Minor clusters extend to 3350 Ma. Similar detrital zircons in Permian-Triassic, Ordovician, Cambrian, and Neoproterozoic sandstones located along the PaleoPacific margin of East Antarctica and southeast Australia further downslope from central Antarctica reflect the upslope GSM-VSH nucleus of the central Antarctic provenance as a complex of 1200-900 Ma (Grenville) mafic granitoids and alkaline rocks and older rocks embedded in 700-460 Ma (Pan-Gondwanaland) fold belts. The wider central Antarctic provenance (CAP) is tentatively divided into a central sector with negative ?Hf in its 1200-900 Ma rocks bounded on either side by positive epsilon-Hf. The high ground of the GSM-VSH in the Permian and later to the present day is attributed to crustal shortening by far-field stress during the 320 Ma mid-Carboniferous collision of Gondwanaland and Laurussia. Earlier uplifts in the ~500 Ma Cambrian possibly followed the 700-500 Ma assembly of Gondwanaland, and in the Neoproterozoic the 1000-900 Ma collisional events in the Eastern Ghats-Rayner Province at the end of the 1300-1000 Ma assembly of Rodinia.

Oxygen isotope values from biogenic apatie (conodont elements and fish teeth) from the Lower Triassic Mianwali Formation (Salt Range, Pakistan)

Recovery from the end-Permian mass extinction is frequently described as delayed, with complex ecological communities typically not found in the fossil record until the Middle Triassic epoch. However, the taxonomic diversity of a number of marine groups, ranging from ammonoids to benthic foraminifera, peaked rapidly in the Early Triassic. These variations in biodiversity occur amidst pronounced excursions in the carbon isotope record, which are compatible with episodes of massive CO2 outgassing from the Siberian Large Igneous Province. Here we present a high-resolution Early Triassic temperature record based on the oxygen isotope composition of pristine apatite from fossil conodonts. Our reconstruction shows that the beginning of the Smithian substage of the Early Triassic was marked by a cooler climate, followed by an interval of warmth lasting until the Spathian substage boundary. Cooler conditions resumed in the Spathian. We find the greatest increases in taxonomic diversity during the cooler phases of the early Smithian and early Spathian. In contrast, a period of extreme warmth in the middle and late Smithian was associated with floral ecological change and high faunal taxonomic turnover in the ocean. We suggest that climate upheaval and carbon-cycle perturbations due to volcanic outgassing were important drivers of Early Triassic biotic recovery.

The distributions, C- and O-isotopic compositions, and frequency of giant-ooids

Early Triassic oceans were characterized by deposition of a number of "anachronistic facies", including microbialites, seafloor carbonate cement fans, and giant ooids. Giant ooids were particularly prevalent in Lower Triassic sections across South China and exhibit unusual features that may provide insights into marine environmental conditions following the end-Permian mass extinction. The section at Moyang (Guizhou Province) contains abundant giant ooids ranging in size between 2 and 6 mm (maximum 12 mm) and exhibiting various cortical structures, including regular, deformed, compound, regenerated and "domed". Preservation of ooid cortical structure is generally good as indicated by petrographic observations, and trace element and carbon isotope analyses suggest that diagenesis occurred in a closed diagenetic system. All ooids exhibit fine concentric laminae, frequently alternating between light-colored coarsely crystalline and dark-colored finely crystalline layers probably reflecting variation in organic content or original mineralogy. Under scanning electron microscope, biomineralized filaments or biofilms and tiny carbonate fluorapatite (CFA) crystals are commonly found in the finely crystalline layers. We infer that the precipitation of CFA was related to adsorption of P via microbial activity on the surfaces of ooids following episodic incursions of deep waters rich in carbon dioxide, hydrogen sulfide and phosphate into shallow-marine environments. Giant ooid precipitation may have been promoted in shallow ramp settings during these events by increased watermass agitation and supersaturation with respect to calcium carbonate, as well as reduced carbonate removal rates through biotic skeletal formation. Spatio-temporal distribution data reveal that giant ooids were widespread in the Tethyan region during the Early Triassic, and that they were most abundant immediately after the end-Permian crisis and disappeared gradually as metazoans repopulated marine environments.

SHRIMP zircon geochronology and geochemistry of Phanerozoic granitoids in southeastern Korea

Phanerozoic granitoids are widespread in the Korean Peninsula and form a part of the East Asian Cordilleran-type granitoid belt extending from southeastern China to Far East Russia. Here we present SHRIMP zircon U-Pb ages and geochemical and Nd isotopic compositions of Late Paleozoic to Early Jurassic granitoid plutons in the northern Gyeongsang basin, southeastern Korea; namely the Jangsari, Yeongdeok, Yeonghae, and Satkatbong plutons. The granite and associated gabbroic rocks from the Jangsari pluton were coeval and respectively dated at 257.3 ± 2.0 Ma and 255.7 ± 1.4 Ma. This result represents the first finding of a Late Paleozoic pluton in South Korea. Three granite samples from the Yeongdeok pluton yielded a slightly younger age span ranging from 252.9 ± 2.5 Ma to 246.7 ± 2.1 Ma. Two diorite samples from the Yeonghae pluton gave much younger ages of 195.1 ± 1.9 Ma and 196.3 ± 1.6 Ma. An Early Jurassic age of 192.4 ± 1.6 Ma was also obtained from a diorite sample from the Satkatbong pluton. The mineral assemblage and Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) relationship indicate that all the analyzed plutons are subduction zone granitoids. Enrichments in large-ion-lithophile-elements and depletions in high-field-strength-elements of these plutons are also concordant with geochemical characteristics typical for the subduction zone magma. The presence of Late Permian to Early Triassic arc system is in contrast with the conventional idea that the arc magmatism along the continental margin of the Korean Peninsula has commenced from Early Jurassic after the termination of Triassic collisional orogenesis. The epsilon-Nd(t) values of the granitoid plutons are consistently positive (2.4-4.6), suggesting that crustal residence time of the basement beneath the Gyeongsang basin is relatively short. Moreover, the reevaluation of previously-published data reveals that geochemical compositions of the Yeongdeok pluton are compatible with those of high-silica adakites; La/Yb = 37.5-114.6, Sr/Y = 138.2-214.0, SiO2 = 62.9-72.0 wt. %, Al2O3 = 15.5-17.0 wt. %, Sr = 562-1173 ppm, MgO = 0.4-1.6 wt. %, Y = 3-6 ppm, Yb = 0.18-0.45 ppm, and Eu/Eu* = 0.92-1.31. The occurrence of adakites in southeastern Korea, and presumably in the Hida belt of central-western Japan, is indicative of a hot subduction regime developing at least partly along the East Asian continental margin during the Permian-Triassic transition period.

Cenozoic terrestrial palynomorphs from the late Oligocene to early Miocene section of sediment core CRP-2/2A (Table 1)

Sparse terrestrial palynomorphs (spores and pollen) were recovered from glacigene Lower Miocene and Oligocene core samples from the Cape Roberts Project (CRP) drillhole CRP-2/2A, Victoria Land Basin, Antarctica. Rarity of palynomorphs probably results from the spares periglacial vegetation in the surrounding landscape at the time of deposition, as well as dilution from rapid sediment accumulation. The Miocene and Late Oligocene vegetation is interpreted as including herb-moss tundra with low-growing woody plants (including Nothofagus and podocarp conifers) in more protected areas, similar to that encountered in the Miocene of CRP-1. Species richness and numbers of specimens increase downhole, a trend that begins very gradually below ~307 mbsf, and increases below ~443 mbsf through the Early Oligocene. These lower assemblages reflect low diversity woody vegetation dominated by several species of Nofhofagus and podocarps, growing in somewhat milder conditions, though still cold temperate to periglacial in the Early Oligocene. The CRP-2/2A core provides new biostratigraphical information, such as the First Appearance Datums (FADS) of Tricolpites sp. a near the Oligocene/Miocene boundary, and Marchantiaceae in the Early/Late Oligocene transition: these are taxa that along with N. lachlaniae, Coptospora spp. and Podocarpidites sp.b characterize assemblages recovered from outcrops of the Pliocene Sirius Group in the Transantarctic Mountains. Some elements of the extremely hardy periglacial tundra vegetation that survived in Antarctica into the Pliocene had their origin in the Oligocene during a time of deteriorating (colder, drier) climatic conditions. The CRP results highlight the long persistence of this tundra vegetation, through approximately 30 million years of dynamically changing climatic conditions. Rare Jurassic and more common Permian-Triassic spores and pollen occur sporadically throughout the core. These are derived from Jurassic Ferrar Group sediments, and from the Permian-Triassic Victoria Group, upper Beacon Supergroup. Higher frequencies of reworked Beacon palynomorphs and coaly organic matter below ~307 mbsf indicate greater erosion of the Beacon Supergroup for this lower part of the core. A color range from black, severely metamorphosed specimens, to light-colored, yellow (indicating low thermal alteration), reworked Permian palynomorphs, indicates local provenance in the dolerite-intruded Beacon strata of the Transantarctic Mountains, as well as areas (now sub-ice) of Beacon strata with little or no associated dolerite well inland (cratonwards) of the present Transantarctic Mountains.

Geochemistry of samples from Valtournenche, Western Italian Alps

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic history, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallization at HT conditions; ages scatter from 263-294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Geochemistry and isotopes at DSDP Hole 79-546

At Site 546, below the Mazagan Escarpment at a water depth of 4 km, 36 m of salt rock was cored from the top of one of a field of salt domes. The core was studied by thin section and a variety of geochemical procedures. The salt rock contains 0.1 to 3% carnallite and lesser amounts of sylvite and polyhalite, which with the corresponding high level of bromide place it within the potash evaporite facies. The bromide profile is of a dominantly marine evaporite deposited in moderately shallow brine which, however, was not repeatedly desiccated. A mineralogical argument suggests that the brine surface was not below sea level. An average of about 5% elastics, with dispersed anhydrite, darken the salt rock to deep shades of red, brown, and gray green. Most of the included materials are in highly deformed boudins or dispersions in the salt rock that has also undergone cataclasis in a subsequent, probably tectonic, deformation. The salt rock is slightly deficient in anhydrite, and the usual separate beds and laminae of anhydrite are virtually absent. Stable isotope ratios of sulfur and oxygen in the sulfate are clearly derived from sea water of Permian to Scythian age, in contrast to the late Triassic or Early Jurassic age of evaporites onshore in Morocco and Portugal and the corresponding evaporites offshore Maritime Canada. In contrast to those evaporites off the axis of Atlantic rifting, the salt at Site 546 may have been deposited in a very early central rift fed by marine waters from Tethys through the Gibraltar or South Atlas fracture zones.

Stable isotopes and element concentrations of samples from the Wadi Bih, United Arab Emirates

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Tab. 2: Average geochemical composition of dolerites from Neuschwabenland

In western Neuschwabenland basic dikes occur in the Jurassic lavas and Permian sediments of Vestfjella as weil as in the Precambrian sedimentary-volcanogenic rock sequence of the Ahlmannryggen and in the Precambrian crystalline complexes of Heimefrontfjella and Mannefallknausane. The concentration of the dikes in Vestfjella is conspicuous. Two main directions of strike perpendicular to each other are recognizable, from which the NE-SW striking one is predominant. The direction of the dikes coincides with the Mesozoic and younger fracture tectonics. Age relationships by structural, petrographical and geochemical observations are confirmed by palaeomagnetic and radiometrie age determinations from PETERS et al. (1986). Considerations on the geochemistry of further dolerite occurrences from Antarctica and other regions of the Gondwana continent are pointed out. Finally comparisons with the analogous South African dike system show the geotectonic significance of the dolerite dikes for the break-up of Gondwana.