Geochemistry of Miocene Alborán Basin volcanism

We present new major and trace element and O-Sr-Nd-isotope data for igneous rocks from the western Mediterranean Alborán Sea, collected during the METEOR 51/1 cruise, and for high-grade schists and gneisses from the continental Alborán basement, drilled during the Ocean Drilling Programme (ODP Leg 161, Site 976). The geochemical data allow a detailed examination of crustal and mantle processes involved in the petrogenesis of the lavas and for the first time reveal a zonation of the Miocene Alborán Sea volcanism: (1) a keel-shaped area of LREE-depleted (mainly tholeiitic series) lavas in the central Alborán Sea, generated by high degrees of partial melting of a depleted mantle source and involving hydrous fluids from subducted marine sediments, that is surrounded by (2) a horseshoe-shaped zone with LREE-enriched (mainly calc-alkaline series) lavas subparallel to the arcuate Betic-Gibraltar-Rif mountain belt. We propose that the geochemical zonation of the Miocene Alborán Basin volcanism results from eastward subduction of Tethys oceanic lithosphere coupled with increasing lithospheric thickness between the central Alborán Sea and the continental margins of Iberia and Africa.

Nd and Sr isotopic compositions and isotope dilution trace element concentrations from 7 sediment cores along the southeast Greenland margin

The Nd and Sr isotopic compositions of Quaternary glacial and glacimarine siliciclastic sediments deposited along the margin of southeast Greenland were determined to assess the roles of the Greenland, Iceland, and more distal ice sheets in delivering detritus to this portion of the northern North Atlantic. The isotopic compositions of detritus generated by portions of the southern Greenland Ice Sheet were defined through measurements of till and trough mouth fan sediments. Massive diamicts from the Scoresby Sund trough mouth fan show a restricted range of e-Nd (-11.8 to -16.6) and 87Sr/86Sr (0.7192-0.7246) consistent with their derivation from mixtures of sediments derived from Paleoproterozoic and/or Caledonian basement and Tertiary Greenland basalts. Further south at Kangerlussuaq, till isotopic compositions covary with the underlying basement type, with low e-Nd values in the inner fiord (-18.1) reflecting the erosion of the local Precambrian gneisses, but with higher e-Nd values (-2.3 to 2.5) found where the trough crosses East Greenland Tertiary basalts. Fine-grained (< 63 µm) sediments deposited along the southeast Greenland margin also show regular spatial isotopic variations. Ambient sediments and ice-rafted detritus in the southern Irminger Basin trend towards low e-Nd values (to ~ -28) and 87Sr/86Sr ratios (~ 0.711 to ~ 0.715) and are likely derived from proximal Archean gneisses of SE Greenland. Further north in the northern Irminger and Blosseville Basins, sediments trend toward much higher e-Nd (> -4) and low 87Sr/86Sr (< 0.709) reflecting a component derived from the local Iceland volcanic rocks and/or the East Greenland Tertiary basalts. In all three regions, the locally-derived detritus is intermixed with sediment with an intermediate e-Nd value (~ -10) and 87Sr/86Sr (~ 0.718) that was likely delivered by icebergs emanating from the Eurasian Ice Sheets and not from eastern Greenland. Deposition of glacial sediments from both proximal and distal (Eurasian) sources occurred adjacent to SE Greenland throughout the past 50 Ka, with periodic increases in IRD deposition at various times including those of Heinrich events 1, 2 and 4. These results suggest that at least the southern portions of the Greenland Ice Sheet experienced periodic instabilities during the Last Glacial period.

Sediment composition in NE Greenland Fjords, Shelf, and Slope

In order to document changes in Holocene glacier extent and activity in NE Greenland (~73° N) we study marine sediment records that extend from the fjords (PS2631 and PS2640), across the shelf (PS2623 and PS2641), to the Greenland Sea (JM07-174GC). The primary bedrock geology of the source areas is the Caledonian sediment outcrop, including Devonian red beds, plus early Neoproterozoic gneisses and early Tertiary volcanics. We examine the variations in colour (CIE*), grain size, and bulk mineralogy (from X-ray diffraction of the <2 mm sediment fraction). Fjord core PS2640 in Sofia Sund, with a marked red hue, is distinct in grain size, colour and mineralogy from the other fjord and shelf cores. Five distinct grain-size modes are distinguished of which only one is associated with a coarse ice-rafting signal - this mode is rare in the mid- and late Holocene. A sediment unmixing program (SedUnMixMC) is used to characterize down-core changes in sediment composition based on the upper late Holocene sediments from cores PS2640 (Sofia Sund), PS2631 (Kaiser Franz Joseph Fjord) and PS2623 (south of Shannon Is), and surface samples from the Kara Sea (as an indicator of transport from the Russian Arctic shelves). Major changes in mineral composition are noted in all cores with possible coeval shifts centred c. 2.5, 4.5 and 7.5 cal. ka BP (±0.5 ka) but are rarely linked with changes in the grain-size spectra. Coarse IRD (>2 mm) and IRD-grain-size spectra are rare in the last 9-10 cal. ka BP and, in contrast with areas farther south (~68° N), there is no distinct IRD signal at the onset of neoglaciation. Our paper demonstrates the importance of the quantitative analysis of sediment properties in clarifying source to sink changes in glacial marine environments.

Sedimentology on two core from the Bransfield Strait area

The raw material for these investigations are samples from marine (sub)surface sediments around the northern part of the Antarctic Peninsula. They had been sampled in the years 1981 to 1986 during several expeditions of the research vessels Meteor, Polarstern and Walther Herwig. 83 box core, gravity core and dredge samples from the area of the Bransfield Strait, the Powell Basin and the northern Weddell Sea have been examined for their grain-size distribution, their mineralogical and petrographical composition. Silt prevails and its clay proportions exceed 25% wt. in water depths greater than 2000 m. The granulometrical results reveal some typical sedimentation processes within the area of investigation. While turbiditic processes together with sediment input from melting icebergs control the sedimentation in the Weddell Sea, the South Orkney Island Plateau and the Powell Basin, the fine grained material from Bransfield Strait mainly relies on marine currents in the shelf area. In addition, the direct sediment input of coarse shelf sediments from the Bransfield Strait into the Powell Basin through submarine canyons could be proven. Variations in the grain-size composition with sediment depth are smalI. The mineral composition of the clay and fine silt fractions is quite uniform in all samples. There are (in decreasing order): illite, montmorillonite, chlorite, smectite, mixed-Iayers, as well as detrital quartz and feldspars. A petrographically based sediment stratigraphy can be established in using the considerable changes in the chlorite- and Ca-plagioclase portions in samples from Core 224. For this sedimentation area a mean sedimentation rate of 7 cm/1000 a is assumed. Remarkable changes in the portions of amorphous silica components - diatom skeletons and volcanic glass shards - appear all over the area of investigation. They contribute between 4-83 % to the clay and fine silt fraction. Several provinces according to the heavy mineral assemblages in the fine sand fraction can be distinguished: (i) a province remarkably influenced by minerals of volcanic origin south and north of the South Shetland Islands; (ii) a small strip with sediment dominated by plutonic material along the western coast of the Antarctic Peninsula and (iii) a sediment controlled by metamorphic minerals and rock fragments in the area of the Weddell Sea and Elephant Island. While taking the whole grain-size spectrum into account a more comprehensive interpretation can be given: the accessoric but distinct appearance of tourmaline, rutile and zircon in the heavy mineral assembly along the northwestern coast of the Antarctic Peninsula is in agreement with the occurrence of acid volcanic rock pieces in the coarse fraction of the ice load detritus in this region. In the vicinity of the South Shetland Islands chlorite appears in remarkable portions in the clay fraction in combination with leucoxene, sphene and olivine, and pumice as well as pyroclastic rocks in the medium and coarse grain fractions, respectively. Amphiboles and amphibole-schists are dominant on the South Orkney Island Plateau. In the sediments of the northwestern Weddell Sea the heavy mineral phases of red spinel, garnet, kyanite and sillimanite in connection with medium to highgrade metamorphic rocks especially granulitic gneisses, are more abundant. A good conformity between the ice rafted rock sampIes and the rocks in the island outcrops could be proven, especially in the vicinity of offshore islands nearby. On the continent enrichments of rock societies and groups appear in spacious outlines: acid effusive rocks in the west of the ice divide on the Antarctic Peninsula, clastic sedimentites at the tip of the Antarctic Peninsula and granoblastic gneisses in central and eastern Antarctica. Coarse grain detritus with more than 1 cm of diameter must have been rafted by icebergs. These rock fragments are classified as rock types, groups and societies. The spacial distribution of their statistically determined weight relations evidently shows the paths of the iceberg drift and in nexus with already known iceberg routes also point to the possible areas of provenance, provided that the density of sample locations and the number of rock pieces are sufficient.

Geochemistry and ages of rock samples from Kirwanveggen, Antarctica

The basement of southern Kirwanveggen (western Dronning Maud Land) is formed by a SSW-dipping section consisting of (from SW to NE): migmatic gneisses; granitoid; low-grade/prograde meta-pelites, meta-psammites and meta-basalts (= "Polaris Formation"); ortho-gneiss; quartzite mylonite; Polaris Formation; quartzite mylonite; meta-turbidites. These units are (partly) separated by at least four SSW-dipping, NE to N directed major thrusts. Most probably, this thrust system is of Pan-African age. Towards north, the section is followed by the molasse-like Urfjell Group, deposited later than approx. 550 Ma and earlier than 450 Ma. Similarities with the Pan-African of the Shackleton Range (thrusting, molasse) led to the assumption, that the East/West Gondwana suture runs from the Shackleton Range towards Sor Rondane (eastern Dronning Maud Land) passing southern Kirwanveggen at its south-east.

Chemical and isotopic compositions of Archean magmatic and metasedimentary rocks and minerals from the Podolia domain, Ukrainian Shield

Zircons from the oldest magmatic and metasedimentary rocks in the Podolia domain of the Ukrainian shield were studied and dated by the U-Pb method on a NORDSIM secondary-ion mass spectrometer. Age of zircon cores in enderbite gneisses sampled in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of terrigenous zircons in quartzites from the Odessa quarry as well as in garnet gneisses from the Zaval'e graphite quarry have age within 3650-3750 Ma. Zircon rims record two metamorphic events around 2750-2850 Ma and 1900-2000 Ma. Extremely low U content in zircons of the second age group indicates conditions of the granulite facies metamorphism in Paleoproterozoic within the Podolia domain. Measured data on orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest existence of the ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of formation and geochemical features of the primordial crust.

SHRIMP U-Pb dating of high-grade migmatites and related magmatites from Northwestern Oates Land, East Antarctica

High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 +/- 10 Ma and 493 +/- 9 Ma. Coexisting zircon gave ages of 500 +/- 4 Ma and 484 +/- 5 Ma for a metatexite (two age populations) and 475 +/- 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 +/- 6 Ma and 482 +/- 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high-amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.

Lithology, mineralogy and geochemistry of ODP Hole 118-735B and 176-735B respectively

Ocean Drilling Program (ODP) Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, penetrated 1508 meters below seafloor with an average recovery of 87%, providing a nearly continuous sample of a significant part of oceanic Layer 3. Based on variations in texture and mineralogy, 12 major lithologic units are recognized in the section, ranging from 39.5 to 354 m thick. The principal lithologies include troctolite, troctolitic gabbro, olivine gabbro and microgabbro, gabbro, gabbronorite and Fe-Ti oxide gabbro, gabbronorite, and microgabbro. Highly deformed mylonites, cataclasites, and amphibole gneisses are locally present, as are small quantities of pyroxenite, anorthositic gabbro, and trondhjemite. Downhole variations in mineral composition, particularly for olivine and clinopyroxene, show a number of cyclic variations. Plagioclase compositions show the widest variations and correspond to different degrees of deformation and alteration as well as primary processes. Downhole chemical variations correspond reasonably well with variations in mineral compositions. Iron and titanium mainly reflect the presence of Fe-Ti oxide gabbros but show some cyclical variations in the lower part of the core where oxide gabbros are sparse. CaO is highly variable but shows a small but consistent increase downhole. MgO is more uniform than CaO and shows a very small downward increase. Sulfur and CO2 contents are generally low, but S shows significant enrichment in lithologic Unit IV, which consists of Fe-Ti oxide gabbro, reflecting the presence of sulfide minerals in the sequence. The lithologic, mineralogical, and geochemical data provided here will allow detailed comparisons with ophiolite sections as well as sections of in situ ocean crust drilled in the future.

Representative analyses of garnet, plagioclase, orthopyroxene, ilmenite, hornblende and biotite for pre-tectonic mafic and metapelitic rocks in Gjelsvikfjella

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P-T-t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M1) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P-T-t path for the Pan-African orogeny. Peak metamorphic (M2b) conditions recorded by most rocks in the area (T = 709-785 °C and P = 7.0-9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M2a). The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent-continent collision at the end of the Mesoproterozoic (M1; 1090-1030 Ma) and again during the Pan-African orogeny (M2a, M2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.

Composition of melt inclusions and age of zircons of plagiogneisses from the Kola Superdeep Borehole

A comprehensive study of melt inclusions and SHRIMP dating of zircons from trondhjemite gneisses of the sequence VIII from the Kola Superdeep Borehole has revealed presence of old primary magmatic crystals with age up to 2887+/-15 Ma. This is not consistent with the previous view, according to which the oldest zircons from the Archean Complex in SG-3 are products of granulite metamorphism. Primary magmatic zircons of early generation (from 2887 to 2842 Ma) formed in deep-seated magma chambers during partial crystallization of CO2-saturated trondhjemite estimates on duration of generation of tonalite-trondhjemite-granite melts through partial melting of mafic rocks.

Table 1: U-Pb analyses of zircons and monazites of paragneisses which belong to the high grade metamorphic basement of Wilson Terrane

Samples of high grade metamorphic basement rocks of Wilson Terrane cropping out in the Deep Freeze Range and on Kay Island were collected during GANOVEX VI to study their isotopic evolution. The age and origin of granulite facies gneisses and of their migmatite host rocks are especially of interest for the interpretation of the geological and tectonic development of North Victoria Land. Another important research aspect is the influence of the polyphase metamorphic evolution on the isotopic systems of whole rocks and minerals like zircon, garnet, orthopyroxene, amphibole and feldspar.

Appendix A: LA-ICP-MS results (Laser-ablation inductively coupled plasma mass spectrometer) from monazite Moacir standard

Three samples of garnet-kyanite paragneiss from the Variscan Ulten Zone (Northern Italy) were studied in detail for U-Th-Pb monazite dating. Monazite in these gneisses is abundant, shows highly variable grain size and occupies different textural positions: within the matrix, as inclusion in garnet and kyanite, within apatite aggregates. Monazite shows different deformation features as a function of the textural position: enclosed (shielded) monazite is generally more fractured than matrix (unshielded) monazite. The integration of textural information with deformation features and in situ U-Th-Pb analyses by LA-ICP-MS indicates that there is no direct correlation between textural site and monazite ages. Old ages of 351-343 Ma, determined on portions of large matrix (unshielded) monazite and on rare domains of monazite shielded by garnet, have been related to a prograde stage of the Variscan metamorphic evolution of the Ulten Zone. Ages of 330-326 Ma, which are related to the thermal peak, are recorded by small matrix monazite, external domains of large matrix monazite, and by (domains of) fractured monazite enclosed in garnet and kyanite. Large, old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation/re-crystallisation partially occurred.