Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic

The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low ⁸⁷Sr/⁸⁶Sr_i). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low ¹⁴³Nd/¹⁴⁴Nd_i and low ⁸⁷Sr/⁸⁶Sr_i andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume     

A late Triassic age for the Rochechouart impact structure,France

Abstract— Argon-40/Argon-39 laser spot fusion dating of pseudotachylyte from the ～25 km diameter Rochechouart impact structure of western-central France yields a matrix age of 214 ± 8 Ma (2s?). Field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process, probably during transient cavity collapse. This new age is considerably older than the previously accepted age of 186 ± 8 Ma for this structure, which was obtained from hydrothermally-altered melt sheet samples. The new age is in accordance with earlier paleomagnetic and fission track data, which indicated that Rochechouart was formed during the late Triassic. Moreover, the new determination is in agreement with the regional geological setting and field relations of the structure. The new age of 214 ± 8 Ma falls within the Norian stage of the Triassic system.     

Hyperspectral image analysis of different carbonate lithologies (limestone,karst and hydrothermal dolomites): the Pozalagua Quarry case study (Cantabria,North‐west Spain)

Ground‐based hyperspectral imaging combined with terrestrial lidar scanning is a novel technique for outcrop analysis, which has been applied to Early and Late Albian carbonates of the Pozalagua Quarry (Cantabrian Mountains, Spain). An image processing workflow has been developed for differentiating limestone from dolomite, providing additional sedimentary and diagenetic information, and the possibility to quantitatively delineate diagenetic phases in an accurate way. Spectral absorption signatures can be linked to specific sedimentary or diagenetic products, such as recent and palaeokarst, hydrothermal karst, (solution enlarged) fractures and different dolomite types. Some of the spectral signatures are related to iron, manganese, organic matter, clay and/or water content. Ground‐truthing accessible parts of the quarry showed that the classification based on hyperspectral image interpretation was very accurate. This technique opens the possibility for quantitative data evaluation on sedimentary and diagenetic features in inaccessible outcrops. This study demonstrates the potential of ground‐based imaging spectroscopy to provide information about the chemical–mineralogical distribution in outcrops, which could otherwise not be established using conventional field methods.     

A pyroxene-bearing meta-ironstone and other pyroxene-granulites from Tonagh Island, Enderby Land, Antarctica: further evidence for very high temperature (> 980°C) Archaean regional metamorphism in the Napier Complex

Abstract A suite of granulites including a meta-ironstone, pyroxenites, and spinel-lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution-recry-stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000°C. Reintegrated pre-exsolution and pre-reaction grain compositions in the meta-ironstone indicate the former coexistence of metamorphic pigeonite (Wo₁₂En₃₈Fs₅₀) and ferroaugite (Wo₃₅En₃₁Fs₃₄) at temperatures in excess of 980°C for pressures of 7 kbar (0.7 GPa) using pyroxene quadrilateral thermometry (Lindsley, 1983). Intra-grain lamellae relationships indicate the exsolution of a second pigeonite (Wo₁₂En₃₅Fs₅₃) from the ferroaugite at temperatures in the range 930–970°C, prior to the c. 720–600°C exsolution of orthopyroxene and clinopyroxene (100) lamellae and later partial recrystallization at similar temperatures. Although pyroxenitic and iherzolitic granulites preserve a much less complete history, reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone. Pyroxene thermometry based on neoblast compositions suggests that recrystallization post-dating a late, low intensity, deformation phase (D3) occurred at temperatures greater than 600°C. These results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the Napier Complex followed by near-isobaric cooling and later deformation under lower-grade granulite facies conditions. Comparison with similar pyroxene data from Fyfe Hills (Sandiford & Powell, 1986) demonstrates further the regional significance of these high temperatures, and implies broadly isothermal metamorphic conditions over a large area (～ 5000 km²) and thickness (6–9 km) of lower crust at c. 3070 Ma.     

Formation of an unusual compact Type A refractory inclusion from Allende

Abstract— We report the results of a study of TS2, an unusual compact Type A inclusion from Allende. A distinctive, major feature of this inclusion is that many of its melilite crystals have no dominant core-rim zoning but instead consist of 50–200 μm patches of Mg-rich melilite (Åk_32–62, median Åk₅₁) set in or partially enclosed by, and optically continuous with, relatively Al-rich melilite (Åk_25–53, median Åk₃₈). The Al-rich regions have jagged, dendritic shapes but occur within crystals having straight grain boundaries. Another unusual feature of this inclusion is the size and spatial distribution of spinel. In many places, especially in the interior of the inclusion, the aluminous melilite encloses numerous, fine (0.5–5 μm) inclusions of spinel and minor perovskite and fassaite. The latter phases also occur as isolated grains throughout the inclusion. Coarse-grained spinel, ～50–150 μm across, occurs in clumps and chains enclosed in relatively Mg-rich melilite, whereas none of the fine spinel grains are clumped together. The sample also contains a spinel-free palisade body, 1.7 × 0.85 mm, that consists almost entirely of Åk-rich (45–65 mol%) melilite. Within the palisade body are two grains of perovskite with extremely Nb-rich (～4–8 wt% Nb₂O₅) cores and rims of typical composition. All phases in this inclusion have chondrite-normalized REE patterns that are consistent with crystal/melt partitioning superimposed upon a bulk modified Group II pattern. We suggest that TS2 had an anomalous cooling history and favor the following model for the formation of TS2. Precursors having a bulk modified Group II pattern melted. Rapid growth of large, dendritic, nonstoichiometric melilite crystals occurred. The melilite trapped pockets of melt and incorporated excess spinel components and TiO₂. Bubbles formed in the residual melt. As crystallization slowed, coarse spinel grew. Some spinel grains collected against bubbles, forming spherical shells, and others formed clumps and chains. Relatively Åk-rich melilite crystallized from the residual melt between dendritic melilite crystals and from melt trapped in pockets and between arms of dendrites, and incorporated the clumps and chains of coarse spinel. Bubbles broke and filled with late-stage melt, their shapes preserved by their spinel shells. Slow cooling, or perhaps an episode of reheating, allowed the early melilite to become stoichiometric by exsolving fine grains of spinel, perovskite and fassaite, and allowed the melilite to form smooth grain boundaries. Dendritic crystals are indicative of rapid growth and the melilite crystals in TS2 appear to be dendritic. Coarse, dendritic melilite crystals have been grown from Type B inclusion melts cooled at ～50–100 °C/h. If those results are applicable to Type A inclusions, we can make the first estimate of the cooling rate of a Type A inclusion, and it is outside the range (2–50 °C/h) generally inferred for Type B inclusions. The rapid cooling inferred here may be part of an anomalous thermal history for TS2, or it may be representative of part of a normal thermal history common to Types A and B that involved rapid cooling early (at high temperatures) as inferred for TS2, and slower cooling later (at lower temperatures), as inferred for Type B inclusions. We prefer the former explanation; otherwise, the unusual features of TS2 that are reported here would be common in Type A inclusions (which they are not).     

Comparison of the Garnet--Orthopyroxene Geobarometer with Recent Experimental Studies, and Applications to Natural Assemblages

The garnet-orthopyroxene geobarometer developed by Harley (1984)in the accompanying paper is compared with other recent experimentaldata on alumina solubility in orthopyroxene, obtained both inend-member and more complex systems. The geobarometer, in partbased on recent reversed data in MgO-Al₂O₃-SiO₂ (Perkins etal., 1981), is in reasonable agreement with other recent dataincluding the reversed data of Perkins & Newton (1980) inCaO-MgO-Al₂O₃-SiO₂. Comparison with the experimental data ofWood (1974) in the FeO-MgO-Al₂O₃-SiO₂ system indicates thatless alumina enters orthopyroxene at a chosen P-T conditionthan suggested by that study. Application of the geobarometer to garnet-peridotite xenoliths,and peridotite massifs within gneiss terrains yields broadlyconsistent and reliable pressure estimates provided the temperatureof equilibration is well constrained. Existing data for garnet-peridotitexenoliths in kimberlite yield P-T estimates which lie near steady-statecontinental geotherms but which lie well to the high-temperatureside of such geotherms at high pressures. These data, however,do not necessarily define a simple ‘kinked geotherm’.It is suggested instead that the xenolith data must be viewedin terms of polygenetic models of equilibration and cation exchangeclosure. Garnet-peridotite bodies enclosed in gneisses in the Alps andNorway yield pressure estimates in the range 20–30 kilobars.Such pressure estimates favour a mantle origin and tectonicemplacement model for these bodies. The most recent data relevantto the ‘country-rock’ orthopyroxene-eclogites fromNorway give pressure estimates (18 ? 8 kb) which do not allowdiscrimination between the in situ model of formation and thetectonic emplacement models. Some suites of crustal granulites yield pressure estimates whichare internally consistent, consistent with the presence of characteristicaluminosilicate polymorphs, and similar to pressures calculatedusing other reliable geobarometers where these are available.However, the geobarometer may underestimate pressures in comparisonwith a previous calibration (Harley & Green, 1982), particularlywhere measured values of K^ga-opx _DFe-Mg are high. This discrepancyresults from the sensitivity of the geobarometer equation torelatively small changes in garnet composition, independentof variations in alumina contents of the orthopyroxenes. A diversesuite of samples from a single locality in Enderby Land, Antarctica,illustrates some of these problems and uncertainties.     

The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section

The geochemistry (major, trace element, O- and Sr-isotope ratios)and petrology of the Transhimalaya, North Himalaya, High Himalayaand ‘Lesser Himalaya’ plutonic belts are comparedbased on the analyses of up to 492 samples. The composite Transhimalaya batholith is subalkaline or monzoniticin character rather than calc-alkaline. Its genesis was probablyclosely related to subduction processes associated with strike-slipmovement. It was emplaced on both sides of the boundary betweenan earlier metavolcanic arc and a continental margin. Two principalperiods of magmatic activity occurred: Upper Cretaceous and,particularly in this region, Eocene at the time of the India-Eurasiacollision when sediments may have become involved in the subductionprocess. Magmatic differentiation, characterized by two superimposedstages of evolution, and hybridization processes, involvingboth basic and acidic magmas, can account for the genesis ofthe different plutonic units. Although a continental contributionis implied, the isotopic data (6.8 < ¹⁸O < 9?2; 0?704< ⁸⁷Sr/⁸⁶Sr, < 0?707) preclude a significant contributionfrom either old crust or surface derived sediments. The North, High and ‘Lesser’ Himalaya plutonic beltsare fundamentally different and correspond to aluminous associationsof two groups of ages (Lower Palaeozoic for the ‘LesserHimalaya’ and part of the North Himalaya; Upper Cenozoicfor the High Himalaya and part of the North Himalaya). Theyare all high-¹⁸O (9 < ¹⁸O < 14) granites and adamelliteswith high initial ⁸⁷Sr/⁸⁶Sr, ratios (0?709 to < 0?740). TheLower Palaeozoic group was generated within the Gondwana continentalcrust, independent of any true orogenesis, with a probable butlimited contribution from the mantle. High Himalaya and NorthHimalaya Cenozoic plutons are directly linked to the activityof the Main Central Thrust. They were derived by similar anatecticprocesses of the same continental source rocks. The small butdistinct chemical and mineralogical differences among the plutonsare related to the increase in the intensity of anatexis ongoing towards the north and the east.     

Paragenetic and mineral-chemical relationships in orthoamphibole-bearing gneisses from Enderby Land, east Antarctica: a record of Proterozoic uplift

Abstract Polymetamorphic orthoamphibole-bearing gneisses from the vicinity of shear zones in Casey Bay, Enderby Land, Antarctica, record both the overprinting of Archaean granulite lithologies by Proterozoic metamorphism and the subsequent evolution of the latter episode during localized deformation.
Mineral chemistry and zoning relationships in orthoamphibole-garnet-kyanite-quartz and later orthoamphibole-garnet-cordierite-quartz assemblages are used together with interpretation of reaction and corona textures to constrain the Proterozoic pressure-temperature path experienced by the rocks. Consideration of reaction topologies, P-T-X(Fe-Mg-A1) relationships in orthoamphibole-bearing assemblages, and standard geothermobarometry indicate that the gneisses underwent a near-isothermal decompression P-T history (steep positive dP/dT) from ± 8 kbar and 700°C to <5.5kbar and 650°C. This uplift path is correlated with the general effects of Rayner Complex metamorphism and deformation which occurred after 1100 Ma in a major erogenic belt south of Casey Bay.