Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a ¹⁴⁷Sm‐¹⁴³Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ¹⁷O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).     

Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

Compositional characteristics and phase equilibria in manganiferous iron formations from a high-grade terrain near Satnuru, Karnataka, India

Manganese-rich and manganese-poor iron formations which occur as thin layers in the Halaguru-Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO₂ contents. The rocks are of four petrographic varieties: (a) quartz-magnetite-orthopyroxene-clinopyroxene, (b) quartz-magnetite-orthopyroxene-clinopyroxene-garnet, (c) quartz-magnetite-clinopyroxene-garnet, and (d) quartz-magnetite-clinopyroxene-garnet-plagioclase. In the orthopyroxene-clinopyroxene pairs, Mn-Mg and Mn-Fe exchange is ideal irrespective of the MnSiO₃ contents of orthopyroxenes (0.6–1.8 mol. % in Mn-poor and 15–25 mol. % in Mn-rich compositions). Mg-Fe exchange in the same pair is however non-ideal. Mn-Fe exchange in orthopyroxene-garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene-garnet and clinopyroxene-garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value. The equilibrium log ffo₂ values in the iron formations, as calculated from the reaction 6FeSiO₃+ O₂= 2Fe₃O₄+ 6SiO₂ are in the range of ?14.2 to ?15.5. Algebraic analysis of variations of f_o2 with composition of phases indicates buffering of O₂ in the rocks. The absence of grunerite in these assemblages is compatible with X_H2O being less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C-O-H system, however, predict high X_H2O values (>0.7) at a_c= 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks.     

巽他弧北部掸邦高原地壳变形、长期块体运动和发震过程:来自大地测量研究结果的约束

本文通过约束大地测量研究来探索掸邦高原及其周围地区现今的地壳变形和长期块体运动,以期提供该地区地球动力学和相关地震危险状况的最新状态。掸邦高原在横向上由西侧的萨干(Sagaing)断裂和东侧的红河断裂这两条主要断裂包围。其中,青藏高原地壳的韧性流挤压被认为是该夹层变形单元变形的主要因素。大地测量清楚地表明,萨干断裂和红河断裂段分别具有约18 mm/a和约45 mm/a右旋运动走滑速率。此外,掸邦高原内部断层体系大地滑移累积表现为1213 mm/a的整体左旋运动速率。我们认为相对于刚性巽他古陆,研究区域的形变分布和长期块体运动主要受区域书架型断层作用控制,其原因是掸邦高原两侧的主断裂(萨干断裂和红河断裂)存在差异性断裂活动。     

Comprehensive Analysis of Fish Assemblages in Relation to Seasonal Environmental Variables in an Estuarine River of Indian Sundarbans

Fish distribution in relation to environmental variables was investigated in the Matla River of Sundarban estuarine system. A total of 64 brackish water species belonging to 38 families showing tropical and subtropical affinities were obtained upon monthly sampling of 1 year. The most abundant species were Harpadon nehereus, Gudusia chapra, Coilia neglecta, Coilia ramcarati and Liza parsia. Fish assemblages showed significant seasonal variation as was revealed from MANOVA. MDS ordination plot based on similarity in the fish assemblages revealed that premonsoonal season was distinguishable from the monsoonal and postmonsoonal seasons. Notable differences were also evident between monsoon and postmonsoon. Canonical correspondence analysis indicated that salinity, acidity, inorganic phosphate concentration and dissolved oxygen were the most important environmental variables in determining the observed variation in fish assemblages. Seasonal succession of fish populations may be related to differences in life cycle and adaptation to prevalent ecological conditions. The study therefore suggests that environmental variations in terms of changing salinity and dissolved oxygen have significant influence in structuring estuarine piscine community in this region.     

Geology, petrochemistry, and genesis of the bimodal lavas of Osham Hill, Saurashtra, northwestern Deccan Traps

The Saurashtra region in the northwestern Deccan continental flood basalt province (India) is notable for compositionally diverse volcano-plutonic complexes and abundant rhyolites and granophyres. A lava flow sequence of rhyolite-pitchstone-basaltic andesite is exposed in Osham Hill in western Saurashtra. The Osham silicic lavas are Ba-poor and with intermediate Zr contents compared to other Deccan rhyolites. The Osham silicic lavas are enriched in the light rare earth elements, and have ε_Nd (t = 65 Ma) values between −3.1 and −6.5 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70709-0.70927. The Osham basaltic andesites have initial ε_Nd values between +2.2 and −1.3, and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70729-0.70887. Large-ion-lithophile element concentrations and Sr isotopic ratios may have been affected somewhat by weathering; notably, the Sr isotopic ratios of the silicic and mafic rocks overlap. However, the Nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas. We suggest that the Osham basaltic andesites were derived by olivine gabbro fractionation from low-Ti picritic rocks of the type found throughout Saurashtra. The isotopic compositions, and the similar Al₂O₃ contents of the Osham silicic and mafic lavas, rule out an origin of the silicic lavas by fractional crystallization of mafic liquids, with or without crustal assimilation. As previously proposed for some Icelandic rhyolites, and supported here by MELTS modelling, the Osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths, due to heating by repetitively injected basalts. The absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of Osham Hill suggests that they reached the surface via separate pathways.     

Palynological characteristics of Kazakhstan Province of Carboniferous Euramerican Region

The paper presents results of palynological studies of the coal-bearing Carboniferous in Central Ka-zakhstan.A quantitative dynamics of major genera of miospores reflects an overall succession of the Carbonifer-ous miospore assemblages.Viséan Serpukhovian assemblages are dominanted by Lophozonotriletes fuscatus,Ac-anthozonotriletes cristifer,A.inclusus,Punctatisporites glabratus,Crassispora ignorata,C.scrupea,Cyclobacu-lisporites trichacanthus,Lycospora breviapiculata,L.echinata and Vallatisporites decrescens.Rare Punctatis-porites bertschoguriensis,Stenozonotriletes marginellus,Vallatisporites appilicatus,Verrucosisporites scrobiculatus,Reticulatisporites cancellatus,Granulatisporites arcuatus are recorded,which are not found in assemblages from overlying sediments.Bashkirian--Moscovian assemblages are dominated by Lycospora subtriquetra,L.pusilla,Vallatisporites ciliaris,Apiculatisporis mollis,Cyclogranisporites testiculatus,Calamospora microrugosa,Schopfi-pollenites principalis,Psilohymena psiloptera,Florinites grandis and Granulatisporites piroformis miospores.Cris-tatisporites latispinus,Angulisporites didymus,Thymospora obscura and Torispora securis are also present,as well as single Apiculatisporis curvispinus,Raistrickia inordinata,Marsupipollenites retroflexus,Entylissa caperata,Potonieisporites grandis and Cordaitina sp.A similar succession of miospore assemblages is recorded in the Carboniferous deposits of Europe and North America,which allows carrying out correlations,despite an endemic characters of many species of Paleozoic miospores from the coal-bearing Carboniferous of the Kazakh-stan Province in the Euramerian Paleofloristic Region.     

Geochemistry of Cu, Co, Ni, Zn, Cd and Cr in the surficial sediments of a tropical estuary, southwest coast of India: a granulometric approach

 Geochemical characteristics of six trace metals – Cu, Co, Ni, Zn, Cd and Cr – in the bulk sediment and sand, silt and clay fractions of a tropical estuary on the southwest coast of India have been studied and discussed. In bulk sediment, the trace metal concentration is controlled mainly by the textural composition of the sample. Mud, sandy mud and sandy silt register higher concentrations of trace metals than that in sand-dominant sediments. The granulometric partitioning studies also re-affirmed the role of particle size in enriching the trace metals. The silt and clay fractions exhibit 7–8 times the enrichment of Cu and Cd compared to that in sand. The enrichment factors of Zn, Cr, Ni and Co in the silt and clay fractions, compared to that in sand, are 5–6, 4–5, 2–5 and 2–3 times, respectively. The trace metals in the sand fraction, particularly Ni and Cr, exhibit strong positive correlation with the heavy mineral content of the samples. It clearly indicates a heavy mineral pathway to the trace metals in the sand fraction. Cu and Co in silt and clay fractions exhibit a marked decrease towards the high saline zones of the estuary. This is attributed to the desorption of Cu and Co from particulate phases during estuarine mixing. Contrary to Cu and Co, the content of Zn in the clay fraction shows a marginal increase towards the estuarine mouth. This could be explained by the influx of Zn-rich contaminant discharges from Zn-smelting industries located slightly north of the estuarine mouth. The released Zn will effectively be held in the lattices of the clay mineral montmorillonite, which also exhibits a marked increase towards the estuarine mouth. The anomalously high values of Cd in some places of the Central Vembanad estuary is attributed to the local pollution. Received: 10 July 1995 · Accepted: 3 June 1996