Low-grade Metamorphism Research Articles

An Early Permian crinoid fauna from Crete*

Early Permian crinoids are reported from a small cape at Bali, north central Crete, Greece. The specimens have undergone low-grade metamorphism, but are moderately well preserved. At the generic level the fauna shows greatest affinity with North American faunas and secondarily with Timor faunas. This is the first Paleozoic crinoid fauna reported from Greece.New taxa introduced are: Synbathocrinus cretensis n. sp., Apographiocrinus nodosus n. sp., Pyndaxocrinus granulatus n. sp., Protencrinus baliensis n. sp., and Artichthyocrinus koenigi n. sp.

Metamorphism and deformation of golpayegan metapelitic rocks, Sanandaj-Sirjan Zone, Iran

The metapelitic schists of the Golpayegan region can be divided into four groups based on their mineral assemblages: (1) garnet-chloritoid schists, (2) garnet schists, (3) garnet-staurolite schists, and (4) staurolite-kyanite schists. Paleozoic pelagic shales experienced progressive metamorphism and polymetamorphism from greenschist to amphibolite facies along the kyanite geotherm. Mylonitic granites are concentrated in the central part of the region more than in other areas, and formed during the dynamic metamorphic phase by activity on the NW-SE striking Varzaneh and Sfajerd faults. The presence of chloritoid in the metapelites demonstrates low-grade metamorphism in the greenschist facies. The textural and chemical zoning of garnets shows three stages of growth and syntectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at greenschist to lower amphibolite facies. Thermodynamic studies of these rocks indicate that the metapelites in the north Golpayegan region formed at 511–618°C and 0.24–4.1 kbar.

Short-wavelength infrared spectroscopy of chlorite can be used to determine very low metamorphic grades

This study investigates the use of short-wavelength infrared reflectance spectroscopy for measuring chlorite physicochemical information of value for determination of very low metamorphic grades. The method is termed chlorite spectral index (CSI) and assessed through quantitative comparison with XRD-derived Arkai and Kubler indices (AI and KI, respectively) in a set of 41 pelitic samples. The four CSI measures investigated measure the depth (D) of the absorption features at 1900, 2250 and/or 2350 nm, and are defined as reflectance 2250D/1900D index (CSI(H 2 O)), reflectance (2350D + 2250D × 1.7)/1900D index (CSI(H 2 O) sum ), reflectance 2250D index (CSI(2250)) and reflectance 2350D + 2250D × 1.7 index (CSI sum ). The results demonstrate the applicability of the method: all CSI measures show a good correlation with the AI, and respective correlation factors range between R 2 = 0.840 (for the CSI(H 2 O)) and R 2 = 0.616 (for the CSI sum ). The CSI measures also show a good correlation with the KI with R 2 = 0.704 (for CSI sum ) to 0.769 (CSI(2250)). By applying the correlation equations to define the metamorphic zone boundaries for each of the CSI measures, 64.3–78.6 % of the samples match the metamorphic grade indicated by AI for the epizone, 47.1–70.6 % for the anchizone and 70–80 % for the diagenetic zone. All CSI measures show a good reproducibility, with an intra sample variance ranging between 2.31 % CSI(H 2 O) sum and 3.34 % (CSI(2250)). CSI is not restricted to pelitic rocks and marls, but can also be applied in very low-grade metabasites. In addition to the existing laboratory method, CSI has application potential in remote sensing applications.

The fine-scale seismic structure of the upper lithosphere within accreted Caledonian lithosphere: implications for the origins of the ‘Newer Granites’

Using the results of recent seismic investigations of the upper continental lithosphere within the Caledonian–Variscan fold belt in SW Ireland a geological model is developed for the late Caledonian–early Devonian accretion history of the crust. It is suggested that this geological model can explain a variety of geological phenomena. These include the origin of the ‘Newer Granite’ series in Britain and Ireland, the partial preservation of lower Devonian ‘Lower Old Red Sandstone’ rocks, within the braided Caledonian fault network and the relatively low metamorphic grade of the Caledonian basement rocks, into which many of the granites are intruded. The proposed model involves a process called ‘incipient delamination’, in which the mantle part of the lithosphere did not detach completely from the accreted crust during the late Caledonian to Acadian orogenic events.

Evidence for homogeneous distribution of osmium in the protosolar nebula

Separate s-, r-, and possibly p-process enriched and depleted components have been shown to host Os in low metamorphic grade chondrites, although no measureable Os isotopic anomalies have yet been discovered for bulk chondrites. Here, iron meteorites from groups IAB, IIAB, IIIAB, IVA and IVB, as well as the main group pallasites are examined. Many of these meteorites show well-resolved anomalies in ε190Os, ε189Os and ε186Osi. The anomalies, however, differ from those observed in chemically extracted components from chondrites, and are interpreted to reflect long-term exposure of the meteorites to cosmic rays, rather than nucleosynthetic effects. A neutron capture model is presented that can well account for observed isotopic variations in 190Os and 189Os. The same model predicts greater enrichment in 186Osi than is observed for at least one iron, suggesting as yet unaccounted for effects, or failings of the model. Despite the variable anomalies resulting from cosmic ray exposure, each of the major meteorite groups examined contains at one member with normal Os isotopic compositions that are unresolved from chondritic compositions. This indicates that some domains within these meteorites were little affected by cosmic rays. These domains are excellent candidates for application of the 182Hf–182W system for dating metal–silicate segregation on their parent bodies. The normal Os also implies that Os was homogeneously distributed throughout the protosolar nebula on the scale of planetesimal accretion, within the current level of analytical resolution. The homogeneity in Os contrasts with isotopic heterogeneity present for other siderophile elements, including Mo, Ru and W. The contrast in the scale of anomalies may reflect a late stage-injection of s- and p- process rich material into the coalescing nebula. Alternately, nebular thermal processing and destruction of some presolar host phases of Mo, Ru and W may also be responsible.

Estratigrafia e geocronologia do greenstone belt de Guarinos, Goiás

The Guarinos greenstone belt is one of the three low metamorphic grade supracrustal rocks assemblage rimmed by orthogneisses of Central Brazil's Archean Block northern limits. The investigation of the stratigraphy of those greenstone belts started by the end of 1970's and underwent improvements during the years as a result of detailed geological mapping. The latest and accepted stratigraphic model for the Guarinos greenstone belt refers its supracrustal rocks under the Guarinos Group, which is subdivided into the Serra do Cotovelo (metakomatiites), Serra Azul (metabasalts), Sao Patricinho (mafic metaturbidites), Aimbe (BIF) and Cabacal (carbonaceous phyllites and impure metarenites) formations. Detailed geologic mapping (1:10,000) and drill-cores during an exploration program by Yamana Desenvolvimento Mineral S.A. in the area allowed a better definition of the metasedimentary package of the Cabacal Formation, which is here formally proposed to be subdivided into a Lower Member of carbonaceous phyllites with basalt lava flows and gondite lenses interlayers, an Intermediate Member of gondite, iron formation, metachert and massive barite lenses, and an Upper Member of carbonaceous phyllites with minor metachert lenses. The impure metarenites, formerly considered as part of the Cabacal Formation, are proposed under the Mata Preta Formation, which is laterally interfingered with the Cabacal Formation. U-Pb LA-ICP-MS geochronological data of detrital zircon grains from the Sao Patricinho mafic metaturbidites and the impure metarenites of the Mata Preta formations indicate that the major source-area of the clastic load had a Siderian to Rhyacian age, with minor contribution from Archean rocks. From the rock assemblage of both units and their contact relationships it is concluded that the basin stage they represent evolved during the world-wide Anoxic Oceanic Event (AOE) that took place during the 2.2 to 2.06 Ga, represented by the carbonaceous phyllites of the Cabacal Formation, coeval with the erosion of a magmatic arc that derived the immature clastic load of the Mata Preta Formation.

Uptake of carbon and sulfur during seafloor serpentinization and the effects of subduction metamorphism in Ligurian peridotites

The concentrations and isotope compositions of carbon and sulfur in Ligurian serpentinites having different metamorphic histories were analyzed in order to investigate the behavior of sulfur and carbon during seafloor serpentinization and during subduction metamorphism. Serpentinites associated with gabbros and with seafloor hydrothermal sulfide deposits in northern Apennine ophiolites that exhibit only slight effects of low-grade metamorphism have high sulfur contents (190–1440ppm total sulfur) and δ34STotal S values of up to 9.8‰ through interaction with hydrothermal fluids in high-temperature ultramafic-hosted hydrothermal systems on the seafloor. Fluid circulation was driven by mafic intrusions and the hydrothermal fluids contained a mixture of basaltic and seawater-derived sulfur. The serpentinites contain 250–480ppm total carbon having δ13CTotal C values of −9.4 to −6.0‰, reflecting mixtures of seawater carbonate (~0‰) and reduced (organic) carbon having δ13C≈−25‰ and derived from multiple sources. Entrainment of cold seawater into the subsurface beneath seafloor hydrothermal sulfide deposits maintained locally low temperatures and enabled microbial reduction of seawater sulfate. Combined with previous work documenting low-temperature serpentinization on the seafloor away from such gabbro-driven high-temperature hydrothermal activity, these results provide an integrated view of serpentinization processes in tectonically extended oceanic basement exposed in oceanic core complexes.High-P–T antigorite serpentinites in the Voltri massif (Ligurian Alps) have sulfur and carbon contents and isotope compositions similar to those of the Apennine ophiolites, indicating that the protoliths underwent similar hydrothermal processes on the seafloor. Recrystallization of chrysotile and lizardite to antigorite and minor partial dehydration to olivine during subduction metamorphism result in no significant changes in the contents or isotope compositions of sulfur or carbon in the serpentinites. We estimate that the uptake of carbon and sulfur by serpentinites in oceanic basement is comparable to or greater than that per unit volume of mafic oceanic crust. The high P–T stability of serpentine can, however, make serpentinites important for transport of carbon and sulfur to greater depths in subduction zones.

Preface: clay mineral diagenesis and very low-grade metamorphic processes. Proceedings of the 2011 Frey–Kübler symposium

In 2002, after the death of Prof. Bernard Kubler (Univ. Neuchâtel) and Prof. Martin Frey (Univ. Basel), Schmidt and Ferreiro Mahlmann published a special issue on ‘‘Diagenesis and Low-Grade Metamorphism’’ in the Swiss Bulletin of Mineralogy and Petrology. Most contributions had been presented during the Symposium on ‘‘Diagenesis and Low-Grade Metamorphism’’ at the EUG meeting 2001 in Strasbourg, France, and had been dedicated to the two most important researchers in that field. The underlying philosophy of the ‘‘Diagenesis and Low-Grade Metamorphism’’ issue, was to provide a compilation of the state of the art in very low-grade metamorphic (VLGM) and low temperature petrologic research to: (i) Geologists who wanted to use low-grade metamorphic petrologic, geochemical, clay mineralogical, isotopic and coal petrographic methods for their specific studies (e.g. in sedimentary geology, basin analysis, hydrocarbon geology, and regional metamorphic geology), (ii) Research scientists interested in methodical and process aspects of diagenesis and low-grade metamorphism, and (iii) Structural geologists with specialization in orogenic research.

Diagenetic and very low-grade metamorphic characteristics of the Paleozoic series of the Istanbul Terrane (NW Turkey)

The Istanbul Terrane along the Black Sea coast in NW Anatolia, is a Gondwana-derived continental microplate, comprising a well-developed Paleozoic succession. Petrographic and X-ray diffraction studies were performed on rock samples from measured sections throughout Ordovician–Carboniferous sedimentary units. Diagenetic-very low-grade metamorphic clastic (shale/mudstone, siltstone, sandstone) and calcareous rocks (limestone, dolomite) mainly contain phyllosilicates, quartz, feldspar, calcite, dolomite, hematite and goethite minerals. Phyllosilicates are primarily represented by illite, chlorite, mixed-layered chlorite–vermiculite (C–V), chlorite–smectite (C–S) and illite–chlorite (I–C). Feldspar is commonly present in the Ordovician and Carboniferous units, whereas calcite and dolomite are abundant in the Silurian and Devonian sediments. The most important phyllosilicate assemblage is illite + chlorite + I–C + C–V + C–S. Illite and chlorite-bearing mixed layer clays are found in all units. The amounts of illites increase in the upper parts of the Silurian series and the lower parts of the Devonian series, whereas chlorite and chlorite-bearing mixed-layers are dominant in the Ordovician and Carboniferous units. Kubler index values of illites reflect high-grade anchimetamorphism for the Early Ordovician rocks, low-grade metamorphism to high-grade diagenesis for the Middle Ordovician–Early Silurian rocks and high-grade diagenesis for the Late Silurian–Devonian units. The K-white micas b cell dimensions indicate intermediate pressure conditions in the Early Ordovician–Early Silurian units, but lower pressure conditions in the Middle Silurian–Devonian units. Illites are composed of 2M1 ± 1Md polytypes in all units, except for Upper Ordovician–Lower Silurian units which involve 1M polytype in addition to 2M1 and 1Md polytypes. The 2M1/(2M1 + 1Md) ratios rise from Devonian to Ordovician together with the increasing diagenetic-metamorphic grade. Chlorites have IIb polytype. In general, crystal-chemical data of clay minerals in the Istanbul Terrane show a gradual increase in the diagenetic/metamorphic grade together with increasing depth. The new data presented in this work indicate that the diagenetic/metamorphic grade of the Paleozoic of the Istanbul Terrane is higher than that of the neighboring Zonguldak Terrane and generated by a single metamorphic phase developed at the end of Carboniferous. This finding contrasts with the metamorphic history of the neighboring Zonguldak Terrane that displays a distinct Early Devonian unconformity and a thermal event.

Diagenesis and very low grade metamorphism in a 7,012 m-deep well Hongcan 1, eastern Tibetan plateau

This study uses clay mineral assemblages, illite “crystallinity” (IC), chlorite “crystallinity” (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7 km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8 m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500 m, and illite/smectite mixed-layer phases extending to a depth of 3,000 m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000 m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2θ (upper and lower boundaries, respectively) corresponding to Kübler’s IC limits at 0.25 and 0.42°2θ. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60 % 2M 1 abundance compared to the 1M type at the surface, to 100 % 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite–dolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118–348 °C is estimated for the well with a pressure facies of low to intermediate type.

Fluid-driven low-grade metamorphism in polydeformed rocks of Avalonia (Arisaig Group, Nova Scotia, Canada)

The Lower Silurian—Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite–smectite and chlorite–smectite mixed-layers are common but Na–K mica and kaolinite occur only in some samples. The identification of illite–smectite mixed-layers in diagenetic samples, with Kubler Index >0.50 Δ°2θ and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW–SE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1 km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200 oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.

Very low-grade metamorphism in the para-autochtonous sedimentary cover of the Pelvoux massif (Western Alps, France)

The metamorphic grade from the para-autochtonous cover of the Pelvoux massif (PM, western Alp, France) was investigated in Priabonian metamarls, through mineral assemblages, simultaneous measurements of illite and chlorite “crystallinity”, K-white mica b cell dimension and K-white mica polytypes content. Kubler (KI) and Arkai (AI) indexes display values characteristic for uppermost diagenetic to low anchizonal conditions. These results are supported by the incomplete transformation of K-white mica polytypes from 1M d and 2M 1 and are in line with the not annealed status of zircon fission tracks in the southeast zone of the Pelvoux massif (Seward et al. 1999). Our findings differ from previous results, which suggested epizonal conditions at several places in the northern part of our field area, but are coherent with the temperature estimation suggested by the mineral assemblage described in the literature. These results are further supported by temperature values deduced from fluid inclusion investigations. A small decrease of KI and AI values is observed within the basal decollement level of the Priabonian cover, when going from the least deformed zones to the most deformed ones. K-white mica b cell dimensions in micas of the area are in the range 9.000–9.040 A, which corresponds to an inferred geothermal gradient of 25–35 °C/km, similar to the values found further north in the Dauphinois domain (Ceriani et al. 2003). This indicates a long steady state heat flow corresponding to a long term metamorphic event like in the eastern Alps.

Chlorite of hydrothermal origin formed in the Strzelin and Borów granites (Fore-Sudetic Block, Poland)

Chlorite, a product of very low- to low-grade metamorphism, is frequently used as a geothermometer because of the fact that its structure and chemical composition can reflect the physical and chemical conditions of its formation. In the hydrothermally altered Strzelin and Borow granites (the Fore-Sudetic Block, Poland) chlorite is ubiquitous. It is found in two forms: spherulitic and post-biotite, and was formed in different ways in different parts of the Strzelin and Borow granites: as result of (1) replacement of biotite or (2) crystallisation from fluid. The chlorite formed in the Borow granite shows a higher Fe content than that in the Strzelin granite, a feature related to the content of Mg and Fe in the host rock. Temperatures of chlorite formation are the lowest for unaltered granite and then gradually increase for slightly, moderately and strongly altered granite and are the highest in hydrothermal veins. This means that the temperature of the hydrothermal fluid was higher than that of the altered granitic bodies. Moreover, the spherulitic chlorite formed at a higher temperature than did the post-biotite chlorite, and is usually smaller because biotite replacement lasted longer than did the crystallisation of spherulitic chlorite directly from hydrothermal fluid. Such pattern are likely to occur in other granitic bodies.

Geochemical Evolution of the Banded Iron Formation-Hosted High-Grade Iron Ore System in the Koolyanobbing Greenstone Belt, Western Australia

The banded iron formation (BIF)-hosted iron ore deposits in the lower greenstone succession of the Koolyanobbing greenstone belt, 50 km north of Southern Cross in Western Australia, are a ∼200 Mt high-grade Fe (>58%) pre-mining resource and represents one of the most important iron ore districts in the Yilgarn craton. Four hypogene alteration (ore-forming) stages and one supergene upgrading event took place: (1) During ore stage 1, LREE-depleted, transition metal-enriched, Mg-Fe (±Ca) carbonates replaced quartz in BIFs. The deposit-scale alteration was most likely induced by devolatilization of sea-floor-altered, Ca-Si-depleted mafic rocks in the vicinity of the BIF during early regional (syn-D 1), very low to low-grade metamorphism and was most strongly developed on reactivated BIF-basalt contacts. (2) Ore stage 2 involved the formation of patchy magnetite ore by a syn-D 2 to-D 4 dissolution of early carbonate. Enrichment of Fe 2O 3total in magnetite iron ore was by a factor of 2 to 2.4, and compatible trace elements in magnetite, such as Ga, V, and Al, were immobile. A subdeposit-scale ferroan talc-footprint proximal to magnetite iron ore in the largest deposit (K deposit) was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the BIF under greenschist facies conditions and potentially high fluid/rock ratio. (3) Magnetite growth, during ore stage 3, forming granular magnetite-martite ore is related to a subsequent hydrothermal event, occurring locally throughout the belt, especially in D 2b faults. (4) Ore stage 4 was associated with Fe-Ca-P-(L)REE-Y-enriched hydrothermal fluids, possibly from a magmatic source such as the postmetamorphic Lake Seabrook granite that crops out about 10 km west of the Koolyanobbing deposits and at the southern margin of the greenstone belt. These Ca-enriched fluids interacted with distal metamorphosed mafic rock and influenced the BIF-ore system in a small number of deposits. They were channelled through regional D 4 faults and caused specularitedolomite- quartz alteration, resulting in Fe grades of up to 68%. (5) Supergene upgrade (ore stage 5) by (further) gangue leaching in the weathering zone was most effective in carbonate-altered BIFs and magnetite ore. This process, together with supergene martitization and goethite replacement of magnetite, led to the formation of high-grade, locally (at K deposit) high P goethite-martite ore. At Koolyanobbing, the two geochemically distinct stages of Archean carbonate alteration clearly controlled the formation of hypogene magnetite-specularite- martite-rich ore and recent supergene modification, including the further upgrade of Fe ore. © 2012 by ECONOMIC GEOLOGY.

Thermally‐induced structural and chemical alteration of organic‐walled microfossils: an experimental approach to understanding fossil preservation in metasediments

The identification and confirmation of bona fide Archean-Paleoproterozoic microfossils can prove to be a challenging task, further compounded by diagenetic and metamorphic histories. While structures of likely biological origin are not uncommon in Precambrian rocks, the search for early fossil life has been disproportionately focused on lesser thermally altered rocks, typically greenschist or lower-grade metamorphism. Recently, however, an increasing number of inferred micro- and macrofossils have been reported from higher-grade metasediments, prompting us to experimentally test and quantify the preservability of organic-walled microfossils over varying durations of controlled heating and under two differing redox conditions. Because of their relatively low-intensity natural thermal alteration, acritarchs from the Mesoproterozoic Ruyang Group were chosen as subjects for experimental heating at approximately 500°C, with durations ranging from 1 to 250 days and in both oxic (normal present day conditions) and anoxic conditions. Upon extraction, the opacity, reflectivity, color, microchemistry, and microstructures of the heated acritarchs were characterized using optic microscopy, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results differ for acritarchs prepared under oxic vs. anoxic conditions, with the anoxic replicates surviving experimental heating longer and retaining biological morphologies better, despite an increasing degree of carbonization with continuous heating. Conversely, the oxic replicates show aggressive degradation. In conjunction with fossils from high-grade metasediments, our data illustrate the preservational potential of organic-walled microfossils subjected to metamorphism in reducing conditions, offer insights into the search for microfossils in metasediments, and help to elucidate the influence of time on the carbonization/graphitization processes during thermal alteration.

Compositional Variations in the Mesoarchean Chromites of the Nuggihalli Schist Belt, Western Dharwar Craton (India): Potential Parental Melts and Implications for Tectonic Settings

The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60–75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86) and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts.

Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: an example from the US Cordillera

40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.

Dating the Tethyan Ocean in the Western Alps with radiolarite pebbles from synorogenic Oligocene molasse basins (southeast France)

For the first time in the Western Alps, radiolarite pebbles collected from Tertiary foreland molasse conglomerates are treated for microfossil extraction and dated. Among forty pebbles collected in the field, seven of them released diagnostic radiolarian assemblages ranging in age from Late Bajocian–Early Callovian to Middle Oxfordian–Early Tithonian. These ages overlap previous biochronological data obtained from in situ localities of the Schistes Lustrés Piemont zone of the French-Italian Alps and triple the number of diagnostic radiolarite samples known so far in this segment of the chain. The diagnostic pebbles are characterized by low grade metamorphism, showing that some eroded thrust-sheets from the oldest parts of the ocean escaped any tectonic burial during the Alpine convergence. Mixing of low and high-grade radiolarites, mafics and ultramafics pebbles implies that various ocean-derived units were exposed. This tectonic scenario involves tight refolding and severe uplift of the Eocene subduction wedge in the Early Oligocene.

Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra, Mars: Implications to the alteration history of Mars

The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1km to 42km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20km in diameter). Most detected minerals formed over relatively high temperatures (100–300°C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.

The Chortis Block--southwestern Mexico connections: U-Pb zircon geochronology constraints

The proposed connections between basement terranes of southwestern México and the Chortís Block of Central America are tested in this paper, which focuses on metamorphic units in northern Chortís that outcrop in southeastern Guatemala. Two chief units are recognized: the mid- to high-metamorphic grade Las Ovejas Complex made up of ortho-, para-gneisses, schists, amphibolites and marbles; and the tectonically juxtaposed, low-metamorphic grade San Diego Phyllite. U-Pb geochronology carried out by LA-ICPMS on zircons separated from both units reveal major differences between them. The high-grade metamorphism and deformation of Las Ovejas Complex is bracketed between ∼27 and ∼37 Ma, which correspond to the U-Pb zircon ages of the oldest cross-cutting dikes and youngest zircons in metasedimentary samples. Las Ovejas samples share xenocrystic and detrital zircon populations of Middle-Late Jurassic, Middle-Late Triassic and Permo-Carboniferous age. In contrast, the San Diego Phyllite contains zircons that overlap at ∼830 to 870 and ∼1150 to 1220 Ma, with the youngest concordant zircons yielding Cambrian age. The contrast in population ages supports the idea that Las Ovejas Complex and San Diego Phyllite are different lithotectonic units tectonically juxtaposed during or after their exhumation. When compared with available U-Pb zircon data for southern México, Las Ovejas Complex show similarities to samples of the Guerrero terrane, mainly the Arteaga Complex and Zihuatanejo subterrane, as well as to the southern and northern portions of the Cuicateco terrane. The San Diego Phyllite shows instead similarities with the Teloloapan subterrane of the Guerrero terrane, the Mexcala Formation, the low-grade units of the Cuicateco terrane and the Grenvillian ages of the Oaxacan Complex metaigneous samples, as well as some of the samples belonging to the Maya Block. We propose a tectonic model in which the Las Ovejas Complex was a portion of a fringing arc located in front of the Cuicateco terrane of southwestern México, removed during the Early Tertiary by docking of the Chortís Block represented by the San Diego Phyllite, and tentatively by similar units recognized elsewhere in the Chortís Block in Honduras and Nicaragua. Eocene-Oligocene metamorphism and deformation of the Las Ovejas Complex would be a result of its displacement off southern México, mainly characterized by sinistral stretching and tectonic Cenozoic transport.