Na-rich Plagioclase Research Articles

Provenance and Tectonic Setting of Late Lower Cretaceous (Albian) Kazhdumi Formation Sandstones (SW Iran)

Provenance and tectonic setting of the Late Lower Cretaceous (Albian) sandstones in SW Iran have been interpreted on the basis of geochemistry (major and trace elements) of 35 samples from Iranian offshore oil fields (Soroosh, Nowrooz, Foroozan and Hendijan). Geochemically, trace-element concentrations, such as La, Th, Sc and Zr, and ratios, such as La/Sc, Th/Sc, La/Co, and Th/Co, of sandstones from Kazhdumi Formation at four localities indicate that they were derived from felsic source rocks and deposited in a passive continental margin. Most major and trace element contents of Kazhdumi sandstones are generally similar to upper continental crust (UCC) values. However, some samples are depleted in major and trace elements (such as K2O, Na2O, Sr and Ba) relative to UCC, which is mainly due to absence or smaller amount of Na-rich plagioclase and K-feldspar and also paucity of clay minerals, which is possibly due to intense weathering and recycling. Enrichments in Zr, negative Sr anomalies and Th/U ratios higher than 4 for Kazhdumi sandstones are further evidence for recycled sources. The study of paleoweathering conditions based on chemical index of weathering (CIW) indicates a moderate to intense weathering of first cycle sediments, or it may alternatively reflect recycling under semi-arid to semi-humid climate conditions in Kazhdumi sandstones. The results of this study suggest that the main source for the Kazhdumi sandstones is likely located in uplifted shoulders of a rifted basin (Neo-Tethys) in its post-rift stage (Arabian basement).

Palaeoarchaean felsic magmatism: A melt inclusion study of 3.45 Ga old rhyolites from the Barberton Greenstone Belt, South Africa

The 3.45Ga old felsic volcanic rocks from the Barberton Greenstone Belt of southern Africa include a submarine succession of felsic lavas and volcaniclastic rocks and shallow subvolcanic intrusions. Sea floor alteration strongly affected rock compositions shortly after emplacement, making comparisons with coeval intrusions challenging. We analysed well-preserved quartz-hosted melt inclusions from the felsic rocks, which offers a unique opportunity to gain insight into pre-alteration melt compositions. Melt inclusion compositions are then compared with whole-rock analyses to evaluate the significance of the volcanic complex in the context of Archaean felsic magmatism in the Barberton Greenstone Belt. Whole-rock immobile trace element concentrations, including high-field strength and rare earth elements, indicate strong similarities with coeval tonalite–trondhjemite–granodiorite intrusions. These same geochemical characteristics distinguish these rocks from other Archaean felsic rocks in the Barberton Greenstone Belt (e.g. the 3.54Ga Theespruit Formation), which have characteristically higher Th, Nb and REE. Low CaO (≤1wt.%) and relatively high Nb and Th (mostly ≤9 and ≤13ppm, respectively) in comparison with least altered whole-rock samples, and the presence of negative Eu anomalies are consistent with the melt inclusions being relatively evolved, rather than representing primitive compositions. Melt inclusion analyses suggest the presence of two melts with varying K2O/Na2O. Melt inclusions from extrusive samples, which contain remnants of K-feldspar phenocrysts, have K2O/Na2O>1 by weight, whereas subvolcanic intrusions, which contain Na-rich plagioclase, have K2O/Na2O<1. Moderately high Cl contents (Cl≤0.7wt.%) of melt inclusions may record interaction of the melting protolith with sea water prior to melting. We speculate that the varying K2O/Na2O in the felsic volcanic rocks was due to melting of a greenstone succession heterogeneously affected by sea floor alteration.

Metamorphic PT path and zircon U–Pb dating of Archean eclogite association in Gridino complex, Belomorian province, Russia

The eclogites from the Gridino eclogite-bearing mélange complex in the Belomorian province, Fennoscandian Shield may represent one of the oldest terrain in the world. The Belomorian province is a superposition of Archean and Palaeoproterozoic collisional orogens, which experienced repeated high-grade metamorphic reworking and intense polyphase tectonic deformation in the early Precambrian. Main purpose of this study is to determine the metamorphic age(s) and evolutionary PT path of eclogite as boudin-like body within the Gridino tectonic mélange. Representative rock materials of well-preserved, plagioclase-free eclogite and associated retrogressed eclogitic rock were sampled from Stolbikha Island in the Gridino area for our research. The eclogite sample mainly comprises omphacite, garnet, amphibole, rare quartz, and some accessory minerals (rutile, magnetite), whereas the retrogressed eclogite sample is made of diopside – Na-rich plagioclase symplectites, amphibole, quartz, and accessory minerals. Thermodynamic modeling points to a clockwise metamorphic PT path, with peak conditions of P=18.5kbar, T=710°C, and a “warm” subduction is inferred thus. LA-ICP-MS zircon U–Pb dating yields diverse metamorphic episodes from the Neoarchean to the Palaeoproterozoic. While metamorphic zircon formed at eclogite-facies condition is identified by studying the trace element distribution and partitioning in and between zircon and co-existing eclogite-facies minerals (omphacite, garnet); consequently, Neoarchean eclogite-facies metamorphic event of ca. 2.70Ga can be distinguished for the Gridino eclogite complex, and a regional-scale thermal event may have occurred in the period of 2.70–2.61Ga.

Algoma-type banded iron formation (BIF), Abitibi Greenstone belt, Quebec, Canada

A typical Algoma-type banded iron formation (BIF) occurs in Orvilliers, Montgolfier, and Aloigny townships in the Abitibi Greenstone belt, Quebec, Canada. The BIF is composed of millimeter to decimeter thick beds of alternating fine-grained, dark gray to black, well laminated, magnetite-rich (and/or hematite) beds and quartz–feldspar metasedimentary (graywacke) beds. The BIF is well defined by magnetic anomalies. These BIF layers are commonly associated with decimeter to meter thick horizons of metasedimentary rocks and mafic to intermediate volcanic rocks, which are locally crosscut by dikes of felsic or mafic intrusive rocks and, as well, narrow dikes of lamprophyre. The upper and lower contacts of the BIF are gradational with the adjacent graywacke. All geological units in the area are metamorphosed to the greenschist facies of regional metamorphism. Magnetite is mainly associated with subordinate amounts of hematite, quartz, Na-rich plagioclase, and muscovite. The fine-grained magnetite content is composed of 77% to 89% of the principal iron oxide minerals present. The magnetite occurs as disseminated idiomorphic to sub-idiomorphic small crystals, which average 20μm±5μm in size. Hematite is the second most abundant iron oxide mineral. Although less abundant, red jasper occurs in cherty horizons with strongly folded fragments and within fault zones. This particular Algoma-type iron formation stratigraphically extends more than 36km along strike. It dips sub-vertically with a true width from 120m to 600m. The origin of the BIF is closely linked to regionally extensive submarine hydrothermal activity associated with the emplacement of volcanic and related subvolcanic rocks in an Archean greenstone belt.

Garnet-forming reactions in felsic orthogneiss: Implications for densification and strengthening of the lower continental crust

Growth of garnet and pyroxene in orthogneiss from the Athabasca granulite terrane (AGT), northern Saskatchewan, provides a model for progressive densification and strengthening of the lower continental crust with implications for the recycling and long-term evolution of continental crust. Two distinct assemblages and textures are preserved in granitic and granodioritic gneiss. Low-strain orthogneiss displays igneous textures and assemblages of Opx+Kfs+Pl+Mag+Qz (± Bt, Hbl, Ilm). High-strain, dynamically recrystallized tectonites have additional garnet, clinopyroxene, and a more Na-rich plagioclase, along with relict orthopyroxene. The reaction (Opx+Ca-rich Pl=Grt+Cpx+Na-rich Pl+Qz) is informally called the “Mary reaction” after documented occurrences in the Mary granitoid batholith. The reaction represents the transition from medium-pressure to high-pressure granulite (Green and Ringwood, 1967), but reaction progress was achieved in these deep crustal rocks along an isobaric cooling path at ca. 1 GPa (35–40 km-depth). Ambient P–T conditions were well within the product (low-T-side) stability field. The abundance of the product assemblage (Grt+Cpx+Na-rich Pl) increases with deformation. Metastable igneous assemblages are widely preserved in low-strain samples. With increasing strain, garnet occurs within recrystallized mantles of plagioclase porphyroclasts, and clinopyroxene occurs in the deformed tails of orthopyroxene crystals. Deformation is interpreted to aid in the breakdown of plagioclase and/or the nucleation of garnet and clinopyroxene. Garnet and pyroxene modes have been observed to exceed 10 vol% in the AGT, but larger amounts are possible because Ca-rich plagioclase and orthopyroxene remnants are widely preserved. Densities increase from ca. 2.6 to ca. 3.0 g/cm3 and modeled P-wave velocities approach 7.0 km/s in felsic rocks. Densities in mafic rocks approach 3.4 cm3. The reaction occurred at least twice in the AGT, 2.6 and 1.9 Ga, and may have occurred at other times during long-term deep crustal residence. Thus, densification can occur incrementally and correspond with the time and intensity of tectonism and concomitant recrystallization. The reaction provides a mechanism for significant densification and strengthening of lower continental crust long after initial stabilization. It may explain the discrepancy between seismic velocities suggestive of mafic/intermediate compositions at lower crustal depths and the observed abundance of felsic rocks in exposed granulite terranes.

Record of S-rich vapors on asteroid 4 Vesta: Sulfurization in the Northwest Africa 2339 eucrite

In the Northwest Africa (NWA) 2339 eucrite, a few high-Fe pigeonite fragments were partially replaced by fine-grained troilite, Mg-rich augite, silica, with minor ilmenite and Mg-rich ferrosilite. The relict pigeonite has distinctly lower Fe/Mn values (17–18) than those (∼30) of augite and Mg-rich ferrosilite in the fine-grained replacement textures and other eucritic pyroxenes in NWA 2339. The few plagioclase grains associated with these Fe-rich pigeonite fragments contain high albite components (Ab17–31). Similar replacement textures are also present in a lithic clast consisting of pyroxene and Na-rich plagioclase (Ab20–25). In this lithic clast, high-Fe subcalcic augite was replaced by troilite, Mg-rich hedenbergite, silica, and minor ilmenite whereas Mg-rich pigeonite remains intact. Troilite also occurs as tiny inclusions in porous regions of plagioclase. Moreover, the porous regions of plagioclase are enriched in Si and depleted in Al, Ca, and Na, compared to relict plagioclase. These textures indicate that the formation of troilite-rich mineral assemblages is related to the reaction between FeSiO3 component in pyroxene and S-rich vapors; whereas, the formation of tiny troilite in porous regions of plagioclase is related to the reaction between Fe2+ in plagioclase and S-rich vapors. X-ray mapping results reveal that during sulfurization of pyroxene and plagioclase, lithophile elements Mn, Al, Ca, Na, and Mg were removed to various extents, very likely by the S-rich vapors. Based on our observations, the S-rich vapors could be dry S–O–P vapors, which most likely have been formed by volatilization of pre-existing S,P-bearing materials due to impact heating on the surface of asteroid 4 Vesta.

The implication of sodium-rich plagioclase minerals contaminating K-rich feldspars aliquots in luminescence dating

Luminescence dating was performed on three different sediments, collected in Italy. Measurements were accomplished on K-rich minerals extract, where we dispensed a small amount of grains on each aliquots. Anomalous fading was measured for each aliquot. We observed a negative correlation between the measured age and associated rate of fading, for each sample. In two samples, a few outliers were identified, where they displayed a lower measured age and fading rate. We positively identified the presence of Na-rich plagioclase minerals dominating the luminescence output of these outliers.

A geochemical investigation of trace elements in well RN-17 at Reykjanes geothermal system, SW-Iceland

Here we present the results of an investigation of well RN-17 at Reykjanes geothermal system (Reykjanes Peninsula, SW-Iceland). We show that the adoption of micro-spot secondary ion mass spectrometry (SIMS) together with bulk-rock techniques, such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and thermal ionisation mass spectrometry (TIMS) carried out on 10 basaltic drill-cuttings over a depth of 3000 m gives us insights into the geochemical characteristics of the well. Whole rock and mineral phases, i.e., plagioclase, clinopyroxene, epidote and amphibole, indicate that the rocks pertaining to the cuttings were affected by hydrothermal alteration that has been able to modify significantly the Sr isotope ratios and the concentration of K, Rb, Cs, Ba and B. Seawater/rock ratio, evaluated for 87Sr/86Sr, achieved its maximum for the shallow cutting RN-17-650, decreasing although not systematically with depth, with the minimum value for the deep cutting RN-17-2800. Moreover, the occurrence of Na-rich plagioclase, epidote and amphibole down to the well bottom are evidence of the persistence of fluid/rock interaction at depth.

Transmission electron microscopy of omphacite and other minerals in eclogites from the CCSD borehole, China: indications for their deformation and temperature history

Seven eclogite samples from 223 to 584 m depths in the Chinese Continental Scientific Drilling main borehole (CCSD-MH) in the Sulu ultrahigh-pressure metamorphic (UH P M) terrane, Eastern China, were studied by transmission electron microscopy (TEM), in order to characterise their microstructures. We observed, among others, omphacite, jadeitic diopside, garnet, amphibole, rutile, Na-rich plagioclase, quartz, K-feldspar, analcime, and diamond (contamination). Omphacite shows a well-known diffusion-controlled disorder-order phase transition which gives rise to antiphase domains (APDs). In our samples we observed a variation of the APDs’ size between ~5 nm and 2 μm which is correlated with the content in jadeite (Jd) component. The broad maximum of ~1–2 μm APDs’ size is centred on Jd50. This size drops to ~5–10 nm for Jd37 and Jd66. The size variation can be explained with the T -path of the respective omphacites. While the large APDs of the omphacites formed and coarsened during subduction and exhumation, the smallest APDs formed and grew during exhumation. In contrast to other eclogite occurrences, deformational defects in omphacites from the CCSD-eclogites are mostly very rare. Occasionally, stacking faults parallel to (010), free dislocations with the Burgers vector [001] and those with 1/2[110] connected with antiphase domain boundaries (APBs), deformation twins on (100) and small-angle grain boundaries were observed. The explanation is that most CCSD-omphacites were strongly affected by recrystallisation, which took place above 600 °C during subduction up to its maximum temperature of ~750 °C and continued during exhumation above 600 °C into the regime of amphibolite facies for several tens of Ma. Garnet shows very rarely crystal defects, while amphiboles usually displayed dislocations and chain multiplicity faults. Rutile of one sample, which contained a few percent Fe, showed fully coherent, nano-sized platelets (Guinier-Preston zones) parallel to (100) and (010). The existence of fluids during retrogression is documented by K-feldspar and analcime in quartz.

Lightning-induced mineral/chemical changes in red pine ( Pinus resinosa)

Red pine ( Pinus resinosa) growing in sandy soil of Glacial Lake Whittlesey age (~ 13–12 ka) in the town of Waterford, Ontario, Canada, north of Lake Erie, was struck by lightning in a fierce thunderstorm, May, 2010. The lightning strike was strong enough to destroy television sets and telephones in nearby houses and two red pines were struck, one split down the middle from above the bole to near the crown. Samples of bark and inner tissue were examined to determine the composition of black silt-size specs seen on the severed material. In addition to minerals found in abundance in the local soil, C clots/spherules, disrupted cellulose structures, blackened tissue, fused disfigured Si and metallic sheets of Na, the latter sourced from melted albite (Na-rich plagioclase), were analyzed. Many silt and sand size particles, sourced from the local soil were presumably electrically-levitated along and within the bark by the lightning discharge. Iron-rich micas, other Fe minerals and a great range of light minerals moistened by torrential rain were among the silts and sands transported to great height and embedded in the bark.

Constructing modal mineralogy from geochemical composition: A geometric-Bayesian approach

Modal mineralogical composition is known to carry more information than major element geochemistry, though the latter is far easier to determine in the lab. Constructing mineral compositions from geochemistry can be seen as a typical end-member problem, where one assumes that some multivariate observations are generated by a convex linear mixture of a few pure end-members: these end-member characteristics as well as the coefficients of the linear mixture for the observations can be then estimated from geochemical data. We propose a mixed geometric-probabilistic solution to this problem. First, we assume known end-members, in number and properties, and study the set of solutions from a purely geometric perspective. Second, we discuss how to select representative solutions from this space, in particular under some distributional assumptions. Third, we allow the end-member properties to randomly vary in a controlled, interpretable fashion. Finally we build a Bayesian model, with a parsimonious parametrization characterizing each of these three steps, that can be treated by conventional Markov-Chain Monte Carlo techniques. In the illustration case study, we apply the method to reconstruct the mineralogy of a set of fluvio-glacial monomictic sediments from an Alpine granitoid massif. Results suggest a trend of enrichment in chlorite, muscovite and Ti-bearing minerals, along with depletion in quartz and feldspar. This is tentatively interpreted as an effect of comminution combined with differential mechanical properties. Moreover, mineral chemistry is estimated to exhibit very low Na in muscovite, Fe-rich garnet, Na-rich plagioclase, K-feldspar with up to 10% Na-component (albite), and biotite with Mg > Fe . The reconstructed modal mineralogy stays in a reasonable agreement with quantitative XRD phase analyses.

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3–1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35–70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20–45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40–50 wt.% SiO 2, 10–35 wt.% Al 2O 3, 1–8 wt.% FeO, and <25 wt.% TiO 2; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.

Spectral emissivity measurements of Mercury's surface indicate Mg- and Ca-rich mineralogy, K-spar, Na-rich plagioclase, rutile, with possible perovskite, and garnet

Mid-infrared 2-D spectroscopic measurements from 8.0 to 12.7 μm of Mercury were taken using Boston University's Mid-Infrared Spectrometer and Imager (MIRSI) mounted on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, 7–11 April 2006. Measurements reported here cover radar bright region C, a dark plains region west of Caloris Basin, and the interior of Caloris Basin. By use of spectral deconvolution with a large spectral library composed of many mineral compositions and grain size separates, we fitted, or “unmixed”, the Mercury spectra. We find mineral suites composed of magnesium-rich orthopyroxene and olivine, Ca-, Mg-, Na-rich clinopyroxene, potassium feldspar, and Na-bearing plagioclase feldspar. Both Ca- and Mg-rich garnet (pyrope and grossular, respectively) are apparently present in small amounts. Opaque minerals are required for spectral matching, with rutile (TiO 2) repeatedly providing the “best fit”. However, in the case of the radar bright region C, perovskite also contributed to a very good fit. Caloris Basin infill is rich in both potassium feldspar and Na-rich plagioclase. There is little or no olivine in the Caloris interior smooth plains. Together with the high alkali content, this indicates that resurfacing magmas were low to intermediate in SiO 2. Data suggest the dark plains exterior to Caloris are highly differentiated low-iron basaltic magmas resulting in material that might be classified as oligoclase basalts.

High-pressure garnet-bearing migmatites from the Chepelare area, Central Rhodope

The association of Ca-rich garnet and Na-rich plagioclase in migmatites is a proper indicator for high-pressure melting. Garnet-bearing leucosome and aplitoid gneiss from the Mechi Chal peak in the Chepelare area of the Central Rhodope comprise a mineral assemblage produced by high-pressure melting: Ca-rich garnet (grossular 22–33 %), Na-rich antiperthitic plagioclase (An15–20), perthitic K-feldspar, quartz, and minor back reaction biotite replacing garnet rims. The approximate conditions of melt crystallization are in the range 1.6–1.9 GPa/800–850°C that is well above the P–T estimates available for the Central Rhodope migmatites. The study focuses attention on the key role of the high-pressure melting in the Alpine metamorphic evolution of the Central Rhodope.

Silicate weathering in temperate forest soils: insights from a field experiment

Few studies of silicate mineral weathering have been conducted in carbonate-bearing temperate forest soils. With climate and vegetation held constant, we compared soil mineralogy and major element chemistry of soil waters from a carbonate-free temperate aspen forest site in the Cheboygan watershed, northern Michigan, with that from carbonate-containing soils from experimental tree-growth chambers (low- vs. high- fertility). All soils were well-drained sands (quartz, Na-rich plagioclase, and K-feldspar) with minor amounts of carbonate present only in the experimentally manipulated soils. The Na+ concentrations in soil waters corrected for atmospheric deposition (Na*) were used to compare relative rates of plagioclase feldspar weathering across sites. In natural soil water profiles, maximum concentrations of Na*, Si, and dissolved organic carbon (DOC) were observed by a depth of 15 cm, a soil zone free of carbonate minerals. Mean Na* and DOC concentrations were different in the three soils, and increased in the order natural soil < low-fertility chambers < high-fertility chambers. While low pH environments are generally viewed as enhancing weathering rates, here higher Na* appears to be related to high DOC, which is consistent with observed increases in active organic functional groups as pH increases. Our results suggest that under a specific vegetative cover, the soil carbon environment affects the weathering flux observed. Our study also suggests that disturbed soils provide an enhanced physical and chemical environment for weathering. Generalized silicate weathering models may benefit from including the enhancing effects of organic anions at moderate pH in addition to precipitation and temperature.

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean–continent transition similar to the presentday western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet þ sodic Al-augite (Na2O 2.5 wt %, Al2O3 12.5 wt %), with accessory graphite, Fe–Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An50–45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene þ plagioclase (An85–72) þ Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine þ plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes þ Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene þ Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu–Hf and Sm–Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T 1100 C and P 2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T 950 , and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase–olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenitebearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation.

Mineral zoning and gold occurrence in the Fortuna skarn mine, Nambija district, Ecuador

The Fortuna oxidized gold skarn deposit is located in the northern part of the Nambija gold district, southern Ecuador. It has been subdivided into four mineralized sites, covering a distance of 1 km, which are named from north to south: Cuerpo 3, Mine 1, Mine 2, and Southern Sector. Massive skarn bodies occur in K–Na metasomatized volcanic and volcaniclastic rocks of the Triassic Piuntza unit. They appear to result from selective replacement of volcaniclastic rocks. Very minor presence of bioclast relicts suggests the presence of subordinate limestone. Endoskarn type alteration with development of Na-rich plagioclase, K-feldspar, epidote, actinolite, anhedral pyroxene, and titanite affects a quartz–diorite porphyritic intrusion which crops out below the skarn bodies in Mine 2 and the Southern Sector. Endoskarn alteration in the intrusion grades into a K-feldspar ± biotite ± magnetite assemblage (K-alteration), suggesting that skarn formation is directly related to the quartz–diorite porphyritic intrusion, the latter being probably emplaced between 141 and 146 Ma. The massive skarn bodies were subdivided into a dominant brown garnet skarn, a distal green pyroxene–epidote skarn, and two quartz-rich varieties, a blue-green garnet skarn and light green pyroxene–garnet skarn, which occur as patches and small bodies within the former skarn types. The proximal massive brown garnet skarn zone is centered on two 060° trending faults in Mine 2, where the highest gold grades (5–10 g/t) were observed. It grades into a distal green pyroxene–epidote skarn zone to the North (Cuerpo 3). Granditic garnet shows iron enrichment from the proximal to the distal zone. Diopsidic pyroxene exhibits iron and manganese enrichment from proximal to distal zones. The retrograde stage is weakly developed and consists mainly of mineral phases filling centimeter-wide veins, vugs, and interstices between garnet and pyroxene grains. The main filling mineral is quartz, followed by K-feldspar, epidote, calcite, and chlorite, with minor sericite, apatite, titanite, hematite, pyrite, chalcopyrite, and gold. Metal and sulfur contents are low at Fortuna, and the highest gold grades coincide with high hematite abundance, which suggests that retrograde stage and gold deposition took place under oxidizing conditions. Fluid inclusions from pyroxene indicate precipitation from high temperature—high to moderate salinity fluids (400 to 460°C and 54- to 13-wt% eq. NaCl), which result probably from boiling of a moderately saline (∼8-wt% eq. NaCl) magmatic fluid. Later cooler (180 to 475°C) and moderate to low saline fluids (1- to 20-wt% eq. NaCl) were trapped in garnet, epidote, and quartz, and are interpreted to be responsible for gold deposition. Chlorite analysis indicates temperature of formation between 300 and 340°C in accordance with fluid inclusion data. It appears, thus, that gold was transported as chloride complexes under oxidizing conditions and was deposited at temperatures around 300°C when transport of chloride complexes as gold carriers is not efficient.

High-pressure mafic granulites in the Trans-North China Orogen: Tectonic significance and age

High-pressure mafic granulites (including retrograded eclogites) have been reported from the Trans-North China Orogen, a Paleoproterozoic orogenic belt along which two discrete continental blocks, referred to as the Eastern and Western Blocks, were amalgamated to form the North China Craton. Extensive metamorphic investigations and geochronology carried out over the last few years provide important insights into the age and significance of these high-pressure granulites, which are critical in understanding of the timing and tectonic processes involved in the assembly of the North China Craton. Most high-pressure mafic granulites in the Trans-North China Orogen preserve the high-pressure granulite facies assemblage garnet + plagioclase + clinopyroxene + quartz, the medium-pressure granulite facies assemblage garnet + plagioclase + clinopyroxene + orthopyroxene ± quartz, the low-pressure granulite facies assemblage orthopyroxene + clinopyroxene + plagioclase ± quartz, and the amphibolite facies assemblage hornblende + plagioclase. Minor high-pressure granulites preserve the early eclogite facies mineral assemblage of garnet + quartz + omphacite pseudomorph (clinopyroxene + Na-rich plagioclase), indicating that they are retrograded eclogites. These mineral assemblages and their P–T estimates define a clockwise P–T path involving near-isothermal decompression and cooling following the peak high-pressure metamorphism, which suggests that they formed during continent–continent collision. Field mapping and geochronology indicate that the precursors of these high-pressure granulites were mafic dykes which were emplaced at ∼1915 Ma and underwent high-pressure granulite facies metamorphism at ∼1.85 Ga. Taken together, the high-pressure granulites in the Trans-North China are considered to have resulted from final collision between the Eastern and Western Blocks to form the North China Craton at ∼1.85 Ga, not at ∼2.5 Ga as recently proposed by some authors.

EXHUMATION HISTORY OF SUBCONTINENTAL GARNET PYROXENITE-BEARING MANTLE FROM THE EXTERNAL LIGURIDE OPHIOLITES (NORTHERN APENNINE, ITALY): IMPLICATIONS FOR RIFTING AND MANTLE EXHUMATION PROCESSES AT OCEAN-CONTINENT TRANSITION

The External Liguride Jurassic ophiolites derive from an ocean-continent transition similar to the West Iberian margin. The mantle sequence consists of spinel and spinel-plagioclase lherzolites recording a lithospheric thinning history culminated into mantle exhumation at the ocean-floor. They represent unroofed subcontinental litosphere of the former Adria-Europe system largely affected by thermochemical erosion, heating and refertilization by asthenospheric melts during the Ligurian Tethys formation (Rampone et al., 1995; Piccardo et al., 2004). Peridotite bodies which escaped melt contamination and intrusion preserve relics of a deep-seated litospheric origin, i.e. occurrence of graphitebearing garnet clinopyroxenite and websterite layers. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites therefore represents a unique tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. Garnet clinopyroxenites are mafic rocks with a primary assemblage of pyrope-rich garnet + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~12.5 wt%), with minor amounts of orthopyroxene, graphite, Fe-Ni sulphides and rutile. Decompression yielded Na-rich plagioclase (An50-45) exsolutions in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An85-72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under synkinematic conditions, at the expenses of two-pyroxene + Alspinel. Mg-rich garnet has been also found in websterite layers, which are commonly characterised by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu-Hf and Sm-Nd mineral isochrons (220 ± 13 Ma and 186 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. The early decompression was associated with significant cooling, corresponding to T = 900-1050°C, and development of the spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase-olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution was characterized by a widespread polyphase brittle deformation under decreasing temperature conditions, coupled with hydration. In particular, the brittle evolution includes an early amphibolite-facies veining stage, followed by low-temperature hydrothermal alteration and serpentinisation associated with polyphase cataclasis. The exhumation was likely accomplished as a two-step process starting during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, possibly including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence was involved into an extensional detachment structure up to sea-floor exhumation, which was associated with development of serpentinisation-related hydrothermal activity. Integrated values of cooling and uplift rates for the Jurassic rifting evolution are ~35°C/Ma and 0.6-1.4 mm yr-1.

Application of fluid inclusions to the study of Bagnore geothermal field (Tuscany, Italy)

A fluid inclusion study of the hydrothermal minerals in two breccias from two wells in the Bagnore geothermal field (Italy) has provided information on the evolution of the fluids, and has also demonstrated that fluid inclusions can be utilized as geothermometers in this geothermal field. Both breccias come from reservoir zones: one (Bagnore 3bis (Bg 3bis)) was cored at a depth of 3111 m below ground level (b.g.l.), whereas the other (Bagnore 22 (Bg 22)) was ejected during a blow-out, probably from a fractured zone present between 2200 and 2300 m b.g.l. The hydrothermal cement of the breccias is mostly made up of quartz, K-feldspar, Na-rich plagioclase, calcite, chlorite and illite. Fluid inclusion studies were carried out on quartz (Bg 3bis and Bg 22 breccias) and adularia (Bg 22 breccia). Three types of fluid inclusions were recognized in the Bg 3bis breccia. Type I (liquid-rich) inclusions trapped an aqueous fluid with a CO 2 concentration (1.7–2.7 mol/kg) that is significantly higher than present-day fluids (0.5 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with less CO 2 (0.6–1.0 mol/kg). Type III (vapor-rich) coexist with type I inclusions, and record an early fluid circulation under boiling conditions. The decrease of the CO 2 (and total gas) concentrations from type I inclusions to type II inclusions, and on to present-day conditions can be related to boiling with gas loss and/or mixing. Only one type of fluid inclusion (type II), with moderate CO 2 concentration (0.7–0.3 mol/kg), was found in the Bg 22 breccia. Boiling and/or mixing explain the variation of the CO 2 content in the Bg 22 reservoir fluid from inclusion formation to modern CO 2 concentration (0.3 mol/kg). The absence of any type I inclusions in Bg 22 breccia may be related to non-uniform CO 2 concentrations in different parts of the field. Present-day temperatures (295±10 °C for Bg 3bis and 320±10 °C for Bg 22) are close or equal to fluid inclusion average total homogenization temperatures (around 290 °C for Bg 3bis and 320 °C for Bg 22), suggesting that fluid inclusions can be useful for estimating local temperatures when direct measurements are not available or dubious.