Microtextural Evidence Research Articles

Recycling argon through metamorphic reactions: The record in symplectites

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12kbar at ~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of 40Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history.

Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Abstract Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ34S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ34SVCDT (Vienna Canyon Diablo troilite) values between +5‰ and −5‰, while the rims show δ34SVCDT values between −20‰ and −24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.

Cryogenian magmatism and crustal reworking in the Southern Granulite Terrane, India

Understanding Neoproterozoic crustal evolution is fundamental to reconstructing the Gondwana supercontinent, which was assembled at this time. Here we report evidence of Cryogenian crustal reworking in the Madurai Block of the Southern Granulite Terrane of India. The study focuses on a garnet-bearing granite–charnockite suite, where the granite shows in situ dehydration into patches and veins of incipient charnockite along the contact with charnockite. The granite also carries dismembered layers of Mg–Al-rich granulite. Micro-textural evidence for dehydration of granite in the presence of CO2-rich fluids includes the formation of orthopyroxene by the breakdown of biotite, neoblastic zircon growth in the dehydration zone, at around 870°C and 8 kbar. The zircon U–Pb ages suggest formation of the granite, charnockite, and incipient charnockite at 836 ± 73, 831 ± 31, and 772 ± 49 Ma, respectively. Negative zircon εHf (t) (−5 to −20) values suggest that these rocks were derived from a reworked Palaeoproterozoic crustal source. Zircon grains in the Mg–Al-rich granulite record a spectrum of ages from ca. 2300 to ca. 500 Ma, suggesting multiple provenances ranging from Palaeoproterozoic to mid-Neoproterozoic, with neoblastic zircon growth during high-temperature metamorphism in the Cambrian. We propose that the garnet-bearing granite and charnockite reflect the crustal reworking of aluminous crustal material indicated by the presence of biotite + quartz + aluminosilicate inclusions in the garnet within the granite. This crustal source can be the Mg–Al-rich layers carried by the granite itself, which later experienced high-temperature regional metamorphism at ca. 550 Ma. Our model also envisages that the CO2 which dehydrated the garnet-bearing granite generating incipient charnockite was sourced from the proximal massive charnockite through advection. These Cryogenian crustal reworking events are related to prolonged tectonic activities prior to the final assembly of the Gondwana supercontinent.

Continuous zircon growth during long-lived granulite facies metamorphism: a microtextural, U–Pb, Lu–Hf and trace element study of Caledonian rocks from the Arctic

Microtextural, U–Pb, trace element and Lu–Hf analyses of zircons from gneisses dredged from the Chukchi Borderland indicate a long-lived, Cambrian–Ordovician, granulite facies metamorphism. These results reveal a complete prograde, peak and cooling history of zircon growth during anatexis. Early increasing temperatures caused modification and Pb-loss of Precambrian zircons by recrystallization and dissolution/re-precipitation of existing grains. Small variations in initial 176Hf/177Hf results (0.282325–0.282042) and flat HREE patterns of these zircons indicate that they grew by dissolution/re-precipitation in the presence of garnet. Zircons subsequently crystallized from a partial melt during peak to post-peak metamorphism from 530 to 485 Ma. A broad range of initial 176Hf/177Hf ratios (0.282693–0.282050) and mineral inclusions within zircons suggest that this phase of growth incorporated Zr and Hf obtained from the breakdown of Zr-enriched phases. Microtextural evidence along with trace element and isotopic data suggests that final growth of metamorphic rims on zircon occurred during slow cooling and crystallization of residual partial melts during the early Ordovician (485–470 Ma). Younger, late Ordovician–Silurian (420–450 Ma) euhedral, oscillatory-zoned, trace element-enriched zircons crystallized within leucocratic veins that intrude the gneisses. Their age corresponds to granitoids dated from this same dredge. The intrusives and veins provide evidence that the Chukchi Borderland rifted from a position near Pearya and northwest Svalbard, which represent the northern continuation of the Caledonian orogen. Evidence for earlier Cambrian metamorphism has not been reported from this region. The age of granulite facies metamorphism reported here represents the earliest phase of deformation in the Arctic Caledonides.

Timescales and mechanisms of formation of amorphous silica coatings on fresh basalts at Kīlauea Volcano, Hawai'i

Young basalts from Kīlauea Volcano, Hawai'i, frequently feature opaque surface coatings, 1–80μm thick, composed of amorphous silica and Fe–Ti oxides. These coatings are the product of interaction of the basaltic surface with volcanically-derived acidic fluids. Previous workers have identified these coatings in a variety of contexts on Hawai'i, but the timescales of coating development, coating growth rates, and factors controlling lateral coating heterogeneity were largely unconstrained. We sampled and analyzed young lava flows (of varying ages, from hours to ~40years) along Kīlauea's southwest and east rift zones to characterize variation in silica coating properties across the landscape. Coating thickness varies as a function of flow age, flow surface type, and proximity to acid sources like local fissure vents and regional plumes emitted from Kīlauea Caldera and Pu'u Ō'ō. Silica coatings that form in immediate proximity to acid sources are more chemically pure than those forming in higher pH environments, which contain significant Al and Fe. Incipient siliceous alteration was observed on basalt surfaces as young as 8days old, but periods of a year or more are required to develop contiguous coatings with obvious opaque coloration. Inferred coating growth rates vary with environmental conditions but were typically 1–5μm/year. Coatings form preferentially on flow surfaces with glassy outer layers, such as spatter ramparts, volcanic bombs, and dense pahoehoe breakouts, due to glass strain weakening during cooling. Microtextural evidence suggests that the silica coatings form both by in situ dissolution–reprecipitation and by deposition of silica mobilized in solution. Thin films of water, acidified by contact with volcanic vapors, dissolved near-surface basalt, then precipitated amorphous silica in place, mobilizing more soluble cations. Additional silica was transported to and deposited on the surface by silica-bearing altering fluids derived from the basalt interior.

A universal power-law scaling exponent for fracture apertures in sandstones

A high-resolution data set of kinematic aperture (opening displacement) of opening-mode fractures, from large (up to 2 m long) quartz-cemented sandstone samples, shows that microfractures are ubiquitous and that most natural-fracture sets are better fit by power-law size distributions than by exponential, normal, or log-normal distributions. The data set includes 3822 fractures within 68 scanlines from eight formations on three continents. Kinematic apertures were measured along scanlines using scanning electron microscope–based cathodoluminescence (SEM-CL) and, for field data, using a hand lens. Microtextural evidence from SEM-CL shows that power law–distributed fractures typically have crack-seal texture and are composed of opening increments having a narrow (characteristic) aperture size range. In contrast, rare non-power-law–distributed fracture populations lack crack-seal texture. Power-law exponents, as measured in one dimension, have values of −0.8 ± 0.1. Most variation among fracture sets results from power-law coefficients, which constitute a scale-invariant measure of fracture intensity. We show how observed scaling patterns can be used to improve estimations of large-fracture spacing in cases where fracture sampling is limited, as by the width of cores. The low (

Recent developments in the analysis of the black mat layer and cosmic impact at 12.8 ka

Recent analyses of sediment samples from “black mat” sites in outh merica and urope support previous interpretations of an impact event that reversed the Late Glacial demise of ice during the Bølling Allerød warming, resulting in a resurgence of ice termed the Younger Dryas (YD) cooling episode. The breakup or impact of a cosmic vehicle at the boundary coincides with the onset of a 1‐kyr long interval of glacial resurgence, one of the most studied events of the Late Pleistocene. New analytical databases reveal a corpus of data indicating that the cosmic impact was a real event, most possibly a cosmic airburst from Earth's encounter with the Taurid Complex comet or unknown asteroid, an event that led to cosmic fragments exploding interhemispherically over widely dispersed areas, including the northern ndes of enezuela and the Alps on the Italian/French frontier. While the databases in the two areas differ somewhat, the overall interpretation is that microtextural evidence in weathering rinds and in sands of associated paleosols and glaciofluvial deposits carry undeniable attributes of melted glassy carbon and e spherules, planar deformation features, shock‐melted and contorted quartz, occasional transition and platinum metals, and brecciated and impacted minerals of diverse lithologies. In concert with other black mat localities in the Western , the etherlands, coastal rance, yria, entral sia, eru, rgentina and exico, it appears that a widespread cosmic impact by an asteroid or comet is responsible for deposition of the black mat at the onset of the glacial event. Whether or not the impact caused a 1‐kyr interval of glacial climate depends upon whether or not the Earth had multiple centuries‐long episodic encounters with the Taurid Complex or asteroid remnants; impact‐related changes in microclimates sustained climatic forcing sufficient to maintain positive mass balances in the reformed ice; and/or inertia in the Atlantic thermohaline circulation system persisted for 1-kyr.

Alteration of plagioclase to nepheline in the Khariar alkaline complex, SE India: Constraints on metasomatic replacement reaction mechanisms

At the Khariar alkaline complex in southeastern India, some tholeiitic amphibolites inter-layered with syenite and nepheline syenite show microtextural evidence of fenitization. Typical metasomatic textures include the replacement of plagioclase by vein nepheline and albitic-plagioclase or by symplectitic nepheline+albite+Na–Al-rich clinopyroxene. Whole rock mass balance calculations indicate that the fenitized variants gained alkalis (Na+K), some large ion lithophile elements (Sr, Rb and Ba) and high field strength elements like Zr and Hf during the metasomatic exchange. The plagioclase replacement textures can be explained by a fluid-mediated coupled dissolution–reprecipitation process whereby the infiltration of alkali-bearing fluid along grain boundaries and deformation-induced intragranular fractures led to the dissolution of the host plagioclase along an inward moving reaction interface. Na+ ions were introduced and Ca2+ ions removed by the fluid phase. The Al and Si released from the breakdown of the anorthite component of the plagioclase combined with Na+ from the fluid to precipitate nepheline+albitic-plagioclase or Na–Al-rich clinopyroxene. Reaction enhanced cracking during metasomatism due to local volume changes in the alteration zone produced a hierarchical system of fractures. Mass-balance calculations using the volume proportion of phases in the plagioclase replacement zones show that SiO2 and Al2O3 were broadly conserved during the replacement; there was significant loss of CaO (ca. 73%) and huge gains of Na2O and K2O. These results indicate that alkali and alkaline elements (Na, K and Ca) were exchanged freely between the fluid phase and the reaction zone whereas Si and Al released during plagioclase dissolution were consumed in the replacement zone itself to form nepheline and albite-rich plagioclase or Na–Al-rich clinopyroxene. The trace element and Sr–Nd isotope signature of the rocks indicate that the fenitizing fluids were derived from the nepheline syenites.

Reconstruction of the pre-Merida glaciation, northwestern Venezuelan Andes

Valley glaciers have been well studied in the northwestern Venezuelan Andes over the last 20-year period but most reconstructions have been limited to the Last Glacial Maximum (LGM). Analyses have been confined to the mapping of moraine positions and the detailed analysis of pedostratigraphic sequences reaching back through the Middle to the Early Wisconsinan (Weichselian) Glaciation. Using bedding macrofeatures, fabric analysis (magnetic azimuths) with mirror images, clast inclination and microtextural evidence much has been learned about the lithology, source of ice, growth of an ice cap in the eastern cordillera and weathered state of surface paleosols. Analysis of two sections of pre-Mérida age, the only two such sections known in the Sierra de Santa Domingo (eastern cordillera), provides new information on lithology, source of the ice and weathered state of sediment of pre-Mérida age. The fabric in these sections is used to deduce the build-up of valley ice in the lower El Caballo Valley from an ice cap similar to one that developed later in Early Mérida time (~ 90 ka to ~ 65 ka). The presence of weathered tills in both sections suggests two glaciations perhaps correlative with the Illinoian and Kansan glaciations of North America. The weathered state of tills and outwash in both sections is assessed on the basis of geochemical (INAA) and mineral composition (XRD) and microtextural evidence determined by SEM/EDS.

Reconstruction of the Early Mérida, pre-LGM glaciation with comparison to Late Glacial Maximum till, northwestern Venezuelan Andes

Valley glaciers have been well studied in the northwestern Venezuelan Andes over the last 20-year period with most sourcing reconstructions limited to the Last Glacial Maximum (LGM) and confined to the mapping of moraine positions and the detailed analysis of pedostratigraphic sequences reaching back into the Middle Wisconsinan (Weichselian) Glaciation. Using bedding macrofeatures, fabric analysis (magnetic azimuths) with mirror images, clast inclination and microtextural evidence determined with the FE-SEM (Field Emission Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy), and micromorphology of the PED5 composite section of Early to Late Mérida age was analyzed to reconstruct the till succession and glacial dynamics. The fabric is used to reconstruct the build-up of valley ice in the lower Mucuñuque Valley, the icecap spreading over the upper catchments of neighbouring drainage basins toward the headwaters of the Mucuchache Valley over a time frame estimated to be < 90 ka and lasting to ∼ 60 ka. Microtextural analysis was invoked to determine if physical differences on quartz and garnet mineral surfaces could be determined between various diamictons such as subglacial, flow and meltout tills, and outwash. Sample identification was made optically and with EDS. The evidence supports the theory that a substantial ice mass – a virtual icecap – existed during the Early Mérida stade of the last glaciation with the ice mass originating and decaying from the upper Mucuñuque Catchment south over some 40° of arc. This is the first evidence for an icecap complex in the northwestern Andes. Comparison between Early and Late Mérida glacial styles indicates that, in the Mucuñuque–Mucuchache corridor of the Mérida Andes glaciation in the LGM was catchment restricted with no evidence of icecap growth.

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred.

Magma–carbonate interaction: An experimental study on ultrapotassic rocks from Alban Hills (Central Italy)

The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO < 3 wt.%) are characterised by low SiO 2 concentration (< 50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H 2O and CaCO 3 in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO 3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO 3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important.

First evidence for ultrahigh‐pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the Eastern Alps

The first evidence for ultrahigh‐pressure (UHP) metamorphism in the Eastern Alps is reported from kyanite eclogites of the Pohorje Mountains in Slovenia. Polycrystalline quartz inclusions surrounded by radial fractures in garnet, omphacite, and kyanite are interpreted to be pseudomorphs after coesite. Abundant quartz rods and needles in omphacite indicate an exsolution from a preexisting supersilicic clinopyroxene that contained a Ca‐Eskola component. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + phengite + quartz/or coesite yields peak pressure and temperature conditions of 3.0–3.1 GPa and 760°–825°C, well within the stability field of coesite, thus supporting the microtextural evidence for UHP metamorphism. This records the highest‐pressure conditions of Eo‐Alpine metamorphism during the Cretaceous orogeny in the Alps, implying a very deep subduction of the continental crust to at least 90–100 km depths. The new data are evidence for a regional southeastward increase of peak pressures in the Lower Central Austroalpine, indicating a south‐ to eastward dip of the subduction zone. Subduction was intracontinental; northwestern parts of the Austroalpine (Lower Central Austroalpine) were subducted under southeastern parts (Upper Central Austroalpine). The subduction zone formed in the Early Cretaceous in the northwestern foreland of the Meliata suture after Late Jurassic closure of the Meliata Ocean and the resulting collision, by a forward subduction shift to a Permian rift.

Bedded jaspers of the Ordovician Løkken ophiolite, Norway: seafloor deposition and diagenetic maturation of hydrothermal plume-derived silica-iron gels

Sedimentary beds of jasper (red hematitic chert) in the Ordovician Lokken ophiolite of Norway are closely associated with volcanogenic massive sulphide (VMS) deposits. The jaspers occur in the immediate hangingwall and laterally peripheral to the large Lokken (25–30 Mt) and small Hoydal (0.1 Mt) VMS deposits, and are exposed discontinuously for several kilometres along strike. Massive or laminated types predominate; jasper-sulphide debris-flow deposits are also abundant near VMS deposits. The jaspers contain hematite-rich laminae showing soft-sediment deformation structures and microtextural evidence that record the presence of a colloidal precursor and an origin as gels. Early textures include: (1) straight or curved chains of hematitic filaments 3–10 µm in diameter and 20–100 µm long; (2) branching networks of 15–25 µm-thick, tubular structures surrounded by cryptocrystalline hematite and filled with quartz and euhedral hematite; (3) small (up to 10 µm) spherules composed of cryptocrystalline hematite and silica; and (4) up to 50 µm silica spherules with hematitic cores. The small filaments seem to have been deposited in varying proportions in the primary laminae, possibly together with hematitic and siliceous microspheroids. Diagenetic changes are represented by polygonal syneresis cracks, and the presence of cryptocrystalline (originally opaline) silica, chalcedony, quartz, carbonate and cryptocrystalline hematite and/or goethite forming botryoidal masses and spheroids <10 µm to 5 mm in diameter. Coarser euhedral grains of quartz, carbonate, and hematite are integral parts of these textures. Bleached, silica-rich jaspers preserve only small relics of fine-grained hematite-rich domains, and locally contain sparse pockets composed of coarse euhedral hematite±epidote. The jaspers are interpreted to record colloidal fallout from one or more hydrothermal plumes, followed by maturation (ageing) of an Si-Fe-oxyhydroxide gel, on and beneath the Ordovician sea floor. Small hematitic filaments in the jaspers reflect bacteria-catalysed oxidation of Fe2+ within the plume. The larger tubular filaments resulted from either microbial activity or inorganic self-organized mineral growth of Fe-oxyhydroxide within the Si-Fe-oxyhydroxide gel after deposition on the sea floor, prior to more advanced maturation of the gel as represented by the spheroidal and botryoidal silica-hematite textures. Bleaching and hematite±epidote growth are interpreted to reflect heat and fluids generated during deposition of basaltic sheet flows on top of the gels.

Metasomatic mechanism of weathering-pedogenesis of carbonate rocks: I. Mineralogical and micro-textural evidence

On the basis of mineralogical, geochemical and micro-textural studies of the typical sections of the red weathering crust of carbonate rocks in the subtropical karst areas of Guizhou Province and Guangxi Autonomous Region, we have found, either on a microscopic or on a macroscopical scale and in different positions of the sections, the most direct and most important mineralogical and micro-textural evidence for the development of metasomatism in the process of weathering-pedogenesis of numerous carbonate rocks. This paper also has expounded for the first time and systematically the mechanism of metasomatism involved in the process of weathering-pedogenesis of carbonate rocks and proposed the sequence of mineral metasomatic evolution in the process of weathering-pedogenesis of carbonate rocks.

Brittle Origins for Disseminated Gold Mineralization inMylonite: Gaocun Gold Deposit, Hetai Goldfield, Guangdong Proince, SouthChina

Gold mineralization in the Hetai goldfield in South China is almost entirely confined to a series of steeply dipping ductile shear zones generated during Hercynian-Indosinian orogenesis of Sinian host rocks. Approximately half of the mineralization is in the form of disseminations in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in veinlets and as sulfide cement to brecciated mesoscopic quartz veins. Heaily influenced by the presence of disseminated mineralization, all publications on Hetai determine a major role for ductile deformation in the introduction of gold into the deposit. The brittle deformation is typically regarded as redistributing, within the host structures, the gold initially introduced as a result of ductile deformation mechanisms. However, textures of the sulfides associated with the gold indicate an entirely postductile deformation timing as they display none of the features attributed to ductile deformation. During the evident brittle deformation phase, competence contrasts at the microscopic scale allowed fracturing to focus on the quartz-sericite domain boundaries of the mylonite foliation, thus permitting the introduction of auriferous fluid into the mylonite to create disseminated mineralization. The control exerted by preexisting structures is further reflected in the restriction of disseminated mineralization to the quartz domains, presumably because the sericite domains accommodated the deformation in a more distributed fashion. Seeral other lines of evidence support an origin of the Hetai deposit in an upper crustal regime. A similar style of disseminated mineralization, to that seen in the mylonite, surrounds microveinlet terminations in the brecciated quartz veins, as well as overprinting pseudotachylite, which is associated with cataclasite. There are also large volumes of unmineralized mylonite indicating no connection between the intensity of strain and gold grade. We conclude that, in structurally controlled deposits, the textural category of disseminated gold mineralization can be generated either under entirely ductile or entirely brittle conditions, and that microtextural evidence has to be carefully analyzed to determine the mode of formation of a deposit.

Late Pleistocene deposits at Broughton Bay, Gower, South Wales: evidence for deposition at a non-marine Devensian ice margin

The results of detailed sedimentological work on Pleistocene deposits at Broughton Bay, west Gower, are presented. Ipswichian raised beach sediments are overlain by a sequence of Devensian glacial and periglacial deposits and by Holocene dune sand. Particle size, clast fabric, lithological and SEM microtextural evidence show that three types of Pleistocene glacigenic deposit are present: (1) a lower, ‘bedded’ till series characterized by fine-grained, shelly diamictons; (2) an upper, stony till; and (3) till re-deposited by solifluction. Facies variations within in situ till were caused by the vertical transfer of materials within the basal ice layers, as marine and estuarine sediments were entrained from the floor of Carmarthen Bay by southward-moving ice. These basal sediments, together with more far-travelled, englacially-transported material, were deposited contemporaneously on west Gower, at or near the maximum limit of the Late Devensian Welsh ice sheet. The sedimentological and dating evidence is best explained by a conventional model of ‘terrestrial’ Late Devensian ice moving southwards onto Gower with a regionally stable landmass and eustatically-lowered sea-level. Reasons for rejecting a glaciomarine origin for the sediments are discussed. Soliflucted till, colluvial sediments and frost cracks at the site reflect periglacial activity towards the end of the Devensian Stage.

Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England

The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west‐central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back‐arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high‐T, low‐P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E‐W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well).The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400‐600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by 40Ar/39Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism.In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high‐T, intermediate‐P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling.Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe‐stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe‐stage deformation documented in the Eastern Acadian) must pre‐date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome‐stage deformation clearly post‐dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370‐375 Ma radiometric ages of late pegmatites and granitoids which cross‐cut all structures.

Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland

The cores of amphibolitic and ultramafic pods within the basement complex of the Nagssugtoqidian mobile belt of East Greenland preserve metastable relics of older high-pressure assemblages. The bulkrock geochemistry of amphibolites indicates that their protoliths derive from low-pressure crystal fractionation (mainly controlled by plagioclase, pyroxene and olivine) of tholeiitic melts of probable MORB affinity. Amphibolites from the cores of the pods are corona-textured and provide microtextural evidence of a polyphase retrogression subsequent to a high- T eclogitic event, producing omphacite-garnet assemblages. Symplectitic intergrowths of Ca-clinopyroxene and plagioclase are the result of the unmixing of the older omphacite, whilst garnet is partly replaced by a fine-grained pseudomorph of orthopyroxene, anorthitic plagioclase and magnetite. This suggests subsequent partial re-equilibration in the intermediate-pressure granulite facies, producing clinopyroxene-orthopyroxene-plagioclase-magnetite assemblages. The final event in the amphibolite facies was controlled by the influx of dominantly hydrous fluids. According to kinetic constraints, the amphibolites developed coronitic or granoblastic textures. The coronas consist of plagioclase and hornblende between remaining garnet and unmixed clinopyroxene. Re-crystallized granoblastic amphibolites do not retain relics; according to the bulk rock chemistry, plagioclase and hornblende coexist with either garnet or clinopyroxene. Adjacent ultramafic rocks conform with this metamorphic evolution, being retrogressed from the spinel ±amphibole lherzolite facies to the tremolite-chlorite peridotite facies. Estimated equilibrium conditions for the final amphibolite facies event are T = 600 + 70°C and P = 5.2 ± 1 kbar. Mafic and ultramafic pods from the basement complex of the Nagssugtoqidian mobile belt of East Greenland probably represent fragments of an oceanic crust which was subjected to a subduction event of Proterozoic age.

Mineralization at the Golden Grove Cu – Zn deposit, Western Australia. I: Premetamorphic textures of the opaque minerals

The competent opaque minerals in the Archaean Golden Grove deposit, pyrite and magnetite, retain pre-regional metamorphic textures despite the lower greenschist-facies grade of metamorphism. The pre-regional metamorphic textures and structures recognized include the development of pyrite and magnetite overgrowths, the replacement of pyrrhotite by pyrite, the conversion of a primary hematite–goethite mineralogy to magnetite and, as a result of thermal metamorphism, further local replacement of pyrrhotite (and sphalerite) by magnetite. Comparisons between pyrite from the Cu-rich mineralization at the base of the deposit and that from the Zn-rich mineralization in the hanging wall indicate that postdepositional modification and recrystallization were more extreme at the base of the deposit. The pre-regional metamorphic textures and structures indicate that pyrite and magnetite overgrowths developed almost immediately after primary precipitation ceased and that overgrowths continued to develop into the late hydrothermal–diagenetic stage of mineralization. A large proportion of the sulphide–Fe-oxide mineralization was formed at shallow depth within the volaniclastic host rocks, but at two horizons (the base and hanging wall) the mineralization formed at or very near the sea floor. These two periods of near sea-floor sulphide precipitation are separated by an oxide-dominated opaque-mineral assemblage, originally hematite–goethite and secondary marcasite but now converted to magnetite and secondary pyrite. The microtextural evidence supports a three-stage evolution of the ore deposit, two sulphide exhalative phases of mineralization separated by a stage of more oxidized hydrothermal activity, or, alternatively, sea-floor weathering during which hematite–goethite formed and marcasite partly replaced earlier formed sulphides.