Lower Greenschist Research Articles

Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA

We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.

Very Low‐Grade Metamorphism of Clastic Rocks from the Meso‐Neoproterozoic and the Paleozoic along the Profile Yueyang‐Linxiang in Northeastern Hunan Province and Its Geological Implications

AbstractThis study uses illite crystallinity, chlorite crystallinity, illite polytypes, the b0 cell‐dimension of K‐white mica, clay mineral assemblages and mineral geothermo‐geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang in northeastern Hunan Province, China. Illite crystallinity Kübler index (KI) of the <2µm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Árkai index (AI) ranges from 0.244 to 1.500°Δ2θ. This indicates that the Meso‐Neoproterozoic and the Lower Paleozoic along the profile Yueyang‐Linxiang were overprinted with diagenesis and anchi‐ to epimetamorphism. Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360°C. The b0 cell dimension values of illites (K‐mica) range from 0.9002 to 0.9054 nm and, on average, at 0.9030 nm for the Meso‐Neoproterozoic. Based on cumulative frequency curves of illite (K‐mica) b0 cell dimension, the peak metamorphic pressure of the Meso‐Neoproterozoic along the profile Yueyang‐Linxiang is derived of an intermediate pressure type. Most illites occur in the 2M1 polytype and some of them in a mixture of 2M1+1M types especially those in the Paleozoic. This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups. However, it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic. Crustal thickening due to “collision” between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso‐Neoproterozoic to ca. 14 km and resulted in a temperature increase in those rocks due to burial. The very low grade to low grade metamorphism overprinting the Meso‐Neoproterozoic implies that the so called “Chiangnania or Jiangnan orogen” was no relative with the “Grenvillian orogeney; instead, it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.

Contrasting coeval paragenesis of gold and scheelite in an orogenic hydrothermal system, Macraes mine, New Zealand

The Macraes deposit (>10 Moz resource) is a Cretaceous orogenic system hosted in the Hyde-Macraes Shear Zone (HMSZ) which was mineralised under lower greenschist facies during later stages of lower greenschist facies metamorphism of host metasedimentary schists. Gold is encapsulated primarily in sulphides that have replaced silicates in ductile shears that are focussed in micaceous rocks. The shears anastomose around structurally competent lenses, and were enhanced by hydrothermal graphite deposition and alteration of albite to muscovite. In contrast, scheelite with minor auriferous sulphides occurs in multigenerational quartz veins that filled fractures in competent lithologies. Hence, scheelite was deposited coevally with gold, from the same hydrothermal fluid, but in different structural settings from most gold at all scales from millimetres to hundreds of metres. Consequentially, there is weak correlation between Au and W at all scales in the deposit. Multigenerational gold and scheelite mineralisation occurred during progressive deformation in the shear zone in two contrasting structural and mineralogical styles in syn-deformationally weakening gold-bearing micaceous shears, and in syn-deformationally hardened competent rocks that became silicified and veined with quartz and scheelite. Hydrothermal fluid flow in the gold-bearing shears occurred at the grain boundary, microshear, and microfracture scales, and was slow (<1m/year), continuous, and pervasive. In contrast, vein formation in more competent lithologies was episodic, locally rapid (>hundreds of m/year), and was controlled by fracture permeability. The Au and W enrichment in the Macraes deposit resulted from regional scale metal mobility, driven by coeval recrystallisation in higher-grade (upper greenschist to amphibolite facies) metamorphism that persisted structurally below the Macraes deposit for at least 10Ma after mineralisation ceased.

A Palaeoproterozoic multi-stage hydrothermal alteration system at Nalunaq gold deposit, South Greenland

Nalunaq is an orogenic, high gold grade deposit situated on the Nanortalik Peninsula, South Greenland. Mineralisation is hosted in shear zone-controlled quartz veins, located in fine- and medium-grained amphibolite. The deposit was the site of Greenland’s only operating metalliferous mine until its closure in 2014, having produced 10.67 t of gold. This study uses a combination of field investigation, petrography and U/Pb zircon and titanite geochronology to define a multi-stage hydrothermal alteration system at Nalunaq. A clinopyroxene-plagioclase-garnet(-sulphide) alteration zone (CPGZ) developed in the Nanortalik Peninsula, close to regional peak metamorphism and prior to gold-quartz vein formation. The ca. 1783–1762-Ma gold-quartz veins are hosted in reactivated shear zones with a hydrothermal alteration halo of biotite-arsenopyrite-sericite-actinolite-pyrrhotite(-chlorite-plagioclase-lollingite-tourmaline-titanite), which is best developed in areas of exceptionally high gold grades. Aplite dykes dated to ca. 1762 Ma cross-cut the gold-quartz veins, providing a minimum age for mineralisation. A hydrothermal calcite-titanite alteration assemblage is dated to ca. 1766 Ma; however, this alteration is highly isolated, and as a result, its field relationships are poorly constrained. The hydrothermal alteration and mineralisation is cut by several generations of ca. 1745-Ma biotite granodiorite accompanied by brittle deformation. A ca. 1745-Ma lower greenschist facies hydrothermal epidote-calcite-zoisite alteration assemblage with numerous accessory minerals forms halos surrounding the late-stage fractures. The contrasting hydrothermal alteration styles at Nalunaq indicate a complex history of exhumation from amphibolite facies conditions to lower greenschist facies conditions in an orogenic belt which resembles modern Phanerozoic orogens.

Neoproterozoic sedimentary basin evolution in southwestern Tarim, NW China: New evidence from field observations, detrital zircon U–Pb ages and Hf isotope compositions

Sedimentary basin evolution is intimately related to tectonic background and thus can serve as one of the most important indicators in deciphering the regional tectonic evolution process. Neoproterozoic volcanic-sedimentary sequences are well preserved along the southwestern margin of the Tarim Block in NW China and provide new insights into the Neoproterozoic tectonic evolution of the Tarim. In this contribution, we report the metamorphic deformation features and systematic detrital zircon U–Pb ages and Hf isotope compositions of the Neoproterozoic strata in SW Tarim. Geochronological data reveal that the greenschist-facies metamorphic and tightly folded Sailajiazitage Group (SG) volcanic-sedimentary sequence deposited during 860–830Ma, the Ailiankate Group (AG) clastic rocks of low greenschist – facies metamorphism and intensive deformation deposited during 820–800Ma, and the unmetamorphosed and undeformed late Neoproterozoic carbonate–clastic–tillite sequences, including the Silu Group (SLG) and the Qiakemakelieke Group (QG), deposited after 760Ma. Two phases of glaciations from the QG in SW Tarim could be equivalent to the Beiyixi glaciation and the Altungol–Tereeken glaciation in north Tarim, respectively. Rock assemblages coupled with geochemistry of the SG bimodal volcanic rocks indicate its deposition in a back-arc basin. Rock assemblages, detrital zircon age spectra and its metamorphism and deformation features suggest that the AG clastic sequence most likely deposited at a foreland basin on the back-arc basin sedimentary package while the middle to late Neoproterozoic SLG and QG carbonate–clastic–tillite sequences deposited at a passive marginal and/or rift basins. The Neoproterozoic evolution process of the southwestern Tarim demonstrates that (1) the Neoproterozoic assemblage of the Tarim basement could have lasted till 760Ma as evidenced by the Aksu blueschist-facies metamorphism and (2) its Pre-Nanhuaian basement could be composed of independent continental terranes. Furthermore, positive εHf(t) deviations in these detrital zircons suggest that the Rodinia plume could have effects on the Neoproterozoic igneous activities.

Provenance signals in metaturbidites of the Paleoproterozoic greenstone belt of the Guiana Shield in Suriname

AbstractThe sedimentological, metamorphic, petrographic and geochemical characteristics of the Armina Formation, part of the Paleoproterozoic Greenstone Belt of Suriname in South America, are described, based on field, geochemical and petrographic evidence obtained through fieldwork along the Marowijne River and study of diamond drill cores from Rosebel Gold Mine (RGM). The metagreywackes show characteristic features of deposition by turbidity currents: coarse-grained, poorly sorted graded greywackes, covered by fine-grained, parallel-laminated phyllitic beds, often with convolute structures and climbing ripples. Their immature character and composition suggest deposition in an arc-trench environment. In the Marowijne River three different facies of metagreywackes are distinguished: (1) the greyish Bonnidoro Falls facies, characterised by common red millimetre-sized pseudomorphs after siderite in the finer beds, (2) the green Paroe Tabiki metagreywacke facies, with decimetre-sized calcsilicate nodules, both metamorphosed in the lower greenschist facies with chlorite as the main mafic mineral, and (3) the grey Armina Falls metagreywacke facies, geochemically similar to the Bonnidoro type but of higher metamorphic grade with biotite as the main mafic mineral. The metagreywackes from the Marowijne River show a predominance of quartz, plagioclase and lithic (tonalitic) clasts, suggesting exhumation of tonalite–trondhjemite–granodiorite plutons before deposition of the turbidites. There is a slight increase in maturity from (1) to (3), suggesting increasing weathering in the source areas. The metagreywackes of the RGM (JZone) have a predominantly metavolcanic origin, suggesting that they have a different provenance area than the Marowijne metagreywackes. Geochemically the spread in composition within each facies is larger than between the facies because of the wide range in grain sizes in each turbidite sequence. A large part of the rocks from the RGM, classified by previous authors as arenites, are geochemically and petrographically metagreywackes. Only a few RGM samples are real arenites, and plot as a separate cluster in geochemical factor score plots because of their low Fe and Na contents.

On the discrimination of semi-graphite and graphite by Raman spectroscopy

Graphite samples from several mines (Austria, Brazil, China, Madagascar, Mozambique, Norway, Russia, Sri Lanka and Ukraine) were investigated by reflectance measurements, X-ray-diffraction, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and Raman spectroscopy. X-ray-diffraction indicates that the structural order of the carbonaceous matter (CM) varies in dependence on the metamorphic rank of the host rock. Semi-graphite is found in lower greenschist facies samples from the Kaisersberg mine (Austria). At the nanoscale, micropores characterize semi-graphite. They are polygonized in the transition zone between semi-graphite and graphite. During progressive graphitization within the greenschist facies, semi-graphite is transformed continuously to graphite. HRTEM images and SAED data show a transformation of the microporous nano-texture to a predominantly lamellar texture. Perfect graphite is observed in samples from the granulite facies. Raman spectroscopy mirrors this evolution. The observed spectral characteristics, the width of the Raman G band and the D1/(G+D1+D2) area ratio of the CM Raman spectrum discriminate semi-graphite and graphite.

Geologia e petrografia do Grupo Alto Jauru na região da Fazenda Retiro Novo, SW do Cráton Amazônico: evidências de um prisma acrescionário estateriano

RESUMO: O Grupo Alto Jauru, localizado no sudoeste do Cráton Amazônico, região da fazenda Retiro Novo, é constituído por biotita-muscovita-quartzo xistos, clorita-biotita-muscovita xistos, granada-cordierita-biotita xisto e estaurolita-andalusita-biotita xisto, associados a granada-sillimanita-biotita gnaisse, biotita gnaisse, anfibolitos e muscovita granito. O grupo apresenta evidências de representar prisma acrescionário dominado por sedimentos. A petrografia e as relações estruturais indicam que foi afetado por dois eventos deformacionais, Dn e Dn+1, com as foliações associadas Sn (xistosidade e bandamento gnáissico) e Sn+1 (clivagem de crenulação) e por três eventos metamórficos (M1, M2 e M3): o primeiro é contemporâneo à Sn de fácies xisto verde inferior; o segundo é associado à fase Sn+1 de fácies xisto verde a anfibolito; o terceiro evento é térmico de fácies anfibolito, resultado da intrusão do Tonalito Cabaçal. A determinação geocronológica obtida pelo método U-Pb (SHRIMP) em zircão forneceu idade de 1819 ± 6,7 Ma para a cristalização do protólito ígneo do biotita gnaisse. Do ponto de vista geotectônico, o Grupo Alto Jauru corresponde a prisma acrescionário formado no Estateriano.

Geological setting of the southern termination of Western Alps

A revision of the stratigraphic and tectonic setting of the southern termination of the Western Alps, at the junction of the Maritime Alps with the westernmost Ligurian Alps, is proposed. In response to the Alpine kinematic evolution, a number of tectonic units formed on the deformed palaeo-European continental margin and were arranged in a NW–SE striking anastomosed pattern along the north-eastern boundary of the Argentera Massif. Because these tectonic units often cut across the palaeogeographic subdivision of the Alpine literature and show only partial affinity with their distinctive stratigraphic features, new attributions are proposed. The Subbrianconnais domain is here intended as a “deformation zone”, and its tectonic units have been attributed to Dauphinois and Provencal domains; furthermore, the Eocene Alpine Foreland Basin succession has been interpreted, based on the affinity of its lithologic characters and age, as a single feature resting above all the successions of the different Mesozoic domains. The Cretaceous tectono-sedimentary evolution of the studied domains was characterized by intense tectonic controls on sedimentation inducing lateral variations of stratigraphic features and major hydrothermal phenomena. Since the early Oligocene, transpressional tectonics induced a NE–SW shortening, together with significant left-lateral movements followed by (late Oligocene–middle Miocene) right-lateral movements along E–W to SE–NW striking shear zones. This induced the juxtaposition and/or stacking of Brianconnais, Dauphinois and Ligurian tectonic units characterized by different metamorphic histories, from anchizonal to lower greenschist facies. This evolution resulted in the arrangement of the tectonostratigraphic units in a wide “transfer zone” accommodating the Oligocene WNW-ward movement of portions of the palaeo-European margin placed at the south-western termination of Western Alps and the Miocene dextral shearing along SE striking faults that bound the Argentera Massif on its NE side.

Pre-collisional accretionary growth of the southern Laurasian active margin, Central Pontides, Turkey

Cretaceous subduction–accretionary complexes crop out over wide areas in the central part of the Pontides, northern Turkey. To the north, the wedge consists of a low-grade metaflysch sequence with blocks of marble, Na-amphibole-bearing metabasite (PT=7–12kbar; 400±70°C) and serpentinite. 40Ar/39Ar phengite ages from the phyllites of the metaflysch are ca. 100Ma. The metaflysch sequence is underlain by oceanic crust-derived HP/LT metabasites and micaschists along a major detachment fault. The metabasites are epidote-blueschists consisting of glaucophane, epidote, titanite, and phengite locally with garnet. Fresh lawsonite-blueschists are exposed as blocks along the detachment fault. Peak metamorphic conditions of a garnet-blueschist are constrained to 17±1kbar and 500±40°C and of a lawsonite-blueschist to 14±2kbar and 370–440°C. 40Ar/39Ar phengite dating on the micaschists constrains the HP/LT metamorphism as 101–92Ma, younging southward. Middle Jurassic (ca. 160Ma) accretionary complexes consisting of blueschist to lower greenschist facies metabasites, marble and volcanogenic metasediment intercalations are exposed at the southern part of the Cretaceous wedge. In the studied area, the North Anatolian Fault forms the contact between Cretaceous and Middle Jurassic HP/LT metamorphic rocks. Wide distribution of Cretaceous subduction–accretionary complexes implies accretionary tectonic continental growth along the Laurasian active margin. High amount of clastic sediment flux into the trench has a major effect on enlarging the wedge during the Albian. Tectonic thickening of the oceanic HP/LT metamorphic sequence, however, was possibly achieved by propagation of the décollement along the retreating slab which can create the space necessary for progressive deep level basal underplating and extension of the wedge for subsequent syn-subduction exhumation.

Dating of detrital zircons from the Dabure clastic rocks: the discovery of Neoproterozoic strata in southern Qiangtang, Tibet

ABSTRACTThis article reports the results of field mapping and the petrology of clastic rocks in the Dabure area, southern Qiangtang, Tibet, together with the results of U–Pb dating of detrital zircons from these rocks. The Dabure clastic rocks are characterized by low compositional and textural maturity, and they have been affected by lower greenschist facies metamorphism. The deposits exhibit the typical features of turbidites. Altogether, 279 detrital zircons were selected for U–Pb dating, and the ages fall into five groups: 550–650, ~800, 900–1100, 1600–1800, and 2300–2500 Ma. In general, the ages of the detrital zircons that are older than ~550 Ma are similar to those found elsewhere in the southern Qiangtang and Himalayan terranes. The most reliable youngest age of a detrital zircon from the Dabure clastic rocks is ~550 Ma. In the southern part of the Tibet Plateau, strata with the same ages and lithologies as the Dabure clastic rocks are widespread, especially in the Himalayan terrane. Combining our data with previous work on the basalts in the Dabure area (the Dabure basalts), we tentatively suggest that the Dabure clastic rocks represent the late Ediacaran (~550 Ma) sedimentary record for the Qiangtang terrane, and that before the late Neoproterozoic the southern Qiangtang terrane was possibly connected to the Himalayan terrane.

Brittle–viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions

Abstract. We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5–6 mm) quartz vein embedded in a phyllonitic matrix to gain insights into the recrystallization mechanisms and the processes of strain localization in quartz deformed under lower greenschist facies conditions, broadly coincident with the brittle–viscous transition. The vein deformed during faulting along a phyllonitic thrust of Caledonian age within the Porsa Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in northern Norway. The phyllonite hosting the vein formed at the expense of a metabasaltic protolith through feldspar breakdown to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the quartz vein acted as a relatively rigid body. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide- and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the (1) hardening, (2) progressive and cyclic increase of the fluid pressure, and (3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms may vary through time in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.

Detrital zircon signatures of the Baltoscandian margin along the Arctic Circle Caledonides in Sweden: The Sveconorwegian connection

New evidence is presented here that the Sveconorwegian Orogen continued northwards from type areas in southwestern Scandinavia along the Baltoscandian outer margin into the high Arctic. The Silver Road (Silvervägen) profile through the Scandinavian Caledonides, located in Sweden along the Arctic Circle at 66–67° N, provides a full section through the tectonostratigraphy of the Baltoscandian margin from the Autochthon, via the Lower Allochthon to the upperment parts of the Middle Allochthon. Metamorphic grade increases upwards through the nappes, being low greenschist facies at lowest levels and increasing to eclogite grade in the highest parts of the Seve Nappe Complex, the latter being related to early Ordovician subduction of the Baltoscandian outermost margin. The sedimentary rocks range in age from Neoproterozoic to Ordovician and provide evidence of the changes of environment from the Baltoscandian platform, westwards out over the Cryogenian rifted margin to the continent-ocean transition zone; also the Ordovician foreland basin. Twelve samples of psammites from the different tectonostratigraphic levels have yielded U/Pb detrital zircon age-signatures that reflect the changing character of their provenance. Autochthonous sandstones are derived from late Paleoproterozoic (1800–1950Ma) crystalline rocks in the vicinity to the east of the thrust front. Ediacaran-early Cambrian quartzites of the Lower Allochthon also yield mainly late Paleoproterozoic zircon signatures, but with subordinate Mesoproterozoic and late Archaean populations, whilst mid Ordovician, W-derived foreland basin turbidites are dominated by Sveconorwegian (950–1100Ma) signatures, with subordinate older Mesoproterozoic to latest Paleoproterozoic populations. All samples from the lower parts of the Middle Allochthon (lacking dolerite dykes) have signatures that are dominated by latest Paleoproterozoic and early Mesoproterozoic ages, with subordinate populations down to Sveconorwegian ages; the latter dominate the overlying Särv nappes and also the Seve Nappe Complex, where c. 945Ma rhyodacites have been previously reported. This evidence of Sveconorwegian source rocks in the hinterland, taken together with previously published detrital zircon data farther south and north of the Arctic Circle, clearly favours the interpretation that the Sveconorwegian Orogen, during the Neoproterozoic, extended along the entire Baltoscandian outer margin into the high Arctic.

The Nickeliferous Archean Madziwa Igneous Complex, Northern Zimbabwe: Petrological Evolution, Magmatic Architecture, and Ore Genesis

The nickeliferous Madziwa Igneous Complex consists of several lenticular, mafic-ultramafic enclaves within late Archean gneissic terrain. The lenses of igneous rock are locally deformed and metamorphosed to lower greenschist grade. They are here interpreted to be remnants of a large, composite, magmatic structure with two main components: (1) a set of narrow (10s–100s of meters wide) dikes intruding the gneissic foliation, and (2) a large lopolith made up of leuconorite. Some dikes contain a differentiated and vertically oriented layered suite comprising a central pyroxenite layer, plus norite as both a continuous marginal layer and intermittent layers within the pyroxenite. The pyroxenite-norite suite has a preliminary U-Pb zircon age of 2684 Ma. Other dikes are made up of diorite and/or ferrodiorite; in places, the dioritic rocks also intrude the pyroxenite-norite suite. The leuconorite lopolith transects both the dikes and the gneissic country rocks, its basal contacts with the pyroxenite-norite suite varying locally from intrusive to gradational to (magmatic) erosional. Although modified in places by secondary mobilization, disseminated Ni-Cu sulfides are primarily hosted within the central (ortho- to mesocumulate) pyroxenite adjacent to internal norite layers. Whole-rock geochemical data establish the comagmatic origin of the principal rock types and indicate an Archean, D-type, basaltic source magma with ca. 8% MgO, the two dioritic rock types representing late, immiscible, silica- and Fe-Ti-P-rich derivatives of pyroxenite-norite(-leuconorite) crystallization. In addition to the major rock types, peridotites occur in several outlying dikes and may represent fractionates of a komatiitic precursor to the basalt. Geologic and geochemical evidence points to bulk assimilation of country rock gneiss by the Madziwa magma and sulfide segregation triggered by felsic contamination. These processes did not occur locally within the dikes but rather in the conduit system prior to the emplacement of the magma charged with sulfide droplets. The unusual vertical layering of the pyroxenite-norite sequence and the localization of the sulfide ores are attributed to the strong outward, cooling gradient across such narrow dikes and to large-scale, lateral movement of sulfide droplets through the solidifying, pyroxene crystal framework ahead of an advancing postcumulus plagioclase crystallization front.

Raman characterization of carbonaceous material in the Macraes orogenic gold deposit and metasedimentary host rocks, New Zealand

Raman spectroscopic and petrographic analyses were performed on samples collected from zones distal and proximal to the Macraes gold deposit in the Otago Schist of New Zealand to characterize the features and possible origins of Carbonaceous Material (CM) and to assess the potential role of CM in the formation of gold deposits. CM is a common component in meta-sedimentary orogenic gold deposits, and it has been proposed that CM contributes to gold mineralization processes, but the details of the mechanisms responsible are not fully understood. Documentation of the origins of the Otago schist CM will improve our understanding of the role of CM in gold deposits.This work has identified four types of CM of varying thermal maturity and origins from prehnite–pumpellyite grade to lower greenschist grade samples. In prehnite–pumpellyite and pumpellyite–actinolite grade rocks, low-maturity CM 1 coexists with framboidal pyrite, indicating an in-situ, sedimentary origin, with a potential association with the source of gold. Low crystallinity CM 2 is also found in low grade samples and is likely to have been deposited from fluids unrelated to gold mobilization. CM 3 is the highest maturity CM recognized. CM 3 is found in samples from the highest metamorphic grades studied (lower greenschist facies), where bands of CM 3 cross cut the foliation, CM 3 is therefore thought to have been transported by fluids, though possibly only at short length scales. CM 4 is less mature than CM 3 and is found in mineralized rocks in association with sulfide minerals and gold. CM 4 is likely to have a depositional origin but its precise role with respect to gold mineralization has not been identified.

Petrological and geochemical features of the Jingtieshan banded iron formation (BIF): A unique type of BIF from the Northern Qilian Orogenic Belt, NW China

The Jingtieshan banded iron formation (BIF) is located in the Northern Qilian Orogenic Belt (NQOB) in NW China. The BIFs are hosted in Mesoproterozoic Jingtieshan Group, a dominantly clastic-carbonate sedimentary formation, and was metamorphosed to lower greenschist facies. The Jingtieshan BIFs include oxide-, carbonate- and mixed carbonate–oxide facies, and consist of alternating iron-rich and silica-rich bands. The BIFs are composed essentially of specularite and jasper, with minor carbonate minerals and barite. The SiO2+Fe2O3 content is markedly high in the oxide facies BIF, followed by FeO, CO2 and Ba, with the other elements usually lower than 1%, suggesting that the original chemical sediments were composed of Fe, Si, CO32− and Ba. The positive correlation between Al2O3, TiO2 and Zr in the BIFs indicates that these chemical sediments incorporate minor detrital components. Oxide facies BIF shows low HFSE, low ∑REE and low Y/Ho. The Post Archean Australian Shale-normalized REE patterns for Jingtieshan BIFs are characterized slight LREE depletion, strong positive Eu anomalies and lack of significant negative Ce anomalies. Siderite in the carbonate- and mixed carbonate–oxide facies BIF shows negative δ13C values varying from −8.4‰ to −3.0‰, and δ18O values show a range of −16.6‰ to −11.7‰. The geochemical signatures and carbon–oxygen isotopes suggest origin from high-temperature hydrothermal fluids with weak seawater signature for the sediments of Jingtieshan BIFs. The absence of negative Ce anomalies and the high Fe3+/∑Fe ratios of the oxide facies BIF do not support ocean anoxia. In contrast to the three main types (Algoma-, Superior- and Rapitan-type) of global BIFs, the Jingtieshan BIFs represent a unique type with features similar to those of sedimentary-exhalative mineralization.

Chlorine isotope behavior during prograde metamorphism of sedimentary rocks

Chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n=25) were collected from a 50 m section of Triassic fluvial and playa-lake strata and Jurassic (Liassic) marine black shales in a well-characterized quarry. Low greenschist to middle amphibolite facies equivalents (n>80) were collected from the Glarus Alps, Urseren Zone, and Lucomagno region. Bulk δ37Cl values are constant within individual sedimentary layers, but vary from −2.0 to +2.4‰ in Triassic rocks and from −3.0 to 0‰ in the black shales. Dolomitic and gypsiferous samples have positive δ37Cl values, but marls and shales are isotopically negative. Bulk Cl contents show only small declines during the earliest stages of metamorphism. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in δ37Cl as their protoliths. Samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in δ37Cl. These data lead to the following conclusions: (1) Terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition. As a result, an unambiguous “sedimentary signature” does not exist in the chlorine stable isotope system. (2) No isotopic fractionation is discernable during metamorphic devolatilization, even at low temperatures. Alpine-style metamorphism thus has little to no effect on bulk chlorine isotopic compositions, despite significant devolatilization. (3) Cl is largely retained in the rocks during devolatilization, contrary to the normally assumed hydrophilic behavior of chlorine. Continuous release of mixed-volatile C–O–H fluids likely affected Cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks. (4) There is no evidence for fluid communication across (meta)sedimentary layers except within the Urseren shear zone. These data contribute to a relatively closed-system view of metamorphic devolatilization from anchizone through mid-amphibolite facies conditions.

Backarc basin and ocean island basalts in the Narooma Accretionary Complex, Australia: setting, geochemistry and tectonics

The Cambrian–Ordovician Wagonga Group contains basalts at Melville Point and Barlings Beach, 20 km south of Batemans Bay, New South Wales. At Melville Point, the succession has basal altered basalts overlain by chert and interbedded siliceous mudstone of the Wagonga Group, in turn overlain by turbidites and chert of the Adaminaby Group with a latest Cambrian to earliest Ordovician age. By contrast, at Barlings Beach, basalt is associated with highly disrupted chert (tectonic mélange), various slivers of mudstone and turbidites, and turbidites of the Adaminaby Group. Immobile elements in the basalts show consistent patterns that allow the magmatic affinity and tectonic setting to be determined in spite of pervasive hydrothermal alteration and subsequent lower greenschist facies metamorphism that accompanied strong folding and multiple foliation development. The Melville Point basalts show Ti/V ratios transitional between arc and MORB and therefore may have formed in either a forearc or backarc basin setting. However, these rocks have higher Ti/V ratios, LREE, Th and Nb than found in forearc basalts and are therefore considered to have formed in a backarc basin setting. In contrast to Melville Point, most basalts at Barlings Beach have a geochemical signature distinctive of ocean island settings like those reported from elsewhere in the Wagonga Group. We believe these rocks developed in a Cambrian backarc basin setting. In the Early to Middle Ordovician, much of the ocean basin was inundated by quartzose turbidites followed by basin destruction with accretion/underplating at a Late Ordovician–early Silurian Benambran subduction zone and formation of the Narooma Accretionary Complex.

Metagabros de la isla Gran Roque (Venezuela). Geoquímica y petrografía

The Gran Roque Island, located in northern Venezuela, is part of the southern Caribbean islands where the basic basement is outcropping. This basement is composed of fine-grained metagabbros, with subophitic relict texture defined by the larger plagioclase laths that enclose the ferromagnesian minerals. The mineral assemblage of chlorite-actinolite-plagioclase-quartz indicates that the rock reached a low metamorphic grade green schist facies. The chemistry of the Gran Roque island metaggabros supports the petrographic identification and characterization. These rocks exhibit tholeiitic affinity and REE flat patterns, which could be correlated to magmas generated at oceanic ridges of N-MORB type

Tectono-metamorphic map of the south-western flank of the Aspromonte Massif (southern Calabria -Italy)

The structural-geological field survey (1:10,000 scale) of the south-western portions of the Aspromonte Massif (southern Calabria, Italy) clearly reveal a nappe-pile stack made of three main tectono-metamorphic units piled-up by means of tectonic contacts with associated cataclastic or mylonitic rocks developed at various crustal depth. These three units are from top to bottom: the Stilo Unit, the Aspromonte-Peloritani Unit (APU) and the Madonna di Polsi Unit. As part of an ongoing project, we have constructed a structural and metamorphic GIS database based on 285 sampling stations, which allows for metamorphic and structural maps to be obtained in the form of the metamorphic zoning of the SU.. This unit, made up of low greenschist- to low amphibolite-facies Palaeozoic metamorphic rocks, lies through a brittle tectonic contact over the APU, which is made up of amphibolite-facies metamorphic rocks intruded by late Hercynian peraluminous both locally overprinted by Alpine type metamorphism. The petrographic-derived metamorphic zonation chart here presented has been created after the detailed petrographic analysis of about 80 thin sections and the recognition of mineral assemblages of a thermal metamorphic aureole (biotite, muscovite and andalusite blastesis) produced by Late Hercynian magmatic bodies intruded into metapelites (Punta d'Atò granitoids, outcropping in the northern part of the mapped area).