Greenschist Unit Research Articles

Geochemical anomaly separation by multi-fractal modeling: case study of Zarshuran gold deposit

Geochemical anomaly separation, using concentration–area method at Zarshuran Au–As deposit, NW Iran, is studied in this work. Lithogeochemical dataset was used in this geochemical survey, which was conducted for exploring Au mineralization in green schist units at Zarshuran Carlin-like gold deposit. Similar surveys were also carried out for Ag and As exploration in these rock units, and obtained results have been interpreted using detailed geological mapping. The correlation between these threshold values and ore grades is clearly interpreted in this investigation. Also, the logarithmic plots show existence of two stages for Au, Ag, and As. According to the SEM studies, it can be concluded that the proposed algorithm is verified technically in order to identify economically valuable Au concentrations in the area under study.

The nature of the Palaeozoic oceanic basin at the southwestern margin of Gondwana and implications for the origin of the Chilenia terrane (Pichilemu region, central Chile)

The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane.

Application of number–size (N-S) fractal model for separation of mineralized zones in Dareh-Ashki gold deposit, Muteh Complex, Central Iran

The goal of this study is to separate different mineralized zones in Dareh-Ashki gold deposit located in Muteh Complex, Central Iran, by using number–size (N-S) fractal model. The N-S log–log plot shows seven geochemical populations and four Au-mineralized zones. Based on obtained results, Au thresholds are 0.17, 0.32, 6.3, and 12.6 ppm which represent weakly, moderately, highly, and extremely mineralized zones in terms of Au grades, respectively. Au values lower than 0.17 ppm illustrate wall rocks. Main mineralization stage of gold commences from 6.3 ppm in this deposit. The moderately mineralized zone with Au values between 0.32 and 6.3 ppm has occupied the biggest part of the studied deposit. However, highly (with Au values between 6.3 and 12.6 ppm) and extremely (higher than 12.6 ppm) mineralized zones have small extension. Correlation between geological model and results from N-S fractal model reveals that the gold mineralized zones specifically the moderately mineralized zone are situated in green schist units.

Normal faulting on Sifnos and the South Cycladic Detachment System, Aegean Sea, Greece

Abstract: We constrain the timing and kinematics of normal faulting on Sifnos, Cyclades, Greece. Penetrative top-to-the-NE kinematic indicators associated with a shallow dipping foliation occur in the deepest tectonic unit (Greenschist Unit). Combined P – T analysis and Rb–Sr dating indicates that this deformation was under way by 29.4 ± 3.4 Ma at P – T conditions of c . 16 kbar and c . 550 °C, and continued to operate at least until 22.8 ± 0.2 Ma at P – T conditions of c . 8 kbar and c . 400 °C. The later top-to-the-south directed Sifnos Detachment developed at the top of the Greenschist Unit and cuts the top-to-the-NE structures. Zircon fission-track ages suggest that deformation on the Sifnos Detachment was largely terminated by about 13–10 Ma. Subsequent extension was accommodated by high-angle normal faults. Regionally the top-to-the-south detachment phase on Sifnos correlates with top-to-the-south extensional faulting on nearby Serifos, which started there at c . 15–13 Ma. Top-to-the-south extensional shearing on Ios to the SE commenced earlier at c . 19–18 Ma, suggesting that top-to-the-south extension propagated westward from Ios to Sifnos and Serifos, or that extension was partitioned into different top-to-the-south detachment systems that operated at different times. Collectively these top-to-the-south detachments are named here the South Cycladic Detachment System, which played a key role in the opening of the Cretan Sea forearc basin.

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high‐pressure rocks in the Central Aegean

AbstractThe blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km−1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.

Evolution of veins and sub-economic ore at Strassegg, Paleozoic of Graz, Eastern Alps, Austria: evidence for local fluid transport during metamorphism

The north-eastern part of the Graz Paleozoic thrust sheet at Strassegg, Austria is characterised by metamorphic assemblages that reflect increasing temperature conditions from 300°C in hangingwall to around 550°C in footwall units. The main quartz and/or carbonate vein generation cuts the schistosity at moderate to low angles is folded with the schistosity and is commonly boudinaged. There is a correlation between vein- and the host-rock mineralogy. Additionally the δ 18O and δ 13C signatures of vein minerals (quartz and/or carbonates) vary systematically with the isotopic signature of the host rock and its minerals. Within individual units, δ 18O of vein quartz has a narrow range of values in contrast with a larger variation in both with δ 18O and δ 13C of carbonates. At Strassegg, the greenschist unit hosts an As–Au mineralisation. Arsenopyrite, pyrite, galena and Pb–(Cu)–Sb–sulfosalts are the dominant sulfides. Fluids associated with vein and ore formation are composed of H 2O–CO 2–NaCl. Formation pressure and temperatures, estimated from fluid inclusion and arsenopyrite thermometry are about 400°C and 4–6 kb. Sulfur isotopes on sulfides and whole rock samples give a narrow range around 0‰ δ 34S. In general, wall rock alteration is minor, although within the greenschist unit small alteration zones (characterised by Fe–dolomite, plagioclase and a halo of increased As-contents) occur around quartz–carbonate veins. The data indicate that the vein material segregated from the host lithologies by fluid circulation, which was largely limited to individual sedimentary formations. We suggest that in this area, metamorphism, vein formation and ore mobilisation were related to enhanced heat flow associated with Late Cretaceous extension.

GRAVITY MODEL FOR BELA OPHIOLITES AT WESTERN MARGIN OF THE INDO-PAKISTAN CONTINENTAL PLATE

The oroclinal axial fold-thrust mountain belt has notable features in the geology of Pakistan. The southern part of this belt is of particular importance because it includes the southern branch of the main Himalayan ophiolitic zone, the Bela ophiolites, and its continuation into the Arabian Sea. A number of geological models and hypotheses have been proposed for explanation of the tectonic setting of the area based on surface geological investigations. A gravity study was carried out to provide a model for the tectonic setting of the investigated area. The results of interactive gravity modeling point to the underthrusting of a narrow segment of the Arabian oceanic plate (Bela-Hingol micro-oceanic plate) under the western rifted margin of the Indo-Pakistan subcontinental plate, characterized by a well developed accretionary wedge, an ophiolite complex and a tectonically eroded green schist unit derived from the over-ridding continental plate.

Rhodope and Vardar: the metamorphic and the olistostromic paired belts related to the Cretaceous subduction under Europe

The Rhodope massif of Bulgaria and Greece is a complex of Mesozoic synmetamorphic nappes stacked in an Alpine active margin environment. A new analysis of the Triassic to Eocene history of the Vardar suture zone in Greece discloses its Cretaceous setting as a subduction trench. We present a geological traverse that takes into account these new observations and runs from the Hellenides to the Balkans, i.e. from the African to the Eurasian sides of the Tethys ocean, respectively. The present review first defines the revisited limits of the Rhodope metamorphic complex. In particular, the lower part of the Serbo-Macedonian massif is an extension of the Rhodope units to the west of the Struma river. Its upper part is separated as the Frolosh greenschist unit, which underlies tectonic slivers of Carpathic-Balkanic type. Several greenschist units, which locally yield Mesozoic fossils, follow the outer limits of the Rhodope. Their former attribution to a stratigraphic cover of the Rhodope has been proven false. They are divided into roof greenschists, which partly represent an extension of the Strandza Jurassic black shales basin, and western greenschists, which mostly derive from the Vardar Cretaceous olistostromic assemblage. The Rhodope complex of synmetamorphic nappes includes Continental Units and Mixed Units. The Continental Units comprise quartzo-feldspathic gneisses in addition to thick marble layers. The Mixed Units comprise meta-ophiolites as large bodies or small knockers. They are imbricated, forming an open dome whose lower, Continental Unit constitutes the Drama window. The uppermost Mixed Unit is overlain by remnants of the European plate. The presentday structure results from combined large-scale thrust and exhumation tectonics. Regional inversions of synmetamorphic senseof-shear indicate that intermediate parts of the wedge moved upward and forward with respect to both the lower and upper plates. A kinematic model is based on buoyancy-driven decoupling at depth between subducted continental crust and the subducting lithosphere. Continuing convergence allows coeval underthrusting of continental crust at the footwall, decoupling at depth, and upward-forward expulsion of a low-density metamorphic wedge above. The continental crust input and its upward return may have lasted for at least the whole of the Early Cretaceous, as indicated by isotopic ages and the deformation history of the upper plate. A Late Eocene marine transgression divides the ensuing structural and thermal evolution into a follow-up uplift stage and a renewed uplift stage. Revision of the limits of the Vardar belt in Greece first resulted in separating the Paikon mountain as a tectonic window below the Vardar nappes. It belongs to the western, Hellenic foreland into which a system of thrust developed downward between 60 and 40 Ma. The eastern limit is a dextral strike-slip fault zone that developed greenschist facies foliations locally dated at 50-40 Ma. Revision of the lithological components discloses the preponderance of Cretaceous volcano-detritic and olistostromic sequences that include metamorphite blocks of Rhodope origin. Rock units that belong to the Vardar proper (ophiolites, Triassic and Jurassic radiolarites, remnants of an eastern Triassic passive margin) attest for a purely oceanic basin. The Guevgueli arc documents the Jurassic change of the eastern Triassic passive margin into an active one. This arc magmatic activity ended in the Late Jurassic and plate convergence was transferred farther northeast to the subduction boundary along which the Rhodope metamorphic complex formed. We interpret the Rhodope and the Vardar as paired elements of a Cretaceous accretionary wedge. They document the tectonic process that exhumed metamorphic material from under the upper plate, and the tectonic-sedimentary process that fed the trench on the lower plate. The history of the Rhodope-Vardar pair is placed in the light of the history of the Tethys ocean between Africa and Europe. The Cretaceous subduction then appears as the forerunner of the present Hellenic subduction, accounting for several shifts at the expense of the lower plate. The Late Eocene shift, at the closure of the Pindos basin, is coeval with the initiation of new uplift and magmatism in the Rhodope, which probably document the final release of the low-density, continental root of the Rhodope from subduction drag.

Rhodope and Vardar: the metamorphic and the olistostromic paired belts related to the Cretaceous subduction under Europe

The Rhodope massif of Bulgaria and Greece is a complex of Mesozoic synmetamorphic nappes stacked in an Alpine active margin environment. A new analysis of the Triassic to Eocene history of the Vardar suture zone m Greece discloses its Cretaceous setting as a subduction trench. We present a geological traverse that takes into account these new observatons and runs from the Hellenides to the Balkans, i.e. from he African to the Eurasian sides of the Tethys ocean, respectively. The present review first defines the revisited limits of the Rhodope metamorphic complex. In particular, the lower part of the Serbo- Macedonian massif is an extension of the Rhodope units west of the Struma river. Its upper part is separated as the Frolosh greenschist unit, which underlies tectonic slivers of Carpathc-Balkanic type. Several greenschist units which locally yield Mesozoic fossils, follow the outer limits of the Rhodope. Their former attribution to a stratigraphic cover of the Rhodope has been proven false. They are divided into roof greenschists, which partly represent an extension of the Strandza Jurassic black shales basin, and western greenschists, which mostly derive from the Vardar Cretaceous olistostromic assemblage. The Rhodope complex of synmetamorphic nappes includes Continental Units and Mixed Units. The Continental Units comprise quartzo-feld-spathic gneisses in addition to thick marble layers. The Mixed Units comprise meta-ophiolites as large bodies or small knockers. They are imbricated, forming an open dome whose lower, Continental Unit constitutes the Drama window. The uppermost Mixed Unit is overlain by remnants of the European plate. The present-day structure results from combined large-scale thrust and exhumation tectonics. Regional inversions of synmetamorphic sense-of-shear indicate that intermediate parts of the wedge moved upward and forward with respect to both the lower and upper plates. A kinematic model is based on buoyancy-driven decoupling at depth between subducted continental crust and the subducting lithosphere. Continuing convergence allows coeval underthrusting of continental crust at the footwall, decoupling at depth, and upward-forward expulsion of a low-density metamorphic wedge above. The continental crust input and its upward return may have lasted for at least the whole of the Early Cretaceous, as indicated by isotopic ages and the deformation history of the upper plate. A Late Eocene marine transgression divides the ensuing structural and thermal evolution into a follow-up uplift stage and a renewed uplift stage. Revision of the limits of the Vardar belt in Greece first resulted in separating the Paikon mountain as a tectonic window below the Vardar nappes. It belongs to the western, Hellenic foreland into which a system of thrust developed downward between 60 and 40 Ma. The eastern limit is a dextral strike-slip fault zone that developed greenschist facies foliations locally dated at 50–40 Ma. Revision of the lithological components discloses the preponderance of Cretaceous volcano-detritic and olistostromic sequences that include metamorphite blocks of Rhodope origin. Rock units that belong to the Vardar proper (ophiolites, Triassic and Jurassic radiolarites, remnants of an eastern Triassic passive margin) attest for a purely oceanic basin. The Guevgueli arc documents the Jurassic change of the eastern Triassic passive margin into an active one. This arc magmatic activity ended in the Late Jurassic and plate convergence was transferred farther northeast to the subduction boundary along which the Rhodope metamorphic complex formed. We interpret the Rhodope and the Vardar as paired elements of a Cretaceous accretionary wedge. They document the tectonic process that exhumed metamorphic material from under the upper plate, and the tectonic-sedimentary process that fed the trench on the lower plate. The history of the Rhodope-Vardar pair is placed in the light of the history of the Tethys ocean between Africa and Europe. The Cretaceous subduction then appears as the forerunner of the present Hellenic subduction, accounting for several shifts at the expense of the lower plate. The Late Eocene shift, at the closure of the Pindos basin, is coeval with the initiation of new uplift and magmatism in the Rhodope, which probably document the final release of the low-density, continental root of the Rhodope from subduction drag.

Fluid and Geochemical Constraints on the Formation and Retrogression of High-Pressure Metamorphic Rocks from Sifnos (Greece)

The Alpine metamorphic evolution of the Cycladic Crystalline Complex of the Aegean aera of Greece is characterized by an Eocene eclogite-to-blueschist facies metamorphism and an Oligocene/ Miocene greenschist-to-amphibolite facies metamorphism. High pressure assemblages are well preserved in the north of Sifnos island (Blueschist Unit), whereas the rest of the island is dominated by greenschist-facies rocks (Greenschist Unit). However, rocks from Sifnos island preserve a complex record of both metamorphic events at all scales. In this paper we combine structural and petrological observations with major and trace geochemistry and stable-isotope data to characterize (a) the fluid involvement during highpressure and greenschist-facies metamorphism and (b) the nature of the retrogression from blueschist to greenschist facies rocks.

Brittle deformation events at the western border of the Bohemian Massif (Germany)

Investigations of brittle deformation structures, present within the crystalline rocks of the Bavarian Oberpfalz, reveal a complex late to post-Variscan crustal evolution. Upper Carboniferous (mainly Westphalian) granites were emplaced into semibrittle to brittle rocks of the ZEV (zone of Erbendorf-Vohenstraus) and the EGZ (Erbendorf greenschist unit), respectively. From both the alignment of the granites and the direction of granite-related tension gashes a north-east-south-west extension must be assumed for the period of magmatic activities. Apart from the granite intrusions, rapid crustal uplift (about 1.5 km/my) led to an increase in the geothermal gradient from 40 °C/km (late Variscan post-granitic). The increased geothermal gradient persisted during the succeeding reverse faulting which results from late Carboniferous (probably Stephanian) east-west and northeast-south-west compression. Although not evidenced directly in the area considered, strike-slip faults seem to have played an important part during the late Variscan crustal evolution, particularly in the Early Permian. The strike-slip events indicate further crustal shortening and indentation under north-south compression. A similar indentation was present in Cretaceous time. After a weak phase of Early Cretaceous reverse faulting, which results from north-south compression, strike-slip faults formed under north-west-south-east and north-south compression. All these faults, in particular the strike-slip faults, seem to be related to the Cretaceous and lowermost Tertiary convergence of the Alpine/Carpathian orogeny. A late stage of crustal extension, characterized by a radial stress tensor (σ2 = σ3), is indicated through high angle normal faults which probably formed during the subsidence of the adjacent Neogene Eger Graben.

Cooling during the exhumation of a blueschist terrane: Sifnos (Cyclades), Greece

Alpine high-pressure metamorphic rocks exposed in the Cyclades were overpinted during their exhumation. We studied the metamorphic evolution of a pervasively overprinted rock sequence, the greenschist unit, exposed in the island of Sifnos. We show that the metamorphic evolution in the Greenschist unit is characterized by the progressive transformation of eclogite-facies rocks through albite-bearing epidote blueschists into greenschists. The evolution of the mineral assemblages during exhumation is shown by a series of chemographic diagrams which are referenced to specific P-T fields in the petrogenetic grid proposed by Evans (1990). P-T conditions corresponding to the major paragenetic groups are estimated also on the basis of mineral chemistry, Fe/Mg exchange thermometry and the use of GEOCALC-PTX and THERMOCALC softwares. A P-T path for the Greenschist unit is presented indicating eclogite-facies conditions of around 480-520 ° C and 12-15 kbar, followed by overprinting in the albite-bearing epidote-blueschist facies at P-T conditions of 8-10 kbar and ca. 480-500 ° C. Pressures of 5.5 kbar and temperatures around 450 ° C are estmated for the subsequent overprint in the greenschist facies. The decompression part of the P-T path is characterized by cooling with no indication of a temperature rise. Thus, the high-pressure metamorphic rocks of Sifnos appears not to follow the thermal evolution path predicted by models which assume whole crust uplift and erosion. On a regional scale the exhumation of the Cycladic blueschist belt is characterized by cooling in the west, but broad clockwise P-T loops with significant heating during decompression are found in the east. We note that cooling affected rocks situated at the deepest levels of the Alpine orogenic wedge (Sifnos, Syros, Tinos) whereas heating has affected rocks exhumed from shallower structural levels (Naxos, Ios)

Amphibole compositions and metamorphic history of the Rand Schist and the greenschist unit of the Catalina Schist, Southern California

The Catalina Schist and Rand Schist are two high P/T terranes in southern California. The Catalina Schist is correlated with the Franciscan Complex and occurs in the continental borderland. It consists of a blueschist-facies melange tectonically overlain by a “greenschist” unit, which, in turn, is overthrust by an amphibolite unit. The greenschist unit itself is inversely zoned from epidote-amphibolite fades at the top through greenschist facies in the center to transitional blueschist-greenschist facies at the base. The Rand Schist is part of the eugeoclinal Pelona-Orocopia Schist terrane, which lies interior to the present continental margin, structurally beneath Precambrian to Mesozoic sialic basement. The Rand Schist is inversely zoned from epidote-amphibolite facies to transitional blueschist-greenschist facies, similar to the greenschist unit of the Catalina Schist.

Evolution of the blueschist and greenschist facies rocks of Sifnos, Cyclades, Greece

The metamorphism on the island of Sifnos is characterized by the Eocene development of a coherent highpressure blueschist terrane and an early Miocene greenschist facies overprint. This study documents the metamorphic evolution of the blueschist assemblages, still preserved in the northern parts of the island, and their subsequent transformation into greenschists in the central and southern parts. The oxygen isotope geothermometry is based on calibrations for quartz, pyroxenes and magnetite (Matthews et al. 1983a) augmented by revised calibrations for the minerals muscovite (Δ Qz−Mu=1.55×106 T −2), epidote (δ Qz−Ep= 1.56+1.92Δ ps)106 T −2), and rutile (Δ Qz−Ru=4.54×106 T −2). Oxygen isotope analyses of minerals from the Blueschist unit of northern Sifnos give consistent fractionations which are independent of rock type. An average temperature of 455° C was obtained, although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions. Analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred. The minerals and whole rocks of the greenschists of central Sifnos are systematically enriched in 18O relative to the blueschist assemblages. Chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of Ca2+ and CO2. The petrologic, isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18O-CO2 enriched aqueous solutions. It is reasonable to assume that this is connected with the Miocene magmatic activity observed throughout the Cyclades. The marbles separating the Blueschist from the Greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation. The pressure of the blueschist metamorphism is estimated at 14±2 kbar, corresponding to a depth of ca. 50 km. The structural style and stratigraphy of Sifnos are suggestive of the subduction of a continental margin sequence. It is clear that the considerable tectonic depression may be associated with continental collision and underthrusting.