Metamorphic Aureole Research Articles

Chemistry of serpentine “polymorphs” in the Pan-African serpentinites from the Eastern Desert of Egypt, with an emphasis on the effect of superimposed thermal metamorphism

The present work deals with some Pan-African serpentinites of Neoproterozoic age from five localities in the Eastern Desert of Egypt namely, Abu Fannani, Fawakhir, Barramiya, Ras Shait and Wadi Ghadir that derivedmostly from lherzolite to harzburgite protoliths. The M-value of antigorite is anindicator of the metamorphic grade which is lowest at Fawakhir (greenschist facies) and highest at Abu Fannani (lower amphibolite facies). Antigoritization during progressive regional metamorphism at Fawakhir is limited and its M-value is much higher than 8.52 indicating crystallization temperatures of 220–250 °C whereas it is ≥300 °C in the rest. Antigorite recrystallized at T ≈ 400–450 °C in the contact metamorphic aureoles due to the emplacement of post-orogenic leucogranites at Fawakhir and Gebel Ghadir. Towards the contact with the granites, M-value of antigorite is low (6.48) compared to 8.52 in the least recrystallized antigorite due to the thermal effect. Antigorite that forms in the thermal aureoles is characterized by two types of substitution; Tschermak substitution (Al3+ and Cr3+ for Si4+ in the tetrahedral sites and Mg2+ in the octahedral sites) and non-Tschermak substitutionin the octahedral sites (2R3+ = 3R2+). Generally, both ortho- and clinochrysotiles are common with MgO contents of 41–41.13 wt% and 39.38–40.93 wt%, respectively. In addition to chlorite, high- and low-Al lizardites are present with almost constant total iron (~0.24 cations) as Fe2+(vi) and variable Mg2+. This suggests that significant R3+ in the octahedral sites are mostly occupied by Al3+ and Cr3+ and not iron. In the lizardite structure, there is Tschermak substitution of Si4+ by some trivalent cations in the tetrahedral sites. Using the TEM images, antigorite is distinguishable from chrysotile and they suggest the presence of polyhedral or polygonal serpentine (spherical or circular with alternating sectors of lizardite). Crack-seal microstructures are displayed by chrysotile and brucite veins developed by extensional shearing that post-dates antigoritization.

Petrogenesis and tectonic setting of the Bazman Batholith, Southeast Iran

The Bazman batholith is located in the south of Lut block, southeastern Iran. The intrusive rocks include alkali granite, intermediate granitoid (quartz-monzonite, quartz-monzodiorite and granodiorite), gabbro and some leucocratic dykes (aplitic) which have been intruded into upper Paleozoic sedimentary rocks and causing a contact metamorphic aureole. In the granitoids, A/NK>1 and A/CNK<1 ratios are indicative of metaluminous nature. Initial 87Sr/86Sr ratios for the Bazman samples (<0.706), are consistent with I-type magmatic sources. Furthermore, based on chemical characteristics and mineralogy, the granitoids are I-type. In the Harker diagrams major and trace elements variations are not consistent with generation of granitoids from gabbros via magmatic differentiation process which is also confirmed by the normalized REEs and multi-element diagrams. In the normalized multi-element diagrams, light ion lithophile elements (LILE) such as K and Rb display positive anomaly whereas high field strength element (HFSE) such as Ti, Nb and Ta represent negative anomaly. Based on major elements geochemistry including low (Na2O+K2O)/(MgO+FeO+TiO2), K2O/Na2O and Al2O3/(MgO+FeO+TiO2) ratios and high CaO/Al2O3 and CaO+FeO+MgO+TiO2 contents, intermediate granitoids are similar to those ofpartial melts of amphibolite/metabasalt sources. In addition, trace elements modeling show that the intermediate granitoids can be produced by dehydration partial melting (10 to 30%) from an amphibolitic source and restite minerals assemblage of plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides. Based on major element geochemistry, the intermediate granitoids are comparable with the partial melts originating from a metabasaltic/amphibolitic source. Geochemical characteristics suggest that the Bazman batholith has been formed in a volcanic arc setting (active continental margin). Therefore, it seems that during upper-Cretaceous, the southern margin of the Lut block was an active continental margin due to the subduction of a Neo-Tethyan slab beneath southern margin of Eurasia. Here we proposed a new model in which the oceanic basin of southern Lut was probably different from the ocean which is now subducting beneath the Makran. Instead it was a closed oceanic basin beneath Jazmurian depression.

Palaeoproterozoic banded iron formationhosted high-grade hematite iron ore deposits of the Transvaal Supergroup, South Africa

Banded iron formation-hosted high-grade (>60 weight percent (wt%) iron (Fe)) hematite ore deposits make up the bulk of the world's iron ore production and reserves. They developed mainly through supergene and/or hydrothermal leaching of silica from the iron formation host rock under oxidizing conditions. Early Palaeoproterozoic iron formations of the Transvaal Supergroup host several such high-grade hematite ore deposits. The largest ones are developed in the Asbesheuwels Subgroup iron formation on the Maremane Dome, between Sishen and Postmasburg in the Northern Cape Province of South Africa. These deposits currently produce virtually all of South Africa's annual supply of about 78 million tonnes (Mt) of high-grade hematite ore, to the local and export market from the Sishen, Khumani, Beeshoek and Kolomela Mines. These represent ancient supergene deposits that formed along the approximately 2.2 to 2.0 Ga unconformity at the base of the Gamagara/ Mapedi red bed succession of the Keis (formerly Kheis) Supergroup. Smaller ancient supergene deposits are also developed along the same unconformity such as where it intersects the Rooinekke Iron Formation of the Koegas Subgroup at the Rooinekke Iron Ore Mine to the south of the Maremane Dome and the intersection of the Hotazel Iron Formation of the Voelwater Subgroup of the Transvaal Supergroup, in the Kalahari Manganese Field to the north of the dome. The supergene ores occur in four types, namely laminated, massive, brecciated and conglomeratic ores, the latter forming part of the Gamagara/Mapedi red bed succession. With the exception of the ore at Rooinekke, which is manganiferous, the supergene ores are of high grade (>60 wt%) and they also have low silica and phosphorous contents. Normalized rare earth elements, in contrast to the banded iron formation protolith, show light rare earth element depletion and the oxygen isotopes are slightly depleted to slightly enriched in 18 O. Hydrothermally enriched deposits include: Thabazimbi, situated in the Penge Iron Formation of the Transvaal outcrop area of the Transvaal Supergroup in the metamorphic aureole of the Bushveld Complex; and the Bovenzeekoebaart and magmatic hydrothermal Nauga East deposit, both located in the Kuruman Iron Formation of the Asbesheuwels Subgroup, along the southern extremity of the Griqualand West outcrop area of the Transvaal Supergroup. These hydrothermal iron ores are all developed at the bottom contact between the host iron formation and underlying carbonaceous shale that marks the transition into the Malmani- Campbellrand carbonate platform succession below. In some instances this contact is faulted. The ore grades upwards into oxidized iron formation, with isolated lenses sometimes developed at higher stratigraphic levels. Fault systems intersect the iron formations and the mineralisation is concentrated between mafic sills, higher up in the iron formations and the bottom shale contact. At Nauga East, mineralisation is developed in contact with a steeply dipping zoned syenite-carbonatite dyke. The hydrothermal ores are of high grade (>60 wt%), but can have higher phosphorous contents of up to 0.25 wt%. Normalized rare earth elements are enriched but similar to that of the host iron formations and the oxygen isotopes are generally depleted in 18 O. The mining of iron ore in South Africa dates from between 800 and 1200 AD, with modern exploration and mining starting from 1916. The first production of high-grade banded iron formation-hosted ore commenced in 1931 from the Maremane Dome and Thabazimbi. The first truly world class iron ore mine in South Africa, Sishen, was opened in 1953 and still operates today. The deposits at Khumani and Welgevonden have gone into production as recently as 2008 and 2011 respectively. Current estimates place the life of mine of the larger deposits at between 15 and 40 years.

Fluid‐fluxed melting and melt loss in a syntectonic contact metamorphic aureole from the Variscan eastern Pyrenees

AbstractOpen‐system behaviour through fluid influx and melt loss can produce a variety of migmatite morphologies and mineral assemblages from the same protolith composition. This is shown by different types of granulite facies migmatite from the contact aureole of the Ceret gabbro–diorite stock in the Roc de Frausa Massif (eastern Pyrenees). Patch, stromatic and schollen migmatites are identified in the inner contact aureole, whereas schollen migmatites and residual melanosomes are found as xenoliths inside the gabbro–diorite. Patch and schollen migmatites record D1 and D2 structures in folded melanosome and mostly preserve the high‐T D2 in granular or weakly foliated leucosome. Stromatic migmatites and residual melanosomes only preserve D2. The assemblage quartz–garnet–biotite–sillimanite–cordierite±K‐feldspar–plagioclase is present in patch and schollen migmatites, whereas stromatic migmatites and residual melanosomes contain a sub‐assemblage with no sillimanite and/or K‐feldspar. A decrease in X Fe (molar Fe/(Fe + Mg)) in garnet, biotite and cordierite is observed from patch migmatites through schollen and stromatic migmatites to residual melanosomes. Whole‐rock compositions of patch, schollen and stromatic migmatites are similar to those of non‐migmatitic rocks from the surrounding area. These metasedimentary rocks are interpreted as the protoliths of the migmatites. A decrease in the silica content of migmatites from 63 to 40 wt% SiO2 is accompanied by an increase in Al2O3 and MgO+FeO and by a depletion in alkalis. Thermodynamic modelling in the NCKFMASHTO system for the different types of migmatite provides peak metamorphic conditions ~7–8 kbar and 840 °C. A nearly isothermal decompression history down to 5.5 kbar was followed by isobaric cooling from 840 °C through 690 °C to lower temperatures. The preservation of granulite facies assemblages and the variation in mineral assemblages and chemical composition can be modelled by ongoing H2O‐fluxed melting accompanied by melt loss. The fluids were probably released by the crystallizing gabbro–diorite, infiltrating the metasedimentary rocks and fluxing melting. Release of fluids and melt loss were probably favoured by coeval deformation (D2). The amount of melt remaining in the system varied considerably among the different types of migmatite. The whole‐rock compositions of the samples, the modelled compositions of melts at the solidus at 5.5 kbar and the residues show a good correlation.

Modeling magmatic accumulations in the upper crust: Metamorphic implications for the country rock

Field exposures of magma chambers tend to reveal contact metamorphic aureoles in the surrounding crust, which width varies from few centimeters to kilometers. The igneous accumulation not only increases the temperature around it, but also weakens its surrounding country rock beyond the brittle–ductile transition temperature. The formation of a ductile halo around the magmatic reservoir may significantly impact into the stability and growth of the magma chamber, as well as into potential dyke injections and processes of ground deformation. In this paper, we examine how a magmatic accumulation affects the country rock through the combination of petrologic and thermal perspectives. For this, we numerically modeled (i) the conductive cooling of an instantaneously emplaced magma chamber within compositionally representative pelitic and carbonate upper crusts, and (ii) the corresponding changes in the viscosity of the host rock potentially leading to ductile regimes. We consider basaltic to rhyolitic magma chambers at different depths with oblate, prolate and spherical geometries. The resulting temperature field distribution at different time steps is integrated with crustal metamorphic effects through phase diagram modeling. Our results indicate that the geometry of the magma accumulations plays a dominant role in controlling the local metamorphic and thermal effects on the country rocks. They conclude that (i) the combination of relatively simple geothermal models with petrologic datasets can generate first order predictions for the maximum metamorphic grade and geometry of magma chamber aureoles; (ii) the possible changes in the mechanical properties of the country rock are not necessarily linked to the petrological changes in contact aureoles; and (iii) the present rheologic outcomes may be used in further studies of magma chamber stability and integrity, which may favor the understanding of the melt transfer throughout the crust.

A large scale investigation into changes in coal quality caused by dolerite dykes in Secunda, South Africa-implications for the use of proximate analysis on a working mine

The coalfields of South Africa contain numerous dolerite intrusions, which affected the quality of the surrounding coal through thermal processes, commonly believed to be controlled by the size of the magmatic body. Data gathered from a working coalfield in Secunda, South Africa, suggest that the relationship between intrusive sills and coal is complex and factors other than intrusion width must be considered in relation to the contact metamorphic effect. The study area contains multiple dolerite intrusions of Karoo age, of which three intrusions occur as sills intruded close to the main coal seam of the. A large database (>8000 boreholes) of coal quality data was used to investigate the presence or absence of a change in coal quality relative to dolerite proximity. Reduction in coal quality was defined using three proximate analysis values, namely the ash, volatile content and dry ash free volatile (DAFV) as defined in the coal industry. The resultant investigation showed no correlation between the position and thickness of the dolerites, and changes in coal quality as measured by proximate analysis. In the absence of a linear relationship between coal quality and dolerite proximity, two processes are proposed to explain the absence of the contact metamorphic effects expected from previous studies. Firstly dolerite emplacement dynamics may influence the size of the metamorphic aureole produced by an intrusion, invalidating intrusion size as a measure of thermal output. Secondly, hydrothermal fluids mobilised by the dolerite intrusions, either from the country rock or the intrusion itself may percolate through the coal and act as the metamorphic agent responsible for changing coal quality, by dissolving the volatile and semi-volatile components of the coal and transporting them to other locations. These two processes are sufficient to explain the lack of a clear “metamorphic effect” related to the dolerite intrusions. However, the perceived lack of a clear correlation between the coal quality parameters and the metamorphic effects associated with dolerite intrusion may also reflect the inadequacies of proximate analysis techniques in quantifying geological processes within the coal.

Origin and geodynamic significance of the early Mesozoic Weiya LP and HT granulites from the Chinese Eastern Tianshan

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U–Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic–pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granulites, and orthopyroxene–clinopyroxene-bearing granulites. Average P–T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600–650°C at 3.2–4.2kbar and peak metamorphism of 750–840°C at 2.9–6.3kbar, indicating a rather high geothermal gradient of ca. 60°C/km. Zircon U–Pb LA–ICP–MS dating revealed metamorphic ages between 244±1 to 237±3Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution.

Syndepositional forced folding and related fluid plumbing above a magmatic laccolith: Insights from outcrop (Lower Cretaceous, Basque-Cantabrian Basin, western Pyrenees)

Emplacement of shallow magmatic laccoliths can result in the development of dome-shaped forced folds and fractures in the overburden and at the depositional surface. These folds and fractures have been described from both outcrop studies and seismic interpretation; however, the main controls on fold growth and fracture development remain unclear and, in some cases, controversial. This paper presents a new approach in the analysis of the growth of laccolith intrusions and forced folds. In this study, deep-water growth deposits at an outcrop in the Basque-Cantabrian Basin (north Iberia) record the coeval formation and evolution of a late Albian forced fold and related clastic dike system above the igneous Larragan laccolith. The vertical succession allows field mapping and characterization of the growth deposits, the underlying forced fold, and the forcible Larragan laccolith at depth. The Larragan laccolith is 600 m wide and 40 m thick and intrudes an organic-rich unit at an original (decompacted) depth of ∼450 m. A multidisciplinary study (sedimentology, structure, petrology, and geochemistry) of the growth deposits reveals that the emplacement of the laccolith created a dome-shaped upward-widening fold, which had a final relief of 18–36 m and a width of ∼850 m at the paleoseabed. Simultaneously, in the hinge zone of the fold, local tensional stress induced the formation of a fracture system, which subsequently acted as conduits for overpressured ascending fluids (methane and water) and fluidized sediments originating from the metamorphic aureole of the laccolith. This plumbing system reached the paleoseabed, where extruded sediments were deposited and methane-derived carbonates precipitated. A comprehensive model is proposed in which incremental emplacement of magma sheets induced pulsating contact metamorphism, folding and fracturing of the overburden, and episodic activity of the fluid plumbing system.

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).

Graphite thermometry in a low-pressure contact aureole, Halifax, Nova Scotia

Abstract Intrusion of the late Devonian South Mountain Batholith, southern Nova Scotia, produced a low-pressure contact metamorphic aureole in its metasedimentary host rocks. The effects of contact metamorphism are particularly well developed in pelitic rocks of the Halifax Group on the eastern margin of the batholith. Contact metamorphic isograds and mineral assemblages suggest low-pressure metamorphism, with P–T conditions at the contact estimated at 2.5–3.0 kbar and ca. 650 °C. In this study, Raman spectroscopy of carbonaceous material (RSCM) was used to obtain temperatures from graphite, which is common throughout the contact aureole. Temperature estimates range from ca. 360 °C just outside the cordierite-in isograd to ca. 640 °C in the sillimanite–K-feldspar zone near the contact, the latter consistent with the temperature estimated from the corresponding silicate mineral assemblage. Three different RSCM calibrations produced very similar results except at the high-temperature end of the observed range. A thermal profile constructed from the RSCM data was used to constrain a 2D numerical model for post-intrusion conductive cooling of the batholith along its eastern margin. Comparison of RSCM vs model thermal profiles suggest that observed differences between the thermal structure of the inner and outer aureole were controlled by the subsurface geometry of the pluton contact. The model predicts that peak temperatures in country rocks within 1 km of the contact were reached within 50 ka of intrusion, but that the outer part of the aureole took 250–500 ka to reach peak temperatures. The results confirm the utility of RSCM thermometry for acquiring temperature data over a range of metamorphic grades.

Triassic warm subduction in northeastTurkey: Evidence from theAğvanis metamorphic rocks

AbstractWithin theTethyan realm, data for the subduction history of thePermo–TriassicTethys in the form of accretionary complexes are scarce, coming mainly from northwestTurkey andTibet. Herein we present field geological, petrological and geochronological data on aTriassic accretionary complex, the Ağvanis metamorphic rocks, from northeastTurkey. TheAğvanis metamorphic rocks form aSSE–NNWtrending lozenge‐shaped horst, ∼20 km long and ∼6 km across, bounded by the strands of the activeNorthAnatolianFault close to the collision zone between theEasternPontides and theMenderes–TaurusBlock. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes ofEarlyEocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°Cand ∼0.60–0.90 GPa.Stepwise40Ar/39Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ∼180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, theAğvanis metamorphic rocks represent the vestiges of theLateTriassic or slightly older subduction in northeastTurkey. EstimatedP–Tconditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐dayCascadia. Contact metamorphic mineral assemblages around anEarlyEocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ∼14 km during theEarlyEocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐micaAr–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision betweenEasternPontides and theMenderes–TaurusBlock.

Detrital anisotropic grandite garnet as an indicator of denudation level of Permian volcanic arc in the provenance of the South Kitakami Belt, Northeast Japan

In this study, the chemical and optical features of detrital garnets from the Middle Permian to Upper Triassic sandstones in the South Kitakami Belt, Northeast Japan, were examined to reveal the tectonic movement in the provenance. The sandstones contain a large amount of detrital grandite garnet grains with a wide range of andradite content. Among them, some grandite garnet grains show optically anisotropic features and rarely oscillatory zoning and sector twinning. The proportion of the detrital anisotropic grandite garnet increases from the Permian to the Middle Triassic and decreases in the Late Triassic. Such grandite garnets with various andradite contents occur in skarn deposits. Isotropic grandite garnets in the early stage of skarn evolution are distributed widely around a pluton, which affects the thermal metamorphism of the surrounding strata. However, anisotropic grandite garnets are formed along veinlets and fractures in the middle to late hydrothermal stage as the pluton cools, and their distribution is limited to a narrower area near the pluton compared to the metamorphic aureoles formed in the early stage. Changes in the chemical and optical features of the detrital garnets indicate a progressive denudation of the plutonic body accompanying skarn deposits in the provenance. The proportion of detrital anisotropic grandite garnet grains among all of the detrital grandite garnet is considered to be a sensitive indicator of the denudation level in a deeper part of the volcanic arc in association with skarn deposits, together with traditional sandstone composition datasets. This newly proposed method should be useful for clarifying the paleogeography during the Permian to Triassic in the East Asian continental margin, associated with uplift and denudation of the Permian volcanic arc, which seems to have been induced by the collision of the North China and South China Blocks.

The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland

The early Ordovician ( c . 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician ( c . 470 Ma). The subduction zone was nucleated on an oceanic transform–fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a suprasubduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform–fracture zone–subduction zone system and NE of a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of mid-ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the greenschist-facies, obducted, oceanic and continental margin rocks trend WSW (the obduction direction).

Mineral chemistry and thermobarometry of plutonic, metamorphic and anatectic rocks from the Tueyserkan area (Hamedan, Iran)

The study area is a part of the NW to SE striking Sanandaj–Sirjan metamorphic belt in western Iran. The Alvand Pluton, consisting of rocks that range in composition from gabbro to granite, is the major magmatic rock complex of this area. Gabbroic rocks include olivine gabbro, gabbro norite, norite and gabbro. Rocks around the Alvand Pluton were subjected to different P–T conditions due to polymetamorphism. Common metamorphic rocks are meta-pelites, but some meta-psammites, meta-basites and meta-carbonates also occur. Slates, phyllites, schists, migmatites and hornfelses are major rock units of meta-pelites in the metamorphic sequence. Based on mineral chemistry, the highest temperature of crystallisation (1300°C) was determined for the olivine gabbros, and the lowest temperature (950°C) was calculated for the hornblende-bearing gabbros. Clinopyroxene–plagioclase barometry suggests that pressures near 5 to 6 kbars prevailed during the crystallisation of the various mafic rocks. P–T estimates yield maximum temperatures of 700–750°C at 5–6 kbars for the high-grade metamorphic rocks from the metamorphic aureole around the pluton. These results indicate that the heat released from the Alvand Pluton (T = 950–1300°C), which intruded the metamorphic rocks at middle and upper crustal levels, was sufficient to cause partial melting leading to formation of the metatexites, diatexites and restite-rich S-type granites. During this process, part of the deep-seated gabbro-dioritic rocks were transported to higher crustal levels by viscous, enclave- and crystal-rich granitic magmas of the partial melting zone.

The evolution of the footwall to the Ronda subcontinental mantle peridotites: insights from the Nieves Unit (western Betic Cordillera)

Strongly heterogeneous deformation and extreme metamorphic gradients characterize the dominantly carbonate Nieves Unit in the footwall to the Ronda mantle extrusion wedge in the western Betic Cordillera. A well-developed foliation and mineral lineation, together with isoclinal intrafolial folds, occur in silicate-bearing, calcite or dolomite marbles within a c . 1.5 km thick metamorphic aureole underlying the peridotites. For the inferred maximum pressure of 300 MPa, petrological investigations allow us to define temperature ranges for the main zones of the metamorphic aureole: &gt;510 °C (probably c. 700 °C) for the forsterite zone; 510–430 °C for the diopside zone; 430–360 °C for the tremolite zone; 360–330 °C for the phlogopite zone. Field structural analysis integrated with petrological, microstructural and electron backscatter diffraction textural data document large finite strains consistent with general shear within the metamorphic aureole, associated with NW-directed thrusting of the peridotites. On the other hand, post-kinematic silicate growth suggests that heat diffusion from the high-temperature peridotites continued after the final emplacement of the Ronda mantle extrusion wedge, leading to final zoning of the metamorphic aureole and to local partial annealing of calcite marble textures, particularly in the highest-temperature zone of the thermally softened footwall carbonates. Following substantial cooling, renewed crustal shortening affected the whole Nieves Unit, resulting in widespread development of NE–SW-trending meso-scale folds.

Country rock monazite response to intrusion of the Searchlight pluton, southern Nevada

We investigated how monazite grains in country rocks responded to the intrusion of the Miocene Searchlight pluton in southern Nevada. Country rock samples were collected from the roof zone and along transects on the flanks (wallrock) of the 16 to 17 Ma pluton. Deep wallrock Ireteba granite monazite grains have patchy secondary growth zones of Searchlight age overprinting primary growth zones of Ireteba age (∼66 Ma). Shallow wallrock Proterozoic gneiss zircon grains define a discordia with an upper intercept age of 1.74 ± 0.02 Ga corresponding to crystallization of the protolith. Proterozoic gneiss monazite grains define a discordia with an upper intercept age of 1.64 ± 0.02 Ga and a poorly-defined lower intercept age of 75 ± 61 Ma that may correspond to the Ireteba intrusion. EMP analyses show that patchy secondary zones in Proterozoic gneiss monazite grains were contemporaneous with intrusion of the Ireteba granite, not the Searchlight pluton. Oxygen isotopes in Ireteba monazite, hydrogen and oxygen isotopes in whole rocks from the Ireteba transect, and oxygen isotopes in whole rocks from the Proterozoic gneiss transect show no systematic pattern related to the contact. No geochemical data support the hypothesis that hydrothermal fluids associated with intrusion of the Searchlight pluton caused monazite in the Proterozoic gneiss or Ireteba granite wallrock to partially recrystallize. Ireteba samples with the most intensely altered monazite were at the greatest paleodepths at the time of Searchlight intrusion and are the most deformed, suggesting that strain caused Ireteba monazite grains to partially recrystallize. In Proterozoic gneiss country rock monazite grains are present on the flanks but absent from the roof zone, suggesting that high fluid fluxes in the roof may have destroyed monazite. Strong focusing of hydrothermal fluid and heat into the roof zone prevented the development of a well-defined contact metamorphic aureole in Ireteba granite and Proterozoic gneiss wallrocks.

New data on the metamorphic petrology of rocks from the Kuekvun uplift (Northern Chukchi Peninsula)

The metamorphic rocks of the Kuekvun uplift are gneisses and schists that developed after Mid-Paleozoic sedimentary sequences, which consist mostly of terrigenous rocks with minor amounts of intermediate to mafic igneous rocks and subordinate carbonates. Two plagiogneiss samples that were selected for detailed analysis were taken from the axial part and flanks of the uplift. The mineral paragenesis and composition of coexisting garnet, biotite, and staurolite indicate a metamorphic temperature of 560–600°C and pressure of 2.5–4 kbar, corresponding to a depth of 8–12 km. These conditions suggest a relatively high geothermal gradient (about 60°C/km), approaching that for contact metamorphic aureoles. Garnets from the axial zone and flanks of the uplift display a similar zoning pattern. The difference is the presence of compositionally contrasting Ca-rich rims in garnets from the axial zone. These rims may be either the result of zoning within the entire metamorphic complex or a late local overprint, e.g., crystallization of granitic plutons, which are common within the study area.

Peraluminous igneous rocks as an indicator of thermogenic methane release from the North Atlantic Volcanic Province at the time of the Paleocene–Eocene Thermal Maximum (PETM)

Unusual cordierite-bearing peraluminous dacites, produced by melting of organic-rich sediments by intrusion of basaltic magma, are found within the North Atlantic Volcanic Province (NAVP). Calculations suggest that formation of the dacites, radiometric dated at 55.9 ± 0.3 Ma and possibly widespread, could have released an average of ∼4,500 Gt (range from 3,000 to 6,000 Gt) of carbon as methane, with a δ13C of about −35‰. Published model results suggest that such a methane release could explain the negative δ13C excursion in the oceans and atmosphere, the extreme global warming, and the marked dissolution of carbonates in the deep oceans that accompanied the concurrent Paleocene–Eocene Thermal Maximum (PETM). Outgassing from melting of sediments and formation of dacites, possibly in conjunction with methane produced in contact metamorphic aureoles and by methane hydrate release, provides a novel way of explaining the PETM and its timing.

Pinite-cordierite from spotted slate of the Brajkovac contact metamorphic aureole (Dudovica locality, central Serbia)

The Paleozoic very low to low-grade metamorphic rocks of the Bukulja-Lazarevac Unit designated as Drina, Golija and Birac formations are contact metamorphosed by the intrusion of the Tertiary Brajkovac granodiorite into spotted slates and hornfelses. In some parts, they are slightly migmatized at the contact. In addition to their outcrops found at the western, eastern and northern parts of the formation, these rocks are also found in boreholes near Dudovica at about 8 km south-west from the pluton. There, at a depth of 110 m, the spotted slates comprise oval to ellipsoid pinite-rich spots which can be regarded as incipient cordierite porphyroblasts (up to 5 mm in diameter) overgrowing the existing regional foliation. They are composed of cryptocrystalline mixture of a very fine sericitic material ? light glassy orange ?film? (some kind of an amorphous gel-like material often mixed with limonite matter) and are abundant in inclusions: minute quartz and dusty ore minerals (magnetite) prevail. In addition, within some spots an increased number of xenotime and monazite inclusions are noted. Minute flakes of neobiotite are formed at the expense of quartz-sericite-chlorite matrix. The secondary chlorite occurring as overgrowths on pinite-cordierite spots shows variable composition (brunsvigite to diabandite). The Mg/Fe+Mg ratio of cryptocrystalline pinitic mixture ranges from 0.14-0.67. The Si vs AlIV+AlVI relations deviate from the ideal muscovite-phengite join due to Tschermak substitution towards chloritic composition or a more complex mixture, including clay minerals (which reflected a decrease of Altot and Si with increase of Fe2+). Obtained data indicates that the cordierite-pinite spots can be related to contact metamorphic processes that occurred within the temperature range 300-450?C.

Time–space relationships among structural and metamorphic aureoles related to granite emplacement: a case study from the Serre Massif (southern Italy)

AbstractTectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10−14s−1within the country rocks during granodiorite intrusion.