Contact Metamorphic Zone Research Articles

Heavy minerals of the aeolian sediments in the East Slovak Basin (Western Carpathians) – Implications for their origin, transport process and sedimentary history

Aeolian sediments usually indicate glacial or periglacial arid-climate conditions, and knowledge of their sedimentary character and mineral composition can reveal the locality’s palaeo-environment and palaeo-geographic history. Reconstruction is provided by heavy-mineral surface micro-textural and geochemical analysis, and this detected the source area, source rocks and transport conditions for the Würmian wind-blown sands which form several dune types in the East Slovak Basin. The heavy-mineral morphology, surface micro-textures and low mineral maturity of the aeolian sediments preserve traces of past sub-aquatic environments and local aeolian transport. The heavy-mineral geochemistry suggests initial detrital derivation from local sources. This was formed by re-worked and re-mobilised Magura Nappe flysch sandstones in the Western Carpathians, with likely contribution of the Pieniny Klippen Belt flysch sediments indicated in sporadic pyrope-rich garnet. The associated detrital pyroxene and amphibole geochemistry denotes primary andesite source rocks which dominate lithology in the surrounding Neogene volcanic mountain chains. These are the Slanské vrchy, Vihorlatské vrchy and Zemplínske vrchy Mountains. The further occurrence of detrital hydrogrossular indicates derivation from contact-metamorphic zones associated with the volcanic rocks. Although different garnet types in the distinct dune profile zones may have resulted from heavy mineral re-sorting during active dune movement, they suggest sources changed by local wind directions. Orientation of basic linear features of the dunes derived from the digital terrain model indicates (paleo)wind generally from the north.

Evolution of Contact Metamorphic Rocks in the Zhoukoudian Area: Evidence from Phase Equilibrium Modelling

The Yanshan intraplate tectonic belt is a tectonic-active area in the central part of the North China Craton that has undergone long-term orogenic evolution. Detailed studies on magmatic activity and metamorphism of this belt are significant for restoring its orogenic thermal evolution process. The Fangshan pluton in the Zhoukoudian area within this tectonic belt is a product of the late Mesozoic Yanshan event. However, there is a lack of detailed research on the metamorphic evolution history of the ancient terrane surrounding the Fangshan pluton subjected to contact thermal metamorphism. To further constrain the metamorphic P–T evolution of contact metamorphism associated with the Fangshan pluton, we collected rock samples in the andalusite–biotite contact metamorphic zone of the Fangshan pluton, and conducted petrographic investigations, geochemical and mineral composition analysis, and phase equilibrium modeling. The phase equilibrium modeling in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system indicates that the peak mineral assemblages of andalusite–biotite schists are pl + q + mu + bi + and ± kfs + ilm + mt, formed at 550 to 610 °C, 1 to 3.5 kbar, and the peak mineral assemblage of garnet–andalusite–cordierite–biotite schists is gt + pl + q + bi + and + cord + ilm + mt, formed at 580 to 620 °C, 1.5 to 2.1 kbar. Therefore, we believe that the rocks in the andalusite biotite contact metamorphic zone of the Fangshan pluton underwent low pressure and medium temperature metamorphism, with the peak metamorphic conditions of about 550–610 °C, <3.5 kbar. The results show that the rocks in contact with the thermal metamorphic zone were rapidly heated by the heat released by the Fangshan pluton, and after reaching the peak metamorphic temperature, they were cooled down simultaneously with the cooling of the rock mass, defined in a nearly isobaric P–T trajectory.

Lacustrine shale oiliness influenced by diabase intrusions in the Paleogene Funing Formation, Subei Basin, China

The research on the correlation between hydrocarbon accumulations and magmatic activities has always attracted aroused much wide attention. Existing research has primarily the hydrocarbon generations capability of source rocks and the quality of reservoirs by diabase intrusions. whereas, rare systematic research has been conducted on the oiliness and enrichment mechanism. To be specific, the diabase intrusive zone, the contact metamorphic zone and the normal shale zone of the Funing Formation in the Gaoyou Sag, Subei Basin were taken as the object of this study. Moreover, in this study, the hydrocarbon generation quality, reservoir quality, and oil-bearing quality of diabase-metamorphic zone-normal shale were evaluated using X-ray diffractions, argon ion polishing-field emission scanning electron microscope, energy spectrum, rock slice/light-sheet microscopic observations, organic geochemical tests, N2 gas adsorption and 2D NMR tests. The results indicated that the intrusive zone, the metamorphic zone, and the normal zone were formed in order by the degree of effect of diabase intrusions. Secondly, the oil content of different parts exhibited significant heterogeneity due to the baking effect of diabase, and the metamorphic zone had the significantly better oil-bearing nature than the intrusive zone. Lastly, a distribution model was proposed for the diabase intrusive zone-metamorphic zone-normal surrounding rock zone.

Multistage zircon growth recording polyphase metamorphic evolution caused by pulsed granitoid intrusions into a low‐P/T type metamorphic belt: P–T–D–t evolution of migmatites in the Ryoke belt, southwest Japan

AbstractWe report contrasting pressure–temperature–time (P–T–t) paths of migmatites developed in the highest‐grade metamorphic zone (Grt–Crd zone) and the contact metamorphic zone (Crd–Kfs zone) of the Mikawa area, Ryoke belt, southwest Japan to discuss the complex P–T–D–t evolution of the middle crust that experienced pulsed granitoid intrusions. In the Grt–Crd zone, sillimanite‐grade high‐T metamorphic condition prevailed from ca. 97 to 87 Ma, followed by cooling to ~500 °C, ~4 kbar. The intrusion of gneissose granitoids below the Grt–Crd zone isobarically reheated the Grt–Crd zone rocks again to the sillimanite‐grade high‐T condition at ca. 84 Ma. This was followed by ca. 71–70 Ma contact metamorphism. Ductile deformation that formed and folded the foliation of migmatites started before ca. 89 Ma and continued at least until ca. 84 Ma in the Grt–Crd zone. On the other hand, ca. 74 Ma age of the Crd–Kfs zone migmatite developed around the Inagawa Granodiorite in addition to ca. 70 Ma age of a syn‐tectonic pegmatite vein revealed that the intrusion of “75–69 Ma granitoids” caused partial melting and locally triggered low‐strain ductile deformation in their contact aureoles. Comparison with other areas of the Ryoke belt suggests that plutono‐metamorphic evolution of the Mikawa and Aoyama areas are similar with each other in that ca. 80 Ma reheating events (i.e., contact metamorphism) are observed, while absence of separate reheating event postdating peak metamorphism in the Yanai area is a rather uncommon feature in the Ryoke belt.

Geophysical Methods of Identification of Ore Control Structures of Orogenic Gold Deposits in Low-Contrast Terrigenous Rocks by Petrophysical Characteristics

The paper presents the analysis of characteristics of abnormal geophysical fields of the ore control of the Malo-Tarynskoe orogenic gold deposit in Verkhoyano-Kolymsky folded area, the Northeast of Russia. It is located in terrigenous rocks of the top Triassic, poorly contrast on petrophysical characteristics. Ore bodies of the Malo-Tarynskoe deposit are presented by the mineralized crushing zones with veinlet-disseminated and vein-disseminated mineralization. To identify the features of manifestation the ore control in geophysical fields land geophysical surveys are conducted by the methods of high-precision magnetic exploration and electroinvestigation. Magnetoprospecting works have established the low-amplitude mosaic structure of the magnetic field of the Malo-Tarynskoe deposit. The main magnetorevolting structure is the Samyr granitoid massif which is allocated with a magnetic minimum (-99 nT). Extreme positive values of the magnetic field are connected with hornfelsed ores in the zone of contact metamorphism (170 nT). Various anomalies as low-magnetic, mainly linear ones caused by explosive violations are distinguished (-12 – 20 nT). In the electric field, the abnormal zones have the linear nature of various extent and intensity. Pro-deleting of anomalies changes from sublongitudinal to northwest. The anomalies of low electrical resistance, generally trace zones of crushing and jointing (208-2300 ohm*m). In general, they are grouped in three “regional” abnormal areas of the northwest and North northwest orientation of low electrical resistance and correlated with the situation of the ore control zones of the Malo-Tarynskoe fault (208 – 1600 ohm*m). Sites with frequent alternativeness and alternating electric anomalies indicate lithologic heterogeneity of the environment (2500 – 10000 ohm*m). The study has shown magnetoprospecting to be effective when mapping magmatic formations and explosive violations. They allow delineating intrusive bodies and allocating hornfelsed zones. The electric profiling application by the method of a median gradient solves a wide range of geological tasks. Apart from allocation of the structures controlling gold mineralization in faint terrigenous ores, electro-investigation allows to carry out the lithologic partition of permafrost rocks in the territories blocked by friable deposits of considerable power.

A long term hydraulic stimulation study conducted at the Salak geothermal field

Awi 18-1 is an injection well drilled in the Cianten Caldera, near the western margin of the Salak (Awibengkok) reservoir in west Java, Indonesia. As the initial well injectivity was low, a long-term hydraulic stimulation was conducted to improve the permeability and establish a better connection to existing natural fractures. A geologic model of the area was built by integration of surface mapping, log and core data, and well performance information. The well penetrated pre- and post-caldera volcanics, the caldera ring fault intrusion and contact metamorphic zone, and marine sedimentary rocks. Permeability was found primarily near the caldera margin in pre-caldera lavas and ring fault intrusion along steeply dipping N to NW and NE trending fractures that were partially sealed by mineral precipitation. A geomechanical simulation model was developed from the geologic model to understand the injectivity evolution mechanisms and behaviours under different injection conditions, and also predict long term future injectivity performance. The model domain contains the completed interval of a deviated injection well (Awi 18-1) and covers the majority of microseismic events observed during the course of the stimulation. It simulates injectivity evolution using fully coupled processes of heat and mass transfers in poro-elastic media. The model was history-matched against the injection data (i.e. pressure) by calibrating rock mechanical and fracture properties. A few what-if scenarios under different operation conditions were simulated to evaluate the effects of injection pressure and temperature on the injectivity. These what-if scenario simulations indicate that both colder injection temperature and higher wellhead pressure lead to better injection efficiency. Also, the higher pressure had prolonged impacts while the colder temperature impacts are limited at the early time.

Reservoir properties and controlling factors of contact metamorphic zones of the diabase in the northern slope of the Gaoyou Sag, Subei Basin, eastern China

Hydrocarbons reserved in the contact metamorphic zones of magmatic rocks are found in rift basins as a new type of unconventional resources. However, their reservoir characteristics are relatively underexplored. On the basis of core samples, thin sections, and analysis data available, this study investigates the reservoir properties of contact metamorphic zones of the diabase from the northern slope of the Gaoyou Sag, Subei Basin in eastern China. The factors that control the reservoir quality are also investigated. In the studied area, diabase intrusion exerts significant influences on contact sedimentary rocks (including contact mudstones and sandstones) as hydrocarbon reservoirs. After contact metamorphism, the impermeable mudstones are altered into permeable metamudstones, including intermediate-metamorphosed hornfelses and low degree-metamorphosed slates. Various types of reservoir spaces, including structural fractures, hydrothermal microfissures, contraction microfissures, cleavage microfissures, intercrystalline micropores, and dissolved micropores, are created in metamudstones. Among them, numerous fractures and microfissures are the main factors of reservoir quality enhancement. Contact sandstones are altered into metasandstones, whose reservoir space types are similar to those of unaffected sandstones. The reservoir spaces mainly include primary intergranular pores, dissolved intergranular pores, dissolved intragranular pores, moldic pores, ultralarge pores, and dissolved micropores.The formation of reservoir spaces in contact metamudstones is mainly controlled by diabase intrusion, and it is not influenced by burial compaction. The controlling mechanisms of this formation include physical fracturing, hydrothermal activity, dissolution, and cooling contraction. The reservoir quality of contact metasandstones is primarily controlled by lithofacies and diagenesis before intrusion. However, later-stage diabase intrusion also exerts considerable control on reservoir quality by causing physical pressure, dissolution, and cementation. Although dissolution increases porosity moderately, it insufficiently compensates for the porosity reduction caused by physical pressure and cementation. Thus, the reservoir quality of contact metasandstones is generally decreased by diabase intrusion. A comprehensive reservoir model of contact metamorphic rocks is proposed based on the preceding analysis. This model serves to predict the distribution of high-quality reservoir in contact metamorphic rocks in the studied area. It can also be applicable to other formations of similar intrusive background.

Hematite‐coated microfossils: primary ecological fingerprint or taphonomic oddity of the Paleoproterozoic?

Microfossils belonging to the 1.88-billion-year-old 'Gunflint-biota' are preserved as carbonaceous and hematitic filaments and spheres that are believed to represent ancient chemolithoautotrophic Fe(II) oxidizing bacteria that grew above a chemocline where ferruginous seawater upwelled into shallow, oxygenated waters. This 'biological' model posits that hematite formed during burial from dewatering of the precursor ferric oxyhydroxides that encrusted Fe(II)-oxidizing bacteria. Here, we present an alternate 'taphonomic' model in which iron-rich groundwaters discharged into buried stromatolites; thus, the mineralization reactions are more informative of diagenetic processes than they are for primary marine conditions. We sampled centimeter-scale columnar stromatolites from both the lower and upper stromatolite horizons of the Biwabik and Gunflint formations, across a range of metamorphic gradients including unaltered to prehnite-pumpellyite taconite, supergene altered ore, and amphibolite-pyroxene grade contact-metamorphic zones. Fossils are rare to very rare and comprise curved filaments that exist in clusters with similar orientations. The filaments from throughout the Biwabik are similar to well-preserved carbonaceous Gunflintia from Ontario. Spheres of Huroniospora are also found in both formations. Microfossils from the least altered sections are preserved as carbon. Prehnite-pumpellyite samples are composed of either carbon or hematite (Fe2 O3 ). Within the contact aureole, filaments are densely coated by magnetite (Fe3 O4 ); the highest grade samples are secondarily oxidized to martite. The consistency in stromatolite microstructure and lithofacies throughout the metamorphic grades suggests they formed under similar environmental conditions. Post-depositional alteration led to replacement of the carbon by iron oxide. The facies association, filament distribution, and lack of branching and attached spherical cells argue against Gunflintia being a direct analogue to common marine, chemolithoautotrophic Fe(II)-oxidizing bacteria. Instead, we propose that the presence of hematite-coated microfossils is a reflection of taphonomic processes and does not necessarily reflect the byproduct of an original microbial ecosystem.

Investigation of Structural Characteristics of Thermally Metamorphosed Coal by FTIR Spectroscopy and X-ray Diffraction

The structural parameters of nine coal samples from a contact metamorphic zone were studied by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The FTIR spectra parameters composed of CH2/CH3, fa, Hal/Har, and (R/C)u indicate distinguishing features, including increase of aromaticity and the loss of aliphatic and oxygen-containing groups with thermally metamorphic evolution of coal. The XRD analysis shows that magmatic intrusion potentially caused rapid changes of structures in coal. With the decrease of distances from the intrusion rocks, the average lateral sizes (La), stacking heights (Lc), and interlayer spacing (d002) of the crystallite structures of coal range from 22.90 to 37.70 A, 12.90 to 23.30 A, and 3.80 to 3.50 A, respectively. Exponential correlations are observed between structural parameters (fa, d002, La, Lc, Hal/H, (R/C)u, and Aar/Aal) and C/H of the coal, suggesting that the structures of coal are controlled by the degree of contact metamorphism.

The Study of Metamorphic Rocks, Zonation and Isogrades in Garnet Rocks in the Hamadan Area

The study area is a part of the Sanandaj- Sirjan metamorphic belt. We can divide Hamadan metamorphic rocks in three groups: regional metamorphic rocks, contact metamorphic rocks and migmatites. In this area we can't completely divide zonation of contact and regional metamorphic. In some places that contact metamorphic has influenced to low degree regional metamorphic rocks, contact metamorphic zonations are clearly appear, but when contact and regional metamorphic have a same degree or regional metamorphic has high degree than contact metamorphic, we can't distinguish them easily. In Hamadan area regional metamorphic zones are Chlorite± Biotite zone (we haven't garnet rocks in this zone), Biotite± Garnet zone (divided in two sub zone, Biotite and Garnet zone), Andalusite zone, Staurolite zone, Staurolite± Andalusite zone, Sillimanite- Muscovite zone and Sillimanite- Potassium feldspar± Cordierite zone, also contact metamorphic zones are Cordierite zone and Cordierite- Potassium feldspar zone.

Origin of the Early-Middle Devonian magmatism in the Sakarya Zone, NW Turkey: Geochronology, geochemistry and isotope systematics

A unique example of isotopically-dated Devonian metagranitoid (the Çamlık metagranite; Okay et al., 1996) crops out in the Biga Peninsula, NW Turkey, although its contact relationships with the country rocks and geodynamic setting have remained to be enigmatic so far. Our field work, however, has shown that a number of metagranitoid bodies similar to the Çamlık metagranite intruded the country rocks and developed contact metamorphic zones, consisting of andalusite and calcsilicate hornfelses, garnet–epidote and diopsite–wollastonite skarns. The country rocks of these metagranitoids are made up of regionally metamorphosed metaclastic successions with subordinate metacarbonate–metachert–metabasites (the Kalabak formation), intercalated with tectonic slices of meta-serpentinites. The metagranitoids and the Kalabak formation are collectively termed here the Havran Unit which forms a NE–SW trending, 20 km wide and 80 km long belt in the Biga Peninsula. The Havran Unit is unconformably overlain by the Late Triassic shallow marine sediments and is in tectonic contact with the Permo-Triassic Karakaya Complex, interpreted as the Palaeotethyan subduction–accretion complex. U–Pb SHRIMP-II and LA–ICP–MS dating of the zircons from four individual plutons yielded crystallization ages ranging from 389.1 ± 2.6 to 401.5 ± 4.8 Ma (i.e. Early to Middle Devonian). Based on their geochemical characteristics, the metagranitoids are divided into two groups named the Çamlık and Yolindi metagranitoids. Both granitoids display distinct subduction signature and plot in the volcanic arc granite and post-collisional granite fields on tectonic discrimination diagrams. 87Sr/ 86Sr ( T ) values of these intrusions vary between 0.707367 and 0.715588 while their 143Nd/ 144Nd ( T ) values range from 0.51166 to 0.51187. Very low values of εNd ( T ) (from −5.3 to −9.1) imply that the Lower to Mid-Devonian metagranitoids in the Havran Unit might have been generated by partial melting of lower continental crust, probably driven by a major magma underplating event. Delamination and/or slab-breakoff models are considered to be the most plausible mechanisms for the genesis of these metagranitoids. The data obtained in this study has shown for the first time the widespread existence of Devonian and older basement in NW Turkey. Regional geological correlations imply that this basement may be an exotic terrane in the Sakarya Zone, emplaced during the Late Triassic, prior to its amalgamation with the Palaeotethyan Karakaya Complex.

Prediction of the Toxic and Harmful Gas along the Sichuan-Tibet Railway

The geological setting of Sichuan-Tibet Railway is complicated, while magmatic and metamorphic rocks are widely distributed. Reference to the other engineering experience of underground works under similar geological conditions at home and abroad, the analysis shows that it exists the hazardous possibility that the inorganic gas are toxic and harmful in the tunnels of Sichuan-Tibet Railway. Combining the conditions that inorganic gas produced and the geological conditions along the Sichuan-Tibet Railway, it analyzes and predicates that CH4, CO2, H2S, SO2, CO, NO2, NH3 and H2 are the main types of poisonous gas along the Sichuan-Tibet Railway. The poisonous gas is mainly distributed in the three suture zones, three crustal fault zones and more than three basement fault zones and mafic and ultrabasic rock zone, granites zone, volcanic zone and contact metamorphic zone. All these zones are the favorable migration point to area and gathering area to the toxic and harmful gas. It should be highly mentioned on the construction of the tunnels.

Polyphase speleogenesis in Lick Creek Cave, Little Belt Mountains, Montana, USA

Lick Creek Cave in northern Montana (USA) is hosted in limestones of the Lower Carboniferous Madison Group near Tiger Butte, an Eocene quartz–syenite porphyry intrusive dome. The cave is located within the zone of contact metamorphism of the dome, which crops out ∼ 300 m from the cave entrance. The cave consists of two genetically distinct cave systems separated by a fracture zone: (1) a ∼ 80 × 50 m dome-shaped cavern in breccias of a Carboniferous paleocave, and (2) anastomosing conduits ∼ 2–10 m across, parallel to the bedding of the Madison Group and extending 100 m up dip to the present cave entrance. The conduits are further subdivided into a tectonised and a maze zone and are variably decorated in several combinations by phreatic isopachous calcite spar cements, with crystals up to several cm long, and with vadose speleothems, including stalactite–stalagmite pairs, flowstone, corallite (cave popcorn), and moonmilk. Our database is comprised of field survey, thin section, XRD, and SEM observations along with 118 δ 18O/ δ 13C analyses and 27 87Sr/ 86Sr measurements from samples of county rock and speleothems. The limestone matrix samples with the heaviest δ 18O/ δ 13C ratios are interpreted as the least recrystallised proxy to Tournaisian seawater. Stable isotope data from other Carboniferous limestones, including paleocave breccias, follow a regional meteoric pathway established elsewhere in the Madison for the Late Carboniferous transition from greenhouse to icehouse conditions. Isopachous calcite spar cements from the conduit zone are interpreted as the result of late-stage, Eocene hydrothermal fluid circulation. Stalactite–stalagmite pairs, flowstone, corallite, and moonmilk carry a signature similar to modern or Quaternary high-alpine meteoric water. Previous workers have determined separate hydrothermal and meteoric δ 18O/ δ 13C stable isotope fields for speleothems in caves in Carboniferous limestones from the Black Hills, South Dakota. We re-define the stable isotope ranges for meteoric and magmatic–hydrothermal calcites based on a comparison of stable isotope data from the Little Belt Mountains with those from the Black Hills. We further propose that the hydrothermal calcite end-member δ 18O composition is around −20‰ PDB, represented by the lowest oxygen isotope values from all data sets, with a corresponding δ 13C of about −7‰ PDB. Sr-isotope data from speleothems, Carboniferous limestone wall rocks, and from the igneous intrusion itself support the interpretation of an Eocene hydrothermal speleogenic event. The integration of petrographic and geochemical data shows that Lick Creek Cave is the result of polyphase speleogenesis in three major episodes: (1) Middle to Late Carboniferous, (2) Eocene, and (3) (sub-)Recent to Recent. The Carboniferous and (sub-)Recent to Recent speleogenesis appear epigenic, i.e., driven by surface-derived waters, whereas the Eocene event was hypogenic, i.e., driven by ascending hydrothermal waters. Each of the three major speleogenic events probably consisted of two or more distinct “phases”, but our database does not permit these phases to be resolved with certainty.

Occurrence of Alaskan‐type mafic‐ultramafic intrusions in the North Qilian Mountains, northwest China: Evidence of Cambrian arc magmatism on the Qilian Block

AbstractField relationships, mineralogy and petrology, whole‐rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan‐type. The mineral chemistry of the chromian spinels (Cr‐spinels) and clinopyroxenes, and the variation trend of the whole‐rock compositional plot in the (Na2O + K2O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass‐resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H2O‐rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform‐type layered sequence, followed by diapiric re‐emplacement. The occurrence of the Alaskan‐type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.

Emplacement age of the Songshugou ultramafic massif in the Qinling orogenic belt, and geologic implications

The Songshugou ultramafic massif is located to the north of the Shang‐Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline‐type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA‐ICP‐MS U−Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled ∼1 m from the massif, the other ∼5 m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (<0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10 ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 506±7 and 510±7 Ma, suggesting that the Songshugou ultramafic massif was emplaced at ∼510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif. To better understand the cooling history of the massif, 40Ar/39Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20 m away from the massif were determined. The 40Ar/39Ar ages increase from 372±15 Ma (JSM‐01) near the massif to disturbed, unreliable ‘plateau’ ages of 474±8 Ma (JSM‐03) and 781±146 Ma (JSM‐04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at ∼1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang‐Dan fault.

Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

AbstractThe H2O and CO2 content of cordierite was analysed in 34 samples from successive contact metamorphic zones of the Etive thermal aureole, Scotland, using Fourier‐transform infrared spectroscopy (FTIR). The measured volatile contents were used to calculate peak metamorphic H2O and CO2 activities. Total volatile contents are compared with recently modelled cordierite volatile saturation surfaces in order to assess the extent of fluid‐present v. fluid‐absent conditions across the thermal aureole. In the middle aureole, prior to the onset of partial melting, calculated aH2O values are high, close to unity, and measured volatile contents intersect modelled H2O–CO2 saturation curves at the temperature of interest, suggesting that fluid‐present conditions prevailed. Total volatile contents and aH2O steadily decrease beyond the onset of partial melting, consistent with the notion of aH2O being buffered to lower values as melting progresses once free hydrous fluid is exhausted. All sillimanite zone samples record total volatile contents that are significantly lower than modelled H2O–CO2 saturation surfaces, implying that fluid‐absent conditions prevailed. The lowest recorded aH2O values lie entirely within part of the section where fluid‐absent melting reactions are thought to have dominated. Samples within 30 m of the igneous contact appear to be re‐saturated, possibly via a magmatically derived fluid. In fluid‐absent parts of the aureole, cordierite H2O contents yield melt–H2O contents that are compatible with independently determined melt–H2O contents. The internally consistent cordierite volatile data and melt–H2O data support the conclusion that the independent P–T estimates applied to the Etive rocks were valid and that measured cordierite volatile contents are representative of peak metamorphic values. The Etive thermal aureole provides the most compelling evidence, suggesting that the cordierite fluid monitor can be used to accurately assess the fluid conditions during metamorphism and partial melting in a thermal aureole.

The hydrothermal system of Central Jebilet (Variscan Belt, Morocco): A genetic association between bimodal plutonism and massive sulphide deposits?

Central Jebilet (Moroccan Variscan Belt) hosts several Cu and Pb–Zn massive sulphide deposits that are associated with a suite of gabbroic and microgranitic, tholeiitic to alkaline, intrusions emplaced 330 Ma ago. The intrusions and ore bodies form structural lineaments within marine Visean shales that are affected by very low to low-grade post-Visean metamorphism and contemporaneous shortening accompanied by the development of conjugate ductile to brittle shear zones. The ductile shear zones are localised in thermally softened aureoles around magmatic intrusions, while brittle deformation is common far from the intrusions. The intrusions have induced a contact metamorphism that reaches the hornblende hornfels facies, and their emplacement was accompanied by hydrothermal activity that leached base metals from the felsic intrusions. The massive sulphide deposits consist of steeply dipping elongate lenses that are located in shear zones 1–1.5 km away from the intrusions. They are dominated by pyrrhotite (up to 90%), sphalerite, galena, chalcopyrite, pyrite and arsenopyrite forming a mylonitic texture. Their wall rocks are altered to syntectonic mineral assemblages similar to those found in the alteration zones associated with the magmatic intrusions. The massive sulphide deposits located near the felsic intrusions are rich in lead and zinc compared to those located near the mafic intrusions, which are copper deposits. These relationships indicate that the whole Central Jebilet hydrothermal system could be described in terms of a lateral secretion of base metals from source zones (i.e. bimodal intrusions) to discharge zones (i.e. the Jebilet sulphide deposits). The metapelites in the contact metamorphic zone around felsic intrusions contain zincian ilmenite that was probably related to interaction of the host rocks with chlorine-rich fluid carrying zinc and other metals leached from the microgranites.

Relationships between contact metamorphism and magnetite formation and destruction in a plutonʼs aureole, White-Inyo Range, eastern California

Magnetic-petrologic studies on metasedimentary rocks from the low-pressure–high-temperature contact-metamorphic aureole of the Joshua Flat–Beer Creek pluton at the southern rim of the Inyo batholith, California, reveal the effect of contact metamorphism on magnetic minerals. The regional metamorphic biotite zone and three different contact-metamorphic zones (andalusite- cordierite, transition, and cordierite–K- feldspar) in the aureole are identified by silicate petrology and their rock magnetic behavior. Phengite barometry yields pressure data in the range of 0.2–0.3 GPa, and Na-in-cordierite thermometry indicates a peak temperature of ∼730 °C for the highest-temperature contact-metamorphic zone. Compositional changes of iron-magnesium silicates and iron oxides correspond to changes in magnetic susceptibility, temper ature dependence of magnetic susceptibility, and AF (alternating field) demagnetization of NRM (natural remanent magnetization) and can be correlated with the metamorphic history of the aureole. Magnetic susceptibility ranges from 0.2 × 10 –3 to 0.5 × 10 –3 SI in the biotite zone and is highly variable in the andalusite-cordierite and transition zones, for which the values range from 0.35 × 10 –3 to 28 × 10 –3 SI. In prograde metamorphism, the assemblage ilmenite + rutile ± hematite transforms to magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograd. In the cordierite–K-feldspar zone and in samples having undergone retrograde metamorphism, prograde magnetite reacts to ilmenite and hematite. This breakdown is associated with decreased magnetic susceptibility (0.2–0.8 × 10 –3 SI) and blurs the magnetite isograd that formed during prograde metamorphism in the outer contact aureole. Retrograde metamorphism in the contact aureole is defined by chlorite composition [ T : 280 and 420 °C, log f (O 2 ):−35 and−28]. Contact-metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite- series plutons. This fact has to be considered when interpreting aeromagnetic data of granitoid rocks.

Lithologic mapping in the Mountain Pass, California area using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data

Evaluation of an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of the Mountain Pass, California area indicates that several important lithologic groups can be mapped in areas with good exposure by using spectral-matching techniques. The three visible and six near-infrared bands, which have 15-m and 30-m resolution, respectively, were calibrated by using in situ measurements of spectral reflectance. Calcitic rocks were distinguished from dolomitic rocks by using matched-filter processing in which image spectra were used as references for selected spectral categories. Skarn deposits and associated bright coarse marble were mapped in contact metamorphic zones related to intrusion of Mesozoic and Tertiary granodioritic rocks. Fe-muscovite, which is common in these intrusive rocks, was distinguished from Al-muscovite present in granitic gneisses and Mesozoic granite. Quartzose rocks were readily discriminated, and carbonate rocks were mapped as a single broad unit through analysis of the 90-m resolution, five-band surface emissivity data, which is produced as a standard product at the EROS Data Center. Three additional classes resulting from spectral-angle mapper processing ranged from (1) a broad granitic rock class (2) to predominately granodioritic rocks and (3) a more mafic class consisting mainly of mafic gneiss, amphibolite and variable mixtures of carbonate rocks and silicate rocks.

Découverte et signification d'une paragenèse à ilménite zincifère dans les métapélites des Jebilet centrales (Maroc)

A zincian ilmenite paragenesis is found in metapelites from a contact metamorphic zone (central Jebilet, Morocco) induced by the emplacement of microgranitic intrusions. The zincian ilmenite is mainly preserved in syntectonic andalusite porphyroblasts. The growth of zincian ilmenite is related either to sphalerite breakdown during prograde metamorphism, or to the pervasive flow of a mineralizing fluid within the metapelites. The chlorine-rich fluid carried zinc and other metals leached in the microgranites, during its flow to discharge zones which were probably the Jebilet sulfide deposits.