Thermal Aureole Research Articles

Thermal properties of carbonatite and anorthosite from the Superior Province, Ontario, and implications for non-magmatic local thermal effects of these intrusions

Igneous intrusions are important to the thermomechanical evolution of continents because they inject heat into their relatively cold host rocks, and potentially change the distribution of radiogenic heat production and thermal properties within the crust. To explore one aspect of the complex evolution of the continental crust, this paper investigates the local thermal effects of two intrusive rock types (carbonatites and anorthosites) on the Archean Superior Province of the Canadian shield. We provide new data on their contrasting properties: rock density near 298 K, thermal diffusivity, and heat capacity up to 800 K (which altogether yield thermal conductivity), plus radiogenic element contents. The volumetrically small carbonatites have widely varying radiogenic heat production (2–56 µW m−3) and moderate thermal conductivity at 298 K (~ 1 to 4 W m−1 K−1) which decreases with temperature. The massive Shawmere anorthosite has nearly negligible radiogenic heat production (< 0.002 µW m−3) and low, roughly temperature-independent thermal conductivity (~ 1.6 W m−1 K−1). Steady-state thermal structures within and around these intrusions, which have quite different shapes and physical properties, were modeled using a pipe geometry for carbonatite and a tabular geometry for anorthosite. We found that the thermal aureoles of these intrusion types persist for hundreds of millions of years after the magmatic heat advected by the intrusions has dissipated. Longevitity of aureoles is due to the high radiogenic element concentrations of the small carbonatite intrusions, and to the low thermal conductivity of the Shawmere anorthosite. Our findings apply to other anorthosite bodies, which vary little in composition and mineralogy, whereas results for carbonatites depend on variations in their radiogenic content.

Syndepositional hydrothermalism selectively preserves records of one of the earliest benthic ecosystems, Moodies Group (3.22 Ga), Barberton Greenstone Belt, South Africa

Abstract The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most non-silica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also – within the resolution of radiometric age data – contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the “just right” degree of heating and very early hydrothermal silicification.

Shale gas leakage in lower Ecca shales during contact metamorphism by dolerite sill intrusions in the Karoo Basin, South Africa

Abstract Contact metamorphism along widespread dolerite sills and dykes, emplaced at 182 to 183 Ma through the sedimentary host rocks of the Karoo Basin, triggered devolatilization of carbon-rich shales of the Lower Ecca Group. Hornfel samples collected from drill cores that intersect dolerite sills were analyzed for mineral phase equilibria, chemistry and porosity to characterize thermal aureoles at various distances from sill intrusions. Andalusite-chiastolite and cordierite porphyroblasts with biotite and muscovite occur within 10 to 20 m of many intrusive contacts. These metamorphic minerals crystallized when host shales attained maximum temperatures ranging between 450 and 600°C. Scanning electron microscopy imaging confirms that the hornfels are compact and that their metamorphic minerals limit porosity along grain boundaries. In few cases intra-mineral porosity occurs within individual crystals such as calcite, andalusite and cordierite. Disequilibrium metamorphic textures such as irregular grain boundaries, and inclusions in andalusite and cordierite reveal that the elevated temperatures were too short-lived to accomplish complete (re)crystallization. Thermal modeling results are consistent with the observed metamorphic mineral assemblages. Gas leakage calculations along a 7 m and a 47 m thick dolerite sill that intrude toward the top of the Whitehill Formation suggest that methane volumes ranging between 8 to 15 Tcf were generated during the sill emplacement. Methane was likely released into the atmosphere through hydrothermal vent complexes that are well preserved in the western Karoo Basin. If such loss was widespread across the entire basin, the implications for paleo-climate change and preserved shale gas reserves in the Karoo Basin of South Africa would be significant.

Effects of igneous intrusions on source rock in the early diagenetic stage: A case study on Beipiao Formation in Jinyang Basin, Northeast China

Abstract Mesozoic intrusive bodies play an important role in the temperature history and hydrocarbon maturation of the Jinyang Basin in northeastern China. The Beipiao Formation, which is the main source rock in Jinyang Basin, was intruded by numerous igneous bodies and dykes in many areas. The effects of igneous intrusive bodies on thermal evolution and hydrocarbon generation and migration in the Beipiao Formation were investigated. A series of samples from the outcrop near the intrusive body were analyzed for vitrinite reflectance (R 0%) and other organic geochemical parameters to evaluate the lateral extension of the thermal aureole. The R 0 values of the samples increase from a background value of ∼0.9% at a distance above 200 m from the intrusive body to more than 2.0% at the vicinity of the contact zone. The width of the altered zone is equal to the thickness of the intrusive body outcropped in the field. Organic geochemical proxies also indicate the intrusive body plays a positive and beneficial role in the formation of regional oil and gas resources. Due to the influence of the anomalous heat from the intrusive body, the hydrocarbon conversion rate of the source rocks of the Beipiao Formation was significantly improved. The accumulation properties and the storage capacity of the shales also greatly improved due to the intrusive body as indicated by the free hydrocarbon migration in the shales. This new understanding not only provides a reliable scientific basis for the accurate assessment of oil and gas genesis and resources in the Jinyang Basin but also provides guidance and reference for oil and gas exploration in the similar type of basin.

Influence of igneous intrusion on coal and gas by means of element geochemistry and vitrinite reflectance method: a case from the Huainan coalfield, northern China

ABSTRACT Material composition changes of coal and gas under igneous intrusion have been one of the hot research fields in global geology. In this study, through in-depth discussion of a case from Huainan coalfield, North China, some important characteristics of thermal altered coal (TAC) are expounded by using the methods of element geochemistry and vitrinite reflectance (Rmax, Ro, and Rmin) calculation. The results show that the profile of coal reflectance/thermal metamorphic temperature (TMT) vs. distance from the intrusion indicates that the thermal aureole range reveals by the thickness of 2.4 m intrusion is approximately 3.24 m. Close to intrusion, the carbon (Cdaf) of coal increases significantly, while hydrogen (Hdaf) and oxygen (Odaf) decreases significantly, and the nitrogen (Ndaf) change is very irregular. After acid treatment, the volatile matter (VMacid) and ash yield (Ashacid) losses of coal have reached 5× and 4× of the ambient zone. The atomic ratios O/C and H/C are parallel to the changes of VMacid. Based on Rmax-Rmin diagram, the pressure effect of TAC is analyzed. In the actual geological model, the coal is mainly subjected to the bearing stress perpendicular to the contact surface instead of the vertical gravity of the magma body, and the applicable conditions of the theoretical model are proposed. Summing up the previous calculation methods of TMT, the exponential function method can be used when the Rmax is <1.0. On the contrary, the previous method can be used. The empirical formula for calculating the carbon isotope (δ 13 CCH4) of gas by using Rmax of TAC shows a negative result. The δ 13 CCH4 of the gas near the contact is relatively positive, which proves to some extent that 13 C of the thermal residual coal and 12 CH4 of the gas are subjected to carbon isotope fractionation. Here, the regression model between the Rmax of TAC and δ 13 CCH4of gas in the case can be expressed as δ 13 CCH4 = 16.777Ln(Rmax)-49.859.

Identification and characterization of hydrothermally altered minerals using surface and space-based reflectance spectroscopy, in parts of south-eastern Rajasthan, India

Imaging spectroscopy has evolved as one of the most significant advancements due to contiguous spectral coverage and higher spectral resolution which enable mineral identification and mineral exploration. Many phyllosilicate and carbonate minerals show specific spectral absorption feature in the wavelength range of visible-to-near-infra-red region of electromagnetic spectrum. These spectral features enable delineation of different mineral assemblages which in turn help in mineral prospecting using hyperspectral imaging spectra. The present study is focussed on evaluation and application of EO-1 Hyperion (hyperspectral) data as an Earth Observation tool for mineral detection and mapping in parts of Udaipur district in south-eastern Rajasthan. Hyperion reflectance imagery of this area was analysed using spectral angle mapper after pre-processing, atmospheric correction and geometric correction. Five endmembers, viz. dolomite, montmorillonite, chlorite, phlogopite and serpentine, were derived from both atmospherically corrected image and from rock samples in the laboratory using ASD field spectroradiometer covering spectral range of 0.4–2.5 µm. The reflectance spectra of endmembers derived from satellite image were initially compared with USGS mineral spectral library, and then after comparing with laboratory-based spectra with respect to absorption features, target minerals were identified which shows more than 70% match with the USGS and laboratory spectra. These minerals were also cross-checked with the reported litho-sequence of the area. Minerals derived from laboratory and image spectra are indicative of hydrothermally altered outer thermal aureole which is also corroborated by litho-structural association of the area.

Hot fluids, burial metamorphism and thermal histories in the underthrust sediments at IODP 370 site C0023, Nankai Accretionary Complex

Abstract Drilling during International Ocean Discovery Program (IODP) Expedition 370 at Site C0023 encountered instances of hydrothermal mineralization from 775 to 1121 m below seafloor. Fluid inclusion homogenization temperatures measured on barite veins within this interval indicate precipitation from fluids with temperatures up to 220 °C, and salinities ten times higher than interstitial water (16–25 c.f. 2.8–3.6 wt % NaCl). Patches of stratabound mineralization (rhodochrosite, calcite, barite and anhydride) are largely confined to the vicinity of veins and have vertical thicknesses and extents that can be explained by precipitation within the thermal aureoles of veins. Thermal maturities measured on petroleum biomarkers in underthrust sediments record a rise from pre-oil window to early oil window thermal maturities. Basin models show that increases in thermal maturity can be explained by burial metamorphism, and kinetic-based calculations suggest that hot fluids would only have had a minimal effect on hydrocarbon generation. However, the movement of hot fluids still has geochemical implications and creates a complex thermal history where both short-duration, localized heating within fracture zones at hot temperatures (~200 °C for less than 1 yr) and heating during burial over long durations (80–110 °C for 0.5–2 Myrs) need to be considered.

Influence of igneous intrusions on the thermal maturity of organic matter in the Sverdrup Basin, Arctic Canada

The influence of igneous intrusions on the trend of thermal maturity with depth in two drillholes, one on Ellesmere Island and the other on Ellef Ringnes Island, Arctic Canada, was assessed using vitrinite and solid bitumen reflectance, Rock-Eval pyrolysis/TOC, and gas composition analysis in order to assess the impact of igneous intrusions on the hydrocarbon system in the area. Thick sequences of Permo-Pennsylvanian strata in Sabine Peninsula, Melville Island, were intruded by two igneous intrusions in the Panarctic Chads Creek B-64 (influenced by one intrusion) and the Panarctic Drake Point D-68 (influenced by two intrusions). Sedimentary strata in Chads Creek B-64 were intruded by a 120 m-thick diabase sill (with temperature of ~1100 °C) of unknown age. This resulted in increased reflectance of vitrinite from 0.90% to 1.65% and of pyrobitumen from 2.90% to almost 5.80% near the sill contact at depth of 3600 m. Solid bitumen outside of the thermal aureole zones and having reflectance values <1.50% followed the extrapolated trend for solid bitumen present in the country rocks. In Panarctic Drake Point D-68, the intrusives are granodiorite sills of 45 m and 75 m thickness and were emplaced during the Upper Jurassic (152 Ma) and Lower Cretaceous (131 Ma). The intrusion temperature was 800 °C. The Vitrinite Rr prior to sill emplacement was <1.0% outside the aureole and increased to approximately 2.0% near the intrusive zone. The rocks were well-lithified prior to sill emplacement. Solid bitumen underwent carbonization after being exposed to a temperature of at least 800 °C. Solid bitumen reflectance also showed a sudden increase from a minimum of 1.50% outside the aureole to approximately 7.0% near the sill contact, a 5× increase as compared to an increase of 2× for vitrinite. This indicates that solid bitumen is a very sensitive indicator of temperature variation, more sensitive than vitrinite. Solid bitumen reflectance prior to the sill emplacement can only be inferred from the solid bitumen below the lower, younger sill because of poor control on solid bitumen in the interval just above the upper, older sill. Reflectance trends above and below the sills appear to be symmetrical despite the superimposition of the effects of one sill on the other. The metamorphic effects of the sill, as determined vertically by the reflectance trend, can be observed for a distance of up to seven times its thickness. The upper, older sill is approximately 45 m thick and the thermal aureole zone above it has a width of 330 m. The thickness of the aureole below the sill is difficult to establish because it has been obliterated by the effect of the lower, younger sill. The thickness of the lower sill is 75 m. An additional factor that influenced solid bitumen reflectance in this case may have been the thickness of the sill, which, in turn, affects the cooling time of the intrusive body. The granodiorite sills are approximately 45–75 m thick and the diabase sill is almost 120 m.

Thickness matters: Influence of dolerite sills on the thermal maturity of surrounding rocks in a coal bed methane play in Botswana

Abstract Global studies suggest that igneous intrusions usually have thermal aureole thicknesses between 30% and 250% of the intrusion thickness. Here, we discuss data and 1D thermal maturity models from nine coreholes (wells) drilled in Botswana to explore for coal bed gas. The coreholes penetrated multiple dolerite sills with thicknesses ranging from 1.2 m to 73.4 m. The sills intruded the sedimentary rocks in the Jurassic period and significantly affected the rank, properties and thermal maturity of Permian coals. The thermal influence of the sills extends for >5–53 m away from their contacts with surrounding rocks. The relatively thick (>13 m) sills have thermal aureole thicknesses of >31–137% of the sill thickness, in line with global observations. However, the relatively thin ( 1000% of the sill thickness, significantly larger than previously reported from most other locations affected by igneous intrusions. Thin sill intrusions in coals and organic-rich shales may lead to the generation of larger volumes of CH4, other hydrocarbon gases, and CO2 than previously thought, affecting the safety in coal mines and gas resources.

Genetic Relationship Between Postcratering Dynamics and Footwall Deposit Formation at Sudbury, Ontario, Canada: Importance for Sulfide Ore Exploration

Research Article| June 01, 2019 Genetic Relationship Between Postcratering Dynamics and Footwall Deposit Formation at Sudbury, Ontario, Canada: Importance for Sulfide Ore Exploration Torben Wüstemann; Torben Wüstemann 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany Search for other works by this author on: GSW Google Scholar Paul Göllner; Paul Göllner 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany Search for other works by this author on: GSW Google Scholar Peter C. Lightfoot; Peter C. Lightfoot 2Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7, Canada Search for other works by this author on: GSW Google Scholar Ulrich Riller Ulrich Riller 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany †Corresponding author: e-mail, ulrich.riller@uni-hamburg.de Search for other works by this author on: GSW Google Scholar Author and Article Information Torben Wüstemann 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany Paul Göllner 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany Peter C. Lightfoot 2Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7, Canada Ulrich Riller 1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany †Corresponding author: e-mail, ulrich.riller@uni-hamburg.de Publisher: Society of Economic Geologists Accepted: 15 Feb 2019 First Online: 24 Jun 2019 Online Issn: 1554-0774 Print Issn: 0361-0128 © 2019 Economic GeologyEconomic Geology Economic Geology (2019) 114 (4): 599–612. https://doi.org/10.5382/econgeo.4655 Article history Accepted: 15 Feb 2019 First Online: 24 Jun 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Torben Wüstemann, Paul Göllner, Peter C. Lightfoot, Ulrich Riller; Genetic Relationship Between Postcratering Dynamics and Footwall Deposit Formation at Sudbury, Ontario, Canada: Importance for Sulfide Ore Exploration. Economic Geology 2019;; 114 (4): 599–612. doi: https://doi.org/10.5382/econgeo.4655 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract High-grade Ni-Cu-Co and precious metal mineralization forms footwall veins in close proximity (<~750 m) to the base of the Sudbury Igneous Complex. Successful exploration for these economically significant sulfide ore deposits benefits from an understanding of the geologic processes associated with their formation, notably within the framework of meteorite impact cratering. Based on examination of drill core and drilling logs of Vale’s Capre 3000 ore zone, we delineate petrographic and microstructural characteristics of sulfide veins and the adjacent host rocks. We show that transport, emplacement, and modification of sulfides are controlled by the dynamic conditions in the thermal aureole of the Sudbury Igneous Complex during early stages of crustal relaxation following impact cratering. This process accounts for the following observations: (1) occurrence of sulfide veins at preexisting structural discontinuities, (2) spatial distribution and thickness of veins, and (3) distinct spatial distribution of partial melting and static recrystallization textures in footwall rocks in the vicinity of sulfide mineralization. We suggest that the identification of high-temperature mineral textures in the footwall rocks lends itself as an exploration tool for footwall deposit identification. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The evolution of quartz veins during the tectonometamorphic development of the Brusque Metamorphic Complex, Brazil

Microstructural analysis and characterization of quartz veins hosted in rocks from the Brusque Metamorphic Complex in the region of Brusque, State of Santa Catarina, have been performed to establish the relations between fluid regimes, tectonic styles and deformation/recrystallization mechanisms. The analysis was based on observations of structural overprinting, spatial structural relations, the origin of the quartz veins and microtectonics studies. Five types of veins were identified: meter-long veins parallel to the regional foliation (V1-veins); massive, meterwide veins present in thermal aureoles (V2-veins); millimeter-wide, erratic veins, also present in thermal aureoles (V3-veins); tabular, undeformed and NW striking (V4-veins); millimeter-wide erratic veins restricted to brittle reactivation of strike-slip shear zones (V5-veins). The regional foliation, developed under garnet zone metamorphic conditions, was more effective in vein production when compared to the steeply dipping mylonitic foliation corridors developed under chlorite zone conditions. Pressure solution was the main deformation mechanism during the regional foliation development. However, granoblastic and decussate textures in hornfels reveal an influence of grain boundary area reduction mechanism. The granoblastic texture in hornfels would have inhibited fluid circulation during dehydration reactions, increasing fluid pressure and promoting massive (V2-veins) and erratic hydraulic fracture-related (V3) veins. In dextral NEtrending strike-slip shear zones, tabular quartz veins (V4-veins) are parallel to tension gashes. Reactivation of strike-slip shear zones under brittle conditions has produced local brecciated millimeter-wide quartz veins (V5-veins). This study underscores the important role of fluids during orogenic evolution near the brittle-plastic transition of the crust, and demonstrates how combined vein and microtextural analysis can reveal the tectonometamorphic history of lowgrade metamorphic rocks.

The influence of the thermal aureole asymmetry on hydrocarbon generative potential of coal beds: Insights from Raniganj Basin, West Bengal, India

The aim of the present study is to uncover hydrocarbon generation potential of an area intruded by lamprophyre dyke. Lamprophyre dyke induced a short-distance asymmetric thermal aureole in coal beds with more intense impact along the dip direction. Large ash yield, carbon content, lower volatile matter content, hydrogen indices, presence of natural coke, disordered kaolinite peaks, less pronounced aliphatic peaks, higher aromaticity under Fourier Transform Infra-Red spectroscopy (FTIR); highly rugged surface and large pores with the aid of Atomic force microscopy (AFM); abundant calcium carbonate mineral phases (calcite, dolomite, ankerite) and high-temperature silica phases (tridymite, sanidine) under X-ray diffraction (XRD) are the consequences of pronounced thermal effect and expulsion of hydrocarbons. Furthermore, the estimated vitrinite reflectance (EVRo) calculated through maximum pyrolysis temperature (Tmax) and measured mean random vitrinite reflectance (VRo) show the maximum difference in the heat-affected coal samples. In addition, parameters for source rock reconstruction and indices for quantification of FTIR spectra i.e. index of aliphaticity (IAL), index of aromaticity (IAR) and index for hydrocarbon generation (IHG) are introduced for coals. The distant (i.e. least-heat-affected) coals collected opposite to the dip direction of the coal bed have a higher potential for hydrocarbon generation. The igneous intrusion thus played a significant role in enhancing the maturity of the organic material, in turn, augmenting the hydrocarbon generation potential for targeting hydrocarbon plays of the area although at the expense of proximal samples.

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

AbstractThe 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.

Pyrometamorphic aureoles of Cretaceous sandstones and shales by Cenozoic basic intrusions, NE Brazil: Petrograhic, textural, chemical and experimental approaches

In this paper, we present the case study of thermal aureoles developed in Cretaceous sedimentary rocks from the Potiguar Basin (NE Brazil) nearby shallow Cenozoic mafic intrusions. The study integrates the field and petrologic study of the metamorphic aureole with experimental work. The pyrometamorphic aureole is discontinuous and less than one hundred meters wide. Along the contact with the intrusions, sandstones, siltstones and shales were transformed into light and dark buchites. The light buchite has acicular clinoenstatite and tridymite, as well as microphenocrysts of sanidine, sekaninaite, and anorthite, within a gray to brown, SiO2- and K2O-rich and Al2O3- and FeO-poor rhyolitic glass. The dark buchite is richer in FeO and Al2O3 and has pyrometamorphic Al-magnetite, hercynite, and Fe-mullite. Buchites derived from shales have a cryptocrystalline to vitreous groundmass and macroscopically resemble tachylites. A series of low-pressure and high-temperature experiments were conducted at 3 kbar and 1000–1200 °C on a piston-cylinder apparatus using sandstone and shale starting materials sandwiched with olivine basalt. The experimental results indicated that melting occurred at approximately 1150 °C, followed by relatively slow cooling until approximately 1000 °C and then by quenching. This multidisciplinary approach suggested that the sedimentary units were submitted to very high temperatures upon contact with the intrusions, and this outcome is corroborated by the mineralogy and textures observed in the natural rocks.

Auréola termal provocada pela intrusão do pluton Totoró em micaxistos do Grupo Seridó, Ediacarano da Província Borborema, NE do Brasil

A Província Borborema, NE do Brasil, é marcada por volumoso plutonismo ediacarano. Neste contexto geológico, o plutão Totoró, um corpo alongado NNE-SSW, compõe-se de uma sequência de tipos básicos a intermediários, incluindo dioritos e gabro-noríticos (mais precoces), granodioritos / tonalitos (predominante), biotita granitos equigranulares, biotita granitos porfiríticos e hololeucogranitos finos a médios. Dados químicos permitem estimar pressões de 1,6-2,7 kbar e temperaturas de 800-900oC para sua colocação. O calor aportado provocou uma auréola termal que se estende até 2 km do contato, constituída por: (i) xisto migmatizado imediatamente adjacente ao contato, com frequentes soleiras de leucossoma subconcordantes com a trama tectônica S2 da encaixante; (ii) uma zona intermediária com sillimanita (± cordierita + granada + biotita); (iii) uma zona com aparecimento de cordierita (± estaurolita ± andaluzita + granada + biotita); e (iv) uma zona externa com micaxisto de fina granulação. Modelamento baseado em petrofísica de rocha mostra que a temperatura na borda do plutão atingiu 688-756°C, diminuindo para 500oC a 1800 m do contato, com tempo de resfriamento estimado de 365.000 anos. Os minerais produzidos no efeito termal delineiam a xistosidade S2, de baixo ângulo (evento D2/M2 na fácies anfibolito ou piroxênio hornfels), os quais são afetados por dobras abertas a fechadas, normais, progredindo para forte transposição e milonitos em zona de cisalhamento NE-SW a N-S, representando o último evento dúctil, retrometamórfico (fácies xisto verde), na região (D3/M3). Os resultados obtidos revelam que pelo menos parte dos batólitos na região em tela se posicionou em níveis crustais relativamente rasos sob condições de alto gradiente geotérmico, porém ainda registrando a fase de arrefecimento da tectônica dúctil regional.

Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith‐Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

AbstractModes of magma emplacement may provide key information on geodynamic‐tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper‐crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper‐level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10–15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite‐country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith‐like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25‐km wide and ~5‐km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under‐accretion of multiple magma sheets deflected from magma pulses that ascended through pipe‐like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source‐derived melt compositions.

Age, petrologic significance and provenance analysis of the Hamedan low-pressure migmatites; Sanandaj-Sirjan Zone, west Iran

ABSTRACTMeta-pelitic rocks with interlayers of meta-psammites within the inner thermal aureole of the Alvand plutonic complex (Sanandaj-Sirjan Zone (SaSZ), western Iran) underwent partial melting; generating various types of migmatites. The mesosome of the Hamedan migmatites is classified into two groups: (1) cordierite-rich and Al-silicate-poor mesosomes and (2) cordierite-poor, Al-silicate-rich groups. Leucosomes are also variable, ranging from plagioclase-rich to K-feldspar-rich leucosomes. Mineral-chemical studies and thermobarometric estimations indicate temperature and pressure of 640–700°C and 3–5 kbar, respectively, for the formation of mesosomes. U–Pb zircon geochronology on 214 grains from the mesosome of migmatites indicates ages of 160–180 Ma (ca ~170 Ma) for zircon metamorphic rims and variable ages of 190–2590 Ma for the inherited detrital zircon cores. Inherited core ages show various age populations, but age populations at 200–600 Ma are more frequent. The age populations of the detrital zircons clarify that the provenance of the younger zircon grains (200–500 Ma) was more likely the Iranian plate, whereas the older grains (600 Ma to >2.5 Ga) may be sourced from both northern Gondwana (such as Arabian-Nubian Shield) and the neighbouring, old cratons like as Africa. We suggest that magmatic activities, especially mafic plutonism at ~167 Ma, are the main triggers for the heat source of metamorphism, partial melting, and migmatization. In contrast to a presumed idea for a Cretaceous regional metamorphic event in the NW parts of the SaSZ, this study attests that the metamorphism should be older and can be associated with Jurassic magmatic pulses.

Making oil from magma

Abstract Petroleum systems within rifted margin basins affected by volcanism continue to remain challenging for the exploration of hydrocarbons, most notably owing to the volume of intrusions that pose imaging, drilling and exploration problems. Typically, intrusions possess small thermal aureoles, but despite this, there is evidence that intrusions could none the less be responsible for the generation of commercial volumes of hydrocarbons. Here we shed new light on this petroleum systems challenge by integrating organic geochemical and Raman spectroscopic techniques to produce potential volumetric data for hydrocarbons generated as a result of igneous intrusion. The results indicate that, in areas with immature source rock intervals, it may be possible for intrusions to generate volumes of oil that would be capable of comfortably filling likely known oil reservoirs. This is a critical step forward in integrating several analytical techniques, indicating that under the right conditions there is the potential for hydrocarbon generation as a result of igneous intrusion.

Thermal controls of lamprophyre sill on hydrocarbon generation outlook of shale beds in Raniganj basin, India

The present work highlights the role of lamprophyre sill on the potential of hydrocarbon generation of shale beds. Here, the authors had systematically collected seven shales for detailed investigations of thermal impact provided by lamprophyre sill both along the dip and opposite to the dip directions of the shale beds. Proximate, ultimate, Rock Eval pyrolysis, organo-petrographic, vitrinite reflectance, Fourier Transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses were performed. Authors have also introduced new parameters like index of aliphaticity (IAL), index of aromaticity (IAR) and index for hydrocarbon generation (IHG) for precise quantification of FTIR spectra of shales, which in turn helpful in indicating the influence of thermal aureole on shale beds. The results indicate more distinct thermal alterations in the heat-affected shales collected along the dip direction of the shale beds owing to more pronounced and extensive thermal impact of lamprophyre sill in the dip direction. The estimated vitrinite reflectance (EVRo) and mean random vitrinite reflectance (VRo) show maximum difference in the heat-affected shales. Low genetic potential (GP), IHG indicate poor potential for heataffected shales along the dip due to intense thermal influence. In contrast, the fairly-affected shales collected opposite to the dip of the bed have higher GP, IHG and hence considered to be excellent source material. The mesopores present in the fairly-affected shales held good amount of hydrocarbons and thus suitable for hydrocarbon exploration and exploitation. The igneous intrusion improves the overall the general maturity of the area although at the expense of the reduction of the hydrocarbon generation potential of the proximal samples.

A combined geochronological approach to investigating long lived granite magmatism, the Shap granite, UK

With the advent of more precise dating methods, it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods. Here we attempt to reconcile a number of dating methods from the c. 5 km2 Caledonian Shap granite, Northern England. The results reveal a more complex and protracted evolution than indicated by application of any single dating method. Zircon U-Pb dates give a weighted mean age of 415.6 ± 1.4 (2σ) Ma. A mafic enclave, dated at 412 ± 2 (2σ) Ma (revised Rb-Sr feldspar age from Davidson et al., 2005), contains resorbed K-feldspar and zircon crystals scavenged from the host crystal mush. These ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 Ma or later during Acadian deformation. Previously reported Re-Os ages on molybdenites associated with magmatic fluids, have given ages of 405.2 ± 1.8 (2σ) Ma (Selby et al., 2008) and confirm the overlap of at least some magmatic activity with Acadian deformation. A similar emplacement age is supported by Rb-Sr whole-rock-mineral and biotite K-Ar dates when adjusted for revised decay constants (402 ± 3 Ma and 401 ± 7 Ma, respectively, Wadge et al., 1978). The lower closure temperatures of these systems relative to the U-Pb system in zircon means that they are more likely to record the timing of final granite emplacement. These data suggest that most zircons grew before final granite emplacement, by about 10 Ma on average. We suggest that the majority of zircon crystals record pre-emplacement magmatic activity within a deeper part of the system. Mafic enclaves and their scavenged cargo of crystals record the assembly of a mid-crustal batholith where crystals remained at least locally mobile at 412 Ma. Gravity data support the existence of an extensive, 1500 km2 intrusive body, originally at about 15 km depth beneath Shap. This batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss, but episodic influxes of silicic magma between c. 412 and 405 Ma are thought to have enabled periods of rejuvenation. These influxes are recorded by complex compositional zoning patterns within K-feldspar megacrysts. The Shap granite itself is likely to represent a rejuvenated crystal slurry, emplaced as a cylindrical cupola above the main magma body during Acadian transpression. This study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages.