Thermal Aureole Research Articles

Anatectic melt volumes in the thermal aureole of the Etive Complex, Scotland: the roles of fluid‐present and fluid‐absent melting

AbstractPelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ∼770 °C and yield <11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H2O, modelled melt modes increase dramatically within ∼13 °C of the solidus, in some cases to >60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H2O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the diorite magma chamber associated with a second phase of intrusion, or by sub‐solidus thermal contraction in the diorite pluton during the early stages of inner‐aureole cooling.

Geological setting, petrology and mineralogy of metabasites in a thermal aureole of Tanvald granite (northern Bohemia) used for the manufacture of Neolithic tools

*Corresponding author This study examined the metabasites in a thermal aureole of the Tanvald granite in the northern part of the Bohemian Massif that were used for manufacturing Neolithic tools. The metabasites form relatively thin bodies from several cm to 2 m thick and are intercalated conformably within the host metasedimentary rocks. Two types of thermally metamorphosed actinolite-hornblende hornfelses (metabasites) were distinguished: 1) fine-grained amphibole-rich metabasite and 2) metabasites with relicts of porphyritic texture. Because of their large lateral extent and geological setting within metasediments, fine-grained metabasites may correspond to former volcanoclastic rocks (meta-tuffs), while the porphyritic metabasites probably represent metamorphosed dolerite sills or dykes. The host rocks are various types of cordieritebiotite spotted schists or andalusite-cordierite-biotite hornfelses. Both groups of metabasites consist of several types of amphiboles, calcic plagioclase (andesine to labradorite) and ilmenite, variably replaced by their alteration products. Apatite, alteration products of ilmenite, magnetite and sulphides are common accessory phases, while epidote and titanite are present only rarely. Relicts of magmatic minerals and minerals from older metamorphic stages are scarce. The peak contact metamorphic conditions were constrained between 540 and 600 °C. Sample of charcoal taken from the base of the Neolithic quarry, where a large amount of waste from making of semi-finished polished tools was found, was dated by radiocarbon dating technique. It gave an age of VERA-2981 6120 ± 35 BP, which places the sampling location in one of the closing stages of quarrying in this part of the site. The correspondence of the petrological and mineralogical characteristics of metabasites from the thermal aureole and stone tools found in Germany and southern Moravia provided evidence that Neolithic tools were traded over distances of several hundred kilometres.

The Late Miocene Guacimal Pluton in the Cordillera de Tilarán, Costa Rica: its nature, age and petrogenesis

*Corresponding author The Guacimal Pluton is situated in the Cordillera de Tilaran in the northwestern Costa Rica. It forms an oval-shaped body strongly elongated in the NW–SE direction. Its dimensions are ~15 × 4–6 km with an exposed surface of 60–70 km 2 . The pluton intruded basic volcanic rocks of the Aguacate Group (Miocene–Pliocene) and is surrounded by a wide thermal aureole of calc-silicate metasomatic rocks. The pluton is mainly formed of monzogranites to granodiorites, which strongly prevail over more basic types occuring scarce and relatively thin dykes and enclaves. The dominant magmatic minerals of this felsic suite are quartz, plagioclase, and K-feldspar with subordinate Mg-rich biotite, amphibole I, and magnetite. Orthopyroxene, Mn-rich ilmenite, Al-poor titanite, rutile, apatite, zircon, thorite, and chalcopyrite are accessories. Secondary minerals, which occur as fillings of miarolitic cavities and interstices, are quartz II, K-feldspar II, epidote, chlorite, actinolite, ilmenite II and Al-rich titanite II. The much less frequent mafic suite (mainly quartz diorite to quartz monzodiorite/monzogabbro) is composed of plagioclase, pargasite, actinolite, K-feldspar, quartz and magnetite, with accessory amounts of opaque minerals, epidote, chlorite, and titanite. The pluton was emplaced at a depth of c. 3 km, crystallized at temperature of c. 760–800 °C under a relatively high oxygen fugacity (1.6–2.1 log units above the NNO buffer). Increased activities of volatiles (H 2 O, F) upon cooling are indicated by the presence of highly aluminous, F-rich titanite and other hydrous silicates in miarolitic cavities. The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline, whereas the mafic suite is nearly exclusively medi um-K calc-alkaline in nature. Laser ablation ICP-MS dating of zircons from two granite samples yielded statistically identical U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma, respectively. The Sr–Nd isotopic compositions are rather primitive ( 87 Sr/ 86 Sr 6 = 0.70380–0.70413, e 6 Nd ~ +7.3 to +7.9). Narrow range of these values rules out open-system processes such as magma mixing or assimilation of isotopically contrasting upper continental crust. Instead, the felsic suite is interpreted as either having crystallized from a highly fractionated melt extracted from a plagioclase–amphibole-dominated crystal mush in a putative deep crustal reservoir or a product of partial melting of older arc-related rocks, such as intermediate lavas or volcaniclastics or immature psammitic sediments rich in volcanic material. The observed variation in the felsic suite was most likely produced by low degrees of closed-system fractional crystallization of an assemblage dominated by feldspars. At least some of the rocks of the volumetrically subordinate mafic suite may represent litholo gies rich in the complementary cumulates.

Timing the structural events in the Palaeoproterozoic Bolé–Nangodi belt terrane and adjacent Maluwe basin, West African craton, in central-west Ghana

The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bolé–Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé–Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to ∼2195Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at ∼2150Ma concluded the development of the Eoeburnean orogenic cycle (DEE).The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé–Nangodi terrane during extension between ∼2137 and 2125Ma. The rifting separated the Abulembire fragment from the Bolé–Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (DE1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the DE1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW–SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the DE2 thrust related folding and stacking in the deformed and granitoid invaded Maluwe basin as well as the single stage sin- and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west- and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the DE2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N–S and E–W) up to 60° with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N–S extension fracturing (DE3) along which mantle derived Wakawaka gabbroids and syenite intruded. The DE1 folding occurred between 2125 and 2122Ma and DE2 before 2119Ma. The tectonic relaxation occurred at 2118Ma. Around 2100Ma, NE–SW directed strike-slip shearing (DE4), fractured the Bolé–Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (DE5) movement which formed some breccias.

SHAFT‐HOLE AXES FROM CAPUT ADRIAE MADE FROM AMPHIBOLE‐RICH METABASITES: EVIDENCE OF CONNECTIONS BETWEEN NORTHEASTERN ITALY AND CENTRAL EUROPE DURING THE FIFTH MILLENNIUM BC

Here we show the results of a study concerning a small group of shaft‐hole axes found in northeastern Italy, made from amphibole‐rich metabasites, fine‐grained and free of phenoblasts. The main mineral phases are amphibole, ranging from actinolite to hornblende, and plagioclase (An10–15 and An70–77). The amphiboles generally show a needle shape and are often radially arranged. Quartz is present in thin veinlets, while ilmenite is widespread in small patches. The petrographic and geochemical features suggest that the axes originate from the southern thermal aureole of Tanvald granite in northern Bohemia. In accordance with this provenance, the typology of the tools shows similarities with the perforated shoe‐last axes spread across Central Europe during the fifth millennium bc and made from similar raw material. For the first time, these axes give evidence of long‐distance (about 800 km) contacts between northeastern Italy and Central Europe during the Neolithic.

Metamorphism and fluid flow in the contact aureole of the Eureka Valley-Joshua Flat-Beer Creek pluton, California

We examined the petrology and stable isotope compositions of metamorphic rocks in the thermal aureole of the Eureka Valley‐ Joshua Flat‐Beer Creek (EJB) composite pluton, White Mountains, eastern California, to determine the physical conditions of contact metamorphism and associated fl uid fl ow. The Jurassic EJB pluton is one of several bodies intruded during protracted Mesozoic magmatism in the White-Inyo Mountains. Emplacement of the pluton rotated and stretched the intruded greenschist-facies Cambrian sedimentary rocks to vertical orientations that are subparallel to the pluton contact. Traverses across the contact aureole included marbles, calc-silicates, schists, and quartzites. Each lithology behaved differently during contact metamorphism. Most dolomite and calcite marbles did not equilibrate with an external fl uid as revealed by original, unshifted δ 18 O and δ 13 C values of 20‰‐25‰ (relative to standard mean ocean water [SMOW]) and ‐1‰ to +2‰ (relative to PeeDee belemnite [PDB]), respectively. Largest isotopic shifts occurred in calc-silicates where δ 18 O decreased to ~15‰. The moderate oxygen-isotope ratio shift suggests that the calc-silicates equilibrated with an aqueous fl uid, magmatic and/or metamorphic, whose oxygen-isotope ratio was buffered by the surrounding silicate metamorphic rocks. Infi ltration of the calc-silicates by an aqueous fl uid in the high-grade part of the aureole is also demonstrated by the presence of grossular garnet and vesuvianite. Schists in the aureole vary in mineralogy and compositions. Harkless and Saline Valley formations have higher Al/K ratios that resulted in more abundant muscovite and andalusite, than the Campito Formation, which contains ubiquitous K-feldspar. On the western side of the pluton, partial melting of the Harkless Formation was driven by fl ow of aqueous fl uids that was focused between the pluton and marbles more distant from the contact. Metamorphic pressure-temperature (P-T) pseudosections for the schists suggest that partial melting has occurred between 2.5 and 3 kbar and >660 °C. The pseudosections also predict a narrow subsolidus cordierite fi eld. However, cordierite , feldspars, and other silicate minerals were altered by pervasive fl uid fl ow through the schists below 500 °C. This fl uid fl ow likely was driven by slow cooling of the large EJB pluton and/or continued magmatism in the White-Inyo Mountains, particularly during later emplacement of the Birch Creek pluton to the west of the EJB pluton.

Petrographic and geochemical effects of sill intrusions on coal and their implications for gas outbursts in the Wolonghu Mine, Huaibei Coalfield, China

Since the early 1980s, fifteen outbursts have occurred in the Huaibei Coalfield of China. These outbursts were reported to be associated with sills. To study the effect of sill intrusions on coal seam and their relationship to methane outbursts, eleven samples from the No. 10 coal seam were taken from the Wolonghu Mine at various distances from a diorite sill. Comparisons were made between unaltered and heat-affected coals using petrographic and chemical data, micropore characteristics, adsorption properties of coal, and gas outburst indexes from field. Approaching the intrusion, vitrinite reflectance levels increased from 2.74% to 5.03%, and the thermal aureole of the sill ~ 60 m (from the sill boundary to sample 9). Three zones along this gradient were identified as corresponding to (1) thermal evolution zone No. 1 (0–5 m from sill), (2) thermal evolution zone No. 2 (5–60 m from sill), and (3) unaltered zone. The methane adsorption capacity of coal samples in the thermal evolution zone No. 2 was generally higher than in the two other zones, and the unaltered zone higher than the thermal evolution zone No. 1. It is concluded that the contact-metamorphism decreased the adsorption capacity of coal and the thermal evolution of sill increased it. The trap effect of sill, combined with the mudstone and siltstone roof and floor of the No. 10 coal seam, provided a seal for the formation of a gas pocket. Abnormally high formation pressures at the No. 10 coal seam led to two outbursts.

Vein development during folding in the upper brittle crust: The case of tourmaline-rich veins of eastern Elba Island, northern Tyrrhenian Sea, Italy

Detailed structural analysis of tourmaline-rich veins hosted in the contact aureole of the ∼6 Ma Porto Azzurro granite in southeastern Elba Island, northern Tyrrhenian Sea is presented. Using geometric features of the veins, the physical conditions at the time of vein formation are estimated, namely the stress ratio (Φ = ( σ 2 − σ 3)/( σ 1 − σ 3)), driving stress ratio ( R′ = ( P f − σ 3)/( σ 1 − σ 3)) and fluid overpressure (Δ P o = P f − σ 3). Two vein sets (A veins and B veins) have been recognized based on orientation and thickness distributions and infilling material. Analysis of vein pole distributions indicates Φ = 0.57 and R′ = 0.24 for the A veins and Φ = 0.58 and R′ = 0.47 for the B veins, and fluid pressures less than the intermediate stress magnitude. Analysis of geometric features of the veins gives estimated fluid overpressures of between ∼16 MPa (A veins) and ∼32 MPa (B veins). We propose a model for the tectonic environment of vein development, in which formation of secondary permeability in the deforming thermal aureole of the Porto Azzurro pluton was controlled by ongoing development of fracture systems in the hinge zone of a regional NNW–SSE trending fold that favored transport and localization of hydrothermal fluids.

Derivation of S and Pb in Phanerozoic Intrusion-Related Metal Deposits from Neoproterozoic Sedimentary Pyrite, Great Basin, United States

The thick (≤8 km), regionally extensive section of Neoproterozoic siliciclastic strata (terrigenous detrital succession, TDS) in the central and eastern Great Basin contains sedimentary pyrite characterized by mostly high δ 34 S values (−11.6 to 40.8‰, >70% exceed 10‰; 51 analyses) derived from reduction of seawater sulfate, and by markedly radiogenic Pb isotopes ( 207 Pb/ 204 Pb >19.2; 15 analyses) acquired from clastic detritus eroded from Precambrian cratonal rocks to the east-southeast. In the overlying Paleozoic section, Pb-Zn-Cu-Ag-Au deposits associated with Jurassic, Cretaceous, and Tertiary granitic intrusions (intrusion-related metal deposits) contain galena and other sulfide minerals with S and Pb isotope compositions similar to those of TDS sedimentary pyrite, consistent with derivation of deposit S and Pb from TDS pyrite. Minor element abundances in TDS pyrite (e.g., Pb, Zn, Cu, Ag, and Au) compared to sedimentary and hydrothermal pyrite elsewhere are not noticeably elevated, implying that enrichment in source minerals is not a precondition for intrusion-related metal deposits. Three mechanisms for transferring components of TDS sedimentary pyrite to intrusion-related metal deposits are qualitatively evaluated. One mechanism involves (1) decomposition of TDS pyrite in thermal aureoles of intruding magmas, and (2) aqueous transport and precipitation in thermal or fluid mixing gradients of isotopically heavy S, radiogenic Pb, and possibly other sedimentary pyrite and detrital mineral components, as sulfide minerals in intrusion-related metal deposits. A second mechanism invokes mixing and S isotope exchange in thermal aureoles of Pb and S exsolved from magma and derived from decomposition of sedimentary pyrite. A third mechanism entails melting of TDS strata or assimilation of TDS strata by crustal or mantle magmas. TDS-derived or assimilated magmas ascend, decompress, and exsolve a mixture of TDS volatiles, including isotopically heavy S and radiogenic Pb from sedimentary pyrite, and volatiles acquired from deeper crustal or mantle sources. In the central and eastern Great Basin, the wide distribution and high density of small to mid-sized vein, replacement, and skarn intrusion-related metal deposits in lower Paleozoic rocks that contain TDS sedimentary pyrite S and Pb reflect (1) prolific Jurassic, Cretaceous, and Tertiary magmatism, (2) a regional, substrate reservoir of S and Pb in permeable and homogeneous siliciclastic strata, and (3) relatively small scale concentration of substrate and magmatic components. Large intrusion-related metal deposits in the central and eastern Great Basin acquired S and most Pb from thicker lithospheric sections.

He and Ar isotopic compositions and genetic implications for the giant Shizhuyuan W–Sn–Bi–Mo deposit, Hunan Province, South China

The Shizhuyuan ore deposit in southern Hunan Province is a world-class W–Sn–Bi–Mo occurrence hosted by Devonian limestone in the thermal aureole of the Qianlishan granite. Mineralization coincided with intrusion of the granite pluton during Mesozoic crustal extension in South China. We present new He and Ar isotope data for volatiles released from pyrite in the Shizhuyuan deposit. Concentrations of 40Ar range from 0.21 to 2.38 × 10−6 cm3 STP 40Ar/g, and 4He concentrations range from 0.8 to 65.1 × 10−6 cm3 STP 4He/g. 3He/4He ratios vary from 0.06 to 1.66 Ra (where Ra is the 3He/4He ratio of air = 1.39 × 10−6) and 40Ar/36Ar ratios range from 293 to 1072. The isotopic compositions of He and Ar indicate that the ore-forming fluids were mantle-derived, modified by air-saturated crustal fluids. Shallow-level boiling increased 3He concentrations, whereas crustal contamination decreased 3He/4He ratios in the magmatic fluids. The occurrence of mantle-derived components in the magmatic fluid indicates that the associated Qianlishan granite is not a typical S-type pluton that formed entirely by crustal melting. We propose that the mineralization was related to mantle upwelling and Mesozoic lithosphere extension of the South China Block.

Crater floor topography and impact melt sheet geometry of the Sudbury impact structure, Canada

Terra Nova, 00, 000–000, 2010 Abstract Based on high-resolution topography data and the geometry of geological contact traces at the surface, we determined the orientation of layer interfaces of the Sudbury Igneous Complex (SIC). Our analysis provides, for the first time, quantitative structural evidence for the thickness variation of its layers, which supports an impact melt origin of the SIC. The present crater floor topography varies up to 400 m over distances of hundreds of metres to a few kilometres, and up to 1500 m over a distance of about 25 km. Crater floor depressions are spatially associated with economically important sulphide mineral deposits, which points to a viable exploration strategy. Finally, observed variations in the width of the thermal aureole imparted by the SIC on its host rocks are due to primary thickness variations of the SIC rather than post-impact deformation.

Computer modeling of granite gneiss diapirism in the Earth’s crust: Controlling factors, duration, and temperature regime

This paper is devoted to the modeling the granite gneiss formation by means of diapiric upwelling. The natural examples of granitic diapirism in the Precambrian granite-greenstone belts and complexes of metamorphic cores are described. A new approach is proposed to describe the partial melting and development of gravity instability in the crustal granitic layer, which experienced heat impact and melting during intrusion of basaltic melt. Rheology of partially melted material and surrounding medium is regarded to be temperature-dependent, following either plasticity or creep (non-Newtonian viscosity) law. Modeling results show that crustal rheology plays a significant role in the character of diapirism (shape of upwelling bodies, duration of the process, and width of thermal aureole). The rates of upwelling within the crust behaving as elastoplastic body are orders of magnitude higher (meters to tens meters per year) than those obtained for creep (viscous) liquid model (0.8 cm/yr). Modeling results revealed that the limiting depth of upwelling of partially crystallized melt, with allowance for temperature dependence of creep, corresponds to the isotherm of 400°C.

Numerical investigation of transient hydrothermal processes around intrusions: Heat-transfer and fluid-circulation controlled mineralization patterns

New insights on the circulation of fluids around magmatic intrusions have been obtained through coupled hydrothermal numerical modelling that takes into account i) a continuous variation of permeability with depth, ii) the period of intrusion emplacement, iii) the physical likelihood of ore deposition using a restricted rock alteration index, and iv) the so-far unexplored pluton floor, and then comparing the results against well-constrained natural cases showing different emplacement depths, high permeability zones (cracked thermal aureoles), faults and plutonic apexes. We show that emplacement depth is a key physical parameter controlling the extent and geometries of advective heat dissipation zones, and that shallow apexes strongly modify the fluid-flow pattern by acting as a focus for convective fluids and mineralization zones. We also show that the cooling phase is not the main convective phase for large plutons commonly associated with long-lived magma emplacement; major advective heat dissipation and mineral deposition zones may also develop before and during the hottest phase of the emplacement, i.e. before magma crystallization. The comparison with natural cases shows that we successfully reproduce, in space and time, the physical conditions required for mineral deposition. In particular, extensional detachment is able to restrain and modify classical fluid-flow patterns induced by coeval intrusion. Finally, even though lacking chemical arguments, we conclude that convection induced by granite emplacement plays a major role in the genesis of granite-related Au deposits. Moreover, the formation of this type of deposit is favoured and controlled by the presence of a fractured thermal aureole around the intrusion.

Ultramafic intrusion triggers hydrothermal explosions at Colle Fabbri (Spoleto, Umbria), Italy

The eroded Colle Fabbri volcano comprises intrusive and extrusive rocks which cover an area of about 10,000 m 2 in the Umbria region, Central Italy. The outcrop is located at the SW boundary of the Umbria Valley Graben (Umbria Region) on an N140 normal fault inside the Intramountain Ultra-Alkaline Province of carbonatite and melilitite rocks of central–southern Italy. A field survey of the outcrop allowed a reconstruction of igneous activity events of this unusual small-scale volcano. It is younger than 0.7 m.y. and rests on Lower Pleistocene conglomerates with intercalated clay beds. A palaeosoil marks the base of the volcanic sequence. It follows several metres of extrusive breccias composed of fragments of thermometamorphosed clay and travertine. Key features of these breccias are mud shells on blocks, plastic mud lumps, slumps, and mechanical injection from one layer into another, as well as plastic and vesiculated, micro-brecciated matrix. The breccias are cemented by a variety of silicate, sulphate, and sulphate–carbonate minerals deposited by intense hydrothermal circulation. The breccias are related to phreatic explosions triggered by the eruption of a superficial hydrothermal system. Up to 1 m thick sill and a dykelet swarm intrude the breccias. In the southern part of the outcrop there is a plug of melilitolitic composition which intrudes the breccias and deforms them upward. A variety of contact breccias is also scattered around the sill and the plug. In some places, hyaloclastite formed when melt invaded water pockets contained in the encasing rocks under hydrostatic pressure conditions. A thermal aureole, which moves up to 10 m away from the contact, is characterised by cordierite–trydimite association, thus indicating high-temperature (>>1000 °C) contact phenomena.

Alpine metamorphism in the central segment of the Western Greywacke Zone (Eastern Alps)

Alpine metamorphism in the central segment of the Western Greywacke Zone (Eastern Alps)The metamorphic pattern of the central Western Greywacke Zone (Austroalpine, Eastern Alps) was investigated by organic matter reflectance, Raman spectroscopy on organic matter and clay mineralogical methods. Raman data map a 10 km wide thermal aureole along the contact zone of the Greywacke Zone to the Penninic Tauern Window. The estimated maximum temperatures of 400 °C to 200 °C decrease from South to North, that is from the contact to the uppermost parts of the Greywacke Zone. This pattern is explained by an Oligocene to Miocene thermal pulse, related to the rapid exhumation of formerly deeply buried rocks of the Penninic unit. During this event, advective heat transport and circulating fluids overprinted the Cretaceous higher anchi- to lower epizonal metamorphic pattern of the central Western Greywacke Zone.

Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New Zealand

AbstractIn southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high‐P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ∼200–1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10–14 kbar. Similar P–T conditions are recorded in high‐P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ∼200–1000 m from the Malaspina Pluton retain medium‐P mid‐amphibolite facies assemblages, despite having been subjected to pressures of 10–14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional‐scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3–4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10–14 kbar. Similar high‐P metamorphic events may therefore be more common than presently recognized.

Implications of incremental emplacement of magma bodies for magma differentiation, thermal aureole dimensions and plutonism–volcanism relationships

Field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets (sills). Emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation, country rock metamorphism and assimilation, and for the formation of large magma chambers in the upper crust. When a magma body begins to grow by slow accretion of sills, each successive intrusion solidifies before the injection of the next one. When the system is thermally mature, sill temperatures equilibrate above the solidus, melts accumulate and older sills can re-melt. The time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body. The transition from a mafic crystal-poor magma to a partially molten rock that retains a highly differentiated melt is fast, whereas the resulting evolved residual melt can reside in the crust for protracted periods. As long as temperatures in the system are relatively low, highly differentiated melts are generated, which may explain the bi-modal character and the absence of intermediate compositions in some magmatic provinces. The level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth. Metamorphism, partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock–magma body boundary. If the magma emplacement rate is low, the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body. Understanding emplacement of magma bodies is fundamental for our understanding of the plutonism–volcanism relationship. Magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions. The time-averaged emplacement rates of plutons are lower than this critical emplacement rate. Eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust. This suggests that the emplacement of magma bodies is a multi-timescale process with the development of large magma chambers corresponding to the highest magma fluxes. Because they control magmatic processes and the impact of magma intrusion on the country rock, future studies should focus on magma emplacement rates and on magma emplacement geometries. These studies should integrate field observation on plutons and geophysical data on active magmatic systems, coupled with laboratory experiments and numerical simulations.

Melilitolite intrusion and pelite digestion by high temperature kamafugitic magma at Colle Fabbri, Spoleto, Italy

A subvolcanic body in Colle Fabbri (Umbria, Central Italy) intrudes sedimentary rocks and the overlaying extrusive breccia. The intrusive rock is melilitolite with essential leucite, kalsilite, and wollastonite plus accessory Ti-garnet, Ti–Al–Fe 3+-clinopyroxene, magnetite, perovskite, rankinite, Si-bearing apatite, Fe–Ni sulphides, and carbonate/zeolite. Bulk chemistry of melilitolite indicates a strong SiO 2-undersaturation, extremely high CaO composition, and strong initial potassic character. Sr and Nd isotopic ratios support a link with the source of kamafugites and carbonatites found in the same region. The igneous contact rock contains variable amounts of quenched clinopyroxene, anorthite, sanidine ± wollastonite, plus spinel and glass. Extremely high temperature melilitolite magma digested pelite country-rock, thus leading to a unique mineral composition. The homogenisation temperatures of mineral inclusions in contact rock demonstrate that crystallisation occurred well above 1230 °C. A large thermal aureole formed in a local clinker-like breccia, characterised by the association of felsic glass + indialite + tridymite ± Ti-magnetite, confirms high-temperature emplacement. Metasomatic/thermometamorphic phenomena favoured zone-specific mineralisation of hydrated Ca-silicates, hydrated complex sulphates and zeolites in breccias and encasing rocks.

Type II diamonds: Flamboyant Megacrysts?

Type II diamonds from Premier are characterised by light carbon isotope-enriched compositions, markedly different from the signatures of the associated Eclogitic (E-type) and Peridotitic (P-type) diamonds at the same locality. This provides strong evidence that these Type II stones represent a third and hitherto unrecognised diamond paragenesis. Their isotopic signature is closely similar to that of rare Orapa diamonds with websterite inclusions, arguing for a common paragenesis. The Orapa websterite garnet inclusions are Ti- and Cr-rich relative to those from other kimberlites, and overlap the compositional field for Orapa garnet megacrysts. These chemical characteristics, coupled with the large size and irregular shape of Type II diamonds, links them to the megacryst suite. This conclusion is supported by evidence which shows that at the low temperature range of megacryst crystallisation, the associated residual liquids, if buffered by mantle wall rocks, at oxygen fugacities between the WM and IW, would be in the diamond P-T-f O2 stability field. The low N concentrations and light cabon isotopic-enriched fingerprint of the Type II stones are consistent with their crystallisation from such evolved magmas. It is demonstrated that in addition to the irregular and often large (megacryst) Type II stones, there are two further suites of N-poor diamonds which can be assigned to either an eclogitic or peridotitic paragenesis on the basis of their inclusions, physical properties and carbon isotopic signatures. Diamonds with complex growth zones and deformation textures can be explained in terms of dynamic mantle processes, linked to formation of the sheared peridotites, within the thermal aureole surrounding the kimberlite magma in which the megacryst suite crystallised. Framesites, fibrous cubic diamonds, and eclogitic diamonds which formed just prior to kimberlite eruption, can also be linked to the processes responsible for crystallisation of the megacryst suite, which is considered to be cognate to the host kimberlite magma.

High-pressure thermal aureoles around two Neoproterozoic synorogenic magmatic epidote-bearing granitoids, Northeastern Brazil

Unusual high-pressure inner thermal aureoles are described from the Minador and Angico Torto epidote-bearing tonalitic plutons that emplaced into greenschist-facies metasedimentary rocks of the Neoproterozoic Cachoeirinha-Salgueiro belt, northeastern Brazil. The foliated pelitic hornfelses display the mineral assemblage garnet, kyanite, staurolite, muscovite, biotite, plagioclase ± quartz. Rare fibrolite is only found very close to the contacts. Hornfelses display steep mineral lineations and steeply-dipping foliations concordant with magmatic contacts. Leucocratic veinlets containing quartz, oligoclase, garnet, kyanite, staurolite, rutile and ilmenite suggest that limited melting conditions were reached very close to magmatic contacts ( T ⩾ 650 °C, P around 8 kbar). These high-pressure hornfelses form a few meters thick, rigid envelopes around the two plutons. Contrary to known examples of kyanite-bearing hornfelses that recorded high-temperature decompression, the nearly isobaric cooling down to ca. 450 °C is constrained by 3.20–3.30 Si contents of retrogressive phengites from both inner hornfelses and ductilely-deformed tonalite at the pluton margins. Isograds and bathograds are, therefore, apparently telescoped due to HP/LT shearing, possibly caused by subsequent differential vertical movements affecting these two solidified plutons. The unusual depth of emplacement of these syn-kinematic calc-alkaline plutons is explained by a tentative geodynamic model involving a pre-620 Ma-subduction setting. Resumen Las aureolas internas que rodean dos plutones tonalíticos emplazados dentro de rocas cajas en facies esquistos verdes del Cinturón-plegado Cachoeirinha-Salgueiro al noreste de Brasil, contienen hornfelses pelíticos foliados con granate, kyanita, estaurolita, muscovita, biotita, plagioclasa ± cuarzo. Fibrolita es rara ó es encontrada solamente cerca de las zonas de contacto. Los hornfelses desarrollaron foliaciones concordantes con buzamiento fuerte cerca de los contactos magmáticos y muestran lineaciones minerales casi verticales. Venillas leucocraticas contienen cuarzo, oligoclasa, granate, kyanita, estaurolita, rutilo e ilmenita sugiriendo que el límite en las condiciones de fusión fueron cercanas a los contactos magmáticos ( T ⩾ 650 °C, P entorno 8 kbar). Estos hornfelses de alta presión forman una cobertura rígida de poco metros entorno a los dos plutones. Contrariamente a lo conocido en diferentes ejemplos de hornfelses con kyanita con registro de descompresión, la isóbara de enfriamiento para cerca de 450 °C está controlada por el contenido de Si 3.20–3.30 de fengitas retrógradas, en los hornfelses internos y en las márgenes de los plutones donde las tonalitas están deformadas ductilmente. Isógradas y batógradas son, por lo tanto, aparentemente telescopadas debido a cizalla de alta presión/baja temperatura, posiblemente causado por subsecuentes movimientos verticales diferenciales afectando estos dos plutones solidificados. El emplazamiento a una profundidad inusual de estos plutones calci-alcalinos syn-cinemáticos es explicado por un modelo geodinámico tentativo que involucra un escenario de subducción pre-620 Ma.