Contact Aureole Research Articles

Geochemistry of granitization and magmatic replacement of basic volcanic rocks in the contact aureole of the Yurchik gabbronorite intrusion, Ganal Range, Kamchatka

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO2, Al2O3, Na2O, K2O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of 143Nd/144Nd and increase of 87Sr/86Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites.

Multi-scale characterization of pore evolution in a combustion metamorphic complex, Hatrurim basin, Israel: Combining (ultra) small-angle neutron scattering and image analysis

Backscattered scanning electron micrograph and ultra small- and small-angle neutron scattering data have been combined to provide statistically meaningful data on the pore/grain structure and pore evolution of combustion metamorphic complexes from the Hatrurim basin, Israel. Three processes, anti-sintering roughening, alteration of protolith (dehydration, decarbonation, and oxidation) and crystallization of high-temperature minerals, occurred simultaneously, leading to significant changes in observed pore/grain structures. Pore structures in the protoliths, and in low- and high-grade metamorphic rocks show surface (Ds) and mass (Dm) pore fractal geometries with gradual increases in both Ds and Dm values as a function of metamorphic grade. This suggests that increases in pore volume and formation of less branching pore networks are accompanied by a roughening of pore/grain interfaces. Additionally, pore evolution during combustion metamorphism is also characterized by reduced contributions from small-scale pores to the cumulative porosity in the high-grade rocks. At high temperatures, small-scale pores may be preferentially closed by the formation of high-temperature minerals, producing a rougher morphology with increasing temperature. Alternatively, large-scale pores may develop at the expense of small-scale pores. These observations (pore fractal geometry and cumulative porosity) indicate that the evolution of pore/grain structures is correlated with the growth of high-temperature phases and is a consequence of the energy balance between pore/grain surface energy and energy arising from heterogeneous phase contacts. The apparent pore volume density further suggests that the localized time/temperature development of the high-grade Hatrurim rocks is not simply an extension of that of the low-grade rocks. The former likely represents the “hot spots (burning foci)” in the overall metamorphic terrain while the latter may represent contact aureoles.

A multidisciplinary study on the emplacement mechanism of the Qingyang–Jiuhua Massif in Southeast China and its tectonic bearings. Part I: Structural geology, AMS and paleomagnetism

During the Cretaceous, the South China Block (SCB) experienced a widely distributed extensional event including numerous plutons emplacement and basin opening. Investigations on the tectonic regime coeval with pluton emplacement, and emplacement mechanism of the pluton remain relatively rare in the SCB. In order to address these questions, a multidisciplinary approach, including field structural and petrographic observations, anisotropy magnetic susceptibility (AMS) and paleomagnetic analyses, was carried out on the Qingyang–Jiuhua granitic massif which intrudes into the Lower Yangtze fold belt in the northeastern part of the SCB. The Qingyang–Jiuhua massif is composed of the granodioritic Qingyang and monzogranitic Jiuhua plutons dated by zircon U–Pb method at ca. 142Ma, and ca. 131Ma, respectively. Our structural observations show that the intrusion of the Qingyang–Jiuhua massif does not modify the fold strike. A weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism. In the contact aureole of the massif, the foliation follows the pluton contour, and the mineral lineation is rare. When present, it exhibits a down-dip attitude. Field and microstructural observations indicate isotropic magmatic textures in most parts of the massif. The AMS analysis of 93 sites reveals weak values for the anisotropy degree (PJ<1.2) and oblate magnetic fabric dominance (T>0) for most of the measured samples. Two principal foliation patterns are identified: horizontal foliations in the center of the plutons, and vertical foliations on the boundaries. Magnetic lineation strike is largely scattered, and weakly inclined at the scale of the entire massif. The paleomagnetic investigations indicate that (a) the younger Jiuhua pluton did not produce a remagnetization in the older Qingyang pluton, (b) no relative movement can be observed between these two plutons, (c) the entire massif did not experience any important relative movement with respect to South China, considering the paleomagnetic uncertainties. Integrating the newly obtained results with previous observations, our study favors a permissive emplacement mechanism for the two plutons, i.e. vertical magma intrusion into an opening space controlled by the NW–SE brittle stretching of the upper crust, which is in agreement with a weak extensional regional tectonic framework of the SCB.

Batholith tectonics: Formation and deformation of ghost stratigraphy during assembly of the mid-crustal Andalshatten batholith, central Norway

The Andalshatten batholith (322 km 2 , >700 km 3 ) is a predominantly granodioritic high-K, calc-alkaline igneous body that was assembled in the mid-crust across four lithologically distinct nappes within the Helgeland Nappe Complex, central Norway. Extensive vertical and horizontal exposures of metamorphic screens and xenoliths within the batholith provide an unparalleled view of the nature of magma emplacement, host rock displacement, and batholith assembly, i.e., batholith tectonics. The mapped intrusion consists of at least five distinct lithologic phases, including schlieren-banded to gneissic granodiorite (11% of batholith area), coarse-crystalline to K-feldspar megacrystic granodiorite (69%), amphibole-bearing diorite (11%), tonalite (2%), and minor leucogranite. Contacts between phases are both sharp and/or gradational and are interpreted to reflect comagmatic behavior over the duration of crystallization of the phases separated by a given contact. New chemical abrasion–thermal ionization mass spectrometry 206 Pb/ 238 U zircon weighted mean ages of 442.67 ± 0.14 Ma and 441.53 ± 0.40 Ma for 2 samples of the voluminous megacrystic granodiorite from disparate localities indicate distinct periods of zircon crystallization separated by ∼1 Ma; titanite ages for these samples are 441.30 ± 0.21 Ma and 436.10 ± 2.80 Ma, respectively. No observable contacts were identified between these two lithologically similar localities. Of the mapped intrusion area, ∼8% (>24 km 2 ) comprises screens (kilometer scale) and xenoliths (subkilometer scale) of metamorphic rocks that reflect the skeletal framework of the host rock nappes into which the granodioritic magmas intruded. This ghost stratigraphy maintains broad continuity with host rock lithology and structural trends. The largest screens show no evidence of internal, emplacement-related ductile deformation, but appear to be rigidly rotated into subparallelism with the western host rock contact, presumably during subsequent magma injection into the batholith. In contrast, xenoliths underwent rotation, translation, and internal deformation in the magma. The scale dependence of synmagmatic deformation of screens and xenoliths is likely the result of smaller blocks becoming thermally equilibrated with the surrounding magma and thus deforming by ductile mechanisms in a magma with increasing yield strength due to crystallization. We interpret the Andalshatten batholith to have been assembled by at least five spatially distinct, elongate batches of magma over a minimum duration 600 ka to 1.7 Ma, including significant recharge events involving dioritic magmas. Local space for batholith assembly was accommodated by brittle and ductile deformation, including viscous flow of host rocks in a dynamothermal contact aureole. Viscous flow was facilitated by reactivation of existing structures (e.g., tightening of interlimb fold angles), recrystallization, and penetrative foliation development, resulting in near-field lateral and downward-directed displacement of host rocks along the western margin during batholith expansion and growth. Emplacement of dioritic magmas added heat and mass to the growing reservoir, enabling significant magnitudes of internal, hypersolidus flow, magmatic foliation development, mechanical mingling, and screen deformation. These observations and data sets are consistent with the hypothesis of multiple recharge events in a magma chamber that was partially molten over reasonably large spatial scales, thereby allowing screens and xenoliths to be incorporated and displaced and/or deformed.

Metamorphic evolution of the contact aureole of the Jhirgadandi pluton, Sonbhadra district, Mahakoshal mobile belt, central India

The metamorphic evolution of the contact aureole around the Late Paleoproterozoic Jhirgadandi pluton in the eastern part of Parsoi Formation of Mahakoshal terrain, central India represents three distinct metamorphic zones, characterized by definite mineral assemblages. The contact-metamorphic event produced the peak-metamorphic mineral assemblages Bt + Qtz + Alb + Sil ± Cd ± Grt ± Mus ± Kfs in the metapelites of inner aureole, Bt + Qtz + And + Mus + Kfs + Plag ± Cd ± Chl in middle aureole and Chl + Mus + Bt ± And + Alb + Qtz ± Ep + Mt ± tourmaline in the outer aureole. The estimated P–T conditions based on detailed geothermobarometric calculations in the thermal metamorphosed rocks are 690 ◦ C/3.4 kbar, 580 ± 15 ◦ C and 487 ± 30 ◦ C in inner aureole, middle aureole and outer aureole, respectively. The variation in metamorphic condition suggests that the shallow crustal level emplacement of Jhirgadandi pluton is responsible for the overprinting of contact metamorphic assemblages (M2) in the low grade metapelites (regional metamorphism M1) of Mahakoshal Group.

Rujevac Sb-Pb-Zn-As polymetallic deposit, Boranja orefield, Western Serbia: native arsenic and arsenic mineralization

Rujevac is a low-temperature hydrothermal polymetallic Sb-Pb-Zn-As vein-type ore deposit, hosted within a volcanogenic-sedimentary zone situated in the Rujevac-Crvene Stene-Brezovica Diabase-Chert Formation (DCF) of the Podrinje Metallogenic District (PMD), Serbia. It is located several kilometers SE from the Boranja contact aureole, which is an integral part of the PMD in Western Serbia. Genetically related to the Tertiary granodioritic magma, the mineral assemblages are characterized by specific features. The mineral association of this deposit consists of sulfides, Pb-Sb(As) sulfosalts, native metals, oxides, hydroxides and gangue minerals. Chemical composition of the ore is very complex, where contents of valuable metals range as follows: Sb (0.17–24.31 wt.%), Zn (0.21–6.29 wt.%), Pb (0.15–6.33 wt.%), As (0.06–1.28 wt.%), Cd (25–747 ppm), Ag (7–408 ppm), Hg (13–473 ppm), and Tl (<1–29 ppm). Electron Probe Microanalyses (EPMA) of native arsenic from both the Rujevac and Stragari deposits showed contents of As up to 98.8 and 97.1 wt.%, with impurity contents of Sb up to 1.3 and 6.6 wt.%, and Tl up to 2 and 1.3 wt.%, respectively. Rhombohedral unit-cell parameters for native arsenic from Rujevac and Stragari deposits amount to: a = 3.760(2), c = 10.555(3) A, V = 129.23(7) A3 and a = 3.763(1), c = 10.560(5) A, V = 129.48(8) A3, respectively. Mineral assemblages, deposition order and genesis of the Rujevac polymetallic deposit were also discussed in detail. Native arsenic mineralization here has been additionally compared with similar well-known global deposits.

Grain-boundary diffusion rates inferred from grain-size variations of quartz in metacherts from a contact aureole

We evaluate a temperature-dependent coefficient for grain-boundary diffusion in quartz aggregates using grain size data from a contact aureole, based on the coupling of a numerical model for the temperature-time history of the contact aureole with a model for the kinetics of diffusion-controlled grain growth. The metachert samples were collected from the contact aureole of the Hanase-Bessho quartz diorite at Hanase Pass, Kyoto, Japan. The quartz grain sizes vary systematically with distance from the quartz diorite. We calculated the temperature-time history using a one-dimensional thermal model, validated by peak metamorphic temperature estimates that are based on the degree of graphitization of carbonaceous material in metapelites, as characterized by Raman microspectroscopy. To minimize the sum of the squares of the errors between the measured and calculated grain sizes, based on the normal grain growth law together with the temperature-time history, we estimated the activation energy and pre-exponential factor in the α-quartz field to be 208 kJ/mol and 1.1 × 10 -8 m 2 /s, respectively, assuming a grain-boundary width of 1 nm. The grain-boundary diffusion rates for temperatures in the greenschist and amphibolite facies are similar to those determined in natural or laboratory grain-coarsening experiments, but differ significantly from those determined in tracer diffusion experiments. During grain-size-sensitive deformation, “effective” grain-boundary diffusion rates may be intermediate between the rates of diffusion along and across the grain boundary, and would be higher than the grain-boundary diffusion rates estimated by grain-coarsening experiments, and lower than those by tracer diffusion experiments.

Structure of the Panzhihua intrusion and its Fe-Ti-V deposit, China

The Panzhihua intrusion in southwest China is part of the Emeishan large igneous province and host of a large Fe-Ti-V ore deposit. In previous interpretations it was considered to be a layered, differentiated sill with the ore deposits at its base. New structural and petrological data suggest instead that the intrusion has an open S-shape, with two near-concordant segments joined by a discordant dyke-like segment. During emplacement of the main intrusion, multiple generations of mafic dykes invaded carbonate wall rocks, producing a large contact aureole. In the central segment, magmatic layering is oriented oblique to the walls of the intrusion. This layering cannot have formed by crystal settling or in-situ growth on the floor of the intrusion; instead we propose that it resulted from inward solidification of multiple, individually operating, convection cells. Ore formation was triggered by interaction of magma with carbonate wall rocks.

Melting of carbonate wall rocks and formation of the heterogeneous aureole of the Panzhihua intrusion, China

The Panzhihua gabbroic intrusion, part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about ∼263 Ma ago. The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite, diopside and garnet skarns. The variation in mineralogy is explained in part by differences in the composition of the protolith, particularly the proportion of silica minerals and clay, and in part by transfer of elements from intruding magmas. The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls, but some contain high Si, Ti, and Fe contents that are interpreted to have come from a magmatic source. Three brucite marbles sampled ∼10 m from the contact of the intrusion and named “enriched brucite marble” have trace element compositions very different from their dolomitic protolith: their rare earth elements are strongly enriched whereas levels of Nb-Ta, Zr-Hf and Ti are very low. These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua, a similarity we take to indicate that the sample underwent partial melting. Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks. However, other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable. We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole. Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite, lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion. Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO2, with potential impact on global climate.

MagmaHeatNS1D: One-dimensional visualization numerical simulator for computing thermal evolution in a contact metamorphic aureole

MagmaHeatNS1D is an IDL (Interactive Data Language) program that is aimed at numerically modeling heat transfer from an igneous intrusion to its host rocks and providing important thermal state information for minerals and organic matters in a contact aureole. It can be used to trace temperature time series, computing the peak temperature, and evaluating organic-matter maturation in a contact aureole. The theoretical basis of the program is a complete one-dimensional heat transfer model, and, hence, the program can allow for numerous potential influencing factors on the heat transfer, involving magma crystallization, volatilization and the supercritical state of pore water, dehydration and decarbonation reactions of host rock matrix, instantaneous and finite-time magma intrusion mechanisms, and hydrothermal convection in host rocks. This ensures that the program can be applicable to study different igneous intrusions in various geological conditions. MagmaHeatNS1D features a graphical user interface for controlling program execution, displaying real-time results, outputting final results, and opening secondary windows which serve to input the model parameters. MagmaHeatNS1D can be used in an intuitive framework for educational and research purposes.

Assimilation of carbonate country rock by the parent magma of the Panzhihua Fe-Ti-V deposit (SW China): Evidence from stable isotopes

The Panzhihua intrusion in southwest China is part of the Emeishan Large Igneous Province and host of a large Fe-Ti-V ore deposit. During emplacement of the main intrusion, multiple generations of mafic dykes invaded carbonate wall rocks, producing a large contact aureole. We measured the oxygen-isotope composition of the intrusions, their constituent minerals, and samples of the country rock. Magnetite and plagioclase from Panzhihua intrusion have δ18O values that are consistent with magmatic equilibrium, and formed from magmas with δ18O values that were 1–2‰ higher than expected in a mantle-derived magma. The unmetamorphosed country rock has high δ18O values, ranging from 13.2‰ (sandstone) to 24.6–28.6‰ (dolomite). The skarns and marbles from the aureole have lower δ18O and δ13C values than their protolith suggesting interaction with fluids that were in exchange equilibrium with the adjacent mafic magmas and especially the numerous mafic dykes that intruded the aureole. This would explain the alteration of δ18O of the dykes which have significantly higher values than expected for a mantle-derived magma. Depending on the exact δ18O values assumed for the magma and contaminant, the amount of assimilation required to produce the elevated δ18O value of the Panzhihua intrusion was between 8 and 13.7 wt.%, assuming simple mixing. The exact mechanism of contamination is unclear but may involve a combination of assimilation of bulk country rock, mixing with a melt of the country rock and exchange with CO2-rich fluid derived from decarbonation of the marls and dolomites. These mechanisms, particularly the latter, were probably involved in the formation of the Fe-Ti-V ores.

Reddish Metagranites from the Gennargentu Igneous Complex (Sardinia, Italy): Insight into Metasomatism Induced by Magma Mingling

the recrystallization of quartz and plagioclase to higher calcium concentrations. The progressive increase in the MgO/(MgO þFeO) of chlorite closer to the contact indicates that this phase also recrystallized.The iron released during chlorite recrystallization was buffered by hematite formation in the pores of metasomatic K-feldspar. The Gennargentu metagranites provide evidence that metasomatic fluids can play a major role in driving metamorphic reactions in contact aureoles. For instance, the expected increase of Ca in plagioclase owing to thermal equilibration was not achieved in the high-T zone of the aureole because of fluid-assisted removal of cations. We conclude that caution should be taken when interpreting the processes that underpin contact metamorphism in terms of thermally driven, ionic diffusion alone, because the role of fluids may be significant, if not overwhelming, in the domains closest to the magmatic source.

Microstructural evolution of carbonaceous material during graphitization in the Gyoja-yama contact aureole: HRTEM, XRD and Raman spectroscopic study

Graphitization of carbonaceous material (CM) in the Gyoja-yama contact aureole, northwest of Kyoto, Japan, was examined using high-resolution transmission electron microscopy (HRTEM), X-Ray Diffractometry (XRD), and Micro-Raman spectroscopy. In this region, the CM in the sedimentary rocks has been transformed from poorly crystalline to well-crystallized graphite. This conversion is evident from its morphological transformation of the grain structure to filament like structure and finally into hexagonal plate structure. These microstructural evolutions closely resemble the graphitization process that is usually observed in regional metamorphism, as reported in previous HRTEM observations. Micro-Raman and XRD results indicate that the graphitization parameters are also comparable to each other, where Lc(002) and La(110) values increase during graphitization in contact metamorphism. However, the d002 values have lower correlation coefficients with R2 compared to other parameters. In addition, the crystal size of fully ordered graphite with a d-spacing of ≤3.36 A in contact metamorphic rocks is approximately 100 A larger than that in regional metamorphic rocks. These features indicate that information of d002 parameter includes not only the peak metamorphic temperature condition, but also several other factors such as deformation, confining pressure, impurities of fluid activities and catalytic effect, which may be the key point to fully understand the evolution of CM to graphite.

The Significance of Partial Melting Processes in Hydrothermal Low Sulfide Cu-Ni-PGE Mineralization Within the Footwall of the Sudbury Igneous Complex, Ontario, Canada

We present new results and review existing data highlighting different aspects of the genetic relationship between partial melting and hydrothermal mineralizing processes in the contact aureole of the Sudbury Igneous Complex. At the basal contact of the igneous complex, in the footwall breccia, crystallized partial melt pods and veins, referred to as footwall granophyres are abundant and intrude all rock types including the breccia matrix, as well as massive Ni-Cu sulfide ore. The final crystallization of these melts was accompanied by the segregation of high salinity fluids dominantly in the temperature range of 450° to 550°C, as revealed by Ti-inquartz thermometry and studies on primary fluid inclusions. In mineralized parts of the footwall breccia there is clear evidence that interaction of partial melts, exsolved fluids, and preexisting magmatic sulfides occurred. In the deeper footwall of the Sudbury Igneous Complex, similar footwall granophyres occur as networks of veins intruding impact brecciated (Sudbury breccia) country rocks and have an intimate spatial association with hydrothermal, low sulfide assemblages highly enriched in Pt, Pd, and Au (with a unique assemblage of PGM, including malyshevite and lisiguangite). It is suggested that partial melting processes were widespread in the contact and proximal footwall environment of the igneous complex and they were important in providing high salinity magmatic fluids to a hydrothermal system responsible for redistribution of base and precious metals. We describe observations which may evidence the migration of partial melts from the contact into the deeper footwall. Furthermore, we suggest a cogenetic model for the precipitation of sulfide-hydrous silicate assemblages in close spatial association to footwall granophyres. We emphasize that the use of footwall granophyres in mineral exploration for Cu-Ni-PGE ores in the footwall of the Sudbury Igneous Complex is clearly justified. Mapping of footwall granophyre vein networks highlights areas that were proximal to the contact environment and point out structural pathways that may have been used by syn- or postgenetic mineralizing fluids in a given area. Widespread partial melting appears to have occurred in the footwall of other large mafic-ultramafic complexes (such as Bushveld and Duluth) and similar genetic relationships of partial melting and ore-forming hydrothermal processes may have possibly existed also in these systems.

Melt infiltration into quartzite during partial melting in the Little Cottonwood Contact Aureole (UT, USA): implication for xenocryst formation

AbstractMelt infiltration into quartzite took place due to generation and migration of partial melts within the high‐grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole (UT, USA). Melt was produced by muscovite and biotite dehydration melting reactions in the BC formation, which contains pelite and quartzite interlayered on a centimetre to decimetre scale. In the migmatite zone, melt extraction from the pelites resulted in restitic schollen surrounded byK‐feldspar‐enriched quartzite. Melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins and ductile shear zones, during intrusion‐related deformation in the contact aureole. The transition between the quartzofeldspathic segregations and quartzite shows a gradual change in texture. Here, thinK‐feldspar rims surround single, round quartz grains. The textures are interpreted as melt infiltration texture. Pervasive melt infiltration into the quartzite induced widening of the quartz–quartz grain boundaries, and led to progressive isolation of quartz grains. First as clusters of grains, and with increasing infiltration as single quartz grains in theK‐feldspar‐rich matrix of the melt segregation. A 3D–μCT reconstruction showed that melt formed an interconnected network in the quartzites. Despite abundant macroscopic evidence for deformation in the migmatite zone, individual quartz grains found in quartzofeldspathic segregations have a rounded crystal shape and lack quartz crystallographic orientation, as documented with electron backscatter diffraction (EBSD). Water‐rich melts, similar to pegmatitic melts documented in this field study, were able to infiltrate the quartz network and disaggregate grain coherency of the quartzites. The proposed mechanism can serve as a model to explain abundant xenocrysts found in magmatic systems.

Numerical modeling of thermal evolution in the contact aureole of a 0.9 m thick dolerite dike in the Jurassic siltstone section from Isle of Skye, Scotland

In this study, we present a heat-transfer-model analysis of thermal evolution in the contact aureole of a 0.9 m thick dolerite dike in the Jurassic siltstone section from Isle of Skye, Scotland based on the constraints from the two types of vitrinite-reflectance geothermometers as well as geochemistry and burial history of host rocks. The predictions from the heat conduction models assuming the finite-time magma intrusion mechanism and pore-water volatilization can match well with both measured vitrinite reflectances and geological conditions of the host rocks. In the region where pore water volatilized and vitrinite reflectance rises with decreasing distance to the dike contact, the bomb geothermometer is consistent with the EASY%Ro model in validating the thermal evolution history of the host rocks, whereas it loses the function of temperature indicator out of the volatilization region. Possibly, the pore-water volatilization influenced the reliability of the bomb geothermometer. The computed total organic carbon contents based on the reconstructed thermal evolution history and the EASY%Ro model present the general agreement with the measured values, demonstrating the availability of the EASY%Ro model in indicating organic-matter transformation ratios at contact metamorphic conditions. ► Volatilization & finite-time intrusion mechanism represent natural conditions. ► Bomb geothermometer is reliable to estimate peak temperature in boiling zone. ► It is consistent with EASY%Ro to calibrate thermal history in boiling zone. ► Reliability of EASY%Ro model at contact metamorphic conditions is verified. ► Estimated TOC based on EASY%Ro model presents good agreement with measurement.

Intrusion and contamination of high-temperature dunitic magma: the Nordre Bumandsfjord pluton, Seiland, Arctic Norway

The Caledonian Nappe Complex of Arctic Norway provides rare insights into the interaction between mafic–ultramafic magmas and the deep continental crust. The Kalak Nappe Complex contains >25,000 km3 of mafic igneous rocks, mostly layered gabbros, making up the 570–560 Ma Seiland Igneous Complex. The complex has been intruded by a series of ultramafic magmatic rocks, including the Nordre Bumandsfjord pluton. Field relationships in this pluton show that extremely fluid, dry, relatively Fe-rich (Fo81) dunite magmas intruded a pile of cumulate gabbros, with block stopping and intrusive brecciation. Diking on scales from mm to metres and extensive melting and assimilation of the gabbros attest to high temperatures, consistent with a 2-km-wide granulite-facies contact aureole. Major- and trace-element trends show that the dunites were progressively contaminated by a cpx-rich partial melt of the gabbros, producing a range of lithologies from dunite through lherzolites to wehrlite. Experimental studies of natural samples at 0.8–1 GPa define the dunite solidus at 1,650–1,700 °C. In the average peridotite, contamination has produced a crystallisation interval of ca 400 °C (1,600–1,200 °C); this would provide large amounts of heat for melting and metamorphism and would maintain the fluidity of the magmas to relatively low T, consistent with field relationships. Thermochemical and dynamic modelling demonstrates that the dunitic primary magmas may represent the last melting of a rapidly ascending diapir of previously depleted subducted oceanic lithosphere. The mafic rocks of the Seiland Complex may already have been extracted from this diapir, and the late dikes of the province may reflect melting of the asthenosphere as the diapir spread out beneath the lithosphere. Ultramafic magmas, abundant in the Archean, may still be more common than usually assumed. However, they would only penetrate to the shallow crust under unusually extensional conditions, where ascent could outpace assimilation.

Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island, Canada

AbstractThis contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ∼50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO2)fluid pseudosections that suggest achievement of CO2‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ13C and δ18O values of carbonate minerals. Although the up to 5‰ decrease of both δ13C and δ18O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ13C is attributed to fluid‐mediated exchange with organic low‐13C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ18O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high‐temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO2)fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling.

A petrologic, geochemical and Sr–Nd isotopic study on contact metamorphism and degassing of Devonian evaporites in the Norilsk aureoles, Siberia

Devonian evaporites and associated sedimen- tary rocks in the Norilsk region were contact metamor- phosed during emplacement of mafic sills that form part of the end-Permian (*252 Ma) Siberian Traps. We present mineralogical, geochemical and Sr-Nd isotopic data on sedimentary rocks unaffected by metamorphism, and meta- sedimentary rocks from selected contact aureoles at Norilsk, to examine the mechanisms responsible for magma-evaporite interaction and its relation to the end- Permian environmental crisis. The sedimentary rocks include massive anhydrite, rock salt, dolostone, calcareous siltstones and shale, and the meta-sedimentary rocks comprise calcareous hornfels, siliceous hornfels and minor meta-anhydrite and meta-sandstone. Contact metamor- phism took place at low pressure and at maximum temperatures corresponding to the phlogopite-diopside stability field. Calcareous hornfels have high CaO, MgO, CO2 ,S O3, low SiO2 and initial Sr isotopic ratios of 0.7079-0.7092, features indicative of calcareous siltstone protoliths. Siliceous hornfels, in contrast, have high SiO2, Al2O3 ,N a 2O, low in other major element oxides and initial Sr isotopic ratios of 0.7083-0.7152, consistent with pelitic or shaley protoliths. Loss of CO2 in a subset of calcareous hornfels can be explained by decarbonation reactions dur- ing metamorphism, but release of SO2 from evaporites cannot be accounted for by a similar mechanism. Occur- rences of wollastonite and a variety of hydrous minerals in the calcareous hornfels are consistent with equilibration with hydrous fluid, which was capable of leaching large quantities of anhydrite in the presence of dissolved NaCl. In this way, substantial sediment-derived sulfur could have been mobilized, incorporated into the magmatic system and released to the atmosphere. The release of CO2 and SO2 from Siberian evaporites added to the variety of toxic gases generated during metamorphism of organic matter, coal and rock salt, contributing to the end-Permian envi- ronmental crisis.

The contact aureole of the South Mountain Batholith in Halifax, Nova Scotia: geology, mineral assemblages, and isograds

The contact aureole at the eastern margin of the South Mountain Batholith (Halifax Pluton) underlies most of the city of Halifax, Nova Scotia. The Halifax Group in the study area includes two lithological units, the Cunard and Bluestone formations. Before intrusion, both had been affected by greenschist facies regional metamorphism and deformed into northeast–southwest-trending, regional upright folds associated with a strong slaty cleavage. Contact metamorphic isograds trend obliquely across the Halifax peninsula, at a high angle to regional structural trends. At 2.5–3 km from the intrusive contact, sparse cordierite spots mark the outer limit of the contact aureole. The biotite-in isograd is marked by the development of biotite within chlorite + muscovite stacks inherited from regional metamorphism. Pyrrhotite is the dominant sulphide mineral throughout the contact aureole. With increasing metamorphic grade, assemblages in both units are marked by increasing modal abundance of cordierite and biotite, with K-feldspar variably developed within ca. 600 m of the contact. However, there is a marked difference in the distribution and appearance of andalusite between the two units. In aluminous pelites of the Cunard formation, idioblastic chiastolite appears before biotite more than 1500 m from the contact. In the less aluminous Bluestone formation, andalusite is present only within ca. 500 m of the contact, where it forms xenoblastic, spongy crystals. In both units, the assemblage andalusite + biotite + K-feldspar ± cordierite is developed near the contact, with local fibrolite and evidence of incipient partial melting. Petrographic constraints suggest pressure–temperature conditions at the contact of ca. 2.5–3.0 kbar (1 kbar = 100 MPa) at ca. 650 °C.