The magmatic feeding system of El Reventador volcano (Sub-Andean zone,Ecuador) constrained by texture,mineralogy and thermobarometry of the 2002 erupted products

After a 26 years long quiescence El Reventador, an active volcano of the rear-arc zone of Ecuador, entered a new eruptive cycle which lasted from 3 November to mid December 2002. The initial sub-Plinian activity (VEI 4 with andesite pyroclastic falls and flows) shifted on 6 and 21 November to an effusive stage characterized by the emission of two lava flows (andesite to low-silica andesite Lava-1 and basaltic andesite Lava-2) containing abundant gabbro cumulates. The erupted products are medium to high-K calc-alkaline and were investigated with respect to major element oxides, mineral chemistry, texture and thermobarometry. Inferred pre-eruptive magmatic processes are dominated by the intrusion of a high-T mafic magma (possibly up to 1165 ± 15 °C) into an andesite reservoir, acting as magma mixing and trigger for the eruption. Before this refilling, the andesite magma chamber was characterized by water content of 5.3 ± 1.0%, high oxygen fugacity (> NNO + 2) and temperatures, in the upper and lower part of the reservoir, of 850 and 952 ± 65 °C respectively. Accurate amphibole-based barometry constrains the magma chamber depth between 8.2 and 11.3 km (± 2.2 km). The 6 October 2002 seismic swarm (hypocenters from 10 to 11 km) preceding El Reventador eruption, supports the intrusion of magmas at these depths. The widespread occurrence of disequilibrium features in most of the andesites (e.g. complex mineral zoning and phase overgrowths) indicates that convective self-mixing have been operating together with fractional crystallization (inferred from the cognate gabbro cumulates) before the injection of the basic magma which then gave rise to basaltic andesite and low-silica andesite hybrid layers. Magma mixing in the shallow chamber is inferred from the anomalous SiO₂–Al₂O₃ whole-rock pattern and strong olivine disequilibria. Both lavas show three types of amphibole breakdown rims mainly due to heating (mixing processes) and/or relatively slow syn-eruptive ascent rate (decompression) of the magmas. The lack of any disequilibrium textures in the pumices of the 3 November fall deposit suggest that pre-eruptive mixing did not occur in the roof zone of the chamber. A model of the subvolcanic feeding system of El Reventador, consistent with the intrusion of a low-Al₂O₃ crystal-rich basic magma into an already self-mixed andesite shallow reservoir, is here proposed. It is also inferred that before entering the shallow chamber the “basaltic” magma underwent a polybaric crystallization at deeper crustal levels.     

Challenges in constraining the P–T conditions of mafic granulites: An example from the northern Trans‐North China Orogen

Some mafic granulites in the Sanggan area of the northern Trans‐North China Orogen (TNCO) have a relatively simple mineralogy with low energy grain shapes that are compatible with an assumption of equilibrium, but the rock‐forming minerals show variations in composition that create challenges for thermobarometry. The mafic granulites, which occur as apparently disrupted dyke‐like bodies in tonalite–trondhjemite–granodiorite gneisses, are divided into two types based on petrography and chemical composition. Type 1 mafic granulites are fine‐ to medium‐grained with an equilibrated texture and an assemblage of plagioclase+clinopyroxene+garnet+magnetite+ilmenite and sometimes minor hornblende±orthopyroxene. Type 2 mafic granulites are coarse‐grained and hornblende bearing with a peak assemblage of garnet+clinopyroxene+plagioclase+hornblende and variably developed coronae and symplectites of plagioclase+hornblende+orthopyroxene partially replacing porphyroblastic garnet±clinopyroxene. SIMS U–Pb dating of metamorphic zircon from two type 1 mafic granulites yields metamorphic ages of c. 1.84 and 1.83 Ga, consistent with published ages of the type 2 mafic granulites. Based on phase equilibrium modelling, we use the common overlap of P–T fields defined by the mineral assemblage limits, and the mole proportion and composition isopleths of different minerals in each sample to quantify the metamorphic conditions. For type 1 granulites, overlap of the mineral proportion and composition fields for each of three samples yields similar P–T conditions of 710–880°C at 0.57–0.79 GPa, 820–850°C at 0.59–0.63 GPa and 800–860°C at 0.59–0.68 GPa. For the type 2 granulites, overlaying the peak assemblage fields for three samples yields common P–T conditions of 870–890°C at 1.1–1.2 GPa. For the retrograde assemblage, overlap of the mineral proportion and composition fields for each sample yields similar P–T conditions of 820–840°C at 0.85–0.88 GPa, 860–880°C at 0.83–0.86 GPa and 880–930°C at 0.89–0.95 GPa. The P–T conditions appear distinct between the two types of mafic granulite, with the mineralogically simple type 1 mafic granulites recording the lowest pressures. However, there are significant uncertainties associated with these results. For the granulites, there are uncertainties related to the determination of modes and composition of the equilibration volume, particularly estimation of O and H₂O contents, and in the phase equilibrium modelling there are uncertainties that propagate through the calculation of mole proportions and mineral compositions. The compound uncertainties on pressure and temperature for high‐T granulites are large and the results of our study show that it may be unwise to rely on P–T conditions determined from the simple intersection of calculated mineral composition isopleths alone. Since the samples in this study are from a limited area—a few hundred square metres—we infer that they record a single P–T path involving both decompression and cooling. However, there is no evidence of the high‐P granulite facies event at 1.93–1.90 Ga that is recorded elsewhere in the TNCO, which suggests that the precursor basic dykes were emplaced late during the assembly of the North China Craton.     

Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

Pre-kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic alteration is challenging because of its usual obliteration by subsequent syn-kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma-old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low-T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post-Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine-age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre-orogenic upper crustal softening controls the deformation patterns in granitic basement units through low-grade metamorphic reactions.     

Modelling white mica pressure‐temperature‐time (P‐T‐t) paths using thermobarometric and 40Ar/39Ar thermochronologic data

White micas are major rock forming minerals in igneous and metamorphic rocks, and their chemical and isotopic variations can be used to determine pressure, temperature and time (P‐T‐t) histories. We apply ⁴⁰Ar/³⁹Ar multi‐diffusion domain modelling to white micas from blueschists blocks in serpentinite matrix mélange from the exhumed Baja California subduction complex. Thermal history models yielded T‐t paths suggesting that ⁴⁰Ar* resides within multiple diffusion domains with varying ⁴⁰Ar* retentivity. Modelled white mica thermal histories and thermobarometric data were used to forward model continuous P‐T‐t paths. P‐T‐t paths are consistent with previous studies and are interpreted to constrain blueschist block exhumation paths within the Baja accretionary wedge. Our P‐T‐t models use temperature controlled ⁴⁰Ar/³⁹Ar step heat data in which argon loss by volume diffusion can be demonstrated, and for which the white mica petrogenesis is known.     

Post-emplacement fluids and pluton thermobarometry: Mount Stuart batholith,Washington Cascades

The effects of post-emplacement infiltration of externally derived, high-temperature fluids into arc-related batholiths are often not well characterized. Such infiltration can have far-reaching effects on the elemental and light isotopic chemistry of a batholith and on its mineral phases. At high temperature, fluid infiltration can be less easily detected, especially if widespread. The Mount Stuart batholith of the Washington Cascades is offered as an example of high-temperature infiltration of high δ¹⁸O fluids derived from its contact aureole. Some of the fluid infiltration coincided with and may have been partly derived from a kyanite-grade, post-emplacement metamorphic event that affected northern portions of the batholith. However, the effects of the fluid infiltration were far reaching and affected the entire margin of the batholith, including southerly portions that did not experience post-emplacement metamorphism. The result led to an oxygen isotopic zonation of the batholith, which is viewed as secondary in origin, with expected effects on mineral chemistry, including derived estimates of thermobarometry, a portion of which is also substantially subsolidus in origin. Our revised emplacement barometry of the Mount Stuart batholith, excluding areas affected by fluid infiltration, demonstrates that it was emplaced at ～350–400 MPa. Soon after emplacement, the batholith was tilted to the north by loading processes and subsequently was righted and unroofed during erosion in the Eocene. Its current near palaeohorizontal orientation has implications for palaeomagnetic studies supporting northward transport (the Baja–British Columbia hypothesis), but these results need further study, given the batholith's complex metamorphic and deformation history and the nature of its dominant magnetic mineralogy (pyrrhotite).     

Mineral chemistry and thermobarometry of Eocene monzogabbroic stocks from the Bafra (Samsun) area in Turkey: implications for disequilibrium crystallization and emplacement conditions

Monzogabbro stocks including felsic enclaves (monzosyenite) around the Bafra (Samsun) area at the western edge of the Eastern Pontides cut Eocene-aged volcanic and sedimentary units. The monzogabbros contain plagioclase, alkali feldspar, clinopyroxene, olivine, hornblende, biotite, apatite, and iron-titanium oxides, whereas the felsic enclaves contain alkali feldspar, plagioclase, hornblende, biotite, clinopyroxene, and iron-titanium oxides. Mineral chemistry data suggest that magmas experienced hydrous and anhydrous crystallization in deep and shallow crustal magma chambers. Several thermobarometers were used to estimate temperatures of crystallization and emplacement for the mafic and felsic magmas. Clinopyroxene thermobarometry yielded 1100–1232 C and 5.9–8.1 kbar for monzogabbros, and 931–1109 C and 1.8–6.9 kbar for felsic enclaves. Hornblende thermobarometry and oxygen fugacity estimates reveal 739–971°C, 7.0–9.2 kbar and 10^?9.71 for monzogabbros and 681–928°C, 3.0–6.1 kbar and 10^?11.34 for felsic enclaves. Biotite thermobarometry shows elevated oxygen fugacity varying from 10^?18.9–10^?11.07 at 632–904°C and 1.29–1.89 kbar for monzogabbros, to 10^?15.99 –10^?11.82 at 719–873°C and 1.41–1.77 kbar for felsic enclaves. The estimated zircon and apatite saturation temperatures are 504–590°C and 693–730°C for monzogabbros and 765–775°C and 641–690°C for felsic enclaves, respectively. These data imply that several phases in the gabbroic and syenitic magmas did not necessarily crystallize simultaneously and further indicate that the mineral compositions may register intervals of disequilibrium crystallization. Besides, thermobarometry contrasts between monzogabbro and felsic enclave may be partly a consequence of extended interactions between the mafic and felsic magmas by mixing/mingling and diffusion. Additionally, the hot felsic magma was close to liquidus conditions (crystallinity < 30%) when injected into cooler mafic magma (crystallinity > 50%), and thus, the monzogabbro stocks reflect hybrid products from the mingling and incomplete mixing of these two magmas.     

Coupled role of deformation and metamorphism in the construction of inverted metamorphic sequences: an example from far‐northwest Nepal

In the Greater Himalayan sequence of far northwestern Nepal, detailed mapping, thermobarometry, and microstructure analysis are used to test competing models of the construction of Himalayan inverted metamorphism. The inverted Greater Himalayan sequence, which is characterized by an increase in peak metamorphic temperatures up structural section from 580 to 720 °C, is divided into two tectonometamorphic domains. The lower domain contains garnet‐ to kyanite‐zone rocks whose peak metamorphic assemblages suggest a metamorphic field pressure gradient that increases up structural section from 8 to 11 kbar, and which developed during top‐to‐the‐south directed shearing. The upper portion of the Greater Himalayan sequence is composed of kyanite‐ and sillimanite‐zone migmatitic gneisses that contain a metamorphic pressure gradient that decreases up structural section from 10 to 5 kbar. The lower and upper portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that spatially coincides with the base of the lowest migmatite unit. Temperatures inferred from quartz recrystallization mechanisms and the opening angles of quartz c‐axis fabrics increase up section through the Greater Himalayan sequence from ～530 to >700 °C and yield similar results to peak metamorphic temperatures determined by thermometry. The observations from the Greater Himalayan sequence in far northwestern Nepal are consistent with numerical predictions of channel‐flow tectonic models, whereby the upper hinterland part evolved as a ductile southward tunnelling mid‐crustal channel and the lower foreland part ductily accreted in a critical‐taper system at the leading edge of the extruding channel. The boundary between the upper and lower portions of the Greater Himalayan sequence is shown to represent a foreland–hinterland transition zone that is used to reconcile the different proposed tectonic styles documented in western Nepal.     

An (in‐)coherent metamorphic evolution of high‐P eclogites and their host rocks in the Chinese southwest Tianshan?

In the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.     

Deformation and fluid flow during orogeny at the palaeo-Pacific active margin of Gondwana: the Early Palaeozoic Robertson Bay accretionary complex (north Victoria Land, Antarctica)

Structural investigations, integrated with X‐ray diffraction, fluid inclusion microthermometry and oxygen‐stable isotope analyses are used to reconstruct the deformation history and the palaeo‐fluid circulation during formation of the low‐grade, turbidite‐dominated Early Palaeozoic Robertson Bay accretionary complex of north Victoria Land (Antarctica). Evidence for progressive deformation is elucidated by analysing the textural fabric of chronologically distinct, thrust‐related quartz vein generations, incrementally developed during progressive shortening and thickening of the Robertson Bay accretionary complex. Our data attest that orogenic deformation was mainly controlled by dissolution–precipitation creep, modulated by stress‐ and strain‐rate‐dependent fluid pressure cycling, associated with local and regional permeability variations induced by the distribution and evolution of the fracture network during regional thrusting. Fracture‐related fluid pathways constituted efficient conduits for episodic fluid flow. The dominant migrating fluid was pre‐to‐syn‐folding and associated with the migration of warm (160–200 °C) nitrogen‐ and carbonic (CO₂ and CH₄)‐bearing fluids. Both fluid advection and diffusive mass transfer are recognized as operative mechanisms for fluid–rock interaction and vein formation during continuous shortening. In particular, fluid–rock interaction was the consequence of dissolution–precipitation creep assisted by tectonically driven cooling fluids moving through the rock section as a result of seismic pumping. The most likely source of the migrating fluids would be the frontal part of the growing accretionary complex, where fluids from the deep levels in the hinterland are driven trough channelization operated by the thrust‐related fracture (fault) systems.     

Carbon isotope exchange during polymetamorphism in the Panamint Mountains, California, USA

Carbon isotope fractionations between calcite and graphite in the Panamint Mountains, California, USA, demonstrate the importance of mass balance on carbon isotope values in metamorphosed carbon-bearing minerals while recording the thermal conditions during peak regional metamorphism. Interbedded graphitic marbles and graphitic calcareous schists in the Kingston Peak Formation define distinct populations on a δ¹³C_(gr)–δ¹³C_(cc) diagram. The δ¹³C values of both graphite and calcite in the marbles are higher than the values of the respective minerals in the schists. δ¹³C values in both rock types were controlled by the relative proportions of the carbon-bearing minerals: calcite, the dominant carbon reservoir in the marble, largely controlled the δ¹³C values in this lithology, whereas the δ¹³C values in the schists were largely controlled by the dominant graphite. This is in contrast to graphite-poor calcsilicate systems where carbon isotope shifts in carbonate minerals are controlled by decarbonation reactions. The marbles record a peak temperature of 531±30 °C of a Jurassic low-pressure regional metamorphic event above the tremolite isograd. In the schists there is a much wider range of recorded temperatures. However, there is a mode of temperatures at c. 435 °C, which approximately corresponds to the temperatures of the principal decarbonation metamorphic reactions in the schists, suggesting that the carbon exchange was set by loss of calcite and armouring of graphite by newly formed silicate minerals. The armouring may explain the relatively large spread of apparent temperatures. Although the modal temperature also corresponds to the approximate temperature of the Cretaceous retrograde event, retrograde exchange is thought less likely due to very slow exchange rates involving well-crystallized graphite, armouring of graphite by silicates during the earlier event, and because of other barriers to retrograde carbon exchange. Thus, only the calcite–graphite carbon isotope fractionations recorded by the marbles demonstrate the high-temperature conditions of the low-pressure Jurassic metamorphic event that was associated with the emplacement of granitic plutons to the west of the Panamint Mountains.