Lower Crustal Conditions Research Articles

P‐Wave Velocities Across the α → β Quartz Transition at Lower Continental Crust Pressure and Temperature Conditions

AbstractThe quartz α → β transition is a displacive phase transition associated with a significant change in elastic properties. However, the elastic properties of quartz at high‐pressure and temperature remain poorly constrained experimentally, particularly within the field of β‐quartz. Here, we conducted an experimental study on the quartz α → β transition during which P‐wave velocities were measured in‐situ at pressure (from 0.5 to 1.25 GPa) and temperature (200–900°C) conditions of the continental lower crust. Experiments were carried out on samples of microcrystalline material (grain size of 3–6 μm) and single‐crystals. In all these, the transition was observed as a minimum in P‐wave velocities, preceded by an important softening while P‐wave velocities measured in the β‐quartz field were systematically lower than that predicted by thermodynamic databases. Additional experiments during which acoustic emission (AE) were monitored showed no significant peak of AEs near or at the transition temperature. Microstructural analysis nevertheless revealed the importance of microcracking while Electron Back‐Scatter Diffraction (EBSD) imaging on polycrystalline samples revealed a prevalence of Dauphiné twinning in samples that underwent the transition. Our results suggest that the velocity change due to the transition known at low pressure might be less important at higher pressure, implying a change in the relative compressibilities of α and β quartz. If true, the velocity changes related to the α → β quartz transition at lower crustal conditions might be lower than that expected in thickened continental crust.

Influence of high‐strain deformation on major element mobility in garnet: Nanoscale evidence from atom probe tomography

AbstractGarnet is a common rock‐forming mineral that occurs in a variety of rock types and over a wide range of pressure (P)–temperature (T) conditions in the Earth's lithosphere. Because garnet is considered a high‐strength mineral stable across an extensive range of conditions (1–25 GPa, <300–2000°C), it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, garnet is commonly used as a geothermobarometer and geochronometer to provide P–T and timing constraints on tectonic events. Herein, we study garnet from an eclogite facies mylonite (central Australia) to investigate the mechanisms of element mobility during high‐strain deformation under relatively dry, lower crustal conditions. Electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) reveal evidence of crystal plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low‐angle grain boundaries developed over length scales of 20–50 μm in the rims of garnet porphyroclasts. Atom probe tomography (APT) analysis of a low‐angle grain boundary within a highly strained portion of a clast shows Ca enrichment and Mg depletion along dislocations, whereas APT data along the rim of a mostly undeformed clast reveal a homogeneous distribution of garnet major components in the specimen matrix with the exception of Ca, Fe and Mg enrichment within a healed microfracture. The above‐mentioned results suggest that under relatively dry conditions, crystal plasticity enhances bulk element mobility via pipe diffusion, highlighting the importance of deformation‐induced microstructures on element mobility, with important implications for the robust and reliable use of garnet as a petrological tool.

Rheology of hydrated plagioclase at lower crustal conditions: Cataclasis, creep and transformational plasticity

Plagioclase feldspar is one of the main rock-forming minerals of the lower continental crust. Assessing its rheology is therefore of particular importance for understanding the mechanical processes acting in convergence zones, where pressure and strain rate increases are often associated with metamorphic reactions and fluid fluxes. In this prospect, we performed axial shortening experiments on “as-is” (∼0.05 wt% H2O) and water-added (∼0.29 wt% H2O) natural plagioclase samples in a Griggs-type apparatus equipped with an acoustic emission monitoring system. Samples were deformed at ∼ 10−6 and ∼ 10−5 s−1, 700–800 °C, and 1.0 GPa in the plagioclase stability field. Experiments have also been performed at 1.5 GPa in order to study the rheological impact of the breakdown reaction Ca-rich plagioclase + water → zoisite + kyanite + quartz + Na-rich plagioclase. Mechanical data indicate that at the conditions studied, the rheology of plagioclase is strongly affected by water content. Microstructural observations confirm that under dry conditions, plagioclase aggregates deform by cataclastic flow. Under wet conditions, dislocation creep appears to be the dominant deformation mechanism, and plagioclase rheology is best described by the following equation, where strain rate, in s−1, is expressed as:ε·=Aσ3.0exp(−(168kJ/mol)/R/T), where σ is the stress in MPa, R is the gas constant, T is the temperature in K, and the pre-exponential term A=10−3.9MPa−3.0s−1. At 1.5 GPa, the reaction is faster under wet conditions but the rheology observed remains identical to that of wet plagioclase in its stability field. In the low water content experiments, plagioclase breakdown induces transient weakening due to fast zoisite nucleation and significant reaction volume change.

The vesiculated layer in the anomalous diogenite Northwest Africa 12973

AbstractDiogenites, which are part of the howardite–eucrite–diogenite (HED) group, are considered to represent rocks from the lower crust and mantle of a differentiated planetary body, likely the asteroid 4 Vesta. The Northwest Africa 12973 (NWA 12973) meteorite was classified as an anomalous diogenite due to the occurrence of a vesiculated layer. This work reports on the petrographic and geochemical study of two fragments of this meteorite, aiming to better constrain the origin of the vesiculated layer. Whereas the interior of NWA 12973 (here called host) presents the typical characteristics of an olivine diogenite, that is, coarse‐grained pyroxene, olivine ribbons, chromite, and accessory phases, the vesiculated layer presents a fine‐grained pyroxene groundmass with local rounded relics of olivine and interstitial chromite and metal, and is characterized by abundant large vesicles. The contact between the vesiculated layer and the host is sharply defined. The composition of individual minerals does not show any significant differences between the host and the vesiculated layer, suggesting in situ melting. Geothermobarometry indicates a slightly higher crystallization temperature at lower pressure for the vesiculated layer, consistent with melting and crystallization under lower crustal conditions upon exhumation. The trigger for the local melting was likely a large impact event, which was responsible for adiabatic decompression in the central area or deep faulting and frictional melting.

Protracted localization of metamorphism and deformation in a heterogeneous lower-crustal shear zone

Deformation of the continental crust is influenced by the pre-existing structural framework, fluid availability, strain rate, and pressure-temperature conditions. We investigate the evolution of a large (hundreds-of-meters wide) heterogeneous shear zone and associated brittle and ductile deformation structures (Lofoten, Norway) using structural analysis, mineral-chemical and microstructural observations, and U-Pb dating. The shear zone developed through a sequence of metamorphic stages: (1) migmatization and granulite-facies metamorphism, (2) eclogite-facies metamorphism, and (3) amphibolite-facies metamorphism. Stage (1) was related to magmatic activity at lower-crustal conditions prior to 1.7 Ga. Stage (2) likely occurred during the early phases of the collision between Baltica and Laurentia (Caledonian collision). Stage (3) was related to shortening during the main phase of the Caledonian collision (∼430–400 Ma) and was accompanied by hydration of the shear zone. We demonstrate how mechanical heterogeneities influence the deformation style from the centimeter to meter scale. Zones with a pre-existing fabric deformed by ductile shearing and folding, whereas homogeneous, dry rocks fractured. Fractures provided precursors for small-scale shear zones. These contrasting deformation styles occurred repeatedly. Consequently, pre-existing structures define the deformation style and serve as conduits for the channelization of fluids over extended periods of time.

U-Pb ages of Gongga Shan granite, western Sichuan and its implication to the tectonic evolutions of eastern Tibet

The eastern margin of the Tibetan Plateau has given rise to much debate about mechanisms of plateau uplift and evolution and, in particular, the role of the lower crust in crustal thickening. Knowledge of the middle to lower crust conditions is critical for evaluating various models of crustal deformation, but data on crustal evolution through time are lacking. Here, we turn to the Gongga Shan granite, an intrusion along the Xianshuihe fault in easternmost Tibet that directly records local Cenozoic crustal conditions. We present 124 U-Pb samples from the Gongga Shan granite (GSG) that prove that the crust has been stepwise producing partial melt from 56 Ma to 4 Ma. According to the age distribution, the GSG can be separated into four major groups with ages of 4–10 Ma, 12–20 Ma, 25–40 Ma, and 43–56 Ma. Combining the timing information with geophysics and low-temperature thermochronology data, we suggest that events younger than 10 Ma may indicate the onset of recent crustal channel flow in the middle to lower crust. In contrast, the youngest 4 Ma ages indicate the ongoing partial melting of the middle crust. The 12–20 Ma events could be related to an earlier stage of crustal channel flow, consistent with the regional large-scale crustal channel flow in central Tibet.

Strain hardening induced by crystal plasticity: A new mechanism for brittle failure in garnets

Garnet is a high-strength mineral stable across a wide range of pressure and temperature conditions and preserves structures that can consequently be used to understand the evolution of stress within the lower crust. Yet, the deformation mechanisms in the brittle-ductile regime of garnet remain ambiguous. Here, we study garnet porphyroclasts from an eclogite facies mylonite (central Australia) to investigate the mechanisms by which garnet is deformed under relatively dry, lower crustal conditions. Electron backscatter diffraction analysis reveals bands of small, relatively strain-free garnet with scattered orientations, outlined by polygonal to lobate high-angle grain boundaries separating the garnet porphyroclasts. Atom probe tomography of a high-angle grain boundary shows Fe enrichment in the form of planar and equally spaced arrays of nanoclusters. Our data demonstrate Fe segregation along grain boundaries of garnet, resulting in the nucleation of Fe-rich nanoclusters that act as barriers for migrating dislocations which could lead to strain-hardening and eventually facilitate mechanical failure.

The origin of olivine basalts from Medvezhya Mount (Avachinsky group of volcanoes, Kamchatka): The evidence of assimilation of sulfide-bearing cumulates

The role and conditions of liquid immiscibility and crystallization of sulfide phase during evolution of subduction related magmas remains to be a debated topic, which bears relevance to mechanisms of porphyry copper deposit formation and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine – the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol. %) middle-K island arc olivine basalts. Olivine with normal zoning predominate (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt. % and decrease of Fe/Mn from 60 to 35 with change of olivine composition from Fo87.8 to Fo78.2. Olivines of this group contain numerous inclusions of minerals of the spinel group varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures for olivines of the prevailing type are 1036–1241°C, for sulfide-bearing olivines – 1010–1062°C. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong oxidation of the magmas. On the contrary, the zoning and composition features of sulfide-bearing olivine suggest its xenogenic origin and probable crystallization at conditions of deep crust from low temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.

Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites

High-conductivity layers of 0.1–1 S/m distributed in the mid-lower crust of the Tibetan Plateau are traditionally explained as the fluid/melt related crustal flow. Dehydration of amphibole-bearing rocks may play an important role in explaining these anomalies. To survey the anomalies' origin, the electrical conductivities of amphibole-bearing samples are measured at 1.5 GPa and 600–1300 K. Our experiments show that dehydration melting of amphibole occurs at about 1100 K. Before dehydration melting of amphibolites, proton conduction together with small polaron conduction dominate the conduction process, whereas ionic conduction plays a more important role after the dehydration melting of amphibolites. The dehydration melting of polycrystalline amphibole yields a conductivity of up to 1 S/m under the lower-crust conditions beneath the Tibetan Plateau. Combining the calculated thermal gradient in the crust of Tibetan Plateau with the petrology proof and seismic data, dehydration melting of garnet amphibolites likely contributes to the high conductivity anomalies within the Tibetan crust. MW and cube model simulation indicates that the melt fraction is 3.8–36 vol% in the partial molten region.

Role of acidic fluids in Earth’s deep lithosphere: Insights from the Neoarchean magmatic roots of the Nilgiri Block, southern India

Fluids play a major role in facilitating igneous/metamorphic processes in the Earth’s crust and mantle. In this study, we investigate the nature and composition of fluids in Earth’s interior by studying the lower crustal rocks. We compare accessory minerals (e.g., apatite, monazite, allanite, and titanite), their texture, mineral reactions and composition among regionally distributed metamorphosed mafic and felsic rocks representing the roots of Neoarchean arc magmatism from the Nilgiri Block of the Southern Granulite Terrane in India. Regional trends in accessory minerals show the formation of monazite, allanite, and titanite in the felsic rocks. Apatite is depleted in REEs in all the rock types, irrespective of the difference in their whole-rock chemistry. Textural features and mineral reactions show that these accessory minerals were affected by fluids present in the lower crustal conditions. By comparing our results with those from previous experimental results, we further show that acidic CO2-H2O-HCl-HF fluids stable in lower crustal conditions could have resulted in these chemical and textural features. Dielectric constant of water is high (10–35 compared to lower crustal conditions) in high-pressure and low-temperature conditions of subduction zones and the upper mantle. Such conditions would enhance dissociation of HCl (compared to lower crust) and result in acidic fluids during dehydration reactions in subduction zones and in the upper mantle. Our results have important implications in understanding the nature and composition of fluids in Earth’s interior and would be helpful to model the tectonic and deep geochemical processes in both early and modern conditions in planetary interiors.

Thermobaric Conditions for Exhumation of Ti-clinohumite Garnetites of the Kokchetav Subduction-Collision Zone (Northern Kazakhstan)

Abstract —A new variety of Ti-clinohumite garnetites (‘garnet peridotites’) of the diamond-bearing Kumdy Kol terrane in the Kokchetav massif has been studied. An ultrahigh-pressure phase, Ti-chondrodite, orthopyroxene and contrast-zoned garnets of retrograde zoning have been found in these rocks for the first time. Ti-chondrodite and orthopyroxene were not described in these rocks earlier, and garnet composition was supposed to be homogeneous. Based on the character of chemical zoning of garnet and orthopyroxene, the peak and three retrograde parageneses for these minerals have been distinguished. Using the garnet–orthopyroxene geothermobarometry, for the Kumdy Kol terrane, based on the quantitative P–T estimates of Ti-clinohumite garnetites, a detailed retrograde P–T trend has been constructed for the first time; this trend includes the UHP peak stage (55–57 kbar, 1075–1120 °С), and three stages of retrograde metamorphism under UHP (32 kbar, 830 °С), HP (21 kbar, 750 °С), and MP (12 kbar, 740 °С) conditions. For the Kumdy Kol terrane this is the most detailed Р–Т trend indicating subisothermal decompression during crustal rock exhumation from the mantle depths (~170 km) to the lower-crust conditions (~36 km). The above-mentioned mineralogical findings allowed reconstructing all the described metamorphism stages, using the same rock and various generations of the same minerals (garnet and orthopyroxene) formed at different stages of tectono–metamorphic evolution of the Kumdy Kol terrane. Preservation of the relics for mineral zoning of the early metamorphic stages seemed to be due to high uplift velocities resulted in rapid decrease in P–T parameters and short duration of regressive metamorphism. The virtually complete identity of the obtained P–T trend for the Kumdy Kol terrane and the P–T trend for the Barchi terrane supports the idea of the common tectono–metamorphic evolution of the whole western (diamond-bearing) part of the Kokchetav massif.

Sulfur dissolution capacity of highly hydrated and fluid-saturated dacitic magmas at the lower crust and implications for porphyry deposit formation

Arc basalts with high H2O content are expected to evolve into highly hydrated felsic melts saturated with aqueous fluids at lower crustal pressures, which are potentially important in providing sulfur (S) and metals for porphyry deposit formation. However, the role of such highly hydrated and fluid-saturated magmas in the transfer of S (and metals) remains rarely studied. To investigate the S dissolution capacity of highly hydrated felsic magmas saturated with fluids at the lower crustal conditions, this study performed experiments with dacite + 10 wt% FeS at H2O contents of 10, 15, and 20 wt%. All the experiments were performed in piston-cylinder apparatus at 1.0 GPa and 950 °C with fO2 conditions imposed by the following solid-state buffers: cobalt-cobalt oxide (CCO), nickel-nickel oxide (NNO), rhenium-rhenium oxide (RRO), MnO-Mn3O4 (MMO), and hematite-magnetite (HM), resulting in a fO2 range of FMQ−1.5 to FMQ+3.3 log units (FMQ = fayalite + magnetite + quartz oxgen buffer). The experimental results show that the H2O content in the felsic melt, in addition to fO2 and melt composition, has a strong effect on the concentration of S in the silicate melt at sulfide ± anhydrite saturation. At a given fO2, the S concentration in the melt increases with increasing H2O content (hydration), resulting in a S concentration of up to ∼8000 ppm in the highly hydrated dacitic melts at fO2 ∼ FMQ+2. We found that high melt hydration at an intermediate fO2 not only dramatically increases sulfide and anhydrite solubilities in the dacitic melts but also promotes the saturation of S-rich fluids. The S concentration in the fluid that corresponds to the highest measured S concentrations in the melt (∼8000 ppm) are ∼5 wt%. These results show that highly hydrated felsic magmas saturated with fluids have a high capacity for S dissolution. Therefore, injection of such fluid-saturated magmas from lower crustal to shallow magma reservoirs is a potential mechanism for the effective transport of S (and perhaps metals) to facilitate subsequent porphyry ore deposit formation.

Pristine microstructures in pseudotachylytes formed in dry lower crust, Lofoten, Norway.

Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved.This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Episodic deformation and reactions in mylonitic high-grade metamorphic granulites from Dronning Maud Land, Antarctica

Microfabrics of mylonitic orthopyroxene-bearing quartzofeldspatic granulites from central Dronning Maud Land, Antarctica, have been investigated to evaluate the deformation behaviour at lower crustal conditions. The microfabric is characterized by a fine-grained matrix of dispersed feldspar, quartz and orthopyroxene, which contains monophase elongate quartz and feldspar aggregates, so-called ribbons, and elongate monocrystalline orthopyroxene ribbon grains. The observed crystallographic preferred orientation (CPO) of orthopyroxene ribbon grains that contain clinopyroxene exsolution lamellae in (100) and kink bands in equidimensional porphyroclasts indicate (100)[001] dislocation glide. Fine-grained matrix orthopyroxene formed by deformation of porphyroclasts and growth in strain shadows during diffusional creep, indicating a separate stage of deformation and reaction. Although dislocation creep of coarse ribbon quartz is indicated by sutured grain boundaries at high angle to the ribbon boundary and few subgrains, the weak CPO is rather unspecific. Fine-grained matrix quartz shows no indication of dislocation creep. We suggest that epitactic growth during quartz-producing mineral reactions played a major role for ribbon formation during diffusional creep. Our study demonstrates the importance of episodic deformation and metamorphic reactions in lower continental crust with transient high stresses allowing for dislocation glide of orthopyroxene enclosed in a fine-grained polyphase matrix undergoing diffusional creep on long term.

Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes

AbstractRecycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and, as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0·94, Mg# olivine = 0·88), high NiO in olivine (∼0·26 wt %) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs SiO2 wt %, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure–temperature at 2·6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE and Ce enrichments and strong Nb and Zr depletions. The trace elements and the pressure–temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

Eight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100 ∘ C, to 800 ∘ C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P- and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes.

Generation of the 105–100 Ma Dagze volcanic rocks in the north Lhasa Terrane by lower crustal melting at different temperature and depth: Implications for tectonic transition

AbstractSuccessively erupted intermediate-felsic rocks with variations in their geochemical compositions indicate physical changes in lower-crust conditions, and the variations can provide important insights into the regional tectonic setting. What triggered the late Early Cretaceous tectonic transition of the central-north Lhasa Terrane remains controversial, hindering the understanding of the mechanisms behind the formation of the central Tibetan Plateau. The sodic Dagze volcanic rocks in the north Lhasa Terrane are characterized by high contents of SiO2 and Na2O, low contents of MgO, Fe2O3, and K2O, and low values of Mg#. However, the trace element compositions of the whole-rocks and their zircons allow the rocks to be divided into two groups. The Group I rocks (ca. 105 Ma) have higher contents of Sr and Ba, higher Sr/Y and La/Yb ratios, and lower contents of Y, Yb, Ti, and Zr than Group II rocks (ca. 100 Ma). Besides, the zircons from Group I rocks have higher values of Yb/Gd and U/Yb, lower values of Th/U, and lower Ti contents than the zircons from Group II rocks. However, the rocks of both groups have identical depleted whole-rock Sr-Nd and zircon Hf isotope values. The geochemical data indicate that rocks of both groups were generated by partial melting of a juvenile lower crust, but the differences in the two groups reflect a transition from deep-cold melting to relatively shallower-hotter melting in the period from ca. 105 to 100 Ma. This transition was synchronous with the rapid cooling of granitoids, topographic uplift, and the shutdown of magmatism in the central-north Lhasa Terrane, and followed by sedimentation and the resumption of magmatism in the south Lhasa Terrane. The above observations collectively indicate that the central-north Lhasa Terrane was under an extensional setting in late Early Cretaceous, and we tentatively suggest that it was in response to lithospheric drip during roll-back of the northward-subducting Neo-Tethyan oceanic plate.

Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

斜長石の高温細粒化と組成累帯構造：2. 等温減圧過程での複変成作用による組成累帯構造の形成

斜長石の高温細粒化と組成累帯構造：2. 等温減圧過程での複変成作用による組成累帯構造の形成

Petrology and geochemistry of the 2014–2015 Holuhraun eruption, central Iceland: compositional and mineralogical characteristics, temporal variability and magma storage

The 2014–2015 Holuhraun fissure eruption provided a rare opportunity to study in detail the magmatic processes and magma plumbing system dynamics during a 6-month-long, moderate- to large-volume basaltic fissure eruption. In this contribution, we present a comprehensive dataset, including major and trace elements of whole-rock and glassy tephra samples, mineral chemistry, and radiogenic and oxygen isotope analyses from an extensive set of samples (n = 62) that were collected systematically in several field campaigns throughout the entire eruptive period. We also present the first detailed chemical and isotopic characterization of magmatic sulfides from Iceland. In conjunction with a unique set of geophysical data, our approach provides a detailed temporal and spatial resolution of magmatic processes before and during this eruption. The 2014–2015 Holuhraun magma is compositionally indistinguishable from recent basalts erupted from the Barðarbunga volcanic system, consistent with seismic observations for magma ascent close to the Barðarbunga central volcano, followed by dyke propagation to the Holuhraun eruption site. Whole-rock elemental and isotopic compositions are remarkably constant throughout the eruption. Moreover, the inferred depth of the magma reservoir tapped during the eruption is consistently 8 ± 5 km, in agreement with geodetic observations and melt inclusion entrapment pressures, but inconsistent with vertically extensive multi-tiered magma storage prior to eruption. The near constancy in the chemical and isotopic composition of the lava is consistent with the efficient homogenization of mantle-derived compositional variability. In contrast, occurrence of different mineral populations, including sulfide globules, which display significant compositional variability, requires a more complex earlier magmatic history. This may include sampling of heterogeneous mantle melts that mixed, crystallized and finally homogenized at mid- to lower-crustal conditions.