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The thermal diffusivity and thermal conductivity of four natural granitoid samples were simultaneously measured at high pressures (up to 1.5 GPa) and temperatures (up to 988 K) in a multi-anvil apparatus using the transient plane-source method. Experimental results show that thermal diffusivity and thermal conductivity decreased with increasing temperature (<600 K) and remain constant or slightly increase at a temperature range from 700 to 988 K. Thermal conductivity decreases 23-46% between room temperature and 988 K, suggesting typical manifestations of phonon conductivity. At higher temperatures, an additional radiative contribution is observed in four natural granitoids. Pressure exerts a weak but clear and positive influence on thermal transport properties. The thermal diffusivity and thermal conductivity of all granitoid samples exhibit a positive linear dependence on quartz content, whereas a negative linear dependence on plagioclase content appears. Combining these results with the measured densities, thermal diffusivity, and thermal conductivity, and specific heat capacities of end-member minerals, the thermal diffusivity and thermal conductivity and bulk heat capacities for granitoids predicted from several mixing models are found to be consistent with the present experimental data. Furthermore, by combining the measured thermal properties and surface heat flows, calculated geotherms suggest that the presence of partial melting induced by muscovite or biotite dehydration likely occurs in the upper-middle crust of southern Tibet. This finding provides new insights into the origin of low-velocity and high-conductivity anomaly zones revealed by geophysical observations in this region.

Magmatic activity of the Emeishan large igneous province (ELIP) of SW China is one of the most significant geological events of the late Paleozoic. The large volume flood basalts plus rare picrites were erupted in Late Permian. Previous studies indicate that the basalts are the derivatives of primary mantle-derived magma by fractional crystallization, but the depths at which this process took place remain unknown. To answer this question, we use phenocryst compositions and mineral-liquid thermobarometers to determine the P-T conditions of the magma reservoirs where crystallization occurred, then use these data to reconstruct the magma plumbing system of the igneous province. Thermobarometric calculations show that most picrite-hosted clinopyroxene phenocrysts crystallized at ∼25 km and 1200-1280°C, whereas most basalt-hosted clinopyroxene phenocrysts crystallized at depths <20 km and temperatures <1200°C. Some picrites containing primitive olivine with Fo up to Fo92 likely formed by eruption of the most primitive magma with composition similar to the primary magma from the deepest reservoir possibly at the Moho. Parental magmas yield mantle potential temperatures of 1740-1810°C, which are the highest such temperatures yet recorded for terrestrial magmas of any age. Less primitive picrites containing both olivine and clinopyroxene phenocrysts formed by eruption of moderately fractionated magma from a reservoir in the middle crust. Basalts and basaltic andesites formed by eruption of the most fractionated magmas from the reservoirs in the upper crust, coinciding with the depths of coeval sulfide ore-bearing and Fe-Ti-V oxide ore-bearing mafic-ultramafic intrusions. The reason that the Emeishan volcanic sequence is dominated by basalts is because most of the mantle-derived magma was trapped in the middle and upper crusts, undergoing variable degrees of crystal fractionation plus crustal contamination before eruption. Primitive picrites are rare because their eruption requires a trans-lithosphere conduit, which is difficult to create and maintain due to increasing lithospheric pressure with depth. The results from this study reveal that magma reservoirs at the crustal levels play a critical role in magma differentiation in a continental setting.

Magma mixing at arc volcanoes is common, but the manner in which mixing or mafic recharge may trigger volcanic eruptions is unclear. We test ideas of eruption triggering for the 1103 ± 13 years B.P. Chaos Crags eruption at the Lassen Volcanic Center, Northern California. We do so by applying mineral-melt and two-mineral equilibria from mafic enclaves and host lavas from six eruptive units of the Chaos Crags eruption to calculate crystallization conditions. Understanding that Chaos Crags are a type location for magma mixing, we estimate these P-T conditions by employing some apparently new methods to reconstruct pre-eruptive liquid compositions-which can be independently verified using various mineral-melt equilibria. We find that crystallization of \"host\" rhyodacite magmas occurs within the upper crust (at pressures of 0-1.7 kbar) over an approximate 300 °C interval (temperatures ranging from 669-975 °C) and that mafic magmas (which occur as enclaves within the host felsic samples) crystallized over an approximate 250 °C temperature interval (ranging from 757-1090 °C), also within the upper crust, though extending to middle-crust depths (0-3.9 kbar). Notably, both host lavas and mafic enclaves contain crystals that are inherited from their opposing end-member, and both magma types contain plagioclase crystals that appear to have equilibrated with the resulting intermediate composition magmas; these intermediate composition plagioclase crystals indicate that some amount of time passed between both the recharge of magma into a felsic reservoir and the mixing event that caused an exchange of crystals before eruption. We propose that mafic recharge-though it may have been the ultimate triggering event-did not immediately precede any of the eruptive events at Chaos Crags. The most mafic (least mixed) enclaves in our collection are nearly aphyric, indicating that they were likely the first melts to enter the system, and quenched upon intrusion into a cold, upper-crust felsic magma. Many high-T olivine grains in enclaves also coexist with clinopyroxene, plagioclase, and amphibole crystals that crystallized from only slightly more evolved liquids, at temperatures that are low enough (e.g., 800-900 °C) to have possibly quenched earlier-formed, high-T Ol crystals, perhaps negating the use of Ol diffusion profiles as a record of mixing-to-eruption timescales (at Chaos Crags, at least, they would only provide minimum times, which could be orders of magnitude less than actual times). And more crystalline enclaves record more mixing and more cooling. It thus appears that recharge is required to reinvigorate an otherwise dormant Chaos Crags system, as described by Klemetti and Clynne (2014), but ∼250 °C of cooling and crystallization, as recorded by many enclaves, provides the immediate cause of eruption-through increased magma overpressure by the exsolution of a fluid phase and increased buoyancy.

U(-Th)-Pb dating of zircon, monazite, and xenotime from metamorphic and igneous rocks at two outcrops along a north-south transect in the Mount Everest region of southern Tibet provide new constraints on the timing and duration of thermal events associated with channel flow and the ductile extrusion of the Greater Himalayan Series (GHS). At the southernmost outcrop in the Kangshung Valley, Th-Pb ages from monazite indicate that prograde metamorphism associated with crustal thickening following the India-Asia collision occurred at least as early as 38.9±0.9 Ma. A subsequent sillimanite-grade metamorphic event at 28.0±1.2 Ma was followed by two phases of leucogranite emplacement at 20.8±0.8 and 16.7±0.4 Ma. At Thongmön, ∼40 km to the northeast of the Kangshung Valley, prograde metamorphism was occurring at ∼25.4 Ma and lasted until 16.1±0.1 Ma, reaching ∼740°C and 5 kbar at 22.4±0.2 Ma. Immediately following metamorphism, two phases of leucogranite were emplaced at 15.2±0.2 and 12.6±0.2 Ma, with an intervening phase of ductile deformation. These data combined with ages from the Rongbuk glacier and Ama Drime range, north and east of Everest and the North Himalayan Mabja dome 100-140 km to the northeast, suggest that GHS metamorphism lasted ∼20 m.yr. and that migmatization and south-directed channel flow peaked around ∼23-20 Ma and ended by ∼16 Ma. The youngest leucogranites crosscut all ductile fabrics related to the Miocene channel flow.

The onset of the Cossato-Mergozzo-Brissago shear zone within the Strona Ceneri Border Zone in the W-Southalpine basement (Italy) and its role in the collapse of the Variscan crust have been the subject of considerable controversy. A set of new petrographic, geochemical and geochronological data from a suite of syn-kinematic migmatitic paragneiss and amphibolites in between the upper and lower crustal sections of the W-Southalpine basement provide new evidence on the thermo-mechanical role played by the middle crust in the evolution of the Permian Southalpine basement. The petrological investigation of these amphibolite-facies rocks and U–Pb ages from monazite crystals, occurring in distinct microstructural positions, provide new P–T-t constraints on the late-Paleozoic tectono-thermal evolution of the Variscan middle crust. The SCBZ units recorded tectonic events from a possible Early Silurian Cenerian (ca. 440 Ma) overprint onto the proto-sedimentary units of the Southalpine basement to the Mid-Permian (ca. 285 Ma) syn-kinematic partial melting event developed close to the CMB shear zone. Phase equilibria modeling is used to constrain the metamorphic conditions recorded by this section of the Variscan basement. Pressure–temperature ( P–T ) isochemical phase diagrams show that, after the ca. 330 Ma Variscan metamorphic peak at P  ≅ 4 kbar and T  < 630 °C, the SCBZ paragneiss experienced isobaric heating up to 700–720 °C at ca. 285 Ma, which led to the formation of a syn-kinematic partial melting event coeval to the emplacement of the Mafic Complex in the lower Ivrea-Verbano Zone. These new geochronological and petrological constraints on the SCBZ paragneiss seem to corroborate the hypothesis that the transition from the stage of mature Variscan orogen to the stage of its collapse developed in the Permian, at ca. 285 Ma. Thus, we argue that the orogenic collapse was probably driven by the rheological weakening of the mid-crustal SCBZ units induced by their syn-tectonic partial melting and, ultimately, by the coeval thermal perturbation of the crust due to the intrusion of the mafic igneous suite at the crust-mantle boundary. Graphical abstract

Fluids are commonly invoked as the primary cause for weakening of detachment shear zones. However, fluid-related mechanisms such as pressure-solution, reaction-enhanced ductility, reaction softening and precipitation of phyllosilicates are not fully understood. Fluid-facilitated reaction and mass transport cause rheological weakening and strain localization, eventually leading to departure from failure laws derived in laboratory experiments. This study focuses on the Miocene Raft River detachment shear zone in northwestern Utah. The shear zone is localized in the Proterozoic Elba Quartzite, which unconformably overlies the Archaean basement, and consists of an alternating sequence of quartzite and muscovite-quartzite schist. In this study, we characterize fluid-related microstructures to constrain conditions that promoted brittle failure in a plastically deforming shear zone. Thin-section analyses reveal the presence of healed microcracks, transgranular fluid inclusion planes and grain boundary fluid inclusion clusters. Healed microcracks occur in three sets, one sub-perpendicular to the mylonitic foliation, and a set of two conjugate microcracks oriented at ∼40–60° to the mylonitic foliation. Healed microfractures are filled with quartz, which has a distinct fabric, suggesting that microcracks healed while the shear zone was still at conditions favourable for quartz crystal plasticity. Transgranular fluid inclusion planes also occur in three sets, similar in orientation to the healed microfractures. Fluid inclusions commonly decorate grain and subgrain boundaries as inter- and intragranular clusters. Our results document ductile overprint of brittle microstructures, suggesting that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.

Strain localization processes in the continental crust generate faults and ductile shear zones over a broad range of scales affecting the long-term lithosphere deformation and the mechanical response of faults during the seismic cycle. Seismic anisotropy originated within the continental crust can be applied to deduce the kinematics and structures within orogens and is widely attributed to regionally aligned minerals, e. g., hornblende. However, naturally deformed rocks commonly show various structural layers (e.g., strain localization layers). It is necessary to reveal how both varying amphibole contents and fabrics in the structural layers of strain localization impact seismic property and its interpretations in terms of deformation. We present microstructures, petrofabrics, and calculate seismic properties of deformed amphibolite with the microstructures ranging from mylonite to ultramylonite. The transition from mylonite to ultramylonite is accompanied by a slight decrease of amphibole grain size, a disintegration of amphibole and plagioclase aggregates, and amphibole aspect ratio increase (from 1.68 to 2.23), concomitant with the precipitation of feldspar and/or quartz between amphibole grains. The intensities of amphibole crystallographic preferred orientations (CPOs) show a progressively increasing trend from mylonitic layers to homogeneous ultramylonitic layers, as indicated by the J Am index increasing from 1.9–4.0 for the mylonitic layers and 4.0–4.8 for the transition layer, to 5.1–6.9 for the ultramylonitic layers. The CPO patterns are nearly random for plagioclase and quartz. Polycrystalline amphibole aggregates in the amphibolitic mylonite deform by diffusion, mechanical rotation, and weak dislocation creep, and develop CPOs collectively. The polymineralic matrix (such as quartz and plagioclase) of the mylonite and the ultramylonite deform dominantly by dissolution-precipitation, combined with weak dislocation creep. The mean P and S wave velocities are estimated to be 6.3 and 3.5 km/s, respectively, for three layers of the mylonitic amphibolite. The respective maximum P and S anisotropies are 1.5%–6.4% and 1.8%–4.5% for the mylonite layers of the mylonitic amphibolite, and 6.0%–6.9% and 4.5%–5.0% for the transition layers; but for the ultramylonite layers, these values increase significantly to 8.0%–9.1% and 5.1%–6.0%, respectively. Furthermore, increasing strain (strain localization) generates significant variations in the geometry of the seismic anisotropy. This effect, coupled with the geographical orientations of structures in the Hengshan-Wutai-Fuping complex terrains, can generate substantial variations in the orientation and magnitude of seismic anisotropy for the continental crust as measured by the existing North China Geoscience Transect. Thickened amphibolitic layers by extensively folding or thrusting in the middle crust can explain the strong shear wave splitting and the tectonic boundary parallel fast shear wave polarization beneath the Hengshan-Wutai-Fuping complex terrains. Therefore, signals of seismic anisotropy varying with depth in the deforming continent crust need not deduce depth-varying kinematics or/and tectonic decoupling.

The areally extensive (>5000 km 2 ), syn-tectonic, ca. 520 Ma, mainly S-type Donkerhuk batholith was constructed through injection of thousands of mainly sheet-like magma pulses over 20–25 Myr. It intruded schists of the Southern Zone accretionary prism in the Damara Belt of Namibia. Each magma pulse had at least partly crystallised prior to the arrival of the following batch. However, much of the batholith may have remained partially molten for long periods, close to the H 2 O-saturated granite solidus. The batholith shows extreme variation in chemistry, while having limited mineralogical variation, and seems to be the world’s most heterogeneous granitic mass. The Nd model ages of ~2 Ga suggest that Eburnean rocks of the former magmatic arc, structurally overlain by the accretionary wedge, are the most probable magma sources. Crustal melting was initiated by mantle heat flux, probably introduced by thermal diffusion rather than magma advection. The granitic magmas were transferred from source to sink, with minimal intermediate storage; the whole process having occurred in the middle crust, resulting in feeble crustal differentiation despite the huge volume of silicic magma generated. Source heterogeneity controlled variation in the magmas and neither mixing nor fractionation was prominent. However, due to the transpressional emplacement régime, local filter pressing formed highly silicic liquids, as well as felsic cumulate rocks. The case of the Donkerhuk batholith demonstrates that emplacement-level tectonics can significantly influence compositional evolution of very large syn-tectonic magma bodies.

Individual fluid inclusions with dense carbon dioxide hosted in quartz from the gold-bearing interval penetrated by the SD-3 Kola Superdeep Borehole were studied using modern techniques. The composition and density of the carbon dioxide fluid were determined by Raman spectroscopy and microthermometry. The density of the fluid is 0.37–1.14 g/cm3 and contains minor admixtures of nitrogen (0.3–1.8 mol %) and water (0.1–0.4 mol %). LA-ICP-MS data indicate that the carbon dioxide fluid inclusions contain high concentrations of Au (1–2611 ppm) and Ag (1–4389 ppm), and high-precision optical data indicate that the high-density CO2 fluid of the inclusions contains Au–Ag nanoparticles. Evidently, gold and silver were transported from the Earth’s mantle to the crust by high-density carbon dioxide fluid in the form of nanoparticles.