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The Malawi Rift is localized within Precambrian amphibolite‐granulite facies metamorphic belts, bounded by up to 150 km long border faults, and generates earthquakes throughout ∼40 km thick crust. Rift‐related faults are inferred to exploit pre‐existing weaknesses that allow rifting of otherwise dry and strong crust. It is unclear what these weaknesses are, and how localization into weak zones can be reconciled with strength required for lower crustal seismicity. We present results of mineral equilibria modeling, which indicate that Proterozoic metamorphism generated dry crust dominated by a quartz‐feldspar assemblage that is metastable at current conditions. For rift propagation to be possible at current cool thermal gradients and in mechanically strong, dry quartzofeldspathic rocks, mid‐ to lower‐crustal strain must be localized into relatively weak, inherited shear zones that deform primarily by aseismic, viscous creep. These shear zones are embedded within high‐strength crust, and interaction between creeping shear zones and enveloped or surrounding rocks may locally increase stress and trigger frictional, seismic slip at mid‐ to lower‐crustal depths. Over time, this interaction may produce a fracture network that allows infiltration of fluids. We therefore suggest that during rifting of previously deformed and metamorphosed crust, major faults are most likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zones. In this case, fluids may infiltrate and locally weaken metastable lower crust, including allowing time‐dependent fluid‐driven seismicity and local partial melting, but length‐scales of this weakening is limited by the scale of the permeability network. Plain Language Summary: In Malawi, East Africa, the Earth's crust is slowly splitting apart. This "rifting" has two unusual characteristics: (a) the crust is thick and strong and therefore difficult to split apart, and (b) there are earthquakes at greater depth and temperature than typical for the continents. We propose that the reason rifting occurs with these two characteristics, in this location, is that the crust is inherited from past periods of mountain building. Based on thermodynamic principles, specifically what minerals are stable under what conditions, we model the composition of Malawi crust in the geological past. We then compare those results to current conditions. We find that the likely inherited composition of Malawi crust is unstable under current conditions, but needs an addition of water to undergo reactions to new, stable minerals. We also note that deformation is only possible in weak zones inherited from past deformation, and these zones will deform by high‐temperature flow of rock, not generating earthquakes. However, flow of local weak zones may induce fracturing in surrounding harder rocks, generating local earthquakes and a fracture system that water can flow along. Over time, this water infiltration can weaken the crust and ultimately allow continental rifting as seen further north in Africa. Key Points: The Malawi crust is mostly dry, metastable and frictionally strong, and rifting requires viscous flow in inherited lower crustal shear zonesMajor faults are likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zonesViscous flow in heterogeneous crust may trigger lower crustal seismicity generating permeability that allows infiltration of external fluids [ABSTRACT FROM AUTHOR]

Abstract Hydration of the subduction zone forearc mantle wedge influences the downdip distribution of seismicity, the availability of fluids for arc magmatism, and Earth's long term water cycle. Reconstructions of present‐day subduction zone thermal structures using time‐invariant geodynamic models indicate relatively minor hydration, in contrast to many geophysical and geologic observations. We pair a dynamic, time‐evolving thermal model of subduction with phase equilibria modeling to investigate how variations in slab and forearc temperatures from subduction infancy through to maturity contribute to mantle wedge hydration. We find that thermal state during the intermediate period of subduction, as the slab freely descends through the upper mantle, promotes extensive forearc wedge hydration. In contrast, during early subduction the forearc is too hot to stabilize hydrous minerals in the mantle wedge, while during mature subduction, slab dehydration dominantly occurs beyond forearc depths. In our models, maximum wedge hydration during the intermediate phase is 60%–70% and falls to 20%–40% as quasi‐steady state conditions are approached during maturity. Comparison to global forearc H2O capacities reveals that consideration of thermal evolution leads to an order of magnitude increase in estimates for current extents of wedge hydration and provides better agreement with geophysical observations. This suggests that hydration of the forearc mantle wedge represents a potential vast reservoir of H2O, on the order of 3.4–5.9 × 1021 g globally. These results provide novel insights into the subduction zone water cycle, new constraints on the mantle wedge as a fluid reservoir and are useful to better understand geologic processes at plate margins.

Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid‐rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non‐collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non‐collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in‐situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic‐Early Cretaceous, and potentially Late Cretaceous‐Paleocene high‐pressure (HP)–low‐temperature metamorphic sequences. Whole‐rock trace element data and in‐situ B isotopes favor serpentinization by a crust‐derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid‐Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back‐arc basins. Subsequent compressional phases trigger short‐lived subduction in the back‐arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non‐collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back‐arc basins. Plain Language Summary: Serpentinites are metamorphic rock products of fluid‐mediated alteration of the mantle. They occur in the ocean floor and the core of mountain belts resulting from continental collisions after the closure of ancient oceanic basins. However, their origin in non‐collisional mountain belts, such as the Andes, remains unclear. To address this conundrum, we studied serpentinite boulders from five river basins in the Ecuadorian northern Cordillera Real. We found that rocks are composed of the high‐temperature serpentine mineral, while spinel is the only original mineral preserved. River basin analysis and boron stable isotopes indicate four potential sources for the studied rocks, juxtaposed to rocks ranging in age from ∼240 to 55 million. Bulk‐rock chemistry and boron isotopes suggest that the serpentinization was triggered by crustal fluids at depths between 80 and 30 km in a subduction zone environment. Through time, the Andes have been characterized by extensional and compressional tectonic phases. These tectonic scenarios enhance the extraction of rocks at deep sections of the Earth along major faults. We propose that Andean serpentinites are fragments of the Earth's mantle sourced from ancient subduction zones and back‐arc basins. Key Points: Serpentinites associated with HP–LT rocks are common in the Andes, but their origin and tectonic significance are not fully understoodOur results in Cordillera Real serpentinites suggest four sources derived from the mantle wedge and obducted ophiolitesSerpentinites in non‐collisional orogens are exhumed during slab rollback and back‐arc basin closure phases [ABSTRACT FROM AUTHOR]

Abstract The Malawi Rift is localized within Precambrian amphibolite‐granulite facies metamorphic belts, bounded by up to 150 km long border faults, and generates earthquakes throughout ∼40 km thick crust. Rift‐related faults are inferred to exploit pre‐existing weaknesses that allow rifting of otherwise dry and strong crust. It is unclear what these weaknesses are, and how localization into weak zones can be reconciled with strength required for lower crustal seismicity. We present results of mineral equilibria modeling, which indicate that Proterozoic metamorphism generated dry crust dominated by a quartz‐feldspar assemblage that is metastable at current conditions. For rift propagation to be possible at current cool thermal gradients and in mechanically strong, dry quartzofeldspathic rocks, mid‐ to lower‐crustal strain must be localized into relatively weak, inherited shear zones that deform primarily by aseismic, viscous creep. These shear zones are embedded within high‐strength crust, and interaction between creeping shear zones and enveloped or surrounding rocks may locally increase stress and trigger frictional, seismic slip at mid‐ to lower‐crustal depths. Over time, this interaction may produce a fracture network that allows infiltration of fluids. We therefore suggest that during rifting of previously deformed and metamorphosed crust, major faults are most likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zones. In this case, fluids may infiltrate and locally weaken metastable lower crust, including allowing time‐dependent fluid‐driven seismicity and local partial melting, but length‐scales of this weakening is limited by the scale of the permeability network.

Gibbs free energy, the fundamental thermodynamic potential used to calculate equilibrium mineral assemblages in geological systems, does not apply to non‐hydrostatically stressed solids. Consequently, there is debate over the significance of non‐hydrostatic stress in petrological and geophysical processes. To help resolve this debate, we consider the effects of non‐hydrostatic stress on the polymorph pairs kyanite/sillimanite, graphite/diamond, calcite/aragonite, and quartz/coesite. While these polymorphs are most relevant to metamorphic processes, the concepts developed are applicable to any single‐component solid reaction. We quantitatively show how stress variations normal to an interface alter equilibrium temperatures of polymorph pairs by approximately two orders of magnitude more than stress variations parallel to an interface. Thus, normal stress controls polymorph stability to first order. High‐pressure polymorphs are expected to preferentially nucleate normal to and grow parallel to the maximum stress and low‐pressure polymorphs, the minimum stress. Nonetheless, stress variations parallel to an interface allow for the surprising possibility that a high‐pressure polymorph can become more stable relative to a low‐pressure polymorph as stress decreases. The effects of non‐hydrostatic stress on mineral equilibrium are unlikely to be observed in systems with interconnected, fluid‐filled porosity, as fluid‐mediated reactions yield mineral assemblages at approximately constant pressures. In dry systems, however, reactions can occur directly between elastic solids, facilitating the direct application of non‐hydrostatic thermodynamics. Non‐hydrostatic stress is likely to be important to the evolution of metamorphic systems, as preferential orientations of polymorphic reactions can generate seismicity and may influence fundamental rock properties such as porosity and seismic anisotropy. Plain Language Summary: Geoscientists are interested in determining at what temperatures and pressures different minerals are stable in the Earth. This is because the breakdown and formation of minerals are important for processes such as the generation of earthquakes and the geochemical cycling of elements such as hydrogen and carbon. However, a key problem is that the calculations we use to determine if minerals are stable assume that pressure is equal in all directions. But if this were true, there would be no mountains or earthquakes which are caused by unequal pressures deep within our planet. This leaves the question of how do we determine mineral stability when pressure is not equal in all directions? In our work, we show that the pressure on each side of a mineral determines whether each side is stable. This means, for example, if one squeezes graphite hard enough in one direction, the squeezed sides will start to form diamond while the other sides remain graphite. Consequently, when pressure is not equal in all directions, minerals will break down or form in certain directions. This could create cracks for earthquakes, or allow water to enter rocks more easily, influencing subsurface fluid flow and element cycling. Key Points: Normal stress determines chemical potential and consequent phase stability on a given interface to first orderInterface‐parallel stresses can influence phase stability for interfaces near a reaction boundaryNon‐hydrostatic thermodynamics are directly applicable to dry systems undergoing solid‐solid reactions [ABSTRACT FROM AUTHOR]

The Vertiskos‐Ograzden metamorphic complex (VOC; W‐Rhodope Metamorphic Domain, RMD) is a key area for reconstructing the geodynamic history of the Hellenides. We distinguish four metamorphic events: (1) an (U)HP eclogite facies metamorphism, succeeded by partial melting during decompression, amphibolite facies reequilibration in the kyanite stability field, and injection of muscovite pegmatites; (2) a LP‐HT event, associated with partial melting within the stability field of sillimanite, cordierite, and K‐feldspar (andalusite at subsolidus conditions) and magmatic intrusions; (3) a prograde moderate high‐pressure (MHP) metamorphism within the kyanite stability field increasing to the NE from epidote amphibolite to upper amphibolite facies; and (4) in the easternmost VOC, a LP amphibolite facies event (new cordierite and andalusite). Considering existing geochronological data, the VOC formed part of successive tectonic settings: (1) the "Variscan" collision zone between "East‐Avalonian" and Armorican continental fragments; (2) the Triassic middle crust of a continental rift zone, (leading to formation of oceanic crust, in the eastern VOC); (3) late Jurassic‐early Cretaceous closure of the oceanic basin and accretion of the VOC to southern European margin and northeastward polarity of subduction/collision; (4) late Cretaceous extension and crustal thinning. Then, subduction of the Neo‐Tethys and Apulian plate underneath Europe occurred, and the VOC plus the upper units of the RMD were part of the upper (European) plate. Large‐scale north dipping low‐angle normal detachments exhumed the VOC from the vicinity of a magmatic arc presently situated in the north RMD and Sredna Gora and Strandja zones and drag it into its present position. Key Points: The Vertiskos‐Ograzden metamorphic complex (W‐Rhodope Domain, RMD) is a key area for reconstructing the geodynamic history of the HellenidesWe distinguish four metamorphic events reflecting Variscan and Alpine tectonic eventsIn the late Cretaceous, large‐scale north dipping low‐angle normal detachments exhumed the VOC from the vicinity of a magmatic arc [ABSTRACT FROM AUTHOR]

ABSTRACT: The development of metamorphic petrology to metamorphic geology in China has a long history. Ancient basement metamorphic rocks are distributed primarily in the North China Craton, the Yangtze Block and Tarim Craton. They are mainly made up of plutonic gneiss and metamorphosed supercrust rock, transformed to granulite facies through Archean Paleoproterozoic. Many of the Paleoproterozoic metamorphic rocks have undergone high-pressure granulite facies metamorphism with a clockwise metamorphic evolution path. The ultrahigh temperature (UHT) granulites from the Late Paleoproterozoic are found in North China Craton. Many high-precision chronological data have allowed preliminary construction of the formation and evolutionary framework of different metamorphic basements. Primarily there are low-temperature and high-pressure blue schist, high-temperature and high-pressure granulite and ultrahigh-pressure (UHP) eclogite facies metamorphic rocks in the Phanerozoic orogenic belt. The discovery of eclogite in the Sulu orogen and a large quantity of coesite in its country rocks show that there was a deep subduction of voluminous continental materials during the collision process between the Yangtze block and the North China Craton in the Early Mesozoic phase. From the studies of, for instance, organic matter vitrinite reflectance, illite crystallinity, illite (muscovite) polytype and illite (muscovite) b dimension, the Late Paleozoic strata in the eastern region of Inner Mongolia and the north-central region of NE China have only experienced diagenesis to an extremely low-grade metamorphism. The discovery of impact-metamorphosed rocks in Xiuyan area of Liaoning province has enriched the type and category of metamorphic rocks in China. The phase equilibrium method has been widely used in the study of metamorphism of middle and high-grade metamorphic rocks. On the basis of existing geologic surveys and monographic study results, different scholars have respectively compiled 1:1500000 Metamorphic Geological Map and Specifications of Qinghai Tibet Plateau and its Adjacent Areas, 1:2500000 Metamorphic Tectonic Map of China, and the 1:5000000 Metamorphic Geological Map and Specifications of China, among others repectively, which have systematically summarized the research results of metamorphic petrology and metamorphic geology in China.

The metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic section of the sole is provided in this publication: these rocks record valuable information in terms of petrological processes and conditions of accretion, with limited retrogression. The petrology of the metamorphic sole shows that it originates from ocean floor and that it is the equivalent of the Shick-Shock volcanics group, metamorphosed to granulite/higher amphibolite facies. Presence of aluminosilicate bearing metapelites allows constraining pressure conditions in a more robust way than in the case of the famous Semail ophiolite (Oman). Thermobarometric estimates for peak metamorphic conditions for the metamorphic sole of the Mont Albert ophiolitic complex indicate temperatures above 800 °C close to the contact with the overlying peridotite, decreasing to ~650 °C within <30 m, at pressure around 1 GPa. Structurally lower subunits of the sole metamorphosed in the amphibolite/higher greenschist facies retain evidence of basaltic protolith accreted to lower pressures. Evidence of partial melting in high-grade subunits, contradictory with the apparent lack of melting products, may be linked to preferential accretion of melt-poor lithologies to the overlying plate, melt-rich lithologies being too weak to allow coupling and accretion to non-serpentinised peridotite. The reconstructed petrological evolution is consistent with stepwise accretion of metamorphic soles in nascent obduction/subduction systems as proposed for the Semail example and other metamorphic soles worldwide: the subducted oceanic plate is metamorphosed up to granulite facies by heat transfer from the still hot overlying peridotite, and accreted to the base of the future ophiolite when the rheological contrast between the two is minimum. Thermobarometric estimates indicate clock-wise pressure-temperature paths. The inferred thermal evolution requires burial of the metamorphic sole below warm, young (or rejuvenated) oceanic lithosphere, consistent with subduction initiation close to the ridge followed by rapid accretion and exhumation of the metamorphic sole. In this context, exhumation along higher geothermal gradient than burial shows that cooling of the mantle wedge above the plate interface takes place slowly compared to exhumation of the metamorphic sole, even though the plate interface itself has been cooling between burial and exhumation of the rocks. • Mineralogy of the metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé) • Kyanite and sillimanite-beaing metapelites constrain thermobarometry • Clockwise pressure-temperature paths and melting with elusive melt remnants • Petrological model with implications for thermal evolution along plate interface [ABSTRACT FROM AUTHOR]

Abstract Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core (HMC), in order to document and investigate the causes of along-strike variations in its Oligocene–Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya. This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ∼30 Ma to 12 Ma, recording (re-)crystallization between 625 °C and 815 °C. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen. Disparities in the duration and timing of metamorphism within the HMC are best explained by along-strike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes. These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger, thicker crystalline core in the eastern Himalaya. Graphical abstract Image 1 Highlights • Investigation of along-strike variations in the Himalayan mountain belt. • Campaign-style titanite petrochronology study analyzing 47 samples from >2000 km. • Results show eastward younging in titanite ages. • Younger, thicker and hotter core of the mountain belt in the east. • Accretion and duplexing along the basal detachment control along-strike trends. [ABSTRACT FROM AUTHOR]

The Antarctic continent constituted the southwestern margin of Gondwana until its break-up in the early Cretaceous, when new margins were created along the separating fragments of South America and Antarctica, forming the Scotia Arc. In the Jurassic, part of this passive continental margin became active with subduction of oceanic lithosphere, leading to the introduction of ocean floor material into the accretionary wedge accompanied by deformation and metamorphism. One of these margins is preserved in the South Orkney Microcontinent, and crops out at the South Orkney Islands. At Powell Island, situated in the center of the South Orkney Islands, a gradual transition from very low-grade metarenite, interlayered with metasiltite and slate of the Greywacke Shale Formation, in the south, to biotite-garnet schist, in the north, belonging to the Scotia Metamorphic Complex, is present. The metamorphic map presents from south to north a pumpellyite muscovite chlorite zone, a garnet zone, a biotite-garnet zone and an abundant biotite-garnet zone. Thermobarometric calculations yielded for the garnet and garnet-biotite zones temperatures between 498 and 517 °C with pressures of 9–11 kbar and for the abundant biotite-garnet zone temperatures between 522 and 550 °C and pressures between 11.8 and 13 kbar. These results align well with earlier obtained data for the lower grade rocks and confirm the idea of metamorphism in an accretionary wedge. The relatively high-pressure is interpreted to be responsible for the inversion of the biotite and garnet isograds, for the albitic composition of plagioclase and for the relatively Ca-rich garnet. P-T conditions fall in the transitional field between greenschist, amphibolite, blueschist and eclogite facies. • The Antarctic continent constituted the southwestern margin of Gondwana. • At Powell Island, situated in the center of the South Orkney Islands. • At Powell Island showing gradual transition from very low grade rocks, in the south, to biotite-garnet schist, in the north. • Powell and Fredriksen islands are interpreted as exposures of part of the Jurassic accretionary prism. • The thermobarometric calculations for temperature values vary between 498 and 550 °C with pressures of 9–13 kbar. [ABSTRACT FROM AUTHOR]

Ludovic Mrazec is one of the most important Romanian geologists because of his fundamental contributions to mineralogy, petrology and tectonics of the Romanian Carpathians and of the Dobrogea Units. He was the founder of the Department of Mineralogy at University of Bucharest (1894) and the founder and organizer of the Geological Institute of Romania (1906), where he started the activity of drawing up the geological map of Romania, scale 1: 500,000. He had fundamental contributions in deciphering the tectonics of the Romanian Subcarpathians by discovering the phenomenon of diapirism and its role in defining the petroleum structures. In the history of Romanian geology, he is well known for his view of the relations between the two groups of crystalline schists in the Southern Carpathians and East Carpathians, through the theory of organic petroleum origin and by organizing the research activity and capitalizing on the Romanian mineral resources, following the elaboration of the first mine laws (1924). [ABSTRACT FROM AUTHOR]

Syros is part of the Attic–Cycladic blueschist belt and consists principally of marbles, schists, and metabasites that have been metamorphosed to the blueschist- to eclogite-facies at about 52 Ma, with a greenschist-facies overprint beginning at about 25 Ma. Distinct, whitish clusters of sub-millimeter-sized grains organized into orthorhombic polyhedral shapes occur in a variety of the blueschists. These clusters are interpreted to be pseudomorphs after lawsonite based on their shape, chemistry, and the presence of rare relict lawsonite in a few of the clusters. A considerable range in observed pseudomorph modes reflects both variety in rock compositions and the extent of greenschist overprinting. Most pseudomorphs contain epidote-group minerals (clinozoisite/epidote), similar to the composition of ideal lawsonite, except for added Fe and lost water. Many contain white mica (phengite, paragonite, muscovite), which requires the addition of K and/or Na to the original lawsonite. In some cases, the pseudomorph is largely mica, so that Ca must have been lost to an epidote mineral, amphibole, or calcite in the matrix. Phengite in the pseudomorphs typically has a lower celadonite content than phengite in the matrix. It appears that the mica has changed its composition by breakdown in the matrix, K diffusion, and regrowth in the pseudomorph. We propose that lawsonite breakdown must be in part driven by the reaction of lawsonite with the celadonite content of phengitic mica in the matrix, to form celadonite-poor (muscovite-rich) white mica in the pseudomorph. The preservation of the lawsonite shape in the pseudomorphs in weakly deformed rocks appears to be due to the relative immobility of Al during the pseudomorphing reactions, with K, Na, and Fe moving to the pseudomorphs and Ca and H2O leaving. The only source of K in these rocks is phengite, which should become less rich in celadonite with an increase in temperature, needed to drive lawsonite breakdown. [ABSTRACT FROM AUTHOR]

The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure‐temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction. Key Points: Exhumed rocks from subduction zones are on average warmer than models predictions for global subduction zonesArbitrary warming below the forearc makes the models conform with the rock record but conflicts with independent observationsThe discrepancy between the rock record and models is explained by preferential exhumation of oceanic crust under warm conditions [ABSTRACT FROM AUTHOR]

Understanding and quantifying the partitioning of elements at low concentrations is important for petrology, as minor and trace elements are used as tracers of many geological processes. The lattice strain model has consequently been found to be very useful, as it links mineral/melt partition coefficients to the elastic properties of the mineral and to the difference in ionic radius between the trace element and the cation it replaces. However, this model has limitations, particularly in terms of describing crystal strain, due to the form of the equation and from the choice of a hard-sphere model. After a review of the thermodynamics of trace element incorporation into crystal sites and equilibrium between mineral phases, we present classical atomistic modelling using transferable empirical potentials. Following incorporation of one (or more) mismatching element(s), crystal strain appears strongly related to the environment of the site of exchange, with anisotropic, vacancy-rich minerals deforming more than densely-packed minerals, due to anisotropic deformation, rotation and tilting of the surrounding polyhedra. As shown by the computed cation–oxygen length in strained crystal sites, bond strain is smaller than the difference in cation radii, and varies between structures, with densely-packed minerals being less strained. Consequently, computed relaxation energies are smaller in less compressible minerals. This has implications for modelling the partitioning of trace elements, and highlights the limits of using continuum mechanics below crystal cell scale. Neither oxides nor silicates have isotropic elastic properties, and at nanoscale they do not deform like continuous material. Predictions of partitioning between mineral and melt (or fluid) remain hampered by several factors, amongst which are (i) knowledge of the thermodynamics properties of the dissolved species (in melts and fluids); (ii) the precision of estimated defect and strain energies; and (iii) the presence of solid solution in natural systems, even when limited to a few percent. The latter may have orders-of-magnitude effects on the calculated partition coefficients due to interactions between strain fields around minor cations, leading to a chemical mixing regime at low concentration different from solid solutions where phase components are found in high proportions. Partitioning between mineral phases in an assemblage is described with similar equations as for mineral/liquid equilibria. In cases where terms linked to fluid species disappear, such as incorporation with similar substitutions in two minerals, the assumption that crystal strain energy is the preponderant term during cation exchange becomes unnecessary and it is preferable to use defect energies rather than strain energies. Application and discussion of these concepts are presented for mineral/melt partitioning and for partitioning between minerals, using garnet/clinopyroxene equilibria. The advantage of atomistic modelling is that it does not rely on fitting. Agreement with experimental data shows predictive accuracy within an order of magnitude, which is poorer than what may be achieved by fitting the lattice strain model to experimental data, but validating the theoretical approach and sufficient for application to some petrology problems. Improvements of the modelling for better application to minerals like augite involve systematic work on the effect of solid solutions on the strain field around defects and imply solving ordering problems. [ABSTRACT FROM AUTHOR]

The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, blocks ranging from centimeters up to ≥100 m in diameter are surrounded by finer-grained matrix that is derived from the blocks. Blocks throughout the mélange represent a diversity of protoliths derived from basalts, cherts and other sediments, and hydrated mantle, but all contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which material within the mélange was metamorphosed. This apparent uniformity of metamorphic grade contrasts with other mélanges, such as the Franciscan Complex, where coexisting rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutile in samples from across the amphibolite facies mélange of the Catalina Schist was measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks, and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents, between 290 and 720 (±10–40) ppm, are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730 (±2–16) °C at an assumed pressure of 1 GPa. These results are broadly consistent with previous thermobarometric estimates. No systematic distribution of temperatures was found, however. Like other mélange zones, material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a more restricted scale compared to some other localities. Progressive metamorphism of mélange matrix is expected to produce rheologically stiffer matrix minerals (such as amphiboles and pyroxenes) at the expense of weaker matrix minerals (sheet silicates), affecting the overall rheological behavior of the mélange, and dictating the scale of flow. The Catalina Schist amphibolite facies mélange matrix appears to provide a snapshot of hotter, stiffer portions of a subduction interface, perhaps more representative of rheological behavior at depths approaching the subarc than is found in some other exhumed mélange zones. [ABSTRACT FROM AUTHOR]

The estimation of the peak metamorphic temperature by Raman spectroscopy of carbonaceous material ( RSCM) is influenced by several bias sources grouped in measuring conditions, spectral processing and sample heterogeneity. The measuring conditions (selected excitation wavelength) and the operator bias during spectral processing have a pronounced impact on the temperature estimate and thus on the comparability and portability of thermometric data obtained by RSCM. Several calibration lines of RSCM geothermometers are published already, but no standardised approach exists. Samples of carbonaceous material bearing metasediments with well-established metamorphic conditions of the central and western Alps compile a reference series. By applying an automated, iterative and randomised curve-fitting approach, a consistent and user input-independent RSCM geothermometer is presented, which covers peak metamorphic temperatures from ca. 160 to 600 °C. The method is hardware independent because the measuring conditions bias is excluded by the use of the reference series and the automated curve-fitting approach reduces the spectral processing bias effectively, increasing the method's comparability and portability. By distributing the reference series and the automated curve-fitting software, a laboratory will be able to derive a laboratory specific calibration line for the RSCM geothermometer. [ABSTRACT FROM AUTHOR]

The formation of accessory allanite, monazite and rutile in amphibolite-facies rocks across the Barrovian sequence of the Central Alps (Switzerland) was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the Lepontine dome. For each of these minerals a specific approach was adopted for geochronology, taking into account internal zoning and U–Th–Pb systematics. In-situ U–Th–Pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the Lepontine dome. Isotope dilution TIMS dating of rutile returns dates in line with this picture, but is complicated by inheritance of pre-Alpine rutile and possible Pb loss during Alpine metamorphism. Allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet. Prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum T recorded by the rock mineral assemblage. Allanite ages vary from 31.3 ± 1.1 Ma in orthogneisses in the East to 31.7 ± 1.1 Ma for a Bündnerschiefer and 28.5 ± 1.3 Ma for a metaquartzite in the central area, to 26.8 ± 1.1 Ma in the western part of the Lepontine dome. These ages are interpreted to date the main deformation events (nappe stacking and isoclinal deformation of the nappe stack), close to peak pressure conditions. The timing of the thermal peak in the Lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of ~ 620 °C. Monazite in the central area yields an age of 22.0 ± 0.3 Ma, which is indistinguishable from the age of 21.7 ± 0.4 Ma from a metapelite in the western part of the Lepontine dome. In the central area some of the classical kyanite‐staurolite-garnet schists directly underlying the metamorphosed Mesozoic sediments contain monazite that records only a pre-Alpine, Variscan metamorphic event of upper greenschist to lower amphibolite-facies conditions dated at ~ 330 Ma. The new age data provide evidence that nappe stacking at prograde amphibolite-facies conditions and refolding of the nappe stack occurred between 32 and 27 Ma, only a few million years after eclogite-facies metamorphism in the Adula-Cima Lunga unit. Amphibolite-facies metamorphism lasted for about 10 My to ~ 22 Ma, allowing for multiple ductile deformation and recrystallization events. The long lasting amphibolite-facies metamorphism requires fast cooling between 20 and 15 Ma in the Central Alps. This fast cooling was not related to an increase in sedimentation rates in the foreland basins, suggesting that tectonic exhumation was responsible for termination of amphibolite-facies metamorphism in the Lepontine dome. [ABSTRACT FROM AUTHOR]

Rutile and titanite commonly form by replacement of ilmenite in metamorphic rocks. Exhumed orthogneiss from the Western Alps show that titanite is mostly stable below 1 GPa while rutile seems to dominate within rocks recrystallized under higher pressures. We herein investigate phase relationships for four granitic compositions with variable CaO content at medium to high-pressure conditions (0.7-1.6 GPa, 450-650 °C) with a focus on ilmenite breakdown and Ti-bearing species formation. Our piston-cylinder experiments show that, in the investigated P- T range, ilmenite reacts during metamorphism above 1.2-1.4 GPa to form rutile. Below this pressure, titanite is the dominant Ti-bearing species for most granitoid compositions. We also show that the position of this reaction curve is strongly influenced by the whole-rock Ca activity. For low-Ca activities, rutile may be stable down to 0.7 GPa (and below) within ilmenite pseudomorphs while the titanite stability field may extend to pressures >1.3 GPa for Ca-richer compositions. Both species may be stable in one single sample depending on the local Ca activity gradients. The finding of metamorphic rutile within metagranitoids with CaO contents >2 wt% can be considered, under certain conditions, as a reliable indicator of high-pressure metamorphism. This study also highlights the importance of improving our knowledge of the phase relationships between rutile and titanite as a function of P- T- X to better interpret the textural and tectonic history in natural samples as well as the meaning of age values yielded by rutile and titanite geochronometers. [ABSTRACT FROM AUTHOR]

The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al2O3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer. [ABSTRACT FROM AUTHOR]

Cordierite commonly contains H2O and CO2 in the channels formed by its ring structure. In many studies cordierite has been shown to have volatile contents and carbon isotope ratios consistent with high-temperature equilibrium, suggesting preservation of protolith carbon isotope ratios and motivating this survey of carbon isotopes in cordierite CO2. Cordierite CO2 from pelitic country rocks in the Etive aureole have δ13C values of -20.70 ± 1.27‰ (n = 10) that are unaffected by a ca. 150 °C thermal gradient and fluid-saturated and undersaturated regimes. These δ13C values are consistent with expected carbon isotope ratios of organic carbon in protolith sediments. Similar lithologies from the Cooma and Huntly aureoles show more variable behavior in a more limited data set, with some rocks preserving organic carbon δ13C values and others that may have been affected by externally derived fluids. In cordierite-gedrite gneisses, carbon isotopes of cordierite (Crd) are distinct from those of cordierite in pelites; when excluding one outlier new data plus those from the literature average δ13C(Crd) = -12.51 ± 2.45‰ (n = 17). These isotope ratios are higher than those of cordierite in typical metasedimentary protoliths and are similar to published carbon isotope ratios of trace carbonate in altered submarine volcanic rocks, which are likely analogs for protoliths of many cordierite-gedrite rocks. Igneous cordierite from granitic plutons have δ13C = -23.61 ± 2.08‰ (n = 13), which is interpreted as reflecting a magmatic carbon budget dominated by organic carbon from sedimentary source rocks. In contrast, small pegmatites reported in the literature have δ13C(Crd) = -10.20 ± 3.06‰ (n = 6), which may indicate derivation from orthogneiss source materials. These new data show that carbon isotopes in cordierite can be used to help understand the protolith of even carbon-poor metamorphic rocks, and can also shed light on carbon in the sources of magmatic rocks. This latter approach has the potential for helping constrain the source rocks of peraluminous granitoids, which is controversial. Determining the extent to which organic carbon δ13C is preserved in granitoids is important for understanding the deep carbon cycle, and could serve as an important constraint in the search for low-δ13C graphite inclusions in Hadean detrital zircons, which have been reported as a potential biosignature for the early Earth. [ABSTRACT FROM AUTHOR]

The central Appalachian Piedmont lies in the critical juncture between the northern and southern Appalachians, portions of the orogen with distinct middle to late Paleozoic accretionary histories. Orogen-scale compilation maps link the central and southern Appalachians, but until recently, limited geochronological data prevented robust tectonic comparisons between high-grade metamorphic rocks in different parts of the orogen. We report the results of in situ U-Th-total Pb monazite geochronology that date significant deformation and metamorphism as middle Silurian (~425 Ma) through middle Devonian (~385 Ma) and demonstrate the diachronous nature of orogen development. The Rosemont Shear Zone is identified as a major tectonic boundary in southeastern Pennsylvania and northern Delaware separating the rifted Laurentian margin from younger rock units that formed in a magmatic arc setting. The Laurentian margin rocks occur in a series of nappes in which the metamorphic grade decreases from the structurally highest nappe to the lowest. The in situ monazite ages show that maximum temperature in the lowest nappe may have been attained some 15 million years after maximum temperature in the highest nappe. We interpret this to be the result of successive nappe emplacement, with the warmer overriding sheets contributing heat to lower levels. Combining geochronologic and thermobarometric results with the geometry of deformation results in a new picture of the tectonic development of the central Appalachian Piedmont that further links the evolution of the southern and northern Appalachians. For the Laurentian margin rocks, tectonism resulted from the approach and collision of peri-Gondwanan terranes during the Silurian to early Devonian in a dominantly sinistral, transpressive tectonic regime. This portion of the Pennsylvania-Delaware Piedmont inboard of the Rosemont Shear Zone is contiguous with comparable rocks in the southern Appalachians. In contrast, arc-related rock units outboard of the Rosemont Shear Zone experienced primarily thermal metamorphism in the Silurian, while crustal thickening and associated regional metamorphism is middle Devonian in age and likely the result of the accretion of Avalonia during the Acadian orogeny. These arc-related and younger rocks probably originated to the north of their present location as part of the northern Appalachians. They were ultimately emplaced in a right-lateral transcurrent regime sometime after the middle Devonian. Thus, it is in this portion of the central Appalachian Piedmont that the northern and southern Appalachians are joined. [ABSTRACT FROM AUTHOR]

Garnet zonation provides an unparalleled record of the pressure-temperature-time-fluid evolution of metamorphic rocks. At extreme temperature conditions >900 °C, however, most elements preserve little zonation due to intracrystalline diffusional relaxation. Under these conditions, slowly diffusing trace elements including P, Na, and Ti have the best chance of recording metamorphic histories. Here we map dramatic zoning patterns of these elements in subducted high-pressure felsic granulite (Saxon Granulite Massif) and ultrahigh-pressure diamondiferous "saidenbachite" (Saxonian Erzgebirge, Bohemian Massif). The results show that garnet replacement via interface coupled dissolution-reprecipitation can strongly affect garnet compositions in subduction zones and that P, Na, and Ti record burial and exhumation histories that are otherwise lost to diffusion. In these samples, P diffuses the slowest, and Ti the fastest. [ABSTRACT FROM AUTHOR]

The Matala Dome (MD), an ENE-trending structure located at the junction between the Pan-African Lufilian and Zambezi belts, is cored by a Gneiss-Schist Unit with uncertain age overlain by a metasedimentary section (Quartzite-Schist Unit, Marble Unit and Carbonate-Siliciclastic Unit) of the Neoproterozoic to Cambrian Katanga Supergroup. The top of the Katangan stratigraphy is represented by synorogenic sedimentary rocks-Upper Siliciclastic Unit. An early event D1 resulted in the development of shallow-dipping metamorphic foliation S1 and pre- to syntectonic growth of garnet and kyanite in the schists of the Quartzite-Schist Unit. Pseudosections and garnet isopleth modelling on schist from this unit defined the peak metamorphism at P = 7.5-9.3 kbar and T = 620-700 °C. U-Pb detrital zircon dating revealed ca. 2.7 Ga source and a high-grade metamorphism during Pan-African times. The S1 foliation was affected by upright folding F2 with ENE-trending axes and associated subvertical crenulation fabric S2 development. The syn-D2 retrogression in the schists is marked by post-S1 staurolite crystallisation and further by chloritisation followed by sericitisation. The D2 event is interpreted to have exhumed the orogenic middle crust and to be responsible for the domal structure of the MD. Ar/Ar dating of muscovite at 529.3 ± 5.6 to 526.3 ± 6.2 is interpreted to date the exhumation event. D2 is correlated with regional N-S shortening event at ca. 530-520 Ma. Based on the lithology, structural record, and time and facies of the metamorphism, a correlation between the MD and the northern part of the Zambezi Belt is suggested. [ABSTRACT FROM AUTHOR]

Crystallographic orientation relationships (CORs) between mineral inclusions and their hosts could potentially deliver information about inclusion formation processes and conditions. Most previous studies are based on small numbers of analyses. This paper uses EBSD to study host-inclusion CORs in an inclusion-rich Permian metapegmatite garnet (Koralpe region, Eastern Alps, Austria), demonstrating the importance of large data sets and of EBSD in particular for the analysis of CORs. The distribution of measured orientations reflects host garnet point group symmetry for 89% of inclusions analyzed (total N = 530). Each inclusion phase (rutile, corundum, and ilmenite) shows at least three different CORs to host garnet. "Statistical" CORs are introduced to describe distributions of inclusion orientations that have one or two degrees of freedom with respect to the host, but still reflect host crystal symmetry. Two end-member characteristics of statistical CORs are distinguished: rotation and dispersion. Most statistical CORs observed show a mixture of both. Each inclusion phase shows at least one statistical COR. Multiple coexisting CORs and statistical CORs are not restricted to rutile. Re-examination of previous garnet-rutile COR studies in light of the new results indicates that COR information may have been overlooked when using small data sets. Variation in COR parameters correlates with broad differences in assumed metamorphic conditions for new and literature samples, suggesting that petrogenetic information may be available if COR formation can be understood. The favorability of the detected CORs cannot be explained by a simple model involving minimization of misfit between lattice planes, implying that other interface properties or the inclusion formation mechanism are important controls on COR development. [ABSTRACT FROM AUTHOR]

Metamorphic chronology or petrochronology has steadily evolved over several decades through ever improving analytical techniques and more complete understanding of the geochemical and petrologic evolution of metamorphosing rocks. Here, the principal methods by which we link metamorphic temperatures (T) and ages (t) are reviewed, focusing primarily on accessory minerals. Methods discussed include textural correlation, inversion of diffusion profiles, chemical correlation, and combined chronologic and thermometric microanalysis. Each method demonstrates remarkable power in elucidating petrologic and tectonic processes, as examples from several orogens illustrate, but limitations must also be acknowledged and help define future research directions. Correlation methods are conceptually simple, but can be relatively non-specific regarding pressure-temperature conditions of formation. A new consideration of errors indicates that modeling of chronologic diffusion gradients provides relatively precise constraints on cooling rates, whereas models of chemical diffusion gradients can lead to large (factor of 2 or more) cooling rate uncertainties. Although arguably the best method currently in use, simultaneous T-t measurements are currently limited to zircon, titanite, and rutile. Directions for future improvement include investigation of diffusion profiles for numerous trace element-mineral systems using now-routine depth profiling. New trace element models will help improve chemical correlation methods. The determination of inclusion entrapment P-T conditions based on Raman spectroscopic measurement of inclusion pressures (“thermoba-Raman-try”) may well revolutionize textural correlation methods. [ABSTRACT FROM AUTHOR]

The maximum-pressure P T conditions ( P max – T ) and prograde P T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated P max – T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for P max < 2 GPa , where only a few of the highest- T model paths overlap petrologic observations and model averages are 100–300 °C colder than average conditions recorded by rocks. Prograde P T paths similarly indicate warmer subduction than typical models. Both petrologic estimates and models have inherent biases. Petrologic analysis may overestimate temperatures at P max where overprinting occurs during exhumation, although P T paths suggest that relatively warm conditions are experienced by rocks on the prograde subduction path. Models may underestimate temperatures at depth by neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and comparison suggest that exhumed high- P rocks provide a more accurate constraint on P T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models appear to be inaccurate, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid–rock interactions and seismicity within subduction zones. [ABSTRACT FROM AUTHOR]

The Phepane Dome is a circular outcrop of metasedimentary rock within the Eastern Lobe of the Bushveld Complex hypothesized to have formed as a diapir, when underlying wallrock rose into the overlying magma body. Interactions between the metasedimentary rock of the Phepane Dome and the magma of the Bushveld Complex were investigated through measurements of oxygen and lithium isotopic compositions, determination of mineral modes and major-element mineral compositions, cathodoluminescence imaging, and dihedral angle analysis. Evidence from cathodoluminescence imaging and dihedral angle analysis suggest that heat transfer during diapirism caused partial melting and complete recrystallization of the Phepane Dome metasedimentary rock. Oxygen isotope analysis of samples from traverses spanning the contact between metasedimentary and igneous rocks demonstrates that relatively minimal exchange of oxygen (over distance ~4 m) occurred across the contact between the Phepane Dome and the surrounding Bushveld magma. The lithium concentrations and isotopic compositions of metasedimentary rock are significantly different from the associated igneous rocks. Lithium isotope analysis of samples from traverses across the contact demonstrates exchange of Li over somewhat greater distances (~60 m) than oxygen, consistent with evidence that suggests a higher diffusivity of Li than most major elements. Models of oxygen diffusion through intergranular melt and aqueous fluid are used to place maximum constraints on the duration of diffusive exchange across the contact, resulting in estimates ranging from 5 kyr to 5 Myr. These values are consistent with previous estimates of the duration of crystallization of the Bushveld Complex and Phepane diapir development. [ABSTRACT FROM AUTHOR]

Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures. [ABSTRACT FROM AUTHOR]

Models for how shortening is accommodated in the India–Asia collision vary between an end member in which largely rigid blocks are extruded eastwards between lithospheric-scale strike-slip faults and an end member in which a hot, weak mid-crustal layer aids distributed deformation. Here the mode of crustal deformation is evaluated by studying the intersection of two main conjugate strike-slip faults at the west end of the Tibetan plateau. Our field mapping suggests that these faults, the right-lateral Karakoram fault and the left-lateral Longmu–Gozha Co fault, an eastward continuation of the Altyn Tagh fault, may be connected by a large-scale, east-dipping listric normal fault, exposing a wedge of mid-crustal rocks in its footwall. Pseudosection modeling of matrix and porphyroclast rim compositions from footwall metamorphic rocks yield late-syntectonic pressures of 640 ± 100 MPa and temperatures around 600 ± 50 °C . Extensive networks of narrow granitic dikes give U–Pb zircon ages as young as 13.7 ± 0.2 Ma , suggesting that footwall rocks remained hot until the late Miocene and were not exhumed until after this time. We infer >40 km heave across the Angmong fault, and suggest that it absorbs effectively all of the slip across the Longmu–Gozha Co fault (which it appears to truncate), so the Longmu–Gozha Co fault is seemingly confined to the upper crust. Similar mechanical decoupling likely occurs throughout the plateau, with strike-slip faulting in western Tibet limited to the upper, brittle part of the crust. [ABSTRACT FROM AUTHOR]

Subduction generated a NW-trending Andean arc along the Californian margin beginning at ~175 Ma. Coeval high-pressure (HP) transformation of oceanic crust in an east-dipping, subduction zone probably accompanied plate convergence, but recovered Franciscan eclogite and garnet blueschist blocks chiefly possess younger recrystallization ages of ~165-150 Ma. These Jurassic HP metamafic rocks were sequestered in a low-temperature environment well into Cretaceous time. Fragments of actinolitic rinds partially surround many such high-grade blocks. Only slightly younger than the HP metamorphism, these rinds reflect metasomatic exchange between metabasalt and serpentinized harzburgite along the dynamic oceanic crust-mantle hanging wall during storage of the mafic rocks at moderate depth. High-grade tectonic blocks later were brought toward the surface in circulating, low-density, sheared mud-matrix mélange and/or in buoyant serpentinite bodies. Most exotic HP metamafic blocks occur in mélanges of the Franciscan Central Belt, reflecting tectonic insertion within the subduction zone--not near-surface additions to the clastic section. However, rare, high-grade clasts in feebly recrystallized Franciscan conglomerates suggest erosion and sedimentary deposition for some HP blocks. The addition of dense metabasaltic olistoliths to the mid- and Upper Cretaceous section requires that these HP material were carried surfaceward first as tectonic fragments, perhaps immersed in low-density serpentinite or mud-matrix mélange, then eroded and transported into the trench. HP rocks are conspicuously lacking in coeval Great Valley strata. Whatever the origin of particular high-grade rocks, widespread post-depositional shearing has largely obliterated their original natures, but all dense metamafic blocks of Jurassic recrystallization age must have been supplied to the Cretaceous Franciscan accretionary complex by entrainment in a low-density, circulating muddy matrix or serpentinite body. The vast majority of exotic HP blocks resides in Central Belt mélanges, and appears to be tectonic rather than olistostromal in origin. [ABSTRACT FROM AUTHOR]