Pressure-temperature-time Paths Research Articles

Pressure–temperature–time path of ultrahigh-pressure metamorphic terranes constrained by zircon petrochronology: A case study of orthogneisses from the northern Sulu belt, China

Reconstruction of pressure–temperature–time (P–T–t) path of volumetrically-dominant orthogneisses in deeply subducted continental crusts is challenging due to their simple amphibolite-facies mineral assemblages. To provide new insights into this issue, a detailed study of mineral inclusions, trace elements and U-Pb ages within robust zircon containers, together with titanite U-Pb dating and petrology were conducted on ultrahigh-pressure (UHP) orthogneisses from the northern Sulu belt. The following P–T–t conditions of three metamorphic stages were constrained: (i) cathodoluminescence (CL)-grey zircon cores (ca. 238–237 Ma) with flat heavy rare earth element (HREE) patterns include garnet, phengite and omphacite that yield estimated P–T conditions of 2.3–2.8 GPa/730–800 °C, corresponding to prograde high-pressure (HP) metamorphism before the UHP peak. (ii) CL-dark zircon mantles (ca. 224 Ma) contain multiphase solid inclusions of plagioclase + K-feldspar + quartz and show extremely low Th/U ratios (≤0.08) and negative Eu anomalies, demonstrating their crystallization from hydrous melt at 2.0–1.8 GPa/890–820 °C during HP eclogite-facies exhumation. In addition, titanite grains are characterized by high LREE contents (1939–5239 ppm), Th/U ratios (0.37–0.81) and crystallization temperatures (830–801 °C), suggesting their peritectic origin. The lower intercept age (220 ± 4 Ma) of titanite is broadly consistent with the ages of the zircon mantles, indicating that they both crystallized from the melt at the same stage. (iii) CL-bright zircon rims (ca. 212–211 Ma) preserve low-pressure mineral inclusions of plagioclase, K-feldspar and quartz, and they show low Th/U ratios (0.03–0.12) and steep HREE patterns with negative Eu anomalies, witnessing amphibolite-facies overprinting at 1.0–0.8 GPa/610–550 °C. Combined with the metamorphic peak conditions of 3.2 GPa/840 °C at ca. 232 Ma constrained by previous studies from the same area, the P–T–t path of the northern Sulu UHP belt is reconstructed. The data reflects two-stage exhumation of this terrane, including an early-stage (5.8 km/m.y.) and a later-stage slower (3.1 km/m.y.) exhumation processes. The faster exhumation during early decompression was probably enhanced by extensive melting activity recorded by the CL-dark zircon mantles from the orthogneisses.

Deformation-enhanced recrystallization of titanite drives decoupling between U-Pb and trace elements

Titanite is a common accessory mineral that is useful in determining both age (U-Pb isotopes) and pressure-temperature (P–T) conditions (trace-element composition: Zr, rare earth elements (REE)). However, titanite has a propensity to recrystallize during metamorphism, fluid flow, and deformation, which can result in modifications to its isotopic and trace-element compositions. This modification has implications for the interpretation of titanite dates and the evaluation of pressure–temperature–time paths. The impact of deformation and recrystallization on trace-element mobility in titanite is investigated through microstructural and compositional mapping of titanite crystals from a sheared orthogneiss within an ultrahigh-pressure domain of the Western Gneiss Region (WGR), Norway. Results show that optically coherent titanite single crystals deformed in the dislocation creep regime and recrystallized by the process of grain-boundary migration, forming aggregates of titanite grains. Some of the aggregate grains record Caledonian-exhumation dates, whereas others have an inherited isotopic composition. Individual grains within the aggregate, regardless of their U-Pb isotopic composition, contain patchy zoning that formed during syn- to post-recrystallization fluid alteration and that is characterized by generally decreasing Ca and Ti and increasing Al and Fe from cores to rims. However, Zr and Sr concentrations are broadly zoned with respect to the long axis of the host crystal, without regard for the aggregate grain boundaries. REE do not show any obvious correlation with microstructure or age. These results indicate that many trace elements in titanite are unaffected by multi-stage, deformation-driven recrystallization; in contrast, Pb is variably mobile in these deformed titanite crystals. The combination of microstructural and high-spatial resolution geochemical and isotopic data reveals the potential extent of decoupling between the U-Pb isotopic system and the behavior of trace elements as pressure–temperature conditions change through time.

Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.

Olivine melting at high pressure condition in the chassignite Northwest Africa 2737

Shock-induced melting textures and assemblages in the chassignite Northwest Africa (NWA) 2737 were investigated. NWA 2737 consists mainly of coarse-grained olivine (Fa19.6–20.5) and minor amounts of low-Ca pyroxene, plagioclase (now maskelynite), and chromite. Several melt inclusions formed through pre-igneous activity are enclosed in the olivine grains, and some melt inclusions were melted again by the impact event. Several olivine fragments are entrained in the melt inclusion and dissociated into spherulitic ferroan–periclase and interstitial fine-grained clinopyroxene assemblages. The olivine fragments were melted once by the shock metamorphism, and ferroan–periclase crystallized from the olivine melt. We expect that bridgmanite crystallized from the residual melt and inverted to clinopyroxene through the subsequent high-temperature event, although we cannot rule out the possibility that the ferroan–periclase and clinopyroxene assemblage is the partial melting product of olivine. Parts of the melted olivine in the melt inclusion are mixed with the glass (having low-Ca pyroxene + a small amount of silica components) filling the matrix portion of the melt inclusion. Then, olivine, clinopyroxene, and tridymite crystallized from the mixed melts. We infer that the olivine–clinopyroxene–tridymite assemblage is the crystallization product from the mixed melts during rapid cooling at low- or ambient-pressure conditions. Olivine grains adjacent to the melt inclusion dissociated to clinopyroxene + ferroan–periclase assemblage. As the distance from the melt inclusion increased, polycrystalline olivine crystal assemblage formed. We expect that the polycrystalline olivine assemblage is the remnant of ringwoodite: metastable ringwoodite formed in the olivine grain by the shock metamorphism, and back transformed into olivine again through high temperature and low pressure conditions. Mineral assemblages in and adjacent to the melt inclusion show evidence for high pressure and high temperature conditions and for low pressure and high temperature conditions. The pressure–temperature–time path recorded in NWA 2737 is not simple, i.e., the ejection path from the impact site is complex.

Pressure–temperature–time path of a metapelite from Mefjell, Sør Rondane Mountains, East Antarctica

Pressure–temperature–time path of a metapelite from Mefjell, Sør Rondane Mountains, East Antarctica

From seafloor spreading to obduction: Jurassic–Cretaceous evolution of the northern branch of the Neotethys in the Northeastern Anatolian and Lesser Caucasus regions

AbstractWe present arguments for an innovative tectonic set-up just prior to the Northern Neotethys obduction event in the NE Anatolian and Lesser Caucasus area. Along the Northern Neotethyan suture (the Ankara–Erzincan–Amasia–Sevan–Akera suture zone), relicts of the northern branch of the Neotethys oceanic domain outcrop as preserved unmetamorphosed slivers obducted over the northern edge of the South Armenian Block (SAB) and Taurides–Anatolides Platform (TAP) margins. Recent studies have shown that the ophiolitic bodies are formed of similar lithologies of Middle Jurassic age, all bearing mid-ocean ridge basalt chemical compositions enriched in large ion lithophile elements. This extensive database supports a model in which these ophiolites are derived from a single obducted nappe. This model is supported by the metamorphic pressure–temperature–time paths of the sole lithologies under the outcrops of the suture zone ophiolites. Palaeontological dating of sediment deposits directly under or sealing the obduction contact also support this model by temporally linking the emplacement of distant ophiolite outcrops. General emplacement during early Late Cretaceous time has been determined. A south-dipping subduction under the SAB shortly predating obduction has recently been proposed from the metamorphic and magmatic evolution preserved in the SAB crystalline basement, founding a model featuring opposite-direction subduction from at least late Middle Jurassic to Early Cretaceous times. The emplacement of alkaline pillow basalts directly on the oceanic crust is dated as Early to mid-Cretaceous. These dates argue the existence of abnormal mantle heat flows which may be responsible for a decrease in the density of the 80 Ma-old oceanic lithosphere prior to its obduction onto the SAB–TAP. We present a detailed review of recent data to further constrain the structural and geodynamic evolution of this sector and to define the tectonic set-up just prior to the obduction event.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Garnet oxygen analysis by SHRIMP-SI: Matrix corrections and application to high-pressure metasomatic rocks from Alpine Corsica

Garnet is a key mineral used to constrain pressure, temperature and age of metamorphic rocks. This contribution reports oxygen isotope measurements in garnet using the SHRIMP-SI ion microprobe. The reproducibility of oxygen isotope analyses on garnet standard UWG2 is ~0.3–0.4‰ (2σ) within and across sessions. The correlation between oxygen isotope measurements and the grossular and andradite components in garnet fits a second-degree polynomial with a maximum bias in δ18O of 2.4 and 8.3‰, respectively. This bias is similar to that determined for other large ion microprobes. Analysis of two additional Mn-rich garnet crystals allowed identification of a separate bias caused by the spessartine component, which can reach a maximum of 2.3‰.The standardisation and correction scheme proposed in this study are applied to garnet crystals from two samples from Alpine Corsica in order to link fluid evolution with the pressure–temperature–time path. The samples have experienced a polyphase metamorphic history, which includes Permian high temperature metamorphism, followed by late However, 92-W2 was re-analysed because of the variability observed in the previously published δ18O values (-0.29 and 0.53‰, Kohn and Valley, 1998). The δ18O value found in this study (0.81 ± 0.44‰, 2s, Table 1) agrees within error with the highest value previously published for this garnet. Eocene high pressure–low temperature metamorphism. Permian Ca-poor garnet cores have high δ18O values (9.9±0.6‰ and 11.1±0.5‰ 2σ, in two samples) with respect to garnet mantles (7.2±0.4‰) and rims (5.4±0.5‰ and 2.2±0.4‰). The dramatic decrease in δ18O from Permian garnet cores to Alpine rims in both samples reflects a combination of external fluid influx and change in sample mineralogy. The low δ18O of the garnet rims that formed at eclogite facies conditions indicates that the metasomatic fluid equilibrated with mafic or ultramafic rocks. This study illustrates that fluid-mediated mass transfer during subduction occurred at lithological contacts between felsic and ultramafic rocks.

Monazite ages and pressure–temperature–time paths from anatectites in the southern Ruby Range, Montana, USA: evidence for delamination, ultramafic magmatism, and rapid uplift at ca. 1780 Ma

The Wolf Creek ultramafic bodies of the southern Ruby Range, southwest Montana, intruded as high-temperature magma during the late stages of an Early Proterozoic orogeny affecting the Great Falls Tectonic Zone of the western Wyoming craton. Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected metapelitic anatectites adjacent to the ultramafic bodies. Leucosomes are mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline, and biotite that was partially resorbed by the melt-producing reaction. Late cordierite and sillimanite replace garnet in the migmatites and other pelitic gneiss in the area. Petrogenic grids for the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system indicate that conditions during anatexis were 750–800 °C, 600–900 MPa. Anatexis occurred during deformation of metapelites and associated rocks adjacent to the larger ultramafic bodies, but crystallization and cooling occurred after deformation, as indicated by the preservation of igneous textures and mesoperthite with undeformed lamellae in the leucosome. Element mapping of monazite using electron-probe micro-analysis (EPMA) identifies four distinct chemical domains of monazite growth during the orogenic event in addition to relict, Archaean to earliest Proterozoic cores. U–Th–Pb dating of the domains establishes two periods of growth of matrix monazite in the paleosome that indicate fabric-forming prograde metamorphism at 1829 ± 17 and 1805 ± 4 Ma (2σ). Coarser, igneous monazites from the leucosome also contain two distinct chemical zones visible in yttrium maps, indicating growth with garnet and then during garnet resorption as decompression led to replacement of garnet by cordierite. The domain ages cannot be statistically distinguished. Together they yield an age of 1782 ± 9 Ma (2σ). The data imply that regional thickening leading to upper amphibolite facies metamorphism occurred prior to 1800 Ma. Thickening was followed after ∼25 Ma by the intrusion of ultramafic magmas, contact anatexis, and then decompression. This sequence is consistent with mantle delamination after orogenic thickening.

Garnet growth during crustal thickening in the Cascades Crystalline Core, Washington, USA

Garnet Sm–Nd and zircon U–Pb ages, and pressure–temperature–time paths elucidate Late Cretaceous crustal thickening which occurred within magmatic arc rocks of the Insular Superterrane. Voluminous tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into upper crustal sedimentary rocks between 96 and 91 Ma, with initial intrusion prior to garnet growth in the metasedimentary rocks. Arc plutonism then shifted northward as crustal thickening commenced. Initial garnet growth, locally with kyanite and staurolite replacing andalusite, at c. 91 Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith, within the Nason Ridge Migmatitic Gneiss. Subsequent heating and garnet growth, which postdates emplacement of large plutons, occurred between 88 and 86 Ma. This late garnet growth occurred at pressures of 6–8 kbar. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. However, later widespread garnet growth at higher pressure probably resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening. We conclude that metasedimentary rocks outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and that the growth of garnet lasted <6 Myr. Heating to temperatures that stabilized garnet after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge (Chiwaukum Schist) which was tectonically thickened and/or overthrust causing loading and thermal relaxation.

Melting of the continental crust during orogenesis: the thermal, rheological, and compositional consequences of melt transport from lower to upper continental crustThis article is one of a selection of papers published in this Special Issue on the the themeLithoprobe—parameters, processes, and the evolution of a continent.

The formation and differentiation of the continental crust occurs at convergent plate margins in accretionary and collisional orogenic belts where sufficient heat is generated to achieve high-grade metamorphism and anatexis. Volumetrically significant H2O-present melting requires an influx of aqueous fluid along zones of high-strain deformation or via fracture networks, or recycling of the fluid dissolved in melt via melt migration and fluid exsolution during crystallization. In contrast, in “dry” crust, melting occurs via hydrate-breakdown melting reactions at higher temperatures than H2O-present melting; volumetrically significant melt production requires temperatures above ∼800 °C. Melting wets residual grains, and anatectic crust becomes porous at a few vol.% melt. Feedback between deformation and melting creates a dynamic rheological environment; as melt volume increases to the melt connectivity transition, which varies but is around 7 vol.% (see discussion later in the text), melt may escape from the source in the first of several melt-loss events with increasing temperature. Major and accessory phase controls on melt production and melt composition for different pressure–temperature–time paths are evaluated using calculated phase equilibria for average pelite. The pristine to slightly retrogressed condition of peritectic minerals in residual crust requires significant loss of melt from the system. The consequences of melt loss are evaluated here. In residual crust, evidence of melt at the grain scale may be preserved in microstructures, whereas evidence of melt extraction pathways at outcrop scale is recorded by leucosome networks. Strain and anisotropy of permeability control the form of mesoscale melt channels with strong anisotropy promoting high-melt focusing. The sequence of structures observed in nature records a transition from storage to drainage; focused melt flow occurs by dilatant shear failure of low-melt-fraction rocks, leading to the formation of networks of channels that allow accumulation and storage of melt and that form the link for melt flow from grain boundaries to ascent conduits. Melt ascent is via ductile-to-brittle fracture; ductile fractures may propagate along foliation as sills or from dilation or shear bands as dikes. Emplacement of horizontal tabular and wedge-shaped plutons occurs around the brittle–ductile transition zone, whereas vertical lozenge-shaped plutons represent crystallization of magma in the ascent conduit. Blobby plutons form by lateral expansion in the ascent conduit localized by thermal or mechanical instabilities.

Comparison of eruptive and intrusive samples from Unzen Volcano, Japan: Effects of contrasting pressure–temperature–time paths

Core samples from the conduit of Unzen Volcano, obtained only 9 years after cessation of the 1991–1995 eruption, exhibit important differences in physical characteristics and mineralogy, and subtle differences in bulk chemistry from erupted samples. These differences in the conduit samples reflect emplacement under a confining pressure where about half of the original magmatic water was retained in the melt phase, maintenance at hypersolidus temperature for some unknown but significant time span, and subsequent subsolidus hydrothermal alteration. In contrast, magma that extruded as lava underwent decompression to 1 atm with nearly complete loss of magmatic water and cooling at a sufficiently rapid rate to produce glass. The resulting hypabyssal texture of the conduit samples, while clearly distinct from eruptive rocks, is also distinct from plutonic suites. Given the already low temperature of the conduit (less than 200 °C, [Nakada, S., Uto, K., Yoshimoto, M., Eichelberger, J.C., Shimizu, H., 2005. Scientific Results of Conduit Drilling in the Unzen Scientific Drilling Project (USDP), Sci. Drill., 1, 18–22]) when it was sampled by drilling, this texture must have developed within a decade, and perhaps within a much shorter time, after emplacement. The fact that all trace-element concentrations of the conduit and the last-emplaced lava of the spine, 1300 m above it, are identical to within analytical uncertainty provides strong evidence that both were produced during the same eruption sequence. Changes in conduit magma that occurred between emplacement and cooling to the solidus were collapse of vesicles from less than or equal to the equilibrium value of about 50 vol.% to about 0.1 vol.%; continued resorption of quartz and reaction of biotite phenocrysts due to heating of magma prior to ascent by intruding mafic magma; breakdown of hornblende; and micro-crystallization of rhyolitic melt to feldspar and quartz. Subsolidus changes were deposition of calcite and pyrite, growth of sericite in anorthite-rich zones of plagioclase, and development of montmorillonite as an alteration product. Significant changes in bulk composition were depletion of Mg, Fe and Na and enrichment in C and S. These changes were due mainly to the breakdown of hornblende and plagioclase, and addition of carbonate and pyrite, respectively. The identical concentrations of REEs in the conduit and surface lava are consistent with low water to rock ratios during alteration. This suggests to us that despite convective hydrothermal removal of heat from the conduit, chemical open-system effects were limited to early loss of magmatic water and later addition of magmatic CO 2 and SO 2 and/or H 2S streaming up the conduit from deeper levels.

The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure–temperature–time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu–Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P–T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P–T-paths. Omphacite blueschists reach maximum P–T-conditions of 17–18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb–Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high (“cold”) P/T gradients. A Rb–Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P–T conditions at 16–18 kbar/340–380 °C. The array of P–T–t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9–20 °C/Ma and 5–6 mm/a, respectively. The derived P–T–t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10–40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P–T–t paths as well as the calculated rates of exhumation and cooling agree well with the P–T–t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15–25 mm/a requires that a minimum amount of 975–1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P–T–t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.

Experimental zircon/melt and zircon/garnet trace element partitioning and implications for the geochronology of crustal rocks

Garnet is the most commonly used mineral in thermobarometry, whereas zircon is the most robust chronometer to date high-grade metamorphic rocks. To provide a basis for correlation of zircon and garnet growth, we determined experimentally the trace element partitioning between zircon, a hydrous granitic melt and garnet at 20 kbar and 800–1000 °C for P, Y, rare earth elements (REE), Zr, Hf, Th and U. In respect to melt, zircon preferentially incorporates all investigated elements apart from REE with atomic number lower than Sm. At this pressure and in the chosen composition, the distribution coefficient between zircon and melt ( D Zrn/Melt) for REE increases with increasing atomic number of the REE and with decreasing temperature. D Yb Zrn/Melt is ∼ 20 at 1000 °C, but more than an order of magnitude higher at 800 °C. The solubility of Zr in hydrous granitic melts buffered by zircon is about a factor of two lower at 20 kbar than what has been previously established for mid-crustal pressures. Large garnet produced in the experiments allowed determination of garnet/melt trace element partitioning ( D Grt/Melt) at temperatures of 800–1000 °C, conditions relevant for partial melting of crustal rocks. There is a systematic increase in D REE Grt/Melt with decreasing temperature. Zircon contains significantly more heavy-REE than garnet at temperatures of 800–850 °C. Zircon/garnet partition coefficients of heavy-REE decrease with increasing temperature from D Lu Zrn/Melt of 12 at 800 °C to 1.4 at 1000 °C. Middle-REE partitioning is close to unity for the whole investigated temperature range. Different empirically determined zircon/garnet partition coefficients from granulites and ultra-high temperature granulites can potentially be explained by the experimentally determined change of partitioning as a function of temperature. These data can assist in establishing equilibrium between garnet and zircon zones in natural rocks, and in the construction of detailed pressure–temperature–time paths in high-grade metamorphic rocks.

A possible model for the lithospheric thinning of North China Craton: Evidence from the Yanshanian (Jura-Cretaceous) magmatism and tectonism

It is well known that the North China Craton was largely formed in the Archean, and was reactivated and transformed during the Jura-Cretaceous (Yanshanian) time into an orogenic belt, which is believed to be related to the lithospheric thinning. Recent debate is centered on the mechanisms and processes of the lithospheric thinning. There are two prevailing models for the lithosphere thinning: (1) thermal erosion or/and chemical metasomatism allowed the lower part of the lithospheric mantle to be transformed into asthenosphere, (2) delamination of the lithospheric mantle, and perhaps also the lowermost crust. In this paper, we attempt to explain how the buoyant cratonic lithosphere may be transformed into a denser one, allowing delamination to take place on the basis of field observation, tectonic analysis and petrologic data on igneous rocks formed during the Yanshanian. We recognize four episodes of contractional deformation that resulted in significant crustal shortening and vertical thickening. The counter-clockwise Pressure–Temperature–time path of the tectonomagmatic events suggests that the underplating basaltic magma may have heated and weakened the existing cold and strong crust. This crustal change in rheology may have facilitated the contractional deformation and crustal thickening. Petrologic data of the contemporary igneous rocks and the lower crustal xenoliths suggest that the crust had reached ∼ 50–65 km in thickness. It suggests that input of large amount of asthenosphere-derived mafic magmas is required to cause crustal melting. Thus, a large amount of eclogite may be formed at the lowermost crust following the transient thickening events. The dense eclogite may trigger the lithosphere delamination.

Grain-size controlin situat high pressures and high temperatures in a diamond-anvil cell

The grain-size distribution and the character of individual grain boundaries in microcrystalline networks play a significant role in material properties, such as melting temperature, diffusion coefficients, resistivity, optical absorption, elastic constants, phase transformation pressure, and so on. In this study, the grain size of NaCl, SiO2 and FeC3 is controlled in situ at high pressures over the entire range of the length scale of crystallinity: single-crystal, micro-/nanocrystalline and amorphous materials within a volume commensurate with the size of the probing X-ray beam. The structure refinement of high-pressure samples from X-ray diffraction data can be significantly improved by controlling grain size by selecting the structure of starting materials and following certain high pressure-temperature-time paths.

Pressure–temperature–time paths of sediment recycling beneath the Tonga–Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga–Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/ 9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2–10, (b) the top 1–22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be–Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ∼800 °C during the breakdown of amphibole±plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10–15% partial melts of depleted mantle after it has been enriched by the addition of 0.2–2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ∼800 °C by the time it attains ∼80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partial melting of the altered oceanic crust.

The pressure–temperature–time path of migmatites from the Sikkim Himalaya

AbstractA combined metamorphic and isotopic study of lit‐par‐lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre‐decompression garnet growth at 23 ± 3 Ma and near‐peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr−1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near‐isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.

Experimental and textural constraints on mafic enclave formation in volcanic rocks

We have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at Southwest Trident volcano, Alaska. Andesite enclaves are present throughout the four dacite lava flows produced by the eruption, and resemble mafic enclaves commonly found in other volcanic rocks. Our experiments replicate the pressure–temperature–time path taken by enclave-forming andesite magma as it is engulfed in dacite during magma mixing. Pressure and temperature information for the andesite and dacite are from [Coombs et al., Contrib. Mineral. Petrol. 140 (2000) 99–118]. The andesite was annealed at 1000°C, and then cooled to 890°C at rates of 110°C h −1, 10°C h −1, and 2°C h −1. Once cooled to 890°C, andesite was held at this lower temperature from times ranging from 1 to 40 h. The andesite that was cooled at the slower rates of 2°C h −1 and 10°C h −1 most resembles enclave groundmass texturally and compositionally. Based on simple thermal calculations, these rates are more consistent with cooling of the andesite groundmass below an andesite–dacite interface than with cooling of enclave-sized spheres. If enclaves do crystallize as spheres, post-crystallization disaggregation must occur. Calculations using the MELTS algorithm [Ghiorso and Sack, Contrib. Mineral. Petrol. 119 (1995) 197–212] show that for incoming andesite to become less dense than the dacite ∼34 volume % of its groundmass must crystallize to undergo ∼18 volume % vesiculation; these values are similar to those determined for Southwest Trident enclaves. Thus such crystallization may lead to ‘flotation’ of enclaves and be a viable mechanism for enclave formation and dispersal. The residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments. Decompression experiments on Southwest Trident dacite suggest an average ascent rate for the eruption of ∼2–3 MPa h −1. An andesite experiment that was cooled and then decompressed at this rate contains melt that matches that of the natural enclaves. It is apparent that decompression (ascent)-induced crystallization occurs in enclaves, but not in the form of microlites as happens in the dacite host, due either to insufficient residence time at chamber temperatures or to the pre-existing microphenocrysts which act as sites for new growth.

Pressure-temperature-time path discontinuity in the Main Central thrust zone, central Nepal

Metapelites collected in central Nepal reveal a discontinuity in metamorphic pressuretemperature-time (P-T-t) paths near the base of the Main Central thrust zone, despite an absence of obvious structural breaks. Garnets in the structurally lowest rocks grew with increasing T and P (loading), whereas garnets 1‐3 km upsection grew with increasing T, but decreasing P (exhumation). Monazite grains in structurally lower rocks yield ionmicroprobe Th-Pb ages of 8‐9 Ma. Structurally higher monazite grains range from 10 to 22 Ma. The P-T-t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca. 8 Ma, but also reflect thermal relaxation following older (20 Ma or older) thrust movement. The Main Central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate.