Structural evolution of the Bayanhongor region, west-central Mongolia

Most of previous models suggest that the Central Asia Orogenic Belt grew southward in the Phanerozoic. However, in the Bayanhongor region in west-central Mongolia, volcanic arc, accretionary prism, ophiolite, and passive margin complexes accreted northeastward away from the Baydrag micro-continent, and hence the region constitutes the southwestern part of a crustal-scale syntaxis close to the west. The syntaxis should be original, because presumably reorientation due to strike-slip faulting can be ignored. It is reconfirmed that the Baydrag eventually collided with another micro-continent (the Hangai) to the northeast. A thick sedimentary basin developed along the southern passive margin of the Hangai micro-continent. This region is also characterized by an exhumed metamorphosed accretionary complex and a passive margin complex, which are both bounded by detachment faults as well as basal reverse faults which formed simultaneously as extrusion wedges. This part of the Central Asia Orogenic Belt lacks exhumed crystalline rocks as observed in the Himalayas and other major collisional orogenic belts. In addition, we identified two phases of deformation, which occurred at each phase of zonal accretion as D1 through Cambrian and Devonian, and a synchronous phase of final micro-continental collision of Devonian as D2. The pre-collisional ocean was wide enough to be characterized by a mid-ocean ridge and ocean islands. Two different structural trends of D1 and D2 are observed in accretionary complexes formed to the southwest of the late Cambrian mid-ocean ridge. That is, the relative plate motions on both sides of the mid-ocean ridge were different. Accretionary complexes and passive margin sediments to the northeast of the mid-ocean ridge also experienced two periods of deformation but show the same structural trend. Unmetamorphosed cover sediments on the accretionary prism and on the Hangai micro-continent experienced only the D2 event due to micro-continental collision. These unmetamorphosed sediments form the hanging walls of the detachment faults. Moreover, they were at least partly derived from an active volcanic arc formed at the margin of the Baydrag micro-continent.     

Experimental and theoretical study of stability of dense hydrous magnesium silicates in the deep upper mantle

Fluid-undersaturated experiments were conducted to determine the phase relations in the simplified peridotite system MgO-SiO₂-H₂O (MSH) at 11.0-14.5 GPa and 800-1400 °C. Stability relations of dense hydrous magnesium silicates (DHMSs) under fluid-undersaturated conditions were experimentally examined. From the fluid-absent experimental results, we retrieved thermodynamic data for clinohumite, phase A, phase E, and hydrous wadsleyite, consistent with the published data set for dry mantle minerals. With this new data set, we have calculated phase equilibria in the MSH system including dehydration reactions. The dehydration reactions calculated with lower water activities of 0.68-0.60 match the fluid-present experiments of this study above 11.0 GPa and 1000 °C, indicating that considerable amounts of silicate component were dissolved into the fluid phase. The calculated phase equilibria illustrate the stability of the post-antigorite phase A-bearing assemblages. In the cold subducting slab peridotite, phase A + enstatite assemblage survives into the transition zone, whereas phase A + forsterite + enstatite assemblage forms hydrous wadsleyite at a much shallower depth of about 360-km. The slab is subducted with no dehydration reactions occurring when entering the transition zone. The phase equilibria also show the high temperature stability of phase E. Phase E is stable up to 1200 °C at 13.5 GPa, a plausible condition in the mantle of relatively low temperature, i.e., beneath subduction zones. Phase E is a possible water reservoir in the mantle as well as wadsleyite and ringwoodite.     

New radiolarian ages from the Troodos ophiolite and their tectonic implications

Abstract Newly obtained radiolarian biostratigraphic age combined with previous isotopic age of the Troodos ophiolite shows that the ophiolite becomes systematically younger from east to west: Turonian, early Campanian, and late Campanian. The youngest late Campanian part of the ophiolite is directly covered by the volcaniclastic sediment derived from an active island arc, whereas the older part is covered by pelagic radiolarite. These facts constitute evidence that the Troodos ophiolite was probably emplaced during the subduction of an active spreading ridge.     

Tale of the Kulet eclogite from the Kokchetav Massive,Kazakhstan: Initial tectonic setting and transition from amphibolite to eclogite

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine‐grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse‐grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four‐stage metamorphic evolution: (I) pre‐eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre‐eclogite stage or coronal eclogite, and so did not identify the four‐stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE‐enriched patterns [(La/Lu)_N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO₂ (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO₂ < 0.9 wt% and FeO < 12.11 wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb–Zr/4–Y and TiO₂–FeO/MgO diagrams, although displacement from the MORB–OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180–200 km at 537–527 Ma, followed by slab breakoff at 526–507 Ma, exhumation and recrystallization at crustal depths is applied to explain the four‐stage evolution of the Kulet eclogite.     

Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks

Accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts, ocean ridges or subduction-related ophiolites. Seamount type basalts are associated with shallow water sediments—mostly reefoidal limestones. Ocean ridge type basalts are generally overlain by pelagic cherts. Subduction-related ophiolitic eruptives, often underlain by gabbroic and ultramafic rocks, are associated with hemipelagic mudstones. The age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin. This relationship has been established for each type of accretionary complex, examples being taken mostly from Japan and the western Pacific rim in order to represent evolutionary processes at a typical active plate margin. In general, the seamount types are older, ridge types are of intermediate age, and the ophiolitic types are by far the youngest, usually close to zero age. Seamount type basalts are accreted by shallower scraping of the seamount's sediment apron together with fragments of seamount basalt, ridge type, by peeling due to permeability contrast, and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient. Accordingly, it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction.     

Arc-type and intraplate-type ridge basalts formed at the trench-trench-ridge tripIe junction: Implication for the extensive sub-ridge mantle heterogeneity

Abstract ' In situ basalts' represent the ridge magmatism at and close to the ancient trench-trench-ridge triple junction. Such basalts in the Amami, Mugi, and Setogawa accretionary complexes, Southwest Japan, were described and analysed. The geochemical data show that the ' in situ basalts' include all the types of basalts, ranging from alkali basalts and high-alumina basalts to tholeiites, and the compositions tend towards intermediate and silicic rocks. The data also reveal that the ridge basalts are indistinguishable both from the island arc and intraplate basalts, no affinities with mid-ocean-ridge basalts. The sub-ridge mantle adjacent to the triple junction had a component of sub-arc mantle, and this mantle heterogeneity can be generated by the formation of a slab window.     

Counter-clockwise prograde P–T path in collisional orogeny and water subduction at the Precambrian–Cambrian boundary: The ultrahigh-pressure pelitic schist in the Kokchetav massif,northern Kazakhstan

A prograde pressure–temperature (P–T) path is estimated for pelitic schists from the latest Precambrian Kokchetav ultrahigh-pressure massif, Kazakhstan, using compositional zoning and mineral inclusions in coarse-grained and inclusion-rich garnets. Ti-bearing inclusions are abundant in garnet and display a zonal distribution. Ilmenite occurs in the inner-core, where most of it makes a composite inclusion with rutile, whereas monomineralic rutile occurs in the outer-core to mantle domains. In the rim region both ilmenite and rutile are present, although in small amounts. Application of the ilmenite-garnet thermometer yields a systematic temperature increase towards rim from 500 to 750 °C. The pressure-sensitive reaction: 3 Fe-Ilm (in Ilm) + Ky + 2 Qtz = 3 Rt + Alm (in Grt) yielded pressures of 1.2–1.3 GPa for the outer-core inclusions.A petrogenetic grid in the K₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O model system was used to estimate the equilibrium compositions of the garnet. The change of the grossular component along the model P–T path expected from the forward modelling is close to the observed compositional profile of the outer-core to rim domains. No constraint is available from thermobarometry in the inner-core; however, the forward modelling of garnet zoning provides information on the early stage of the P–T path during the garnet growth.The estimated P–T path is counter-clockwise in the prograde stage with a steep bend at around 700 °C and 1.2–1.5 GPa. This is similar to the metamorphic P–T gradient of the Kokchetav massif. This result contrasts markedly with the traditional clockwise P–T path in many collisional metamorphic terranes, and is regarded to represent a subduction geotherm at the Precambrian–Cambrian boundary. The P–T path proposed in this study also supports the models for the recovery of the “snowball Earth” from late-Proterozoic glaciation through effect of water in the solid Earth mantle.     

Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

We estimated metamorphic conditions for the  6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.

Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from  230 °C in the volcanic unit to  380 °C at the bottom of the gabbro unit, systematically  200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.

Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).     

Granite subduction: Arc subduction, tectonic erosion and sediment subduction

Continental growth has been episodic, reflecting the episodic nature of mantle dynamics as well as surface dynamics of the Earth, the net result of which is exhibited by the present mantle with two huge reservoirs of TTG rocks, one on the surface continents and the other on the D″ layer on the Core-Mantle Boundary (CMB). During the early half of the Earth history, the felsic continental crust on the surface which formed in an intra-oceanic environment has mostly been subducted into the deep mantle, except in the rare case of parallel arc collision. The growth history of continental crust shows that with its simultaneous formation, a considerable amount must have also been subducted. Such ongoing subduction processes can be seen in the western Pacific region, through tectonic erosion, arc subduction, and sediment-trapped subduction.