ABSTRACT Records of high-pressure/low-temperature (H P -L T ) metamorphic interfaces are not common in Precambrian orogens. It should be noted that the association of H P -L T metamorphic interfaces and strongly deformed ocean plate stratigraphy that form accretionary prisms between trenches and magmatic arcs are recognized as hallmark signatures of modern plate tectonics. In East Africa (Tanzania), the Paleoproterozoic Ubendian-Usagaran Belt records a H P -L T metamorphic interface that we consider as a centerpiece in reviewing the description of tectonic units of the Ubendian-Usagaran Belt and defining a new tectonic model. Our new U-Pb zircon age and the interpretations from existing data reveal an age between 1920 and 1890 Ma from the kyanite bearing eclogites. This establishment adds to the information of already known H P -L T metamorphic events at 2000 Ma, 1890–1860 Ma, and 590–520 Ma from the Ubendian-Usagaran Belt. Arc–back-arc signatures from eclogites imply that their mafic protoliths were probably eroded from arc basalt above a subduction zone and were channeled into a subduction zone as mélanges and got metamorphosed. The Ubendian-Usagaran events also record rifting, arc and back-arc magmatism, collisional, and hydrothermal events that preceded or followed H P -L T tectonic events. Our new tectonic subdivision of the Ubendian Belt is described as: (1) the western Ubendian Corridor, mainly composed of two Proterozoic suture zones (subduction at 2000, 1920–1890, Ma and 590–500 Ma) in the Ufipa and Nyika Terranes; (2) the central Ubendian Corridor, predominated by metamorphosed mafic-ultramafic rocks in the Ubende, Mbozi, and Upangwa Terranes that include the 1890–1860 Ma eclogites with mid-ocean ridge basalt affinity in the Ubende Terrane; and (3) the eastern Ubendian Corridor (the Katuma and Lupa Terranes), characterized by reworked Archean crust.

Abstract Pre-Cenozoic rocks of the Japanese islands are largely composed of latest Palaeozoic to Cretaceous accretionary complexes and Cretaceous granitic intrusives. Exposures of older rocks are restricted to a limited number of narrow terranes, notably the Hida, Oeyama and Hida Gaien belts (Inner Zone of SW Japan), the Kurosegawa Belt (Outer Zone of SW Japan) and the South Kitakami Belt (NE Japan). In these belts, early Palaeozoic basement rocks are typically overlain by a cover of middle Palaeozoic to Mesozoic shelf facies strata. This chapter describes these basement inliers and their cover, grouping them under four subheadings: Hida, Oeyama, Hida Gaien and South Kitakami/Kurosegawa belts. Although opinions are varied among authors whether the Unazuki Schist should be placed in the Hida Belt (TT) or in the Hida Gaien Belt (KT & NM) sections, this chapter will describe the Unazuki Schist in the Hida Belt section.

Abstract Accretionary complexes (AC) form at convergent plate margins by the subduction of oceanic plate underneath the continental plate (Fig. 2b.1). The oceanic plate is created at the mid-oceanic ridge, and moves to the trench while accumulating pelagic sediments. After arriving at the trench, where the pelagic sediments are covered by continent-derived clastic materials, the plate is subducted and part of the sediments accrete to the continental plate, producing fault stacking and several types of mèlanges (Fig. 2b.1). The characteristic AC succession re?ects the ocean plate stratigraphy (OPS) (Matsuda & Isozaki 1991) starting with basaltic basement covered by radiolarian ribbon chert, then siliceous mudstone and ?nally coarse clastic rocks.

Three high-grade tectonic blocks, including jadeite-bearing retrograded eclogite, pumpellyite-rich retrograded eclogite, and clinopyroxene-bearing garnetamphibolite, are newly described in the jadeitite-bearing New Idria serpentinite body. Petrologic analyses reveal two contrasting peak metamorphic stages—eclogite facies metamorphism (M \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(_{1}^{E}\) \end{document} ) characterized by garnet + omphacite (∼48 mol% jadeite) + rutile ± epidote + quartz, and amphibolite-facies metamorphism (M \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(_{1}^{A}\) \end{document} ) characterized by garnet + hornblende + augite (∼14 mol% jadeite) + rutile + quartz. Both peak metamorphic events are overprinted by very low -T blueschist-facies minerals (M 2 ), which include glaucophane, lawsonite, pumpellyite, jadeitite (up to 94 mol% jadeite), chlorite, and titanite. Garnet-clinopyroxene geothermometry yields T = ∼580–620 °C at P > 1.3 GPa for the M \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(_{1}^{E}\) \end{document} stage and T = ∼630–680 °C at P = ∼0.8–1.0 GPa for the M \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(_{1}^{A}\) \end{document} stage. The jadeite- and lawsonite-bearing phase equilibria constrain metamorphic conditions of P > 1.0 GPa at T = ∼250–300 °C for the M 2 stage that is probably synchronous with the formation of nearby jadeitite within serpentinite. The presence of eclogite blocks suggests that the New Idria serpentinite diapir was initiated at mantle depths. The wide range of P-T conditions of tectonic blocks supports the idea that the New Idria serpentinite diapir rose from mantle depths and enclosed tectonic blocks at various mantle-crustal levels during diapiric upwelling and extrusion.

Early Cretaceous lawsonite eclogites and related high-pressure rocks occur as tectonic inclusions within serpentinite mélange south of the Motagua fault zone, Guatemala. Petrologic and microtextural analyses of mafic high-pressure rocks reveal three metamorphic stages linked to several deformational textures. The prograde stage represents an incipient eclogitization and is preserved in prograde garnet, along with an older S 1 –S 2 foliation. The prograde assemblage is garnet ( X Mg = ∼0.22) + omphacite (∼52 mol% jadeite) or jadeite (∼83 mol % jadeite) + lawsonite + chlorite + rutile + quartz ± phengite (3.6 Si p.f.u.); some rocks also have ilmenite and rare ferro-glaucophane. Lawsonite in garnet of some lawsonite eclogites contains rare pumpellyite inclusions. The presence of synmetamorphic brittle deformation, inclusions of pumpellyite, Fe 2+ -Mg distribution coefficients between omphacite inclusions and adjacent garnet with Ln( K D ) = 2.7–4.5, and garnet-clinopyroxene-phengite thermobarometry suggest that eclogitization initiated at temperature ( T ) = ∼300 °C and pressure ( P ) > 1.1 GPa, and continued to T = ∼480 °C and P = ∼2.6 GPa. In contrast, the retrograde eclogite-facies assemblage is characterized by reversely zoned garnet rims and omphacite ± glaucophane + lawsonite + rutile + quartz ± phengite (3.5 Si p.f.u.) along the S 3 foliation. Garnet-phengite-clinopyroxene thermobarometry yields P = ∼1.8 GPa and T = ∼400 °C. The youngest, blueschist-facies assemblage (glauco-phane + lawsonite + chlorite + titanite + quartz ± phengite) locally replaces earlier mineral assemblages along S 4 crenulations. The inferred prograde P - T trajectory lies near a geotherm of ∼5 °C km −1 , comparable to the calculated thermal and petrologic structure of the NE Japan subduction zone. These petrologic characteristics indicate: (1) the basalt-eclogite transformation may occur at T = ∼300 °C in cold subduction zones, (2) glaucophane-bearing prograde assemblages are rare during incipient eclogitization in cold subduction zones, and (3) the chlorite-consuming reactions that form Fe-Mg-Mn garnet are more effective than the lawsonite-consuming reaction that forms a grossular component. At depths of ∼100 km in cold subduction zones, dehydration embrittlement may be caused by such chlorite-consuming reactions.

The Haiyangsuo area of the NE Sulu ultrahigh-pressure terrane, eastern China, consists of gneisses with minor granulite and amphibolite layers, metagabbros, and granitic dikes. The peak-stage assemblages of the granulites (garnet + orthopyroxene + clinopyroxene + plagioclase ± pargasite ± biotite ± quartz) formed at >750 °C and 9–11 kbar and were overprinted by amphibolite-facies phases characterized by well-developed corona layers of | garnet | amphibolite + quartz | at contacts between plagioclase and clinopyroxene or orthopyroxene, as well as by the exsolution of (orthopyroxene + ilmenite + amphibole) from clinopyroxene. These textures indicate a near-isobaric cooling history of the granulite-bearing gneiss terrane. The metagabbro preserves a relict igneous assemblage (orthopyroxene + clinopyroxene + plagioclase + pargasite ± ilmenite ± quartz) in its core, but in its margins has a metamorphic corona texture similar to the granulite that formed at ∼600–700 °C and 7–10 kbar. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons indicates that the protolith age of the garnetbiotite gneiss is older than 2500 Ma, whereas the granulite-facies metamorphism (the first regional metamorphic event) occurred at 1846 ± 26 Ma. Gabbro intrusion took place at 1734 ± 5 Ma, and the formation of amphibolite assemblages in both metagabbro and granulite occurred at ca. 340–370 Ma. Both gneiss and metagabbro were intruded by granitic dikes, with one dated at 158 ± 3 Ma. These data, together with a lack of eclogitic assemblages, suggest that this granulite-amphibolite–facies complex is exotic relative to the Triassic Sulu high-pressure–ultrahigh-pressure terrane; juxtaposition took place in Jurassic time.