The Kamuikotan zone in Hokkaido, Japan: tectonic mixing of high-pressure and low-pressure metamorphic rocks

Abstract. In the Kamuikotan zone, central Hokkaido, Japan, two distinct types of metamorphic rocks are tectonically mixed up, along with a great quantity of ultramafic rocks; one type consists of high-pressure metamorphic rocks, and the other of low-pressure ones. The high-pressure metamorphic rocks are divided into two categories. (1) Prograde greenschist to glaucophaneschist facies rocks derived from mudstone, sandstone, limestone, a variety of basic rocks such as pillow and massive lavas, hyaloclastite and tuff, and radiolarian (Valanginian to Hauterivian) chert, among which the basic rocks and the chert, and occasionally the sandstone, occur as incoherent blocks (or inclusions) enveloped by mudstone. (2) Retrograde amphibolites with minor metachert and glaucophane-calcite rock, which are tectonic (or exotic) blocks enclosed within prograde mudstone or serpentinite, or separated from these prograde rocks by faults. The K-Ar ages of the prograde metamorphic rocks (72, 107 and 116 Ma on phengitic muscovites) are younger than those of the retrograde rocks (109, 132, 135 and 145 Ma on muscovites, and 120 Ma on hornblende). The low-pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian (Tithonian) chert with the metamorphic grade ranging from the zeolite facies, through the greenschist (partly, actinolite-calcic plagioclase) facies to the amphibolite (partly, hornblende-granulite) facies. The low-pressure metamorphism has a number of similarities with that described for'ocean-floor'metamorphism. The tectonic evolution of such a mixed-up zone is discussed in relation to Mesozoic plate motion.     

Petrogenesis of Franciscan glaucophane schists and associated metamorphic rocks,California

Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite.All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions.The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.     

关于大别山榴辉岩相变质作用

大别山产出的榴辉岩相岩石包括石榴橄榄岩、榴辉岩、榴云片岩、榴辉片麻岩、榴玉英岩和榴辉大理岩等不同系列,它们均分布于花岗质片麻岩中。矿物共生序列研究表明,榴辉岩相岩石经历了从绿帘角闪岩相、柯石英榴辉岩相、角闪榴辉岩相、绿帘角闪岩相到绿片岩相的演化过程。花岗质片麻岩及变质火山—沉积岩系并未经历超高压变质作用,但却与榴辉岩相岩石经历了同一期绿帘角闪岩相变质事件,证明二者在地壳范围内发生了构造合并     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.     

Tectonic setting, petrology and geochronology of jadeite + glaucophane and chloritoid + glaucophane schists from north-west Turkey

The north-west Turkish blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble. The depositional age of the blueschist protoliths are probably Palaeozoic to Mesozoic, while the age of the high-pressure/low-temperature metamorphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient blueschist metamorphism and by spinel peridotite slices. The metaclastic rocks with regional jadeite and glaucophane, which comprise the lower part of the blueschist unit, make up an over 1000-m-thick coherent sequence in the Kocasu region of north-west Turkey. Rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole + Iawsonite but no garnet indicate lawsonite blueschist facies metamorphism. The blueschist metaclastics in the Kocasu region are practically free of calcium and ferric iron and closely approximate the NFMASH system in bulk composition. Two low-variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane + paragonite. The metaclastics comprise up to several-metres-thick layers of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glaucophane assemblage, even in Fe²⁺-rich compositions, is stable in the jadeite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field. Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jadeite, paragonite and chlorite indicate metamorphic P-T conditions of 20 ± 2 kbar and 430 ± 30 d? C, yielding geothermal gradients close to 5d? C km^-1, one of the lowest geotherms recorded. Blueschists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km. Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 ± 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48-53-Ma-old granodiorite bodies that were emplaced at 10 km depth. This suggests that the exhumation of blueschists by underplating of cold continental crust, and normal faulting at the blueschist-peridotite, interface occurred during the Late Cretaceous to Palaeocene (88-53 Ma).     

南苏鲁造山带的超高压变质岩及岩石化学研究

在南苏鲁造山带核部，古老的表壳岩和花岗质侵人岩经历了三叠纪的超高压变质作用，在超高压变质岩石抬升过程中经历了强烈的角闪岩相退变质作用改造。据岩相学和岩石化学研究，可以区分出六大类典型超高压变质岩：榴辉岩、石榴石橄榄岩、石英硬玉岩、石榴石多硅白云母片岩、硬玉石英岩和石榴石绿辉石文石岩。这些岩石的角闪岩相退变质产物分别是斜长角闪岩、蛇纹岩、长英质片麻岩、长石石英云母片岩、石英岩和大理岩。地球化学研究揭示，榴辉岩的原岩很可能是形成在大陆内部构造环境的拉斑玄武岩，而石榴石橄榄岩可能是起源于亏损的残余地幔。石英硬玉岩原岩包括正变质的花岗岩和奥长花岗岩、副变质的酸性火山碎屑岩和长石石英砂岩。大面积分布的古老花岗岩很可能是形成在大陆或大陆边缘环境。长石石英云母片岩、石英岩和大理岩的原岩为沉积岩，与副变质的长英质片麻岩和基性火山岩—起构成了古老的表壳岩组合。双峰式的酸性和基性火山岩组合的存在也证明部分表壳岩是形成在大陆环境。因此，可以推测南苏鲁造山带核部的超高压变质岩原岩为形成在大陆板内环境的沉积岩—酸性和基性火山岩—花岗岩和奥长花岗岩建造。     

Petrology of ultrahigh-pressure rocks from the southern Su-Lu region, eastern China

Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

Current mercury ore formation on Mendeleyev

The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone.

The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic compositions. Mineral assemblages of the metamorphosed mafic rocks include: (1) coesite (as pseudomorphs) + garnet + omphacite + rutile + zoisite; (2) jadeite + quartz (coesite) + garnet + kyanite ± paragonite; (3) garnet + omphacite + barroisite + rutile; and (4) garnet + glaucophane + lawsonite. The upper unit is characterized by sheets of serpentinite that contain lawsonite-bearing metarodingite and rare calcium-rich eclogite. A metamorphosed melange containing blocks of ultramafic, eclogite, and quartz-jadeite rocks is situated between the two units.

The UHP metamorphic event that affected the lower unit is characterized by recumbent folding and shear zones. Subsequent large-scale, left-lateral strike-slip movements deformed both tectonic units. These deep-crustal metamorphic structures are oriented at high angles relative to the younger, N-S-trending Main Uralian thrust and the left-lateral strike-slip movement that displaced the Maksyutov block.     

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex,Spain)

At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO₂ contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO₂ ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H₂O–CO₂ fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H₂O and molecular CO₂ are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H₂O–CO₂ fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.     

苏北高压变质带绿片岩中石榴石内文石包裹体的发现

苏北高压变质带位于苏鲁超高压变质带东南缘,露头多为绿片岩相的中—晚元古代云台岩群,典型的蓝片岩仅见于灌云县杨集的钻孔岩心中。本文通过拉曼光谱研究,在连云港地区的绿片岩内石榴石中发现文石(CaCO3)包裹体。文石呈细小包裹体残留于细粒半自形石榴石斑晶中。这些石榴石作为低扩散的刚性矿物,经历了多期变质变形作用,变质反应证据保存在石榴石的成分环带及包体矿物组合中。文石包裹体的存在证明苏北高压变质带北部的、即本区出露的这套岩系是早期经历了高压变质作用、晚期又叠加了绿片岩相变质作用的高压变质地体。     

Ultra-high-pressure metamorphism of carbonate rocks in the Dabie Mountains, central China

Abstract Widespread ultra-high-P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra-high-P metamorphic event occurred on a regional scale during Triassic collision between the Sino-Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite-bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra-high-P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine-grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite-bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra-high-P metamorphic terrane. Retrograde minerals are fine grained and replace coarse-grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An₁₈) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively. The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra-high-P conditions are consistent with the coexistence of grossular-rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low X_CO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.     

青藏高原北部可可西里狮头山含硬玉岩类的基本特征及地质意义

描述了可可西里山南狮头山含硬玉岩类的岩石学、矿物学特征。含硬玉岩类的原岩为辉长岩，围岩为石炭—二叠系。由于变质作用不彻底，保留有原岩的矿物组合。典型的高压变质矿物组合为：钠长石 硬玉 霓石 蓝闪石 绿帘石 绿泥石。硬玉SiO2、FeO偏低，Al2O3、Na2O明显偏高，变质矿物组合中以富钠矿物为主，缺少石英，与国内外含硬玉岩类的原岩成分、变质矿物组合均不相同，是一种新的高压低温硬玉岩类。狮头山高压变质带向西延至若拉岗日、大横山一带，与北侧的西金乌兰—金沙江板块缝合带(西段)平行产出，以此推测缝合带南侧的羌北—昌都板块向北消减于巴颜喀拉板块之下。     

Petrology of the Non-mafic UHP Metamorphic Rocks from a Drillhole in the Southern Sulu Orogenic Belt, Eastern-Central China

The Drillhole ZK703 with a depth of 558 m is located in the Donghai area of the southern Sulu ultrahigh-pressure (UHP) metamorphic belt, eastern China, and penetrates typical UHP eclogites and various non-mafic rocks, including peridotite, gneiss, schist and quartzite. Their protoliths include ultramafic, mafic, intermediate, intermediate-acidic, acidic igneous rocks and sediments. These rocks are intimately interlayered, which are meters to millimeters thick with sharp and nontectonic contacts, suggesting in-situ metamorphism under UHP eclogite facies conditions. The following petrologic features indicate that the non-mafic rocks have experienced early-stage UHP metamorphism together with the eclogites: (1) phengite relics in gneisses and schists contain a high content of Si, up to 3.52 p.f.u. (per formula unit), while amphibolite-facies phengites have considerably low Si content (<3.26 p.f.u.); (2) jadeite relics are found in quartzite and jadeitite; (3) various types of symplectitic coronas and pseud     

Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

A high-grade blueschist tectonic block from the Franciscan Complexof the northeast Diablo Range shows evidence of three episodesof retrograde blueschist facies metamorphism ± deformationdeveloped under progressively declining P-T conditions. Thefirst retrograde metamorphism involved formation of an outerrind of actinolite + chlorite + rutile ± phengite, andthe growth of coarse-grained chlorite + pumpellyite within theblock. During the second event the rind and outer edge of theblueschist were folded, sheared and fractured, and primary glaucophanewithin the blueschist was replaced by albite, medium-grainedchlorite, and glaucophane-crossite. The third retrograde metamorphismwas marked by the pseudomorphic replacement of rind actinoliteby aragonite and quartz. Aragonite also crystallized extensivelywithin th block, accompanied by lawsonite, chlorite, jadeiticpyroxene, and crossite; this last mineral assemblage is identicalto that of the surrounding Franciscan metasedimentary rocks.Features characteristic of the first and second retrograde metamorphicevents are readily observed in other high-grade tectonic blocksof the Franciscan Complex and the correlative Otter Point Formationof Oregon. In contrast, evidence of a third retrograde metamorphism,matching that of presently associated Franciscan and Otter Pointrocks, has been found in some but not all blocks examined sofar. The high P/T conditions of prograde metamorphism and the availablemetamorphic age determinations suggest that the tectonic blocksoriginally formed in a pre-Franciscan subduction zone setting.Fragments of blueschist and eclogite from this metamorphic terrainwere tectonically incorporated in a serpentinite diapir, andthey developed alteration rinds through interaction with theenclosing ultramafic rock. The available data suggest that theexamined exotic block and at least some others were transferredto the Franciscan as detritus from a body of serpentinite thatreached the earth's surface. Such blocks were then resubductedand metamorphosed along with their presently associated sedimentarysequences.     

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge,SW Japan

To understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.     

Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships

In the southeastern Reynolds Range, central Australia, a low- P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high-grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz-bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.     

Retrogressive Microstructures:A Key to Post Collisional Uplift and Extension Tectonics of Ultrahigh-Pressure Metamorphic Rocks from Dabie Orogenic Belt,China

ThereisanunanimousunderstandingthattheUHPmetamorphisminDabie-SuluareaswasformedduringtheTri-assiccollisioneventbetweentheYangtzeandNorthChinacratons(Congetal.,l994$Zhouetal-,l996;Liouetal.,l994)-Eclogiteisthemostimportantrocktypeamongtheul-trahigh-pressure(UHP)metamorphicrocks.Accordingtotheiroccurrencethereare3typesofeclogites:typeIisen-clavesinultramaficintrusionsandasmembersinthelayeredmafic-ultramaficcomplex(Jahnl998);typeIispodsorlay-ersinthehighlymetamorphicsupracrustals(mainlying…     

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits,California: updates on distribution and characteristics

ABSTRACT

Sedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.     

Stratigraphic and metamorphic inversions in the central Menderes Massif: a new structural model

The Menderes Massif is a large area of dominantly Tertiary metamorphic rocks in western Turkey. It is bordered in the west by the Cycladic Metamorphic Complex with Eocene high-pressure/low-temperature (HP/LT) metamorphism. In the Central Menderes the AydLn mountains are made up of a thrust stack of Eocene age. At the base of the thrust stack, greenschist-facies Paleozoic metasediments of the Menderes Massif form an inverted stratigraphic sequence. The Barrovian-type metamorphism is also inverted with garnet-bearing metapelites lying over the lower-grade biotite-bearing metapelites. The P-T conditions in the garnet zone are estimated as 530°C and 8 kbar. This schist sequence of the central Menderes Massif is interpreted as the inverted lower limb of a major southward closing recumbent fold, with the southern Menderes Massif representing a section from the near hinge of this fold. The Paleozoic metamorphic rocks of the central Menderes Massif are tectonically overlain by gneiss klippen possibly originating from the sheared and southward translated core of the Menderes fold. Lying also tectonically over the Paleozoic metamorphic rocks is a major thrust sheet belonging to the Cycladic metamorphic complex. It consists of garnet micaschist, Mesozoic marble, serpentinite and amphibolitised eclogite. Although it has a highly sheared internal structure, it probably represents an initially coherent sequence that has undergone HP/LT metamorphism during the Eocene. The AydLn mountains are dominated by contractional structures with subordinate extensional structures.