Metamorphism of the Bikou Group in the Shanxi-Gausu-Sichuan Border Region1

Abstract The Bikou Group on the Shaanxi-Gansu-Sichuan border is composed of Mid-Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist facies. Three metamorphic zones, i.e. zones A, B, and C, may be distinguished on the basis of the field distribution of metamorphic rocks and the variation of b₀ values of muscovite. Blueschists are characterized by coexistence of sodic amphiboles and epidote and occur as stripes or relict patches in extensive greenschists of zone A. Studies of metamorphic minerals such as amphiboles, chlorite, epidote and muscovite and their textural relationships indicate that blueschists and greenschists were not formed under the same metamorphic physico-chemical conditions. The blueschist facies was formed at temperatures of 300-400°C and pressures of 0.5–0.6 GPa. The greenschist facies in zones A and B has similar temperatures but its pressure is only 0.4 GPa or so. The transition from the blueschist to greenschist facies is a nearly isothermal uplift process. The rock and mineral assemblages of the Bikou Group indicate that the blueschist facies metamorphism of the group might be related to crustal thickening or A-subduction accompanying the closure of an intracontinental small ocean basin.     

Parageneses and compositions of amphiboles from Franciscan jadeite–glaucophane type facies series metabasites at Cazadero, California

Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair. Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted. Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe₂O₃-rich metabasites. The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM₄ of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.     

Greenschist amphiboles from Haast river,New Zealand

A section across the Haast Schist Group in the Southern Alps of New Zealand shows a sequence of metamorphosed eugeosynclinal sediments. Meta-basic rocks (greenschists) have been studied to determine the nature of the actinolite-hornblende transition and to investigate the change in amphibole composition through the Metamorphic Facies Series.Electron microprobe analyses of 21 representative amphiboles, including 3 amphibole pairs can be shown to support theories of a miscibility break in the calciferous amphibole solid solution series. The existence of a miscibility break is further supported by the widespread appearance, even at low metamorphic grades, of exsolution lamellae in actinolite and hornblende amphiboles.Amphibolite facies amphiboles differ from greenschist facies amphiboles in that (a) there are increased amounts of Ti entering the lattice and (b) that there is an increased occupancy of the A site at higher metamorphic grades.     

Blue amphiboles and their significance for the metamorphic history of the Pan-African Gariep belt, Namibia

The metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan-African Gariep orogenic belt in south-western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro-) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio-riebeckite with little ferro-glaucophane component in a highly metasomatized albite-rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low- P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio-riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.     

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.     

Glaucophane-bearing assemblage overprinted by greenschist-facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with Na^B from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between Na^B= 0.92 and Na^B= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.     

内蒙古头道桥地区蓝闪石片岩和相关变质岩石岩石学特征及其构造意义

内蒙古头道桥地区出露了一套经高压变质形成的岩石组合。本次研究通过岩相学和矿物化学分析,根据矿物组合的不同,识别出蓝片岩、绿片岩两种不同类型的岩石类型。其中,蓝片岩的矿物组合为角闪石（蓝闪石、蓝透闪石）+绿帘石+钠长石+绿泥石+石英+赤铁矿±多硅白云母±方解石±榍石;绿片岩的矿物组合为绿泥石+钠长石+石英±绿帘石±角闪石（阳起石、镁角闪石、蓝透闪石、冻蓝闪石等）±多硅白云母±赤铁矿。确定了蓝片岩的峰期变质级别为绿帘-蓝闪片岩相,峰期变质温度为400~600℃,压力为1.2~1.4 GPa。绿片岩的峰期变质级别为绿帘-角闪岩相。结合前人研究成果,认为蓝片岩和绿片岩的形成与额尔古纳地块和兴安地块的碰撞拼合有关。     

Mineralogy of New Caledonian metamorphic rocks

The compositions of metamorphic pyroxenes from blueschists in northern New Caledonia are investigated. Aegerine-augite occurs in siliceous metasediments and aegerine in some low-grade sodic basic schists. Calcic metamorphic pyroxene (omphacite and chloromelanite) appears first in metabasalts in higher grades of the lawsonite zone and is widespread in metamorphosed igneous rocks and quartzofeldspathic gneisses of the epidote zone. Omphacites in basic rocks have higher Mg∶Fe ratios and are less jadeitic than omphacites from adjacent interbedded quartzofeldspathic gneisses. With increasing metamorphic grade pyroxenes become more jadeitic and diopsidic at the expense of their acmite component. Elemental partitioning between coexisting pyroxenes, garnets and amphiboles from in situ regional metamorphic rocks is generally regular, suggesting equilibrium crystallization. Omphacite appears to be a stable phase within blueschist facies over a temperature range of at least 350° to 550° C. The “eclogitic” assemblage almandine-omphacite is stable within the earth's crust in metamorphosed sediments and igneous rocks over a temperature range of 400° to at least 550° C. No estimate of absolute pressures involved in metamorphism in the Ouégoa district can yet be made.     

羌塘中部高压变质带的退变质作用及其构造侵位

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界.本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代.研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母40Ar-39Ar坪年龄为219±2Ma.高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.     

SOME TRENDS IN THE STUDY OF CARBONATE SEDIMENTS IN THE FEDERAL REPUBLIC OF GERMANY

A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (～600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO₂, and P₂O₅ contents and possess geochemical characteristics of within-plate basalts; some of these diabasic rocks contain sodic clinopyroxene and amphibole as primary phases and have minor pumpellyite, albite, epidote, chlorite, and calcite as the prehnite/pumpellyite-facies metamorphic assemblage. This prehnite/pumpellyite-facies overprint did not affect the host rocks of the blueschist-facies lithologies.

K-Ar and Rb-Sr ages of phengite and whole rocks from pelitic schists are ～690 to 728 Ma, and a ⁴⁰Ar/³⁹Ar age of crossite from the blueschist is 754 Ma. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the blueschist and cross-cutting dike rocks, clearly demonstrating that conditions required for blueschist-facies metamorphism were attained and ceased at least 700 Ma. The northward-increasing metamorphic grade of the small blueschist terrane may reflect northward subduction of an accretionary complex beyond the northern edge of the Tarim craton. Abundant subparallel diabasic dikes indicate a subsequent period of Pre-Sinian rifting and diabasic intrusion along the northern margin of Tarim; a Sinian siliciclastic and carbonate sequence was deposited unconformably atop the Aksu Group and associated diabase dikes.     

Phase equilibria in rocks of the paleoproterozoic banded iron formation (BIF) of the Lebedinskoe deposit, Kursk Magnetic Anomaly, and the petrogenesis of BIF with alkali amphiboles

BIF with alkali amphibole at the Lebedinskoe iron deposits, the largest in Russia, were metamorphosed at 550°C and 2–3 kbar and contain ferriwinchite, riebeckite, actinolite, grunerite, and aegirine-augite. All reaction textures observed in the rocks were produced during the prograde metamorphic stage and represent the following succession of mineral replacements: Gru → Rbk, Act → Win → Rbk. Data obtained on the textural relations and compositional variations of Ca, Ca-Na, and Na Al-free amphiboles point to the complete miscibility in the actinolite-ferriwinchite and ferriwinchite-riebeckite isomorphic series. Riebeckite is formed in BIF during the prograde metamorphic stage, with the participation of a fluid insignificantly enriched in Na⁺ and at increasing oxygen fugacity. The critical factors controlling the development of alkali amphiboles and Ca-Na pyroxenes in carbonate-bearing BIF is the oxygen activity and the presence of at least low concentrations of Na⁺ ions in the fluid. The minerals contain Fe³⁺, and all reactions producing them are oxidation reactions. The origin of riebeckite late in the course of the mineral-forming process is caused by the Ca²⁺Mg²⁺ → Na⁺Fe³⁺ heterovalent isomorphic replacement in calcic and calcic-sodic amphiboles and by the oxidation of grunerite in the presence of a fluid enriched in Na ions.     

Blueschist facies metamorphism of peralkaline rhyolites from the Tenda crystalline massif (northern Corsica): evidence for involvement in the Alpine subduction event?

The Tenda crystalline massif (northern Corsica) is a fragment of the western Corsica basement involved in the Alpine orogeny. Rhyolite dykes crosscutting the gabbroic complex of Bocca di Tenda (southern sector of the Tenda crystalline massif) show an unusual metamorphic mineral assemblage, defined by jadeite‐bearing (up to 46 mol percentage) aegirine, riebeckite, celadonite‐rich phengite (Si=3.50–3.65 apfu), quartz, albite and K‐feldspar. Jadeite‐bearing aegirine and riebeckite mostly occur as coronas around jadeite‐free aegirine and arfvedsonite, respectively, which both are relics of igneous origin. This metamorphic assemblage reflects the peralkaline compositions, which are characterised by anomalously high contents of SiO₂ and Na₂O, and negligible CaO and MgO. The evolved rocks of the gabbroic sequence (quartz‐diorites to tonalites) and the surrounding granitoids are characterised by the development of riebeckite/ferroglaucophane, epidote, celadonite‐rich phengite and albite, thus pointing to a metamorphic crystallization in the epidote‐blueschist facies. In all the studied rocks, metamorphic reactions were controlled by fluid‐assisted mass‐transfer through grain boundaries and microfractures. The different mineral assemblages allow the peak P–T metamorphic conditions to be constrained to between 0.8 GPa/300 °C and 1.1 GPa/500 °C. These estimates attest to a geothermal gradient (dT/dP) of 10–13 °C km^?1 and indicate that the Tenda crystalline massif was buried to a minimum depth of 27 km during the Alpine orogeny. The blueschist facies recrystallization in the Tenda crystalline massif has been related to the cessation of an eastward‐dipping subduction event.     

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area,northern Iran

The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.     

Metamorphic and tectonic evolution of a structurally continuous blueschist‐to‐Barrovian terrane,Sivrihisar Massif,Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have ⁴⁰Ar/³⁹Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ～200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ～1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.     

Thermal-baric structure and P–T history of the Brooks Range metamorphic core, Alaska

Documentation of pressure–temperature (P–T) histories across an epidote‐amphibolite facies culmination provides new insight into the tectono‐thermal evolution of the Brooks Range collisional orogen. Thermobarometry reveals that the highest grade rocks formed at peak temperatures of 560–600 °C and at pressures of 8–9.5 kbar. The thermal culmination coincides with the apex of a structural dome defined by oppositely dipping S2 crenulation cleavages suggesting post‐metamorphic doming. South of the thermal culmination, greenschist facies and lowermost epidote‐amphibolite facies rocks preserve widespread evidence for an early blueschist facies metamorphism. In contrast, no evidence for an early blueschist facies metamorphism was found in similar grade rocks of the northern flank, indicating that the southern flank underwent initial deeper burial during southward underthrusting of the continental margin. Thus, while the dome shows a symmetric distribution of peak temperatures, the P–T paths followed by the two flanks must have varied. This variation suggests that final thermal re‐equilibration to greenschist and epidote–amphibolite facies conditions did not result from a simple process of southward underthrusting followed by thermal re‐equilibration from the bottom upward. The new data are inconsistent with a previous model that invokes such re‐equilibration, along with northward thrusting of epidote–amphibolite facies rocks over lower grade rocks presently on the southern flank of the culmination, to produce an inverted metamorphic field gradient. Instead, it is suggested that following blueschist facies metamorphism, rocks of the southern and northern flanks were juxtaposed, during which time the more deeply buried south flank was partially emplaced above rocks to the north, where they escaped Albian epidote–amphibolite facies overprinting. Porphyroblast growth, which post‐dates the main fabric on the north flank of the culmination may be the result of Albian thermal re‐equilibration following this deformation. Post‐metamorphic doming resulted from a combination of Albian‐Cenomanian extension and Tertiary deformation.     

大别地区的变质作用及与碰撞造山过程的关系

大别造山带从南到北可分为５个变质构造单元：扬子北缘蓝片岩带、突松变质杂岩带、南大虽碰撞杂岩带、北大别变质杂岩带和北淮阳变质带。各个变质构造单元中不同岩石的变质作用可划分为３种类型：（１）超高压型。以含柯石英（及金刚石）的榴辉岩为代表,仅见于南大别碰撞杂岩带中,这类岩石的ＰＴ轨迹反映洋壳Ｂ型俯冲的特点。（２）高压型。见于大别山南部的蓝片岩带、宿松变质杂岩带和南大别杂岩中的变质沉积岩及部分片麻岩中,与     

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.     

Possible depth limit for underplating by a seamount

The Cazadero metabasite allochthon is located within the central melange belt of the northern California Coast Ranges, U.S.A. It composed mainly of pillowed and massive mafic flows, hyaloclastite and minor pelagic sediments. It is about 3 × 15 km² in extent, and is in fault contact with Late Mesozoic fore-arc flysch deposits of the Great Valley sequence. Three metamorphic zones were mapped in the allochthon, as follows-in order of increasing metamorphic grade: the lawsonite, the pumpellyite and the epidote zone. Ca---Na pyroxenes are ubiquitous; they were used to estimate the P-T path, together with the oxygen isotope geothermometry of Taylor and Coleman. The result shows a bending point in the P-T path for blueschist facies metamorphism of the Cazadero allochthon. At shallow depths, lower grade metamorphism proceeded under predominant pressure-increase from 4 to 7 kbar at nearly constant T of about 150–200 °C, whereas after passing the bending point higher-grade metamorphism occurred with increasing temperature from 200 to 350 °C at nearly constant P of about 7–8 kbar.

The bending point reflects the depth of transfer of underplating materials from the oceanic lithosphere and its overlying sediments to the hanging wall plate. Either seamount volcanics and pelagic sediments, or mixed packages of fragmented oceanic crust with trench-fill turbidites, were subducted beneath the North American plate. They were subjected to early lawsonite- and pumpellyite-zone metamorphism with very low geothermal gradient. We infer that when they reached about 20 km depth, underplating occurred and the metamorphosed rocks became part of the hanging wall. Heat conduction from the overlying hydrated mantle wedge caused a temperature increase at nearly constant depth. The earlier recrystallized seamount volcanics and pelagic sediments were then metamorphosed at a much higher geothermal gradient. The Cazadero allochthon has suffered extremely high P/T subduction zone metamorphism and it preserves very well the record of Cretaceous subduction-underplating processes.     

High-pressure metamorphism in metabasites from the Betic Cordilleras (S.E. Spain) and its evolution during the Alpine orogeny

The Nevado-Filábride complex is the lowest tectonic unit of the Betic Zone sensu stricto (ss) of the Betic Cordilleras (S.E. Spain). The upper series of this complex consists of a metamorphosed sequence intruded by basic and ultrabasic igneous rocks. High-pressure metamorphism in the eclogite and blueschist facies is recorded in the metabasites, but this was partially obliterated by further successive metamorphic stages in the almandine-amphibolite and greenschist facies.Coronitic and granoblastic eclogites appear side by side in the large stocks of basic rocks. The coronitic eclogites originate from coarse-to medium-grained olivine gabbros, and the granoblastic eclogites from fine-grained basic rocks (dolerites and porphyritic basaltic rocks). Higher chemical mobility and rate of diffusion, as well as the availability of fluids during the eclogite facies metamorphism, are responsible for the greater degree of recrystallization found in the granoblastic eclogites. The availability of fluids during this metamorphic stage was controlled by the difference in the hydration of the protolith and by variable proximity to surrounding water-rich metasediments.The minerals in the eclogites are chemically homogeneous, suggesting that they are almost completely equilibrated, even in the coronitic eclogites. The estimated equilibrium P-T conditions were found to be the same (approximately 550° C at 12 kbar pressure) in both coronitic and granoblastic eclogites, and it has, therefore, been deduced that the coronitic eclogites do not represent the first and lower-grade step of a prograde metamorphism in which the granoblastic eclogites are the higher-grade step.No relationship was found between shearing and eclogite crystallization. Nevertheless, a first fabric/foliation developed in the later blueschist facies stage, and syntectonic growth of the minerals was detected in glaucophane-bearing rocks.The further metamorphic evolution of the metabasites from high-to intermediate-pressure conditions is documented by the formation of minerals belonging to albiteepidote and almandine-amphibolite facies assemblages. The application of the amphibole zonation model, in order to deduce the P-T path, does not give realistic values.High-pressure metamorphism is related to an early subduction event in the Betic Cordilleras, with a later more-or-less isothermal uplift to shallower levels.