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1.
Prograde eclogites from the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt, central Shikoku, southwest Japan 总被引：1，自引：0，他引：1
Abstract Prograde eclogites occur in the Tonaru epidote amphibolite mass in the Sambagawa Metamorphic Belt of central Shikoku. The Tonaru mass is considered to be a metamorphosed layered gabbro, and occurs as a large tectonic block (approximately 6.5 km × 1 km) in a high-grade portion of the Sambagawa schists. The Tonaru mass experienced high- P /low- T prograde metamorphism from the epidote-blueschist facies to the eclogite facies prior to its emplacement into the Sambagawa schists. The estimated P – T conditions are T = 300–450°C and P = 0.7–1.1 GPa for the epidote-blueschist facies, and the peak P – T conditions for the eclogite facies are T = 700–730°C and P ≥ 1.5 GPa. Following the eclogite facies metamorphism, the Tonaru mass was retrograded to the epidote amphibolite facies. It subsequently underwent additional prograde Sambagawa metamorphism, together with the surrounding Sambagawa schists, until the conditions of the oligoclase–biotite zone were reached. The high- P /low- T prograde metamorphism of the eclogite facies in the Tonaru mass and other tectonic blocks show similar steep d P /d T geothermal gradients despite their diverse peak P – T conditions, suggesting that these tectonic blocks reached different depths in the subduction zone. The individual rocks in each metamorphic zone of the Sambagawa schists also recorded steep d P /d T geothermal gradients during the early stages of the Sambagawa prograde metamorphism, and these gradients are similar to those of the eclogite-bearing tectonic blocks. Therefore, the eclogite-bearing tectonic blocks reached greater depths in the subduction zone than the Sambagawa schists. All the tectonic blocks were ultimately emplaced into the hanging wall side of the later-subducted Sambagawa high-grade schists during their exhumation. 相似文献
2.
The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface. 相似文献
3.
Hideki Masago 《Island Arc》2000,9(3):358-378
Abstract In the Barchi–Kol area, located at the westernmost part of the Kokchetav ultrahigh pressure (UHP) to high-pressure (HP) massif, northern Kazakhstan, metabasites from the epidote amphibolite (EA) facies to the coesite eclogite (CEC) facies are exposed. Based on the equilibrium mineral assemblages, the Barchi–Kol area is divided into four zones: A, B, C and D. Zone A is characterized by the assemblage: epidote + hornblende + plagioclase + quartz, with minor garnet. Zone B is characterized by the assemblage: garnet + hornblende + plagioclase + quartz + zoisite. Zone C is defined by the appearance of sodic–augite, with typical assemblage: garnet + sodic–augite + tschermakite–pargasite + quartz ± plagioclase ± epidote/clinozoisite. Zone D is characterized by the typical eclogite assemblage: garnet + omphacite + quartz + rutile, with minor phengite and zoisite. Inclusions of quartz pseudomorph after coesite were identified in several samples of zone D. Chemical compositions of rock-forming minerals of each zone were analyzed and reactions between each zone were estimated. Metamorphic P-T conditions of each zone were estimated using several geothermobarometers as 8.6 ± 0.5 kbar, 500 ± 30 °C for zone A; 11.7 ± 0.5 kbar, 700 ± 30 °C for zone B; 12–14 kbar, 700–815 °C for zone C; and 27–40 kbar, 700–825 °C for zone D. 相似文献
4.
Mineral chemistry,P-T-t paths and exhumation processes of mafic granulites in Dinggye,Southern Tibet 总被引：4，自引：0，他引：4
LIU Shuwen ZHANG Jinjiang SHU Guiming & LI Qiugen The Key Laboratory of Orogenic Belts Crustal Evolution Ministry of Education School of Earth Space Sciences Peking Uni-versity Beijing China 《中国科学D辑(英文版)》2005,48(11):1870-1881
Lower crustal high grade metamorphic rocks have been successively found at Pamirs nearby the western Himalayan syntaxis, Namjagbarwa and Dinggye nearby the eastern Himalayan syntaxis and the central segment of the Himalayan Orogenic Belt, respec-tively[1―4]. In particular, some researchers deduced that there were probably eclogites at some locations[5]. Moreover, some geochronological data of these lower crustal granulites also have been accumulated. For example, the high-pressure granulit… 相似文献
5.
Aragonite and omphacite-bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite-rich and graphite-poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite-poor layer contains a 1.5-cm ivory-colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO3 end-member: the FeCO3 and MnCO3 components are both less than 0.3 mol% and the SrCO3 component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P > 1.1–1.3 GPa and T = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite-bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized. 相似文献
6.
Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China 总被引：6，自引：0，他引：6
Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula. 相似文献
7.
Kyanite-anthophyllite schist and southwest extension of the Dabie Mountains ultrahigh- to high-pressure belt 总被引：1，自引：1，他引：1
Abstract Kyanite-anthophyllite schist preserves the first record of high pressure in the amphibolite-facies unit of the SW Dabie Mountains, whereas ultrahigh- and high-pressure (UHP and HP) metamorphism has been well documented by the occurrence of coesite, diamond and mafic eclogite in the SE Dabie Mountains. Textural evidence indicates that minerals of the kyanite-anthophyllite schist formed mainly in two stages: (i) garnet + kyanite + antho-phyllite + rutile formed at pressure in excess of 1.2 GPa at T < 650°C; (ii) cordierite±staurolite formed by reaction of anthophyllite + kyanite at P < 0.5 GPa, T∼530°C. Plagioclase and ilmenite replaced garnet and rutile respectively during decompression. In a still later stage, secondary biotite recrystallized, accompanied by sillimanite replacing kyanite, and spinel replacing staurolite. The P-T information suggests that the amphibolite unit in the SW Dabie Mountains is part of the Triassic collision belt between the Sino-Korean and Yangtze cratons. The P-T paths of the UHP eclogite in the eastern Dabie Mountains and the HP kyanite-anthophyllite schist in the SW Dabie Mountains show similar decompression and equivalent late stage Barrovian-style metamorphism. Emplacement of voluminous granitoid at middle crustal levels between 134–118 Ma contributed to the development of the Barrovian-type metamorphism in the Dabie Mountains. 相似文献
8.
Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan 总被引：3，自引：0，他引：3
Abstract Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma. 相似文献
9.
According to the field occurrences and petrological study, the low temperature eclogite facies metamorphic rocks in Western Tianshan of Xinjiang can be divided into five types: (i) massive glaucophane-epidote eclogites and glaucophane-paragonite eclogites; (ii) schistose or gneissic mica eclogites; (iii) banded calcite eclogites; (iv) pillow glaucophane eclogites; (v) garnet-omphacite quartzites. Their eclogite facies metamorphism has undergone four stages of evolution: (i) pre-peak lawsonite-blueschist facies stage,T = 350–4000°C,P = 0.7–0.9 GPa; (ii) peak eclogite facies stage,T = 530 ± 20°C,P = 1.6–1.9 GPa; (iii) retrograde epidote-blueschist facies stage, T=500–530°C,P = 0.9–1.2 GPa and (iv) retrograde blueschist-greenschist facies stage,T= 450–550°C,P= 0.7–0.8 GPa. The metamorphic PT path of Western Tianshan eclogites is characterized by clockwise ITD resulting from the subduction of Tarim plate northward to Yili-Central Tianshan plate followed by fast uplift to the surface. But there were at least two stages of blueschist facies retrograde metamorphism overprinted during their uplift. 相似文献
10.
Toshio Nozaka 《Island Arc》1999,8(2):154-167
Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite–actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P–T conditions are estimated to be within 200–350°C and 5–7 kbar. These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P–T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramafic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan. 相似文献
11.
Shunsuke Endo 《Island Arc》2010,19(2):313-335
Evidence for eclogite‐facies metamorphism is widespread in the Western Iratsu body of the oceanic subduction type Sanbagawa Belt, Southwest Japan. Previous studies in this region focused on typical mafic eclogites and have revealed the presence of an early epidote‐amphibolite facies metamorphism overprinted by a phase of eclogite facies metamorphism. Ca‐rich and titanite‐bearing eclogite, which probably originated from a mixture of basaltic and calc‐siliceous sediments, is also relatively common in the Western Iratsu body, but there has been no detailed petrological study of this lithology. Detailed petrographic observations reveal the presence of a relic early epidote‐amphibolite facies metamorphism preserved in the cores of garnet and titanite in good agreement with studies of mafic eclogite in the area. Thermobarometric calculations for the eclogitic assemblage garnet + omphacite + epidote + quartz + titanite ± rutile ± phengite give peak‐P of 18.5–20.5 kbar at 525–565°C and subsequent peak‐T conditions of about 635°C at 14–16 kbar. This eclogite metamorphism initiated at about 445°C/11–15 kbar, implying a significantly lower thermal gradient than the earlier epidote‐amphibolite facies metamorphism (～650°C/12 kbar). These results define a P–T path with early counter‐clockwise and later clockwise trajectories. The overall P–T path may be related to two distinct phases in the tectono‐thermal evolution in the Sanbagawa subduction zone. The early counter‐clockwise path may record the inception of subduction. The later clockwise path is compatible with previously reported P–T paths from the other eclogitic bodies in the Sanbagawa Belt and supports the tectonic model that these eclogitic bodies were exhumed as a large‐scale coherent unit shortly before ridge subduction. 相似文献
12.
High-pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk–Ulo Complex) with fault-bounded slices of shale, sandstone, chert, basalt, limestone, conglomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K–Ar ages. Small amounts of eclogite, glaucophane rock, garnet–amphibolite and jadeite–quartz–glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite–garnet–glaucophane–phengite–quartz equilibrium, peak pressure and temperature of the jadeite–quartz–glaucophane rock are P = 22 ± 2 kbar and T = 530 ± 40 °C. The estimated P–T conditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was ∼ 7.0 °C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz–mica schists at lower or middle crustal depths. 相似文献
13.
Nobuhiko Nakano Yasuhito Osanai Masaaki Owada Yasutaka Hayasaka Tran Ngoc Nam 《Island Arc》2009,18(1):126-143
The Kontum Massif in central Vietnam is composed of various metamorphic complexes including a high-temperature southern part (Kannak and Ngoc Linh complexes) and a low- to medium-temperature northern part (Kham Duc complex). The Kham Duc complex exhibits Barrovian-type medium-pressure metamorphism evidenced by kyanite- and/or staurolite-bearing metapelites. The garnet–gedrite–kyanite gneiss, which is the focus of the present study, preserves several mineral parageneses formed during a prograde and retrograde metamorphic history: staurolite + quartz in gedrite, garnet + gedrite + kyanite in the matrix, and spinel + cordierite symplectite between gedrite and sillimanite. The calculated semiquantitative petrogenetic grid reveals peak pressure conditions of 620–650°C at 1.1–1.2 GPa and peak temperature conditions of 730–750°C at 0.7–0.8 GPa. The monazite U–Th–Pb electron microprobe ages of the garnet–gedrite–kyanite gneiss and associated gneisses yield 246 ± 3 Ma for the Kham Duc complex, which is similar to the age of the high- to ultrahigh-temperature metamorphism in the adjacent Kannak and Ngoc Linh complexes of the southern Kontum Massif. The present results indicate that both the Barrovian-type and ultrahigh-temperature metamorphism occurred simultaneously in the Kontum Massif during an event strongly related to Permo–Triassic microcontinental collision tectonics in Asia. 相似文献
14.
Abstract High‐ to ultrahigh‐pressure metamorphic (HP–UHPM) rocks crop out over 150 km along an east–west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet‐like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north‐dipping, fault‐bounded orogen‐parallel units (I–IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high‐pressure amphibolite through quartz–eclogite and coesite–eclogite to diamond–eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780–1000°C, P = 37–60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730–750°C, P = 11–14 kbar; Unit IV: T = 530°C, P = 7.5–9 kbar) and bottom (Unit I: T = 570–680°C; P = 7–13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top‐to‐the‐north) and top (top‐to‐the‐south) of the pile. The orogen‐scale architecture of the massif is sandwich‐like, with the HP–UHPM nappe group juxtaposed across large‐scale subhorizontal faults, against underlying low P–T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP–UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower‐grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30–40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab‐breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP–UHPM rocks. 相似文献
15.
Ultra-high pressure metamorphic rocks in the Dabie-Su-Lu region, China: Their formation and exhumation 总被引：17，自引：0，他引：17
Cong Bolin Wang Qingchen Zhai Mingguo Zhang Ruyuan Zhao Zhongyan Ye Kai 《Island Arc》1994,3(3):135-150
Abstract Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra-high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie-Su-Lu region. The authors introduce and discuss a two-stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre-eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm-Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino-Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two-stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks. 相似文献
16.
Takeshi Ikeda 《Island Arc》2002,11(3):185-192
Abstract The present paper is reporting on the regional occurrence of orthopyroxene-bearing basic rocks from the Ryoke Metamorphic Belt in the Yanai district, southwest Japan. Their localities are confined to the highest-grade zone of the area (i.e. the garnet–cordierite zone, where garnet coexists with cordierite, K-feldspar and biotite in pelitic rocks). Orthopyroxene coexists with quartz and hydrous minerals such as biotite, cummingtonite and hornblende, and in some cases with clinopyroxene, suggesting that the highest grade of the Ryoke metamorphism reached a low-temperature subfacies of the granulite facies, contrary to the upper amphibolite facies as previously asserted. 相似文献
17.
Ultrahigh-pressure garnet lherzolite from Chijiadian, Rongcheng County, in the Su-Lu region of eastern China 总被引：3，自引：0，他引：3
Abstract Petrological studies of a serpentinized garnet lherzolite body in Rongcheng of the Su-Lu region of eastern China revealed unusually high pressure. Spinel lherzolite probably in a subducting slab was transformed to garnet lherzolite at mantle depth. During exhumation, they were subsequently subjected to the granulite and then amphibolite overprinting and a phase of serpentinization. The peak P–T conditions of the garnet lherzolite estimated after detailed analysis of the metamorphic texture are 4–5 GPa and 820°C or 5–6 GPa and 780°C, depending on the chosen geothermobarometers. The lower dP/dT of the garnet lherzolite can be interpreted as the results of subduction of an old (say 100 Ma older than the time of collision) and cold, slab underneath the margin of the Sino–Korean craton. 相似文献
18.
H. Austrheim 《Earth and Planetary Science Letters》1987,81(2-3)
The granulite facies assemblages of the anorthositic rocks of the Bergen Arcs (stable at 800–900°C and 10 kbar) have been transformed to eclogite facies assemblages (stable at 700–750°C and 16–19 kbar) in the vicinity of Caledonian shear zones. This section of the root zone of the Caledonian mountain chain reveals a deep polymetamorphic crust where Precambrian granulites (mean density 3.02 g/cm3) and Caledonian eclogites (mean density 3.19 g/cm3) alternate on a scale of meters over a minimum area of 3 × 12 km. Detailed mapping of three localities shows that eclogites account for up to 30–45% of the rock volume. The stabilitization of the eclogite mineralogy is controlled by fluids penetrating these deep crustal shear zones. The eclogitization is independent of preexisting compositional variation in this anorthosite-norite complex. The Bergen Arcs example suggests that the amount of eclogite versus granulites in the lowermost crust is a function of deformation and fluid access, rather than being controlled byT, P and rock composition alone. These relationships may explain the gradual increase in seismic velocity observed in some deep crustal sections and also the complex reflection pattern obtained from the lowermost crust in many areas. 相似文献
19.
Abstract The mantle-derived xenoliths entrained in the Pliocene basanite from Baengnyeong Island, South Korea, are spinel lherzolites and spinel harzburgites. The overall compositional range of the Baengnyeong xenoliths matches that of the post-Archean xenoliths of lithospheric mantle origin from eastern China, but without any compositional evidence for a refractory Archean mantle root. Mineral compositions of the xenoliths have been used to estimate the equilibrium temperatures and pressures, and to construct a paleogeothermal gradient of the source region. The xenolith-derived paleogeotherm is constrained from about 820°C at 7.3 kbar to 1000°C at 20.6 kbar. Like those from the post-Archean Chinese xenoliths of lithospheric mantle origin, the Baengnyeong geotherm is considerably elevated relative to the conductive models at the depth of the crust–mantle boundary, reflecting a thermal perturbation probably related to lithospheric thinning. There is no significant P / T difference between harzburgite and lherzolite, which suggests that the harzburgites are interlayered with lherzolites within the depth interval beneath Baengnyeong Island. 相似文献
20.
A discussion on the boundary between diagenesis and metamorphism with reference to zeolite facies 总被引：1，自引：0，他引：1
Abstract A Triassic formation in the Turpan-Hami and Ordos basin of China gives two typical examples of occurrence of laumontite belonging to diagenetic facies. Sandstones of the Upper Triassic Yanchang Formation in the eastern sector of the Ordos basin are formed at the mature stage of mesodiagenesis at temperatures in the range of 71-120°C, while the temperatures of the oil-bearing beds Chang-6 and Chang-7 where laumontite is present are about 81-88°C. The laumontite-bearing beds of the Middle Triassic in the Hami depression are presently at the supermature stage of organic matter, the corresponding diagenetic temperature being about 140 °C. The term zeolite facies should be rejected, and the features of metamorphic stage should be clearly denned, such as closely packed grains, absence of pores and cements, and characteristic mineral assemblages including sericite, epidote, muscovite, illite (2M1 ), chlorite (IIb, β= 97°), dickite, pyrophyllite, graphite, chlorozeolite formed at temperatures of 200°C and Ro values of 2.5%; the stage is also characterized by schistosity and illite crystallinity of IC = 0.42°Δ 2θ. The anchimetamorphic zone is characterized by some characteristic minerals such as paragonite, rectorite, albite, laumontite, illite (1M), chlorite (Ib, P = 90°+ IIb, β= 97°) formed at temperatures of 130-200°C and Ro values of 1.3-2.5% as well as other quantitative parameters of organic matter. 相似文献