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1.
The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm?1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω1 > 8.5 cm?1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone. 相似文献
2.
Tsutomu Ota Masaru Terabayashi Christopher D. Parkinson and Hideki Masago 《Island Arc》2000,9(3):328-357
Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km. 相似文献
3.
Petrogenesis of garnet lherzolite, Cima di Gagnone, Lepontine Alps 总被引：10，自引：0，他引：10
Garnet lherzolite at Cima di Gagnone has chemical and mineralogical properties similar to those of other garnet lherzolites in the lower Pennine Adula/Cima Lunga Nappe (Alpe Arami, Monte Duria). The Cima di Gagnone occurrence encloses mafic boudins that belong to an eclogite-metarodingite suite common in the numerous neighboring ultramafic lenses. The ultramafic rocks at Cima di Gagnone, including the garnet lherzolite, are interpreted as tectonic fragments of an originally larger lherzolite body that underwent at least partial serpentinization prior to regional metamorphism. This lherzolite body cycled through at least three metamorphic facies: greenschist or blue-schist (as antigorite serpentinite) → eclogite (as garnet lherzolite), pre-Alpine or early Alpine → amphibolite facies (as chlorite-enstatite-tremolite peridotite), Lepontine metamorphism. Relics of titanoclinohumite in the garnet peridotite, as also recorded by Möckel near Alpe Arami, are consistent with this metamorphic history, since they indicate a possible connection with Pennine antigorite serpentinites, e.g., Liguria, Piedmont, Zermatt-Saas, Malenco, Pustertal, all of which have widespread titanoclinohumite belonging to the antigorite paragenesis. Estimated pressures in excess of 20 kbar and temperatures of 800°±50°C for the garnet lherzolite assemblage are not inconsistent with conditions inferred for Gagnone and Arami eclogites. These conditions could have been reached during deep subduction zone metamorphism. It is shown by calculation that the effects of Fe and Cr on the location of the garnet lherzolite/spinel lherzolite phase boundary largely counter-balance each other. 相似文献
4.
Abstract Jadeite‐bearing eclogites and associated blueschists locally crop out in a greenschist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca‐poor/Fe‐rich core and Ca‐rich/Fe‐poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (Xjd = 0.34–0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre‐exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure (P)‐temperature (T) conditions of 0.9 GPa/390°C?1.4 GPa/560°C for the stage of the garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation, and 2.2 GPa/495°C‐2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix. The estimated P‐T conditions and continuous changes of mineral parageneses imply a counterclockwise P‐T path which is a combination of (i) an early prograde stage of high‐pressure/low‐temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax‐of‐subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle–slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge. 相似文献
5.
The Heilongjiang Group: A Jurassic accretionary complex in the Jiamusi Massif at the western Pacific margin of northeastern China 总被引：45，自引：0，他引：45
Fu-Yuan Wu Jin-Hui Yang Ching-Hua Lo Simon A. Wilde De-You Sun Bor-Ming Jahn 《Island Arc》2007,16(1):156-172
Abstract The tectonic setting of the Eastern Asian continental margin in the Jurassic is highly controversial. In the current study, we have selected the Heilongjiang complex located at the western margin of the Jiamusi Massif in northeastern China for geochronological investigation to address this issue. Field and petrographic investigations indicate that the Heilongjiang complex is composed predominately of granitic gneiss, marble, mafic‐ultramafic rocks, blueschist, greenschist, quartzite, muscovite‐albite schist and two‐mica schist that were tectonically interleaved, indicating they represent a mélange. The marble, two‐mica schist and granitic gneiss were most probably derived from the Mashan complex, a high‐grade gneiss complex in the Jiamusi Massif with which the Heilongjiang Group is intimately associated. The ultramafic rocks, blueschist, greenschist and quartzite (chert) are similar to components in ophiolite. The sensitive high mass‐resolution ion microprobe U‐Pb zircon age of 265 ± 4 Ma for the granitic gneiss indicates that the protolith granite was emplaced coevally with Permian batholiths in the Jiamusi Massif. 40Ar/39Ar dating of biotite and phengite from the granitic gneiss and mica schist yields a late Early Jurassic metamorphic age between 184 and 174 Ma. Early components of the Jiamusi Massif, including the Mashan complex, probably formed part of an exotic block from Gondwana, affected by late Pan‐African orogenesis, and collided with the Asian continental margin during the Early Jurassic. Subduction of oceanic crust between the Jiamusi block and the eastern part of the Central Asian Orogenic Belt resulted in the formation of a huge volume of Jurassic granites in the Zhangguangcai Range. Consequently, the collision of the Jiamusi Massif with the Central Asian Orogenic Belt to the west can be considered as the result of circum‐Pacific accretion, unrelated to the Central Asian Orogenic Belt. The widespread development of Jurassic accretionary complexes along the Asian continental margin supports such an interpretation. 相似文献
6.
Abstract According to field occurrence and P-T condition, eclogites of southern Henan and northern Hubei Provinces can be divided into two types: medium temperature (MT) and low temperature (LT) eclogites. MT eclogite occurs as layers or lenticular bodies within migmatized gneiss of the Dabie Group. This study is the first to report an occurrence of the assemblages coesite and kyanite + talc in this area. Garnet exhibits a distinct prograde compositional zoning and has mineral inclusions with rotational textures indicating syntectonic growth. Five evolutionary stages are outlined. (1) Pre-eclogite stage, determined by the inclusions of barroisite + zoisite + quartz in the cores of zoned garnets. (2) Eclogite stage, characterized by garnet + omphacite + kyanite ± talc + coesite + rutile, represents the peak metamorphism. The peak conditions are estimated to be T = 600-700°C, P >27 kb. (3) Glaucophane stage, without an appearance of plagioclase, is assigned to a transitional stage. Blades of glaucophane form rims around garnet grains as a result of the reaction talc + jadeite = glaucophane. This marks the beginning of retrograde metamorphism. (4) Symplectite stage, where eclogitic minerals break down, and Amp + Pl symplectite develops around garnet or omphacite; (5) Later retrograde stage is represented by epidote-amphibolite assemblages. Low temperature eclogite appears as blocks in the Qijiaoshan Formation (part of the Susong Group). Four stages can be identified: (1) Pre-eclogite stage, amphibole + epidote + sphene inclusions occur in garnet core; (2) Eclogite stage, consists of garnet + omphacite + rutile + quartz + phengite + glaucophane + zoisite. The peak conditions are T = 490-560°C, P <15 kb; (3) Symplectitic stage, is characterized by the breakdown of eclogitic minerals; (4) Greenschist facies stage, is recorded by a greenschist facies assemblage. The difference between the two types of eclogites suggests contrasting processes. A model is proposed whereby partial melting of continental crust and the emplacement of tonalite occurs during the exhumation of ultrahigh-pressure eclogite terrain. 相似文献
7.
Akira Ishiwatari Yuki Yanagida Yi‐Bing Li Teruaki Ishii Satoru Haraguchi Kazuto Koizumi Yuji Ichiyama Masaru Umeka 《Island Arc》2006,15(1):102-118
Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body. 相似文献
8.
Abstract Various modes of occurrence of talc were identified in piemontite‐quartz schists collected from schist and eclogite units in the Kotsu area of the Sanbagawa Belt, eastern Shikoku, Japan. They can be classified into the following types: (A) matrix and (B) pull‐apart talc. The matrix talc is associated with aegirineaugite or glaucophane in the eclogite unit and with albite or chlorite in the schist unit. The pull‐apart talc is developed at the pull‐apart of microboudin structures of Na‐amphibole, along with albite or chlorite in samples from both units, suggesting that the pull‐apart talc was formed by Na‐amphibole consuming reactions in both units. The talc–aegirineaugite–phengite association is found in a thin layer (a few millimetres thick), with higher Na2O/(Na2O + Al2O3 + MgO) ratio in the ANM (Al2O3–Na2O–MgO) diagram projected from phengite, epidote and other minerals, in the eclogite unit. Crystals of aegirineaugite have decreased jadeite content [= 100 × Al/(Na + Ca)] and increased aegirine content [= 100 × (Na – Al)/(Na + Ca)] from the core (ca Jd40Aeg40Di20) to the rim (ca Jd23Aeg53Di24), and are replaced by winchite and albite in varying degrees at the crystal margins. Na‐amphibole is glaucophane/crossite, commonly rimmed by Al‐poor crossite or winchite at the margin in the eclogite unit, although it is relatively homogeneous crossite in the schist unit. These textures suggest that the talc‐phengite‐(aegirineaugite or glaucophane) assemblage equilibrated during an early stage of metamorphism and the pull‐apart talc was formed at a later stage in the eclogite unit. A plausible petrogenetic grid in the NCKFe3+MASH system with excess piemontite (regarded as epidote), hematite, quartz and water, pseudosection analysis for the aegirineaugite‐bearing layer and the observed mineral assemblages suggest that the talc‐aegirineaugite‐phengite assemblage is stable under high pressure conditions (ca 560–580°C and 18–20 kbar). The pull‐apart talc was formed at ca 565–580°C and 9.5–10.5 kbar by the reaction of glaucophane/crossite + paragonite = talc + albite during the decompression stage, suggesting that the piemontite‐quartz schist in the eclogite unit experienced high‐pressure metamorphism at ca 50–60 km depth and was then exhumed to ca 30 km depth under nearly adiabatic conditions. 相似文献
9.
Electrical conductivity σ of two ultramafic rocks (a spinel lherzolite and a garnet peridotite) has been investigated to melting temperature at 1 bar under known oxygen fugacity environment. The electrical conductivity of the two rocks is found to increase with degree of partial melting and an ～ 15% melt fraction is necessary for the electrical conductivity to increase by ～ 1 order of magnitude. For a given melt fraction electrical conductivity of a spinel lherzolite is lower than that of a garnet peridotite and may be attributed to the differences in the composition of the melts formed. 相似文献
10.
Josephinite is a terrestrial iron-nickel alloy with an intergrown magnesium silicate, and arsenide and sulphide phases, and andradite garnet; several specimens have been found to contain elemental silicon and CaO · 2“FeO”. Josephinite is not awaruite, an iron-nickel mineral formed by serpentinization of ultramafic rocks. Because of its geologic setting and unique mineralogy we propose that josephinite might have originated in the region of the coremantle boundary, was transported via a deep-mantle “plume” and diatreme mechanism into lithosphere mantle that has been emplaced in the Klamaths by ophiolite obduction. Regardless of such a hypothesis, we report here the discovery of terrestrial silicon occurring with josephinite, which seems to preclude a lithosphere environment of origin for josephinite. 相似文献
11.
Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prp25–35), sodium augite (Jd10–25) with a small amount of ilmenite. There are two stages of retrometamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and lherzolite. Due to intensive serpentinitization, only 5%–40% of the relic olivine (Fo92–93) are preserved. The orthopyroxenes are Mg-rich (En87–93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe?Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988)K D=4.06–5.28;T=793–919°C,P=1.5 GPa are estimated for the garnet pyroxenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emplaced into the lower crust in the solid state and then uplifted to the shallow depth. Obviously, this kind of garnet pyroxenite must be petrogenetically related to its host rock. The REE distribution pattern and the Ni?Co?Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle rocks. 相似文献
12.
Recently, garnet pyroxenite enclaves within peridotites occurring near Raobazhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized peridotites. Major mineral components comprise: garnet (Prp25–35), sodium augite (Jd10–25) with a small amount of ilmenite. There are two stages of retrometamorphism: the retrogressive granulite facies mineral assemblage is superimposed by that of amphibolite facies. The host rocks of the garnet pyroxenite are spinel peridotites, including spinel harzburgite and lherzolite. Due to intensive serpentinitization, only 5%–40% of the relic olivine (Fo92–93) are preserved. The orthopyroxenes are Mg-rich (En87–93) with bending of cleavages and granulation at their margins showing intracrystalline plasticity. On the basis of garnet-clinopyroxene Fe−Mg exchange equilibrium geothermometry proposed by Ellis & Green (1979) and Krogh (1988)K D=4.06–5.28;T=793–919°C,P=1.5 GPa are estimated for the garnet pyroxenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emplaced into the lower crust in the solid state and then uplifted to the shallow depth. Obviously, this kind of garnet pyroxenite must be petrogenetically related to its host rock. The REE distribution pattern and the Ni−Co−Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle rocks.
相似文献13.
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
14.
Petrological modeling is a powerful technique to address different types of geological problems via phase-equilibria predictions at different pressure–temperature-composition conditions. Here, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high-pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan and (2) forward-modeling the mineral–melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). We calculated the peak metamorphic conditions at ~2.0–2.3 GPa and ~550–630 °C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5–2.8 GPa and ~640–750 °C. According to our models, the quartz eclogite experienced partial melting after peak metamorphism. In terms of the petrological evolution of the subducted uppermost portion of the oceanic crust along the warm Nankai geotherm, our models show that fluid release occurs at ~20–60 km, likely promoting high pore-fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown. Our petrological models predict partial melting at >60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at these conditions. Modeled melt compositions correspond to high-SiO2 adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in southwest Japan, suggesting that the modeled melts may serve as an analog to explain adakite petrogenesis. 相似文献
15.
Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan 总被引：2，自引：0，他引：2
Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe3+? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO2 contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin. 相似文献
16.
Adi Maulana Andrew G. Christy David J. Ellis Akira Imai Koichiro Watanabe 《Island Arc》2013,22(4):427-452
We present the first data on bulk‐rock major and trace element compositions for a suite of eclogite‐ and blueschist‐facies rocks from the Bantimala Complex, Indonesia, with the aim of better constraining the protolith origins and nature of the subducted crust. The eclogites can be classified into two groups: glaucophane‐rich eclogite and glaucophane‐free eclogite, whereas the blueschists are divided into albite–epidote glaucophanite and quartz–glaucophane schists. SiO2 contents of the eclogites are 43.3–49.6 wt%, with Na2O + K2O contents 3.7–4.7 wt%. The blueschists show a wider range of compositions, with SiO2 = 40.7–63.8 wt% and Na2O + K2O = 2.7–4.5 wt%. Trace element data suggest that the eclogite protoliths include both enriched and normal mid‐oceanic ridge basalt (E‐MORB and N‐MORB) and also gabbroic cumulates. The blueschists show more variation in protoliths, which include N‐MORB, Oceanic Island Basalt (OIB) and Island Arc Basalt (IAB). Plots of element concentrations against the immobile Zr show considerable mobility of large ion lithophiles but not of high field‐strength elements during high‐pressure metamorphism, and indicate that the high SiO2 content of some blueschists is probably due to metasomatism by a LILE‐rich siliceous aqueous fluid. Strong correlations between K, Rb, Ba and Cs suggests that enrichment of these elements occurred by a single process. All the protoliths were subducted, metamorphosed to blueschist/eclogite‐facies and subsequently exhumed. It is noteworthy that the samples deduced to have come from thicker‐crust environments (OIB, IAB) were subducted to shallower depths (blueschist‐facies) than MORB‐derived samples, all except one of which reached eclogite‐facies conditions. The geochemical data of this study demonstrate the variety of ocean floor types that were subducted under the southeast margin of Sundaland in the late Jurassic period. 相似文献
17.
Palaeomagnetic results from 27 sites at five localities within the dismembered Baër-Bassit ophiolite of northern Syria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of the Arabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within one of these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressive increase in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation (exceeding 200°). Although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differential thrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodos ophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partially rotated Baër-Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the Late Tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slip fault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed in the study are therefore of composite origin, and reflect the complex development of structural fabrics within the ophiolite. 相似文献
18.
Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high‐grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote‐amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle‐derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted. 相似文献
19.
A.M. McCaig 《Earth and Planetary Science Letters》1983,63(3):459-473
A high-temperature contact is described between the basal pargasite-bearing spinel-lherzolites of the Bay of Islands ophiolite complex and underlying garnet-granulite facies metagabbros of its dynamothermal aureole. Three distinct high-temperature hydrous assemblages occur in the basal mylonites of the peridotite, and spinel- and garnet-bearing corona textures indicative of increase in pressure under constant or increasing temperature conditions are described for the first time from the uppermost part of the aureole. On the basis of garnet-clinopyroxene geothermometry and garnet-forming reactions in metabasic rocks, P?T conditions of 7–11 kbar, 750–850°C are estimated for rocks on both sides of the contact. Steep inverted gradients in both temperature and pressure of equilibration occur in the aureole, which most likely represents a thinned, overturned and metamorphosed section through an ophiolite sequence. It is proposed that the aureole formed in a low-angle shear zone cutting the oceanic crust and upper mantle.Age data shows that the Bay of Islands Complex was 30–40 Ma old and therefore relatively cold at the time of formation of the aureole. Prolonged (> 1 Ma) shear heating must therefore have occurred at high shear stresses and movement rates (≥ 1 kbar, 10 cm/yr) to produce the high contact temperatures. The displacement surface probably initiated as a discrete fault, evolving into a viscous shear zone with time. Downward movement of the locus of shearing into weaker lithologies and finally thrusting of the ophiolite-aureole complex over cold sediments accounts for the preservation of steep metamorphic gradients in the aureole.The observed pressures at the ophiolite-aureole contact are 3–7 kbar in excess of the expected load pressure from the present thickness of the ophiolite. The cause of the pressure excess was removed before formation of lower-grade parts of the aureole. Possible explanations are tectonic thinning of the ophiolite during displacement or more likely emplacement of nappes on top of the ophiolite before formation of the aureole. A model involving detachment of the ophiolite slice from below a subduction zone can account for the high pressures, rapid uplift and erosion during displacement, and the coincidence of K-Ar ages of amphiboles from the aureole and the sheeted dyke complex of the ophiolite. 相似文献
20.
Masaki Enami Jun‐Ichi Kimura Motohiro Tsuboi Yui Kouketsu Takayoshi Nagaya Shuaimin Huang 《Island Arc》2019,28(1)
Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks. 相似文献