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1.
The Late Cretaceous Alihoca ophiolite in the Inner Tauride suture zone (ITSZ) of South-Central Turkey represents part of a single ophiolitic thrust sheet that originated from the Inner Tauride ocean. The ophiolite contains upper mantle peridotites, cumulate wehrlites, layered-to-isotropic gabbros, and microgabbroic-to-doleritic dikes. An ophiolitic mélange beneath the Alihoca ophiolite includes blocks of limestone, peridotite, dolerite, basalt, and deep-sea sedimentary rocks (radiolarite, chert) in a matrix comprising sheared serpentinite and mudstone. Isotropic gabbro and dolerite dike rocks show enrichment in Sr, K, Rb, Ba, and Th (LILE) and depletion of Ta, Nb, Zr, Ti, and Y (HFSE), indicating an island arc tholeiite (IAT) affinity. Relatively younger dolerite rocks display low TiO2 (<0.5 wt.%) contents, concave REE profiles with low HREE concentrations, and high LREE values, typical of boninitic affinities. The Alihoca ophiolite, hence, displays an IAT to boninitic geochemical progression in its magmatic evolution, reminiscent of many other Tethyan ophiolites in the region. It represents the remnant of a forearc oceanic crust, which developed during the early stages of subduction within the Inner Tauride ocean. Volcanic, volcano-sedimentary, and sedimentary rocks of the Uluk??la–Çamard? basin north of the ITSZ disconformably overlie the mafic-ultramafic rocks of the Alihoca ophiolite. Pillowed and massive lavas of the latest Cretaceous–Palaeocene Uluk??la Formation have alkaline basalt-to-basaltic andesite compositions, displaying relatively enriched LILE and LREE patterns with negative Nb and Ta anomalies. These geochemical features suggest that magmas of the Uluk??la–Çamard? volcanic rocks formed from partial melting of a metasomatized lithospheric mantle. This melting event was triggered by the influx of asthenospheric heat through a slab breakoff-induced window in the downgoing Tethyan oceanic lithosphere.  相似文献   

2.
The Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.  相似文献   

3.
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world. The mechanism of continental growth and tectonic evolution of the CAOB remain debated. Here we present an overview of Early Paleozoic ophiolitic mélanges, calc-alkaline intrusions, and metamorphic rocks in West Junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the Junggar Ocean, a branch of the southern Paleo-Asian Ocean (PAO). The Early Paleozoic ophiolitic mélanges are composed of ultramafic-mafic rocks, cherts, pelagic limestones, basaltic breccias and tuffs. The mafic rocks from these ophiolitic mélanges are divided into MORB-like and OIB-like types. The MORB-like rocks were formed in a fore-arc setting, but the OIB-like mafic rocks were formed by the intra-plate magmatism related to mantle plume activities. The Early Paleozoic intrusions are occurred as small stocks with a dominant composition of diorite, trondhjemite, and granodiorite. These granitoids display (high-K) calc-alkaline affinities, and have high and positive εNd(t) and εHf(t) values, formed in an arc-related setting. The metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from ~500 Ma to ~460 Ma. Their protoliths are calc-alkaline andesite and tholeiitic and alkaline basalts, formed in an arc related and seamount setting, respectively. It is clearly show that the West Junggar was under an intra-oceanic subduction regime during the Early Paleozoic, and the initial subduction of the southern PAO might have occurred in the Early Cambrian. Based on our observations, and in combination with previous work, we propose the plume-induced subduction initiation model for the Early-Middle Cambrian tectonic evolutionary of the Junggar Ocean. Our new model not only shed light on subduction initiation dynamics of the southern PAO, but also contribute to tectonic evolution of the CAOB.  相似文献   

4.
The Nanlinshan and Banpo mafic–ultramafic intrusions belong to the prominent Yunxian-Jinghong magmatic belt in the western part of the Simao Block, one of several Gondwana-derived continental fragments assembled by the closure of multiple Tethyan oceans. Different petrogenic models including ophiolites, MORB-type cumulates and Alaskan-type complexes have been proposed for these intrusions. In order to better constrain possible origins, we have undertaken an integrated geochronological, petrological and geochemical study of both intrusions. Precise CA-ID-TIMS zircon U–Pb dating indicates that the Nanlinshan and Banpo intrusions have similar ages of ~298 and 295?Ma, respectively, confirming that they are the oldest intrusive rocks in the Yunxian-Jinghong magmatic belt. A comparison between whole rock compositions and the compositions of major silicate minerals, including trace elements in clinopyroxene, reveals that (1) the ultramafic rocks of these intrusions are crystal cumulates of a relatively primitive magma, (2) the associated gabbroic and dioritic rocks are the products of more fractionated liquids, and (3) the parental magmas of these rocks were all depleted in some high field strength trace elements including Nb, Zr and Hf. Both intrusions are also characterized by elevated εNd values between +3.4 and +6.6. The positive εNd values coupled with negative Nb and Zr–Hf anomalies are consistent with the interpretation that these two intrusions are the products of subduction-related basaltic magmatism. The results from this study suggest that subduction of the Paleo-Tethys Ocean along the western margin of the Simao Block initiated as early as ~298?Ma and that the Simao Block and the Northern Qiangtang Block of the Tibet Plateau are separate Gondwana-derived continental fragments instead of a single fragment as previously reported.  相似文献   

5.
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6.
Several major volcanic zones are distributed across the eastern North China Craton, from northwest to southeast: the Greater Xing’an Range, Jibei-Liaoxi, Xishan, and Songliao Basins, and the Yanji, Huanghua, and Ludong volcanic zones. The Huanghua depression within the Bohai Bay Basin was filled by middle Late Mesozoic volcanic rocks and abundant Cenozoic alkaline basalts. Zircon LA-ICP-MS and SHRIMP U–Pb dating show that basic–intermediate volcanic rocks were extruded in the Early Cretaceous of 118.8 ± 1.0 Ma (weighted mean 206Pb/238U age), before Late Cretaceous acid lavas at 71.5 ± 2.6 Ma. An inherited zircon from andesite has a Paleoprotoerozoic core crystallization age of 2,424 ± 22 Ma (206Pb/207Pb age) indicating that the basement of the Bohai Bay Basin is part of the North China Craton. Early Cretaceous basic and intermediate lavas are characterized by strong enrichments in LREE and LILE and depletions in HREE and HFSE, indicating a volcanic arc origin related to oceanic subduction. Depletion in Zr only occurs in basic and intermediate volcanic rocks, while depletions in Sr and Ti exist only in acid samples, indicating that the acid series is not genetically related to the basic–intermediate series. Formation ages and geochemical features indicate that the Late Cretaceous acid lavas are products of crustal remelting in an extensional regime. Combined information from all these volcanic zones shows that subduction-related volcanic rocks were generated in the Jibei-Liaoxi and Xishan volcanic zones during the Early Jurassic, about 60 Ma earlier than their analogues extruded in the Huanghua and Ludong volcanic zones during the Early Cretaceous. This younging trend also exists in the youngest extension-related volcanism in each of these zones: Early Cretaceous asthenosphere-derived alkaline basalts in the northwest and Late Cretaceous in the southeast. A tectonic model of northwestward subduction and continuous oceanward retreat of the Paleo-Pacific Plate is proposed to explain the migration pattern of both arc-related and post-subduction extension-related volcanic rocks. As the subduction zone continuously migrated, active continental margin and backarc regimes successively played their roles in different parts of North China during the Late Mesozoic (J1–K2).  相似文献   

7.
Sub-ophiolitic greenschist facies metamorphic rocks occur at the sole of ophiolite slices and as blocks in the mélange zone beneath the Andaman ophiolite. These are represented by metabasics as actinolite schist to actinolite–chlorite schist and metasediments as garnetiferous quartzo-feldspathic mica–chlorite schist and piemontite quartzite to piemontite bearing quartz–muscovite–chlorite schist to muscovite–quartz-chlorite schist. Actinolite occurs along the schistosity and also as porphyroblasts. Syn to post-tectonic garnet shows no compositional zoning and represent almandine–spessartine solid solution (Alm44–47, Sps23–27, Gros13–17, Pyr9–10). The metabasics are enriched in LILE and depleted in Zr and Y compared to N-MORB.The lithological features suggest that residual heat was the main heat source for greenschist-facies metamorphism. Top part of the subducting slab and overlying trench sediments were metamorphosed and dislocated by the close spaced thrusts in an accretionary prism setting. The field association indicates that metamorphism and the uplift of metamorphic rocks along with ophiolite slices were bracketed in between Cretaceous and Oligocene period. These processes were later than the Pre-Cretaceous emplacement of the ophiolites of Sumatra and Java.  相似文献   

8.
《Gondwana Research》2010,17(3-4):414-430
The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H2O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.  相似文献   

9.
The Central Andean gravity high, a relic of an old subduction complex?   总被引：1，自引：0，他引：1  
The Central Andean gravity high (CAGH) is a positive anomaly in isostatic residual gravity with its center located at the western flank of the Central Andes at about 24°S. The gravity was analyzed by various methods to draw quantitative conclusions about the sources of this anomaly and their process of formation. Methods include the analysis of the gravity gradients, power spectrum, wavelength filters, and Euler deconvolution.Numerical investigations of gravity field in the area of the CAGH indicate the presence of a dense body of nearly 400 km length and about 100–140 km width, that masses lie at varying depths between 10 and 38 km. A correlation between the location of the residual anomalies and the topographic lows in the area between the Salars de Atacama and Pipanaco is observed, which indicates the strong influence of the anomalous-causing rocks of the CAGH within the formation process of the Andean orogen. An influence of these causing bodies of rock on the trend of Holocene volcanic arc is likely. Genesis of the anomalous dense formations of rock could be traced back to Ordovician–Silurian time when a pre-Andean subduction zone is postulated in the region of northern Chile with its corresponding volcanic arc in the region of the CAGH.

Zusammenfassung

El campo de gravedad alto de los Andes Centrales (CAGH) consiste en una pronunciada anomalía positiva de la gravedad isostática, cuyo centro se encuentra en el borde oeste de los Andes Centrales a los 24°S. En este estudio se analizó el campo de gravedad mediante distintos métodos, de manera de poder establecer conclusiones cuantitativas sobre el causante de esta anomalía y el proceso de formación de este causante.La investigación numérica de las anomalías gravimétricas del CAGH indica la presencia de un cuerpo de alta densidad con aproximadamente 400 km de largo y 100–140 km de ancho, que se encuentra a profundidades variables entre 10 y 38 km. Se observa una correlación entre la posición de la anomalía residual y los bajos topográficos en los areas de Salares de Atacama, Arizaro, Antofalla y Pipanaco, la cual indica una fuerte influencia de rocas productoras de la anomalía en el CAGH, dentro del proceso de formación del orógeno andino. Es probable que estos cuerpos de rocas causantes de la anomalía tengan incluso influencia en el alineamiento del arco volcánico holocénico. La generación de cuerpos de rocas con una densidad anómala puede remontarse al Ordovícico–Silúrico, tiempo para el que postula una subducción pre-Andina en la región del norte de Chile y que corresponde con el arco volcánico en la región del CAGH.  相似文献   

10.
Focused, interdisciplinary research in the subseafloor environment has advanced our understanding of fluid flow paths and the interrelationship between fluid pressure and deformation, heat flow, and diagenesis. Recent investigations in the Pacific Ocean, the Gulf of Mexico, and the North Sea provide examples of high rates of fluid flow in the permeable basement, with the flow affecting properties of overlying sediments. There are also some indications that flow could continue into subduction. Studies on continental slopes provide evidence for sedimentation-driven overpressures that can destabilize slopes. These studies particularly emphasize the contributions of lateral flow driven by uneven loading. Similarly, evidence continues to build that stratigraphic horizons and faults help to dewater subduction zones. A recent development is the monitoring of pore pressure, temperatures, flow, and fluid chemistry, which aids the understanding of transient behavior and response to strain events.  相似文献   

11.
Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.  相似文献   

12.
The distribution of ore‐deposit types in different sectors of the circum‐Mediterranean realm that have been affected by subduction processes since the Cretaceous varies in space and time. Sectors involved in W‐directed subduction (Sardinia, Apennines–Maghrebides, Internal Betics, Tyrrhenian, Western–Eastern Carpathians) are dominated by relatively low‐sulphidation epithermal (±VMS) deposits. Orogens formed by NE‐directed subduction (Dinarides–Hellenides–Pontides–Anatolides–Taurides; DHPAT) were initially dominated by pluton‐related porphyry–skarn–high‐sulphidation epithermal associations. These distinct metallogenic styles can be related to the systematic tectono‐magmatic asymmetry of E–NE‐ and W‐directed subduction systems and are analogous to the relationship observed in circum‐Pacific belts. Exceptions to this simple pattern occurred in the DHPAT in the Cenozoic, when deposit associations typical of both E‐directed and W‐directed systems were formed. Such exceptions are interpreted to reflect superimposition of contrasting subduction trends and inheritance from earlier metallogenic stages (Apuseni) or the interference of subduction processes with subduction‐unrelated extension (Hellenides, West Anatolia).  相似文献   

13.
Levin  B. W.  Rodkin  M. V.  Sasorova  E. V. 《Doklady Earth Sciences》2017,476(1):1109-1112
Doklady Earth Sciences - The role played by the Earth’s rotation is very important in problems of physics of the atmosphere and ocean. The importance of inertia forces is traditionally...  相似文献   

14.
The Güira de Jauco metamorphic sole, below the Moa-Baracoa ophiolite (eastern Cuba), contains strongly deformed amphibolites formed at peak metamorphic conditions of 650–660°C, approximately 8.6 kbar (~30 km depth). The geochemistry, based on immobile elements of the amphibolites, suggests oceanic lithosphere protholiths with a variable subduction component in a supra-subduction zone environment. The geochemical similarity and tectonic relations among the amphibolites and the basic rocks from the overlying ophiolite suggest a similar origin and protholith. New hornblende 40Ar/39Ar cooling ages of 77–81 Ma obtained for the amphibolites agree with this hypothesis, and indicate formation and cooling/exhumation of the sole in Late Cretaceous times. The cooling ages, geochemical evidence for a back-arc setting of formation of the mafic protoliths, and regional geology of the region allow proposal of the inception of a new SW-dipping subduction zone in the back-arc region of the northern Caribbean arc during the Late Cretaceous (ca. 90–85 Ma). Subduction inception was almost synchronous with the main plume pulse of the Caribbean–Colombian Oceanic Plateau (92–88 Ma) and occurred around 15 million years before arc-continent collision (75 Ma–Eocene) at the northern leading edge of the Caribbean plate. This chronological framework suggests a plate reorganization process in the region triggered by the Caribbean–Colombian mantle plume.  相似文献   

15.
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16.
RenzoSartori 《《幕》》2003,26(3):217-221
A deep, narrow, and distorted Benioff zone, plunging from the Ionian Sea towards the southern Tyrrhenian basin, is the remnant of a long and eastward migrating subduction of eastern Mediterranean lithosphere. From Oligocene to Recent, subduction generated the Western Mediterranean and the Tyrrhenian back-arc basins, as well as an accretionary wedge constituting the SouthernAoenninic Arc.In the Tyrrhenian Sea, stretching started in late Miocene and eventually produced two small oceanic areas: the Vavilov Plain during Pliocene (in the centralsector) and the Marsili Plain during Quaternary (in the southeastern sector). They are separated by a thicker crustal sector, called the Issel Bridge. Back-arc exten-sion was rapid and discontinuous, and affected a land locked area where continental elements of various sizesoccurred. Discontinuities in extension were mirrored bychanges in nature of the lithosphere scraped off to form the Southern Apenninic Arc. Part of the tectonic units of the southern Apennines, accreted into the wedge from late Miocene to Pliocene, had originally been laid down on thinned conti-nental lithosphere, which should constitute the deep portion of the present slab. After Plio-cene, only Ionian oceanic lithosphere wassubducted, because the large buoyancy of thewide and not thinned continental lithosphere of Apulia and Africa (Sicily) preserved the seelements from roll back of subduction. After Pliocene, the passively retreating oceanic slabhad to adjust and distort according to the geometry of these continental elements.The late onset of arc volcanism in respect to the duration of extension in the Tyrrhenian-Ionian system may find an expla-nation considering an initial stage of subduc-tion of thinned continental lithosphere. The strong Pleistocene vertical movements that occurred in the whole southeastern system(subsidence in the back-arc basin and upliftin the orogenic arc) may instead be related to the distortion of the oceanic slab.  相似文献   

17.
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18.
The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

19.
The thermal structure of subduction zones exerts a major influence on deep-seated mechanical and chemical processes controlling arc magmatism, seismicity, and global element cycles. Accretionary complexes exposed inland may comprise tectonic blocks with contrasting pressureetemperature(Pe T)histories, making it possible to investigate the dynamics and thermal evolution of former subduction interfaces. With this aim, we present new Lue Hf geochronological results for mafic rocks of the Halilbag?Complex(Anatolia) that evolved along different thermal gradients. Samples include a lawsoniteeepidote blueschist, a lawsoniteeepidote eclogite, and an epidote eclogite(all with counter-clockwise Pe T paths),a prograde lawsonite blueschist with a "hairpin"-type Pe T path, and a garnet amphibolite from the overlying sub-ophiolitic metamorphic sole. Equilibrium phase diagrams suggest that the garnet amphibolite formed at w0.6 -0.7 GPa and 800 -850℃, whereas the prograde lawsonite blueschist records burial from 2.1 GPa and 420℃ to 2.6 GPa and 520℃. Well-defined Lue Hf isochrons were obtained for the epidote eclogite(92.38 ± 0.22 Ma) and the lawsoniteeepidote blueschist(90.19 ± 0.54 Ma),suggesting rapid garnet growth. The lawsoniteeepidote eclogite(87.30 ± 0.39 Ma) and the prograde lawsonite blueschist(ca. 86 Ma) are younger, whereas the garnet amphibolite(104.5 ± 3.5 Ma) is older.Our data reveal a consistent trend of progressively decreasing geothermal gradient from granulite-facies conditions at ~104 Ma to the epidote-eclogite facies around 92 Ma, and the lawsonite blueschist-facies between 90 Ma and 86 Ma. Three Lue Hf garnet dates(between 92 Ma and 87 Ma) weighted toward the growth of post-peak rims(as indicated by Lu distribution in garnet) suggest that the HP/LT rocks were exhumed continuously and not episodically. We infer that HP/LT metamorphic rocks within the Halilbag?Complex were subjected to continuous return flow, with "warm" rocks being exhumed during the tectonic burial of "cold" ones. Our results, combined with regional geological constraints, allow us to speculate that subduction started at a transform fault near a mid-oceanic spreading centre. Following its formation, this ancient subduction interface evolved thermally over more than 15 Myr, most likely as a result of heat dissipation rather than crustal underplating.  相似文献   

20.
The thermal structure of subduction zones exerts a major influence on deep-seated mechanical and chemical processes controlling arc magmatism, seismicity, and global element cycles. Accretionary complexes exposed inland may comprise tectonic blocks with contrasting pressure–temperature (P–T) histories, making it possible to investigate the dynamics and thermal evolution of former subduction interfaces. With this aim, we present new Lu–Hf geochronological results for mafic rocks of the Halilba?? Complex (Anatolia) that evolved along different thermal gradients. Samples include a lawsonite–epidote blueschist, a lawsonite–epidote eclogite, and an epidote eclogite (all with counter-clockwise P–T paths), a prograde lawsonite blueschist with a “hairpin”-type P–T path, and a garnet amphibolite from the overlying sub-ophiolitic metamorphic sole. Equilibrium phase diagrams suggest that the garnet amphibolite formed at ～0.6–0.7 GPa and 800–850 °C, whereas the prograde lawsonite blueschist records burial from 2.1 GPa and 420 °C to 2.6 GPa and 520 °C. Well-defined Lu–Hf isochrons were obtained for the epidote eclogite (92.38 ± 0.22 Ma) and the lawsonite–epidote blueschist (90.19 ± 0.54 Ma), suggesting rapid garnet growth. The lawsonite–epidote eclogite (87.30 ± 0.39 Ma) and the prograde lawsonite blueschist (ca. 86 Ma) are younger, whereas the garnet amphibolite (104.5 ± 3.5 Ma) is older. Our data reveal a consistent trend of progressively decreasing geothermal gradient from granulite-facies conditions at ～104 Ma to the epidote-eclogite facies around 92 Ma, and the lawsonite blueschist-facies between 90 Ma and 86 Ma. Three Lu–Hf garnet dates (between 92 Ma and 87 Ma) weighted toward the growth of post-peak rims (as indicated by Lu distribution in garnet) suggest that the HP/LT rocks were exhumed continuously and not episodically. We infer that HP/LT metamorphic rocks within the Halilba?? Complex were subjected to continuous return flow, with “warm” rocks being exhumed during the tectonic burial of “cold” ones. Our results, combined with regional geological constraints, allow us to speculate that subduction started at a transform fault near a mid-oceanic spreading centre. Following its formation, this ancient subduction interface evolved thermally over more than 15 Myr, most likely as a result of heat dissipation rather than crustal underplating.  相似文献   

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