Contrasting modes of eclogite and blueschist exhumation in a retreating subduction system: The Tasmanides,Australia

Blueschists and eclogites located in the Tasmanides of eastern Australia preserve evidence of contrasting modes of exhumation. A review of structural, metamorphic, geochronological and geochemical data indicates that these HP metamorphic rocks can be sub-divided into three main groups: (i) eclogite–blueschists with calc-alkaline and tholeiitic affinities contained within thick sedimentary sequences (called continental HP rocks); (ii) moderate-pressure (< 9 kbar) blueschist of arc to MORB-type composition within sedimentary or serpentinite mélange zones (called accretionary HP rocks) and (iii) eclogites of MORB-type composition with or without a pervasive blueschist overprint contained within serpentinite (called exotic HP rocks). Three different modes of exhumation can be ascribed to the different rock types, namely: (i) exhumation influenced by the buoyancy of continental slabs; (ii) exhumation of accretionary HP rocks by corner flow and/or extensional collapse in the accretionary wedge or (iii) discontinuous exhumation of eclogites triggered by slab rollback and trench retreat. We suggest that a dominant west-dipping, eastward migrating subduction zone can explain the distribution and formation of HP metamorphic rocks in the Tasmanides.Thermobarometric and geochronological data from eclogites and blueschists in the Peel–Manning Fault System (New England Orogen) also provide evidence for discontinuous exhumation of subducted oceanic rocks. These data indicate that eclogites were exhumed from depths of ~ 70 km to ~ 30 km during the Ordovician (490–470 Ma), with terminal exhumation and exposure along the Peel–Manning Fault system probably occurring during the Permian. Based on these timing constraints, we suggest a model where HP rocks reside between depth-dependant exhumation circuits for considerable lengths of time.     

An early Paleozoic Subduction Channel in the North Qilian Mountains,Northwestern China: Structural,Petrological and Chronological Constraints

Detailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel.     

Geochemical characteristics and age of metamorphic sole rocks within a Neotethyan ophiolitic mélange from Konya region (central southern Turkey)

Palaeo- and Neo-Tethyan-related magmatic and metamorphic units crop out in Konya region in the south central Anatolia. The Neotethyan assemblage is characterized by mélange and ophiolitic units of Late Cretaceous age. They tectonically overlie the Middle Triassic–Upper Cretaceous neritic to pelagic carbonates of the Tauride platform. The metamorphic sole rocks within the Konya mélange crop out as thin slices beneath the sheared serpentinites and harzburgites. The rock types in the metamorphic sole are amphibolite, epidote-amphibolite, garnet-amphibole schist, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and quartz-amphibole schist. The geochemistry of the metamorphic sole rocks suggests that they were derived from the alkaline (seamount) and tholeiitic (E-MORB, IAT and boninitic type) magmatic rocks from the upper part of the Neotethyan oceanic crust. Four samples from the amphibolitic rocks yielded ⁴⁰Ar/³⁹Ar isotopic ages, ranging from 87.04 ± .36 Ma to 84.66 ± .30 Ma. Comparison of geochemistry and geochronology for the amphibolitic rocks suggests that the alkaline amphibolite (seamount-type) cooled below 510 ± 25 °C at 87 Ma whereas the tholeiitic amphibolites at 85 Ma during intraoceanic thrusting/subduction. When all the evidence combined together, the intraoceanic subduction initiated in the vicinity of an off-axis plume or a plume-centered spreading ridge in the Inner Tauride Ocean at 87 Ma. During the later stage of the steady-state subduction, the E-MORB volcanics on the top of the down-going slab and the arc-type basalts (IAT/boninitic) detached from the leading edge of the overriding plate, entered the subduction zone after ~2 my and metamorphosed to amphibolite facies in the Inner Tauride Ocean. Duration of the intraoceanic detachment (~87 Ma) and ophiolite emplacement onto the Tauride-Anatolide Platform (Tav?anl? Zone), followed by subsequent HP/LT metamorphism (~82 Ma) spanned ~5 my in the western part of the Inner Tauride Ocean.     

Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan,Kazakhstan and Kyrgyzstan)

The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐P (HP) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of ?11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.     

Ionprobe U-Pb zircon ages from the high-pressure/low-temperature mélange of Syros, Greece: age diversity and the importance of pre-Eocene subduction

The Cycladic blueschist belt in the central Aegean Sea has experienced high‐pressure (HP) metamorphism during collisional processes between the Apulian microplate and Eurasia. The general geological and tectonometamorphic framework is well documented, but one aspect which is yet not sufficiently explored is the importance of HP mélanges which occur within volcano‐sedimentary successions. Unresolved issues concern the range in magmatic and metamorphic ages recorded by mélange blocks and the significance of eventual pre‐Eocene HP metamorphism. These aspects are here addressed in a U‐Pb zircon study focusing on the block–matrix association exposed on the island of Syros. Two gneisses from a tectonic slab of this mélange, consisting of an interlayered felsic gneiss‐glaucophanite sequence, yielded zircon ²⁰⁶Pb/²³⁸U ages of 240.1 ± 4.1 and 245.3 ± 4.9 Ma, respectively, similar to Triassic ages determined on zircon in meta‐volcanic rocks from structurally coherent sequences elsewhere in the Cyclades. This strongly suggests that parts of these successions have been incorporated in the mélanges and provides the first geochronological evidence that the provenance of mélange blocks/slabs is neither restricted to a single source nor confined to fragments of oceanic lithosphere. Zircon from a jadeitite and associated alteration zones (omphacitite, glaucophanite and chlorite‐actinolite rock) all yielded identical ²⁰⁶Pb/²³⁸U ages of c. 80 Ma. Similar Cretaceous U‐Pb zircon ages previously reported for mélange blocks have been interpreted by different authors to reflect magmatic or metamorphic ages. The present study adds a further argument in favour of the view that zircon formed newly in some rock types at c. 80 Ma, due to hydrothermal or metasomatic processes in a subduction zone environment, and supports the interpretation that the Cycladic blueschist belt records both Cretaceous and Eocene HP episodes and not only a single Tertiary HP event.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Early Cretaceous exhumation of high‐pressure metamorphic rocks of the Sistan Suture Zone,eastern Iran

The Sistan Suture Zone (SSZ) of eastern Iran is part of the Neo‐Tethyan orogenic system and formed by convergence of the Central Iranian and Afghan microcontinents. Ar Ar ages of ca. 125 Ma have been obtained from white micas and amphibole from variably overprinted high‐pressure metabasites within the Ratuk Complex of the SSZ. The metabasites, which occur as fault‐bounded lenses within a subduction mélange, document peak‐metamorphic conditions in eclogite or blueschist facies followed by near‐isothermal decompression resulting in an epidote–amphibolite‐facies overprint. ⁴⁰Ar/³⁹Ar step heating experiments were performed on a phengite + paragonite mixture from an eclogite, phengites from two amphibolites, and paragonite from a blueschist; ‘best‐fit’ ages from these micas are, respectively, 122.8 ± 2.2, 124 ± 13, 116 ± 19 and 139 ± 19 Ma (2σ error). Barroisite from an amphibolite yielded an age of 124 ± 10 Ma. The ages are interpreted as cooling ages that record the post‐epidote–amphibolite stage in the exhumation of the rocks. Our results imply that both the high‐pressure metamorphism and the epidote–amphibolite‐facies overprint occurred prior to 125 Ma. Subduction of oceanic lithosphere along the eastern margin of the Sistan Ocean had therefore begun by Barremian (Early Cretaceous) times. Copyright © 2008 John Wiley & Sons, Ltd.     

Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran

The Amassia–Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This blueschist mélange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. ⁴⁰Ar/³⁹Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.     

Geochronology of the Duguer range metamorphic rocks,Central Tibet: implications for the changing tectonic setting of the South Qiangtang subterrane

The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected for in situ zircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments.     

Did the Turonian–Coniacian plume pulse trigger subduction initiation in the Northern Caribbean? Constraints from 40Ar/39Ar dating of the Moa-Baracoa metamorphic sole (eastern Cuba)

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.     

Late Cretaceous eclogitic high-pressure relics in the Bitlis Massif

A new occurrence of eclogites was found in the Kesandere valley in the eastern most part of the Bitlis complex, SE Anatolia. These high-pressure (HP) relics were preserved in calc-arenitic metasediments within the high-grade metamorphic basement of the Bitlis complex. The eclogitic parageneses were strongly overprinted during decompression and heating. These new eclogites locality complements the evidence of blueschist-facies metamorphism documented recently in the meta-sedimentary cover sequence of this part of the Bitlis complex. Thermodynamic calculations suggest peak conditions of ca. 480–540 °C/1.9–2.4 GPa. New U/Pb dates of 84.4 ± .9 and 82.4 ± .9 Ma were obtained on zircons from two Kesandere eclogite samples. On the basis of geochemical criteria, these dates are interpreted to represent zircon crystallization during the eclogitic peak stage. Kesandere eclogites differ from those previously described in the western Bitlis complex (Mt. Gablor locality) in terms of lithologic association, protolithic origin, and peak P–T conditions (600–650 °C/1.0–2.0 GPa, respectively). On the other hand, eclogitic metamorphism of Kesandere metasediments occurred shortly before blueschist-facies metamorphism of the sedimentary cover (79–74 Ma ⁴⁰Ar/³⁹Ar white mica). Therefore, the exhumation of Kesandere eclogites started between ca. 82 and 79 Ma, while the meta-sedimentary cover was being buried. During this short time span, Kesandere eclogite were likely uplifted from ~65 to 35 km depth, indicating a syn-subduction exhumation rate of ~4.3 mm/a. Subsequently, eclogite- and blueschist-facies rocks were likely retrogressed contemporarily during collision-type metamorphism (around 72–69 Ma). The Bitlis HP rocks thus sample a subduction zone that separated the Bitlis–Pütürge (Bistun?) block from the South-Armenian block, further north. To the south, Eocene metasediments of the Urse formation are imbricated below the Bitlis complex. They contain (post Eocene) blueschists, testifying separation from the Arabian plate and southward migration of the subduction zone. The HT overprint of Kesandere eclogites can be related to the asthenospheric flow provoked by subducting slab retreat or break off.     

White mica K–Ar geochronology of Sanbagawa eclogites from Southwest Japan: implications for deformation-controlled K–Ar closure temperature

White mica (phengite and paragonite) K–Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84–89 Ma (Seba, central Shikoku), 78–80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82–88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K–Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K–Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K–Ar ages are attributed to excess ⁴⁰Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high ⁴⁰Ar/³⁶Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme ⁴⁰Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K–Ar closure temperature.     

Modelling white mica pressure‐temperature‐time (P‐T‐t) paths using thermobarometric and 40Ar/39Ar thermochronologic data

White micas are major rock forming minerals in igneous and metamorphic rocks, and their chemical and isotopic variations can be used to determine pressure, temperature and time (P‐T‐t) histories. We apply ⁴⁰Ar/³⁹Ar multi‐diffusion domain modelling to white micas from blueschists blocks in serpentinite matrix mélange from the exhumed Baja California subduction complex. Thermal history models yielded T‐t paths suggesting that ⁴⁰Ar* resides within multiple diffusion domains with varying ⁴⁰Ar* retentivity. Modelled white mica thermal histories and thermobarometric data were used to forward model continuous P‐T‐t paths. P‐T‐t paths are consistent with previous studies and are interpreted to constrain blueschist block exhumation paths within the Baja accretionary wedge. Our P‐T‐t models use temperature controlled ⁴⁰Ar/³⁹Ar step heat data in which argon loss by volume diffusion can be demonstrated, and for which the white mica petrogenesis is known.     

Prolonged Variscan to Alpine history of an active Eurasian margin (Georgia, Armenia) revealed by Ar/Ar dating

Variscan to Alpine magmatic activity on the North Tethys active Eurasian margin in the Caucasus region is revealed by ⁴⁰Ar/³⁹Ar ages from rocks sampled in the Georgian Crystalline basement and exotic blocs in the Armenian foreland basin. These ages provide insights into the long duration of magmatic activity and related metamorphic history of the margin, with: (1) a phase of transpression with little crustal thickening during the Variscan cycle, evidenced by HT-LP metamorphism at 329–337 Ma; (2) a phase of intense bimodal magmatism at the end of the Variscan cycle, between 303 and 269 Ma, which is interpreted as an ongoing active margin during this period; (3) further evolution of the active margin evidenced by migmatites formed at ca. 183 Ma in a transpressive setting; (4) paroxysmal arc plutonic activity during the Jurassic (although the active magmatic arc was located farther south than the studied crystalline basements) with metamorphic rocks of the Eurasian basement sampled in the Armenian foreland basin dated at 166 Ma; (5) rapid cooling suggested by similar within-error ages of amphibole and muscovite sampled from the same exotic block in the Armenian fore-arc basin, ascribed to rapid exhumation related to extensional tectonics in the arc; and finally (6) cessation of ‘Andean’-type magmatic arc history in the Upper Cretaceous. Remnants of magmatic activity in the Early Cretaceous are found in the Georgian crystalline basement at c. 114 Ma, which is ascribed to flat slab subduction of relatively hot oceanic crust. This event corresponds to the emplacement of an oceanic seamount above the N Armenian ophiolite at 117 Ma. The activity of a hot spot between the active Eurasian margin and the South Armenian Block is thought to have heated and thickened the Neo-Tethys oceanic crust. Finally, the South Eurasian margin was uplifted and transported over this hot oceanic crust, resulting in the cessation of subduction and the erosion of the southern edge of the margin in Upper Cretaceous times. Emplacement of Eocene volcanics stitches all main collisional structures.     

北祁连榴辉岩相变沉积岩的特征及其构造意义

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表.     

喜马拉雅造山带两种不同类型榴辉岩与印度大陆差异性俯冲

印度与亚洲大陆新生代碰撞-俯冲形成的喜马拉雅造山带核部由高压和超高压变质岩组成.超高压榴辉岩分布在喜马拉雅造山带西段,由石榴石、绿辉石、柯石英、多硅白云母、帘石、蓝晶石和金红石组成.超高压榴辉岩的峰期变质条件为2.6～2.8GPa和600～620℃,其经历了角闪岩相退变质作用和低程度熔融.超高压榴辉岩的进变质、峰期和退变质年龄分别为～50Ma、45～47Ma和35～40Ma,指示一个快速俯冲与快速折返过程.高压榴辉岩产出在喜马拉雅造山带中-东段,由石榴石、绿辉石、多硅白云母、石英和金红石组成.高压榴辉岩的峰期变质条件为>2.1GPa和>750℃,叠加了高温麻粒岩相退变质作用与强烈部分熔融.高压榴辉岩的峰期和退变质年龄可能分别是～38 Ma和14～17 Ma,很可能经历了一个缓慢俯冲与缓慢折返过程.喜马拉雅造山带两种不同类型榴辉岩的存在表明,印度与亚洲大陆约在51～53Ma碰撞后,印度大陆地壳的西北缘陡俯冲到了地幔深度,导致表壳岩石经历了超高压变质作用,而印度大陆地壳的东北缘平缓俯冲到亚洲大陆之下,导致表壳岩石经历了高压变质作用.     

西南天山变质俯冲杂岩带的内部结构——来自木扎尔特剖面的启示

西南天山高压-超高压变质带是世界上少有的经历深俯冲的增生杂岩带,是古天山洋向北俯冲的结果。针对该俯冲杂岩带内部结构的研究目前仍存在争论。本文以木扎尔特地区一条长约4 km的南北向剖面为例,对西南天山高压-超高压变质带的野外特征、矿物学和变质演化研究进行了综述。目前的研究表明,木扎尔特地区存在超高压和高压两类硬柱石榴辉岩,但绝大部分都经历了强烈的退变质和变形改造,被蓝片岩相或绿片岩相矿物组合取代。这些变基性岩在空间上构成北部和南部两个榴辉岩带,二者为构造接触。木扎尔特超高压硬柱石榴辉岩与其围岩经历相似的峰期压力,构成西南天山超高压带的西端。与东侧阿克牙孜地区超高压榴辉岩相比,它们在变形特征、岩石组合和变质演化方面表现出一定的独特性,很可能说明深俯冲板片在折返过程中沿构造带走向存在差异变质-变形演化。这些基础研究对全面认识冷俯冲增生杂岩带的变质演化及其俯冲和折返的地球动力学机制具有重要意义。     

Anatomy of a subduction complex: architecture of the Franciscan Complex,California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.     

P–T–time (phengite Ar closure) history of spatially close-outcropping HP and UHP oceanic eclogites (southwestern Tianshan): implication for a potential deep juxtaposing process during exhumation?

ABSTRACT

The Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable ⁴⁰Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

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