Continental Margin Subduction Research Articles

Long-lived Northern Hemisphere convergence systems driven by upper-mantle thermal inhomogeneity

Abstract Plate reconstructions reveal that two secular centers of convergence formed beneath eastern Eurasia and North America no later than 200 Ma. The cause of these convergence centers, which featured flat subduction, slab stagnation, and/or continental margin subduction, remains uncertain. Here, we propose that upper-mantle thermal inhomogeneity, particularly an anomalously cool Northern Hemispheric upper mantle, was a fundamental driver of this long-lived convergence. By considering the pattern of observed thermal inhomogeneity, our numerical models show that flow-induced asymmetrical subduction will tend to develop toward cold mantle domains, even when the subducting plate is buoyant. The models can reproduce the diverse subduction styles observed in the Northern Hemisphere by including proposed pre-subduction plate distributions and/or properties.

Subduction development along extinct mid-ocean ridges versus weakened passive continental margins

Where subduction zones develop is a fundamental issue in subduction dynamics. Intra-oceanic settings and continental margins are the main regions for subduction development, and various tectonic settings are further proposed based on these two categories, including extinct mid-ocean ridges and passive continental margins. Mid-ocean ridges often locate adjacent to passive continental margins since they formed in successive extension processes. However, it remains enigmatic how these two tectonic settings compete in terms of subduction initiation and development. Here, we use a 2-D numerical model to investigate how subduction develops in the tectonic setting where mid-ocean ridges and passive continental margins are juxtaposed. The results exhibit three oceanic plate subduction types: continental margin subduction, ridge-inversed subduction, and a transitional type featured by double subduction along ridges and margins. The physical parameters that influence the formation of the three types of subduction, including the geometry of weakened passive margins, the cooling age of extinct mid-ocean ridges, the direction and magnitude of convergence rates, and the ridge-margin distance, are systematically investigated. Small cooling ages, pushing from oceanic plates, short ridge-margin distances, and strong continental margins favor ridge-inversed subduction, while large cooling ages, pushing from overriding plates, long ridge-margin distances, and weakened continental margins facilitate continental margin subduction. We emphasize the importance of preexisting weakness along the passive margins, which primarily promote subduction development along margins. Our model results shed new light on the natural examples of subduction development along extinct mid-ocean ridges and weakened passive margins.

U-Pb zircon ages from metasedimentary and plutonic rocks in the Bras d'Or terrane of Cape Breton Island, Nova Scotia, Canada: insights into the Ediacaran–Cambrian tectonomagmatic evolution of Ganderia

The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620–600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma.

Geochemistry, Lu–Hf garnet ages, and P–T conditions of blueschists from the Meliatic and Fatric nappes, Western Carpathians: Indicators of Neotethyan subduction

Basaltic rocks from ophiolitic mélanges provide information on geodynamic setting, origin, and later tectonometamorphic conditions. This paper resolves the P–T conditions and timing of high-pressure metamorphism in an accretionary wedge that formed during the Late Jurassic closure of the Neotethyan Meliata Basin. Blueschist-facies metabasites of the Meliatic Bôrka Nappe and the Albian conglomerate pebbles of the Fatric Klape Nappe contain rare assemblages of garnet in association with glaucophane, phengite, rutile, and epidote (±albite). Here, we compare a Lu–Hf garnet age from the Meliatic Bôrka Nappe in the southern margin of the Inner Western Carpathians (IWC) with a garnet age from inferred Meliatic blueschists of the Klape Nappe overlying the IWC northern margin. The Hačava type locality of the Bôrka Nappe hosts calc-alkaline type metabasite (∼VAB-C; εNd(245) = +0.9) embedded within Middle to Upper Triassic marbles of the Neotethyan Meliata Basin northern continental margin. The investigated Klape Nappe island arc tholeiite (∼VAB-T; εNd(240) = +5.9) blueschist pebble, and associated deepwater metasedimentary rock pebbles were found in a conglomerate layer of unmetamorphosed Albian flysch. The garnet ages of 153.95±0.69 Ma and 152.1±1.5 Ma correspond to closure of the Meliata Basin during southward intraoceanic and continental margin subduction. This was followed by the exhumation of HP blocks in serpentinite mélange and the formation of an accretionary wedge with included anchimetamorphosed Jurassic flysch. The P–T conditions of the blueschists were constrained by Perple_X modelling to be 520°C and 1.55 GPa for the Bôrka Nappe, and 490–510°C and 1.68–1.72 GPa for a pebble from the Klape Nappe conglomerate. The similarity of garnet dates and metamorphic conditions between the two samples suggest that the blueschists formed during the Late Jurassic Meliatic subduction. The north-vergent Meliatic nappes are the inferred source of the Albian flysch conglomerates deposited in the foreland Fatric Basin. This material was subsequently transported by the Fatric Klape Nappe to the IWC orogenic front during the Turonian, following the closure of the Fatric Basin.

Geochemistry and Petrogenesis of the Wadhrai Granite Stock of the Malani Igneous Suite in Nagar Parkar Area, SE Pakistan

The Wadhrai granite stock is a part of the Nagar Parkar Igneous Complex, an extension of the Neoproterozoic Malani Igneous Suite of western Rajasthan. It is occupied by a petrographically uniform granite comprising perthite, plagioclase, quartz, with small quantities of biotite, opaque oxides, titanite, and secondary minerals. The rocks are sparingly porphyritic and contain dykes of microgranite, aplite, and rare pegmatite. In the south-central part, parallel sheets and swarms of mafic dykes, and in the western part very fine-grained felsic sheets intrude the body. The granite is metaluminous to peraluminous and characterized by high silica (73–76 wt%), and alkalis (7–9 wt%), and low CaO (0.15–1.4 wt%), MgO (0.15–0.38 wt%), Th (7–12 ppm), and U (1–2 ppm). On geochemical discriminant diagrams, it classifies mostly as A-type (with rather high Y/Nb (8.6 to 2.4, average 5.2) and low Nb/Ga and Ce (typical of A2-type), but sparingly as I-type. Chondrite-normalized patterns show enrichment in LREE over HREE, and small negative Eu anomalies, whereas mantle-normalized spidergrams display higher LILE over HFSE, distinct troughs for Nb, Sr, P, Ti, and spikes for La, Ce, Nd, Sm and Tb. The granite magma was possibly derived from a tonalite-granodiorite-dominated crustal source. Based on the above-mentioned geochemical evidence, it is interpreted that the source rocks of the magma of the Wadhrai granite likely developed initially in a continental margin subduction setting and underwent partial melting in a continental extensional environment.

Investigation of geochemical characteristics and hydrothermal alteration zone mapping supported by remote sensing of a porphyry copper prospect, southeastern Iran: integrated applications to exploration

The area discussed in this exploration methodology case study is situated along the Sistan Suture Zone where the granitoid suite is a mantle-derived multiphase intrusive complex. One of the characteristics of this region is the presence of large areas of sulfide-bearing, quartz-rich stockwork and pyritic veins. Geochemical findings show that it is limited to the calc-alkaline to shoshonitic series intrusions and is associated with a volcanic arc (I-type) formed within an active continental margin subduction setting. The associated intrusive complex has characteristics consistent with Cu productive porphyries, supported by high K-adakitic Sr/Y, La/Yb, Y and Al 2 O 3 /TiO 2 geochemical signatures. The stockwork mineralization includes the hypogene (chalcopyrite and bornite), with locally superimposed supergene (covellite, malachite, goethite and hematite) zones. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images (visible-near infrared (VNIR), and shortwave infrared (SWIR) bands) and mixture tuned matched filtering (MTMF) algorithm enable identification of each hydrothermal alteration type, especially where pictures were innovatively classified using a concentration–number (C–N) fractal method. Four alteration types are evident (i.e. phyllic, argillic and propylitic), as well as secondary (supergene) jarosite, that are associated with gossans, which are an indicator of the hypogene pyritic shell. The propylitic alteration envelopes the phyllic and argillic varieties, forming a belt around the pyritic shell; alteration assemblages were confirmed by X-ray powder diffraction (XRD) analysis. Finally, all results show a mineralization alteration pattern within the case study region that is similar to those of known porphyry copper and associated molybdenum- and gold-bearing systems in this region of Iran and worldwide.

Magma hybridization and crust-mantle interaction revealed by mineralogical and geochemical footprints: a case study from the South Qilian Accretionary Belt at the northern Tibetan Plateau

ABSTRACT The relationship between mafic magmatic enclaves (MMEs) and host granitoids is ongoing debate due to their significant roles in understanding deep magmatisms and geodynamic processes. This paper reports on Early Palaeozoic granitoids and associated MMEs from the Delingha pluton in the South Qilian Accretionary Belt (NW China) to constrain their petrogenesis and draw implications. The MMEs, occurring as ellipsoidal to rounded shapes, are andesitic and have the same crystallization age (~445 Ma) and mineral assemblage as the host granitoids. However, minor isotopic differences between them and compositional and textural disequilibria on minerals (e.g. plagioclase and amphibole) do not support a cumulate origin that the MMEs were formed at earlier stage of the same magmatic system, but forcefully reflect magma hybridization processes between mafic and felsic magmas, during which the hybridized magma was broken up into discrete enclaves by convective motion in the host felsic magma. Accordingly, the primitive compositions of the MMEs likely originated from metasomatized mantle wedge source, while mantle-derived melt underplated and partially melted the lower continental crust to form the host rocks with a certain mantle contribution. Thus, the Delingha pluton is the product of crust-mantle interaction at an active continental margin subduction setting rather than syn-collisional environment, indicating that subduction of the South Qilian Ocean located between the Central Qilian Block and the Oulongbuluke Block may have lasted till to ~445 Ma.

The Late Triassic-Jurassic magmatic belt and its implications for the double subduction of the Neo-Tethys Ocean in the southern Lhasa subterrane, Tibet

The southern Lhasa subterrane (SL), located between the Yarlung-Tsangpo Suture Zone and the Luobadui-Milashan Fault, preserves a complete Late Triassic-Jurassic magmatic record related to Neo-Tethys Ocean evolution. Therefore, the SL is key to understanding the geodynamic processes occurring during the controversial early-stage evolution of this ocean. Here, we present new zircon U–Pb data and a detailed synthesis of geochronological, geochemical, and Sr-Nd-Pb-Hf isotope data from Late Triassic-Jurassic igneous rocks in the SL to constrain the early-stage evolution of the Neo-Tethys Ocean. The data suggest that these igneous rocks can be divided into two groups. Group 1 magmatism lasted from ca. 243 Ma to 145 Ma, with three stages at ca. 243–200 Ma, 199–165 Ma, and 164–145 Ma, and is characterized by high positive εHf(t) and εNd(t) values and low (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values. Group 2 magmatism lasted from ca. 225 Ma to 160 Ma, with two stages at ca. 225–190 Ma and 189–160 Ma, and is characterized by lower εHf(t) and εNd(t) values and higher (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values than Group 1. We suggest that the Triassic-Jurassic magmatic belt in the SL was controlled by double subduction; the Group 1 igneous rocks formed in an intraoceanic arc setting, while the Group 2 igneous rocks formed in a continental margin arc setting. An integrated interpretation of magmatism, ophiolites, sedimentary rocks, and metallogenesis within the SL constrains the geodynamic processes that occurred during the early-stage tectonic evolution of the Neo-Tethys Ocean as follows. (1) During the Middle-Late Triassic, the initiation of intraoceanic subduction between Gondwana and the SL produced the ~243–200 Ma magmatism of Group 1. Subsequently, a new continental margin subduction system formed on the southern margin of the SL, which induced the ~225–190 Ma magmatism of Group 2. (2) With ongoing subduction, slab roll-back of the intraoceanic subduction system resulted in the ~199–165 Ma magmatic flare-up of Group 1. Additionally, slab roll-back of the continental margin subduction system resulted in extension of the overriding plate and formed the ~189–160 Ma magmatic flare-up of Group 2. (3) Finally, the slab roll-back associated with intraoceanic subduction transitioned to flat subduction and produced the 164–145 Ma magmatism of Group 1, and porphyry Cu-Au mineralization developed during the stress transition period. In summary, the double subduction system in the early-stage evolution of the Neo-Tethys Ocean controlled the tectonic, magmatic, and metallogenic events in the SL.

2.8–1.7 Ga history of the Jiao-Liao-Ji Belt of the North China Craton from the geochronology and geochemistry of mafic Liaohe meta-igneous rocks

The assembly and long-term evolution of the Eastern Block of the North China Craton are poorly constrained. Here we use bulk rock geochronological and geochemical data from mafic meta-igneous rocks (hornblendites, amphibolites and a metagabbro) of the Liaohe Group to reconstruct the Neoarchean to Paleoproterozoic history of the Jiao-Liao-Ji Belt, located between the Longgang and Nangrim blocks that together form the Eastern Block of the North China Craton. The mafic/ultramafic meta-igneous rocks have intrusive or tectonic contacts with the Liaoji granitic rocks (~2.2–2.0 Ga), which form the basement of the Jiao-Liao-Ji Belt. The major and trace element data indicate that the protoliths had calc-alkaline composition and formed along an active continental margin subduction zone. The mafic rocks form a whole-rock 176Lu/177Hf isochron with an age of 2.25 ± 0.31 Ga, overlapping with UPb zircon ages for mafic and granitic rocks from the Jiao-Liao-Ji Belt and consistent with being the emplacement age of the mafic protoliths along the active continental margin. In contrast, the whole-rock 147Sm/144Nd isochron age of 2.83 ± 0.18 Ga is likely to reflect the average age of the lithospheric mantle source from which the mafic/ultramafic protoliths were extracted. Together with geological evidence, we propose that the southwestern portion of the Longgang Block was an active continental margin since at least the early Paleoproteorozic. Literature age data from metamorphic zircons show that peak granulite metamorphism took place at ~1.96–1.88 Ga, resulting from the collisional event that fused the Longgang and Nangrim blocks into the Eastern Block of the North China Craton. Our bulk-rock 207Pb/206Pb age of 1824 ± 19 Ma and our 87Rb/86Sr age of 1671 ± 58 Ma reflect retrograde (cooling) stages during the exhumation of the Jiao-Liao-Ji Belt after the orogenesis.

Late-Mesozoic tectonic evolution and magmatic history of the Zhe-Gan-Wan region, SE China: evidence from the Shiling rhyolite and other Yanshanian granitoids

Yanshanian granitoids (178–120 Ma, Jurassic to Cretaceous), which intruded into Precambrian crystalline base- ment and Paleozoic strata, are widely distributed at the junction of the Zhejiang, Jiangxi and Anhui provinces (the ZGW region) in southern China, along with coeval volcanic rocks. This paper summarizes zircon U–Pb age data, geochemi- cal characteristics and Sr-Nd isotopic characteristics of the Late Jurassic to Middle Cretaceous granitoids and volcanic rocks from the ZGW region. We demonstrate that members of the Shiling rhyolite formed during two different periods of magmatic activity at 154.7 ± 2.5 Ma and 139–134 Ma and that igneous rocks from the different periods have distinct geochemical characteristics. Jurassic igneous rocks of the ZGW region have relatively low SiO2and high Al 2O3contents, and show enrichment of large ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs). They are strongly enriched in LREE and depleted in HREEs with weakly negative Eu anomalies and strongly negative Nb, Ta anomalies. Rb and Y concentrations follow the trends of I-type and S-type granites. By contrast, Cretaceous igneous rocks of this region are characterized by high SiO2and low Al2O3contents with negative Eu anomalies. They have typical wing- shaped rare earth element (REE) distribution patterns and show enrichment of Rb, Th, U, Nb, Ta and depletion of Ba, Sr, P and Ti. They have affinity to A-type peraluminous granites or highly-fractionated felsic rocks. Overall, the igneous rocks evolved from high-Sr low-Yb to low-Sr high-Yb, which might reflect the evolution of the tectonic setting from subduction to lithospheric extension/thinning, i. e., a transition from a continental margin subduction setting during the Late Jurassic to a within-plate extensional setting during the Early Cretaceous, at c. 142 ± 3 Ma. The repeated alternation between lava extrusion and extension and extensional fault kinematics in the late Mesozoic is related to the changes of direction and rate of plate movement of the Izanagi and Pacific plates.

Origin and Age Determination of the Neotethys Meliata Basin Ophiolite Fragments in the Late Jurassic–Early Cretaceous Accretionary Wedge Mélange (Inner Western Carpathians, Slovakia)

This study reports the Neotethyan Meliata Basin ophiolite fragments in the Late Jurassic–Early Cretaceous accretionary wedge mélange in the southern part of the Inner Western Carpathians (IWC). Here we present new lithostratigraphical, petrographical, geochemical, and geochronological data obtained from the mélange blocks used to reconstruct the Meliaticum paleotectonic zones in a tentative evolutionary model of this accretionary wedge. The Dobšiná mélange block continental margin carbonatic and siliciclastic sediments have calc-alkaline basalt intercalations. The basalt Concordia age dated to 245.5 ± 3.3 Ma by U–Pb SIMS on zircon most likely indicates the pre-oceanic advanced early Middle Triassic continental rifting stage. The evolving marginal oceanic crust is composed of Middle to Upper Triassic cherty shales to radiolarites. The detrital zircon U–Pb SIMS Concordia ages of 247 ± 4 Ma and 243 ± 4 Ma from a cherty shale, and the xenocryst zircon population Concordia age of 266 ± 3 Ma from a 0.5 m thick “normal” mid-ocean ridge (N-MOR) basalt layer in this cherty shale reveal the connection of the oceanic basin to the adjacent rifting continental margin. The chertified reddish limestone transition to radiolarite indicates syn-rift basin deepening. Upwards, regular alternating N-MOR basalts and radiolarites are often disturbed by peperite breccia horizons. The Nd isotope values of these basalts (εNd240 = 7–8) are consistent with their chondrite normalized rare earth element (REE) patterns and indicate a depleted mantle source. The Triassic ophiolitic suite also comprises rare ocean island (OI) basalts (εNd240 = 5) and serpentinized subduction unrelated peridotites. The Middle to Late Jurassic shortening and southward intra-oceanic and continental margin subduction at approximately 170–150 Ma enhanced the formation of the trench-like Jurassic flysch succession which preceded the closure of the Meliata Basin. The flysch sediments form a mélange matrix of olistolithic unsubducted, obducted, and MP–HP/LT metamorphosed exhumed blocks of the Triassic to Lower Jurassic successions. Blocks of peridotites, rodingites, blueschists, greenschists, rare amphibolites, deep-water shaly sediments and shallow- to deep-water carbonates are typical members of the mélange. The Meliatic accretionary wedge mélange nappe outliers were incorporated in the IWC orogenic wedge in the late Early Cretaceous according to metamorphic rutile U–Pb SIMS ages of 100 ± 10 Ma determined from a Jaklovce metabasalt.

Chapter 13 Permian–Triassic felsic tuffs in South Island, New Zealand: significance for oceanic and active continental margin subduction

Abstract Felsic tuffs play an important role in the Permian–Triassic geology of the Eastern Province in South Island. In the Brook Street Terrane, primary felsic tuff is minor in the south (e.g. Takitimu Mountains) but abundant in the north (Grampian Formation, Nelson area). Felsic fallout tuff dominates one interval of the Maitai Group (Early Triassic Kiwi Burn Formation), south of the Alpine Fault, but is otherwise mainly redeposited by gravity flows. The Murihiku Terrane is characterized by two main intervals of felsic fallout tuff, the Middle Triassic Gavenwood Tuffs and the Late Triassic Bare Hill Tuff Zone, south of the Alpine Fault (e.g. Hokonui Hills and south Otago coast). Counterparts north of the Alpine Fault (Richmond Group) are mainly reworked, with terrigenous admixtures. Tuffaceous sediments are also abundant in the late Middle–early Late Triassic Willsher Group (south Otago coast). Based on combined field, petrographical, semi-quantitative X-ray diffraction (XRD) and chemical evidence, the felsic tuffs of the Brook Street Terrane in the south are interpreted as small-scale eruptions of fractionated oceanic-arc-type magmas. In contrast, the Triassic felsic tuffs of the Murihiku Terrane, Willsher Group and Maitai Group erupted violently and episodically in proximal to distal segments of the SE Gondwana continental margin.

Nature and evolution of the Cretaceous basins in the eastern margin of Eurasia: A case study of the Gyeongsang Basin, SE Korea

The Gyeongsang Basin is the largest among the non-marine Cretaceous sedimentary basins in Korea and Japan, formed by sinistral strike-slip movement induced by the oblique subduction of the paleo-Pacific plate under the eastern Asian continental margin, and has an evolutionary history of about sixty million years. SHRIMP and LA-MC-ICPMS U-Pb analyses were performed on the detrital zircons separated from the constituent formations of the Gyeongsang Basin. We determined the maximum depositional ages of the four formations based on their ages of the youngest groups as follows; 118.0 ± 2.6 Ma (n = 4) for the Hasandong Formation, 108.7 ± 0.8 Ma (n = 11) for the Chilgok Formation, 106.0 ± 0.2 Ma (n = 63) for the Silla Conglomerate, 105.4 ± 0.4 Ma (n = 14) for the Haman Formation, and 99.9 ± 0.7 Ma (n = 5) for the Jindong Formation, belonging to the period from Aptian to Cenomanian. The pre-Cretaceous detrital zircons of the lower to middle Gyeongsang Supergroup indicate their derivation from the Yeongnam Massif and the Okcheon Metamorphic Belt surrounding the Gyeongsang Basin. At around 115 Ma, as reflected as enhanced proportion of such ages in the detrital zircons and occurrence of farther inland igneous activities, the tectonic environment of Gyeongsang Basin changed to intra-arc due to the more active reestablishment of subduction-related igneous activity. However, the upper Gyeongsang Supergroup has a new age component of the mid-Permian, along with changes in the direction of paleocurrent flowing from the northeast and east, indicating sediment supply from SW Japan with rocks of that age. The changes in the direction of the paleocurrent flow seem to indicate that the center of igneous activity has moved in the direction of the subduction zone to the east. As the intensity of igneous activity continued to increase, the Gyeongsang Basin was closed by the filling of the basin by volcanic activity. The Cretaceous basins of the Korean Peninsula and southwest Japan show similar evolutionary histories, suggesting a type model of the creation and the evolution of basin around the continental margin subduction zone; creation of sedimentary basin by pull-apart or rifting in the backarc region, evolution to intra-arc basin by the landward progress of subduction-related igneous activity, and finally the closure of the basin due to the advent of highly enhanced igneous activity.

Eclogites from the Marun-Keu Complex, Polar Urals, Russia: a record of hot subduction and sub-isothermal exhumation

Abstract Lenses and boudins of eclogitized and non-metamorphic gabbroic and ultramafic rocks occurring in quartzofeldspathic gneisses and migmatites of the Marun-Keu Complex, Polar Urals, are of central interest for the study of fluid-controlled processes in the subducting continental lithosphere. Petrology and mineral chemistry, combined with multi-equilibrium geothermobarometry and phase-equilibrium modelling, applied to mafic eclogitic rocks show a prograde evolution from c. 685°C, 1.89 GPa and a H 2 O = 0.6 to c. 745°C, 2.13 GPa and a H 2 O = 1 followed by near-isothermal decompression down to c. 1.63 GPa. The absence of pre-eclogitic H 2 O-bearing mineral inclusions in garnet and omphacite suggests that the primary magmatic rocks have not been affected by prograde metamorphism during subduction due to the lack of fluid. We suggest that the Marun-Keu Complex represents a coherent crustal block that experienced the same P – T evolution but provides a metamorphic record of different P – T parameters because of an uneven introduction of aqueous-bearing fluids during subduction and exhumation. The tectonometamorphic history of the Marun-Keu Complex is best attributed to the relatively hot regime of the continental margin subduction beneath an island arc, whereas the blueschist-facies part of the complex towards the north could be related to the closure of the Uralian Ocean.

The tectonic setting and evolution of the 2.7 Ga Kalgoorlie–Kurnalpi Rift, a world-class Archean gold province

The Yilgarn Craton results from three major mantle input events (at ca 3.0–2.9, 2.8 and 2.7 Ga) that have interacted with > 3.0 Ga continental crust. Zircon geochronology and Sm-Nd isotopic data subdivide the craton into an older Yilgarn proto-craton and the younger, more primitive Eastern Goldfields Superterrane (EGST). Formation of the Kalgoorlie-Kurnalpi Rift (KKR) within the EGST was associated with the 2.7 Ga event, which exploited weakened crust at the eastern margin of the Yilgarn proto-craton where thick sequences of komatiite and basalt were erupted between ca 2710 and 2690 Ma in the Kalgoorlie Terrane. Calc-alkaline volcanism in the Kurnalpi Terrane began at ca 2730 Ma and continued to ca 2690 Ma, overlapping rifting and plume-related volcanism in the Kalgoorlie Terrane. Deposition of siliciclastic sedimentary rocks within basins at ca 2660 resulted from an intra-orogenic extensional event and coincided with the transition from High-Ca to Low-Ca granite magmatism and peak emplacement of intrusions with a metasomatised mantle source component. Most aspects of the KKR are satisfied by broadly coincident plume-related magmatism in the Kalgoorlie Terrane and westward subduction to the east of the Burtville Terrane. Geochemical characteristics of 2730–2700 Ma calc-alkaline volcanism and 2685–2630 Ma low-SiO2 and alkali-rich intrusions support models for a continental margin subduction zone setting. World-class gold deposits formed in reactivated margins of the KKR, which became flux zones for mantle-derived magmas, hydrothermal fluids and heat during 2675–2620 Ma orogenesis. The orogenic gold mineralisation can be subdivided into proximal intrusion-related and distal-source deposits.

Geochemistry and tectonic setting of the Golabad granitoid complex (SW Nain, Iran)

The Oligo-Miocene Golabad granitoid complex intrusive into the Eocene volcanic rocks occurs in the Urumieh-Dokhtar Magmatic Arc (UDMA) in Iran. According to microscopic and chemical studies, the granitoid complex consists of three different rock types: 1) plutonic rocks comprising diorite, quartz diorite, granodiorite and granite; 2) volcanic rocks composed of basalt, andesite basalt, ± pyroxene bearing andesite and rhyolite, and 3) pyroclastic rocks. The main mineral constituents of these rocks are mostly plagioclase (oligoclase and andesine), quartz, K-feldspar, amphibole (magnesio-hornblende and actinolite-hornblende) and Mg-biotite. In addition, apatite, titanite, zircon, and opaque minerals are common accessory minerals. The studied enclaves are classified as mafic micro-granular enclaves (MME) with monzodiorite compositions. Geochemically, the rocks in this study represent medium to high-K calc-alkaline series, metaluminous and I-type nature.Plotting, the chemical composition of plagioclase on the An-Ab-Or ternary diagram, the temperature of crystallization is estimated to range from 700 to 900 °C at a pressure of 4.5 Kbar. High TiO2 values of biotites from the Golabad granitoid complex suggest magmatic origin and the crystallization temperature is estimated to range from 700 to 750 °C. The amphiboles according to their chemical analysis, are classified as igneous amphiboles generated in high oxygen fugacity conditions. The chemical data of the amphiboles and biotites pointed out to the I- type nature of the Golabad granitoid complex emplaced in an active continental margin subduction setting. The amphibole crystallization pressure was estimated by Al in amphibole varies from 1.09 to 2.28 Kbar. Using the calculated pressure the depth of the formation of the Golabad granitoid complex estimated from 4 to 9 Km.

Structure of the (ultra)high-pressure Western Gneiss Region, Norway: Imbrication during Caledonian continental margin subduction

Structure of the (ultra)high-pressure Western Gneiss Region, Norway: Imbrication during Caledonian continental margin subduction

Recycling of palaeo-Pacific subducted oceanic crust related to a Fe–Cu–Au mineralization in the Xu-Huai region of North Anhui-Jiangsu, East China: Geochronological and geochemical constraints

ABSTRACTIn this study, Early Cretaceous skarn deposits and genesis of their host diorite/monzodiorite porphyry in the Xuzhou-Huaibei (Xu-Huai) region, northern Anhui-Jiangsu have been discussed by detailed geochemical work. In-situ zircon U–Pb dating of the diorites related to Fe–Cu–Au deposits shows that they were formed between 131.4 ± 1.5 Ma and 130.8 ± 1.8 Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry with similarity to that of arc-related igneous rocks. The diorite porphyry was probably derived from typical arc magmas related to continental margin subduction characterized by light rare earth elements (LREEs) enrichment and HFSE depletion. REEs compositions of apatite in the diorite porphyry indicate that the dioritic magma was produced from the metasomatized subcontinental mantle by slab-derived fluids. The magma was proven to be a high oxygen fugacity; thus, it was particularly conducive to the precipitation of Fe, Cu, Au and other ore-forming elements. The δ34S values of pyrite and chalcopyrite of Fe–Cu–Au ores range from −0.2‰ to 2.8‰, indicating that the sulphur in the ore was probably derived from deep-seated magmas. Integrated with geochronological and geochemical analyses, we suggest that the Early Cretaceous igneous suites associated with Fe–Cu–Au deposits in the Xu-Huai region are related to recycling subduction of Pacific oceanic crust.

New data on the period of existence of the continental margin subduction zone in the Middle Urals

U–Pb (SIMS) dating of zircons from granite of the Petrokamensk gabbro–granitoid complex, marking termination of the Devonian island–arc magmatism in the Verkhisetsk–Tura Zone of the Middle Urals, gave an age of 386 ± 3 Ma. Zircons from the West Verkhisetsk granitoids, the formation of which corresponds to the beginning stage of magmatism of the continental margin in the region, yielded ages of 386.6 ± 4.1 and 381.8 ± 6.0 Ma. This suggests that the change in the regime of the island–arc geodynamic mode to an active continental margin took place at the boundary of the Middle and Late Devonian. The upper boundary of existence of the continental margin subduction zone is determined by the closure of the Ural paleo-ocean and the beginning of continental collision processes, which resulted in accumulation of flysch in the Ural foreland trough from the second half of the Bashkirian Stage.

Origin of the late Early Cretaceous granodiorite and associated dioritic dikes in the Hongqilafu pluton, northwestern Tibetan Plateau: A case for crust–mantle interaction

We present a detailed study of geochronology, mineral chemistries, bulk-rock major and trace element abundances, and Sr–Nd–Hf isotope compositions of the granodiorite and associated dioritic dikes in the Hongqilafu pluton at the northwestern margin of the Tibetan Plateau. The granodiorite and dioritic dikes yielded zircon U–Pb ages of ~104Ma and ~100Ma, respectively. The dioritic dikes comprise varying lithologies of gabbroic diorite, diorite porphyry and granodiorite porphyry, exhibiting a compositional spectrum from intermediate to felsic rocks. Their mineral compositions display disequilibrium features such as large major element compositional variations of plagioclase, clinopyroxene and amphibole crystals. These dioritic dikes are enriched in incompatible elements (Ba, Rb, Th, U, K) and Sr–Nd–Hf isotopes (87Sr/86Sri: 0.7066 to 0.7071, εNd(t): −5.3 to −7.4, εHf(t): −3.6 to −6.2). We suggest that the dioritic dikes were most likely derived from partial melting of mantle wedge metasomatized by the subducted/subducting seafloor with a sediment component, followed by AFC processes with fractional crystallization of clinopyroxene, amphibole and plagioclase and assimilation of lower continental crust. The mantle-wedge derived magma parental to the dioritic dikes underplated and induced the lower continental crust to melt, forming the felsic crustal magma parental to the granodiorite with mantle melt signatures and having more enriched isotope compositions (87Sr/86Sri: 0.7087 to 0.7125, εNd(t): −9.5 to −11.6, εHf(t): −10.3 to −14.1) than those of the dioritic dikes. The Hongqilafu pluton is thus the product of mantle–crust interaction at an active continental margin subduction setting over the period of several million years. This understanding further indicates that the closure timing of the Shyok back-arc basin and the collision between the Kohistan–Ladakh Arc and the Karakoram Terrane may have taken place later than ~100Ma.