Continental Subduction Zones Research Articles

Petrographic and isotopic studies in Kamoo area, northeast isfahan, Iran

The study area is located 5.5 km north-east of Kamo County and 30 km north-east of Meymeh, Isfahan. Kamo Index comprises an anticline with a northwest-southeast trend, the outcrop of which consists of siltstone-sandstone and Jurassic shale rocks that lie along a cretaceous unit. Based on field studies as well as petrographic and geochemical investigations, mineral assemblages and minerals in the area include chalcopyrite, pyrite, marcasite, and magnetite as primary ores and malachite, bornite, covellite, chalcocite, hematite, goethite, and hydrous iron oxides as secondary ores. The secondary ores have been formed by oxidation and biogenic processes. The primary textures are granular, disseminated, vein-shaped, and layered and the secondary textures are boxed and martitized, shear zones, replacements, and veins. Tectonic, metamorphic, and oxidation factors are involved in the formation of these textures. Due to tectonic activity in this area and the presence of faults, replacement, vein, and shear textures, which have developed after mineralization, are observed. Among alterations, hematite, propylitic, sericitic, limonitic, and kaolinitic are highly prevalent and they are important in the formation of iron deposits. The sub-alkaline granitoids of the region are related to the continental subduction-zone orogenic series. Studies on the decomposed iron samples from Kamo show that most of the samples are in the sub-alkaline series and the chemical and mineralogical composition of the magma derived from this series is calc-alkaline with a large amount of iron. According to mineralogical, lithological, and geochemical evidence and based on isotopic results, Kamo deposit is a skarn-type deposit.

Syn-exhumation magmatism in an active continental margin above a continental subduction zone: Evidence from Late Triassic mafic igneous rocks in the southeastern North China Block

AbstractSyn-subduction arc magmatism is absent above continental subduction zones, providing important constraints on the nature of petrogenetic processes during continental collision. Nevertheless, we have identified syn-exhumation mafic igneous rocks in an active continental margin above a continental subduction zone, where the South China Block (SCB) was deeply subducted beneath the North China Block (NCB) for collisional orogeny in the Triassic. These mafic igneous rocks occur in the southeastern margin of the NCB, showing consistent Late Triassic zircon U-Pb ages of ca. 219–218 Ma, coeval with exhumation of the deeply subducted continental crust. These rocks are categorized into two series of sub-alkaline and alkaline, all exhibiting arc-like trace element distribution patterns, highly enriched radiogenic Sr-Nd-Hf and high zircon O isotope compositions. In particular, they exhibit two-stage whole-rock Nd and Hf model ages and zircon Hf model ages of Paleoproterozoic, which are comparable to those of ultrahigh-pressure metamorphic rocks with the SCB affinity in the Dabie-Sulu orogenic belt. Such geochemical features indicate that these mafic igneous rocks were derived from partial melting of ultramafic metasomatites generated by reaction of felsic melts from the subducted SCB with the mantle wedge peridotite beneath the NCB. The geochemical differences in element and isotope compositions between the two series igneous rocks can be mainly ascribed to different proportions of the crustal component in the metasomatites, which is verified by quantitative modellings of the geochemical transfer in the continental subduction zone. The systematic variations in some geochemical variables such as Fe/Mn, Zn/Fe, and Nb/Ta ratios indicate pyroxenite-rich and hornblendite-rich lithologies, respectively, for the sub-alkaline and alkaline series igneous rocks. With the tectonic extension for exhumation of the deeply subducted continental crust in the Late Triassic, the fertile and enriched metasomatites in the mantle wedge underwent partial melting for the syn-exhumation mafic magmatism in the southeastern NCB. Therefore, the mafic igneous rocks in the active continental margin not only record the crust-mantle interaction in the continental subduction zone, but also witness the generation of syn-exhumation magmatism in the late stage of continental collision.

Geochemistry, petrogenesis, zircon U[sbnd]Pb geochronology and Sr[sbnd]Nd isotopic composition of Kuh-e-Shah volcanic rocks: Implications for an active continental margin along with eastern Iran during the Paleogene

The Kuh-e-Shah complex includes the Paleogene volcano-plutonic belt of the Lut Block, eastern Iran. The volcanic rocks which outcropped in this complex mainly consist of trachy-andesites, andesites, and basaltic-andesites that mineralogically contain plagioclase, pyroxene, hornblende, and minor biotite and olivine. Geochemically, they have features typical of high-K calc-alkaline to shoshsonitic magmas with enrichment in large ion lithophile elements (LILE), and depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE). Chondrite-normalized REE plots show enrichment in light REE (4.9 < LaN/YbN < 11.6), Nb depletion and the slight negative Eu anomalies (Eu/Eu* = 0.80–0.99). Tectonic discrimination diagrams are used to infer a volcanic arc setting related to a continental subduction zone. It is concluded that the studied volcanic rocks have resulted from FC of a parental magma which formed by partial melting of the subducted oceanic crust and the overlying mantle wedge with spinel-lherzolite composition. Zircon UPb dating indicates an age of 38.6 to 38.9 Ma for volcanic rocks (Middle Eocene, Bartonian). Initial 87Sr/86Sr and 143Nd/144Nd ratios (0.704350–0.704820 and 0.512619–0.512779, resp.), are compatible with parental melts formation in a subduction mantle wedge. The ɛNdi values (+0.60 to +3.73) are in the range of mantle-derived melts. We suggest that volcanic rocks of the Lut Block are part of the Paleogene volcanism resulted from the Sistan oceanic crust subduction under the Lut Block during the Cretaceous.

Magnesium-carbon isotopes trace carbon recycling in continental subduction zone

Subduction of the oceanic crust is an important mechanism for transporting the surface carbon into the deep mantle. Subduction of the continental crust is generally preceded by subduction of the oceanic slab, which may carry different sources of crustal carbon into the same subduction zone with enhanced carbon recycling at the convergent plate boundary. Thus, it is significant to identify whether crustal carbon was recycled into the mantle along continental subduction zones. In this contribution, we report for the first time the Mg and C isotope compositions of post-collisional mafic igneous rocks from the Hong'an and Dabie orogens in east-central China. There are relatively low δ26Mg values of −0.77‰ to −0.24‰ and high δ13C values of −4.0‰ to −1.8‰ in association with very high C contents of 9712–10,177 ppm for the Hong'an rocks, indicating recycling of the early subducted, carbonated oceanic crust into the mantle. In contrast, there are relatively high δ26Mg values of −0.23‰ to −0.18‰ and low δ13C values of −5.9‰ to −4.1‰ in association with relatively low C contents of 711–2313 ppm for the Dabie rocks, demonstrating recycling of the crustal carbon from the subsequently subducted continental lithosphere. These interpretations are verified by quantitative modeling of the geochemical data for the two orogens of mafic igneous rocks. The results suggest that addition of 30–40% carbonated oceanic crust-derived materials and 10–20% continental crust-derived materials to the mantle sources can produce the Mg–C–Sr–Nd–Hf isotope compositions of mafic igneous rocks in the Hong'an and Dabie orogens, respectively. Therefore, the different types of crustal carbon were incorporated into the mantle wedge at different depths by the successive subduction of oceanic and continental slabs. In particular, the carbon recycling is evident in the continental subduction zone. As such, carbon fluxes in and out of the mantle in continental subduction zones also need to be taken into account when constraining the global C cycle.

Accretionary complex: Geological records from oceanic subduction to continental deep subduction

Accretionary complex was usually formed by offscraping of the subducting crustal material over the trench and thus often referred to as subduction zone melange. The structure, composition and forming process of accretionary wedges can provide important insights into the evolution history of ocean basin, ocean-continent material cycle, continental accretion and thus contribute to understanding of the origin of plates and the growth of continents. Accretionary complex is characterized by a block-in-matrix structure associated with imbricate thrusts and isoclinal folds, diversified metamorphic types and intense water-rock interactions, which are distinct to the traditional stratigraphy. Since the proposal of the concept of accretionary wedge over a hundred years ago, great progress has been made in a variety of research focuses, such as the identification of the distribution of accretionary complexes, their compositions and formation mechanisms, the affinities of the matrix and igneous rocks, the recognition of the Ocean Plate Stratigraphy (OPS), the reconstruction of oceanic basin, the dynamic background of the tectonic evolution, the relationship between subduction zone and orogenic belt and, in particular, the accretionary complexes in continental subduction zones. These studies have significantly improved our understanding of the plate tectonic theory. Challenges remain in the identification of ancient accretionary complexes, the detailed analysis of accretionary complex zones, the accretion characteristics during continental collision, and the geochemical tracing of water-rock interaction during the accretion. China contains representative orogenic belts and accretionary complex zones in the world, and its geological records provide the best opportunity to make new breakthroughs in understanding of the plate tectonics.

Petrogenesis of gem sapphire in a pegmatite-aplite vein from the Alvand batholith, Western Iran

In the Hamadan region, Sanandaj-Sirjan Zone (SaSiZ), W Iran, a pegmatite-aplite vein occurs that contains up to 15 vol% sapphire. The Hamadan sapphire is black to blue, and translucent to opaque, which reveals its semi-gem and gem quality. On the basis of petrographic studies, the Hamadan sapphire-bearing pegmatite-aplite vein represents alkali syenitic to syenitic composition and its adjacent granitoids have a range of composition from syenitic to quartz monzonite and granodiorite. New petrographic, whole-rock geochemical data and δ18O values [9.30–12.45‰VSMOW (Vienna Standard Mean Ocean Water)] suggest crustal origin for the peraluminous, S-type and alkaline sapphire-bearing pegmatite-aplite vein. The presence of syngenetic quartz inclusions in the sapphire, and some similar physical-gemological signatures (e.g., parting patterns and angles) of the Hamadan sapphire and kyanite support the generation of the sapphire at the expense of kyanite. In addition, a secondary metasomatic stage may have played a minor role in formation of the sapphire grains. The results indicate an important role for the mafic magmas in triggering crustal anatexis and gem crystallization in continental subduction zone settings.

Syn-exhumation melting of the subducted continental crust: Geochemical evidence from early Paleozoic granitoids in North Qaidam, northern Tibet

Syn-exhumation partial melting of the deeply subducted continental crust commonly occurs in collisional orogenic settings. The resulting felsic melts not only form granitoids, but can also modify the composition of the mantle wedge above continental subduction zones. This study investigates crustal anatexis and its role in granitoid formation and crust–mantle interaction during continental collision using new whole-rock geochemical, Sr–Nd–Hf isotope, and zircon U–Pb geochronological and Lu–Hf isotope data for early Paleozoic granitoids of the North Qaidam area of northern Tibet, a collisional orogen that contains ultrahigh-pressure (UHP) metamorphic rocks. Zircon U–Pb dating of the granitoids in this area yielded ages between 416 ± 6 Ma and 393 ± 2 Ma, coeval with the exhumation of the deeply subducted continental crust. This indicates that these granitoids formed as a result of syn-exhumation magmatism during the continental collision. Relict zircons in the granitoids yield concordant Neoproterozoic and early Paleozoic U–Pb ages that are consistent with the protolith and metamorphic ages of UHP metaigneous rocks in the orogen, respectively. The granitoids have calc-alkaline to high-K calc-alkaline compositions, with total alkali contents of 4.07–8.09 wt%. They are also metaluminous to weakly peraluminous with A/CNK ratios of 0.86–1.14. They are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), yielding arc-like trace element patterns. They also have high (87Sr/86Sr)i ratios (0.7058–0.7195) and negative εNd(t) values (−7.1 to −1.4) and have whole-rock and zircon εHf(t) values of −2.8 to +2.7 and − 8.6 to +4.7, respectively. These data indicate that the granitoids are compositionally similar to the UHP metaigneous rocks in the collisional orogen, suggesting a genetic link. This in turn indicates that the granitoids in the study area formed from magmas generated by the partial melting during the exhumation of deeply subducted continental crust. The resulting granitic melts not only generated the syn-exhumation granitoids in this area, but also caused metasomatism associated with crust–mantle interaction in continental subduction channels.

The study of subduction channels: Progress, controversies, and challenges

The subduction channel is defined as a relatively thin and weak zone with coherent kinematics between the descending and overriding plates during subduction. The materials in the channel, showing the characteristics of melange, consist of low-density, low-viscosity, highly sheared metasediments and/or serpentinite-rich matrix with relatively rigid blocks. The channel materials flow downwards and upwards, forming a kind of channel convection. Based on geophysical and geological observations on active oceanic subduction zones, combined with numerical simulations, the subduction channel model was initially set to shallow depths With the development of numerical simulation technology and fossil subduction zone investigations, the subduction channel model was extended to depths of 80–100 km to describe deeper geodynamic environments of oceanic subduction zones and explore the exhumation of high to ultrahigh pressure metamorphic rocks. It is also extended to continental subduction zones in order to explain the exhumation of metamorphic rocks in different grades as well as fluid activity and crust-mantle interaction during continental subduction. Although there are a series of differences in material composition and slab/mantle interface property between the upper and lower plates, continental subduction channels exhibit many similarities in structure and dynamics to oceanic subduction channels. Existing studies have provided a fundamental understanding of the structural and dynamic characteristics of subduction channels, but many problems still need to be solved and clarified. In particular, the fundamental characteristics of subduction channels obtained by numerical simulation need to be confirmed and modified through geological investigations of fossil subduction channels.

Geochemistry, geochronology and Sr–Nd–Hf isotopes of two types of Early Cretaceous granite porphyry dykes in the Sulu orogenic belt, eastern China

Late Mesozoic granitic rocks are widely distributed in the Sulu orogenic belt, but the source, tectonic affinity, and associated geodynamic setting that produced the respective magmas remain controversial. To provide insights into these issues, we present field-based petrological, whole-rock major and trace element and Sr–Nd isotope geochemical, zircon U–Pb dating, and Lu–Hf isotope studies on two types of granite porphyry dykes that are newly recognized from the central Sulu belt. U–Pb dating of magmatic zircons from both types yields consistent ages that vary between 124 ± 2 and 118 ± 2 Ma, constraining the timing of intrusion as Early Cretaceous. The granitic rocks have high-K calc-alkaline peraluminous compositions with low Mg# values and are characterized by fractionated rare earth element patterns with strong depletion in high field strength elements. Compared with type I of the granite porphyry dykes, type II exhibits higher SiO2 but slightly lower Na2O and K2O abundances, contains higher Rb/Sr and 87Sr/86Sr ratios, and shows more pronounced negative Eu, Sr, and Ba anomalies. Both types I and II have high initial 87Sr/86Sr ratios of 0.709–0.711 and negative εNd(t) values of −19.8 to −18.4. The magmatic zircons possess negative εHf(t) values of −29.1 to −20.8, with mostly Neoarchean Hf model ages of 3001–2478 Ma. These features, together with the presence of Neoproterozoic inherited zircons, indicate that the two types of granite porphyries successively crystallized from a joint granite magma that derived from partial melting of the continental crust of the Yangtze Craton. Therefore, an interrelationship between the granite porphyry dykes and massive magmatic granitoids from adjacent regions in the Sulu belt may be documented, recording evidence of a joint ancient crustal reworking and recycling in a fossilized continental subduction zone during the Early Cretaceous.

Tracing subduction zone fluids with distinct Mg isotope compositions: Insights from high-pressure metasomatic rocks (leucophyllites) from the Eastern Alps

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<−1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries.

Experimental interaction of granitic melt and peridotite at 1.5 GPa: Implications for the origin of post-collisional K-rich magmatism in continental subduction zones

To decipher the genesis of post-collisional magmatism in continental subduction zones, we conducted a series of experiments on the interaction of granitic melt derived from continental crust with harzburgite at 1.5 GPa and temperatures ranging from 950 °C to 1250 °C. The two end-members were homogeneously mixed with different crustal melt/peridotite ratios and under water-saturated or water-undersaturated conditions. Interaction takes place in two stages: the initial reaction, followed by re-melting of the metasomatized product. In the first stage, crustal melt reacts with the harzburgite, consuming olivine and forming orthopyroxene and phlogopite. In the second stage, re-melting of the metasomatized product is dominated by the incongruent melting of both orthopyroxene and phlogopite, leading to increasing peritectic olivine production as melting progresses. Upon reaction with the peridotite and melting of the metasomatized product, initially granitic melts evolve in composition to trachytic and trachyandesitic. The crustal melt/peridotite ratio does not appear to affect the SiO2 content of melt that equilibrates with olivine and orthopyroxene, whereas the SiO2 content of the melt is higher when olivine is absent from the residue. K2O in the melt is buffered by coexisting phlogopite: this buffering effect of phlogopite weakens at higher crustal melt/peridotite ratio and in water-undersaturated conditions, leading to higher K2O in the melt. Trace element compositions of the hybrid melt generally inherit the characteristics of the crustal melt: most LILEs and transition elements are buffered by phlogopite, with Rb and V having the highest partition coefficients. Our experimental melts show compositional parallels with mantle-derived post-collisional magmas in the Variscan orogen, verifying the involvement of granitic continental crust melt in their source.

Geochemical evidence from coesite-bearing jadeite quartzites for large-scale flow of metamorphic fluids in a continental subduction channel

Metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab-mantle interface in subduction channels. It is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels. This assumption is challenged by a combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements in coesite-bearing jadeite quartzites from the Dabie orogen, China. Although the target samples were collected from different outcrops in an area of ∼50 km2, zircon U-Pb dating yields similar discordia lines with not only consistent upper intercept ages of 1.9–2.0 Ga but also consistent lower intercept ages of 224–235 Ma. This indicates the same Paleoproterozoic protolith and the same Triassic metamorphism for these jadeite quartzites. The O isotope analysis of mineral separates and whole-rock yields variable δ18O values from 6.3‰ to 9.4‰, indicating involvement of supracrustal components. Except for one outlier at −0.43‰, all the other rocks give variable δ26Mg values from −0.16‰ to 0.61‰, much higher than normal mantle values. The whole-rock Mg isotopes show significant positive correlations not only with MgO contents but also with Rb/La, Rb/Gd and Rb/Nb ratios, but a negative correlation with Na2O contents. These observations indicate that the middle Paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh-pressure metamorphism. They would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the TTG provenance. In view of the spatial occurrences of the target samples, the metamorphic fluids would have flowed inside the continental subduction channel on a large scale. This is the first report of the large-scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels.

Serpentinite-derived low δ7Li fluids in continental subduction zones: Constraints from the fluid metasomatic rocks (whiteschist) from the Dora-Maira Massif, Western Alps

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.

Water in Garnet Pyroxenite from the Sulu Orogen: Implications for Crust‐Mantle Interaction in Continental Subduction Zone

AbstractMineral water contents, together with the elements and isotopes of minerals and whole‐rock, were determined for garnet pyroxenites enclosed by ultrahigh‐pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The results suggest that the garnet pyroxenites were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424–660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water‐rich clinopyroxene. Calculated whole‐rock H2O/Ce ratios are 63–145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene‐rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the ultramafic metasomatites are an important water reservoir in the mantle wedge.

Deformation-enhanced HP-UHP metamorphic transformation of two-pyroxene granite in ductile shear zones at Yangkou, Sulu UHP metamorphic belt, China

We report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure (HP-UHP) metamorphic transformation of a two-pyroxene granite in deep continental subduction zone. Integrated structural and petrologic study of rocks from the two-pyroxene granite block and shear zones in the block in the UHP shear complex at Yangkou, Sulu UHP metamorphic belt, reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene, augite, plagioclase, K-feldspar, quartz and biotite. Metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics. In contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to HP-UHP phase minerals. Dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite, which increases reaction surface area and enhances metamorphic transformation. Fluid released by biotite transformation to HP-UHP minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone. Deformation conditions are estimated ca 546 °C and pressure 2.5 GPa.

Metasomatic flow of metacarbonate-derived fluids carrying isotopically heavy boron in continental subduction zones: Insights from tourmaline-bearing ultra-high pressure eclogites and veins (Dabie terrane, eastern China)

Abstract Tourmaline, one of the most important hosts of boron (B) in crustal rocks, has rarely been found in natural high-pressure (HP) and ultra-HP (UHP) eclogites. Here, we report the first finding of tourmaline-bearing UHP eclogites and veins in the Dabie terrane, China. These distinctive samples occur exclusively in impure UHP marbles. Investigations on the eclogite-vein-marble system provide important insights into the origin, flow, and metasomatic effects of eclogite-facies, 11B-rich fluids in continental subduction zones. Petrologic and geochemical evidence indicates that tourmaline in the eclogites (Tur-E) formed by metasomatism due to the infiltration of a B-rich fluid under conditions of ca. 2.2–2.6 GPa and 610–660 °C. The HP veins, containing euhedral tourmaline (Tur-V) and occurring at the eclogite-marble contacts or in the interiors of eclogite lenses, represent the crystallized products of the infiltrating fluid. Systematic compositional variations along the profile from the interiors of the eclogite lenses to their margins and mass-balance calculations indicate large inputs of B, carbon, large ion lithophile elements, and light rare earth elements to the eclogites during fluid infiltration. All types of tourmaline, including the Tur-E and Tur-V as well as minor amounts of tourmaline in the marbles (Tur-M), have similar compositions (dravitic) with XMg values [= Mg/(Mg + Fe)] of 0.7–0.8. In situ analyses using laser ablation multicollector inductively coupled plasma mass spectrometry show that all of tourmaline has high δ11B values (ranging from +6 to +15‰), which suggest a 11B-rich isotope signature for the infiltrating fluid. The tourmaline B isotope data, together with the detailed field observations and whole-rock Sr-Nd isotopes, reveal that the infiltrating fluid was derived from the impure marbles. Influx of such fluid was highly channelized and was mainly achieved along lithologic boundaries between UHP marbles and eclogites or along fractures in the eclogites. This study indicates that impure marbles are an important reservoir of isotopically heavy B in deeply subducted continental slabs. This point may be of particular importance because most B reservoirs in subducted continental crust typically have light B isotopic compositions. Our results highlight that subducted metacarbonate rocks could liberate eclogite-facies, B-rich, high-δ11B fluids and thus might exert important effects on the cycle of B and its isotopes in subduction zones.

Magmatic fluid input explaining the geochemical anomaly of very high arsenic in some southern Tibetan geothermal waters

Tibet is characterized by extremely high terrestrial heat flow and widely distributed hydrothermal systems, among which many are discharging geothermal waters with extremely high arsenic concentrations from over 10 mg/L to up to 126 mg/L. The distribution of these high arsenic waters is basically in accordance with the location of the Indian continental subduction zone. A detailed analysis indicates that host rock leaching alone cannot explain the observed arsenic anomaly. Instead, substantial contribution of arsenic from the underlying magma chambers is postulated, which are likely mantle-derived intrusions that have been severely contaminated by the very thick crustal material below southern Tibet, possibly including deep-seated arsenic-rich sedimentary rocks. Alternatively, the partial melts at crustal depths reflected by the detected low-velocity and/or high-conductivity anomalies in southern Tibet, serving as the magmatic heat source of the high‑arsenic hydrothermal systems, may also form indirectly under the high-temperature conditions generated by mantle upwelling or due to crustal thickening. Low sulfide concentrations further promote high dissolved arsenic concentrations with little formation of thioarsenates and no precipitation of limiting arsenic-sulfide phases. In contrast, mantle magmas less affected by crustal contamination, serving as the heat source of many hydrothermal systems in rift zones and hot spots worldwide where the crust is thinner, are releasing magmatic fluids lower in arsenic and higher in mantle sulfur species. Overall, the existence of a magmatic heat source alone does not ensure the formation of geothermal waters with very high arsenic concentrations, and geothermal waters from sedimentary rock-hosted systems are neither necessarily arsenic-rich. The most critical factor controlling the arsenic concentration of geothermal water discharging from a magmatic hydrothermal system is the geological genesis of the magma fluid and its chemical composition.

Mountain Building Orogeny in Precollision Hot Backarcs: North American Cordillera, India‐Tibet, and Grenville Province

AbstractIn this article I discuss the new perspectives on continental collisional orogeny from recent evidence that most continental subduction zones have 200‐ to 1,000‐km‐wide uniformly hot backarcs. Such hot backarcs are involved in most current and past continental collisions. I give the examples of the current India‐Asia and ancient Grenville orogens. (1) In continental collision, at least one side of the closing ocean has a backarc sufficiently hot and weak to be deformed by normal plate driving forces, in contrast to cool stable lithosphere that is too strong. Most deformation is concentrated on the subduction zone side of collision zones. (2) Precollision backarcs are hot enough to generate a ductile detachment in the lower crust, facilitating subsequent thrusting of the incoming stable continent under the upper‐middle crust of the backarc. (3) In hot backarcs there is a precollision low‐viscosity channel in the lower crust that facilitates lateral flow from beneath collision‐generated thickened crust. In continental collision orogenic belts, the high temperatures indicated by ductile deformation, metamorphism, and igneous activity are usually ascribed to heating by the orogenic process. I argue that the primary heat source is the uniformly hot backarc that predates collision. Most continental backarcs have temperatures of 800–850 °C at a 35‐km Moho, compared to ~450 °C for normal stable crust, and lithosphere thicknesses of 60–70 km compared to about 200 km. High temperatures that weaken the lithosphere may not be a consequence of orogeny but rather a precollisional requirement for most orogenic deformation and crustal thickening.

Element Transfer and Redox Conditions in Continental Subduction Zones: New Insights from Peridotites of the Ulten Zone, North Italy

The orogenic peridotites and pyroxenites of the Ulten Zone (north Italy) record multistage metasomatism by crust-derived melts and fluids within a Late-Variscan mantle wedge. We acquired new major and trace element data as well as garnet and whole-rock iron speciation for a representative suite of samples, with the aim to further constrain element cycling and the redox state attending the development of the major mineralogical and textural rock types that occur within the Ulten Zone. Initially, spinel peridotites were refertilized by mafic melts in the hot and shallow mantle wedge, followed by garnet formation as the peridotites were carried towards a cool, subducting slab of continental crust by corner flow. Upon exhumation, ingress of aqueous, crust-derived fluids provoked amphibole-forming reactions, which caused gradual consumption of garnet and clinopyroxene and transformation from coarse- to fine-grained assemblages. Since Si, Al, Na, Ti, Ca and HREE formerly stored in reactants were not fully accommodated in newly-formed phases, these elements were partially removed from the bulk-rock, generating more depleted compositions resembling residues after partial melting. Unexpectedly, the remaining garnet retains low Fe3+/Sigma Fe (<0046) even after the bulk-rocks became strongly enriched in Fe3+ during metasomatism and retrogression (Fe3+/Sigma Fe = 011-023), which was mostly stored in coexistent amphibole and interstitial serpentine. Low Fe3+/Sigma Fe in garnet is consistent with Delta logfO(2) = FMQ-1.7 to FMQ-0.3 at 2GPa. Combined with previous studies, this is evidence for garnet growth within a heterogeneously oxidized mantle wedge, reflecting a variable extent of percolation by oxidizing aqueous fluids. During metasomatism, concomitant variable enrichment in LILE, LREE and some HFSE, and significant compositional differences between sampling localities, reflect both variable fluid/rock ratios at small spatial scales but also indicate chromatographic effects that likely relate to different positions relative to the subducting crust releasing fluids into the mantle wedge. Hydration by dilute fluids during retrogression did not result in additional enrichment in fluid-mobile elements, but caused further replacement of garnet and clinopyroxene. This study highlights the control that changing mineralogies, developed in response to interaction with various crustal melts and fluids under variable pressure-temperature and redox conditions in a continental subduction zone, exert on the retention or release of major and trace elements in peridotite. In particular, formation and/or persistence of amphibole and dolomite, as documented in the present study, suggest that the subduction-modified mantle wedge is an efficient trap for volatiles and fluid-mobile elements.

Magma origins and geodynamic implications for the Makran-Chagai arc from geochronology and geochemistry of Bazman volcano, southeastern Iran

Bazman is an isolated voluminous (3497 m elevation; >300 km2) composite volcano that is located within the western segment of the Makran-Chagai arc. This arc obliquely tracks the modern EW-trending trench offshore the Oman sea coast of Iran and Pakistan, with several Quaternary stratovolcanoes (Bazman and Taftan in SE Iran, and Kuh-i-Sultan in western Pakistan) superimposed on a preexisting Neogene arc. High spatial resolution and high sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry (SIMS) yielded four clusters of zircon ages for Bazman volcanic rocks which range from ca. 8.6 Ma to 41 ka. Lava and pyroclastic materials erupted between 8.6 and 5.9 Ma forming a thick Neogene volcanic succession (Ngv). These deposits were intruded by hypabyssal andesite-dacite domes at ca. 4.1 Ma. The next younger group of the Quaternary Bazman center comprises several eruptive pulses at 1.37 Ma, 1.13 Ma, 0.943 Ma, 0.853 Ma and 0.635 Ma, which constructed the main edifice around a central vent through alternating eruptions of lava and pyroclastic rocks (Qtv and Qa2). Subsequently, ignimbrite flows also sourced from the main vent erupted at 0.59 and 0.47 Ma, and covered the earlier erupted deposits. The youngest dated eruptive phase (Qa3) comprises the emplacement of dacitic to andesitic lava around the top of volcano. Based on Uranium-series (238U-230Th) disequilibrium in Qa3 zircons, model ages of ca. 41 ka define a maximum eruption age of these lavas, which tapped a long-lived magma reservoir with zircon crystallization ages predating eruption by 10’s to 100’s of ka. Low- medium-K calc-alkaline basaltic-andesitic eruptions formed parasitic cones located on northeastern flanks of the edifice as the youngest unit (Qa4) of Bazman. Both Neogene and Quaternary rocks from Bazman volcano exhibit homogeneous trace element characteristics, including enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and heavy rare earth elements (HREE), respectively. High Th/Yb and negative anomalies of Ta, Nb and Ti are consistent with an enriched mantle source in a continental subduction zone. Trace element modeling for the most primitive basalts erupted at Bazman suggests an origin of parental magmas through ∼15% partial melting of an enriched spinell-lherzolite mantle, whereas the genesis of Bazman acidic volcanic rocks with SiO2 > 62% involves partial melting in the middle/lower continental crust which became subsequently overprinted through assimilation – fractional crystallization (AFC) processes. Compositional continuity of Quaternary Bazman volcanic rocks with their Neogene precursors implies a steady magma source above the subducted Oman slab. Consequently, subduction geometry in the Makran-Chagai arc has experienced little modification over the past few million years.