Depletion In HFSE Research Articles

Recycling of subducted carbonates: Formation of the Taohuala Mountain carbonatite, North China Craton

Carbonatitic magmatism plays a significant role in Earth's carbon cycle, which is also a lithoprobe of crust-mantle interaction, mantle metasomatism and partial melting. Due to different mineral assemblages and geochemical compositions, and diverse tectonic settings, the origin of carbonatite has long been debated. At subduction zones, sediments (including carbonates) are subducted into the mantle with the downgoing oceanic slab. However, the detailed mechanism of how subducted carbonates contribute to carbonatitic magmatism remains unclear. Here we present geochronological, geochemical and isotopic study on the Taohuala Mountain carbonatite at the southern margin of the Alxa Block, North China Craton. The classification of carbonatite from the Taohuala Mountain relies strongly on the observations of obvious intrusion contact relationships and flow structures in field outcrop. The Taohuala Mountain carbonatite has SiO2 ranging from 2.37wt.% to 11.45wt%, high CaO (45.93–53.86wt%) and low MgO (0.51–4.39wt%), and is characterized by enrichment of LILE (Ba, Sr), depletion of HFSE (Nb, Ta, Zr, Hf), and slightly negative Ce and Eu anomalies. Carbonates in the samples have high 87Sr/86Sr (0.70686–0.70694) and low 143Nd/144Nd (0.511635–0.511924). Remarkably, the highly fractionated δ18OVSMOW (11.83–25.92‰) indicates components of both sedimentary and mantle origin. Detailed zircon in situ U-Pb dating and oxygen isotope analysis exhibit contrast ages and δ18OVSMOW from core to rim, i.e., old ages (mainly>800Ma), high Th/U (mainly>0.5) and low δ18OVSMOW (6.37–11.44‰) in cores (inherited), whereas young ages (~400Ma), low Th/U (mainly<0.01) and high δ18OVSMOW (20.04–24.54‰) in rims, suggesting that the Taohuala Mountain carbonatite may have been generated from melting of subducted sedimentary carbonates. Considering all these evidences, and that the collision along Qilian Mountains was older than the carbonatite, we propose that a large volume of sedimentary carbonates subducted and remained in the lithospheric mantle under the Alxa block during the closure of the Paleo-Qilian Ocean. Subsequently, the carbonatite was formed by melting of carbonates with minor contributions from the mantle during the breakoff or rollback of the Paleo-Asian oceanic slab.

Petrogenesis of the Sanshilipu gabbro complex in the Shangdan Suture: New constraints on evolution of the Qinling Orogen

The Sanshilipu gabbro complex is located along the Shangdan Suture, which is important to understanding the evolution of the Qinling Orogen, Central China. The complex is mainly composed of norite, gabbro, and gabbro–diorite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon dating yields a weighted mean 206Pb/238U age of 414 ± 2.8 Ma, interpreted to represent the crystallization age of the gabbro complex. The complex has SiO2 contents ranging from 48.6 to 51.3 wt% with enrichment of Na relative to K, showing calc‐alkaline feature. The studied samples are characterized by an enrichment of large‐ion lithophile elements (Ba, Sr, and Pb) and depletion of HFSE (Nb, Ta, Zr, Hf, and Ti) and a large range of Nb/Ta ratios (3.7 to 11.8). They also have early flattened‐LREE distribution patterns. These geochemical characteristics indicate that the Sanshilipu gabbro complex was formed in an island‐arc environment. They have slightly enriched Sr–Nd isotope compositions with (87Sr/86Sr)i = 0.7047–0.7063 and ɛNd(t) from −0.44 to −4.63, indicating that the formation of the complex was involved in some subducted sediments and oceanic crust and fluid released from the subducted slab. The identification of island‐arc‐originated mafic rocks suggests that the onset time of the collision between the Yangtze Craton and the North China Craton should be younger than 415 Ma.

Petrogenesis of the Early Palaeozoic granitoids from the Yunkai massif, South China block: implications for a tectonic transition from compression to extension during the Caledonian orogenic event

AbstractA comprehensive geochronological and geochemical study was carried out on the gneissic monzogranites, porphyritic granodiorites and charnockites in the Gaozhou complex of the Yunkai massif in the southern part of the South China block to better understand the Early Palaeozoic tectonic regime of the South China block. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicates an age of 453.2 ± 5.1 Ma to the formation of the gneissic monzogranites, whereas the porphyritic granodiorites and charnockites were generated at 437.0 ± 1.5 Ma and 435.2 ± 2.2 Ma, respectively. The gneissic monzogranites show geochemical features consistent with the high-K, calc-alkaline rock series and are strongly peraluminous. They have SiO2contents ranging from 67.75 to 69.65 wt. % and relatively low CaO contents (1.66–1.94 wt. %). Their REE patterns are fractionated with enriched LREEs and negative Eu anomalies. The samples also show enrichment in LILEs (e.g. Rb and K) and Pb, and depletion in Sr, Ba and HFSEs (e.g. Nb, Ta, Ti and P). They haveεNd(t) values of −8.2 to −7.7. Conversely, the porphyritic granodiorites and charnockites are characterized as medium-K, calc-alkaline rock series and weakly to strongly peraluminous. They exhibit pronounced depletions in HFSEs and positive Pb anomalies. Compared to the earlier gneissic monzogranites, these rocks have relatively lower SiO2(65.50–69.36 wt. %), but higher CaO contents (3.34–4.05 wt. %), and have slightly lowerεNd(t) values (−9.1 to −8.4). Petrography and geochemical compositions of the gneissic monzogranites indicate that they are S-type granite and likely formed by partial melting of Neoproterozoic to Early Palaeozoic immature metagreywackes; whereas The porphyritic granodiorites and charnockites are A-type granite and likely derived from low degrees of partial melting of the dry, granulitic residue depleted by prior extraction of granitic melt. The new data for the Caledonian granitoids in the Yunkai massif suggest that they were formed in a post-collisional tectonic setting. They represent the earliest post-collisional alkaline magmatism reported so far in the Yunkai massif, and thus indicate a tectonic regime switch, from compression to extension, as early as the Late Ordovician to Early Silurian (~450–435 Ma).

P.1.l.030 - Sexual dimorphic effect of the invalidation of GHS-R on GH ultradian secretion and meal pattern

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.

U-Pb geochronology of Martín García, Sola, and Dos Hermanas Islands (Argentina and Uruguay): Unveiling Rhyacian, Statherian, Ectasian, and Stenian of a forgotten area of the Río de la Plata Craton

The Río de la Plata Craton is one of the three major cratons of South America. The craton is largely covered by sedimentary basins where its most exposed area is Buenos Aires-Piedra Alta Province (Chernicoff et al., 2014). This province includes the Martín García Island in the confluence of Uruguay River and the Río de la Plata estuary. Despite to be a reference area for the craton the Martín García Island lacks modern geological investigation. We present U-Pb SIMS (secondary ion mass spectrometer) geochronological data on zircon and titanite, as well as Hf isotope determinations on zircon, from rocks of Martín García Island (Argentina), Sola and Dos Hermanas Islands (Uruguay) and from Paso Severino Formation (Uruguay). We investigated: 1) Rhyacian intermediate-to acidic plutonic, arc-type rocks of the Florida Belt dated between 2090 Ma and 2115 Ma, derived from juvenile Neoarchean crust (TDMHf: 2.52 Ga; average εHf: +3.62); 2) Rhyacian metadacite (San José metamorphic belt) of 2127 Ma; 3) Statherian metagabbros of 1724–1734 Ma, with Transplatense inheritance; 4) Early Ectasian metagabbro of 1392 Ma, with Rhyacian inheritance; and 5) Stenian metagabbros of 1193 Ma (TDMHf: 2.00 Ga, εHf: 0.1). Most of the dated orthogneisses show Brasilian-age (from 778 to 550 Ma) Pb loss in the zircons, interpreted to be caused by shearing or uplifting during Neoproterozoic. The results show that the area is not exclusively Rhyacian in age but encompasses Statherian, Ectasian, and Stenian gabbros. The last two are interpreted as distal intrusions associated to the Sunsás Orogen. All post-Transplatense metagabbros have geochemical signature of island arc basalt derived from primitive mantle with enrichment of LILE and depletion of HFSE. These characteristics point to the recurrence of magma chambers intermittently active during the Rhyacian, Statherian, Ectasian, and Stenian, all with a similar source. The term “Transplatense” is used to replace “Trans-Amazonian” events that occurred within the craton.

Crust–mantle interaction during Early Jurassic subduction of Neo-Tethyan oceanic slab: Evidence from the Dongga gabbro–granite complex in the southern Lhasa subterrane, Tibet

Gabbro–granite complexes that form in active continental margins provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas and offer important insights into crust–mantle interaction. In this paper, we present detailed petrographic, mineralogical, whole-rock geochemical, Sr–Nd isotopic, zircon U–Pb, and Hf isotopic data for the Dongga gabbro–granite complex of the southern Lhasa subterrane, with the aim of elucidating the petrogenesis of these rocks and gaining insights into the processes of crust–mantle interaction associated with the northward subduction of the Neo-Tethyan oceanic slab. The Dongga complex consists predominantly of tonalite and granodiorite with abundant mafic microgranular enclaves (MMEs) and lesser amounts of hornblende gabbro. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 178.0±1.5Ma and 192.3±1.5Ma for the hornblende gabbros, and 178.1±1.7Ma and 178.8±1.8Ma for the tonalites and MMEs, respectively. Geochemically, the gabbroic rocks have low SiO2 and Na2O+K2O concentrations and high Mg# values. These gabbroic rocks are enriched in large ion lithophile elements (LILEs; e.g., Ba, K, Pb, and Sr) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P, Zr, and Hf). The tonalites are subalkaline and metaluminous and are also characterized by an enrichment of LILEs and LREEs and a depletion of HFSEs. The MMEs are silica poor and alkali rich and have broadly similar trace element signatures to those of the host tonalites. The gabbroic rocks, tonalites, and MMEs have similar initial 87Sr/86Sr ratios (ISr) of 0.7035–0.7036 and positive εNd(t) values of +5.3 to +6.8. However, zircon εHf(t) values from the hornblende gabbros (+14.0 to +18.2) are somewhat higher than those of the tonalites (+11.4 to +15.0) but are similar to the MMEs (+14.4 to +18.2). The combined petrological, geochemical, and isotopic features of the Dongga gabbro–granite complex suggest that the southern Lhasa subterrane was located in an active continental margin setting during the Early Jurassic. The gabbroic rocks were generated by the hydrous partial melting of a depleted mantle wedge metasomatized by aqueous fluids released from the subducted Neo-Tethyan oceanic slab. The tonalites and MMEs were produced by the mixing of mafic magmas related to the gabbroic rocks and felsic magmas derived from the remelting of the newly formed juvenile lower crust. The underplating of mantle-derived magmas and the resulting crustal melting and magma mixing occurred during the Early Jurassic subduction of the Neo-Tethyan oceanic slab. The mantle-derived magmas not only provided substantial heat to melt the lower crust but also mixed with the crust-derived magmas to form granitoids and MMEs.

Water in garnet pyroxenite from the Sulu orogen: Implications for crust-mantle interaction in continental subduction zone

Mineral water contents, together with the major and trace element compositions of minerals and whole-rock, were determined for garnet pyroxenites enclosed by ultrahigh-pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The garnet pyroxenites have low SiO2 contents of 40.25 to 46.68wt% and MgO contents of 10.99 to 14.79wt%. They are characterized by enrichment in LREE and LILE (Ba, Sr, Pb) but depletion in HFSE (Nb, Zr) and HREE. They 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 1213ppm for clinopyroxene, and 55 to 1476ppm 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 424–660ppm, 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. Garnet also has the high water contents, suggesting its importance in hosting water at mantle depths. 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 presence of ultramafic metasomatites in the mantle wedge would enhance its water storage and thus reduce the water transport into deeper mantle by subduction.

Geochemical constraints on the tectonic setting of the Sonakhan Greenstone Belt, Bastar Craton, Central India

The Neo-Archean Sonakhan Greenstone Belt (SGB) located in the north-eastern fringes of Bastar craton, Central India, is dominated by Basalts, Andesites, Dacites and Rhyolites association. Partial melting modeling on the SGB metabasalts indicates that these rocks were derived by 20% melting of spinel peridotite. Fractional crystallisation modeling with REE reveal that the most evolved samples represent the product of fractional crystallization of least evolved magma with 35% plagioclase, 35% clinopyroxene, 20% olivine, 5% magnetite and 5% ilmenite as fractionating minerals with 40% remaining magma. Depletion of HFSE with reference to the LILE and LREE/HFSE ratios and Nb, Zr anomalies in the multi-element diagram of the mafic rocks of SGB indicate Island arc magmatic setting. The enriched Th/Yb values further substantiate that the mantle arrays were modified by subduction-related fluids or melts. The general conclusions drawn indicate that the metabasalts from the SGB were formed as a result of subduction of an intraoceanic lithosphere in a fore-arc suprasubduction zone environment.

Petrochemistry and zircon U-Pb geochronology of granitic rocks in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand: Implications for petrogenesis and tectonic setting

Carboniferous biotite granite, Late Permian hornblende granite, and Triassic biotite-hornblende granite, all of which belong to the Eastern Granite Belt, expose in the Wang Nam Khiao area, Nakhon Ratchasima, northeastern Thailand. The Carboniferous biotite granite is dominated by quartz, K-feldspar, plagioclase, and biotite. The Late Permian hornblende granite contains dominant assemblages of plagioclase, quartz, K-feldspar, hornblende, and minor amount of biotite, while the Triassic biotite-hornblende granite consists of quartz, plagioclase, K-feldspar with small amounts of biotite, and hornblende. The REE patterns with steep decrease from light to heavy REE together with the LILE (e.g. K, Sr) enrichment and depletion of some particular HFSE (e.g. Nb, Ti) indicate low degree of partial melting. Mineral chemistry of biotite and hornblende in the granites reflects crystallization from hydrous calc-alkaline arc-derived magmas possibly formed by subduction. Amphibole-plagioclase thermometry and Al-in-hornblende barometry indicate that the Late Permian hornblende granite and the Triassic biotite-hornblende granite may have equilibrated at 3.0–5.8kbar/700–820°C and 2.0–3.2kbar/600–750 °C, respectively, in the middle-upper crust (about 10–15km depth). Zircon U-Pb geochronology of the Carboniferous biotite granite, Late Permian hornblende granite and Triassic biotite-hornblende granite yielded intrusion ages of 314.6–284.9Ma, 253.4Ma, and 237.8Ma, respectively, which implies multiple episodes of arc-magmatism formed by Palaeo-Tethys subduction beneath Indochina Terrane during Late Carboniferous/Early Permian, Late Permian and Middle Triassic.

Pamir Plateau formation and crustal thickening before the India-Asia collision inferred from dating and petrology of the 110–92 Ma Southern Pamir volcanic sequence

The formation of the Pamir is a key component of the India-Asia collision with major implications for lithospheric processes, plateau formation, land-sea configurations and associated climate changes. Although the formation of the Pamir is traditionally linked to Cenozoic processes associated with the India-Asia collision, the contribution of the Mesozoic tectonic evolution remains poorly understood. The Pamir was formed by the suturing of Gondwanan terranes to the south margin of Eurasia, however, the timing and tectonic mechanisms associated with this Mesozoic accretion remain poorly constrained. These processes are recorded by several igneous belts within these terranes, which are not well studied. Within the Southern Pamir, the Albian-Turonian volcanic rocks and comagmatic plutons of the Kyzylrabat Igneous Complex (KIC) provide an important and still unconstrained record of the Pamir evolution. Here we provide the age, origin and the geodynamic setting of the KIC volcanics by studying their petrology, zircon U-Pb geochronology, geochemistry and isotope composition. 17 samples from the KIC volcanics yield U-Pb ages spanning from 92 to 110Ma. The volcanics are intermediate to acidic in composition (SiO2=56–69wt%) and exhibit high-K calc-alkaline and shoshonitic affinity (K2O/Na2O=1–2.2wt%). They show enrichment in LILE and LREE and depletion in HFSE and HREE with negative Ta, Ti and Nb anomalies, suggesting an arc-related tectonic setting for their formation. Low εNd(t) values (from −9.1 to −4.7), relatively high 87Sr/86Sr(i) ratios (0.7069–0.7096) and broad range of zircon εHf values (from −22.6 to −1.5) suggest a mixture of different magma sources. These features suggest that volcanics were derived by crustal under- or intraplating of an enriched subduction-related mantle shoshonitic magmas, by heating and partial melting of the lower crust, and by mixing of both magma components. Our results further imply that the KIC volcanics represent a shoshonitic suite typical of an evolution from active continental arc to post-collisional setting with a steepening of the Benioff zone and thickening of the crust toward the back-arc. This setting is best explained by the subduction - collision transition along the Shyok suture due to accretion of the Kohistan island arc to the Karakoram. This suggests that a significant part of the crustal shortening and thickening accommodated in the Pamir occurred in the Mesozoic before the India-Asia collision with implications for regional tectonic models. This further suggests the Pamir was already a major topographic feature with potentially important paleoclimate forcing such as the monsoonal circulation.

Origin of the mafic microgranular enclaves (MMEs) and their host granitoids from the Tagong pluton in Songpan–Ganze terrane: An igneous response to the closure of the Paleo-Tethys ocean

The Songpan–Ganze terrane is mainly composed of a Triassic sedimentary sequence and late Triassic–Jurassic igneous rocks. A large number of plutons were emplaced as a result of tectono-magmatic activity related to the late stages of Paleo-Tethys ocean closure and ensuing collision. Granitoids and their hosted mafic enclaves can provide important constraints on the crust–mantle interaction and continental crustal growth. Mesozoic magmatism of Songpan–Ganze remains enigmatic with regard to their magma generation and geodynamic evolution. The Tagong pluton (209Ma), in the eastern part of the Songpan–Ganze terrane, consists mainly of monzogranite and granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (ca. 208–209Ma). The pluton comprises I-type granitoid that possesses intermediate to acidic compositions (SiO2=61.6–65.8wt.%), high potassium (K2O=3.2–4.1wt.%), and high Mg# (51–54). They are also characterized by arc-type enrichment of LREEs and LILEs, depletion of HFSEs (e.g. Nb, Ta, Ti) and moderate Eu depletions (Eu/Eu*=0.46–0.63). Their evolved zircon Hf and whole-rock Nd isotopic compositions indicate that their precursor magmas were likely generated by melting of old lower continental crust. Comparatively, the MMEs have lower SiO2 (53.4–58.2wt.%), higher Mg# (54–67) and show covariation of major and trace elements, coupled with field and petrographic observations, such as the disequilibrium textures of plagioclase and amphibole, indicating that the MMEs and host granitoids were originated from different magma sources but underwent mafic–felsic magma mixing process. Geochemical and isotopic data further suggest that the precursor magma of the MMEs was formed in the continental arc setting, mainly derived from an ancient metasomatized lithospheric mantle wedge.The Triassic granitoids from the Songpan–Ganze terrane show remarkable temporal–spatial-petrogenetic affinities to the counterparts of subduction zones in the Yidun and Kunlun arc terranes, plausibly support a double-sided subduction of the Paleo-Tethys ocean. The mixing mechanism for the formation of the Tagong pluton was likely associated with the break-off of a subducted slab of the Paleo-Tethys ocean, which triggered subsequent upwelling of hot asthenosphere beneath accreted arc fragments and induced lithospheric mantle-derived magmas suffice to underplate and mix with the lower crust-derived felsic magma. Collectively, the late Triassic igneous rocks record significant crustal growth and continental development as response to the final demise of the Paleo-Tethys ocean (ca. 210Ma), and marks the last episode of orogenic magmatism in the Songpan–Ganze terrane after which the region entered into post-orogenic phase of evolution.

Petrogenesis of the Late Cretaceous Tholeiitic Volcanism and Oceanic Island Arc Affinity of the Chagai Arc, Western Pakistan

AbstractThe Late Cretaceous Chagai arc outcrops in western Pakistan, southern Afghanistan and eastern Iran. It is in the Tethyan convergence zone, formed by northward subduction of the Arabian oceanic plate beneath the Afghan block. The oldest unit of the Chagai arc is the Late Cretaceous Sinjrani Volcanic Group. This is composed of porphyritic lava flows and volcaniclastic rocks, and subordinate shale, sandstone, limestone and chert. The flows are fractionated low‐K tholeiitic basalts, basalticandesites, and andesites. Relative enrichment in their LILE and depletion in HFSE, and negative Nb and Ta and positive K, Ba and Sr anomalies point to a subduction‐related origin. Compared to MORB, the least fractionated Chagai basalts have low Na2O, Fe2O3T, CaO, Ti, Zr, Y and 87Sr/86Sr. Rather than an Andean setting, these results suggest derivation from a highly depleted mantle in an intraoceanic arc formed by Late Cretaceous convergence in the Ceno‐Tethys. The segmented subduction zone formed between Gondwana and a collage of small continental blocks (Iran, Afghan, Karakoram, Lhasa and Burma) was accompanied by a chain of oceanic island arcs and suprasubduction ophiolites including Semail, Zagros, Chagai‐Raskoh, Kandahar, Muslim Bagh, Waziristan and Kohistan‐Ladakh, Nidar, Nagaland and Manipur. These complexes accreted to the southern margin of Eurasia in the Late Cretaceous.

Carboniferous-Permian tectonic transition envisaged in two magmatic episodes at the Kuruer Cu-Au deposit, Western Tianshan (NW China)

The Western Tianshan in NW China is one of the most important gold provinces in the Central Asian Orogenic Belt (CAOB). The recently discovered Kuruer Cu-Au deposit has been interpreted to represent a transition from high-sulfidation epithermal to porphyry mineralization system. In this study, we present new LA-ICP-MS zircon U-Pb ages for the many magmatic rock types at Kuruer, including the Dahalajunshan Formation andesitic tuff (333.2±1.6Ma), diorite porphyry (269.7±2.0Ma), slightly-altered (264.4±2.6Ma) and intensively-altered (270.5±2.5Ma) albite porphyry. These ages reveal two distinct magmatic episodes: The Early Carboniferous Dahalajunshan Formation (wall rocks) andesitic tuff samples contain narrow ranges of SiO2 (60.29–61.28wt.%), TiO2 (0.96–0.98wt.%), Al2O3 (16.55–16.57wt.%) and Fe2O3T (5.36–5.57wt.%). The tuff is characterized by LREE enrichment and HFSE depletion, as well as LREE/HREE enrichment ((La/Yb)N=8.31–8.76) and negative Eu anomalies (δEu=0.64–0.76). Zircon εHf (t) values are 5.4–8.2, and two-stage Hf model ages (TDM2) are 821–1016Ma, indicating partial melting of a moderately depleted mantle wedge with Precambrian continental crustal input. The ore-forming Middle Permian diorite porphyry and (quartz) albite porphyry have variable major oxide compositions (e.g., SiO2=53.09–53.12wt.% for the diorite porphyry, 70.84–78.03wt.% for the albite porphyry, and 74.07–75.03wt.% for the quartz albite porphyry) but similar chondrite-normalized REE and primitive mantle-normalized multi-element patterns. These porphyries display LREE enrichment and HFSE depletion, as well as elevated LREE/HREE enrichment and negative Eu anomalies. The positive zircon εHf(t) values (11.7–15.9 for the diorite porphyry, 8.9–14.9 for the albite porphyry) and young two-stage Hf model ages (TDM2) (282–542Ma for the diorite porphyry, 337–717Ma for the albite porphyry) indicate a major juvenile continental crustal involvement. We propose that the Carboniferous and Middle Permian magmatism was formed in a continental arc and post-collisional settings, respectively, with the latter episode responsible for the Cu-Au mineralization.

Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of the ∼635 Ma mafic intrusions in the northern margin of the Yangtze Block: A link of metasomatized subcontinental lithospheric mantle and Ni-Cu-(PGE) sulfide mineralization

A number of ∼635Ma mafic-ultramafic intrusions occur in the Nanyang region and Suizhou-Zaoyang (Suizao) region along the northern margin of the Yangtze Block. The intrusions in the Suizao region are mainly composed of troctolite, olivine gabbro and gabbronorite. The parental magmas of the intrusions are estimated based on the non-modal melting using two fresh samples with the lowest REE concentrations and the results show that the parental magmas may have La/Sm of ∼2.5, Sm/Yb of ∼2.5 and La/Yb of ∼6.2 and could be produced by 5–10% of partial melting of the mantle composed of spinel-garnet (50:50) lherzolite. The estimated compositions of the parental magmas also display the enrichment of light REE and weak depletion of HFSE (Th, U, Nb and Ta) on the primitive mantle-normalized trace element patterns and have Ba/Nb, Ba/La and Ba/Th distinctly higher than MORB and OIB, consistent with the derivation from metasomatized subcontinental lithospheric mantle (SCLM) rather than convecting mantle. The rocks of the intrusions have radiogenic 187Os/188Os(i) of 0.137–0.247, which are likely attributed to low degrees of crustal contamination and no involvement of convecting mantle-derived magmas. They consistently have εNd(t) values of −5.4 to −10.4, much lower than the value of +2.5 for the mantle inferred from Neoproterozoic evolutionary trend and could be interpreted as the mixing of the parental magmas with 7–10% of Archean basement of the Yangtze Block. The rocks of the intrusions commonly have <0.1wt% S and 0.08–17.5ppb PGE (Os, Ir, Ru, Rh, Pt and Pd). They have Cu/Pd (7900–6,500,000) much higher than the value of the primitive mantle (∼7000), which is likely resulted from the retention of sulfide in the mantle source due to the low degrees of partial melting of the mantle. We conclude that the low PGE concentrations of the rocks can be attributed to the sulfide retention in metasomatized SCLM source and no substantial interaction of metasomatized SCLM with the melts from convecting mantle. The ∼635Ma mafic intrusions in the Suizao region may thus have little potential for economically important Ni-Cu-(PGE) sulfide mineralization.

Petrological and geochemical features of the early Paleozoic granitic gneisses and iron ores in the Tianhu iron deposit, Eastern Tianshan, NW China: Implications for ore genesis

This paper reports whole-rock geochemical, zircon U–Pb and Hf isotopic data for ore-hosted granitic gneisses, mineral compositions of oxides, and sulfur isotopic data for sulfides in iron ores from the Tianhu deposit, central part of the Eastern Tianshan. Our results can provide crucial constraints on the genesis of granitic gneisses and early Paleozoic tectonic setting of the Eastern Tianshan. LA-ICP-MS U–Pb dating on magmatic zircons yielded weighted mean 206Pb/238U ages of 463 to 438Ma, interpreted as the crystallization ages of the granitic protoliths and the formation ages of the Tianhu Group. Zircon U–Pb age of ore-hosted granitic gneiss (ca. 459Ma) can provide reliable constrains on upper limit for iron mineralization age in the Tianhu deposit. Geochemical characteristics suggest that the protoliths of the Tianhu granitic gneisses are metaluminous to weakly peraluminous high-K calc-alkaline granitic rocks, exhibiting typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Zircon Hf isotopic compositions show a positive trend from ~463 to ~438Ma, indicating that ~460Ma magmas came from both ancient and juvenile sources, whereas ~438Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425Ma. The formation of these early Paleozoic granitoids was in response to subduction processes, suggesting that subduction of Junggar Ocean probably began in the Early Ordovician and lasted until Late Silurian.Pyrite and pyrrhotite in iron ores have δ34SCDT values from +4.6 to +15.7‰, which are consistent with the marine source, but inconsistent with the magmatic source or those involved evaporites in skarn iron deposit. Geological, geochemical, and isotopic data suggest that the Tianhu iron ores were formed by volcano-sedimentary processes in a subduction environment during the early Paleozoic time, and Tianhu is a metamorphosed volcano-sedimentary iron deposit, rather than a skarn iron deposit as previously proposed.

Diopside, apatite, and rutile in an ultrahigh pressure impure marble from the Dabie Shan, eastern China: A record of eclogite-facies metasomatism during exhumation

Petrography and mineral chemistry of an ultrahigh pressure (UHP) impure marble from the Dabie Shan, eastern China, was studied using cathodoluminescence color imagery, X-ray mapping, electron microprobe analytics, laser ablation inductively coupled plasma mass spectrometry and phase equilibrium calculations. Diopside in the marble shows two generations of fluid-altered zones, which contain more Na, Al, Fe, Ti, Sr, Y, REE, and transition metal elements (TMEs: Sc, V, Co, Ni, Zn) and less Mg, Ca, and K compared to the original diopside. Apatite contains oriented monazite inclusions and high Cl contents (1.2–2.5wt%) with negative Eu anomalies (EuN/EuN⁎=2EuN/(SmN+GdN)=0.61–0.68). Rutile in the matrix displays fluid-mediated microstructures and becomes depleted in Zr and Fe compared to rutile inclusions in dolomite and diopside. All these microstructural and geochemical features are demonstrated to be caused by eclogite-facies metasomatism during exhumation of the marble. Two Na-Al-Fe-rich aqueous fluids have been involved in the metasomatism in tandem, as indicated by the two fluid-altered diopside zones and their major element compositional differences compared to the original diopside. Based on available partition coefficients of trace elements between clinopyroxene and aqueous fluid, both metasomatic fluids were calculated to display enrichments in LILE (Cs, Rb, B, Pb, Sr, Li) and LREE and depletions in HFSE (Th, U, Nb, Ta, Zr, Hf), HREE, and TME (V, Cr, Ni). A recently-determined Cl-OH exchange coefficient for apatite-aqueous fluid was applied to the high-Cl apatite and indicates a high chlorinity (≥30wt%) for one (or both) of the metasomatic fluids. Either the decompression breakdown of peak hydrous minerals (such as lawsonite) in coexisting eclogite and/or paragneiss or the dehydration of the subducted oceanic crust before the collision between the Yangtze and North China blocks may have given rise to such metasomatic fluids. This study implies that pervasive external fluid-rock interaction during exhumation, before the amphibolite-facies overprint, is much more common than previously thought in the Dabie-Sulu UHP rocks. Moreover, important constraints are provided on the fluid nature and behavior in subduction zones.

Paleoproterozoic evolution of the arc–back-arc system in the east Sarmatian Orogen (East European Craton): Zircon SHRIMP geochronology and geochemistry of the Losevo volcanic suite

The East European Craton has a thick sequence of volcano-sedimentary rocks preserved in the Losevo belt that developed along the junction between Sarmatian and Volgo-Uralian microcontinents. The major lithologies of the Losevo terrain (LT) are a dominant bimodal volcanic suite and a basalt–andesite–dacite–rhyolite assemblages (BADR). The LT rocks have been divided from lower to upper sequences into the Terrigene, Strelitsa and Podgornoye Formations, but the stratigraphic subdivisions have not been geochronologically tested. Here we present geochemistry and SHRIMP zircon geochronology of volcanic rocks from the LT. The volcanic suite from the Terrigene Formation has tholeiitic and calc-alkaline affinites, significant enrichment in LILE and LREE and strong depletion in HFSE with eNd(t)=+ 2.6, whereas the felsic dikes display an A-type affinity, with typical enrichment in Zr, Nb, Y, and depletion in Sr and Ti, fractionated REE patterns, and strong negative Eu anomalies with eNd(t) in the range of -0.5 to 2.6. The bimodal volcanic suite of the Strelitsa Formation is composed of tholeiites displaying minor depletion in LREE, slight enrichment of LILE, no or weak depletion of Nb resembling transition MORB with eNd(t)=+3.0 to +3.6) and rhyolites with high LREE/HREE, high Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (eNd(t)=+1.8 to +2.9) resembling slab-derived high pressure adakitic melts. The volcanic rocks of the Podgornoye Formation are bimodal with tholeiitic chemistry, lack enrichment in LILE and LREE and have a slight depletion in HFSE (eNd(t)=+3.7) together with rhyolites having high LREE/HREE, moderate Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (eNd(t)=+2.1 to +2.6) resembling slab derived relatively low-pressure adakite-like melts. The BADR assemblage has significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. Geochronological data indicate that the early LT volcanic rocks were formed during the early (Terrigene Formation) stage of intra-continental arc with a continental basement whereas the Strelitsa bimodal volcanic rocks were formed during a middle stage of back-arc extension and the Podgornoye bimodal volcanic rocks and BADR were formed during a later stage intra-oceanic arc. The identification of a 2170 to 2120 Ma back-arc basin in the East Sarmatian Orogen together with broadly coeval arcs indicate that the eastern margin of the Sarmatia was active with an arc–back-arc environment. Our new data suggest that the initial melts of the bimodal suite were adakitic derived by slab melting, followed by mantle metasomatism, whereas the basaltic magmas formed in an island arc setting. The LT and similar-aged volcanic belts in other terrains are considered to represent the initial (2.1–2.0 Ga), subduction-related growth of the Paleoproterozoic Columbia supercontinent.

Delamination of lithospheric mantle evidenced by Cenozoic potassic rocks in Yunnan, SW China: A contribution to uplift of the Eastern Tibetan Plateau

New zircon U–Pb ages, mineral chemical data, whole-rock geochemistry and Sr–Nd isotopes from the potassium-rich intrusions in the Yunnan area, SW China, were determined to provide constraints on the uplift of the Eastern Tibetan Plateau. The intrusive rocks consist of shoshonitic syenites (high-Mg syenites, low-Mg syenites and syenite porphyries) and potassic granitoids (granite porphyries). Zircon LA-ICP-MS U–Pb dating indicates coeval emplacement ages of ~35Ma. The shoshonitic syenites have alkaline affinities and the enrichment in LILEs and LREEs (e.g. La, Sr, U, Pb), with depletion of HFSEs (e.g. Nb, Ti, Ta) and weak Eu anomalies. They display uniform Sr–Nd–Lu–Hf isotopic compositions with similar initial 87Sr/86Sr ratios (0.7073–0.7079), enriched εNd(t) values (−6.8 to −4.3) and mostly negative zircon εHf(t) values ranging from −4.6 to +0.1. The high-Mg syenites have high MgO, Fe2O3T, TiO2, CaO, Cr, Ni concentrations and relatively high Mg# (60–68), indicating an origin from enriched lithospheric mantle. The low-Mg syenites and syenite porphyries are geochemically distinct with the high-Mg syenites, but the insignificant variations in major elements, linear trends of La against (La/Yb)N and similar Sr–Nd isotopic compositions to the high-Mg syenites suggest that they were produced by different degrees of partial melting of the same enriched mantle source. The potassic granitic intrusions are sub-alkaline with a strongly peraluminous character. They display an S-type granite affinity, with high Al2O3/TiO2 and low CaO/Na2O and K2O/Al2O3 ratios, suggesting a pelitic source. They are LREE-enriched and have relatively flat HREE patterns with weakly negative Eu anomalies and positive Rb, U, and Pb anomalies and negative Nb, Ta, and Ti anomalies. They have relatively high initial 87Sr/86Sr ratios (0.7143) and enriched Nd isotopic compositions [εNd(t)=−4.1]. Their zircon εHf(t) values (−4.0 to +0.09) and old two-stage Hf model ages (TDMc=1.16–1.36Ga) suggest that they were generated by partial melting of an ancient K-rich crust. The petrogenesis of the potassic intrusive rocks with their uniform Nd model ages (TDM=968–1168Ma) indicates that the lithospheric mantle beneath Yunnan was enriched by metasomatism related to ancient oceanic subduction. Considering the widespread and coeval nature of potassium-rich rocks in Eastern Tibet, we consider that delamination of metasomatized lithospheric mantle beneath Yunnan in the Eocene was a result of the India–Asia collision, which triggered partial melting of both the lithospheric mantle and the K-rich crust, and contributes to uplift of the Eastern Tibetan Plateau.

In situ elemental and isotopic study of diorite intrusions: Implication for Jurassic arc magmatism and porphyry Cu-Au mineralisation in southern Tibet

The Gangdese porphyry Cu-Au belt (South Tibet, China) in the eastern Tethyan metallogenic belt hosts many deposits, which formed in subduction and collisional settings. Several porphyry Cu-Au deposits have been discovered in the Xietongmen region, including a large porphyry Cu-Au deposit at Xiongcun. However, the formation mechanism for the Xietongmen Cu-Au deposits and their igneous host rocks remain enigmatic. This study reports whole-rock major- and trace-element compositions, Sr-Nd isotopes, in situ zircon Hf- and plagioclase Sr-isotope compositions, and major- and trace-element compositions of amphiboles to constrain the magmatic emplacement and evolution related to the porphyry Cu-Au mineralisation in the Xietongmen region. New zircon U-Pb ages show that the Xietongmen diorites formed at 185Ma, consistent with the age of Cu-Au mineralisation. The Xietongmen diorites are characterised by enrichment in LILEs and LREE, and depletion in HFSEs. These features suggest that the diorites were produced by arc magmatism in an oceanic-slab subduction environment. In situ isotopic compositions exhibit radiogenic εHf(t) (+9.8 to +14.4) and low initial 87Sr/86Sr values (0.7039–0.7043); whole-rock results show positive εNd(t) (+5.26 to +5.31) and unradiogenic initial 87Sr/86Sr values (0.7042–0.7043). These isotopic results are similar to coeval mantle-derived hornblende gabbros and the Sangri volcanic rocks in the Gangdese belt. Amphibole clots in the Xietongmen diorites have higher MgO and Ni contents, with AlT>1.0, Mg#>0.5, Ni>143ppm, and REE patterns different from those of magmatic and idiomorphic amphiboles in the host diorites, suggesting that the clots are xenocrysts. All the petrological and geochemical evidence suggests that the Xietongmen mineralised diorites were derived from a Jurassic hydrous mantle wedge induced by slab dehydration. These magmas contain higher contents of water and other volatiles and have higher oxidation states. These conditions favored the transfer of metals (such as Cu, Au) to the upper crust, and then formed the Jurassic porphyry Cu-Au deposits.

Palaeoproterozoic granitic magmatism in the northern segment of the Jiao-Liao-Ji Belt: implications for orogenesis along the Eastern Block of the North China Craton

ABSTRACTAs the northern segment of the Jiao-Liao-Ji Belt (JLJB), the Palaeoproterozoic Liaoji Belt is a key region for deciphering the formation and evolution of the North China Craton (NCC). In this study, we present the geochronology, geochemical, and isotopic studies on the monzogranitic gneiss, which is one of the major lithotectonic elements of the Liaoji Belt. LA-ICP-MS zircon U–Pb dating reveals that the studied monzogranitic gneisses were formed in the period of 2213–2178 Ma. They are in tectonic contact with the Palaeoproterozoic volcano-sedimentary rocks in the field. The monzogranitic gneisses belong to the high-K calc-alkaline series, and are metaluminous to peraluminous. They have 10,000 Ga/Al ratios of 2.63–3.14 with an average of 2.90, and are thus classified as aluminous A-type granites. Their εNd(t) values vary from −3.4 to +2.5, indicating heterogeneous source region. The monzogranitic gneisses are characterized by enrichment in LREE and LILE (e.g. Rb, Ba, Th, and K) and depletion in HREE and HFSE (such as Nb, Ta, and Ti), and are typical to magmatism in active continental margins formed in a subduction-related tectonic setting. Taking into account their A-type affinity and regional geological data, we suggest that the monzogranitic gneisses were most probably generated in a local extensional back-arc environment during subduction.