Adakitic Geochemical Signatures Research Articles

The role of intra-oceanic arc in the growth of continental crust: Evidence from the Early Paleozoic Bainaimiao arc

The Early Palaeozoic arc-related formations in the northern margin of the North China Craton (NCC) belonging to the Bainaimiao arc belt is an important part of the eastern Central Asian Orogenic Belt (CAOB). The tectonic evolution of this belt remains controversial. Here we present detailed data on the geochronology and geochemistry on a newly identified Early Palaeozoic igneous suite from the Faku area in northern Liaoning. Zircon U–Pb dating of the Faku volcanic and intrusive rocks yielded weighted mean 206Pb/238U ages of ca. 458 Ma and 433–405 Ma, respectively, which represent their crystallization (eruption) times. Zircon Hf isotopic analyses show a low negative to positive εHf (t) values of − 0.33 to + 12.4, implying depleted characteristics of the source. The geochemical features of Laolingshan basaltic andesites display higher TiO2 and lower P2O5 contents, similar to that of mid-ocean ridge basalts, and relatively high Zr/Y and low Ta/Yb ratios, suggesting oceanic island arc basalt affinity. Therefore, we suggest that the Laolingshan basaltic andesites were formed in an intra-oceanic environment. The Bachagou granodiorites, with adakitic geochemical signatures, belong to the calc-alkaline and high-K calc-alkaline series, implying formation in an active continental margin tectonic setting. The Baerhushan granodiorites, which geochemically belong to S-type granites, were probably formed in a syn-collisional environment influenced by arc–continent collision. The Donggou granitoids, which have higher SiO2 and alkali and lower MgO and P2O5, show A-type granite features. Geochemical characteristics of the Donggou A-type granite suggest formation in an extensional setting caused by orogenic intermittent extension. The Baerhushan gabbro diorites, which display properties of both mid-ocean ridge and continental rift basalts, were produced under post-orogenic extensional tectonic setting after the arc–continent collision. Our study traces the Early Palaeozoic tectonic evolution of the south-eastern segment of the CAOB and the related accretionary tectonics.

Middle Devonian Evandale porphyry Cu-Mo (Au) deposit, southwestern New Brunswick, Canada: Analysis of petrogenesis to potential as a source for distal intrusion-related epithermal gold mineralization

The 391.2 ± 3.2 Ma (U-Pb on zircon) Evandale Granodiorite of southwestern New Brunswick is quite a unique intrusive rock in the southern part of the northeastern Appalachians. This research examines the relationship between the porphyry Cu-Mo-(Au) and epithermal systems with Evandale Granodiorite and environs. This Middle Devonian multi-phase I-type granite intrudes along the northern boundary of the deformed Silurian sedimentary and mafic volcanic rocks of the Mascarene Basin. This granitoid is identified as an oxidized I-type system with adakitic geochemical signatures and associated porphyry Cu-Mo-(Au) mineralization, possibly linked to slab failure sourced from decompression melting of the lower crust caused by post-collisional uplift, similar to the Magaguadavic Granite to the southwest. The Evandale adakitic intrusion is divided into two phases that consist of (1) a calc-alkalic coarser phase ranging from medium- to coarse-grained, seriate to porphyritic granodiorite to monzogranite with metaluminous to peraluminous features, and (2) the later stage alkali-calcic fine-grained (to aplitic) locally porphyritic phase ranging from a monzogranite to syeno-granite with peraluminous characteristics. The primary mineralogy of the intrusive rocks consists of K-feldspar + albite + plagioclase + quartz + biotite + hornblende + apatite ± muscovite ± zircon, along with several alteration phases, such as biotite, chlorite, sericite, and calcite. Geochemical analyses (using INAA, ICP-MS, and XRF) indicate that both phases of the Evandale granodiorite show a high concentration of Cu (108 ppm) and Au (33 ppb) associated with the presence of pyrite, chalcopyrite, and arsenopyrite in the fine-grained dyke samples. Up to 6 ppm Mo was contained within the granitic phase, whereas lesser amounts of Mo were confined to the fine grained to aplite phases. Microprobe analyses of primary biotite as both phenocrysts and groundmass shows 0.55 avg. wt.% Cl and 0.21 avg. wt.% F, which are relatively concentrated compared to other high-grade porphyry deposits. Hydrothermally altered rocks, veins, and sulfides associated with the porphyritic suite result from buoyant magmatic-hydrothermal fluids from a deeper intrusion exsolving fluids (quenched porphyry related), depressurizing, and reacting with the host coarser grained granodiorite and even some of the dykes locally. Based on the geothermometry and geobarometry of amphibole (actinolite-magnesiohornblende), the crystallization temperature and pressure of the Evandale coarse-grained granite and aplite are 674–755 °C, 0.2–0.7 kbar (20–70 MPa) and 838 °C, 0.42–1.4 kbar (42–140 MPa), respectively. This is consistent with biotite formation at temperatures ranging from 670° to 760 °C, based on Ti-in-biotite thermometry. Also, zircon saturation temperatures of the earlier coarse-grained porphyritic granites and later fine-grained porphyry to aplitic dykes are 736 °C and 787 °C, respectively. Spatially and temporally associated gold deposits beside Evandale and within this region of southwestern New Brunswick suggests that these oxidized I-type intrusions are a potential source for distal, low sulfidation epithermal gold deposits in the region.

Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

AbstractThe origins of Cenozoic granites in the Himalaya are key to understanding the evolution of the Himalayan orogen. However, it is unclear whether these granites represent primary melts, and the nature of their magma source is controversial. Here, we present a systematic element and Sr–Nd–B–Mo isotope study of Cenozoic granites from the Yardoi area in the eastern Tethyan Himalaya, China. These granites can be divided into two groups: mid‐Eocene porphyritic two‐mica granites with low SiO2 contents (65.9−69.6 wt.%) and adakitic geochemical signatures, and mid‐Eocene to Miocene equigranular granites with high SiO2 contents (71.6−75.5 wt.%). The high‐SiO2 granites (HSG) have similar Sr−Nd isotope compositions to the low‐SiO2 granites (LSG), but they have distinct δ11B values of −19.4‰ to −11.4‰ and −10.6‰ to −6.89‰. This indicates that the two groups have different sources, with the LSG derived by partial melting dominantly of metamafic rocks at thickened lower crustal conditions, and the HSG generated by partial melting of the mid‐crust metasedimentary rocks with less enriched Nd isotope compositions. The δ98/95Mo of the LSG and HSG are highly variable with values of −0.68‰ to 0.12‰ and −1.13‰ to 0.46‰, respectively. δ11B values of the HSG correlate positively with δ98/95Mo and Sr/Y values and correlate negatively with K2O, Rb, Zr, and Rb/Sr, reflecting the addition of external metamorphic fluids during anatexis of the metapelites. The B–Mo isotope data robustly suggest source‐controlled compositional diversity of the Himalayan granites, which could provide clues to the physical and geochemical responses during the evolution of a large orogen.

Zircon Chemistry and Oxidation State of Magmas for the Duobaoshan-Tongshan Ore-Bearing Intrusions in the Northeastern Central Asian Orogenic Belt, NE China

The Duobaoshan (DBS)-Tongshan (TS) porphyry Cu–(Mo) deposit (4.4 Mt Cu, 0.15 Mt Mo) is located in the northeastern part of the central Asian orogenic belt (CAOB) in northeastern China. It is hosted by early Ordovician dioritic to granodioritic intrusions which are characterized by the subduction-related geochemical signatures including high concentrations of large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and low concentrations of heavy REEs (HREEs) and high-field -strength elements (HFSEs), such as Nb, Ta, Zr and Ti in bulk rock compositions. Furthermore, they show adakitic geochemical signatures of high Sr/Y ratios (29~55) due to high Sr (290~750 ppm) and low Y (<18 ppm). Zircon trace element abundances and published Sr-Nd-Hf isotope data of these rocks suggest that the parental magmas for these ore-bearing intrusions were rich in H2O and formed by partial melting of a juvenile lower crust/lithospheric mantle or metasomatized mantle wedge during the northwestward subduction of the Paleo-Asian Ocean before the collision of the Songnen block with the Erguna-Xing’an amalgamated block in the early Carboniferous. Values of Ce4+/Ce3+ and Ce/Nd in zircons are 307~461 and 14.1~20.3 for mineralized granodiorites, and 231~350 and 12.4~18.2 for variably altered diorite and granodiorites in DBS, whereas those for DBS-TS microgabbros are 174~357 and 7.4~22, and 45.9~62.6 and 5.0~5.8 for the early Mosozoic Qz-monzonites, respectively. Zircon Eu/Eu* values are high and similar among mineralized granodiorites (~0.6), altered diorite and granodiorites (~0.6) and the Mesozoic Qz-monzonites (~0.8), whereas the values are low and variable for the DBS-TS microgabbros (0.3~0.6). The magma oxidation state calculated from zircon chemistry and whole rock compositions are FMQ +1.0 to +1.5 in mineralized samples, and FMQ +2.4 to +4.2 in altered samples. The values are comparable to those for the fertile intrusions hosting porphyry Cu-Mo-(Au) deposits in the central and western CAOB and elsewhere in the world. Elevated oxidation state is also observed in the TS microgabbros, FMQ +1.4 to +1.9, and the early Mesozoic Qz-monzonites, FMQ +2.4 to +2.5. Comparison of zircon geochemistry data from porphyry deposists elsewhere suggests that positive Ce anomalies are generally associated with fertile intrusions, but not all igneous rocks with high Ce anomalies are Cu fertile. The findings in this study are useful in exploration work and evaluating oxidation state of magmas for porphyry Cu-(Mo) deposits in the region and elsewhere.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt <1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.

Timing of formation and tectonic setting of Paleozoic granitoids in the eastern Mongol–Okhotsk Belt: Constraints from geochemical, U[sbnd]Pb, and Hf isotope data

The Mongol–Okhotsk Belt is one of the major structural elements in East Asia. However, many aspects of the evolution of this structure from the oceanic domain to the suture zone remain unresolved. The most controversial of these aspects are the timing and polarity of subduction processes as well as the timing of the final closure of the Mongol–Okhotsk Ocean. Here, we present whole-rock geochemical and UPb geochronological data for granitoids in the eastern Mongol–Okhotsk Belt to constrain their timing of formation and tectonic setting. Our new data suggest three stages of granitic magmatism in the eastern part of the Mongol–Okhotsk Belt: Cambrian (ca. 511 Ma), Middle Devonian (ca. 393 Ma), and Late Permian–Early Triassic (257–250 Ma). Cambrian (511 ± 4 Ma) plagiogranites in the Galam Terrane have adakitic geochemical signatures, and εHf(t) values of zircons from these granites vary mostly from +8.8 to +14.8 with Hf model ages (tHf(DM)) ranging from 0.77 to 0.53 Ga. These plagiogranites most likely represent the roots of an island arc that formed above a NW-dipping (present-day coordinates) subduction zone beneath the edge of the Siberian Craton. Middle Devonian (393 ± 7 Ma) granites from the Selemdzha Terrane, as well as Late Permian–Early Triassic (257 ± 4, 253 ± 3, 253 ± 2, 252 ± 3, and 250 ± 4 Ma) granodiorites and granites from the Selemdzha and Tokur terranes, have supra-subduction whole-rock geochemical signatures. Slight negative and positive zircon εHf(t) values of −6.1 to +8.4, tHf(DM) ages of 1.1–0.6 Ga, and tHf(C) ages of 1.4–0.6 Ga suggest that the studied rocks contain a continental crustal component. The Middle Devonian and late Permian–Early Triassic granodiorites and granites formed either in a continental island arc or on an active continental margin. During the Middle Devonian and at the late Permian–Early Triassic boundary, the Mongol–Okhotsk oceanic crust was subducting beneath the edge of the Siberian Craton. Thus, middle Cambrian, Middle Devonian, and Permian–Triassic granitoids in the eastern Mongol–Okhotsk Belt reflect three stages of subduction, meaning that the accretionary wedge terrane assemblages started to form during the Cambrian then continued during the Devonian and at the Permian–Triassic boundary, but was not started at Jurassic as previously proposed.

Determining the Geodynamic Setting of Adakitic Granitoids Using Geochemical Data

Discriminant analysis was performed for representative sets of igneous rocks with adakitic geochemical signatures (granitoids of Archean tonalite–trondhjemite–granodiorite suites, island-arc adakites, and adakites and adakitic granitoids of collisional to postcollisional geodynamic settings). It was shown that the granitoids of Archean tonalite–trondhjemite–granodiorite suites are significantly different from islandarc adakites, as well as from collisional to postcollisional adakites and adakitic granitoids. The following discriminant function was proposed for the geodynamic classification of island arc and collisional-postcollisional adakites and adakitic granitoids on the basis of chemical composition: DF3 =–1.69324TiO2–0.25537Al2O3–0.21269FeO* + 0.06076MgO–0.09796CaO + 0.47377Na2O + 0.29270K2O + 3.57821P2O5 + 0.00431Rb + 0.00036Sr + 0.03119Y + 0.00006Zr + 0.01088Nb–0.00048Ba + 0.01366La + 0.0004Ce + 0.02319Nd–0.18584Sm + 1.29135Eu–0.62229Gd + 0.3819Dy + 2.06583Er–2.62769Yb + 1.6464.

Metasomatized asthenospheric mantle contributing to the generation of Cu-Mo deposits within an intracontinental setting: A case study of the ∼128 Ma Wangjiazhuang Cu-Mo deposit, eastern North China Craton

Cu-Mo deposits are commonly associated with calc-alkaline porphyries and derived from subduction-modified lithosphere in magmatic arcs. Here we report the Wangjiazhuang Cu-Mo deposit, which is peculiarly associated with an alkaline quartz monzonite that originated from a metasomatized asthenospheric mantle within an intracontinental setting in the eastern North China Craton (NCC). The deposit was formed at ca. 128.3±0.7Ma (2σ), basically coincident with the emplacement of the host quartz monzonite (128.8±1.0Ma, 2σ). Pegmatitic vein-type and stockwork-/disseminated-type mineralizations were identified in the deposit, of which the former is shallowly situated and characterized by coarse and well-crystallized Cu- and Mo-bearing sulfides+quartz+biotite+K-feldspar assemblages, whereas the latter is deeply situated and characterized by stockworks or Cu- and Mo-bearing sulfides disseminated in the altered quartz monzonite. Separation between Cu-rich and Mo-rich ore bodies at shallow depth was also observed. The Cu-Mo mineralization is typically associated with potassic-silicic alteration. High-temperature fluid inclusions with halite/sylvite daughter minerals (having homogenization temperatures of 287–466°C and salinities of 33.8–55.3wt% NaCl equivalent) suggest a magmatic origin of the ore-forming fluids. Differentiation of the fluids into vapor-rich and liquid-rich phases occurred during the magmatic-hydrothermal processes, as indicated by the coexistence of vapor-rich, liquid-rich and daughter mineral-rich fluid inclusions with similar homogenization temperatures. LA-ICP-MS analysis of the fluid inclusions shows that Cu is highly partitioned into the vapor-rich fluid inclusions, whereas Mo is mainly concentrated in the daughter mineral-rich fluid inclusions. This implies that phase differentiation of the ore-forming fluids was likely responsible for the separation of Cu-rich and Mo-rich ore bodies. The presence of hematite in the fluid inclusions also suggests high oxidation state of the ore-forming fluids. Based on the variations of δ34S in sulfides and Al contents in quartz, continuous reduction of the oxidized ore-forming fluids, coupled with pH change of the fluids during water-rock interaction, led to the deposition of the ore-forming metals.The host quartz monzonite shows adakitic geochemical signatures. Fractional crystallization of low-Mg amphiboles contributed to the adakitic signatures. According to the mixing modeling using Sr-Nd isotopes, the quartz monzonite was mainly derived from asthenospheric mantle-derived magma mixed by ∼15–20% lower crust components at the base of the lower crust. Zircon saturation thermometry and Ti-in-quartz thermobarometer suggest that the rock was emplaced at the temperature of ∼746°C and the depth of ∼4km. High water (H2O≥4wt%) and metal (Cu is up to 840ppm and Mo is up to 11.9ppm) contents as well as high oxidation state (fO2>fayalite-magnetite-quartz oxygen buffer) are featured in the quartz monzonite, indicating that the asthenospheric mantle source had been hydrated and oxidized by the water released from the stagnant Paleo-Pacific slab in the transition zone (410–610km). Such modification by the deep stagnant oceanic slab beneath the eastern NCC thus not only triggered the remarkable thinning of the Archean lithospheric keel (loss of >120km), but also promoted the asthenospheric mantle to become the favorable magma and metal source for the Cu-Mo deposits in the study region.

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.

The distribution, geochronology and geochemistry of early Paleozoic granitoid plutons in the North Altun orogenic belt, NW China: Implications for the petrogenesis and tectonic evolution

Abundant early Paleozoic granitoid plutons are widely distributed in the North Altun orogenic belt. These rocks provide clues to the tectonic evolution of the North Altun orogenic belt and adjacent areas. In this paper, we report an integrated study of petrological features, U–Pb zircon dating, in situ zircon Hf isotope and whole-rock geochemical compositions for the Abei, 4337 Highland and Kaladawan Plutons from north to south in the North Altun orogenic belt. The dating yielded magma crystallization ages of 514Ma for the Abei Pluton, 494Ma for the 4337 Highland Pluton and 480–460Ma for the Kaladawan Pluton, suggesting that they are all products of oceanic slab subduction because of the age constraint. The Abei monzogranites derived from the recycle of Paleoproterozoic continental crust under low-pressure and high-temperature conditions are products of subduction initiation. The 4337 Highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust. The Kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive εHf(t) values, and the Kaladawan monzogranite-syenogranite are derived from partial melting of Neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle. These results, together with existing data, provide significant information about the evolution history of oceanic crust subduction during the 520–460Ma. The initiation of subduction occurred during 520–500Ma with formation of Abei Pluton; subsequent transition from steep-angle to flat-slab subduction at ca.500Ma due to the arrival of buoyant oceanic plateaus, which induces the formation of 4337 Highland Pluton. With ongoing subduction, the steep-angle subduction system is reestablished to cause the formation of 480–460Ma Kaladawan Pluton. Meanwhile, it is this model that account for the temporal–spatial distribution of these early Paleozoic magmatic rocks in the North Altun orogenic belt.

The calc-alkaline and adakitic volcanism of the Sabzevar structural zone (NE Iran): Implications for the Eocene magmatic flare-up in Central Iran

A major magmatic flare-up is documented along the Bitlis–Zagros suture zone in Eocene–Oligocene times. The Cenozoic magmatism of intraplate Central Iran is an integrant part of this tectono-magmatic scenario. The Cenozoic magmatism of the Sabzevar structural zone consists of mostly intermediate to felsic intrusions and volcanic products. These igneous rocks have calc-alkaline and adakitic geochemical signatures, with nearly coincident zircon U–Pb and mica Ar–Ar ages of ca. 45Ma. Adakitic rocks have quite low HREE and high Sr/Y ratio, but share most of their geochemical features with the calc-alkaline rocks. The Sabzevar volcanic rocks have similar initial Sr, Nd and Pb isotope ratios, showing their cogenetic nature. Nd model ages cluster tightly around ~0.2–0.3Ga. The geochemistry of the Sabzevar volcanic rocks, along with their isotopic signatures, might strangle that an upper mantle source, metasomatized by slab-derived melts was involved in generating the Sabzevar calc-alkaline rocks. A bulk rock trace element modeling suggests that amphibole–plagioclase–titanite-dominated replenishment-fractional crystallization (RFC) is further responsible for the formation of the middle Eocene Sabzevar adakitic rocks. Extensional tectonics accompanied by lithospheric delamination, possibly assisted by slab break-off and melting at depth was responsible for the Eocene formation of the Sabzevar magmatic rocks and, more in general, for the magmatic “flare-up” in Iran.

Early Paleozoic intrusive rocks from the eastern Qilian orogen, NE Tibetan Plateau: Petrogenesis and tectonic significance

Voluminous Early Paleozoic intrusive rocks occur in the Qilian orogenic belt, northeastern margin of the Tibetan Plateau. Their petrogenesis can provide insights into the tectonic evolution of Qilian and its adjacent areas. This paper carries out an integrated study of U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions for the Bamishan and Heishishan plutons in the eastern Qilian orogen. Both the Bamishan and Heishishan plutons consist of mafic, intermediate and felsic intrusive rocks. U–Pb zircon dating yielded the magma crystallization ages of 459–449Ma for the Bamishan pluton and 447–438Ma for the Heishishan pluton. Their geochemical and Sr–Nd–Hf isotopic data show that the parental magmas of the Bamishan monzogranites and quartz diorites were generated by slab (MORB and overlying sediments) melting. The Bamishan hornblende gabbros have extremely high Cr (1159–1163ppm), Ni (220–353ppm) and Mg# (73–74), indicating a cumulate origin. The Heishishan granodiorite–granite has adakitic geochemical signatures and they were produced by partial melting of the mafic lower crust. The Heishishan hornblende gabbro and quartz diorites were derived from the enriched lithospheric mantle metasomatized by subduction-related fluid/melt. In light of the available data, we propose that there was an Early Paleozoic ocean basin between the Central-South Qilian and Qaidam–West Qiling belts. The northward closure of this basin and subsequent continental collision and post-collisional processes resulted in the generation of the Early Paleozoic magmatism in the eastern Central Qilian. The Late Ordovician to Early Silurian (~460–440Ma) intrusive rocks in the eastern Central Qilian formed in a post-collisional setting. A slab break-off model may account for the magma generation.

Origin of two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton: implications for early Mesozoic lithosphere thinning

New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO2 and K2O, moderate initial 87Sr/86Sr, low negative εNd(t) and εHf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al2O3, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial 87Sr/86Sr; high negative εNd(t) and εHf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.

Zircon U–Pb and phlogopite 40Ar–39Ar age of the Chengchao and Jinshandian skarn Fe deposits, southeast Hubei Province, Middle–Lower Yangtze River Valley metallogenic belt, China

The Chengchao and Jinshandian deposits in the southeast Hubei Province are the two largest skarn Fe deposits in the Middle–Lower Yangtze River Valley metallogenic belt (MLYRVMB), China. They are characterized by NW-striking orebodies that are developed along the contacts between the Late Mesozoic granitoid and Triassic carbonate and clastic rocks. New sensitive high-resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of the mineralization-related quartz diorite and granite at Chengchao yield ages of 129 ± 2 and 127 ± 2 Ma, respectively, and those at Jinshandian of 127 ± 2 and 133 ± 1 Ma, respectively. These results are interpreted as the crystallization age of these intrusions. Hydrothermal phlogopite samples from the skarn ores at Chengchao and Jinshandian have the plateau 40Ar–39Ar ages of 132.6 ± 1.4 and 131.6 ± 1.2 Ma, respectively. These results confirm that both intrusions and associated skarn Fe mineralization were formed contemporaneously in the middle Early Cretaceous time. New zircon U–Pb and phlogopite 40Ar–39Ar ages in this study, when combined with available precise geochronological data, demonstrate that there were two discontinuous igneous events, corresponding to two episodes of skarn Fe-bearing mineralization in the southeast Hubei Province: (1) 140–136 Ma diorites and quartz diorites and 141–137 Ma skarn Cu–Fe or Fe–Cu deposits and (2) 133–127 Ma quartz diorites and granites and 133–132 Ma skarn Fe deposits. This scenario is similar to that proposed for the entire MLYRVMB. The intrusions related to skarn Fe deposits show obviously petrological and geochemical differences from those related to skarn Cu–Fe or Fe–Cu deposits. The former are quartz diorite and diorite in petrology and have similar adakitic geochemical signatures and in equilibrium with a garnet-rich residue, whereas the latter are petrologically granite and quartz diorite that are distinguishable from adakitic rocks and in equilibrium with a plagioclase residue. These features indicated that two episodes of magmatism and the formation of skarn Fe-bearing deposits in the southeast Hubei Province, MLYRVMB, might be associated lithosphere thinning induced by asthenosphere upwelling during the Late Mesozoic.

Early Permian plutons from the northern North China Block: constraints on continental arc evolution and convergent margin magmatism related to the Central Asian Orogenic Belt

Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper, the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock e Nd(t) values ranging from −17.4 to −9.3 and zircon e Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher e Nd(t) values from −11.1 to −9.3 and e Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower e Nd(t) value of −17.4 and zircon e Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic.

Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis

Based on a compilation of published sources, rocks referred to as adakites show the following geochemical and isotopic characteristics: SiO2 ≥56 wt percent, Al2O3 ≥15 wt percent, MgO normally <3 wt percent, Mg number ≈0.5, Sr ≥400 ppm, Y ≤18 ppm, Yb ≤1.9 ppm, Ni ≥20 ppm, Cr ≥30 ppm, Sr/Y ≥20, La/Yb ≥20, and 87Sr/86Sr ≤0.7045. Rocks with such compositions have been interpreted to be the products of hybridization of felsic partial melts from subducting oceanic crust with the peridotitic mantle wedge during ascent and are not primary magmas. High Mg andesites have been interpreted to be related to adakites by partial melting of asthenospheric peridotite contaminated by slab melts. The case for these petrogenetic models for adakites and high Mg andesites is best made in the Archean, when higher mantle geotherms resulted in subducting slabs potentially reaching partial melting temperatures at shallow depths before dehydration rendered the slab infusible. In the Phanerozoic these conditions were likely only met under certain special tectonic conditions, such as subduction of young (≤25-m.y.-old) oceanic crust. Key adakitic geochemical signatures, such as low Y and Yb concentrations and high Sr/Y and La/Yb ratios, can be generated in normal asthenosphere-derived tholeiitic to calc-alkaline arc magmas by common upper plate crustal interaction and crystal fractionation processes and do not require slab melting. An assessment of several arc volcanic suites from around the world shows that most adakite-like compositions are generated in this way and do not reflect source processes. Similarly, rare adakite-like intrusive rocks associated with some porphyry Cu deposits are the evolved products of extensive crustal-level processing of calc-alkaline basalt-andesite-dacite-rhyolite series magmas. If slab melts contribute to such magmas, their geochemical signatures would have been obliterated or rendered ambiguous by subsequent extensive open-system processes. In Archean terranes, where adakitic and high Al tonalite-trondhjemite-granodiorite (TTG) magma series rocks are more common, porphyry Cu deposits are rare and, where found, are associated with normal calc-alkaline suites rather than adakites. The two different magma series are compositionally distinct in terms of several major and trace element parameters. Common upper plate magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) affecting normal arc magmas can be demonstrated to explain the distinctive compositions of most adakite-like arc rocks, including high Mg andesites and especially those rare examples associated with porphyry Cu deposits. In contrast, slab melting can in most cases neither be proved nor disproved and is therefore unsatisfactory as a unique factor in porphyry Cu genesis.

U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Garze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement

The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U-Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have mag- matic crystallization ages of 221 ± 3.8 Ma and 216 ± 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb-Sr-Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan- Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.

U–Pb zircon SHRIMP ages, geochemical and Sr–Nd–Pb isotopic compositions of intrusive rocks from the Longshan–Tianshui area in the southeast corner of the Qilian orogenic belt, China: Constraints on petrogenesis and tectonic affinity

The Longshan–Tianshui area is located in the southeast corner of the Qilian orogenic belt. Three intrusions (the Yanjiadian diorite, the Caochuanpu and the Guanshan granites) in this area provide insights into regional tectono-magmatic events, petrogenesis and tectonic evolution. Based on U–Pb zircon SHRIMP dating, the magma emplacement ages of the Yanjiadian diorite, the Caochuanpu and the Guanshan granites are 441±10, 434±10 and 229±7 Ma, respectively. The Yanjiadian diorites have geochemical characteristics similar to island-arc-type rocks, but their formation resulted from northward subduction of the Huluhe (Erlanping) back-arc basin. The Caochuanpu granites with K 2O/Na 2O<0.9, Y of <6.9 ppm, Yb of <0.39 ppm, Sr/Y of 36–97 and (La/Yb) N of 34.8–80.0 exhibit adakitic geochemical signatures. Combined with Pb–Sr–Nd isotope data, the magma for the Caochuanpu granites was suggested to be derived from partial melting of previously underplated Neoproterozoic mafic rocks in the Qinling (or Longshan–Tianshui) island-arc lower crust. The partial melting resulted from island-arc lower crustal thickening due to northward subduction of the Qinling ocean and closure of the Huluhe back-arc. The Guanshan granites with (La/Yb) N of 17.5–42.4, Sr/Y of 15.8–50.5, Y of 7.8–15.7 ppm and Yb of 0.64–1.31 ppm also display some adakitic geochemical signatures. However, the geochemical and Pb–Sr–Nd isotopic compositions of the Guanshan granites are quite distinct from those of the Caochuanpu granites. It is proposed that magma for the Guanshan granites was derived from partial melting of subducted South Qinling–West Qinling continental crust during Triassic continental collision between the North China and the South China plates. The result provides an example that the adakitic rocks can be generated from partial melting of subducted continental crust. The tectono-magmatic events and petrogenesis of intrusive rocks in the Longshan–Tianshui area are in agreement with those of the North Qinling tectonic unit in the Qinling orogenic belt. Therefore, the Qilian orogenic belt corresponds to the North Qinling tectonic unit in a regional tectonic framework. Both the Qilian and the Qinling belts had a similar tectonic evolution.