Low εNd Research Articles

Petrogenesis of Jurassic fractionated I-type granites in Southeast China: Constraints from whole-rock geochemical and zircon U–Pb and Hf–O isotopes

Whole-rock geochemical and Sr–Nd–Hf isotopic data and zircon in situ U–Pb ages and Hf–O isotopes are reported for twelve granitic intrusions from inland to coastal areas of the Cathaysia Block, Southeast (SE) China, in order to constrain their magma sources and petrogenesis. They are mainly composed of biotite granites with minor granodiorites. Zircon U–Pb dating by LA-ICPMS and SIMS methods gave emplacement ages of 165–154Ma for these rocks, which are metaluminous to weakly peraluminous (A/CNK=0.95–1.10), with variable SiO2 contents and high K2O contents. They are all depleted in high field strength elements (HFSEs) and enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and show negative Sr, Ba and Eu anomalies in the spidergrams. Petrographic and geochemical features suggest the granodiorites that contain hornblende to be typical of I-type, while the biotite granites without hornblende are fractionated I-type granites. Their variable major and trace element concentrations are due to fractional crystallization of feldspar, biotite, hornblende, and accessory minerals, including apatite, allanite and monazite. Moreover, our data indicate significant variations in isotopic compositions of these granitoids from inland to coastal areas. The inland granites, from the Lapu, Fogang, Xinfengjiang and Baishigang plutons, have relatively high initial 87Sr/86Sr ratios (0.7105–0.7155) and low εNd(t) (−11.2 to −7.3) and εHf(t) (−10.9 to −6.3) values for whole rocks, and variable δ18O values of 7.69–9.25‰ and εHf(t) values of −13.5 to −6.7 for zircons. In contrast, the coastal granites, from the Longwo, Wushikeng, Lianhuashan, Shigushan, Chiliao, Hulutian, Mantoushan and Fenghuang plutons, show relatively low initial 87Sr/86Sr ratios (0.7056–0.7110) and high εNd(t) (−7.6 to −6.2) and εHf(t) (−6.5 to −2.8) values for whole-rocks, together with εHf(t) values of −7.2 to 3.9 and δ18O values of 6.69–8.38‰ for zircons. The spatial isotopic variations suggest that these Jurassic granitoids were generated by different-percentage mixing of mantle- and ancient (Paleoproterozoic) crust-derived magmas, with subsequently minor crustal assimilation. Our results highlight that Jurassic fractionated I-type granites occur widely from inland to coastal areas of SE China, arguing against the previous view that Jurassic granites occur only in inland while coastal granites are mainly Cretaceous. The widespread occurrence of Jurassic granitoids in SE China is attributed to ascending of mantle-derived magmas, which provide heat for partial melting of lower crustal materials, in an extensional setting.

Late Cretaceous high-Mg# granitoids in southern Tibet: Implications for the early crustal thickening and tectonic evolution of the Tibetan Plateau?

This study presents new major and trace element, plus Sr–Nd and zircon U–Pb isotope data for the Zhongcang granitic plutons, which are located to the south of the Yongzhu–Asuo ophiolite belt within the northwestern part of the central Lhasa subterrane, Tibetan Plateau. These data provide new insights into the Late Cretaceous tectonic evolution of southern Tibet. The Zhongcang plutons are dominated by granodiorites and granites that yield zircon U–Pb emplacement ages of 94–88 Ma. They can be further divided into metaluminous and peraluminous subtypes. The metaluminous rocks have adakite-like geochemical signatures, including high SiO2, Al2O3, and Sr concentrations, and low Yb and Y concentrations, and high Sr/Y and (La/Yb)N ratios. These rocks also have negative εNd(t) values (−3.17 to −0.17), variable initial 87Sr/86Sr(i) ratios (0.705927–0.707668), and high K2O and Th concentrations, suggesting that they were not derived from the partial melting of subducted oceanic crust in an arc setting. The Zhongcang adakitic rocks have higher MgO and Cr concentrations and Mg# values than do contemporaneous intrusive rocks derived from a region of thickened lower crust within the central Lhasa subterrane. These data suggest that the Zhongcang adakitic rocks were generated by the partial melting of a delaminated thickened lower crust within a Late Cretaceous continental setting. In comparison with the Zhongcang adakitic rocks, the peraluminous rocks have significant negative Eu and Sr anomalies and lower εNd(t) values (−4.06 to −6.64). This, combined with their high Mg# values, and Cr concentrations, suggests that the peraluminous units formed from primitive magmas similar to those that formed the Zhongcang adakitic rocks, but modified by contamination with ancient crustal material and by fractional crystallization of plagioclase and apatite during uprising and/or emplacement. The Zhongcang high-Mg# granitoids provide robust evidence for Late Cretaceous crustal thickening prior to India–Asia collision.

Interaction between felsic and mafic magmas in the Salmas intrusive complex, Northwestern Iran: Constraints from petrography and geochemistry

The Salmas plutonic complex, in the northernmost part of Sanandaj-Sirjan Zone of Iran, provides evidence for magma interaction processes. The complex contains mafic-intermediate, hybrid and felsic rocks which intruded into the Paleozoic metamorphic complex. They show typical relationships described in many mafic–felsic mingling and mixing zones worldwide, such as mafic microgranular enclaves (in felsic and hybrid rocks), mafic sheets, and hybrid rocks. The mafic microgranular enclaves (MMEs) are characterized by fine-grained, equigranular and hypidiomorphic texture and some special types of microscopic textures, e.g., quartz xenocrysts, oscillatory-zoned plagioclase, small lath-shaped plagioclase in large plagioclase, spike zones in plagioclase and spongy-cellular plagioclase textures, rounded plagioclase megacrysts blade-shaped biotite, acicular apatite. The mafic sheets and MMEs in granites (MME-Gr), which indicated magma mingling structures, show ISr values and εNd(i) similar to diorites. The hybrid rocks and their mafic enclaves (MME-H) show isotope signatures similar to each other. Granites have isotope signatures [higher 87Sr/86Sr(i) (0.70788–0.71075) and lower εNd(i) (−2.4 to −4.2)] distinct to those of the all rock types and MMEs. Major, trace and REE modeling show that hybrid rocks are generated via 40–60% mixing of mafic (dioritic) and felsic (granitic) end-members.All the geochemical data suggest that underplating of dioritic magma, which has been produced by fractional crystallization of gabbros, under the lower crust caused its melting to make felsic (granitic) magma. Injection of dioritic magma into the base of the felsic magma chamber and a limited mixing of two end-members, the lower crust-derived magma and mantle-derived melts, formed hybrid magma and their enclaves. Injections of new mafic magma pulses into hybrid magma generated mafic enclaves into them. The injections of denser dioritic magma pulses into a felsic magma chamber and spreading of it at the level of rheological contrast have been formed mafic sheets and MMEs in granites.

Tectono-magmatic evolution of the Gaoligong belt, southeastern margin of the Tibetan plateau: Constraints from granitic gneisses and granitoid intrusions

The Gaoligong belt is located in the southeastern margin of the Tibetan plateau, and is bound by the Tengchong and Baoshan blocks. This paper presents new data from zircon geochronology, geochemistry, and whole-rock Sr–Nd–Pb–Hf isotopes to evaluate the tectonic evolution of the Gaoligong belt. The major rock types analysed in the present study are granitic gneiss, granodiorite, and granite. They are metaluminous to peraluminous and belong to high-K, calc-alkaline series. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircons from nine granitic rocks yielded crystallization ages of 495–487Ma, 121Ma, 89Ma, and 70–63Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high εNd(t) and εHf(t) values of −1.06 to −3.45 and −1.16 to 2.09, and model ages of 1.16Ga to 1.33Ga and 1.47Ga to 1.63Ga, respectively. Their variable 87Sr/86Sr and Pb values resemble the characteristics of the Early Paleozoic Pinghe granite in the Baoshan block. Our data suggest that the rocks were derived from the break-off of the Proto-Tethyan oceanic slab between the outboard continent and the Baoshan block, which induced the partial melting of Mesoproterozoic pelitic sources mixed with depleted mantle materials. (2) Early Cretaceous granodiorites with low εNd(t) and εHf(t) values of −8.92 and −4.91 with Nd and Hf model ages of 1.41Ga and 1.49Ga, respectively. These rocks have high initial 87Sr/86Sr (0.711992) and lower crustal Pb values, suggesting that they were derived from Mesoproterozoic amphibolites with tholeiitic signature, leaving behind granulite residue at the lower crust. (3) Early Late Cretaceous granites with low εNd(t) and εHf(t) values of −9.58 and −4.61 with Nd and Hf model ages of 1.43Ga and 1.57Ga, respectively. These rocks have high initial 87Sr/86Sr (0.713045) and lower crustal Pb isotopic values. These rocks were generated from the partial melting of Mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere, following the closure of the Bangong–Nujiang Ocean and collision of the Lhasa–Qiangtang blocks. (4) Late Cretaceous to Paleogene granitic gneisses with low εNd(t) and εHf(t) values of −4.41 to −10 and −5.95 to −8.71, Nd model ages ranging from 1.08Ga to 1.43Ga, and Hf model ages from 1.53Ga to 1.67Ga, respectively. These rocks show high initial 87Sr/86Sr (0.713201 and 714662) and lower crustal Pb values. The data suggest that these rocks are likely related to the eastward subduction of the Neo-Tethyan Oceanic slab, which induced partial melting of Mesoproterozoic lower crustal metagreywacke. The results presented in this study from the Gaoligong belt offer important insights on the evolution of the Proto-Tethyan, Bangong–Nujiang, and Neo-Tethyan oceans in the southeastern margin of the Tibetan Plateau.

Petrogenesis of orogenic lamproites of the Bohemian Massif: Sr–Nd–Pb–Li isotope constraints for Variscan enrichment of ultra-depleted mantle domains

During convergence of Gondwana-derived microplates and Laurussia in the Palaeozoic, subduction of oceanic and continental crusts and their sedimentary cover introduced material of regionally contrasting chemical and isotopic compositions into the mantle. This slab material metasomatised the local mantle, producing a highly heterogeneous lithospheric mantle beneath the European Variscides. The eastern termination of the European Variscides (Moldanubian and Saxo-Thuringian zones of Austria, Czech Republic, Germany and Poland) is unusual in that the mantle was modified by material from several subduction zones within a small area. Orogenic lamproites sampled this lithospheric mantle, which has a chemical signature reflecting extreme depletion (low CaO and Al2O3 contents and high Mg-number) followed by strong metasomatic enrichment, giving rise to crust-like trace element patterns, variable radiogenic 87Sr/86Sr(330) (0.7062–0.7127) and non-radiogenic Nd isotopic compositions (εNd(330)=−2.8 to −7.8), crustal Pb isotopic compositions, and a wide range of δ7Li values (−5.1 to +5.1). This metasomatic signature is variably expressed in the lamproites, depending on the extent of melting and the nature of the source of the metasomatic component. Preferential melting of the metasomatically enriched (veined) lithospheric mantle with K-rich amphibole resulted in lamproitic melts with very negative, crust-like δ7Li values, which correlate positively with peralkalinity, HFSE contents and lower εNd. Both the higher degree of melting and progressive consumption of the metasomatic component reduce the chemical and isotopic imprints of the metasomatic end member. The very positive δ7Li values of some lamproites indicate that the source of these lamproites may have been modified by subducted oceanic lithosphere. Fresh olivine from the Brloh (Moldanubian) lamproitic dyke shows very high Fo (up to 94%) and very high Li contents (up to 25ppm), demonstrating that the extremely depleted and later enriched lithospheric mantle may have contributed significantly to the Li budget of the lamproites. The regional distribution of lamproites with contrasting chemical and isotopic fingerprints mimics the distribution of the different Variscan subduction zones.

Geochemistry, geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu*=0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8ppm) and HREE contents (e.g., Yb=1.29–1.64ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.

Petrogenesis and tectonic significance of the late Triassic mafic dikes and felsic volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibet Plateau

We present zircon U–Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228–218Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~8–9Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns are similar to those of the present-day ocean island basalts (OIBs) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076–0.7104), low εNd(t) (−2.18 to −3.46), low εHf(t) (−2.85 to −4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71–17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213–0.7550) and less negative εNd(t) (−3.83 to −5.09) and εHf(t) (−3.06 to −3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb–Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45–50% crustal materials and 50–55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting, induced melting of the prior metasomatized mantle lithosphere and the existing crust. This work represents our ongoing effort in understanding the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.

Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan, Kyrgyzstan

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi–Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic–carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid-oceanic ridge (MORB-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO2, LREE (La/Smn=1.9), and Nb (Zr/Nbav.=10), have differentiated HREE (Gd/Ybn=2.0), medium to low εNd (~5.7) and are characterized by clear Nb positive anomalies in normalized multi-element plots (Nb/Thpm=1.3, Nb/Lapm=1.1). The OIBs formed by relatively low degrees of melting (<5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORB-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher εNd values (~9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several seamount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian–Bashkirian.

The Dovyren intrusive complex (northern Baikal region, Russia): isotope–geochemical markers of contamination of parental magmas and extreme enrichment of the source

The Dovyren intrusive complex includes the ore-bearing (Cu–Ni–PGE) Yoko–Dovyren layered pluton (728 Ma, up to 3.4 km in thickness), underlying ultramafic sills, and comagmatic leuconorite and gabbro-diabase dikes. Studies of Sr–Nd–Pb isotope systems were carried out for 24 intrusive rocks and five associated low- and high-Ti basalts. The high-Ti basalts show 0.7028 ≤ (87Sr/86Sr)T ≤ 0.7048 and 4.6 ≤ εNd(T) ≤ 5.8, similar to the values in MORB. The intrusive basic and ultrabasic rocks are geochemically similar to the low-Ti formation, making a compact cluster of compositions with extremely high ratios of radiogenic Sr and Pb isotopes and low εNd values. The maximum enrichment in radiogenic Sr is shown by the rocks near the pluton bottom ((87Sr/86Sr)T = 0.71387 ± 0.00010 (2σ); εNd(T) = –16.09 ± 0.06), which are the products of crystallization of the most primitive high-Mg magmas. The above-located dunites, troctolites, and gabbro show lower enrichment, probably because of the contamination of the host rocks during the filling of the magma chamber and/or because of the slight heterogeneity of the source. Calculations of the proportions of mixing of the parental melt with carbonate terrigenous material have shown that the variations in the Sr and Nd isotope ratios are due to the incredibly high contamination of the sediments, up to 40–50%. This contradicts the succession of the main rock types in the Yoko–Dovyren pluton in accordance with the crystallization of picrite-basaltic magma. The contribution of 5–10% high-Ti component seems more likely and suggests interaction between two isotopically contrasting magmas in this province in the Late Riphean. In general, the minor variations in εNd(T) of the intrusive rocks and metavolcanics (–14.3 ± 1.1) testify to the isotopically anomalous source of the low-Ti magmas. The time variation trend of εNd and geochemical features of the Dovyren rocks indicate that the products of melting of 2.7–2.8 Ga suprasubduction mantle might have been the massif protolith. Thus, the Dovyren parental magmas formed from a much older (sub)lithospheric source in the Late Riphean. The source was initially enriched in a mafic component with a low Sm/Nd ratio and was isolated from the convecting mantle and mantle melting processes for ~2 Gyr. The existence of such a long-living and at least twice reactivated lithospheric substratum is confirmed by the fact that the Nd isotope evolution trend of the initially nonanomalous mantle protolith includes not only the Dovyren rocks but also the Paleoproterozoic gabbro of the Chinei pluton and the Archean enderbites of the Baikal region.© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Petrogenesis and tectonic setting of Triassic granitoids in the Qiangtang terrane, central Tibet: Evidence from U–Pb ages, petrochemistry and Sr–Nd–Hf isotopes

Triassic granitoids, including the ∼220Ma Shuanghu and ∼210Ma Rongma granitoids studied here, are widely distributed around the Longmu–Shuanghu suture and in the Qiangtang terrane, central Tibet. The majority of these granitoids can be classified as high-K calc-alkaline in nature and yield negative Ba and Sr anomalies on primitive mantle-normalized diagrams. In addition, they are: enriched in light rare earth elements (LREE) ((La/Yb)N=1.61–21.79); strongly enriched in large ion lithophile elements (LILE: e.g., Cs, Rb, and K), and depleted in high field strength elements (HFSE: e.g., Nb and Ti). Magma mixing played a role in the genesis of the Shuanghu granodiorites, as indicated by the occurrence of dioritic enclaves and the wide range in zircon Hf compositions (εHf(t)=−15.0 to −2.5). The I-type Shuanghu granodiorites and S-type Shuanghu and Rongma granites might have been derived from melting of southern Qiangtang crust given the high initial Sr (0.7131–0.7272), low εNd(t) (−8.9 to −11.1) and zircon εHf(t) values (−15 to −7.2). The granitoids may have formed during melting of southern Qiangtang crust, heated by upwelling asthenosphere mantle, a result of break-off and delamination of the Paleo-Tethys slab in a collisional setting.

Tectonic record, magmatic history and hydrothermal alteration in the Hercynian Guérande leucogranite, Armorican Massif, France

The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southern part of the Armorican Massif. At the scale of the intrusion, this granite displays structural heterogeneities with a weak deformation in the southwestern part, whereas the northwestern part is marked by the occurrence of S/C and mylonitic extensional fabrics. Quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both E–W and N–S stretching. Therefore, during its emplacement in an extensional tectonic regime, the syntectonic Guérande granite has probably experienced some partitioning of the deformation. The southwestern part is characterized by a muscovite–biotite assemblage, the presence of restites and migmatitic enclaves, and a low abundance of quartz veins compared to pegmatite dykes. In contrast, the northwestern part is characterized by a muscovite–tourmaline assemblage, evidence of albitization and gresenization and a larger amount of quartz veins. The southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone. The granite samples display continuous compositional evolutions in the range of 69.8–75.3wt.% SiO2. High initial 87Sr/86Sr ratios and low εNd(T) values suggest that the peraluminous Guérande granite (A/CNK>1.1) was formed by partial melting of metasedimentary formations. Magmatic evolution was controlled primarily by fractional crystallization of K-feldspar, biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7±1.3Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.

Petrogenesis of the Chagangnuoer deposit, NW China: a general model for submarine volcanic-hosted skarn iron deposits

The Chagangnuoer deposit is a typical submarine volcanic rock-hosted skarn iron deposit, where orebodies mainly occur in andesitic rocks of the Dahalajunshan Formation (DF) with skarns well developed around orebodies. The volcanic rocks of the DF in the Chagangnuoer deposit display calc-alkaline characteristics. The ore-bearing andesitic rocks have high 87Sr/86Sr(i) (0.7058–0.7117) and low εNd(t) (−3.51 to 1.67). They probably formed through mixing of basaltic melts and the induced crustal melts. LA-ICP-MS U–Pb zircon ages of 250 and 305Ma are obtained for the granite and granodiorite in the Chagangnuoer deposit, respectively, which are significantly younger than the timing of the skarn formation (316Ma). These age data indicate that the granitoids have no contribution to the skarn and associated iron mineralization. This paper proposes a new genetic model for submarine volcanic rock-hosted skarn iron deposits, in which the iron mineralization, skarn formation and volcanic magmatism are necessary aspects of the same system; the iron separates and concentrates from the silicate magma in the form of Fe(II) carbonate complex. While this conceptual model is largely based on observations on the Chagangnuoer deposit, it may have general significance for skarn-type iron deposits associated with submarine volcanic rock sequences and warrants further testing and improvement.

Separating biogeochemical cycling of neodymium from water mass mixing in the Eastern North Atlantic

The radiogenic neodymium (Nd) isotope ratio 143Nd/144Nd (expressed in εNd) is being used as a tracer in paleo and modern ocean circulation. However, the mechanisms controlling input, distribution, and internal cycling are far from understood. For example, globally, Nd concentration ([Nd]) commonly follows patterns of nutrient tracers, generally increasing with depth below the thermocline, while εNd, tends to reflect the water masses, which has often been referred to as the ‘Nd-paradox’. Here we present dissolved Nd isotopes and concentrations at unprecedented vertical and spatial resolution from the eastern part of the US GEOTRACES North Atlantic Zonal Transect (Gulf of Cadiz – Mauritanian Shelf – Cape Verde Islands).The [Nd] of all samples ranges from 12.3 to 36.7 pmol/kg, with lowest [Nd] usually found within the layer of highest chlorophyll-a levels (chl-max), suggesting removal through scavenging. The Nd isotope compositions range between εNd=−13.4 and −9.9, with lower values at the surface within the extension of the Saharan dust plume and a benthic nepheloid layer (BNL). Less negative values are found in oligotrophic surface waters, Mediterranean Outflow Water (MOW), and near the Cape Verde Islands. Overall, water mass mixing derived from εNd is best visible at the Strait of Gibraltar, where MOW enters the Atlantic Ocean. Most of the sub-thermocline εNd varies within a small range with poor water mass distinction due to the dominance of North Atlantic Deep Water. High surface [Nd] associated with more negative εNd is interpreted to be the result of dust deposition and dissolution. Local [Nd] maxima with no apparent change in εNd compared to ambient seawater, observed within a zone of minimum oxygen concentration (OMZ) at ∼500 m depth off Mauritania, suggest minor input of lithogenic Nd but a rather high contribution through desorption of previously scavenged Nd. That is, Saharan dust in this area has only a minor influence on the isotope composition of water below the uppermost surface layer. The low εNd values and elevated [Nd] within the BNL on the other hand indicate release from detrital material. Our new high-resolution dataset gives valuable insights into the Nd distribution near continental margins, influenced by high atmospheric inputs and changing oxygen conditions. We show how vertical and lateral processes overlap to produce the observed Nd distribution, bringing us forward in understanding the Nd-paradox.

Geochronology and geochemical constraints on petrogenesis of Early Paleozoic granites from the Laojunshan district in Yunnan Province of South China

The Early Paleozoic Orogen in South China has been a topic of hot debate; in particular with regard to the relative contributions of crustal and mantle sources to the generation of granites. To address this issue, we report geochronological and geochemical data for three related suites of granites (Tuantian, Suite 1, Nanlao, Suite 2, and Laochengpo, Suite 3) from the Laojunshan district in the southeast Yunnan Province, western South China Block (SCB). LA-ICP-MS U-Pb dating of zircons indicates magmatic crystallization ages of ~436Ma, ~430Ma and ~427 Ma for Suites 1, 2, and 3 granites, respectively. The εHf(t) values of magmatic zircons in all three suites are heterogeneous, ranging between +3 and −14. All three suites also contain inherited zircon cores with Proterozoic U-Pb and Hf model ages. Bulk compositions form well-defined mixing arrays, with Suites 1 and 2 being strongly peraluminous (A/CNK >1.1), whereas Suite 3 is weakly peraluminous (A/CNK=1.0–1.1). Suites 1 and 2 have lower TFe2O3, Al2O3, MnO, MgO, CaO, TiO2, Na2O, Mg# and Nb/Ta but higher K2O, Rb/Sr, Rb/Ba and εNd(t) than those of Suite 3. These results suggest that Suite 1 and Suite 2 were formed by partial melting of Proterozoic metasedimentary rocks with little or no input of mantle-derived materials, but Suite 3 was derived from a mixture of a crustal melt with mantle derived mafic magma. The apparent paradox that the mantle-derived component has lower εNd(t) values than the crustal components is consistent with the very low εNd(t) values found in mantle-derived basalts in the SCB. The Wuyi-Yunkai orogenic collapse may have caused lower crustal melting of the Cathaysia Block. The early Paleozoic granitoid magmatism with mafic magma input indicates an extensional environment in the western SCB, in response to post-collisional orogenic collapse. Asthenospheric upwelling and basaltic underplating probably contributed heat and melts triggering felsic magmatism in the western SCB.

Zircon U–Pb ages and Hf–O isotopes, and whole-rock Sr–Nd isotopes of the Bozhushan granite, Yunnan province, SW China: Constraints on petrogenesis and tectonic setting

The Bainiuchang silver-polymetallic ore deposit is a super-large deposit in the western part of the South China tungsten–tin province (or the Nanling tungsten–tin province). The deposit is spatially and temporally associated with the Bozhushan granite pluton. Our new data indicate that the Bozhushan granitoids formed at 86–87Ma. The granitoids are geochemically consistent with A-type granite. The Bozhushan pluton consists predominantly of biotite granite that is characterized by weakly peraluminous to metaluminous compositions and high alkali contents (Na2O+K2O=7.51–9.06wt.%). The granitic rocks are enriched in large-ion lithophile elements (LILE) Rb, Th, U, and K, but relatively depleted in Ba and Sr. In addition, they have high Zr+Nb+Ce+Y contents (310–478ppm) and high 10,000× Ga/Al ratios (2.7–3.1). The temperatures of the parental magmas for the Bozhushan granites are estimated to be 790–842°C based on the zircon saturation thermometer. Isotopically, the Bozhushan granites are characterized by elevated initial 87Sr/86Sr ratios (0.7126–0.7257) and low εNd values (−11.2 to −12.4), and high δ18O values (7.91–9.58‰) and low εHf values (−9.5 to −6.1) for zircon crystals, which indicate a dominant continental crustal source. The two-stage Hf model ages vary from 1.53 to 1.86Ga. The isotopic compositions support the interpretation that the granitic rocks formed by melting of the Meso- and Neoproterozoic metasedimentary basements of the Cathaysia block. These results, together with geological records in the other parts of the western Cathaysia block, suggest that the formation of the Bozhushan A-type granites is related to lithospheric extension and asthenospheric upwelling that are associated with the change of plate motion in Late-Cretaceous.

Petrogenesis of Cretaceous igneous rocks from the Duolong porphyry Cu–Au deposit, central Tibet: evidence from zircon U–Pb geochronology, petrochemistry and Sr–Nd–Pb–Hf isotope characteristics

The Duolong porphyry Cu–Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au) was recently discovered in the southern Qiangtang terrane, central Tibet. Here, new whole‐rock elemental and Sr–Nd–Pb isotope and zircon Hf isotopic data of syn‐ and post‐ore volcanic rocks and barren and ore‐bearing granodiorite porphyries are presented for a reconstruction of magmas associated with Cu–Au mineralization. LA–ICP–MS zircon U–Pb dating yields mean ages of 117.0 ± 2.0 and 120.9 ± 1.7 Ma for ore‐bearing granodiorite porphyry and 105.2 ± 1.3 Ma for post‐ore basaltic andesite. All the samples show high‐K calc‐alkaline compositions, with enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE: Cs and Rb) and depletion of high field strength elements (HFSE: Nb and Ti), consistent with the geochemical characteristics of arc‐type magmas. Syn‐ and post‐ore volcanic rocks show initial Sr ratios of 0.7045–0.7055, εNd(t) values of −0.8 to 3.6, (206Pb/204Pb)t ratios of 18.408–18.642, (207Pb/204Pb)t of 15.584–15.672 and positive zircon εHf(t) values of 1.3–10.5, likely suggesting they dominantly were derived from metasomatized mantle wedge and contaminated by southern Qiangtang crust. Compared to mafic volcanic rocks, barren and ore‐bearing granodiorite porphyries have relatively high initial Sr isotopic ratios (0.7054–0.7072), low εNd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [εHf(t) of 1.5–9.7], possibly suggesting more contribution from southern Qiangtang crust. Duolong volcanic rocks and granodiorite porphyries likely formed in a continental arc setting during northward subduction of the Bangong–Nujiang ocean and evolved at the base of the lower crust by MASH (melting, assimilation, storage and homogenization) processes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

The source of Mesozoic granitoids in South China: Integrated geochemical constraints from the Taoshan batholith in the Nanling Range

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic granitic intrusions from the Taoshan batholith in the Nanling Range, South China. The results not only place constraints on the origin of granitoids but also provide insights into the effect of melting temperature on granitoid compositions. LA-ICPMS zircon U–Pb dating yields weighted 206Pb/238U ages of 230±2Ma for the Caijiang intrusion, 167±2Ma for the Huangpi intrusion, 152±3Ma for the Daguzhai intrusion, and 146±3Ma for the Luobuli intrusion. Relict zircon cores with older Mesozoic to Paleozoic U–Pb ages are present in some samples. These Mesozoic granitoids are weakly to strongly peraluminous with A/CNK ratios of 1.05 to 1.23. They exhibit arc-like trace element distribution patterns, with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion of HFSE (e.g., Nb, Ta, Ti). They show high whole-rock initial 87Sr/86Sr ratios of 0.7101 to 0.7156 and low εNd(t) values of −11.2 to −8.8 as well as negative zircon εHf(t) values of −14.3 to −4.8 and Paleoproterozoic Nd-Hf model ages. These results suggest their derivation from partial melting of ancient continental crust. Furthermore, they have high zircon δ18O values of 8.6 to 10.6‰ and calculated whole-rock δ18O values of 10.8 to 12.5‰. Along with their peraluminous features, high K2O/Na2O ratios, arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions, these Mesozoic granitoids are unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. There are significant differences in both major and trace elements between the Caijiang intrusion of Triassic age and the Huangpi, Daguzhai and Luobuli intrusions of Jurassic age. While the Triassic granites exhibit high (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but low Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with high zircon saturation temperatures, the Jurassic granites have low (Fe2O3)T+MgO, P2O5, Th, LREE and Zr+Nb+Ce+Y contents but high Al2O3/TiO2, Rb/Ba and Rb/Sr ratios in association with low zircon saturation temperatures. These correlations indicate that melting temperature and thus extent of partial melting have played an important role in dictating the compositions of granitoids. Therefore, the geochemical compositions of S-type granitoids are dictated not only by the composition of source rocks but also by the temperature of partial melting.

Petrogenesis of the Late Jurassic Laomengshan rhyodacite (Southeast China): constraints from zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf isotopes

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb–Zn–Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, and Sr–Nd–Pb isotopic compositions, and in situ Hf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb–Zn–Au orefield. Zircon U–Pb dating yields a weighted average age of 156.7 ± 1.6 Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg# = 42.4–47.5). Samples display high (87Sr/86Sr)i values (0.71165–0.71176), low εNd(T) values (−10.7 to −10.3), old Nd model ages (TDM = 1.73–1.86 Ga), and relatively homogeneous Pb isotopic compositions [(206Pb/204Pb)i = 18.365–18.412, (207Pb/204Pb)i = 15.663–15.680, and (208Pb/204Pb)i = 38.625–38.666]. The zircons exhibit enriched εHf(T) values (−16.22 to −9.86) and old two-stage Hf model ages (TDM2 = 1.82–2.22 Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite.

U–Pb zircon geochronology, geochemical and Sr–Nd–Hf isotopic compositions of the Early Indosinian Tongren Pluton in West Qinling: Petrogenesis and geodynamic implications

The West Qinling Orogenic Belt (WQOB), an important part of the Qinling-Dabie-Sulu Orogen (Central Orogen), is essential to understand the prolonged evolution of the northeastern branch of the Paleo-Tethys in East Asia. Zircon LA-ICP-MS U–Pb age and Lu–Hf isotopes, bulk-rock major and trace element, and Sr–Nd isotope data for the granodiorite intrusion from the Tongren area, West Qinling are presented. LA-ICP-MS zircon U–Pb dating analyses from two samples constrain the time of crystallization of the granodiorite to ca. 241Ma. Combined with newly published age data from other coeval intrusive rocks in West Qinling, it is concluded that an Early Indosinian magmatic event occurred in West Qinling. In-situ zircon Hf isotope data on the two samples dated exhibit εHf(t) values of −0.6 to −5.8 and −0.8 to −3.9, with corresponding TDMC ages of 1312 to 1637Ma and 1321 to 1516Ma, respectively. Granodiorites are characterized by relatively high Mg# values (51.4–58.0), intermediate SiO2 (63.75–67.04wt.%), high Al2O3 (14.24–15.58wt.%) and total alkalies (6.60–8.00wt.%) and relatively low A/CNK values (0.93–1.10). Granodiorites have I-type, high-K to shoshonitic, and metaluminous to weakly peraluminous character, showing bulk-rock geochemical features of arc-related granitoids. They are enriched in large-ion lithophile and light rare-earth elements, and depleted in some high-field-strength elements. The rocks show uniformly high (87Sr/86Sr)t (0.707616–0.708237) and low εNd(t) values (−7.9 to −7.5). Estimated crystallization temperatures for the rocks range from 720±48°C (Ti-in-zircon thermometer) to 759±25°C (zircon saturation thermometer). The data suggest that magmas were generated by dehydration melting of a mafic lower crustal component with additional input of a mafic component derived from the subcontinental lithospheric mantle. We conclude that the Tongren granodiorite formed in a subduction-related regime in response to slab roll-back of the northward-subducting A’nyemaqen–Mianlue oceanic lithosphere.

Southern Tibetan Oligocene–Miocene adakites: A record of Indian slab tearing

Oligocene–Miocene granitoids exposed within the 1500-km-long Gangdese arc of southern Tibet exhibit adakitic compositions. Five plutonic samples from the southeastern Lhasa terrane near Namche Barwa area were analyzed to determine their geochemical characteristics and to better understand the geodynamic evolution of the eastern Himalayan syntaxis orogen. The samples, yielded U–Pb zircon ages in the range of 30–26 Ma, are intermediate to silicic in composition and have elevated K 2 O (1.5–8.9), Th/La ratios (0.14–1.10), low MgO (< 2%) and Mg# (< 50), as well as high initial 87 Sr/ 86 Sr i (0.7062–0.7103) and low εNd(t) (− 10.9 to − 1.0) isotopic compositions. In addition, these granitic and granodioritic rocks have high Sr/Y (66–306) and La/Yb (25–312) ratios, a characteristic of adakitic rocks. In-situ zircon εHf(t) isotopic compositions are in the range of + 8.1 to − 1.9. Within the framework of the Tibetan–Himalayan orogenesis we attribute these rocks to represent partial melting and mixing products of two end-member components of the lower Tibetan crust: the roots of the relatively juvenile Gangdese arc crust and newly-underplated high-potassium mafic magmas. Adakitic magmatism initiated at 30–26 Ma near the eastern Himalayan syntaxis and systematically decreases in age to the west to 18–9 Ma near Shigatse. We attribute this temporal–spatial distribution of adakitic magmatism within the Gangdese arc, along with the regional Oligo-Miocene geology, to the progressive tearing of the Indian plate. Based on the decrease in age of adakitic magmatism from east to west we hypothesize that the tear initiated beneath the eastern Himalayan syntaxis and propagated westward. • Adakitic granitoids formed at 30–26 Ma around eastern Himalayan syntaxis. • Indian crustal materials besides juvenile crust were involved in generation of these granitoids. • Indian slab tearing initiated beneath eastern Himalayan syntaxis and propagated westward.