Late Paleozoic volcanic record of the Eastern Junggar terrane, Xinjiang, Northwestern China: Major and trace element characteristics, Sr–Nd isotopic systematics and implications for tectonic evolution

The Eastern Junggar terrane of the Central Asian Orogenic Belt includes a Late Paleozoic assemblage of volcanic rocks of mixed oceanic and arc affinity, located in a structurally complex belt between the Siberian plate, the Kazakhstan block, and the Tianshan Range. The early history of these rocks is not well constrained, but the Junggar terrane was part of a Cordilleran-style accreted arc assemblage by the Late Carboniferous. Late Paleozoic volcanic rocks of the northern part of the east Junggar terrane are divided, from base to top, into the Early Devonian Tuoranggekuduke Formation (Fm.), Middle Devonian Beitashan Fm., Middle Devonian Yundukala Fm., Late Devonian Jiangzierkuduke Fm., Early Carboniferous Nanmingshui Fm. and Late Carboniferous Batamayineishan Fm. We present major element, trace element and Sr–Nd isotopic analyses of 64 (ultra)mafic to intermediate volcanic rock samples of these formations. All Devonian volcanic rocks exhibit remarkably negative Nb, Ta and Ti anomalies on the primitive mantle-normalized trace element diagrams, and are enriched in more highly incompatible elements relative to moderately incompatible ones. Furthermore, they have subchondritic Nb/Ta ratios, and their Zr/Nb and Sm/Nd ratios resemble those of MORBs, characteristics of arc-related volcanic rocks. The Early Devonian Tuoranggekuduke Fm., Middle Devonian Beitashan Fm., and Middle Devonian Yundukala Fm. are characterized by tholeiitic and calc-alkaline affinities. In contrast, the Late Devonian Jiangzierkuduke Fm. contains a large amount of tuff and sandstone, and its volcanic rocks have dominantly calc-alkaline affinities. We therefore propose that the Jiangzierkuduke Fm. formed in a mature island arc setting, and other Devonian Fms. formed in an immature island arc setting. The basalts from the Nanmingshui Fm. have geochemical signatures between N-MORB and island arcs, indicating that they formed in a back-arc setting. In contrast, the volcanic rocks from the Batamayineishan Fm. display geochemical characteristics of continental intraplate volcanic rocks formed in an extensional setting after collision. Thus, we propose a model that involves a volcanic arc formed by northward subduction of the ancient Junggar ocean and amalgamation of different terranes during the Late Paleozoic to interpret the formation of the Late Paleozoic volcanic rocks in the Eastern Junggar terrane, and the Altai and Junggar terranes fully amalgamated into a Cordilleran-type orogen during the end of Early Carboniferous to the Middle–Late Carboniferous.     

Crustal contamination of mafic magmas: evidence from a petrological, geochemical and Sr–Nd–Os–O isotopic study of the Proterozoic Isortoq dike swarm, South Greenland

The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe–Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 °C and solidus temperatures are 850–930 °C. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from −3 to +1 log units relative to the fayalite–magnetite–quartz buffer.

The rocks have MgO contents <6 wt.% with Mg# between 53 and 17. Primitive mantle-normalized trace element patterns show a relative enrichment of LIL elements with Ba peaks and Nb troughs. Clinopyroxenes show a general enrichment in REE relative to chondritic values with variable slightly positive to prominent negative Eu anomalies. Two of the dikes were dated with Sm–Nd three-point isochrons at 1190±44 and 1187±87 Ma, respectively. Initial ⁸⁷Sr/⁸⁶Sr ratios of mafic mineral separates range from 0.70289 to 0.70432 and initial _Nd values vary from +0.3 to −10.7. Whole-rock initial ¹⁸⁷Os/¹⁸⁸Os ratios are highly variable including very radiogenic values of up to 7.967. δ¹⁸O_v-smow values of separated clinopyroxene and amphibole range from +5.2‰ to +6.2‰ and fall within the range of typical mantle-derived rocks, although mixing with a lower crustal component is permitted by the data. Using energy-constrained assimilation-fractional crystallization (EC-AFC) modeling equations, the Sr–Nd isotope data of the more radiogenic samples can successfully be modeled by addition of up to 10% lower crustal granulite-facies Archean gneisses as contaminants. The Os isotopic data also suggest the involvement of old radiogenic crust. In accordance with seismic data, we conclude that a wedge of Archean crust extends from West Greenland further to the south below the present erosion level.     

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.     

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t_DM model ages of 2.17–1.91 Ga and ε_Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t_DM model ages of 2.13–1.42 Ga and ε_Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr–Nd isotope compositions

The widespread Mesozoic granitoids in South China (135,300 km²) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n = 7) and FZC (n = 6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO₂ content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI = 0.97–1.08 and εNd(T) = −8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI > 1.1 and εNd(T) < −11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI < 1.0 and εNd(T) > −6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T) > −8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites.     

The Cadomian Orogeny in Saxo-Thuringia, Germany: geochemical and Nd–Sr–Pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance

The Cadomian basement and the Cambro-Ordovician overstep sequence in Saxo-Thuringia is characterized by clastic sedimentation from the Late Neoproterozoic to the Ordovician. Magmatism in the Avalonian–Cadomian Arc preserved in Saxo-Thuringia occurred between ca. 570 and 540 Ma. Peri-Gondwanan basin remnants with Cadomian to Early Palaeozoic rocks are exposed as very low-grade metamorphosed rocks in six areas (Schwarzburg Anticline, Berga Anticline, Doberlug Syncline, North Saxon Anticline, Lausitz Anticline, and Elbe Zone). A hiatus in sedimentation between 540 and 530 Ma (Cadomian unconformity) is related to the Cadomian Orogeny. A second gap in sedimentation occurred during the Upper Cambrian (500 to 490 Ma) and is documented by a disconformity between Lower to Middle Cambrian rocks and overlying Tremadocian sediments. Major and trace-element signatures of the Cadomian sediments reflect an active margin (“continental arc”), those of the Ordovician sediments a passive margin. The Cambrian sediments have inherited the arc signature through the input of relatively unaltered Cadomian detritus. Initial Nd and Pb isotope data from the six Saxo-Thuringian areas demonstrate that there is no change in source area with time for each location, but that there are minor contrasts among the locations. (1) Cadomian sediments from the Lausitz Anticline, the Doberlug Syncline and the Elbe Zone have lower ²⁰⁷Pb/²⁰⁴Pb than all other areas. (2) The core of the Schwarzburg Anticline, which is overprinted by greenschist facies conditions and detached, is isotopically heterogeneous. One part of its metasedimentary units has less radiogenic Nd than sediments from other low-grade units of similar age in the same area. (3) Cadomian sediments from the Schwarzburg Anticline show an input of younger felsic crust. (4) The Rothstein Group shows distinct input of young volcanic material. Also, (5) Cadomian sediments from the Lausitz Anticline, the Elbe Zone and parts of the North Saxon Anticline are characterized by input from an old mafic crust. Nd isotope data of the remaining areas yield average crustal residence ages of the sediment source of 1.5–1.9 Ga, which suggests derivation from an old craton as found for other parts of the Iberian–Armorican Terrane Collage. Similarly, the Pb isotope data of all areas indicate sediment provenance from an old craton.The rapid change of lithologies from greywacke to quartzite from the Late Neoproterozoic (Cadomian basement) to the Ordovician does not reflect changes in sediment provenance, but is essentially due to increased reworking of older sediments and old weathering crusts that formed during various hiatus of sedimentation. This change in sediment maturity takes its chemical expression in lower overall trace-element contents in the quartzite (dilution effect by quartz) and relative enrichment of some trace-elements (Zr, MREE, HREE due to detrital zircon and garnet). The Rb–Sr systematics of the quartzites and one Ordovician tuffite was disturbed (most likely during the Variscan Orogeny), which suggests a lithology-controlled mobility of alkali and calc-alkali elements. By comparison with available data, it seems unlikely that only Nd T_DM model ages are useful to distinguish between West African and Amazonian provenance. Nd T_DM model ages of 1.5 to 1.9 Ga in combination with paleobiogeographic aspects, age data from detrital zircon, and palaeogeographic constraints, especially through tillites of the Saharan glaciation in the Hirnantian, strongly indicate a provenance of Saxo-Thuringia from the West African Craton.     

Genesis of Pb–Zn Mineralization Beneath the Xiangshan Uranium Orefield,South China: Constraints from H–O–S–Pb Isotopes and Rb–Sr Dating

The volcanic‐hosted Xiangshan uranium orefield is the largest uranium deposit in South China. Recent exploration has discovered extensive Pb–Zn mineralization beneath the uranium orebodies. Detailed geological investigation reveals that the major metallic minerals include pyrite, sphalerite, galena, and chalcopyrite, whilst the major non‐metallic minerals include quartz, sericite, and calcite. New δ¹⁸O_fluid and δD_fluid data indicate that the ore‐forming fluids were mainly derived from magmatic, and the sulfide δ³⁴S values (2.2–6.9‰) suggest a dominantly magmatic sulfur source. The Pb isotope compositions are homogeneous (²⁰⁶Pb/²⁰⁴Pb = 18.120–18.233, ²⁰⁷Pb/²⁰⁴Pb = 15.575–15.698, and ²⁰⁸Pb/²⁰⁴Pb = 37.047–38.446). The ⁸⁷Sr/⁸⁶Sr ratios of sulfide minerals range from 0.7197 to 0.7204, which is much higher than volcanic rocks and fall into the range of metamorphic basement. Lead and strontium isotopic compositions indicate that the metallogenic materials probably were derived from metamorphic basement. Pyrite Rb–Sr dating of the ores yielded 131.3 ± 4.0 Ma, indicating that the Pb–Zn mineralization occurred in the Early Cretaceous.     

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708) and inhomogeneous negative _Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ¹⁸O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial ⁸⁷Sr/⁸⁶Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.

Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.     

Rb–Sr and Sm–Nd isotopic systematics of the Hayachine–Miyamori ophiolitic complex: Melt generation process in the mantle wedge beneath an Ordovician island arc

The Hayachine–Miyamori (HM) ophiolitic complex in the Kitakami Mountains, northeastern Japan consists of ultramafic tectonite and cumulate members. The most fertile lherzolites have mineral and trace element compositions similar to those of abyssal peridotites. They show 350–430 Ma Nd depleted mantle model ages, which are within the range of the K–Ar emplacement ages obtained from intrusive gabbroic rocks, suggesting a partial melting event just before the emplacement. The measured ¹⁴³Nd/¹⁴⁴Nd ratio of clinopyroxene in the tectonite peridotites shows positive correlation with ¹⁴⁷Sm/¹⁴⁴Nd and decreases with increasing refractoriness, which cannot be explained by a simple melting and melt extraction to a various extent followed by radiogenic ingrowth. It clearly suggests influx of a melt/fluid enriched in highly incompatible trace elements during melting. Time corrected isotopic compositions of the HM complex exhibit a clear island arc signature with uniform initial isotopic ratio (⁸⁷Sr/⁸⁶Sr = 0.7035–0.7041, ε_Nd = + 7.8–+ 5.0). Application of an open-system melting model to the observed trace element abundances in clinopyroxene suggests influx of three distinct agents to the HM mantle with the following characteristics: (1) moderate enrichment in highly incompatible elements with negative anomalies of Sr and Zr; (2) extensive enrichment of highly incompatible elements with positive Sr and negative Zr anomalies; and (3) extensive enrichment of highly incompatible elements with positive anomalies of Sr and Zr. These characteristics cover a variety of slab-derived components proposed in the literatures, suggesting the agents responsible for the open-system melting in the HM ophiolite might represent full spectrum of slab-derived components from back-arc to fore-arc regions of the Ordovician island arc system.     

Hf–Nd isotopic decoupling in continental mantle lithosphere beneath Northeast China: Effects of pervasive mantle metasomatism

Nd–Hf isotopic decoupling has frequently been observed in the continental and oceanic mantle, but its origin remains controversial. Here we present combined elemental and Sr–Nd–Hf isotopic study on peridotite xenoliths entrained in Cenozoic basalts from Shuangliao and Jiaohe in Northeast China, which provides insight into this issue. The data reveal a heterogeneous lithospheric mantle beneath Northeastern China, consisting of fertile (type I) to strongly refractory (type II) peridotites. Type I peridotites are largely shielded from late metasomatism, thus preserving information of depletion events. Nd model age suggests a Proterozoic lithospheric mantle beneath NE China. Type II peridotites are mostly refractory harzburgites and show ubiquitous enrichment of incompatible elements. They are further divided into two sub-groups. Clinopyroxenes from type IIa samples have high and wide Lu/Hf (0.34–1.3) and very radiogenic Hf isotopic ratios (ε_Hf = 44.4–63.8). Hf concentration is generally low (0.12–0.43 ppm) and plots along or slightly above the modeled partial melting depletion trend. In contrast, Nd content in type IIa clinopyroxenes is significantly higher than the modeled concentrations in residues at a given degree of melt depletion. The difference in enrichment of Hf and Nd translates to decoupling of Lu/Hf–Sm/Nd ratios and of Nd–Hf isotopes (ε_Nd = −1.3 to 8.4). Clinopyroxenes from most of type IIb peridotites have relatively low Lu/Hf ratios (0.04–0.24) and coupled Nd–Hf isotopes. Both Hf and Nd plot significantly above the depletion trend; their concentrations are governed by the equilibrium partitioning between percolating melt and peridotites. The distinct geochemical characteristics of type IIa and type IIb clinopyroxenes may have resulted from chromatographic percolation of small volumes of silicate melts, in which percolation fronts of incompatible elements are dependent on their relative incompatibilities.     

Geochronology and Sr–Nd–Hf isotopes of the Mesozoic granitoids from the Great Xing'an and Lesser Xing'an ranges: implications for petrogenesis and tectonic evolution in NE China

The Great Xing′an and Lesser Xing′an ranges are characterized by immense volumes of Mesozoic granitoids. In this study, we present major and trace element geochemistry, U–Pb geochronology and systematic Sr–Nd–Hf isotopes for the representative samples, in order to constrain their petrogenesis and the tectonic evolution in NE China. The granitoids generally have high SiO₂ (66.5–78.8 wt.%) and Na₂O + K₂O (7.0–8.9 wt.%) contents and belong to high‐K calc‐alkaline to shoshonitic series. All of them show enrichment in Rb, Th, U, Pb and light rare earth elements (LREE), and depletion in Nb, Ta, P and Ti. Zircon U–Pb dating suggests that there was continuous magmatism in both the Great Xing′an Range and the Lesser Xing′an Range during the Jurassic–Early Cretaceous interval. Seven Jurassic granitoids have (⁸⁷Sr/⁸⁶Sr)_i values of 0.704351 to 0.707374, with ϵ_Nd(t) values of −3.4 to 2.4 and ϵ_Hf(t) values of 0.8 to 11.3, indicating that they originated from mixed sources involving depleted mantle and pre‐existing crustal components. One Early Cretaceous sample yields (⁸⁷Sr/⁸⁶Sr)_i value of 0.706184, ϵ_Nd(t) value of 0.6, and ϵ_Hf(t) values of 7.0 to 8.2, which is in accordance with previous studies and indicates a major juvenile mantle source for the granitoids in this period. In the Jurassic, the magmatism in the Great Xing′an Range was induced by the subduction of the Mongol–Okhotsk Ocean, while the contemporaneous magmatism in the Lesser Xing′an Range was related to the subduction of the Palaeo‐Pacific Ocean. In the Early Cretaceous, extensive magmatism in NE China was probably attributed to large‐scale lithospheric delamination. Copyright © 2014 John Wiley & Sons, Ltd.     

Petrogenesis of the Yangchang Mo‐bearing granite in the Xilamulun metallogenic belt,NE China: geochemistry,zircon U–Pb ages and Sr–Nd–Pb isotopes

The Yangchang granite‐hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA‐ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc‐alkaline with K₂O/Na₂O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large‐ion lithophile elements (LILEs) and depleted in high‐field‐strength elements (HFSEs) with negative Nb and Ta anomalies in primitive‐mantle‐normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar ²⁰⁶Pb/²⁰⁴Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the ε_Nd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo‐Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.     

Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China

The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at 507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km², composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO₂=69.85–72.35%, K₂O/Na₂O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)_N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb=16.707–17.055, ²⁰⁷Pb/²⁰⁴Pb=15.239–15.326 and ²⁰⁸Pb/²⁰⁴Pb=37.587–37.853, which are similar to that of the continental lower crust. Their _Nd(t) values range from −16 to −20, and depleted-mantle Nd model ages (T_DM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.     

Emplacement age and isotope geochemistry of Sung Valley alkaline–carbonatite complex, Shillong Plateau, northeastern India: implications for primary carbonate melt and genesis of the associated silicate rocks

The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite Nd (+0.7 to +1.8) and Sr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite.     

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. ⁴⁰Ar/³⁹Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K₂O + Na₂O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (⁸⁷Sr/⁸⁶Sr = 0.70454–0.71264, ¹⁴³Nd/¹⁴⁴Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (⁸⁷Sr/⁸⁶Sr = 0.70466–0.70892, ¹⁴³Nd/¹⁴⁴Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial ⁸⁷Sr/⁸⁶Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (³He/⁴He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.     

Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks

Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/Yb_CN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (_Nd from −9.3–−20.1) isotopic compositions. T_DM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb=16–17.3, ²⁰⁷Pb/²⁰⁴Pb=15.1–15.6, ²⁰⁸Pb/²⁰⁴Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.     

SIMS zircon U‐Pb dating from bentonites in the Penglaitan Global Stratotype Section for the Guadalupian–Lopingian boundary (GLB), South China

The Penglaitan section, as the Global Stratotype Section for the Guadalupian–Lopingian boundary (GLB), displays continuous deposition with a complete succession of pelagic conodont zones across the GLB. However, there is no reliable radiometric age from the Penglaitan section itself to constrain the GLB. Here, we report SIMS zircon U‐Pb ages from two bentonite layers (Bed 7c) in the Penglaitan Global Stratotype Section near the GLB. The sample PL‐62‐1 yields a weighted mean ²³⁸U/²⁰⁶Pb age of 257.1 ± 2.2 Ma, and the sample PL‐62‐2 yields a weighted mean ²⁰⁶Pb/²³⁸U age of 257.0 ± 4.2 Ma. Therefore, we consider 257.0–257.1 Ma as the age of deposition of Bed 7c (the end of the C. postbitteri postbitteri conodont Zone, ca. 86 cm above the GLB), and, considering the depositional rate of chert, we suggest 258.6 Ma as the age of the GLB. Copyright © 2015 John Wiley & Sons, Ltd.     

Conodont biostratigraphy across the Permian–Triassic boundary at the Dawen section, Great Bank of Guizhou, Guizhou Province, South China: Implications for the Late Permian extinction and correlation with Meishan

Several continuous Permian–Triassic boundary (PTB) sections are well exposed in the interior of the Great Bank of Guizhou (GBG) on the east limb of the Bianyang syncline, Luodian County, Guizhou Province, South China. Fourteen conodont taxa are identified, including Clarkina kazi, Clarkina lehrmanni n. sp., Clarkina taylorae, Clarkina zhejiangensis, Hindeodus eurypyge, Hindeodus inflatus, Hindeodus sxlatidentatus, Hindeodus parvus erectus, Hindeodus parvus parvus, Hindeodus postparvus, Hindeodus praeparvus, Hindeodus typicalis, Isarcicella staeschei, and Merrillina ultima based on a detailed study of the Permian-Triassic interval at the Dawen section. The first occurrence (FO) of H. parvus parvus in the lower Daye Formation, at about 7.45 m above the contact surface between the Upper Permian skeletal packstone and a calcimicrobial framestone unit, is correlated with the Permian–Triassic boundary; the occurrence of H. eurypyge, H. praeparvus and M. ultima immediately below and H. postparvus above the interval supports this interpretation. A morphometric analysis of 31 Hindeodus specimens helped distinguish H. parvus erectus from H. parvus parvus and H. postparvus. Correlation with the Meishan section (PTB GSSP) using both conodont biostratigraphy and carbon isotopes, indicates that the major extinction at the two localities is simultaneous and coincides with the top of the skeletal packstone at Dawen. The contact between the skeletal packstone and the calcimicrobialite is very irregular and has previously been interpreted as a dissolution surface and correlated with a surface in the lower part of bed 27 at Meishan. Our results confirm this interpretation and reveal that the dissolution event postdated the extinction.     

Constraints from monazite and xenotime growth modelling in the MnCKFMASH‐PYCe system on the P–T path of a metapelite from Shillong‐Meghalaya Plateau: implications for the Indian shield assembly

The Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north‐trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back‐arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back‐arc setting.