Source characteristics of the ∼2.5 Ga Wangjiazhuang Banded Iron Formation from the Wutai greenstone belt in the North China Craton: Evidence from neodymium isotopes

Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive ε_Nd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative ε_Nd(t) values, whereas Al-rich BIF samples possess consistently positive ε_Nd(t) features. Significantly, the associated supracrustal rocks in the study area have positive ε_Nd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (ε_Nd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (ε_Nd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (ε_Nd(t) > 0).     

Geochemistry,petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen,South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834 Ma) and the late Neoproterozoic (770–750 Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive ε_Nd(t) values (+ 2.7 to + 6.6) whereas mafic rocks exhibit negative ε_Nd(t) values (− 5.8 to − 0.9). The basaltic rocks show TiO₂ contents of 0.62–0.69 wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show ε_Nd(t) values (− 1.23 to + 3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO₂ contents of 1.01–1.33 wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma.     

Petrogenesis of Carboniferous adakites and Nb-enriched arc basalts in the Alataw area,northern Tianshan Range (western China): Implications for Phanerozoic crustal growth in the Central Asia orogenic belt

Carboniferous volcanic rocks in the Alataw area, Northern Tianshan Range (Xinjiang), consist of early Carboniferous (ca. 320 Ma) adakites and Nb-enriched arc basalts and basaltic andesites (NEBs), and late Carboniferous (ca. 306–310 Ma) mainly high-K calc-alkaline andesites, dacites and rhyolites. The adakites are calc-alkaline, and characterized by high Na₂O/K₂O (1.52–3.32) ratios, negligible to positive Eu anomalies, strong depletion of heavy rare earth elements (e.g., Yb = 0.74–1.47 ppm) and Y (6.7–14.9 ppm), positive Sr and Ba but negative Nb and Ti anomalies, and relatively constant ε_Nd(T) values (+ 3.4–+ 6.6) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7035–0.7042). Some andesitic and dacitic adakite samples exhibit high MgO contents similar to magnesian andesites. The NEBs are sodium-rich (Na₂O/K₂O = 2.03–8.06), and differ from the vast majority of arc basalts in their higher Nb, Zr, TiO₂ and P₂O₅ contents and Nb/Th, Nb/La and Nb/U ratios, and minor negative to positive anomalies in Ba, Nb, Sr, Zr and Ti. They have the highest ε_Nd(T) values (+ 6.4–+ 11.6) but varying (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7007–0.7063). The high-K calc-alkaline suite is similar to typical ‘normal’ arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative Eu, Nb, Sr and Ti anomalies. They have ε_Nd(T) values (+ 1.2–+ 6.4) and (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7018–0.7059). Geochemically, they are similar to coeval I-type granitoids in the Alataw area. Given the presence of early Carboniferous ophiolites in the Northern Tianshan Range, and the isotopically inappropriate compositions of Proterozoic metamorphic basement in the Alataw area, we argue that the Alataw adakites were most probably related to the melting of young subducted crust of the Northern Tianshan Ocean. The NEBs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab-derived fluids. The later high-K calc alkaline suite was generated by AFC processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids. The larger range of isotopic compositions exhibited by both the NEB and high-K suite, relative to the adakites, suggests that the mantle wedge was heterogeneous prior to slab- or fluid-mediated metasomatism.Continental crustal growth of the Central Asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 Ma and possibly afterwards. The results of this study suggest that other Carboniferous Nb-enriched basalts in the Tianshan Range were generated by subduction processes rather than by intraplate tectonics as previously proposed.     

Petrogenesis of gold-mineralized magmatic rocks of the Taerbieke area,northwestern Tianshan (western China): Constraints from geochronology,geochemistry and Sr–Nd–Pb–Hf isotopic compositions

Many Late Paleozoic Cu–Au–Mo deposits occur in the Central Asian Orogenic Belt (CAOB). However, their tectonic settings and associated geodynamic processes have been disputed. This study provides age, petrologic and geochemical data for andesites and granitic porphyries of the Taerbieke gold deposit from the Tulasu Basin, in the northwestern Tianshan Orogenic Belt (western China). LA-ICP-MS zircon U–Pb dating indicates that the granitic porphyries have an Early Carboniferous crystallization age (349 ± 2 Ma) that is broadly contemporaneous with the eruption age (347 ± 2 Ma) of the andesites. The andesites have a restricted range of SiO₂ (58.94–63.85 wt.%) contents, but relatively high Al₂O₃ (15.39–16.65 wt.%) and MgO (2.51–6.59 wt.%) contents, coupled with high Mg^# (57–69) values. Geochemically, they are comparable to Cenozoic sanukites in the Setouchi Volcanic Belt, SW Japan. Compared with the andesites, the granitic porphyries have relatively high SiO₂ (72.68–75.32 wt.%) contents, but lower Al₂O₃ (12.94–13.84 wt.%) and MgO (0.10–0.33 wt.%) contents, coupled with lower Mg^# (9–21) values. The andesites and granitic porphyries are enriched in both large ion lithophile and light rare earth elements, but depleted in high field strength elements, similar to those of typical arc magmatic rocks. They also have similar Nd–Hf–Pb isotope compositions: ε_Nd(t) (+0.48 to +4.06 and −0.27 to +2.97) and zircons ε_Hf(t) (+3.4 to +8.0 and −1.7 to +8.2) values and high (²⁰⁶Pb/²⁰⁴Pb)_i (18.066–18.158 and 17.998–18.055). We suggest that the Taerbieke high-Mg andesitic magmas were generated by the interaction between mantle wedge peridotites and subducted oceanic sediment-derived melts with minor basaltic oceanic crust-derived melts, and that the magmas then fractionated to produce the more felsic members (i.e., the Taerbieke granitic porphyries) during late-stage evolution. Taking into account the Carboniferous magmatic record from the western Tianshan Orogenic Belt, we suggest that the formation of the Early Carboniferous andesites and granitic porphyries in the Taerbieke area were related to the Paleo-Junggar Oceanic plate southward subduction under the Yili–Central Tianshan plate. The close association of the Early Carboniferous magmatic rocks and Au mineralization in the Taerbieke area suggests that the arc magmatic rocks in the Tulasu basin may have a high potential for Au mineralization.     

In situ zircon U–Pb and Hf–O isotopic results for ca. 73 Ma granite in Hainan Island: Implications for the termination of an Andean-type active continental margin in southeast China

We report in the paper integrated analyses of in situ zircon U–Pb ages, Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes for the Longlou granite in northern Hainan Island, southeast China. SIMS zircon U–Pb dating results yield a crystallization age of ∼73 Ma for the Longlou granite, which is the youngest granite recognized in southeast China. The granite rocks are characterized by high SiO₂ and K₂O, weakly peraluminous (A/CNK = 1.04–1.10), depletion in Sr, Ba and high field strength elements (HFSE) and enrichment in LREE and large ion lithophile elements (LILE). Chemical variations of the granite are dominated by fractional crystallization of feldspar, biotite, Ti–Fe oxides and apatite. Their whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7073–0.7107) and ε_Nd(t) (−4.6 to −6.6) and zircon ε_Hf(t) (−5.0 to 0.8) values are broadly consistent with those of the Late Mesozoic granites in southeast China coast. Zircon δ¹⁸O values of 6.9–8.3‰ suggest insignificant involvement of supracrustal materials in the granites. These granites are likely generated by partial melting of medium- to high-K basaltic rocks in an active continental margin related to subduction of the Pacific plate. The ca. 73 Ma Longlou granite is broadly coeval with the Campanian (ca. 80–70 Ma) granitoid rocks in southwest Japan and South Korea, indicating that they might be formed along a common Andean-type active continental margin of east–southeast Asia. Tectonic transition from the Andean-type to the West Pacific-type continental margin of southeast China likely took place at ca.70 Ma, rather than ca. 90–85 Ma as previously thought.     

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.     

The Late Neoproterozoic rift-related metarhyolite–basalt association of the Glushikha trough (Yenisei Ridge): petrogeochemical composition,age, and formation conditions

New data are presented on the petrogeochemical composition, age, and formation conditions of the Late Neoproterozoic metarhyolite–basalt association of the Glushikha trough (Yenisei Ridge). The association is localized within the subaerial and shallow-water terrigenous-carbonate sediments of the Orlovka Group, which overlies Proterozoic rocks with unconformity. The felsic volcanics are essentially potassic and enriched in Rb, U, Th, and Fe. They show a weakly fractionated REE pattern with a prominent negative Eu anomaly. The basalts and picrite basalts have higher contents of Ti, Fe, P, HFSE, REE, U, Th, Ba, and Sr, and their spidergrams show no Nb or Ta depletion with respect to Th and LREE. These rocks have the petrochemical parameters of intraplate magmatic associations in continental rift zones. New geochronological data (SHRIMP II) on single zircon grains from the felsite porphyry of the metarhyolite–basalt association (717 ± 15 Ma) indicate Late Neoproterozoic volcanism in the Yenisei part of the Central block of the Trans-Angara region. According to Sm–Nd isotopic data, the rhyolites originate from Paleoproterozoic crust (T_Nd(DM) = 1757 Ma; T_Nd(DM-2st) = 1651 Ma; ∑ _Nd(T) = ? 2.7). The Orlovka volcanosedimentary rocks are rift-related, as evidenced by the following facts: (1) localization of the volcanosedimentary rocks in a narrow fault-line trough; (2) bimodal rhyolite-basaltic composition of the volcanics; and (3) petrology and geochemistry of the picrite basalts and basalts, typical of intraplate environments. The studies show that Late Neoproterozoic rifting and intraplate plume magmatism took place not only in the Tatarka–Ishimba fault zone but also in the Yenisei fault zone of the Yenisei Ridge.     

Paleoproterozoic bimodal post-collisional magmatism in the southwestern Amazonian Craton,Mato Grosso,Brazil: Geochemistry and isotopic evidence

This paper discusses geological and geochemical aspects of a Paleoproterozoic volcano-plutonic association that crops out in southwestern Amazonian Craton, Mato Grosso, Brazil. The study area was divided into undeformed and deformed domains, based on structural and geochronology studies. The undeformed domain is composed mainly of felsic explosive and effusive flows. Inter-layered mafic flows of basalts and sedimentary rocks are also present. The deformed domain is mainly composed of titanite and hornblende-bearing monzogranite to syenogranite and biotite monzogranite, while granodiorite is less common. U–Pb single zircon analyses yielded ages of 1.8–1.75 Ga in granites and felsic volcanic rocks for both domains. Basalts from the undeformed domain are phaneritic, fine-grained, and are often hydrothermally altered. They show tholeiitic affinity and are LREE enriched. Their trace element composition resembles those of within-plate associations. The ε_Nd (t = 1.75 Ga) for all these rocks are positive, ranging from 0.12 to 1.49, which reflect a juvenile source. The felsic volcanism comprises subalkaline rocks with high K contents and is divided into two groups: crystal enriched ignimbrites and effusive rhyolites. REE patterns of effusive rocks show negative-Eu anomalies and are smooth in the ignimbrites. Trace element patterns similar to those of the effusive rocks and ignimbrites are found in magmatic rocks derived from sources affected by subduction-related metasomatism. Hornblende and biotite granites occur in the deformed felsic plutonic domain. These rocks show evidence of low-temperature metamorphism and deformation, and in some places, of hydrothermal alteration. The LREE/Nb (or Ta) ratios of these rocks are consistent with those observed in granites of post-collisional settings. The ε_Nd (t = 1.75 Ga) values are slightly negative on average, with few positive values (?1.41 to +2.96). These data are interpreted as indicative of a magmatism produced during a post-collisional event from mixed sources: a metasomatised mantle and a Paleoproterozoic continental crust. An intracontinental shearing with age of 1.7–1.66 Ga created the Teles Pires–Juruena lineament which partially controlled this magmatism.     

Magma mixing origin for the Aolunhua porphyry related to Mo–Cu mineralization,eastern Central Asian Orogenic Belt

Many Cu–Mo–Au deposits are considered to be related to adakitic porphyries formed in non-arc settings, e.g., in collisional orogenic zones and intra-plate environments, but their genesis is still under discussion. The Aolunhua porphyry complex and its related Mo–Cu deposit from the eastern Central Asian Orogenic Belt (CAOB) provide important insights into this issue. The porphyries are characterized by high Sr (496–705 ppm) and Sr/Y and La/Yb ratios similar to those of typical adakitic rocks, and low I_Sr ratios (0.7049–0.7052) and positive ε_Nd(t) (+ 0.5 to + 1.4) and ε_Hf(t) (+ 3.5 to + 9.8) values. These features, along with the occurrence of mafic microgranular enclaves (MMEs), compositional and textural disequilibrium of plagioclase phenocrysts and relatively high Mg# values (45–52), indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc-type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids/melts. High Sr/Y and La/Yb ratios are indicative of contribution from enriched mantle-derived materials (with high LILEs; e.g., Sr, La), which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende. MMEs hosted by the ore-bearing porphyry have zircon U–Pb ages of ca. 132 Ma, similar to those of the host rocks. The enclaves have elevated Mg^# (56–63), LILEs (e.g., Sr = 660–891 ppm), LREE (La_N = 68–150, (La/Sm)_N = 3.0–4.0, (La/Yb)_N = 12.0–19.6) and ratios of radiogenic isotopes of Nd- and Hf (ε_Nd = + 0.7 to + 1.6; ε_Hf = + 3.3 to + 10.9), suggesting that their parental magmas were derived from the metasomatized mantle source. The Mo–Cu mineralization was probably related to the high water content, high oxygen and sulfur fugacity of hybrid magma. Formation of the adakitic porphyries and related Mo–Cu deposits of the eastern CAOB could be related to the Early Cretaceous lithospheric extension, caused by the subduction of the Paleo-Pacific plate and its induced reactivation of juvenile arc-type lower crust.     

Incremental growth and origin of the Cretaceous Renjiayingzi pluton,southern Inner Mongolia,China: Evidence from structure,geochemistry and geochronology

The Renjiayingzi intermediate-acid pluton is located along a pre-existing ENE–WSW-trending dextral shear zone that forms part of the Xar Moron suture zone that marks the final closure of the Paleo-Asian Ocean. The pluton is composed of three small intrusions, which from northwest to southeast, are named the Shuangjianshan (SI), the Qianweiliansu (QI) and the Xingshuwabeishan (XI) intrusions. LA-ICPMS zircon U–Pb dating of a pyroxene diorite from the SI yields an age of 138 ± 1 Ma; the SHRIMP zircon U–Pb age of a tonalite from the QI records an age of 134 ± 2 Ma, whereas LA-ICPMS zircon U–Pb dating of a monzogranite from the XI has an age of 126 ± 1 Ma, suggesting the entire pluton was built up by three separate emplacement events that young to the ESE: this is further supported by the contact relations. Incremental growth of plutons by amalgamation of repeated small magma pulses is the most viable emplacement model. The pluton was probably emplaced by updoming of the roof along previous tensile fractures and by upward stacking of the three intrusions. The SI and QI have similar U–Pb ages and geochemical characteristics, and most likely had the same magma source and underwent similar petrogenetic processes. They have high MgO concentrations at low silica contents, are enriched in large ion lithophile elements, depleted in high field strength elements, have negative ε_Nd(t) values of −1.8 to −3.7, with Nd model ages of 1.07–1.19 Ga. Pyroxene diorites of the SI also have variable zircon ε_Hf(t) values (from −0.8 to +6.1), indicating that they were mainly derived from juvenile crust with minor crustal contamination and clinopyroxene-dominated fractional crystallization. The late monzogranites from the XI show weak negative ε_Nd(t) values of −2.3 to −2.5, young Nd model ages of 0.99–1.00 Ga, positive zircon ε_Hf(t) values (+1.3 to +4.6) and higher SiO₂ and K₂O contents, with strong depletion in Eu, P and Ti, indicating derivation from a distinct petrogenetic process from the two earlier intrusions. The monzogranites were the result of partial melting of juvenile crust in response to mantle-derived magma underplating, together with plagioclase-dominated fractional crystallization.     

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia,North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.     

Ordovician igneous and metamorphic units in southeastern Puna: New U–Pb and Sm–Nd data and implications for the evolution of northwestern Argentina

New field, petrological, geochemical, and geochronological data (U–Pb and Sm–Nd) for Ordovician rock units in the southeastern Puna, NW Argentina, indicate two lithostratigraphic units at the eastern–northeastern border of salar Centenario: (1) a bimodal volcanosedimentary sequence affected by low- to medium-grade metamorphism, comprising metasediments associated with basic and felsic metavolcanic rocks, dated 485 ± 5 Ma, and (2) a plutonic unit composed of syenogranites to quartz-rich leucogranites with U–Pb zircon ages between 462 ± 7 and 475 ± 5 Ma. Felsic metavolcanic and plutonic rocks are peraluminous and show similar geochemical differentiation trends. They also have similar Sm–Nd isotopic compositions (T_DM model ages of 1.54–1.78 Ga; ε_Nd(T) values ranging from −3.2 to −7.5) that suggest a common origin and derivation of the original magmas from older (Meso-Paleoproterozoic?) continental crust. Mafic rocks show ε_Nd(T) ranging from +2.3 to +2.5, indicating a depleted mantle source. The data presented here, combined with those in the literature, suggest Ordovician magmatism mainly recycles preexisting crust with minor additions of juvenile mantle-derived material.     

Geochronology and geochemistry of Cretaceous Nanshanping alkaline rocks from the Zijinshan district in Fujian Province,South China: Implications for crust–mantle interaction and lithospheric extension

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for Nanshanping alkaline rocks from the Zijingshan district in southwestern Fujian Province (the Interior or Western Cathaysia Block) of South China. The Nanshanping alkaline rocks, which consist of porphyritic quartz monzonite, porphyritic syenite, and syenite, revealed a Late Cretaceous age of 100–93 Ma. All of the rocks show high SiO₂, K₂O + Na₂O, and LREE but low CaO, Fe₂O₃^T, MgO, and HFSE (Nb, Ta, P, and Ti) concentrations. These rocks also exhibit uniform initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7078 to 0.7087 and ε_Nd(t) values of −4.1 to −7.2, thus falling within the compositional field of Cretaceous basalts and mafic dikes occurring in the Cathaysia Block. Additionally, these rocks display initial Pb isotopic compositions with a ²⁰⁶Pb/²⁰⁴Pb_i ratio of 18.25 to 18.45, a ²⁰⁷Pb/²⁰⁴Pb_i ratio of 15.63 to 15.67, and a ²⁰⁸Pb/²⁰⁴Pb_i ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (ε_Hf(t) = −11.7 to −3.2), which are different from those of the basement rocks, we suggest that Nanshanping alkaline rocks were primarily derived from a subduction-related enriched mantle source. High Rb/Sr (0.29–0.65) and Zr/Hf (37.5–49.2) but relatively low Ba/Rb (4.4–8.1) ratios suggest that the parental magmas of these rocks were most likely formed via partial melting of a phlogopite-bearing mantle source with carbonate metasomatism. The relatively high SiO₂ (62.35–70.79 wt.%) and low Nb/Ta (10.0–15.3) ratios, positive correlation between SiO₂ and (⁸⁷Sr/⁸⁶Sr)_I, and negative correlation between SiO₂ and ε_Nd(t) of these rocks suggest that the crustal materials were also involved in formation of the Nanshanping alkaline rocks. Combined with geochemical and isotopic features, we infer magmatic processes similar to AFC (assimilation and fractional crystallization) involving early fractionation of clinopyroxene and olivine and subsequent fractionation of biotite-dominated assemblages coupled with a lesser amount of crustal contamination, thereby forming the Nanshanping alkaline rocks. The Nanshanping alkaline rocks appear to be associated with an extensional environment in the Cathaysia Block. This extensional regime could have resulted in the slab break-off and rollback of the subducting paleo-Pacific plate and the upwelling of the asthenospheric mantle, which induced partial melting of the enriched lithospheric mantle in the Cretaceous.     

Gold-hosting high Ba-Sr granitoids in the Xincheng gold deposit,Jiaodong Peninsula,East China: Petrogenesis and tectonic setting

The Xincheng deposit is the only large gold deposit with a proven reserve of >200 t gold hosted by the Early Cretaceous granitoids in northwest Jiaodong Peninsula, East China. The granitoids hosting this ore deposit comprise an inner medium- to fine-grained quartz monzonite and an outer medium- to coarse-grained monzogranite with distinctive K-feldspar megacrysts. LA–ICP–MS zircon dating yields U–Pb ages of 128 ± 1 to 132 ± 1 Ma and 127 ± 2 to 129 ± 1 Ma, for the quartz monzonite and the monzogranite, respectively. The Early Cretaceous ages obtained in our study are comparable with the 126–130 Ma age range reported for the Guojialing granitic suite. The monzogranites, typical high Ba–Sr granites, possess high SiO₂ (70.89–73.35%), K₂O (3.85–4.32%), total alkalis (K₂O + Na₂O = 8.08–8.68%), Sr (634–888 ppm), Ba (1395–2111 ppm) and LREE (59.43–145.88), with low HREE and HFSE contents and insignificant Eu anomalies. The rocks display markedly high Sr/Y (114–297) and (La/Yb)_N (20–79) ratios. They have low MgO (0.23–0.62%), Cr (0.4–8.33 ppm) and Ni (0.47–2.92 ppm) contents. The typical high Ba–Sr signatures of the outer acidic monzogranites are also shared by the inner intermediate-acidic quartz monzonites, with a relatively higher abundance of these elements. The plagioclases in the quartz monzonites and monzogranites are oligoclase–andesine with An contents of 11.7–44.5%, and oligoclase with An contents of 12.9–29.3%, respectively, which both show the reverse zoning texture. The quartz monzonites have zircon ε_Hf(t) values of −21.3 to −13.9 (average −18.7), which are less negative and show larger variations than those of the monzogranites (ε_Hf(t) = −24.7 to −18.1, average −19.5). Detailed elemental, mineralogical and isotopic data suggest that the high Ba–Sr quartz monzonites and monzogranites were most likely generated by partial melting of the basement rocks of the Jiaobei terrane accompanied by crustal assimilation, with minor addition of the intermediate magma derived from the partial melting of juvenile mafic lower crust formed by the earlier underplating of mantle magma, and the quartz monzonites may represent the path of intermediate magma inputting into felsic magma. In combination with previous investigations, we suggest subduction of the paleo-Pacific slab beneath the North China Craton (NCC) and associated asthenosphere upwelling were most likely the mechanism associated with the generation of the high Ba–Sr granites.     

Shrimp U–Pb zircon geochronology,geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province,SW China

The Cida A-type granitic stock (∼ 4 km²) and Ailanghe I-type granite batholith (∼ 100 km²) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO^⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (ε_Nd(t) = − 0.25 to + 1.35 and (⁸⁷Sr/⁸⁶Sr)_i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε_Nd(t) ratios (− 6.34 to − 6.26) and high (⁸⁷Sr/⁸⁶Sr)_i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.     

Evolution of the Solonker suture zone: Constraints from zircon U–Pb ages,Hf isotopic ratios and whole-rock Nd–Sr isotope compositions of subduction- and collision-related magmas and forearc sediments

The Solonker zone in northern Inner Mongolia (China) is considered as the suture between the North China Craton and the South Mongolian microcontinent. Two magmatic belts are recognized along the suture zone: a subduction-related magmatic belt (represented by the Baolidao arc rocks), and a younger, collision-related granite belt (represented by the Halatu granites). We use zircon U–Pb ages, zircon in-situ Hf isotopic analyses and whole-rock Nd–Sr isotopic data of the two magmatic belts and related forearc sediments (the Xilinhot metamorphic complex) to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks. A gabbroic diorite (BLD-1) of the Baolidao arc was dated at 310 ± 5 Ma (by SHRIMP). This sample shows an ε_Nd(t) value of +2.5 and I_Sr of 0.7052. Hf isotopic analyses on 25 zircons from the same sample show ε_Hf(t) = +5.4 to +11.5. Another diorite sample (XH-2) of the same arc from south of Xilinhot displays even more “depleted” isotopic compositions, with ε_Nd(t) = +5.6 and I_Sr = 0.7037. The main population of zircons from this sample have highly variable and depleted Hf isotopic compositions (ε_Hf(t) = 0–18.3). The large variation in Hf isotopic composition of zircons (with largely the same crystallization age) from a single pluton is explained by a mixing process between depleted mantle-derived magma and continental crust in an active continental arc setting. The Halatu granite (HLT-2) was dated at 234 ± 7 Ma (by SHRIMP). Zircons from the granite also show a large variation of ε_Hf(t) values (+9.1 to ?26), despite most samples having whole-rock ε_Hf(t) > +2. The large variation in ε_Hf(t) values suggests that the granite formed probably by partial melting of two source regions – a dominant juvenile crust and a subordinate old continental crust. Most zircons from the Xilinhot metamorphic complex show ages comparable with those of the Baolidao arc rocks, suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism. Some zircons, however, show Precambrian ages that fall into two groups: one with ages of 780–900 Ma, resembling those from the South Mongolian microcontinent, and the other with ages of 1524–2900 Ma, similar to those of the North China Craton. Thus, the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents, and metamorphosed subsequently during collision in the late Paleozoic. Our zircon age data thus constrain timing of collision between the South Mongolian microcontinent and the North China Craton to have been between 296 and 234 Ma.     

Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution

Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean ²⁰⁶Pb/²³⁸U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO₂ contents (45.2–52.7 wt.%) and total alkaline (K₂O + Na₂O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/Yb_N = 0.86–1.1), together with their high ε_Nd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/Yb_N = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low ε_Nd(t) values (+ 3.4 to + 5.4) and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give ε_Hf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai.     

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 495–487 Ma, 121 Ma, 89 Ma, and 70–63 Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high ε_Nd(t) and ε_Hf(t) values of − 1.06 to − 3.45 and − 1.16 to 2.09, and model ages of 1.16 Ga to 1.33 Ga and 1.47 Ga to 1.63 Ga, 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 ε_Nd(t) and ε_Hf(t) values of − 8.92 and − 4.91 with Nd and Hf model ages of 1.41 Ga and 1.49 Ga, 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 ε_Nd(t) and ε_Hf(t) values of − 9.58 and − 4.61 with Nd and Hf model ages of 1.43 Ga and 1.57 Ga, 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 ε_Nd(t) and ε_Hf(t) values of − 4.41 to − 10 and − 5.95 to − 8.71, Nd model ages ranging from 1.08 Ga to 1.43 Ga, and Hf model ages from 1.53 Ga to 1.67 Ga, 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 and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli–Erguna area of Inner Mongolia,China: Geochronological,geochemical and Hf isotopic evidence

U–Pb dating and Hf isotopic analyses of zircons from various granitoids, combined with major and trace element analyses, were undertaken to determine the petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli–Erguna area of Inner Mongolia, China. The Neoproterozoic granitoids are mainly biotite monzogranites with zircon U–Pb ages of 894 ± 13 Ma and 880 ± 10 Ma, and they are characterised by enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, K) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and heavy rare earth elements (HREEs). The Late Devonian granitoids are dominantly syenogranites and mylonitised syenogranites with zircon U–Pb ages of 360 ± 4 Ma, and they form a bimodal magmatic association with subordinate gabbroic rocks of the same age. The Late Devonian syenogranites have A-type characteristics including high total alkalis, Zr, Nb, Ce and Y contents, and high FeOt/MgO, Ga/Al and Rb/Sr ratios. The Carboniferous granitoids are mainly tonalites, granodiorites and monzogranites with U–Pb ages varying from 319 to 306 Ma, and they show very strong adakitic characteristics such as high La/Yb and Sr/Y ratios but low Y and Yb contents. The Late Permian granitoids are dominated by monzogranites and syenogranites with zircon U–Pb ages ranging between 257 and 251 Ma. Isotopically, the ε_Hf(t) values of the Neoproterozoic granitoids range from +4.3 to +8.3, and the two-stage model ages (T_DM2) from 1.2 to 1.5 Ga. The Late Devonian granitoids are less radiogenic [ε_Hf(t) from +12.0 to +12.8 and T_DM2 from 545 to 598 Ma] than the Carboniferous [ε_Hf(t) from +6.8 to +9.5 and T_DM2 from 722 to 894 Ma] and Late Permian granitoids [ε_Hf(t) from +6.1 to +9.4 and T_DM2 in the range of 680–895 Ma]. These data indicate (1) the Neoproterozoic granitoids may have been generated by melting of a juvenile crust extracted from the mantle during the Mesoproterozoic, probably during or following the final stages of assembly of Rodinia as a result of the collision and amalgamation of Australia and the Tarim Craton; (2) the Late Devonian granitoids may have formed by partial melting of a new mantle-derived juvenile crust in a post-orogenic extensional setting; (3) the Carboniferous granitoids appear to have been produced by melting of garnet-bearing amphibolites within a thickened continental crust during and following the collision of the Songnen and Erguna–Xing’an terranes; and (4) the Late Permian granitoids may have been generated by melting of garnet-free amphibolites within the Neoproterozoic juvenile continental crust, probably in the post-collisional tectonic setting that followed the collision of the North China and Siberian cratons.     

Reprint of “Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area,eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks”

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0 Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246 Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have ε_Hf(t) values varying from − 0.7 to 7.6, which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs. ε_Hf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (T_DM = 2.9–3.4 Ga). In addition, the remarkably high ε_Hf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO₂ (46.5 to 52.8 wt.%) and high MgO (5.68 to 10.9 wt.%) contents, crust-like trace element features and low ε_Nd(t) values (− 4.5 to − 0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.