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Secondary ion mass spectrometry (SIMS) has been applied to analyze a wide range of materials for earth science research due to its high sensitivity, high precision and capacity for in situ micro-analysis. This technique operates under ultra-high vacuum (UHV) conditions, especially for water content measurements on nominally anhydrous minerals (NAMs). However, UHV conditions are hard to achieve when using epoxy mounts, which degas readily during the vacuum processes. In this study, we developed a novel sample preparation method using a tin-based alloy instead of epoxy resin. Using a tin alloy mount coupled with an automatic liquid nitrogen refilling system, the results show that the vacuum conditions have been significantly improved from ∼1.2 × 10−8 mbar to ∼1.9 × 10−9 mbar. The background for mineral water content, measured with a CAMECA IMS 1280-HR SIMS installed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), is lowered to ＜10 ppm. Moreover, tin alloy mounts are harder and easier to polish than indium mounts, which was commonly used for previous UHV analyses. A relief of ＜2 μm is yielded for tin alloy mounts, which is essential for high precision isotope analysis. Our results also show that the routine external precision of oxygen isotope ratios is better than 0.15‰ (1SD), comparable with that obtained using epoxy mounts. [ABSTRACT FROM AUTHOR]

The occurrence of extensional-related A-type granites with a high-K nature in some early Precambrian blocks hints a possible link with the orogeny, and may imply the ending of the orogenic processes and the beginning of craton stabilization. The newly recognized late Paleoproterozoic A-type granite, the Guijiayu pluton composed of monzogranites in the Xiaoqinling area of the southern margin of the North China Craton (NCC), is a promising locus to investigate the late Paleoproterozoic tectonic evolution of the NCC. LA-ICP-MS zircon U–Pb dating shows that the Guijiayu pluton was emplaced at ∼1.80 Ga. Petrologically, the Guijiayu granites contain perthite, annite and calcic amphibole but are lack of alkali mafic minerals. Geochemically, all the rocks are enriched in silicon and total alkali, and depleted in MgO, CaO and P 2 O 5 , with high FeOt/MgO, TiO 2 /MgO and 10,000 Ga/Al ratios and high HFSEs (i.e., Zr, Nb, Ce, Y) concentrations, as well as negative Eu, Sr, P and Ti anomalies. They are also characterized by high melting temperatures and metaluminous to weakly peraluminous natures. Taken together, the Guijiayu granites show an affinity to aluminous A-type granite. Their geochemical characteristics (e.g., obviously negative Eu and Sr anomalies), along with the low zircon ε Hf ( t ) values (−13.9 to −7.0) and whole-rock ε Nd ( t ) values (−5.81 to −5.65), indicate that the Guijiayu aluminous A-type granite was derived from partial melting of the crustal calc-alkaline granitoids dominated by the granodiorite of the Taihua Group in the study area. Combined with the regional data, especially for the identification of the clockwise P–T paths of Paleoproterozoic metamorphism in the southern margin of the NCC, even coeval granites and regional retrograde metamorphism in other areas in the NCC, it is most likely that the peak of continent–continent collision had finished in the study area at least before ∼1.80 Ga, and that the generation of the Guijiayu aluminous A-type granite was related to the underplating of mafic magmas in a post-collisional regime. [ABSTRACT FROM AUTHOR]

Abstract: Siliceous high-Mg basalt (SHMB) is a rare rock type that occurs mainly at or near the Archean–Proterozoic boundary. It shares some geochemical similarities with Phanerozoic boninites, but there is a clear distinction. Whether the petrogenesis of SHMB resembles that of Phanerozoic boninites or is related to the komatiitic magmatism is controversial. Neoarchean SHMBs are identified for the first time from the Taishan granite–greenstone terrane within the Eastern Block of the North China Craton (NCC). Zircon U–Pb dating indicates that they were emplaced at ∼2.54Ga, contemporaneous with the generation of sanukitoids and adakitic rocks, but later than the eruption of the Late Archean (2.71Ga) komatiites and komatiitic basalts in the area. The high MgO (>8%), high SiO2 (>52%), and Al2O3/TiO2 ratio (12.4–45.0), together with low TiO2 (<0.5%) and HFSE contents and strong enrichment in LREE and LILE in the Taishan SHMBs are comparable with typical Phanerozoic boninites, except for distinct HREE depletion, lack of U-shaped REE patterns and conspicuous positive Zr anomalies. In conjunction with their more depleted Nd isotopic characteristics (ɛ Nd(t =2.54Ga)=+4.42 to +1.05) relative to Late Archean komatiites in the region, it suggests that these SHMBs were derived from partial melting of refractory depleted mantle which experienced earlier basalt extraction and was subsequently enriched in LILE and LREE by subduction-related metasomatization, rather than the products of assimilation–fractional crystallization (AFC) of komatiitic magma. A slab-derived adakitic melt was likely the metasomatizing agent, along with minor aqueous fluids released from the subducting oceanic slab. In combination with regional studies, the generation of these magmas was probably related to slab rollback, which is ascribed to the arrival of an oceanic plateau and/or residual thickened lithospheric keel at the subduction zone at that time. This mechanism might have played a crucial role in the formation of Archean granite–greenstone belts and was an important factor in continental crustal growth, particularly during the Late Archean. [Copyright &y& Elsevier]

Abstract: A series of Late Neoarchean high Ba–Sr granites with a potassium-rich signature has been identified from the Taishan granite–greenstone terrane (TSGT), within the Eastern Block of the North China Craton (NCC). LA-ICP-MS zircon dating shows that the Wanghailou, Hujiazhuang and Xiajiazhuang granitic intrusions were emplaced at ~2.54Ga, ~2.52Ga and ~2.47Ga, respectively. Geochemically, they are alkali-rich, with high K2O contents and high K2O/Na2O ratios, revealing their high-potassium calc-alkaline nature. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to typical A-, I- and S-type granites, and show negative anomalies in HFSE (Nb, Ta, Ti) and weak Eu anomalies. Their geochemical and isotopic characteristics, such as extremely low Y and Yb concentrations, high Sr/Y and (La/Yb)cn ratios, strongly fractionated REE patterns and elevated εNd(t) and εHf(t), imply that the parental magmas were derived directly from partial melting of the sub-arc enriched mantle at depths >15kbar with residual garnet in the source. This mantle was previously depleted lithospheric mantle and was subsequently metasomatized by subduction-related fluids and/or melts. Fractional crystallization and crustal assimilation of these magmas during ascent resulted in their variable geochemical and isotopic characteristics. Taking into account the space–time patterns of late Archean magmatism in the area, an active subduction regime is favored to account for the generation of these potassic high Ba–Sr granites in the TSGT. The two-stage Nd and Hf model ages of these granites suggest that continental crust older than 2.80Ga is preserved beneath the TSGT. In combination with previous data from the region, it is noted that, apart from rapid crustal growth at ~2.7Ga, other additions of mantle-derived magma, such as minor 2.6–2.5Ga sanukitoids, siliceous high-Mg basalts (SHMBs) and high Ba–Sr granites, also made a significant contribution to continental crustal evolution in the TSGT. [Copyright &y& Elsevier]

Abstract: Three groups of coeval plutonic rocks with different petrogenetic histories and geochemical features have been recognized in the Late Archean Taishan granite–greenstone terrain (TSGT) in the Eastern Block of the North China Craton. Zircon U–Pb dating indicate that they were emplaced contemporaneously at around 2.54Ga. Geochemically, all of the three groups have high Al2O3 and Sr concentrations, low Y and Yb concentrations, and high Sr/Y and La/Yb ratios. They have high-SiO2 concentrations ranging from 57.56 to 67.98wt%, indicating that they are typical high-SiO2 adakites. Granodiorites from the Taishan area and monzodiorites from the Zoucheng area have similar geochemistries. However, the Zoucheng monzodiorites have wider ranges in element concentrations than the Taishan granodiorites, with the latter having higher Mg numbers. The Yishui monzodiorites have higher MgO, Cr, and Ni concentrations, higher Mg numbers, and lower SiO2 concentrations, but similar REE and spidergram patterns to the Zouchang and Taishan intruisions. These geochemical features indicate that their parental magmas were all derived from the partial melting of a downgoing oceanic slab. The Zoucheng high-SiO2 adakitic rocks were probably produced by this process alone; however, the Taishan and Yishui high-Si adakite groups were formed by interaction between primary adakitic melts and overlying peridotitic mantle. In combination with coeval sanukitoid or low-SiO2 adakitic magmatism in the study area, the rock association indicates partial melting of a peridotitic mantle wedge that was metasomatized by adakitic melts and aqueous fluids derived from the subducting slab, the presence of these three high-SiO2 adakite groups supports a genetic model involving slab subduction in the Late Archean (∼2.54Ga). The adakitic intrusions and associated rocks in the region postdate the earlier (∼2.7–2.6Ga) voluminous TTG rocks suggesting that the angle of subduction changed from flat or shallow subduction, to steeper subduction likely related to the arrival of an oceanic plateau or thickened lithospheric keel at the subduction zone. [Copyright &y& Elsevier]

Abstract: Numerous intrusive bodies of ultramafic–mafic to felsic compositions are exposed in association with volcanic rocks in the Emeishan large igneous province (LIP), southwestern China. In this paper, we present new elemental and isotopic data for the Permian mafic dykes from the Panxi region which is located in the western part of the Emeishan LIP. The characteristics of major and trace elements and Sr–Nd isotopes, in combination with isotopic ages, suggest that these mafic dykes were originated from a similar mantle source related to an upwelling mantle plume and their formation was involved in a process of variable degrees of fractional crystallisation coupled with assimilation of enriched crustal materials. The mafic dyke samples from the Panxi region show some systematic variations in geochemical compositions. The gabbros in Yanyuan contain the lowest REE (e.g., La=5–12 ppm), the highest MgO (8.38–21.33%) and compatible elements (e.g. Cr=104–2610 ppm, Ni=135–496 ppm), and display depleted Sr–Nd isotopic ratios (initial 87Sr/86Sr=0.703752–0.703844; ε Nd(t)=+4.87 to +5.13). These gabbros likely represent an undifferentiated composition similar to primary melts derived from a depleted mantle source. In contrast, the gabbro samples from Panzhihua and Huili have more enriched LREE abundances and incompatible elements, and display depleted to slightly enriched Sr and Nd isotopic signatures (initial 87Sr/86Sr=0.704354–0.705436; ε Nd(t)=−0.29 to +4.58), though their trace element spidergram patterns are similar to those of the Yanyuan samples. These gabbros were formed probably through the removal of a few percent of olivine, clinopyroxene, plagioclase and apatite from the melts that formed the Yanyuan gabbros. All the mafic intrusives in the region exhibit elemental and Sr–Nd isotopic features generally comparable with those of the nearby volcanic rocks. In addition, their emplacement immediately preceded or was synchronous with the main pulse of volcanism that formed lavas over a large area along the western margin of the Yangtze Craton. These lines of evidence suggest that there is a genetic link between the mafic dykes and flood basalts in the Emeishan LIP, and the both share a common mantle source related to the Emeishan plume. [Copyright &y& Elsevier]

A-type granites are a spectrum of especial felsic rocks that are produced commonly within an extensional environment. They are rare before middle Paleoproterozoic but present abundantly since the late Paleoproterozoic, probably indicating a causal change of tectonics and geodynamic mechanism in the continental evolution of the Earth. An early Mesoproterozoic A-type granite belt was recognized in the southern margin of the North China Craton (NCC) in recent decades. The occurrence of this type of rocks indicates a possible spatial–temporal link between the tectonic evolution of the NCC and the breakup of the Columbia supercontinent during the Mesoproterozoic. The Zhangjiaping granitic pluton is located in the southern margin of the NCC and one part of the A-type granite belt. It can be divided into three zones according to their textural and compositional differences of the rocks with a gradually increasing degree of evolution inwards: wall zone, intermediate zone and core zone. LA-ICP-MS zircon U–Pb dating shows that it was emplaced at ca. 1.53 Ga. Petrologically, the Zhangjiaping granites contain perthite and calcic amphibole but are lack of alkali mafic minerals. Geochemically, all the Zhangjiaping granitic rocks show an affinity to fractionated aluminous A-type granites, with enrichment in silicon and total alkali, and depletion in MgO, CaO and P 2 O 5 with negative Eu, Sr and Ti anomalies, as well as high FeOt/MgO, TiO 2 /MgO and Ga/Al ratios and high HFSEs (i.e., Zr, Nb, Ce, Y) concentrations. Such signatures, along with their low ε Hf ( t ) values (−12.5 to −4.0) of all the granites from the Zhangjiaping pluton, obviously indicate that the Zhangjiaping aluminous A-type granites were derived from partial melting of the crustal calc-alkaline granitoids (i.e., the granodiorites of the Taihua Complex in the study area) and experienced fractional crystallization during magmatic evolution. Their generation would be related to the underplating of mafic magmas in an extensional regime. Integrated with regional data, the Zhangjiaping aluminous A-type granites were the products of anorogenic magmatism likely corresponding to the breakup of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

Whether the voluminous Early Cretaceous magmatism in the Tengchong and Baoshan blocks, western Yunnan, China, was related to subduction of the Meso-Tethys or the Neo-Tethys is controversial. Here, we report SHRIMP and LA-ICP-MS U Pb zircon ages, bulk-rock major and trace elements, and Sr Nd isotope data from volcanic rocks in the Lameng area of the Baoshan block, SW China, investigate their genesis, and discuss the evolution of the Tengchong and Baoshan blocks. These rocks yield U Pb zircon ages of 120.3 ± 1.7 and 120.8 ± 1.4 Ma and constitute a bimodal volcanic suite (group 1 and group 2). Group 1 samples show alkaline basaltic signatures (SiO 2 = 46.2–53.1 wt.%), enrichments in Zr, Hf, Ti, and light rare earth elements, and slight depletions in Nb, Ta, and heavy rare earth elements. They have ε Nd (t) = −1.0–+3.7 and initial 87Sr/86Sr = 0.70503–0.70709. Group 2 samples display higher silica contents (SiO 2 = 61.7–65.0 wt.%) and evolved ε Nd (t) (−4.3 to −3.6) and initial 87Sr/86Sr (0.70653–0.70678) values. These features suggest that the group 1 melts were derived from an asthenospheric mantle source mixed with subcontinental lithospheric mantle components and that the group 2 samples were produced by partial melting of the lower crust in a continental back-arc setting. From an affinity with widespread coeval magmatic rocks in the Tengchong and Lhasa blocks, we suggest that the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks had closed by 120 Ma and that the Early Cretaceous magmatism in the Tengchong and Baoshan blocks formed in an arc–back-arc system associated with Neo-Tethyan slab rollback. • The first report on the Early Cretaceous bimodal volcanic rocks in the Baoshan block. • The Tengchong and Baoshan blocks constituted an arc–back-arc system at ca. 120 Ma. • The Bangong-Nujiang Ocean had closed in the Early Cretaceous. [ABSTRACT FROM AUTHOR]

Abstract: The Xiaodingxi and Manghuihe volcanic sequences represent key components of the Lancangjiang igneous zone in southwest China. Their petrogenesis provides important constraints on the tectonic evolution of the eastern Paleotethys ocean. The basaltic rocks from the Xiaodingxi and Manghuihe sequences yield SHRIMP zircon U–Pb weighted mean ages of 214±7Ma and 210±22Ma, respectively, which is 15–20Ma younger than the ages of the syn-collisional granite magmatism (230–241Ma). Samples from the volcanic sequences are dominated by alkaline basalts and basaltic andesites, and can be geochemically classified into two groups. Group 1 samples, mainly from the Xiaodingxi sequence and the lower part of the Manghuihe sequence, are characterized by low MgO (1.49–7.50wt.%) and Zr/Nb (9.4–15.3), and high Al2O3 (15.95–18.39wt.%). They are enriched in LILE and LREE contents and depleted in HFSE, and have 87Sr/86Sr(t) ratios of 0.705473–0.706972, εNd(t) of −1.47–0.75, and similar Pb isotopic compositions to the global average composition of pelagic sediments. In contrast, Group 2 samples from the middle–upper parts of the Manghuihe sequence have similar Al2O3 (16.62–18.23wt.%) but higher MgO (8.08–11.74wt.%) and Zr/Nb (15.9–23.9) than those of Group 1 samples. They exhibit relatively flat REE patterns, significantly negative Nb–Ta and Th–U anomalies and positive Sr anomalies. In comparison with Group 1, Group 2 samples show higher Cr, Ni contents and εNd(t) values (1.17–5.02), and lower 87Sr/86Sr(t) and Pb isotopic ratios (Δ8/4=43.2–59.8 and Δ7/4=11.8–19.8). The geochemical data suggest that Group 1 samples might be the differentiated product of primitive high MgO and low Al2O3 melts originating from a refractory modified mantle with the involvement of 5–10% recycled pelagic sediments. The parental magma for Group 2 samples may have been derived from a plagioclase-rich, garnet-free source comprising 80–85% fluid-metasomatized and 15–20% asthenospheric components. Based on all available data, a tectonic model involving eastward subduction in the Permian and collision in the Triassic can be proposed for the evolution of the eastern Paleotethys ocean. During the late Triassic, the progressive upwelling of the asthenospheric mantle, shortly after slab detachment, may have led to the melting of the metasomatized mantle wedge, resulting in the post-collisional Group 1 and Group 2 magmas. [Copyright &y& Elsevier]

Abstract: Paleoproterozoic (∼1770Ma) mafic dikes occur throughout the Trans-North China Orogen. These dikes can be divided into three geochemically distinct groups. Group 1 with FeOt of 12.73–18.06%, (Nb/La)N 0.18–0.39, (Th/La)N of, 0.60–1.24, ε Nd(t) of −5.1 to −2.4, and an enrichment in LILE’s and a depletion in HFSE’s. Group 2 has flat REEs and incompatible elements patterns, and possesses (Nb/La)N =0.64–0.87, (Th/La)N =0.61–1.20 and ε Nd(t)=−1.7 to 1.1. Group 3 has low FeOt contents (8.19–11.57%) and (Th/La)N ratios (0.29–0.72), but similar (Nb/La)N ratios and ε Nd(t) values to those of Group 1. Petrological and geochemical data suggest that these dikes may have originated from different sources. Group 1 could be derived from a “re-fertilized” lithospheric source incorporating a continental basaltic component trapped from a earlier phase of subduction. Group 2 was likely a product of a subduction-modified lithospheric mantle hybridized by recycled gabbroic lower crust. By contrast, Group 3 originated from a mixture of subduction-modified lithospheric mantle with a MORB component. In combination with the available data, we propose that the sources of the mafic dikes were initially modified by the ∼1.85Ga subduction/collision event that assembled the North China Craton, and partially melted under subsequent rifting in response to the upwelling convective mantle. [Copyright &y& Elsevier]

Abstract: The Indosinian granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Samples collected from Hunan Province can be geochemically classified into two groups. Group 1 is strongly peraluminous (A/CNK>1.1), similar to S-type granites, and Group 2 has A/CNK=1.0–1.1, with an affinity to I-type granites. Group 1 has lower FeOt, Al2O3, MgO, CaO, TiO2 and ε Nd(t) values but higher K2O+Na2O, Rb/Sr, Rb/Ba and 87Sr/86Sr(t) than those of Group 2. Samples of both groups have similar LREE enriched pattern, with (Eu/Eu⁎)=0.19–0.69, and strongly negative Ba, Sr, Nb, P and Ti anomalies. Geothermobarometry study indicates that the precursor magmas were emplaced at high-level depth with relatively low temperature (734–827 °C). Geochemical data suggest that Group 1 was originated from a source dominated by pelitic composition and Group 2 was from a mixing source of pelitic and basaltic rocks with insignificant addition of newly mantle-derived magma. Eight granitic samples in Hunan Province are dated at the cluster of 243–235 and 218–210 Ma by zircon U–Pb geochronology. Together with recent zircon U–Pb ages for other areas in the SCB, two age-clusters, including 243–228 Ma just after peak-metamorphism (∼246–252 Ma) and 220–206 Ma shortly after magma underplating event (∼224 Ma), are observed. It is proposed that in-situ radiogenic heating from the over-thickened crust induced dehydrated reaction of muscovite and epidote/zoisite to form the early Indosinian granites in response to the isostatic readjustments of tectonically thickened crust. Conductive heating from the underplating magma in the postcollisional setting triggered the formation of late Indosinian granites. Such a consideration is supported by the results from FLAC numerical simulation. [Copyright &y& Elsevier]

The wide occurrence of Paleoproterozoic mafic dikes in the basement of the Trans-North China Orogen provides important constraints on the major ∼1800Ma tectonothermal event of the North China Craton. These mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites, and can be geochemically classified into three groups: The Groups 1 and 2 rocks are characterized by high total FeO contents (12.69–15.61%), of which the Group 1 rocks are more enriched in LILE and LREE contents and depleted in HFSE ((Th/Nb)n = 2.5–3.8, (La/Yb)cn = 3.0–5.8; (Nb/La)n = 0.27–0.38) and have more radiogenic Nd isotopic ratios (εNd(t) = -3.40 to -5.14), than the Group 2 rocks ((Th/Nb)n = 0.8–1.2, (La/Yb)cn = 1.5–2.1, (Nb/La)n = 0.65–0.87, εNd(t) = -0.60 to -1.67). In contrast, the Group 3 rocks are typified by low FeOt (7.86–11.04%) and high MgO (5.62–9.56%) contents. Higher (La/Yb)cn (4.22–7.13) ratios, less radiogenic Nd isotopic ratios (εNd(t) = -2.75 to -5.52), and more significant Th–U and Nb–Ta depletion are also apparent features. 40Ar/39Ar geochronology of three representative samples from each group yielded plateau ages of 1780.7 ± 0.5Ma, 1765.3 ± 1.1Ma and 1774.7 ± 0.7Ma, respectively. These elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement. Instead, they most likely originated from variable sources under different degrees of partial melting. The Group 1 rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction-related fluids, whereas the Group 2 rocks originated from a hybridized source involving ca. 40% subduction-modified lithospheric mantle, similar to the Group 1, and ca.60% N-MORB component. In contrast, the Group 3 rocks show significant Th–U depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction. Based on all the available data, we propose a tectonic model of a full subduction-syncollision-postcollision-rifting cycle for the evolution of the NCC between 1870 and 1765Ma. A variety of source regions beneath the southern Taihang Mountains initially developed during the period of subduction/collision, and subsequent extension/rifting resulted in the melting of the subduction-modified lithospheric mantle, which produced widespread Paleoproterozoic mafic dikes on the Trans-North China Orogen in response to upwelling of convective mantle. [Copyright &y& Elsevier]