Low‐temperature thermochronology of the Mt Painter Province,South Australia

Apatite fission track results are reported for 26 outcrop samples from the Mt Painter Inlier, Mt Babbage Inlier and adjacent Neoproterozoic rocks of the northwestern Curnamona Craton of South Australia. Forward modelling of the data indicates that the province experienced variable regional cooling from temperatures >110°C during the Late Palaeozoic (Late Carboniferous to Early Permian). The timing of this cooling is similar to that previously reported from elsewhere in the Adelaide Fold Belt and the Curnamona Craton, suggesting that the entire region underwent extensive Late Palaeozoic cooling most likely related to the waning stages of the Alice Springs or Kanimblan Orogenies. Results from the Paralana Fault Zone indicate that the eastern margin of the Mt Painter Inlier experienced a second episode of cooling (～40–60°C) during the Paleocene to Eocene. The entire region also experienced significant cooling (less than ～40°C) during the Late Cretaceous to Palaeogene in response to unroofing and/or a decrease in geothermal gradient. Regional cooling/erosion during this time is supported by: geomorphological and geophysical evidence indicating Tertiary exhumation of at least 1 km; Eocene sedimentation initiated in basins adjacent to the Flinders and Mt Lofty Ranges sections of the Adelaide Fold Belt; and Late Cretaceous ‐ Early Tertiary cooling previously reported from apatite fission track studies in the Willyama Inliers and the southern Adelaide Fold Belt. Late Cretaceous to Palaeogene cooling is probably related to a change in stress field propagated throughout the Australian Plate, and driven by the initiation of sea‐floor spreading in the Tasman Sea in the Late Cretaceous and the Eocene global plate reorganisation.     

Mesoproterozoic arc magmatism in SE India: Petrology,zircon U–Pb geochronology and Hf isotopes of the Bopudi felsic suite from Eastern Ghats Belt

The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U–Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab₇ Or₉₃), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An_23–24 Ab₇₅). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal ²⁰⁷Pb/²⁰⁶Pb ages of 1582 (MSWD = 1.4) for the rapakivi granite 1605 ± 3 Ma (MSWD = 3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.28171–0.28188, with εHf(t) values of −2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (T_DM2) ranging from 2.15 to 2.47 Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic Rodinia assembly.     

A newly defined Late Ordovician magmatic‐thermal event in the Mt Painter Province,northern Flinders Ranges,South Australia

Magmatism,metamorphism and metasomatism in the Palaeoproterozoic‐Mesoproterozoic Mt Painter Inlier and overlying Neoproterozoic Adelaidean rocks in the northern Flinders Ranges (South Australia) have previously been interpreted as resulting from the ca 500 Ma Delamerian Orogeny. New Rb–Sr, Sm–Nd and U–Pb data, as well as structural analysis,indicate that the area also experienced a second thermal event in the Late Ordovician (ca 440 Ma). The Delamerian Orogeny resulted in large‐scale folding, prograde metamorphism and minor magmatic activity in the form of a small volume of pegmatites and leucogranites. The Late Ordovician event produced larger volumes of granite (the British Empire Granite in the core of the inlier) and these show Nd isotopic evidence for a mantle component. The high‐temperature stage of this magmatic‐hydrothermal event also gave rise to unusual diopside‐titanite veins and the primary uranium mineralisation in the basement, of which the remobilisation was younger than 3.5 Ma. It is possible that parts of the Mt Gee quartz‐hematite epithermal system developed during the waning stages of the Late Ordovician event. We suggest that the Ordovician hydrothermal system was also the cause of the commonly observed retrogression of Delamerian metamorphic minerals (cordierite, andalusite) and the widespread development of actinolite, scapolite, tremolite and magnetite in the cover sequences. Deformation during the Late Ordovician was brittle. The recognition of the Late Ordovician magmatic‐hydrothermal event in the Mt Painter Province might help to link the tectonic evolution of central Australia and the southeast Australian Lachlan Fold Belt.     

Geochronology of the Gabbro–mafic Microgranular Enclaves–granite Associations in the Gejiu District, Yunnan Province and Their Geodynamic Significance

Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites (porphyritic granite and equigranular granite) and akaline rocks. The ages of the granites and akaline rocks which are considered to have genetic connecting with the mineralization have been comfirmed, but the gabbro-mafic microgranular enclaves-granite assemblage’s ages are still unknown. By means of LA-ICP-MS zircon U-Pb dating, the data of Shenxianshui equigranular granite, the mafic microgranular enclave in Jiasha area, the host rock of the mafic microgranular enclaves and the Jiasha gabbro are around ~80 Ma. Besides the above mentioned data, a group of new ages at ~30 Ma were discovered in this study, which is from gabbro and mafic microgranular enclaves. Based on the previous data and the new data gained this time, we suggest the major geochronology framework of the magmatism and mineralization events in Gejiu area is ~80 Ma, which is consistent with the Late Cretaceous magmatism and mineralization events in the whole southeast Yunnan and west Guangxi area and they were suggested to belong to the same geotectonic setting in late Yenshannian. And the new ages of the ~30 Ma obtained in this study is considered to represent a responding to the complicate tectonic evolution history of the Tibetan orogenic events in Cenozoic.     

Geochronology,geochemistry and Sr–Nd–Hf isotopes of the Haisugou porphyry Mo deposit,northeast China,and their geological significance

The Haisugou Mo deposit is located in the northern part of the Xilamulun Mo–Cu metallogenic belt in northeastern China. The Mo mineralization mainly occurs as quartz-molybdenite veins within the Haisugou granite, which was emplaced into rocks of the Early Permian Qingfengshan Formation. Zircon U–Pb dating by LA–ICP-MS of the granite yields a crystallization age of 137.6 ± 0.9 Ma, suggesting emplacement during the peak time of Mo mineralization in eastern China, broadly constrained to ca. 150–130 Ma, when tectonic stresses shifted from compression to extension. Whole-rock geochemical data suggest that the granite belongs to the high-K calc-alkaline series, and is characterized by relatively high LREE; low HREE; depletion of Ti, Ba, and Nb; and a moderate negative Eu anomaly. The zircon ε_Hf(t) and whole-rock ε_Nd(t) values for the intrusion range from +4.5 to +10.0 and +0.2 to +1.6, respectively, indicating that the magma originated from the juvenile lower crust source derived from depleted mantle, with some component of ancient continental crust. The granite is also characterized by initial (⁸⁷Sr/⁸⁶Sr)_i ratios ranging from 0.7040 to 0.7074, which suggest some contamination by the upper crust during the ascent of the primitive magma. Moreover, it can be recognized from the whole-rock major and trace element data that significant fractional crystallization occurred during magmatic evolution, with the separation of plagioclase and K-feldspar. Because Mo is an incompatible element and tends to concentrate in the melt during crystallization, fractionation processes likely played an important role in the formation of the Haisugou Mo deposit.     

Petrogenesis and metallogenic potential of the Jurassic porphyries in the northwest Zhejiang Province,South China: insights from SHRIMP zircon geochronology,geochemistry, and Sr–Nd–Hf isotopes of the Huangbaikeng ore-bearing porphyries

The northwest Zhejiang Province is a key domain for providing deep insight into the crust–mantle interaction and tectonic evolution of the South China block. In this paper, we collect geochemical, geochronological, and isotopic data of the Jurassic porphyries in this region, and investigated the Huangbaikeng ore-bearing porphyry in the Tongcun Mo–Cu deposit, using it as an example to uncover the porphyry petrogenesis and evaluate their metallogenic potential. Two varieties of the Huangbaikeng porphyry were distinguished: the medium- to coarse-grained type and medium- to fine-grained type. Zircon Sensitive High-Resolution Ion Microprobe U–Pb dating indicates that they were emplaced at 161.8 ± 2.8 and 162.7 ± 3.5 Ma, respectively, which are consistent with the molybdenite Re–Os ages of 163.9–161.8 Ma. The inherited zircons age spectrum significantly recorded a series of geological events, for example, assembly and breakup of the Columbia and Rodinia supercontinent, and the Triassic collision of Yangtze and North China blocks. Whole rock Sr–Nd and Jurassic zircon Hf isotopic data yield mostly negative ε_Hf(t) values (0.5 to ?8.4) and ε_Nd(t) values (?0.79 to ?4.82). Besides the Huangbaikeng porphyry, all the Jurassic porphyries in the northwest Zhejiang Province have a wide range of SiO₂ contents (76.78–60.91 wt.%). They do not contain typical aluminous minerals (e.g. cordierite and garnet), and are mainly metaluminous to weakly peraluminous with high Na₂O, low FeO^T/MgO, and Zr + Nb + Ce + Y concentrations in composition. They thus fit the I-type granite definition. Some major and trace elements show strong correlations with SiO₂, possibly indicating extensive fractional crystallization during their magma evolution. Tectonic discriminations imply that these plutons were likely formed in a volcanic arc regime possibly related to subduction of the Palaeo-Pacific plate. Sr–Nd–Hf isotopic data suggest a mixed source of the Mesoproterozoic crust and 30–50% mantle components. Compared with the adjacent Dexing Cu-bearing porphyies, which have more positive ε_Hf(t) and ε_Nd(t) values with more significant mantle components (55–70%), the Jurassic porphyries in the northwest Zhejiang Province probably lack metallogenic potential to form a giant porphyry copper deposit as Dexing.     

Geochronology–geochemistry of the Cida bimodal intrusive complex,central Emeishan large igneous province,southwest China: petrogenesis and plume–lithosphere interaction

The Cida complex is situated in the Panxi region and is predominantly composed of mafic-ultramafic and syenitic rock units; minor amounts of intermediate rocks occupy the contact zone between the two major rock types. The intermediate unit is mineralogically heterogeneous and typically exhibits a mottled structure. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb zircon dating shows that the mafic-ultramafic rocks and syenitic rocks formed almost coevally (243 ± 0.77 Ma and 240.5 ± 0.76 Ma, respectively). These ages may represent the end phase of the Emeishan large igneous province (ELIP) magmatism. Most of these three rock types possess alkaline and metaluminous affinities. The mafic-ultramafic, syenitic, and intermediate units have K₂O + Na₂O contents of 1.85–5.16, 6.55–10.46, and 9.55–11.54 wt.%, and SiO₂ contents of 40.06–46.70, 61.74–68.54, and 51.57–54.13 wt.%, respectively. The mafic-ultramafic unit displays ocean-island basalt (OIB)-like primitive-mantle-normalized incompatible element patterns, coupled with low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7048–0.7064), positive ?_Nd(t) (0.32–2.23), and zircon ?_Hf(t) (4.53–14.17) values, consistent with a mafic plume-head origin, whereas one exceptional sample with negative ?_Nd(t) (–0.22) can be interpreted as due to the involvement of considerable amounts of enriched subcontinental lithospheric mantle. The relatively low (La/Yb) _N ratios (3.40–7.69) reflect a spinel-facies lherzolite source. The syenitic unit is characterized by enrichment in large ion lithophile elements (e.g. Rb, K, Pb) and light rare earth elements (LREEs), relative to high field strength elements (e.g. Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), respectively. These features, together with their metaluminous affinities, low SiO₂ contents, lower initial ⁸⁷Sr/⁸⁶Sr ratios (0.7043), positive ?_Nd(t) (0.18), and zircon ?_Hf(t) (2.63–10.09) values as well as modelling of REEs, can be plausibly explained by crustal partial melting of juvenile basic materials beneath the Yangtze Block. In contrast, the field, petrographic observations, and geochemical signatures (e.g. the linear correlations between FeO* and MgO, K/Ba and Rb/Ba ratios) suggest that the intermediate unit may have resulted from magma mixing between the syenitic and basaltic magmas that in turn had evolved from a parental mafic-ultramafic liquid. Thus, the formation of the Cida complex can be attributed to the plume–lithosphere interaction plus partial melting of juvenile basic lower crust in response to heating of underplated plume-derived basaltic magma.     

Geochronology,geochemistry and Sr–Nd–Hf–S–Pb isotopes of the Early Cretaceous Taoxihu Sn deposit and related granitoids,SE China

The Taoxihu deposit (eastern Guangdong, SE China) is a newly discovered Sn polymetallic deposit. Zircon U-Pb dating yielded 141.8 ± 1.0 Ma for the Sn-bearing granite porphyry and 145.5 ± 1.6 Ma for the biotite granite batholith it intruded. The age of the granite porphyry is consistent (within error) with the molybdenite Re–Os isochron age (139.0 ± 1.1 Ma) of the Sn mineralization, indicating a temporal link between the two. Geochemical data show that the granite porphyry is weakly peraluminous, contain high Si, Na and K, low Fe, Mg, Ca and P, and relatively high Rb/Sr and low K/Rb values. The rocks are enriched in Rb, Th, U, K, and Pb and depleted in Ba, Sr, Ti and Eu, resembling highly fractionated I-type granites. They contain bulk rock initial ⁸⁷Sr/⁸⁷Sr of 0.707371–0.707730 and εNd(t) of −5.17 to −4.67, and zircon εHf(t) values from −6.67 to −2.32, with late Mesoproterozoic T_DM2 ages for both Nd and Hf isotopes. This suggests that the granite porphyry was likely formed by the partial melting of the crustal basement of Mesoproterozoic overall residence age with minor mantle input.δ³⁴S_CDT values of the Taoxihu chalcopyrite and pyrite range from 0.1 to 2.1‰ (average: 0.9‰), implying a dominantly magmatic sulfur source. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the Taoxihu sulfide ores are 18.497–18.669, 15.642–15.673 and 38.764–38.934, respectively, indicating a mainly upper continental crustal lead source with minor mantle contribution. The highly fractionated and reduced (low calculated zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N¹ values) nature of the ore-forming granitic magma may have facilitated the Sn enrichment and played a key role in the Sn mineralization. We propose that the ore-forming fluids at Taoxihu were of magmatic-hydrothermal origin derived from the granite porphyry, and that both the granite porphyry and the Sn mineralization were likely formed in an extensional setting, possibly related to the subduction slab rollback of the Paleo-Pacific Plate.     

Geochronology and geochemistry of the Shilu Cu–Mo deposit in the Yunkai area,Guangdong Province,South China and its implication

Shilu is a large porphyry–skarn deposit in the Yunkai district in Guangdong Province, South China. The Shilu granitic intrusion in the mine area is a granodiorite which is genetically related to Cu mineralization. Plagioclase in the granodiorite has a zoned texture and is mainly andesine with minor amounts of labradorite, whereas the K-feldspars exhibit Carlsbad twins and some are also characterized by a zonal texture. K-feldspars from the granodiorite show high contents of Or (87–92 wt.%) with minor Ab (8–13 wt.%) and negligible An value of 0–0.3 wt.%. Biotite can be classified as magnesio-biotite, and is characterized by Mg-rich [Mg/(Mg + Fe) = 0.54–0.60] and Al^VI-low (average values = 0.11). Hornblende is chiefly magnesiohornblende and tschermakite. LA-ICP-MS zircon U–Pb age of the Shilu granodiorite is 107 ± 0.7 Ma, which is consistent with molybdenites Re–Os age of 104.1 ± 1.3 Ma. Geochemical data indicate that the Shilu granodiorite is silica-rich (SiO₂ = 63.43–65.03 wt.%) and alkali-rich (K₂O + Na₂O = 5.45–6.05 wt.%), as well as calcium-rich (CaO = 4.76–5.1 wt.%). Trace element geochemistry results show enrichments in large ion lithophile elements (e.g., Rb, K, and Ba) and depletions in some high field strength elements (e.g., Nb, P, Ta, and Ti). The total rare earth element (REE) content of the granodioritic rocks is low (∑ REE < 200 ppm), and is characterized by light REE enrichment [(La/Yb)_N > 9] and moderately negative Eu anomalies (Eu/Eu* = 0.83–0.90). These mineralogical, geochronological, and geochemical results suggest that the Shilu granodiorite has a mixed crust–mantle source with a geochemical affinity to I-type granitoids. Hornblende thermobarometry yielded magmatic crystallization temperatures of 686–785 °C and crystallization pressures between 1.0 and 2.34 kbar, which is converted to depths in a range of 3.31 to 7.71 km. Biotite thermobarometry yielded similar temperatures and lower pressures of 680–780 °C and 0.8–2 kbar (depth 2.64–6.6 km), respectively. The parent magma had a high oxygen fugacity. The Shilu granodiorite has a relatively low ε_Nd/t–t value and high (⁸⁷Sr/⁸⁶Sr)_i value, and Nd isotopes yield two-stage depleted mantle Nd model ages of 969–1590 Ma. Our new data, combined with previous studies, imply that the granodiorite and the associated Shilu Cu–Mo deposit was formed in an extensional environment, closely related to remelting of residual subducted slab fragments in the Jurassic.     

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex,Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO₂ contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (⁸⁷Sr/⁸⁶Sr) _t (t?=?260 Ma) ratios and ?_Nd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and??0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (⁸⁷Sr/⁸⁶Sr) _t . Their ?_Nd(t) versus (Th/Nb)_PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)_PM versus (Yb/Sm)_PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.     

Geochronology and geochemistry of Late Triassic bimodal igneous rocks at the eastern margin of the Songnen–Zhangguangcai Range Massif,Northeast China: petrogenesis and tectonic implications

Abstract

New zircon laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectroscopy U–Pb ages, and Hf isotope and whole-rock geochemical data are reported for Mesozoic igneous rocks from the eastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China, in order to document the petrogenesis of the igneous rocks and reconstruct the early Mesozoic tectonic setting of the region. Zircons from five representative igneous rocks are euhedral–subhedral and display oscillatory growth zoning or striped absorption in cathodoluminescence images, suggesting a magmatic origin. The dating results indicate that granite, gabbro, and rhyolite from the eastern Songnen–Zhangguangcai Range Massif formed during Late Triassic (204–211 Ma). The Late Triassic granitoids and rhyolites have an affinity to A-type granites or rhyolites. Their zircon ε_Hf(t) values and Hf two-stage model ages range from –3.8 to +3.8 and from 999 to 1485 Ma, respectively, indicating that their primary melts were derived from the partial melting of the Meso-Proterozoic crust. The geochemistry of coeval gabbros, which reflects primary magma composition, shows a significant large ion lithophile element (e.g. Ba and Sr) enrichment and high field strength element (i.e. Zr, Hf, Nb, Ta, and Ti) depletion. Based on zircon ε_Hf(t) values (–4.2 to +2.8) and Hf single-stage model ages (746–1031 Ma), we conclude that the mafic magma is the product of partial melting of lithospheric mantle that was metasomatically enriched by fluids derived from the subducted oceanic crust. The Late Triassic magmatism along the eastern margin of the Eurasian continent has bimodal magma compositions, indicating an extensional setting after the final closure of the Palaeo-Asian Ocean rather than being related to subduction of the Palaeo-Pacific Plate beneath the Eurasian continent. The occurrence of Late Triassic igneous rocks on the eastern side of the Mudanjiang Fault suggests that this fault does not represent the suture zone between the Songnen–Zhangguangcai Range and Jiamusi massifs.     

Geochemistry,Sr–Nd–Pb–Hf isotopes systematics and geochronological constrains on petrogenesis of the Xishan A-type granite and associated W–Sn mineralization in Guangdong Province,South China

The Xishan deposit, located in the western Guangdong Province in South China, is a quartz-vein type W-Sn deposit with an average Sn grade of 0.1–0.4 wt%. The deposit is temporally and spatially associated with Xishan alkali feldspar granite. The W–Sn mineralization is present mainly as veins that are hosted by the granite. In this paper we present new zircon U–Pb age, whole-rock geochemical data, Sr–Nd–Pb–Hf isotopic data and Re–Os age in order to constrain the nature and timing of magmatism and mineralization in the Xishan mining district with implications on geodynamic settings. LA–ICP–MS zircon U–Pb analyses yielded an age of 79.14 ± 0.31 Ma for the alkali feldspar granite, consistent with the molybdenite Re–Os age of 79.41 ± 1.11 Ma. The alkali feldspar granite shows high contents of SiO₂ (71.52–76.25 wt%), high total alkalis (Na₂O + K₂O = 9.35–13.51 wt%), high field strength elements (e.g. Zr = 95.4–116 ppm, Y = 97.1–138 ppm, Nb = 36.1–55.5 ppm, Ga = 97.1–138 ppm), and rare earth elements (total REE = 171.8–194.0 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 3.23–3.82) suggesting that it has the geochemical characteristics of A-type granite and shows an A₂ subtype affinity. Sr–Nd isotopes of the alkali feldspar granite show that (⁸⁷Sr/⁸⁶Sr)_i values range from 0.7111 to 0.7183, and the ε_Nd(t) values and Nd model ages (T_2DM) vary from −6.8 to −6.5 and 1414 to 1433 Ma, respectively. The Pb isotopic compositions are variable, with ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values ranging from 18.783 to 18.947, 15.709 to 15.722 and 38.969 to 39.244, respectively, indicating that the alkali feldspar granite was derived from a mantle-crust mixed source. In situ Hf isotopic analyses reveal that the alkali feldspar granite has ε_Hf(t) values ranging from −9.69 to −0.04 and two-stage Hf model ages from 1145 Ma to 1755 Ma, indicating that the alkali feldspar granite was formed by the partial melting of Mesoproterozoic crusts of the Cathaysia Block with additions of mantle-derived materials. These results, together with previously presented regional geological relationships, suggest that the formation of the Xishan granite and associated W–Sn mineralization is related to lithospheric extension and asthenospheric upwelling that are attributed to a directional change of Pacific plate motion.     

Zircon U–Pb and Hf isotopic study of Mesozoic felsic rocks from eastern Zhejiang,South China: Geochemical contrast between the Yangtze and Cathaysia blocks

The Jiangshan–Shaoxing Fault Zone (JSFZ) in Zhejiang Province has been proposed to represent a suture between the Yangtze and Cathaysia blocks in South China. In this study, in-situ zircon U–Pb and Hf isotopic analysis and whole-rock major- and trace-element measurement of early to middle Cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone. Twelve Cretaceous granitoid bodies were sampled from the NW and SE sides of the fault zone, respectively, with composition ranging from diorite to granite (SiO₂ = 56.2–76.6 wt.%). These granitoids yielded U–Pb ages ranging from 135–100 Ma, with a systematic variation in zircon Hf isotopic compositions (ε_Hf(t) = + 6.9 to –7.0 in the NW side vs. + 1.9 to ? 12.9 in the SE side). The T_DM2 values for the granitoids from the NW side are 0.34 to 1.33 Ga, with two peaks at ca. 876 and 1170 Ma respectively, whereas those from the SE side are 0.70 to 1.62 Ga, with a single peak at ca. 1126 Ma. The Hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the Yangtze and Cathaysia blocks, supporting that the JSFZ is possibly a suture zone between the two blocks. Our results together with the available geological data suggest that the Mesoproterozoic materials are important for both the Yangtze and Cathaysia basement and the Neoproterozoic magmatic activities were important in the Yangtze Block, possibly related to the break-up of the Rodinia supercontinent, but less significant in the Cathaysia Block. This may imply that the two blocks have not completely juxtaposed in the Neoproterozoic.     

Zircon U–Pb age,Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic–felsic Complex at the southeastern margin of the Yangtze Block

This work presents an integrated study of zircon U–Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic–felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7093–0.7127, low εNd(t) values ranging from −5.6 to −5.4 and corresponding Nd model ages (T_2DM) of 1.6 Ga. Their zircon grains have εHf(t) values ranging from −2.7 to 2.6 and model ages of 951–1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7053–0.7058, εNd(t) values of 0.2–1.6 and corresponding T_2DM of 1.0–1.1 Ga. Their zircon grains have εHf(t) values ranging from 3.2 to 6.1 and model ages of 774–911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7065–0.7117, εNd(t) values from −5.7 to −1.9 and Nd model ages of 1.3–1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd–Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.     

The U–Pb ages and Hf isotopes of detrital zircons from Hainan Island,South China: implications for sediment provenance and the crustal evolution

In situ U–Pb dating and Hf isotopic of detrital zircons from beach sediments of Yalong Bay were analyzed to trace sedimentary provenance and reveal the crustal evolution of Hainan Island in South China. The grain size distribution of the sediments displays a clear single-peak feature, indicating the sediments were formed under the same condition of hydrodynamic force. The detrital zircons had Th/U ratios of greater than 0.1, and REE pattern displayed a positive Ce anomaly and a negative Eu anomaly, indicating that these zircons are predominantly of magmatic origin. The U–Pb spectrum of detrital zircons mainly peaked at the Yanshanian (96–185 Ma), Hercynian–Indosinian (222–345 Ma) and Caledonian (421–477 Ma). A portion of the detrital zircons were of Neoproterozoic origin (728–1,003 Ma), which revealed that the basement in the eastern region of Hainan Island was mainly of Neoproterozoic, with rare Archean materials. The positive ε _Hf(t) values (0 to +10.1) of the Neoproterozoic detrital zircons indicated that the juvenile crust grew in the southeastern Hainan Island mainly during the Neoproterozoic period. The Neoproterozoic orogeny in the southeastern part of the island (0.7–1.0 Ga) occurred later than in the northwestern region of the island (1.0–1.4 Ga). Importantly, the Grenvillian orogeny in the southeastern area of Hainan Island shared the same timing with that of the western Cathaysia Block; i.e., both areas concurrently underwent this orogenic event, thereby forming a part of the Rodinia supercontinent. Afterwards, the crust experienced remelting and reworking during the Caledonian Hercynian–Indosinianand Yanshanian accompanied by the growth of a small amount of juvenile crust.     

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.     

Textural and micro-analytical insights into mafic–felsic interactions during the Oruanui eruption,Taupo

Quenched juvenile mafic inclusions (enclaves) are an occasional but informative component in the deposits of large felsic eruptions. Typically, the groundmasses of these inclusions rapidly crystallize as the mafic magma is chilled against a more voluminous, cooler felsic host, providing a physical and chemical record of the nature and timing of mafic–felsic interactions. We examine mafic inclusions of two compositional lineages (tholeiitic and calc-alkaline) from deposits of the 25.4 ka Oruanui eruption (Taupo, New Zealand). 2-D quantitative textural data from analysis of back-scattered electron images reveal a marked diversity in the groundmass textures of the inclusions, including median crystal sizes (amphibole: 14–45 µm; plagioclase: 21–75 µm) and aspect ratios (amphibole: 1.7–4.2; plagioclase: 2.1–4.0), area number densities (amphibole: 122–2660 mm^?2; plagioclase: 117–2990 mm^?2), area fractions (?) of minerals (?_plag?=?23–45%, ?_amph?=?0–28%, ?_cpx?=?0–6%, ?_oxides?=?0.6–5.5%), and the relative abundance of plagioclase and amphibole (?_plag/?_amph?=?1.0–4.6). Textural parameters vary more significantly within, rather than between, the two compositional lineages, and in some cases show marked variations across individual clasts, implying that each inclusion’s cooling history, rather than bulk composition, was the dominant control on textural development. Groundmass mineral compositions are also diverse both within and between inclusions (e.g. plagioclase from An_34–92, with typical intra-clast variability of ~?20 mol%), and do not correlate with bulk chemistry. Diverse groundmass textures and mineral and glass chemistries are inferred to reflect complex interplay of a range of factors including the degree and rate of undercooling, bulk composition, water content and, possibly, intensive variables. Our data are inconsistent with breakup of a crystallizing ponded mafic layer at the base of the Oruanui melt-dominant body, instead implying that each inclusion partially crystallized as a discrete body with a unique cooling history. Extensive ingestion of mush-derived macro-crystals suggests that mechanical breakup of mafic feeder dikes occurred within a transition zone between the mush and melt-dominant magma body. In this zone, the mush lacked yield strength, as has been inferred from field studies of narrow (meters to few tens of meters) mush-melt transition zones preserved in composite intrusions. Evidence for plastic deformation of inclusions during eruption and the abundance of fresh residual glass in inclusions from all eruptive phases suggest that the inclusions formed syn-eruptively, and must have been formed recurrently at multiple stages throughout the eruption.     

Provenance and tectonic setting of Neoproterozoic sedimentary sequences in the South China Block: evidence from detrital zircon ages and Hf–Nd isotopes

Neoproterozoic sedimentary sequences in the South China Block provide great opportunity to examine the tectonic evolution and crustal accretion during this period. This study presents U–Pb ages and Hf isotope composition of detrital zircons and Nd isotope composition of whole rocks of the Neoproterozoic sequences from the Yangtze Block, part of the South China Block. Age patterns of detrital zircons imply that the source area experienced three major periods of magmatic activity at 2,300–2,560, 1,900–2,100 and 770–1,000?Ma and two major episodes of juvenile crust accretion at 2,600–3,400 and 770–1,000?Ma. The maximum age of the Gucheng glaciation can be restricted at?~768?Ma from the youngest detrital zircon ages, probably corresponding to the Kaigas glaciation rather than to the Sturtian glaciation. High La/Sc ratio and low Cr/Th, Sc/Th and Co/Th ratios of the sedimentary rocks point to a derivation from dominantly felsic upper continental crustal sources, whereas large variation of ε_Nd(t) and ε_Hf(t) values indicates that mantle-derived magmatic rocks also provided material to the sedimentary sequences in different degrees. The shift in ε_Nd(t) values of whole rocks and U–Pb age spectra of detrital zircons records the evolution from a back-arc to retro-arc foreland to a rift basin. Age distribution of detrital zircons from the Neoproterozoic sequences, compared with those of the major crustal blocks of Rodinia, implies that the position of the Yangtze Block was probably adjacent to northern India rather than between Australia and Laurentia before the breakup of the Rodinia supercontinent.