Shao-Yong Jiang

Early Cambrian ocean anoxia in South China

Arising from: M. Wille, T. F. Nägler, B. Lehmann, S. Schröder & J. D. Kramers 453, 767–769 (2008)10.1038/nature07072; Wille et al. replyThe cause of the most marked changes in the evolution of life, which define the first-order stratigraphic boundary between the Precambrian and the Phanerozoic eon, remains enigmatic and a highly topical subject of debate. A global ocean anoxic event, triggered by large-scale hydrogen sulphide (H2S) release to surface waters, has been suggested by Wille et al., on the basis of two data sets from South China and Oman, to explain the fundamental biological changes across the Precambrian/Cambrian (PC/C) boundary. Here we report a new precise SHRIMP U–Pb zircon age of 532.3 ± 0.7 million years (Myr) ago (Fig. 1) for a volcanic ash bed in the critical unit that reflects the ocean anoxic event, the lowermost black shale sequence of the Niutitang Formation in the Guizhou Province, South China. This age is significantly younger than the precise PC/C boundary age of 542.0 ± 0.3 Myr ago, approximately 10 Myr younger than the extinction of the Ediacaran fauna, and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the PC/C boundary.

Contrasting origins of late Mesozoic adakitic granitoids from the northwestern Jiaodong Peninsula, east China: implications for crustal thickening to delamination

Two suites of granitoids, the Late Jurassic (158 ± 3 Ma) Linglong suite and the Early Cretaceous (130–126 Ma) Guojialing suite, crop out in the northwestern Jiaodong Peninsula, eastern China. The Linglong suite is a monzogranite, comprising alkali feldspar, plagioclase, quartz and Fe-rich biotite. The Guojialing suite includes at least five plutonic bodies of both granodiorite and monzo-granite. The rocks are composed of plagioclase, alkali feldspar, quartz, Mg-rich amphibole and Mg-rich biotite. Both the Linglong and Guojialing suites have adakitic affinity. They are enriched in LREE with high La/Yb ratios and show positive Eu anomalies. The rocks are also enriched in LILE and depleted in HFSE with high Sr/Y ratios. The Linglong granite shows very uniform Sr–Nd isotopic compositions with initial 87 Sr/ 86 Sr ratios of 0.7119–0.7126 and e Nd (T) values of −21.3 to −21.6, which are similar to those of the local Neoarchaean basement. The Guojialing suite has variable initial 87 Sr/ 86 Sr ratios (0.7108–0.7120) and e Nd (T) values (−10.8 to −17.2), which are distinct both from those of the Neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area. Detailed petrological and geochemical data indicate that the Linglong suite was derived by partial melting of Neoarchaean metamorphic lower-crustal rocks at depth of > 50 km with a eclogite residue, whereas the Guojialing suite was formed by the reaction of delaminated eclogitic crust-derived melt with the upwelling asthenospheric mantle. The petrogenesis of these two contrasting adakitic granitoids suggests intensive lower-crustal delamination during Early Cretaceous times, following a crustal thickening process from the late stage of the Early Jurassic to early stage of the Late Jurassic with crustal thickness of 50 km, respectively.

Early Paleozoic depositional environment and intraplate tectono-magmatism in the Cathaysia Block (South China): Evidence from stratigraphic, structural, geochemical and geochronological investigations

The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolite-facies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative εNd(t) values (−9.7 to −13.7) and two-stage εNd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206Pb/238U ages of 435 ± 4 Ma, 424 ± 5 Ma, 428 ± 3 Ma and 427 ± 2 Ma. All the zircon εHf(t) values are negative (−6 to −9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subduction-collision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.

Petrology and geochemistry of shoshonitic plutons from the western Kunlun orogenic belt, Xinjiang, northwestern China: implications for granitoid geneses

Abstract A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+Fe T ) and Fe 3+ /Fe 2+ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+Fe T ) and Fe 3+ /Fe 2+ ratio. The rocks show SiO 2 contents of 52.77–71.85% and high K 2 O+Na 2 O (mostly >8%, average 9.14%), K 2 O/Na 2 O (mostly >1, average 1.50) and Fe 2 O 3 /FeO (0.85–1.51, average 1.01) and low TiO 2 contents (0.15–1.12%, average 0.57%). Al 2 O 3 contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning.

Geochronology and geochemistry of Neoproterozoic mafic rocks from western Hunan, South China: implications for petrogenesis and post-orogenic extension

The Neoproterozoic mafic rocks in western Hunan, South China, form a NNE-striking mafic rock belt for which outcrops are found predominantly in Guzhang, Qianyang and Tongdao. Samples from Qianyang and Tongdao yielded ion microprobe U–Pb zircon ages of 747 ± 18 Ma and 772 ± 11 Ma, respectively. The mafic rocks are geochemically divided into two subtypes. Ultramafic rocks from Tongdao are depleted in Nb and Ti, with decoupled Nd–Hf isotopes, and geochemical features similar to the c. 761 Ma mafic–ultramafic rocks from Longsheng, northern Guangxi. Their e Nd (t) value is −2.91, implying an enriched mantle source. Alkaline mafic rocks from Qianyang and Guzhang have high values of TiO 2 , total alkali, some high field strength elements and (La/Yb) N , and low Zr/Nb, La/Nb, Sm/Nd and 143 Nd/ 144 Nd ratios as well as coupled Nd–Hf isotopes. They are geochemically similar to ocean island basalts and show fractional crystallization of Fe–Ti oxides, olivine and pyroxene in the mafic magma. The c. 760 Ma mafic rocks in western Hunan may be the products of post-orogenic magmatism. After the Jinningian (Sibao) orogenic process, the upwelling of the deep asthenospheric mantle caused by the break-off and detachment of the subducted oceanic slab led to extension in the area. The extension might have taken place earlier in the Tongdao and Longsheng areas, which led to the partial melting of the lithospheric mantle that had been metasomatized during early oceanic subduction to generate a relatively large amount of sub-alkaline rocks. However, the less alkaline mafic rocks in Qianyang and Guzhang might have been generated in the relatively later stage of the extension, and may have resulted from a small degree of partial melting of the asthenospheric mantle.

Petrogenesis of Late Jurassic Qianlishan granites and mafic dykes, Southeast China: implications for a back-arc extension setting

A late Mesozoic belt of volcanic-intrusive complexes occurs in Southeast China. The Qianlishan granites are distributed in the northwest of the belt. The pluton is composed of porphyritic biotite granite (153 Ma) and equigranular biotite granite (151 Ma) and was intruded by granite-porphyry dykes (144 Ma) and mafic dykes such as lamprophyre and diabase (142 Ma). The granitic rocks, consisting mainly of K-feldspar, plagioclase, quartz and Fe-rich biotite, have SiO 2 contents of 72.9–76.9%, and are enriched in alkalis, rare earth elements (REE), high field strength elements (HFSE) and Ga with high Ga/Al ratios, but depleted in Ba, Sr and transition metals. Trace-element geochemistry and Sr–Nd isotope systematics further imply that the Qianlishan granitic magmas were most probably derived by partial melting of Palaeo- to Mesoproterozoic metamorphic lower-crustal rocks that had been granulitized during an earlier thermal event. These features suggest an A-type affinity. The Qianlishan lamprophyre and neighbouring coeval mafic dykes (SiO 2 = 47.9–53.8 wt%) have high MgO and compatible element contents. These rocks also have high K 2 O contents and are enriched in alkalis, light REE, large ion lithophile elements, and depleted in HFSE. They have low initial e Nd values and relatively high initial 87 Sr/ 86 Sr ratios. We suggest a subduction-modified refractory lithospheric mantle (phlogopite-bearing harzburgite or lherzolite) for these high-Mg potassic magmas. The Qianlishan diabases (SiO 2 = 48.4–48.7 wt%) are alkaline and have high TiO 2 and total Fe 2 O 3 contents, together with the positive initial e Nd value, suggesting derivation from fertile asthenopheric mantle (phlogopite-bearing lherzolite). A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Qianlishan granites and associated mafic dykes. Between 180 and 160 Ma, Southeast China was a continental arc, forming the 180–160 Ma plutons of the late Mesozoic volcanic-intrusive complex belt, and the lower-crust was granulitized. Since 160 Ma the northwestern belt has been in a back-arc extensional setting as a consequence of slab roll-back, resulting in the lithosphere thinning and an influx of asthenophere. The upwelling asthenosphere, on the one hand, induced the local lithospheric mantle to melt partially, forming high-Mg potassic magmas, and on the other hand it underwent decompression melting itself to form alkaline diabase magma. Pulsatory injection of such high-temperature magmas into the granulitized crustal source region induced them to partially melt and generate the A-type magmas of the Qianlishan granitic rocks.

Two subgroups of A-type granites in the coastal area of Zhejiang and Fujian Provinces, SE China: age and geochemical constraints on their petrogenesis

Late Cretaceous (90–100 Ma) A-type granites are widespread in the coastal area of the Zhejiang and Fujian Provinces, SE China. According to mineralogical and geochemical characteristics, the A-type granites in this belt can be further divided into aluminous and peralkaline subgroups. The aluminous subgroup often contains aluminous-rich minerals (e.g. spessartine and Mn-rich muscovite), while the peralkaline subgroup usually contains riebeckite, arfvedsonite and aegirine. Geochemically, the aluminous A-type granites show lower Nb, Zr, Ga, Y and REE abundances, and lower FeO*/MgO and Ga/Al than the peralkaline subgroup. When they occur in the same area, the two subgroups of A-type granites display quite similar initial Nd isotopic compositions, which are indicative of mixing of ancient basement crustal rocks with variable amounts of mantle materials. Integrated geological and geochemical investigations indicate that both the aluminous and the peralkaline magmas are highly evolved and reflect the residual liquids left after high degrees of fractional crystallisation in a deep magma chamber. The present authors suggest that the mineralogical and geochemical differences between the aluminous and peralkaline subgroups are likely to have been generated via different differentiation paths controlled by varying fluorine contents of the parent magmas.

CHEMICAL AND RB-SR, SM-ND ISOTOPIC SYSTEMATICS OF TOURMALINE FROM THE DACHANG SN-POLYMETALLIC ORE DEPOSIT, GUANGXI PROVINCE, P.R. CHINA

Abstract The Dachang Sn-polymetallic ore deposit (Guangxi, China) is one of the largest Sn deposits in the world, which occurs in Devonian siliceous rocks and limestones as stratiform, massive and vein orebodies. Various models for the ore genesis of the deposit have been proposed, and the major debate focuses on whether the stratiform Sn–Pb–Zn orebodies are syn-sedimentary in origin or they were products of the Yanshanian magmatic-hydrothermal event (∼100 Ma) like the vein-type ores. In this paper, we present a chemical and Rb–Sr, Sm–Nd isotopic study of tourmaline, which occurs both in the stratiform orebodies and in veins associated with the granite in the Dachang deposit, and use these data to constrain the origin of the tourmaline and associated mineralisation. Two types of tourmaline occur in the Dachang deposit: (I) tourmaline from the stratiform Sn–Pb–Zn ores and their host siliceous rocks; (II) quartz-tourmaline veins in or near the granite. Group I tourmalines are Mg-rich dravites with low Fe/(Fe+Mg) ratios (0.01–0.25) and variable Na/(Na+Ca) ratios (0.62–1.0); group II tourmalines are Fe-rich schorls with high Fe/(Fe+Mg) and Na/(Na+Ca) ratios (0.81–0.88 and 0.92–0.97, respectively). Group II tourmalines have low REE contents and LREE-enriched patterns with negative Eu anomalies (Eu/Eu*=0.06 to 0.35). In contrast, group I tourmalines have higher and more variable REE contents with HREE-enriched patterns and negative to positive Eu anomalies (Eu/Eu*=0.18 to 3.64). They also have different contents for other trace elements such as U, Th, Zr, Hf, and Sn, which may reflect the nature of the hydrothermal fluids that circulated deep into the footwall lithologies. Group I tourmalines have 87 Sr / 86 Sr ratios of 0.71339–0.71818 and 143 Nd / 144 Nd ratios of 0.51201–0.51210; while the group II tourmalines have more variable 87 Sr / 86 Sr ratios (0.71187–0.72735) and slightly higher 143 Nd / 144 Nd ratios (0.51210–0.51224). The Sr and Nd isotopic data display different mixing trends, suggesting different origins for the two types of tourmalines. Group I tourmalines are thought to have formed during the Devonian from deeply circulating submarine hydrothermal fluids. The fluids leached Sr and Nd from the footwall lithologies (with high 87 Sr / 86 Sr and low 143 Nd / 144 Nd ) and mixed Sr and Nd derived from the overlying Devonian host sedimentary rocks (with low 87 Sr / 86 Sr and intermediate 143 Nd / 144 Nd ). Group II tourmalines formed from Yanshanian magmatic-hydrothermal fluids, with their Sr and Nd derived from a granitic source (remelted crustal rocks with high 87 Sr / 86 Sr and relatively high 143 Nd / 144 Nd ) and again mixing with Sr and Nd derived from host Devonian sedimentary rocks.

Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia

Abstract Tourmaline is a commonly found gangue mineral in Archean greenstone belt lode-gold deposits worldwide. In this paper, we report major, trace, and rare-earth element (REE) and boron isotopic compositions of tourmaline from two major amphibolite-grade Archean gold deposits (Big Bell and Mount Gibson; Murchison Province, Yilgarn Craton, Western Australia), in an attempt to better understand the hydrothermal gold formation processes that were also responsible for tourmalinization. Tourmalines in these two major deposits all belong to the dravite–schorl series, and tend to be Mg-rich where closely associated with gold and sulfide ores. The ore-related tourmalines also contain high concentrations of ore metals such as Au, Ag, Pb, and Cu. In the Big Bell deposit, tourmalines occur along the foliation in lode schists, and show low Fe/(Fe+Mg) ratios of 0.11–0.30, TiO 2 contents of 0.15–0.73 wt.%, and negligible MnO ( δ 11 B values of −17.7‰ to −15.0‰. In the Mount Gibson deposit, the tourmalines closely associated with the sulfide ores are the most Mg-rich, with low Fe/(Fe+Mg) ratios of 0.19–0.22, while tourmalines from garnet-bearing biotite–muscovite schists that overly the lode-gold orebody are more Fe-rich (Fe/(Fe+Mg)=0.30–0.63). Tourmalines from an amphibolite schist are Mg-rich, with Fe/(Fe+Mg) ratios (0.27–0.34) that lie between the above two groups of tourmalines. The compositional trend from Fe-rich tourmalines in the sulfide-poor schists to Mg-rich in the sulfide ores at Mount Gibson is similar to that observed in massive sulfide ore deposits worldwide. Alternatively, the Mg-rich nature of the sulfide-related tourmalines may be due to metamorphic reactions that are accompanied by Fe incorporation into sulfides, leaving coexisting silicates more Mg-rich. The REE geochemistry of the tourmalines from the Mount Gibson ore zone schists displays strong HREE enrichments and positive Eu anomalies with Eu/Eu* ratios up to 2.4, which are similar to tourmalines associated with massive sulfide deposits. The ore-related tourmalines disseminated in the host schists show δ 11 B values of −18.6‰ to −17.0‰, whereas one coarse-grained tourmaline vein has a lower δ 11 B value of −21.9‰ to −21.4‰. These data support hypotheses which suggest that the schists that host the gold mineralization are the metamorphosed equivalents of a base metal-rich, hydrothermally altered, seafloor horizon that is typical of VMS-style mineralization, with the boron possibly derived from leaching of footwall sedimentary and volcanic rocks during hydrothermal circulation.

New carbon isotope stratigraphy of the Ediacaran–Cambrian boundary interval from SW China: implications for global correlation

The Yangtze Platform preserves relatively thick carbonate successions and excellent fossil records across the Ediacaran–Cambrian boundary interval. The intensely studied Meishucun section in East Yunnan was one of the Global Stratotype Section candidates for the Precambrian–Cambrian boundary. However, depositional breaks were suspected in the section and the first appearance of small shelly fossils could not be verified. The Laolin section located in NE Yunnan is more continuous and shows great potential for global correlation of carbon isotope features across the Precambrian–Cambrian boundary. However, the stratigraphic framework and correlations were controversial. We studied and systematically sampled the Laolin section and present here new carbon isotope data for this section. The Laolin section consists of, in ascending order, the Baiyanshao dolostone of the Dengying Formation, the Daibu siliceous dolostone, Zhongyicun dolomitic phosphorite, lower Dahai dolostone and upper Dahai limestone of the Zhujiaqing Formation, and the black siltstone of the Shiyantou Formation. Our data reveal a large negative δ 13 C excursion (−7.2‰, L1′) in the Daibu Member, which matches the previously published data for the Laolin section, and a large positive excursion (+3.5‰, L4) in the Dahai Member, which was not shown in the published data. The excursion L1′ correlates well with the similarly large negative excursion near the first appearance of small shelly fossils in Siberia and Mongolia. Similar magnitude excursions are also known from Morocco and Oman, for which there are no robust fossil constraints but from where volcanic ash beds have been dated precisely at 542 Ma, thus confirming a global biogeochemical event near the Ediacaran–Cambrian boundary. Our data also indicate that deposition was more continuous at the Laolin section compared with the Meishucun section, where there are no records of a comparable negative excursion near the Ediacaran–Cambrian boundary, nor any comparable positive excursion in the Dahai Member. Therefore, the Laolin section has proven potential to be a supplementary Global Stratotype Section for the Ediacaran–Cambrian boundary on the Yangtze Platform.