扬子地块西、北缘中元古代地层的划分与对比

广泛分布于扬子地块西缘和北缘的中元古代地层经历了强烈的变形和绿片岩相的变质改造。根据形成时代,该区的中元古代可以识别出两个阶段,中元古代早期(1.8~1.4Ga)和中元古代晚期(1.4~1.0Ga)。中元古代早期的地层包括大红山群、东川群、河口群和通安组(1~4段),中元古代晚期的地层主要由分布在扬子地块西南缘的昆阳群、会理群和分布于扬子地块北缘的神农架群和打鼓石群组成。新的锆石原位定年结果表明,通安组的凝灰岩形成于1744±14Ma左右,河口群角斑岩形成于1659±23Ma左右,侵入会理群天宝山组的辉长辉绿岩形成于1026±7Ma左右。根据岩石组合、形成环境以及年代学资料,中元古代早期的大红山群、东川群、河口群和通安组(1~4段)形成时代相近,地层组成基本相同,它们都含有与岩浆热液有关的铁氧化物铜金(IOCG)矿床或层状铜矿床(SSC),都在1.75~1.45Ga期间形成于大陆裂解环境。扬子地块北缘的火地垭群也可能属于中元古代早期地层。中元古代晚期地层在扬子地块西缘北缘均有分布,其中的昆阳群和会理群大体形成于1.2~1.0Ga,神农架群和打鼓石群形成于1.4~1.0Ga,它们的顶界可能延伸到新元古代早期。在中元古代晚期的地层中含有大量叠层石,表明它们形成于温暖潮湿的浅海环境。除上述的中元古代晚期地层之外,云南元谋地区的苴林群、川西的登相营群、通安组五段等也属于中元古代晚期的地层。     

滇中姚安老街子板内富碱火山岩锆石SHRIMP U-Pb年代学及火山机构划分

滇中姚安老街子板内富碱火山-岩浆杂岩体属于哀牢山-金沙江富碱侵入岩带的一部分,因其富含Pb-Ag-Au多金属矿床和特殊大地构造位置被地质学家广泛关注。尽管前人在矿床学(金属赋存状态和物质来源)、岩石学、地球化学等方面展开大量的研究,但关于姚安富碱火山岩体火山机构和岩相学以及形成时代等却少有研究。本文对老街子火山-岩浆杂岩体进行详细的岩相学地质调查和剖面实测,划分出完整的次火山岩相(含集块正长斑岩)、火山-沉积相(凝灰质砾岩和砂岩)和火山喷发相火山机构,其中将火山喷发相细分为溢流亚相(熔结凝灰岩和假白榴石粗面岩)和爆发亚相(含角砾和集块粗面质熔结凝灰岩)。并对粗面质凝灰岩和熔结凝灰岩进行锆石SHRIMP U-Pb精确年代学研究,最终分别获得~(206)Pb/~(238)U年龄加权平均值为33.63±0.50Ma和33.27±0.38Ma,代表着火山岩喷发形成时代为渐新世。该期火山岩的报道为老街子Pb-Ag-Au矿区深部外围找矿预测提供新思路,也为青藏高原东南缘新生代岩石圈构造演化和地球动力学过程研究提供新的窗口。     

Origin of highly siderophile and chalcogen element fractionations in the components of unequilibrated H and LL chondrites

Osmium isotopic compositions, abundances of highly siderophile elements (HSE: platinum group elements, Re and Au), the chalcogen elements S, Se and Te and major and minor elements were analysed in physically separated size fractions and components of the ordinary chondrites WSG 95300 (H3.3, meteorite find) and Parnallee (LL3.6, meteorite fall). Fine grained magnetic fractions are 268-65 times enriched in HSE compared to the non-magnetic fractions. A significant deviation of some fractions of WSG 95300 from the 4.568 Ga ¹⁸⁷Re-¹⁸⁷Os isochron was caused by redistribution of Re due to weathering of metal. HSE abundance patterns show that at least four different types of HSE carriers are present in WSG 95300 and Parnallee. The HSE carriers display (i) CI chondritic HSE ratios, (ii) variable Re/Os ratios, (iii) lower than CI chondritic Pd/Ir and Au/Ir and (iv) higher Pt/Ir and Pt/Ru than in CI chondrites. These differences between components clearly indicate the loss of refractory HSE carrier phases before accretion of the components. Tellurium abundances correlate with Pd and are decoupled from S, suggesting that most Te partitioned into metal during the last high-temperature event. Tellurium is depleted in all fractions compared to CI chondrite normalized Se abundances. The depletion of Te is likely associated with the high temperature history of the metal precursors of H and LL chondrites and occurred independent of the metal loss event that depleted LL chondrites in siderophile elements. Most non-magnetic and slightly magnetic fractions have S/Se close to CI chondrites. In contrast, the decoupling of Te and Se from S in magnetic fractions suggests the influence of volatility and metal-silicate partitioning on the abundances of the chalcogen elements. The influence of terrestrial weathering on chalcogen element systematics of these meteorites appears to be negligible.     

Magmatic hydrothermal origin of the Hadamengou-Liubagou Au-Mo deposit,Inner Mongolia,China: Constrains on geology,stable and Re-Os isotopes

The Hadamengou-Liubagou Au-Mo deposit is the largest gold deposit in Inner Mongolia of North China. It is hosted by amphibolite to granulite facies metamorphic rocks of the Archean Wulashan Group. To the west and north of the deposit, there occur three alkaline intrusions, including the Devonian-Carboniferous Dahuabei granitoid batholith, the Triassic Shadegai granite and the Xishadegai porphyritic granite with molybdenum mineralization. Over one hundred subparallel, sheet-like ore veins are confined to the nearly EW-trending faults in the deposit. They typically dip 40° to 80° to the south, with strike lengths from hundreds to thousands of meters. Wall rock alterations include potassic, phyllic, and propylitic alteration. Four distinct mineralization stages were identified at the deposit, including K-feldspar-quartz-molybdenite stage (I), quartz-pyrite-epidote/chlorite stage (II), quartz-polymetallic sulfide-gold stage (III), and carbonate-sulfate-quartz stage (IV). Gold precipitated mainly during stage III, while Mo mineralization occurred predominantly in stage I. The δD_H2O and δ¹⁸O_H2O values of the ore-forming fluids range from −125‰ to −62‰ and from 1.4‰ to 7.5‰, respectively, indicating that the fluids were dominated by magmatic water with a minor contribution of meteoric water. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal carbonate minerals vary from −10.3‰ to −3.2‰ and from 3.7‰ to 15.3‰, respectively, suggesting a magmatic carbon origin. The δ³⁴S_CDT values of sulfides from the ores vary from −21.7‰ to 5.4‰ and are typically negative (mostly −20‰ to 0‰). The wide variation of the δ³⁴S_CDT values, the relatively uniform δ¹³C values of carbonates (typically −5.5‰ to −3.2‰), as well as the common association of barite with sulfides suggest that the minerals were precipitated under relatively high fo₂ conditions, probably in a magmatic fluid with δ³⁴S_ƩS ≈ 0‰. The Re-Os isotopic dating on molybdenite from Hadamengou yielded a weighted average age of 381.6 ± 4.3 Ma, indicating that the Mo mineralization occurred in Late Devonian. Collectively, previous ⁴⁰Ar-³⁹Ar and Re-Os isotopic dates roughly outlined two ranges of mineralizing events of 382–323 Ma and 240–218 Ma that correspond to the Variscan and the Indosinian epochs, respectively. The Variscan event is approximately consistent with the Mo mineralization at Hadamengou-Liubagou and the emplacement of the Dahuabei Batholith, whereas the Indosinian event roughly corresponds to the possible peak Au mineralization of the Hadamengou-Liubagou deposit, as well as the magmatic activity and associated Mo mineralization at Xishadegai and Shadegai. Geologic, petrographic and isotopic evidence presented in this study suggest that both gold and molybdenum mineralization at Hadamengou-Liubagou is of magmatic hydrothermal origin. The molybdenum mineralization is suggested to be associated with the magmatic activity during the southward subduction of the Paleo-Asian Ocean beneath the North China Craton (NCC) in Late Devonian. The gold mineralization is most probably related to the magma-derived hydrothermal fluids during the post-collisional extension in Triassic, after the final suturing between the Siberian and NCC in Late Permian.     

Magmatic–hydrothermal processes in Sangdong W–Mo deposit,Korea: Study of fluid inclusions and 39Ar–40Ar geochronology

The Sangdong scheelite–molybdenite deposit in northeast South Korea consists of strata-bound orebodies in intercalated carbonate-rich layers in the Cambrian Myobong slate formation. Among them, the M1 layer hosts the main orebody below which lie layers of F1–F4 host footwall orebodies. Each layer was first skarnized with the formation of a wollastonite + garnet + pyroxene assemblage hosting minor disseminated scheelite. The central parts of the layers were subsequently crosscut by two series of quartz veining events hosting minor scheelite and major scheelite–molybdenite ores, respectively. The former veins associate amphibole–magnetite (amphibole) alteration, whereas the latter veins host quartz–biotite–muscovite (mica) alteration. Deep quartz veins with molybdenite mineralization are hosted in the Cambrian Jangsan quartzite formation beneath the Myobong formation. In the Sunbawi area, which is in close proximity to the Sangdong deposit, quartz veins with scheelite mineralization are hosted in Precambrian metamorphic basement. Three muscovite ³⁹Ar–⁴⁰Ar ages between 86.6 ± 0.2 and 87.2 ± 0.3 Ma were obtained from M1 and F2 orebodies from the Sangdong deposit and Sunbawi quartz veins. The Upper Cretaceous age of the orebodies is concordant with the published ages of the hidden Sangdong granite, 87.5 ± 4.5 Ma. This strongly suggests that the intrusion is causative for the Sangdong W–Mo ores and Sunbawi veins.Fluid inclusions in the quartz veins from the M1 and F2 orebodies, the deep quartz-molybdenite veins, and the Sunbawi veins are commonly liquid-rich aqueous inclusions having bubble sizes of 10–30 vol%, apparent salinities of 2–8 wt% NaCl eqv., and homogenization temperatures of 180–350 °C. The densities of the aqueous inclusions are 0.70–0.94 g/cm³. No indication of fluid phase separation was observed in the vein. To constrain the formation depth in the Sangdong deposit, fluid isochores are combined with Ti–in–quartz geothermometry, which suggests that the M1 and F2 orebodies were formed at depths of 1–3 km and 5–6 km below the paleosurface, respectively. The similarity of the Cs (cesium) concentrations and Rb/Sr ratios in the fluid inclusions of the respective orebodies indicate an origin from source magmas having similar degrees of fractionation and enrichment of incompatible elements such as W and Mo. High S concentrations in the fluids and possibly organic C in the sedimentary source likely promoted molybdenite precipitation in the Sangdong orebodies, whereas the scheelite deposition in the deep quartz–molybdenite veins hosted in the quartzite is limited by a lack of Ca and Fe in the hydrothermal fluids. The molybdenite deposition in the Sunbawi quartz–molybdenite veins hosted in the Precambrian metamorphic basement rocks was possibly limited by a lack of reducing agents such as organic C.     

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling,China: insights from LA-ICP-MS zircon U–Pb ages,Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO₂ (63.59–76.22%) and low P₂O₅ (0–0.2%) concentrations, a high K₂O/Na₂O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous ε_Hf(t) (?4.425 to 1.067 for Xujiaba and ?4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.     

赣西北大雾塘钨矿区地质特征及Re-Os同位素年代学研究

通过对大湖塘钨矿田的大雾塘钨矿区辉钼矿Re-Os同位素年代学的研究,测得辉钼矿的w(Re)为0.3368×10~(-6)~8.256×10~(-6),获得的5个模式年龄比较一致,介于(136.6±2.2)Ma~(138.4±2.4)Ma,加权平均年龄为(137.7±2.7)Ma(MSWD=0.07)。将5个模式年龄进行等时线年龄计算,获得一条相关性较好的~(187)Re-~(187)Os等时线,计算得到辉钼矿Re-Os等时线年龄为(137.9±2.0)Ma(MSWD=0.20),与加权平均年龄一致,可代表辉钼矿的形成年龄。结合石门寺和狮尾洞矿区典型矿床地质、地球化学特征和成岩作用时空关系,认为大雾塘矿床的形成是大湖塘钨矿田的第二期次(140 Ma)大规模成矿作用的产物,2期成矿作用可能是大湖塘钨矿田巨量成矿元素堆积的重要原因之一。     

海南省新村钼矿床LA-ICP-MS锆石U-Pb和辉钼矿Re-Os年龄及其地质意义

新村钼矿床是海南省近年来发现的具中型规模的钼矿床。文章对赋矿二长花岗岩进行LA-ICP-MS锆石U-Pb同位素测年,获得~(206)Pb/~(238)U年龄加权平均值为(102.0±1.5)Ma(MSWD=2.3);对辉钼矿进行Re-Os同位素定年,获得5件样品的模式年龄范围为(97.29±1.43)Ma~(98.52±1.55)Ma,加权平均年龄为(97.84±0.64)Ma,等时线年龄为(98.90±3.40)Ma,成岩年龄与成矿年龄在误差范围内一致。辉钼矿的Re含量和锆石Hf同位素特征指示新村钼矿的成岩成矿物质来自于壳幔混源。新村钼矿床的成岩成矿年龄与海南岛最重要的钼成矿期(95~105 Ma)一致,属中国东部早白垩世晚期—晚白垩世早期钼成矿事件的一部分,与该时期岩石圈大规模拉伸减薄、软流圈上涌及强烈壳-幔作用密切相关。     

Protracted thrusting followed by late rapid cooling of the Greater Himalayan Sequence,Annapurna Himalaya,Central Nepal: Insights from titanite petrochronology

The Greater Himalayan Sequence (GHS) has commonly been treated as a large coherently deforming high‐grade tectonic package, exhumed primarily by simultaneous thrust‐ and normal‐sense shearing on its bounding structures and erosion along its frontal exposure. A new paradigm, developed over the past decade, suggests that the GHS is not a single high‐grade lithotectonic unit, but consists of in‐sequence thrust sheets. In this study, we examine this concept in central Nepal by integrating temperature–time (T–t) paths, based on coupled Zr‐in‐titanite thermometry and U–Pb geochronology for upper GHS calcsilicates, with traditional thermobarometry, textural relationships and field mapping. Peak Zr‐in‐titanite temperatures are 760–850°C at 10–13 kbar, and U–Pb ages of titanite range from c. 30 to c. 15 Ma. Sector zoning of Zr and distribution of U–Pb ages within titanite suggest that diffusion rates of Zr and Pb are slower than experimentally determined rates, and these systems remain unaffected into the lower granulite facies. Two types of T–t paths occur across the Chame Shear Zone (CSZ). Between c. 25 and 17–16 Ma, hangingwall rocks cool at rates of 1–10°C/Ma, while footwall rocks heat at rates of 1–10°C/Ma. Over the same interval, temperatures increase structurally upwards through the hangingwall, but by 17–16 Ma temperatures converge. In contrast, temperatures decrease upwards in footwall rocks at all times. While the footwall is interpreted as an intact, structurally upright section, the thermometric inversion within the hangingwall suggests thrusting of hotter rocks over colder from c. 25 to c. 17–16 Ma. Retrograde hydration that is restricted to the hangingwall, and a lithological repetition of orthogneiss are consistent with thrust‐sense shear on the CSZ. The CSZ is structurally higher than previously identified intra‐GHS thrusts in central Nepal, and thrusting duration was 3–6 Ma longer than proposed for other intra‐GHS thrusts in this region. Cooling rates for both the hangingwall and footwall of the CSZ are comparable to or faster than rates for other intra‐GHS thrust sheets in Nepal. The overlap in high‐T titanite U–Pb ages and previously published muscovite ⁴⁰Ar/³⁹Ar cooling ages imply cooling rates for the hangingwall of ≥200°C/Ma after thrusting. Causes of rapid cooling include passive exhumation driven by a combination of duplexing in the Lesser Himalayan Sequence, and juxtaposition of cooler rocks on top of the GHS by the STDS. Normal‐sense displacement does not appear to affect T–t paths for rocks immediately below the STDS prior to 17–16 Ma.     

广东锡坪钼铜多金属矿床辉钼矿Re-Os同位素定年及其地质意义

广东锡坪钼铜多金属矿床位于钦杭成矿带的西南端,为一大型的斑岩型矿床。本文利用辉钼矿Re-Os同位素定年方法对锡坪钼铜多金属矿床4件辉钼矿样品进行了成矿年代学测定,获得的模式年龄为85.15~88.34Ma,加权平均值为86.1±2.3Ma,对应的等时线年龄为89.9±3.4Ma,模式年龄和等时线年龄结果在误差范围内基本一致,指示锡坪钼铜多金属矿床的成矿时限为晚白垩世。锡坪钼铜多金属矿辉钼矿样品的Re含量较低,表明其成矿物质可能主要来自于壳源。钦杭成矿带成岩成矿作用以中生代燕山期为主,存在180~150Ma、110~80Ma两个爆发期,两期成岩成矿作用可能均与太平洋板块的俯冲有一定的关系。锡坪钼铜多金属矿床是在岩石圈伸展减薄环境下发生的大规模成矿作用的产物。