The significance of magmatic erosion for bifurcation of UG1 chromitite layers in the Bushveld Complex     

《Ore Geology Reviews》2017

One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex.  相似文献   

Assimilation of sulfate and carbonaceous rocks: Experimental study,thermodynamic modeling and application to the Noril’sk-Talnakh region (Russia)     

《Ore Geology Reviews》2017

Most of the intrusions in the Noril’sk-Talnakh region (Siberia) are hosted in thick sedimentary sequences including abundant evaporitic and terrigenous sedimentary rocks. Three mafic-ultramafic intrusions in this region contain unusually thick massive sulfide deposits, which represent one of the world’s largest economic concentrations of Ni, Cu and PGE. The interaction of Siberian magmas with sulfate and organic matter-rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated. Here we clarify, by both experimental petrology and thermodynamic modeling, how sulfate and organic matter assimilation occur in mafic-ultramafic magmas, affecting magma composition, crystallization and sulfide saturation.Interaction experiments were conducted at conditions relevant to the emplacement of Noril’sk type intrusions (1200 °C, ∼80 MPa) to simulate the assimilation of sulfate and/or organic compounds by ultramafic magmas. We used a picrite from Noril’sk1 intrusion, and coal and anhydrite from the area as starting materials. The experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt, which increases the magma’s sulfur content, but suppresses sulfide saturation and reduces olivine crystallization. Extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0.9 ± 0.1 wt% S. Conversely, coal assimilation promotes sulfide segregation and magma crystallization, while decreasing the dissolved H₂O content of the melt and increasing the amount of coexisting fluid phase.We also employed gas-melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the S content of the magma, and investigate the role of temperature, pressure, and initial gas content of the magma in the assimilation process. We quantify how sulfate assimilation strongly oxidizes the magma and increases its S content; both effects are intensified by increasing pressure (from 50 to 100 MPa in this study), decreasing temperature (from 1350 to 1200 °C in this study), and decreasing amounts of fluid phase initially coexisting with the magma (from 2 to 0 wt%). The interaction with organic matter (CH in this study) induces a strong reduction of the magma, even for extremely low degrees of assimilation (few tenths of wt%), and the dehydration of the melt.We therefore suggest that in the Noril’sk-Talnakh district (1) additional S was supplied to mantle derived magmas by the assimilation of evaporitic rocks, and was transported during magma ascent in the form of dissolved, oxidized S; (2) a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments. This mechanism can potentially produce massive sulfide deposits by important sulfate assimilation (locally higher than 3 wt% CaSO₄) and minor organic matter assimilation (few tenths of wt% CH); however, if one of the two steps does not occur, or the assimilation in (1) is not large enough, disseminated sub economic or no sulfide deposits are produced. We conclude that exceptional conditions favoring substantial assimilation of sediments are needed to generate exceptional ore deposits like those of the Noril’sk-Talnakh district.  相似文献   

巴布亚新几内亚利希尔岛拉多拉姆(Ladolam)金矿矿床地质特征及找矿标志     

陈丹丹 《地质与勘探》2017,53(1):187-197

巴布亚新几内亚利希尔岛拉多拉姆金矿处于太平洋板块与印澳板块的板块边界部位,为西南太平洋地区最大的碱性侵入岩有关的低硫型浅成低温热液型金矿。矿床赋存在Luise火山内,成矿与更新世碱性二长闪长岩侵入体有关,该岩体形成于俯冲带环境下被流体交代的上地幔的部分熔融。矿床的矿化可分为三个阶段,在岩体侵入后形成斑岩铜矿化,并在之后逐渐过渡为浅成低温热液型金矿化。该类矿床在成矿过程和矿体的分布明显受到区域性构造、蚀变带和成矿有关角砾岩等成矿结构面的控制,在找矿标志上具有特定的物化探和地质特征,具有较好的找矿远景。  相似文献   

东昆仑石头坑德镁铁-超镁铁质岩体矿物学特征及成矿指示   总被引：2，自引：0，他引：2  

张照伟 《地质与勘探》2017,53(5):825-837

石头坑德镁铁-超镁铁质岩体位于东昆仑造山带东段五龙沟地区,邻近昆中大断裂,主要岩性为辉石岩、橄辉岩、橄榄岩、辉长岩等,铜镍矿体主要赋存于Ⅰ号岩体的辉石岩、橄辉岩和橄榄岩中。岩石地球化学特征表明,其镁铁比值(m/f)变化范围在2.77~6.01之间,属铁质系列的镁铁-超镁铁岩,有利于成铜镍矿。稀土总量总体较低,轻稀土元素之间分馏强,重稀土元素之间分馏弱。岩石总体富集LILE(Rb、Ba、Th、U),贫HFSE(Nb、Ta、Zr、Y),具有明显的Rb、U正异常及Nb、Ta的负异常。超镁铁岩类橄榄石中的Ni普遍亏损,指示深部存在硫化物的熔离作用,在橄榄石结晶之前,大量的Ni进入到硫化物中,极有利于形成富矿岩浆或矿浆。可见,伴随同化混染作用的发生,岩浆中S达到过饱和,进而硫化物发生不混溶作用,富含硫化物的岩浆运移至东昆仑造山带邻近昆中断裂构造薄弱的部位成岩成矿。铜镍矿体多赋存在含橄榄石的超镁铁质岩体内,指示石头坑德岩体深部存在高度富集铜镍的部位,是下一步的找矿方向。  相似文献   

Differential host use affects fecundity of the gastropod Crepidula onyx          下载免费PDF全文

Amber R. Von Tungeln  Bruno Pernet 《Marine Ecology》2017,38(2)

For epibiotic or symbiotic marine invertebrates, alternative host species may differ substantially in quality, and under some circumstances such differences in host quality may lead to the evolution of increased host specificity. However, the fitness consequences of alternative hosts for epibiotic or symbiotic marine invertebrates have rarely been quantified. In Southern California, the gastropod Crepidula onyx is often found as an epibiont on either bay mussels (Mytilus galloprovincialis) or cone snails (Conus californicus). These hosts differ greatly in maximum size, with possible effects on size at sex change and final size in Cr. onyx, and thus on fecundity. Further, Cr. onyx on the two hosts differ in shell shape, possibly affecting the size of the ctenidium, which Cr. onyx uses for suspension feeding. We examined these potential effects of host use on fitness components in Cr. onyx. Epibionts on mussels reached much larger average sizes than did those on cone snails; further, epibionts on mussels often completed sex change at much larger sizes than did those on cone snails. On average, mussel epibionts had threefold higher average fecundities than did cone snail epibionts. Although there was a slight difference in shell shape between epibionts on the two host species, there was no difference in the scaling of ctenidium area with body size for Cr. onyx from the two hosts. The large average differences in fecundity in epibionts associated with the two alternative hosts suggests that there may be strong selection on host choice at larval settlement.  相似文献   

吉林省西林河脆-韧性剪切带及其与金矿化的关系     

赵凤桐  徐光荣  梁宜林  赵宏光  王好均 《地质与资源》1996,5(2):108-113

西林河脆-韧性剪切带是区域上富尔河-古洞河岩石圈深断裂带的重要组成部分,总体走向330°,由东部(F₁,F₂,F₃)、中部(F₄,F₅)和西部(F₆)3条脆-韧性剪切断层带组成。东部带以脆性变形为主,延长约4km;中部带除脆性变形外兼有塑性变形,延长约5km;西部带以塑性变形为主,总延长5.5km。构造岩比较发育,分为脆性变形构造岩系列和塑性变形构造岩系列。构造岩的变形和退变作用同时进行。西林河脆-韧性剪切带反映了西林河金矿的成矿时代为燕山期,是金成矿的导矿和容矿构造。本着就矿找矿的原则,根据成矿地质条件,在已有的6个矿体的延伸方向进行追索找矿;根据边缘成矿作用观点,可在矿区内太古宙灰色片麻岩与元古宙浅变质岩的接触带进行找矿。确立了6条找矿标志。  相似文献   

Ca. 1.7 Ga Magmatism on Southwestern Margin of the Yangtze Block: Response to the Breakup of Columbia     

GENG Yuansheng  DU Lilin  KUANG Hongwei  LIU Yongqing 《《地质学报》英文版》2020,94(6):2031-2052

This paper presents some data of the Jiaopingdu gabbro and Caiyuanzi granite at the southwestern margin of the Yangtze Block, on the geochemical compositions, zircon LA–ICP–MS U–Pb ages and Hf isotopic data. The Jiaopingdu gabbro gives the age of 1721 ± 5 Ma, the Caiyuanzi granite 1732 ± 6 Ma and 1735 ± 4 Ma, and the Wenjiacun porphyry granite 1713 ± 4 Ma, suggesting nearly contemporaneous formation time of the gabbro and granite. The bimodal feature is demonstrated by the gabbro SiO2 content of 44.64–46.87 wt% and granite 73.81–77.03 wt%. In addition, the granite has high content of SiO2 and Na2O + K2O, low content of Al2O3 and CaO, enriched in REEs (except Eu) and Zr, Nb, Ga and Y, depleted in Sr, implying it belongs to A-type granite geochemistry and origin of within-plate environment. The zircon εHf(t) of the granite and gabbro is at the range of 2–6, which is near the 2.0 Ga evolution line of the crust, implying the parent magma of the gabbro being derived from the depleted mantle and a small amount of crustal material, and the parent magma of the granite from partial melting of the juvenile crust and some ancient crustal material at the same time. Compared with 1.8–1.7 Ga magmatism during breakup of other cratons in the world, we can deduce that the Columbia has initially broken since ca. 1.8 Ga, and some continental marginal or intra-continental rifts occurred at ca. 1.73 Ga.  相似文献   

西秦岭何家庄—老虎窑岩体U-Pb年龄和成因及其对板块俯冲时间的限定          下载免费PDF全文

谭人文  王永  陈柏林  石永红  高允  申景辉 《中国地质》2020,47(4):1155-1172

何家庄—老虎窑岩体位于勉略缝合带以北,靠近秦岭微板块北部的商丹断裂带西段内部,两者的主体岩性为花岗闪长岩。本文对何家庄—老虎窑岩体进行了LA-ICP-MS锆石U-Pb同位素定年和地球化学特征以及Hf同位素特征研究。结果表明,西部何家庄附近花岗闪长岩侵位时间为235～240 Ma,东部老虎窑附近花岗岩侵位时间为239 Ma,两者均侵位于早三叠世。何家庄—老虎窑岩体花岗闪长岩均为高钾钙碱性系列,表现出强过铝质特征,属于I型花岗岩。两者岩浆锆石的εHf(t)值都是以负值为主,包含一到两个正值,地壳模式年龄为中元古代(1255～1754 Ma),指示它们的源岩有地幔物质的加入,可能是以中元古代古老地壳的部分熔融为主的壳源物质与地幔物质混合的岩浆。且表现出与埃达克质岩相似的地球化学特征,反映其岩浆可能起源于俯冲洋壳在高压环境下部分熔融产生的溶体与由于底侵作用被橄榄岩混染的下地壳部分熔融产生的高钾溶体的岩浆混合。两者的侵位年龄和成因证明了扬子板块与秦岭微板块在235～240 Ma的这个期间处于俯冲阶段。  相似文献   

Apatite U–Pb geochronology and whole rock,Sr–Nd–Pb isotopic geochemistry of XV mafic-ultramafic intrusion,Bafq, Central Iran: Implications for petrogenesis and tectonic setting     

Sakine Amraei  Mohammad Yazdi  Liang Qiu  Chang-Zhi Wu  Lei Chen  Bertrand Moine  Majid Ghasemi Siani  Qihui Zhang  Shahrokh Rajabpour 《Island Arc》2024,33(1):e12514

The XV mafic-ultramafic intrusion is located in the western part of the Posht-e-Badam Block (PBB) within the Central Iranian Micro-Continent (CIMC). Petrographically, the intrusion is composed of gabbro and pyroxenite. Apatite U–Pb dating has established the crystallization age of this intrusion to be 363 ± 67 Ma. The XV intrusive rocks are tholeiitic to slightly calc-alkaline in nature and are characterized by an enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and Heavy Rare Earth Elements (HREE), respectively. The major oxide elements display continuous trends relative to SiO₂. The ⁸⁷Sr/⁸⁶Sr(i) ratios range from 0.7045 to 0.7056, and the εNd(i) values range from 2.63 to 3.30. In addition, the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios exhibit a narrow range, varying from 18.68 to 18.70, 15.67 to 15.71, and 38.84 to 38.99, respectively. The geochemical and isotopic characteristics suggest that the parental magma was derived from a Sub- Continental Lithospheric Mantle (SCLM) that was modified by oceanic slab-derived components. The locations of the XV intrusive rocks in εNd(i) versus TDM (Ga) and Nb/La versus discrimination diagrams further support this conclusion. Fractional crystallization is identified as the dominant process influencing the formation of distinct lithological units within the XV intrusive rocks. Our newly presented isotopic and geochronological data, when considered in the regional context, suggest that the XV intrusive rocks were formed in an extensional tectonic setting. In this scenario, upwelling from the asthenospheric mantle induced heating, leading to the melting of previously subduction-modified SCLM. Comparative analysis with previously published ages indicates that extensional magmatism in the PBB continued into the Middle Paleozoic.  相似文献   

斯氏并殖吸虫第一中间宿主拟钉螺属一新种记述 (中腹足目, 盖螺科)          下载免费PDF全文

程由注  吴小平  李莉莎  林陈鑫  江典伟 《海洋科学》2009,33(10):97-99

为调查斯氏并殖吸虫拟钉螺宿主,现场采集螺标本,进行形态分类和生态考察及螺体寄生虫检查.发现拟钉螺属(Tricula)一新种,命名为建欧拟钉螺(Tricula jianouensis sp.nov.),螺壳高3.125 mm,壳宽1.600 mm,体螺层高1.125 mm,壳口长径1.275 mm,壳口宽径0.925 mm.螺口外缘翘起呈铲状;轴缘与外缘形成较窄夹角,内唇嵴明显高出,与体螺层间有一沟状隙.齿舌每一横列有7枚齿,齿式:2-1-2/2-2·3-1-3·11(14)·14(15).该螺充当斯氏并殖吸虫第一中间宿主,斯氏并殖吸虫尾蚴感染率为0.11%.  相似文献