Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit,Quadrilátero Ferrífero,Brazil     

《Ore Geology Reviews》2008,33(3-4):629-650

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.  相似文献   

Dehydration and melting during continental collision: Constraints from element and isotope geochemistry of low-T/UHP granitic gneiss in the Dabie orogen     

Qiong-Xia Xia  Yong-Fei Zheng  Li-Gang Zhou 《Chemical Geology》2008,247(1-2):36-65

A combined study of petrography, whole-rock major and trace elements as well as Rb?Sr and Sm?Nd isotopes, and mineral oxygen isotopes was carried out for two groups of low-T/UHP granitic gneiss in the Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during exhumation of deeply subducted continent. Zircon δ¹⁸O values of ? 2.8 to + 4.7‰ for the gneiss are all lower than normal mantle values of 5.3 ± 0.3‰, consistent with ¹⁸O depletion of protolith due to high-T meteoric-hydrothermal alteration at mid-Neoproterozoic. Most samples have extremely low ⁸⁷Sr/⁸⁶Sr ratios at t₁ = 780 Ma, but very high ⁸⁷Sr/⁸⁶Sr ratios at t₂ = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during Neoproterozoic magma emplacement and the metamorphic dehydration during Triassic continental collision. Rb–Sr isotopes, Th/Ta vs. La/Ta and Th/Hf vs. La/Nb relationships suggest that Group I gneiss experienced lower degrees of hydrothermal alteration, but higher degrees of dehydration, than Group II gneiss. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREEs but no correlations between Nb and LILEs (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILEs during the dehydration. Thus the correlation between Nb and LREEs is inherited from protolith rather than caused by metamorphic modification. Relative to Group I gneiss, Group II gneiss has stronger negative Eu anomaly, lower contents of Sr and Ba but higher contents of Rb, Th and U. In particular, Nb correlates with LILEs (e.g., Rb, Sr, Ba, Th and U), but not with LREEs (La and Ce). This may indicate decoupling between the dehydration and LILEs transport during continental collision. Furthermore, dehydration melting may have occurred due to breakdown of muscovite during “hot” exhumation. Group II gneiss has extremely low contents of FeO + MgO + TiO₂ (1.04 to 2.08 wt.%), high SiO₂ contents of 75.33 to 78.23 wt%, and high total alkali (Na₂O + K₂O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Almost no UHP metamorphic minerals survived; felsic veins of fine-grain minerals occurs locally between coarse-grain minerals, resulting in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. In contrast, Group I gneiss only shows metamorphic dehydration. Therefore, the two groups of gneiss show contrasting behaviors of fluid–rock interaction during the continental collision.  相似文献   

Zircon U–Pb and Hf isotopic study of gneissic rocks from the Chinese Altai: Progressive accretionary history in the early to middle Palaeozoic     

Min Sun  Chao Yuan  Wenjiao Xiao  Xiaoping Long  Xiaoping Xia  Guochun Zhao  Shoufa Lin  Fuyuan Wu  A. Kröner 《Chemical Geology》2008,247(3-4):352-383

Gneissic rocks in the Chinese Altai Mountains have been interpreted as either Paleozoic metasedimentary rocks or Precambrian basement. This study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a NE–SW traverse in the northwestern Chinese Altai. Petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from I-type granites formed in a subduction environment. Three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources. Zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 Ma, some give Neoproterozoic ages, and a few yield discordant Paleoproterozoic to Archean ages. Zircon Hf isotopic compositions indicate that both juvenile/mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived. In contrast, zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids, and define single populations for each sample with mean ages between 380 and 453 Ma. The general absence of Precambrian inheritance and positive zircon ?Hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas. Our data can be interpreted in terms of a progressive accretionary history in early to middle Palaeozoic times, and the Chinese Altai may possibly represent a magmatic arc built on a continental margin dominated by Neoproterozoic rocks.  相似文献   

Evolution of subcontinental lithospheric mantle beneath eastern China: Re–Os isotopic evidence from mantle xenoliths in Paleozoic kimberlites and Mesozoic basalts     

Hong-Fu Zhang  Steven L. Goldstein  Xin-Hua Zhou  Min Sun  Jian-Ping Zheng  Yue Cai 《Contributions to Mineralogy and Petrology》2008,155(3):271-293

The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.  相似文献   

东喜马拉雅构造结岩体冷却的⁴⁰Ar/³⁹Ar年代学研究   总被引：2，自引：1，他引：1  

龚俊峰  季建清  陈建军  桑海清  李宝龙  刘一多  韩宝福 《岩石学报》2008,24(10):2255-2272

  相似文献   

橄榄岩—熔体相互作用过程中Re-Os体系的变化趋势: 对华北新生代地幔橄榄岩Re-Os年龄含义的启示   总被引：1，自引：0，他引：1  

张宏福 《岩石学报》2008,24(11):2457-2467

橄榄岩的Re-Os同位素体系作为探讨岩石圈地幔形成年龄的重要手段已受到人们越来越多的重视。然而,Re-Os体系是否会受到橄榄岩的低温蚀变如蛇纹石化和地幔交代作用或橄榄岩—熔体相互作用的影响并不十分清楚。本文在总结国际上典型造山带橄榄岩剖面研究和地幔橄榄岩捕虏体研究的基础上,发现橄榄岩的Re-Os同位素体系基本不受蛇纹石化等低温蚀变作用的影响; 但橄榄岩—熔体相互作用会造成橄榄岩的Re-Os体系的变化即Re或Os的带入。当熔体/岩石比例较小时(≤1),橄榄岩—熔体相互作用会造成橄榄岩的Re含量显著增加,而放射成因的Os同位素即¹⁸⁷Os增量有限。但当熔体/岩石比例很大时(≥1),会造成¹⁸⁷Os同位素显著增加。同时,橄榄岩的Re含量与中等程度不相容主、微量元素如Al、Ti、Yb的正相关关系可以通过橄榄岩的两阶段演化模式来解释,即早期的部分熔融残留和后期的橄榄岩—熔体相互作用共同影响的结果。橄榄岩的Re含量与Cr含量的正相关关系进一步说明了这一问题。上述结果对华北东部岩石圈地幔的演化过程有重要的启示作用。因为华北东部古生代时期的确存在太古宙岩石圈地幔,然而新生代时期地幔橄榄岩捕虏体普遍具有元古宙而非太古宙的事实证明橄榄岩—熔体的相互作用的确广泛存在,从而造成岩石圈地幔橄榄岩变年轻。因此,新生代地幔捕虏体的Re-Os同位素年龄并不是岩石圈地幔的形成年龄,而是混合作用的结果。汉诺坝地幔橄榄岩的Re含量和Sr、Nd同位素组成符合橄榄岩—多元熔体相互作用的混合趋势,其橄榄岩的全岩Al₂O₃含量和橄榄石Fo的反相关性能够通过橄榄岩—熔体相互作用来解释。  相似文献   

Environmental Redox Changes of the Ancient Sea in the Yangtze Area during the Ordo-Silurian Transition     

YAN Detian  CHEN Daizhao  WANG Qingchen  WANG Jianguo  CHU Yang 《《地质学报》英文版》2008,82(3):679-689

Extensive organic-matter （OM） rich facies （black shales） occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections （Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province） straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon （TOC）, pyrite sulphur, and different species of Fe, including dithionite-extractable Fe （FED）, pyrite Fe （FeP）, HCl-extractable Fe （FeH）, and total Fe （FeT）, in black shales, as well as other redox proxies, such as the SIC ratio, the ratio between highly reactive Fe （FeHR = FeD ＋ FeP） and FeT, and the FeP/（FeP ＋ FeH） ratio, known as the degree of pyritization （DOP）. In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios （0.20-0.77; avg. 0.45）, high DOP values （0.21-0.72; avg. 0.54）, and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low FeHR/FeT ratios （0.10-0.35; avg. 0.21）, low DOP values （0.11- 0.40; avg. 0.28）, and SIC values are clustering on the normal marine value （SIC = 0.36）. The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios （0.32-0.49; avg. 0.41）, high DOP values （0.46-0.68; avg. 0.53） and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters  相似文献   

Late Neoproterozoic,Ordovician and Carboniferous events recorded in monazites from southern-central Madagascar     

《Precambrian Research》2006,144(3-4):278-296

The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U + Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630–2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550–560 Ma. Most ages fall in two populations at 420–460 and 490–500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300–310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar.  相似文献   

大别山-苏鲁碰撞造山带构造几何学、运动学和岩石变形分析   总被引：14，自引：7，他引：14  

林伟王清晨  石永红 《岩石学报》2005,21(4):1195-1214

按照构造几何学特点和运动学特征我们把大别山-苏鲁造山带的分为三个构造单元:南部,中部和北部。造山带南部为一套构造堆叠体系;中部为一个混合岩穹窿,浅变质的砂岩、板岩和片岩构成了大别山-苏鲁造山带的北部构造单元。造山带南部的构造堆叠体系主要由前陆褶皱带构成:未变质的新元古代-早三叠世的沉积地层;由“宿松群”北部和苏北地区的“海州群”构成的高压变质岩石单元及含柯石英和金刚石的超高压变质岩石单元。造山带中部的混合岩穹隆由大别山地区的罗田穹隆和苏鲁地区的莱西-栖霞穹隆构成。同样大别山北部的浅变质“佛子岭-卢镇关群”和胶东地区浅变质的“蓬莱群”构成了造山带北部的构造堆叠体系。同时大别山和苏鲁两个构造地体均经历了相似的多期构造变形:沿 NW-SE 向矿物拉伸线理发育的上部指北的剪切变形代表着造山带主变形期的变形;早期向南逆冲的韧性剪切变形和沿中部混合岩穹隆边缘发育的重力滑脱变形体系,后者代表了混合岩穹隆形成时的垂向缩短作用。正是由于构造几何学和多期变形的可对比性决定了这两个变质地体具有相同的地球动力学背景。  相似文献   

华北东南部中生代岩石圈地幔性质、组成、富集过程及其形成机理   总被引：30，自引：7，他引：30  

张宏福周新华  范蔚茗孙敏 郭锋英基丰  汤艳杰张瑾 牛利锋 《岩石学报》2005,21(4):1271-1280

本文通过对我国华北东南部中生代幔源岩浆活动的时空分布规律及其地球化学特征的系统总结来进一步厘定该地区中生代岩石圈地幔的性质和组成,并通过与华北内部如鲁中地区中生代岩石圈地幔的对比研究探讨华北东部岩石圈的时空演化规律、富集过程及其形成机理。幔源岩石的 Sr-Nd-Ph 同位素特征表明华北东部中生代岩石圈地幔存在明显的时空不均匀性,其中心部位如鲁中地区以弱富集地幔为主体;而东南部如鲁西南和胶东地区则为类似 EM2型地幔(~(87)Sr/~(86)Sr_i 可高达0.7114)。华北东南部中生代岩石圈地幔随时间的演化特征也很明显。这些幔源岩石的地球化学特征和玄武岩中地幔岩捕虏体(橄榄岩和辉石岩)和捕虏晶(橄榄石和辉石)的组成和结构特征皆证明华北东南部中生代岩石圈地幔曾受到过富硅熔体的强烈改造。橄榄岩-熔体的相互反应是该区岩石圈改造和组成转变的重要方式,从而造成古生代高镁橄榄岩转变为晚中生代低镁橄榄岩和辉石岩。进入岩石圈地幔的熔体具下/中地壳物质重熔的特征,从而导致该区晚中生代岩石圈地幔的快速富集。有关华北东部中生代岩石圈减薄和改造的时限、过程和机制等问题也进行较详细的讨论。  相似文献