Petrology and thermobarometry of mafic granulites and migmatites from the Chafalote Metamorphic Suite: New insights into the Neoproterozoic P–T evolution of the Uruguayan—Sul-Rio-Grandense shield

This paper reports a study of the metamorphic evolution of pelitic, semi-pelitic migmatites and mafic granulites of the Chafalote Metamorphic Suite (CMS), Uruguay, which represents the southernmost exposures of high-grade metamorphic rocks in the Dom Feliciano Belt, Uruguain—Sul-Rio-Grandense shield, South America. This belt is one of the Brasiliano orogens that crop out along the Brazilian and Uruguayan Atlantic margin, and the CMS is one of several disconnected segments of supracrustal rock in a dominantly granitic terrain. Petrological evidence from CMS mafic granulites and semi-pelitic migmatites indicates four distinct metamorphic assemblages. The early prograde assemblage (M₁) is preserved only as inclusions in porphyroblasts of the peak-metamorphic (M₂) assemblage. Peak-metamorphism was followed by near-isothermal decompression (M₃), which resulted in symplectites and coronitic textures in the mafic granulites and compositional zoning of Ca in garnet (decreasing rimwards) and plagioclase (increasing rimwards) in the semi-pelitic migmatites. The retrograde metamorphic assemblage (M₄) is represented by hydration reaction textures replacing minerals of the M₂ and M₃ assemblages. Average P–T calculations using the program THERMOCALC and conventional thermobarometric methods yield peak-metamorphic (M₂) P–T conditions of 7–10 kbar and 830–950 °C, near-decompressional (M₃) P–T conditions of 4.8–5.5 kbar and 788–830 °C and M₄ retrograde P–T conditions of 3–6 kbar and 600–750 °C. The calculated P–T path for the CMS rocks is ‘clockwise’ and incorporates a near-isothermal decompression segment followed by minor cooling, consistent with a history of crustal thickening followed by extensional collapse at ca. 650–600 Ma. The metamorphism recorded by rocks of this crustal segment may be correlated with 650 Ma metamorphism in the Coastal Terrane of the Kaoko Belt in Namibia, being the first unequivocal match between South America and Africa provided by crystalline rocks south of the Congo Craton.     

关于峨眉山大火成岩省一些重要问题的讨论

峨眉山大火成岩省(ELIP)近几年来取得了许多新的进展,但在一些重要的问题上仍存在着争论,笔者列举了ELIP火山喷发的时间、分布范围、高钛和低钛玄武岩、层状岩体与玄武岩以及成矿作用的关系、大火成岩省与生物绝灭的关系以及与地幔柱的关系等方面存在的问题,以及由此引发的相关争论.这些问题的解决对于大火成岩省研究的深入将会起到重要作用.     

青海省洪水河铁矿床地质特征及成因初探

洪水河铁矿床位于东昆仑早古生代——晚古生代——早中生代复合适山系之昆中花岗——变质杂岩带。通过分析研究认为,洪水河铁矿床的主矿体呈层状、似层状、透镜状顺层产于蓟县纪狼牙山组钙质绿泥千枚岩中,矿体规模大,矿体厚度、连续性、矿石品位相对稳定。其成矿地质背景和矿床地质特征表明,该矿床属于沉积变质型铁矿床。     

Compositions of four unusual CM or CM-related Antarctic chondrites

To identify chemical group affinities and infer the occurrence of thermal metamorphism or aqueous alteration in their histories, we quantified 43 trace elements in the CM or CM-related Antarctic carbonaceous chondrites EET 96010, LAP 02277, MET 01070, and WIS 91600. We also analyzed LAP 02206, a CV chondrite, to add to our comparison database. We present whole-rock oxygen isotope data for LAP 02206, LAP 02277, and MET 01070 to complement our trace element results. With these data, we confirm the CV classification of LAP 02206 and CM or CM-like classification for the other four chondrites in this study. On the basis of moderately volatile element content, our results show that EET 96010 experienced open-system heating, while any heating LAP 02277 and MET 01070 may have experienced was in a chemically closed-system. WIS 91600, on a trace element basis, appears to be CM-like material. Our analyses support the idea that CM material has experienced a wide variety of post-accretionary processing.     

云南宝丰寺岩体锆石微量元素特征及地质意义

云南宝丰寺岩体地处扬子陆块西缘金沙江-红河断裂与程海-宾川断裂夹持部位,属于金沙江-红河富碱斑岩带组成部分,由细晶黑云母花岗斑岩(FCBGP)和粗晶花岗斑岩(CGGP)组成。通过对宝丰寺岩体开展的锆石Ti温度、锆石Ce~(4+)/Ce~(3+)值和δEu和δCe值研究结果表明:两类岩石锆石Ti温度相差不大(FCBGP为618.55℃—792.75℃,均值为692.42℃;CGGP为560.35℃～820.41℃,均值为714.13℃),与S型花岗岩浆形成温度相似,反映宝丰寺岩体属S型花岗岩;锆石Ce~(4+)/Ce~(3+)值、δEu和δCe值总体较高(FCBGP的Ce~(4+)/Ce~(3+)值、δEu和δCe分别为24.51～1190.06、0.32～0.49和10.17～1231.19,均值分别为390.36、0.42和264.29;CGGP的Ce~(4+)/Ce~(3+)值、δEu和δCe分别为50.13～494.56、0.44～0.58和24.87～262.76,均值分别为232.98、0.49和141.82),与金沙江-红河富碱斑岩带含矿岩体特征值类似,表明宝丰寺岩体形成于高氧逸度环境,有利于岩浆体系中的成矿金属元素保留至岩浆分异演化的晚阶段,有利于形成斑岩型矿床。     

Impact cratering: The South American record – Part 1

The Earth’s impact record is known to be rather limited in both time and space. There are ca. 190 impact structures currently known on Earth, representing a minor fraction of all the impact events that contributed to the initial formation of our protoplanet, and then to formation and modification of the surface of the planet. Moreover, the distribution of impact structures on Earth is manifestly uneven. One continent that stands out for its relatively small number of confirmed impact structures and impact ejecta occurrences is South America. The limited impact record for this large continent makes a robust case that there is a significant potential for further discoveries. Significant information on the impact record of South America is dispersed in different types of publications (journal articles, books, conferences abstracts, etc.), and in several languages, making it difficult to access and disseminate it among the geoscientific community. We aim to present a summary of the current knowledge of the impact record of this continent, encompassing the existing literature on the subject. It is published in two parts, with the first one covering an up-to-date introduction to impact cratering processes and to the criteria to identify/confirm an impact structure and related deposits. This is followed by a comprehensive analysis of the Brazilian impact structures. The Brazilian impact record accounts for the totality of the large structures of this kind currently confirmed in South America. The second part will examine the impact record of other countries in South America, provide information about a number of proposed impact structures, and review those that already have been discarded as not being formed by impact.     

新疆北山早二叠世岩浆型铜镍硫化物矿床 综合信息勘查模式

新疆北山发育有20～30个产于晚古生代北山裂谷带中的早二叠世超基性岩岩体。自2008年以来在坡北基性-超基性杂岩体中发现坡一大型镍矿床和坡十中型镍矿床。利用本项目资料结合前人研究成果,文章从构造、岩体基本特征、围岩条件、矿物学特征、化探异常特征、地球物理参数等多个方面总结了含矿岩体(相)与非含矿岩体(相)的区别。研究发现新疆北山铜镍矿矿床受深大断裂控制,多产于深大断裂一侧的次级断裂中。在预查阶段或预查以前,可以首先利用1∶20万和1∶5万勘查地球化学数据,对Cu、Ni、Co、Cr异常区进行调查研究,同时辅以ETM遥感线环解译、1∶5万航磁资料的化极和向上延拓图(关注高磁异常)和1∶5万重力资料的布格重力异常(关注△GB-142×10 g.u的区域)来圈定区域上的基性超基性岩体。地表的孔雀石蚀变、黄钾铁矾和镍华是寻找铜镍矿的重要线索。在普查阶段,若存在纯橄岩且岩相分异良好的杂岩体,则更有可能发现铜镍矿。同一地区,具有高Cr尖晶石且尖晶石Fe3+/∑Fe0.3的基性-超基性杂岩体,成铜镍矿的潜力更大;含最超基性或接近最超基性的岩相的岩体成铜镍矿的可能性更大;含有高Fo、高NiO、低FeO*、低CaO、低TiO2的橄榄石且低Fe、Ca含量辉石的岩相是铜镍矿勘查的优选岩相。围岩有硫化物、硫酸盐或石墨层的超基性岩体的成矿潜力要大于围岩为无硫化物方解石大理岩的超基性岩体的成矿潜力。在此基础上,瞬变电磁(TEM)方法获得的瞬变响应感应电动势的垂直分量DBz/Dt(1.2)数据和可控源声频大地电磁可以用来推断深部超基性岩体的分布,是深部验证的重要依据。三维磁化率反演可以为矿区的定位预测或确定超基性岩体的延伸情况提供一定的依据。井中磁测可以作为防止漏矿的一种补充方法。在详查阶段,通过精细钻孔编录和三维软件查明矿体的空间展布趋势规律可为定位预测提供依据。文章总结了针对新疆北山地区铜镍硫化物矿床的综合信息勘查模型和勘查策略,并对硫化物饱和机制进行了探讨。     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.     

中国镍矿床地质特征与矿产预测

我国是镍矿资源消费大国,同时又是镍矿资源相对匮乏的国家。为了科学地评估我国镍矿资源潜力,指导找矿勘查工作,进而提高我国镍矿资源的自我保障能力,国土资源部在2006-2013年部署了包括镍矿在内的25个矿种的全国矿产资源潜力评价项目。该项目以矿产预测类型为主线,通过分析我国镍矿的矿床地质特征和时空分布规律,对代表预测要素的地质、物探、化探和遥感信息进行综合,最终形成预测模型,并在此基础上采用相似类比的思想圈定预测区,估算资源量。结果表明：（1）我国镍矿包括岩浆型、风化壳型和沉积型3种矿产预测类型,其中以岩浆型镍矿占绝对主导地位,其查明资源储量约占全国镍矿资源总量的93%;（2）我国共划分了30个成镍带,其中重点成镍带13个,大部分集中在西部地区;（3）不同预测类型对应的预测模型,其主要预测要素存在明显差异,岩浆型镍矿最重要的预测要素是基性超基性岩体,风化壳型镍矿最重要的预测要素是超基性岩顶部形成的风化壳,沉积型镍矿最重要的预测要素是寒武系下统黑色岩系;（4）全国圈定了524个镍矿最小预测区,在此基础上,全国共圈定98个镍矿3级预测区,在3级预测区中,大于10万t的镍矿预测区共有35个;（5）镍矿累计查明镍资源储量共计1 345万t,预测镍资源量共计2 820万t。     

北大别罗田榴辉岩的减压出溶结构与超高压变质作用

岩石学研究表明北大别罗田榴辉岩经过了榴辉岩相、麻粒岩相和角闪岩相变质作用,表现出多种减压出溶结构.特征性的减压出溶结构有单斜辉石和石榴石中分别发育石英+角闪石+斜长石等与金红石+单斜辉石+角闪石等针状矿物出溶体,以及锆石中含有柯石英残晶.这些进一步证明北大别南部（罗田一带）同北部（如黄尾河、百丈岩等地）榴辉岩一样经过了>5～7CPa的超高压变质作用.