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1.
Summary The Mesoproterozoic diamondiferous Majhgawan pipe of central India is re-examined in the light of new and recently published petrological, geochemical and isotope data. This investigation reveals that its tectonic setting is similar to that of lamproites and orangeites (Group II kimberlite of southern Africa) and not that of a typical kimberlite. The petrography and mineralogy are comparable to lamproite and to some extent to orangeite, whereas the major element geochemistry is more akin to that of kimberlite. Trace element geochemistry is closer to that of lamproite but Nd isotope systematics are atypical of lamproite or orangeite. The inferred petrogenesis of the Majhgawan pipe is also similar to that of other such potassic metasomatised mantle magmas without any strong affinity to a particular clan/group.It is demonstrated in this study that the Majhgawan pipe shares the petrological, geochemical and isotope characteristics of all three rock types. It is therefore suggested to constitute a transitional kimberlite–orangeite (Group II kimberlite)–lamproite rock. The existence of such transitional magmas in space and time in other cratons, outside India, is also highlighted. The name majhgawanite is proposed for this rock – keeping in mind the antiquity of the Majhgawan pipe, its intriguing petrological and geochemical characteristics and also on the basis of Indias legacy for introducing diamond to the world – to designate such mafic potassic-ultrapotassic transitional rock types so as to distinguish them from the classical kimberlite, lamproite or orangeite.It is concluded that the correlations between kimberlite petrography, geochemistry and isotopic types (viz., Group I and II), as established for kimberlites in southern Africa, need not be necessarily valid elsewhere. Hence, the recommendations of I.U.G.S. on classification of kimberlite, orangeite and lamproite are clearly inadequate when dealing with the transitional mafic potassic ultrapotassic rocks. It is further stressed that mineralogical, geochemical and isotopic aspects of mafic potassic-ultrapotassic rocks need to be considered in unison before assigning any name as the nomenclature of such exotic and rare alkaline rock types invariably implies economic and tectono-magmatic (regional) significance. 相似文献
2.
目前所知产金刚石的岩石类型包括金伯利岩、钾镁煌斑岩、榴辉岩、蛇绿岩套、碱性超基性杂岩、碱性超基性煌斑岩和橄榄岩类(方辉橄榄岩、纯橄榄岩等) 等偏碱性超镁铁质岩石, 而有经济价值的金刚石原生矿床仅见于金伯利岩和钾镁煌斑岩中, 除此之外的其他岩石类型中仅见有少量微粒金刚石.金伯利岩和钾镁煌斑岩都起源于地幔深部, 就此意义上讲, 二者是同源的, 但其岩石化学成分、主要矿物组成、产出大地构造背景以及同位素资料等, 却存在着比较明显的差异.由此构成了金刚石原生矿床的两个成矿系列: 金伯利岩成矿系列和钾镁煌斑岩成矿系列.金伯利岩成矿系列又可以根据其化学成分划分为3个亚系列, 即: 高Cr, Ti, Mg成矿亚系列, 低Cr, Ti, Mg成矿亚系列和介于二者之间的一种具有复杂化学成分的成矿亚系列.钾镁煌斑岩成矿系列则可以根据其主要矿物组成, 划分出橄榄石钾镁煌斑岩成矿亚系列、白榴石钾镁煌斑岩成矿亚系列以及介于两者之间的白榴石-橄榄石钾镁煌斑岩成矿亚系列共3种次级成矿系列.与此同时, 无论是金伯利岩成矿系列, 还是钾镁煌斑岩成矿系列, 又都可以根据其野外地质产状, 划分为以下3个成矿亚系列(形成时间从早到晚) : (1) 火山沉积凝灰岩成矿亚系列; (2) 火山凝灰角砾岩成矿亚系列; (3) 火山-次火山侵入相成矿亚系列. 相似文献
3.
Diamond exploration focuses on geochemical analysis of indicator minerals that are more abundant than diamond itself. Among such indicators, low-Cr (Cr2O3 < 1 wt%) garnets from mantle eclogites are problematic since they overlap compositionally with many lower-crust-derived garnets also transported by kimberlite. Misclassification of these garnets may create “false positive” mantle signatures and possible misdirection of exploration efforts. Statistical solutions using major elements in low-Cr garnet (Hardman et al. in J Geochem Explor 186:24–35, 2018) provide improved error rates for the discrimination of low-Cr crustal and mantle garnets recovered from kimberlite. In this study we analysed a large suite of garnets (n = 571) from both crustal and mantle settings, already characterised for major elements, for a wide range of trace elements by laser ablation inductively-coupled plasma mass spectrometry and use these new data along with literature data (n = 169) to evaluate the effectiveness of adding trace elements to garnet-based diamond exploration programs. A new garnet classification scheme, initially using a major-element based filter, uses garnet Sr contents and Eu anomalies to help identify low-Cr garnets that are misclassified using major element methods. Combined with existing methods, our new trace element classifiers offer improvement in classification error rates for low-Cr, crustal and mantle garnets to as low as 4.7% for calibration data. 相似文献
4.
Four statistical techniques for modelling landslide susceptibility were compared: multiple logistic regression (MLR), multivariate adaptive regression splines (MARS), classification and regression trees (CART), and maximum entropy (MAXENT). According to the literature, MARS and MAXENT have never been used in landslide susceptibility modelling, and CART has been used only twice. Twenty independent variables were used as predictors, including lithology as a categorical variable. Two sets of random samples were used, for a total of 90 model replicates (with and without lithology, and with different proportions of positive and negative data). The model performance was evaluated using the area under the receiver operating characteristic curve (AUC) statistic. The main results are (a) the inclusion of lithology improves the model performance; (b) the best AUC values for single models are MLR (0.76), MARS (0.76), CART (0.77), and MAXENT (0.78); (c) a smaller amount of negative data provides better results; (d) the models with the highest prediction capability are obtained with MAXENT and CART; and (e) the combination of different models is a way to evaluate the model reliability. We further discuss some key issues in landslide modelling, including the influence of the various methods that we used, the sample size, and the random replicate procedures. 相似文献
5.
The available geological, geochronological and isotopic data on the felsic magmatic and related rocks from South Siberia, Transbaikalia and Mongolia are summarized to improve our understanding of the mechanisms and processes of the Phanerozoic crustal growth in the Central Asian mobile belt (CAMB). The following isotope provinces have been recognised: ‘Precambrian’ ( TDM=3.3–2.9 and 2.5–0.9 Ga) at the microcontinental blocks, ‘Caledonian’ ( TDM=1.1–0.55 Ga), ‘Hercynian’ ( TDM=0.8–0.5 Ma) and ‘Indosinian’ ( TDM=0.3 Ga) that coincide with coeval tectonic zones and formed at 570–475, 420–320 and 310–220 Ma. Continental crust of the microcontinents is underlain by, or intermixed with, ‘juvenile’ crust as evidenced by its isotopic heterogeneity. The continental crust of the Caledonian, Hercynian and Indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments. The crustal growth in the CAMB had episodic character and important crust-forming events took place in the Phanerozoic. Formation of the CAMB was connected with break up of the Rodinia supercontinent in consequence of creation of the South-Pacific hot superplume. Intraplate magmatism preceding and accompanying permanently other magmatic activity in the CAMB was caused by influence of the long-term South-Pacific plume or the Asian plume damping since the Devonian. 相似文献
6.
This paper examines ‘folk’ evaluation and classification of firewood as a means towards understanding man-firewood relations in Freetown. It demonstrates how access to ‘wood views’ renders comprehensible identifiable spatial and temporal patterns in firewood exploitation and utilisation. It is argued that the extension of the ethno-scientific approach beyond mere identification of wood categories to the examination of the relationship between cognition and action generates information capable of informing efforts towards short-term gains and/or long-term development policy aimed at a more efficient management and utilisation of the firewood resource. The article concludes by emphasising the need for an understanding of the view both from ‘above’ (technological forestry) and ‘below’ (folk ecology) of man-firewood relationships in the Freetown area in order that meaningful co-operative modifications based on a blend of sodo-cultural and ecological considerations, on the one hand, and economic and technological factors on the other, can be effected. 相似文献
7.
Pre-1.6 Ga rocks comprise around 45% of the onshore area of Western Australia (WA), constituting the West Australian Craton (WAC) (including the Archean Yilgarn and Pilbara Cratons) and the western part of the North Australian Craton (NAC). These areas provide the conditions suitable for diamond formation at depth, and numerous diamondiferous lamproite and kimberlite fields are known. As emplacement ages span close to 2500 Ma, there are significant opportunities for diamond-affinity rocks being present near-surface in much of the State, including amongst Phanerozoic rocks. WA’s size, terrain, infrastructure and climate, mean that many areas remain underexplored. However, continuous diamond exploration since the 1970s has resulted in abundant data. In order to advance future exploration, a comprehensive database of results of diamond exploration sampling (Geological Survey of Western Australia 2018) has been assessed. The Yilgarn and Pilbara Cratons have spinel indicators almost exclusively dominated by chromite (>90% of grains), whereas (Mg,Fe,Ti)-bearing Al-chromites account for more of the indicator spinels in the NAC, up to 50% of grains at the Northern Territory (NT) border. Increasing dominance of Al in chromites is interpreted as a sign of weathering or a shallower source than Al-depleted Mg-chromites. Garnet compositions across the State also correlate with geological subdivisions, with lherzolitic garnets showing more prospective compositions (Ca-depleted) in WAC samples compared to the NAC. WAC samples also show a much broader scatter into strongly diamond-prospective G10 and G10D compositions. Ilmenites from the NAC show Mg-enriched compositions (consistent with kimberlites), over and above those present in NT data. However, ilmenites from the WAC again show the most diamond-prospective trends. Numerous indicator mineral concentrations throughout the State have unknown sources. Due in part to the presence of diamondiferous lamproites, it is cautioned that some accepted indicator mineral criteria do not apply in parts of WA. For example Ca-depleted garnets, Mg-depleted ilmenites and Cr-depleted and Al-absent clinopyroxenes are all sometimes associated with strongly diamondiferous localities. Quantitative prospectivity analysis has also been carried out based on the extent and results of sampling, age of surface rocks relative to ages of diamond-prospective rocks, and the underlying mantle structure. Results show that locations within the NAC and with proximity to WA’s diamond mines score well. However, results point to parts of the WAC being more prospective, consistent with mineral chemical data. Most notable are the Hamersley Basin, Eastern Goldfields Superterrane and the Goodin Inlier of the Yilgarn Craton. Despite prolific diamond exploration, WA is considerably underexplored and the ageing Argyle mine and recent closure of operations at Ellendale warrant a re-evaluation of diamond potential. Results of mineral chemistry and prospectivity analysis make a compelling case for renewed exploration. 相似文献
8.
Primary and placer gold deposits are mined from the Pan-African Adola volcano-sedimentary sequence, in southern Ethiopia. Two major mineralized belts can be recognized: the Megado (‘Gold Belt’) and the Kenticha Belts. The Kenticha Belt is also known for its rare metal mineralization. Extensive exploration of the area resulted in two most important primary gold deposits of Lega Dembi and Sakaro. The primary gold deposits are classified into four classes based on their geological setting: - auriferous veins, lodes, stockworks and silicified zones disseminated in schistose rocks - gold associated with quartzite - gold mineralization confined to conglomerates and meta-arkoses - auriferous quartz veins in high grade gneiss rocks This classification provides a useful guide for future exploration programme 相似文献
9.
Distinctly different groundmass mineralogy characterise the hypabyssal facies, Mesoproterozoic diamondiferous P3 and P4 intrusions from the Wajrakarur Kimberlite Field, southern India. P3 is an archetypal kimberlite with macrocrysts of olivine and phlogopite set in a groundmass dominated by phlogopite and monticellite with subordinate amounts of serpentine, spinel, perovskite, apatite, calcite and rare baddeleyite. P4 contains mega- and macrocrysts of olivine set in a groundmass dominated by clinopyroxene and phlogopite with subordinate amounts of serpentine, spinel, perovskite, apatite, and occasional gittinsite, and is mineralogically interpreted as an olivine lamproite. Three distinct populations of olivine, phlogopite and clinopyroxene are recognized based on their microtextural and compositional characteristics. The first population includes glimmerite and phlogopite–clinopyroxene nodules, and Mg-rich olivine macrocrysts (Fo 90–93) which are interpreted to be derived from disaggregated mantle xenoliths. The second population comprises macrocrysts of phlogopite and Fe-rich olivine (Fo 81–89) from P3, megacrysts and macrocrysts of Fe-rich olivine (Fo 85–87) from P4 and a rare olivine–clinopyroxene nodule from P4 which are suggested to have a genetic link with the precursor melt of the respective intrusions. The third population represents clearly magmatic minerals such as euhedral phenocrysts of Fe-rich olivine (Fo 85–90) crystallised at mantle depths, and olivine overgrowth rims formed contemporaneously with groundmass minerals at crustal levels. Close spatial association and contemporaneous emplacement of P3 kimberlite and P4 lamproite is explained by a unifying petrogenetic model which involves the interaction of a silica-poor carbonatite melt with differently metasomatised wall rocks in the lithospheric mantle. It is proposed that the metasomatised wall rock for lamproite contained abundant MARID-type and phlogopite-rich metasomatic veins, while that for kimberlite was relatively refractory in nature. 相似文献
10.
本文对具有经济的金伯利岩和金云火山岩金刚石原生矿两大类型模式特征和产出的构造部位进行了研究.在此基础上,对我省金刚石成矿构造条件进行了分析,并阐述了我省寻找金伯利岩和金云火山岩两大类型金刚石原生矿有利地区. 相似文献
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