Ore geology, mineralogy and geochemistry of a fault-controlled hydrothermal clay-Li deposit hosted by Precambrian metasedimentary rocks in south China
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Precambrian time is the whole of geologic time from the very beginning of the earth history until the earliest fossiliferous Cambrian beds were deposited. Precambrian time covers almost 90% of the total length of time that has passed since the formation of the earth. Until recently, however, this long period of geologic time was among the least known segments of the geologic record.The actual absence of fossils in Precambrian rocks makes it very difficult to correlate rocks of one locality to those of the others or to identify the age of geological formations from different localities. By introducing the dating methods based on radioactive decay, reliable age data on minerals and rocks have been accumulated, especially since 1950. Precambrian is now outstanding in availability of a very notable number of exact ages, among which the oldest ones are estimated at 3, 500 million years. The Precambrian rocks are exclusively found in the vast shield areas of the world. In the African continent, they occupy 57% of the whole continent in its areal distribution.In recent years, researches on the Precambrian rocks in the African continent have made a remarkable progress, especially on the following five points:(1) Since the improvement and spread of radiometric dating techniques, various Precambrian orogenic belts and early Palaeozoic one have been dated. Stratigraphic successions of the Precambrian system were greatly revised. Fairly well correlation of Precambrian rocks from one region to another is done throughout the African continent.A. Holmes (1963) wrote vividly this situation of drastic revision of the stratigraphy on Precambrian system in Africa as follows: “For me, probably the most dramatic and unexpected surprise of a decade packed with surprises was the announcement of the great age of the Bushveld Complex, about 2, 000 million years, and the consequent realization that the Transvaal Group of strata must be older still. Until 1901 the Transvaal ‘System’ was correlated on lithological grounds with the Palaeozoic Cape ‘System’. Then for over half a century the Transvaal ‘System’ was confidently thought to be of late Precambrian age and, lithologically, a typical representative of the Algonkian. Yet it has turned out to be immensely older than such characteristically Archean rock sequences as the Grenville of the Canadian shield and Svecofennian of the Baltic shield.”(2) The almost all Precambrian rocks of the African continent have been hitherto considered to represent the Precambrian Craton (Shield). Recently, time, character, and areal distribution of the Precambrian orogenic cycles in Africa have been confirmed, and the following five orogenic cycles have been recognized.a. Upper Luanyi Cycle (more than 3, 000m. y. ago)b. Shamvaian Cycle (2, 700-230m. y. ago)c. Limpopo Cycle (2, 150-1, 650m. y. ago)d. Kibaran Cycle (1, 290-850m. y. ago)e. Katangan Cycle (620-485m. y. ago)The Katangan belt of orogenesis, late Precambrian to early Palaeozoic in age, is shown to be extensively developed throughout Africa. Of recent years, awareness of the significance of this Katangan Cycle has been growing. From an important but essentially local feature, it has grown to the status of a “Pan-African thermo-tectonic episode.”The Kibaran belt of east and central Africa is also probably extended to the Orange River belt and to Natal in South Africa.These Katangan and Kibaran belts represent a distinctive regime of younger orogens consisting of mobile zones which have suffered orogenic deformation from time to time during the past ca. 1, 200m. y., and this younger tructural regime is readily differentiated from older cratons which have remained stable over the past ca. 1, 500m. y.a large part
Radiometric dating
Geologic time scale
Geologic record
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Ultrahigh‐pressure eclogite transformed from mafic granulite in the Dabie orogen, east‐central China
Abstract Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic ( c . 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.
Omphacite
Protolith
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Protolith
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Abstract. The two major lithology or gneiss components in the polycyclic granulite terrain of the Eastern Ghats, India, are the supracrustal rocks, commonly described as khondalites, and the charnockite-gneiss. Many of the workers considered the khondalites as the oldest component with unknown basement and the charnockite-protoliths as intrusive into the khondalites. However, geochronological data do not corroborate the aforesaid relations. The field relations of the hornblende- mafic granulite with the two gneiss components together with geocronological data indicate that khondalite sediments were deposited on older mafic crustal rocks. We propose a different scenario: Mafic basement and supracrustal rocks were subsequently deformed and metamorphosed together at high to ultra-high temperatures – partial melting of mafic rocks producing the charnockitic melt; and partial melting of pelitic sediments producing the peraluminous granitoids. This is compatible with all the geochronological data as well as the petrogenetic model of partial melting for the charnockitic rocks in the Eastern Ghats Belt.
Charnockite
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Basement
Lithology
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Mafic granulite, garnet peridotite, and garnet pyroxenite occurred as slices or lenses within dominant felsic granulite, and they together constitute a high‐pressure metamorphic terrane in the Bashiwake unit, South Altyn Tagh, Northern Tibet, China. Previous studies focused on the metamorphic evolution, and geothermobarometry results indicated that the mafic granulite has experienced high pressure/(ultra‐)high temperature (HP/(U)HT) metamorphism, followed by a medium pressure (MP) granulite‐facies overprint. However, the nature and petrogenesis of the mafic granulite in the dominant felsic granulite are poorly known. Combining the previous geothermobarometry results with the petrographic observations, mineral chemistry, and pseudosection modelling in this study, at least four stages were suggested for the metamorphic evolution of the mafic granulites in the South Altyn Tagh, including the eclogite‐facies stage (3–4 GPa, 910–1000°C), high pressure–ultrahigh temperature (HP–UHT) metamorphism, an isothermal decompression, and subsequent MP granulite‐facies overprint. The U–Pb dating of zircons yielded two age clusters: one age cluster at ca. 500 Ma, representing the retrograde age of HP–UHT metamorphism after the eclogite‐facies stage, and another age cluster of ca. 900 Ma that represented the age of the protolith for the mafic granulite. This indicated that the protolith of the mafic granulite was formed in the early Neoproterozoic and then was taken to extreme temperatures and pressures during the early Palaeozoic orogenic event. The elemental abundances of the mafic granulites in the Bashiwake area clearly indicated that they were higher in FeO and TiO 2 , but were significantly lower in MgO, Cr, and Ni than those of associated garnet peridotites/pyroxenites, and they showed LREE‐enriched patterns with slightly positive Eu anomalies. Sr–Nd isotopic data suggested a basaltic magmatic origin with crust contamination for the protolith of the mafic granulite. Integrating these results together with previous studies, we suggest that the mafic granulites were derived from the basaltic magma intrusion in the continental crust during the Neoproterozoic and subsequently suffered a common HP/UHT metamorphism with felsic crust rocks in the early Palaeozoic (ca. 500 Ma) after the eclogite‐facies metamorphism related to the continental collision (>500 Ma).
Felsic
Geothermobarometry
Protolith
Petrogenesis
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Early Earth
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Omphacite
Felsic
Diopside
Recrystallization (geology)
Hornblende
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Similarity (geometry)
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"‘Upper Precambrian Correlations’ and ‘Upper Precambrian-Cambrian Boundary’: Activities in Sweden." Geologiska Föreningen i Stockholm Förhandlingar, 101(1), pp. 75–76
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