Geochemistry and Isotope Composition of Paleoproterozoic Granites and Felsic Volcanics of the Elash Graben: Evidence of the Heterogeneity of the Early Precambrian Crust

2021 ◽  
Vol 62 (10) ◽  
pp. 1175-1187
Author(s):  
A.D. Nozhkin ◽  
O.M. Turkina ◽  
K.A. Savko
Keyword(s):  
Isotope Composition ◽  
Volcanic Rocks ◽  
Rare Metal ◽  
Temporal Association ◽  
Metal Deposit ◽  
Wide Range ◽  
Rare Metal Deposit ◽  
Geochronological Study ◽  
Felsic Volcanic ◽  
Felsic Volcanic Rocks

Abstract —The paper presents results of a petrogeochemical and isotope–geochronological study of the granite–leucogranite association of the Pavlov massif and felsic volcanics from the Elash graben (Biryusa block, southwest of the Siberian craton). A characteristic feature of the granite–leucogranites is their spatial and temporal association with vein aplites and pegmatites of the East Sayan rare-metal province. The U–Pb age of zircon from granites of the Pavlov massif (1852 ± 5 Ma) is close to the age of the pegmatites of the Vishnyakovskoe rare-metal deposit (1838 ± 3 Ma). The predominant biotite porphyritic granites and leucogranites of the Pavlov massif show variable alkali ratios (K2O/Na2O = 1.1–2.3) and ferroan (Fe*) index and a peraluminous composition; they are comparable with S-granites. The studied rhyolites of the Tagul River (SiO2 = 71–76%) show a low ferroan index, a high K2O/Na2O ratio (1.6–4.0), low (La/Yb)n values (4.3–10.5), and a clear Eu minimum (Eu/Eu* = 0.3–0.5); they are similar to highly fractionated I-granites. All coeval late Paleoproterozoic (1.88–1.85 Ga) granites and felsic volcanics of the Elash graben have distinct differences in composition, especially in the ferroan index and HREE contents, owing to variations in the source composition and melting conditions during their formation at postcollisions extension. The wide range of the isotope parameters of granites and felsic volcanic rocks (εNd from +2.0 to –3.7) and zircons (εHf from +3.0 to +0.8, granites of the Toporok massif) indicates the heterogeneity of the crustal basement of the Elash graben, which formed both in the Archean and in the Paleoproterozoic.

The genesis of agates and amethyst geodes

2019 ◽  
Vol 57 (6) ◽  
pp. 867-883 ◽  
Author(s):  
Ingrid N. Kigai
Keyword(s):  
Lava Flow ◽  
Volcanic Rocks ◽  
Silica Sol ◽  
Low Grade ◽  
Wide Range ◽  
Diluted Solution ◽  
Mafic Magmas ◽  
Felsic Volcanic ◽  
Low Pressures ◽  
Felsic Volcanic Rocks

Abstract Practically all aspects of agate genesis generate debate. The time is ripe to clarify the most important enigmas concerning the environments of formation of agates and the related famous amethyst geodes of Brazil and Uruguay. Agates form over a wide range of temperatures, from those of basaltic and andesitic melts (about 1100 °C) down to about 50 °C, and at rather low pressures. Their formation in liquid mafic magmas is indicated by a correlation between (1) the orientation of amygdules and the inclination of onyx banding in them and (2) the attitude of amygdules in the lava flow layers. The correlation arises because lava moves at a different rate close to and far from the upper and lower rims of a flow. The alkaline supercritical fluid fills gas vesicles in lavas and dissolves silica, mainly, from ambient lava or rock to produce a silica sol. If the pressure on the fluid causes percolation of water from amygdules, the sol coagulates on the walls of the vesicle to form a concentric lining. If the pressure in amygdules falls below the maximum osmotic pressure of a sol (about 0.1 MPa for a silica sol), percolation of fluid stops, and coagulation leads to the formation of horizontal onyx banding. Multiple repetitions of precipitation of various gel layers can be caused by overlapping fresh flows upon the cooling older agate-bearing lava flow. In a submarine setting, phase separation of the fluid and the formation of a film of gel between vapor (or diluted solution) and brine stimulate the osmotic processes, which result in growth of hollow membrane tubes and branching moss-like arrays at the bottom of amygdules. Some agates exhibit numerous channels as a result of repeated extrusion of fluid or gel from inner zones to the periphery of amygdules that were compressed under the burden of new flows. Previously, such channels were interpreted to be feeding channels for silica supply in amygdules. Periodic compression of amygdules after percolation of fluid from them requires no additional supply of silica because the volume of the amygdules is reduced in proportion to the loss of fluid. The concentric and horizontal banding and mossy textures of agates from the lithophysae of felsic volcanic rocks were created during active volcanism as well. The agates from dissolution-induced cavities in carbonate rocks and the famous amethyst druses of Brazil and Uruguay formed at the moderate temperatures associated with low-grade burial metamorphism, as indicated by the lack of moss textures and onyx banding.

scholarly journals U-Pb geochronology of felsic volcanic rocks hosted in the Gafo Formation, South Portuguese Zone: the relationship with Iberian Pyrite Belt magmatism

2008 ◽  
Vol 72 (5) ◽  
pp. 1103-1118 ◽  
Author(s):  
D. R. N. Rosa ◽  
A. A. Finch ◽  
T. Andersen ◽  
C. M. C Inverno
Keyword(s):  
Inductively Coupled Plasma ◽  
Volcanic Rocks ◽  
Iberian Pyrite Belt ◽  
Classification Problems ◽  
Inductively Coupled ◽  
Two Samples ◽  
Geochronological Study ◽  
Radiometric Ages ◽  
Felsic Volcanic ◽  
Felsic Volcanic Rocks

AbstractFelsic volcanic rocks exposed in the Frasnian Gafo Formation, in the Azinhalinho area of Portugal, display very similar geochemical signatures to volcanic rocks from the Iberian Pyrite Belt (IPB). located immediately to the south. The similarities include anomalously low high field-strength elements (HFSE) concentrations, possibly caused by low-temperature crustal melting, which translate into classification problems.A geochronological study, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon grains from these rocks, has provided concordia ages of 356±1.5 Ma and 355±2.5 Ma for two samples of rhyodacite porphyry, and 356±1.4 Ma for a granular rhyodacite. These results show that volcanism at Azinhalinho was broadly contemporaneous with IPB volcanism, widely interpreted as being of Famennian to Visean age. Considering that the host rocks of the Azinhalinho volcanic rocks are Frasnian, and therefore deposited synchronously with the Upper Devonian Phyllite-Quartzite Group sedimentation in the IPB basin, the radiometric ages imply that the Azinhalinho felsic rocks are intrusive and likely represent conduits or feeders to the volcanism of the IPB.

The Duck Pond and Boundary Cu–Zn deposits, Newfoundland: new insights into the ages of host rocks and the timing of VHMS mineralization

2010 ◽  
Vol 47 (12) ◽  
pp. 1481-1506 ◽  
Author(s):  
Vicki McNicoll ◽  
Gerry Squires ◽  
Andrew Kerr ◽  
Paul Moore
Keyword(s):  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Isotopic Data ◽  
Sulphide Ores ◽  
Intrusive Rocks ◽  
Felsic Volcanic ◽  
Host Rocks ◽  
Felsic Volcanic Rocks

The Duck Pond Cu–Zn–Pb–Ag–Au deposit in Newfoundland is hosted by volcanic rocks of the Cambrian Tally Pond group in the Victoria Lake supergroup. In conjunction with the nearby Boundary deposit, it contains 4.1 million tonnes of ore at 3.3% Cu, 5.7% Zn, 0.9% Pb, 59 g/t Ag, and 0.9 g/t Au. The deposits are hosted by altered felsic flows, tuffs, and volcaniclastic sedimentary rocks, and the sulphide ores formed in part by pervasive replacement of unconsolidated host rocks. U–Pb geochronological studies confirm a long-suspected correlation between the Duck Pond and Boundary deposits, which appear to be structurally displaced portions of a much larger mineralizing system developed at 509 ± 3 Ma. Altered aphyric flows in the immediate footwall of the Duck Pond deposit contained no zircon for dating, but footwall stringer-style and disseminated mineralization affects rocks as old as 514 ± 3 Ma at greater depths below the ore sequence. Unaltered mafic to felsic volcanic rocks that occur structurally above the orebodies were dated at 514 ± 2 Ma, and hypabyssal intrusive rocks that cut these were dated at 512 ± 2 Ma. Some felsic samples contain inherited (xenocrystic) zircons with ages of ca. 563 Ma. In conjunction with Sm–Nd isotopic data, these results suggest that the Tally Pond group was developed upon older continental or thickened arc crust, rather than in the ensimatic (oceanic) setting suggested by previous studies.

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