Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites
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Leucogranite
Isochron dating
Abstract Thirteen samples of Värmland granites from south-eastern Värmland, Sweden, have been subjected to a Rb—Sr dating study. Most of the studied samples subdivide into two groups defining two separate isochrons of closely similar age but different initial 87Sr/86Sr ratios. Five samples from a southern sub-area of Filipstad granite around Filipstad and Karlskoga have yielded a Rb-Sr age of 1700 ± 37 Ma at an initial 87Sr/86Sr ratio of 0.7068 ± 0.0021. Five samples from the northern sub-area of Hagfors—Ekshärad—Uvanå, together with two samples from the vicinity of Kristinehamn, form another isochron of 1689 ± 30 Ma with an initial Sr-isotopic ratio of 0.7021 ± 0.0006. One sample could not be assigned to either isochron and is intermediate between them. The differentiation of Värmland-group granites into separate complexes and types is also brought out by field data and geochemical characteristics.
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1984)
Metamorphosed intrusive rocks (metagabbro and metatonalite), and nonmetamorphosed aplite dyke, all of which intrude into the coarse-grained biotite-hornblende gneiss, are exposed in the Urushiyama area along the Takahara River, the main part of the Hida metamorphic belt. RbSr mineral isochron ages of 216±10 Ma and 222±2 Ma are given for the coarse-grained biotitehornblende gneiss and metagabbro, respectively. On the contrary, the whole rock isochrons of the metagabbro and metatonalite show 415±144 Ma and 332±74 Ma, respectively. These whole rock ages correspond to the ages of intrusion of the gabbro and tonalite and they were re-equilibrated with respect to the Rb-Sr system approximately 220 Ma by the regional metamorphism. Whole rock samples of aplite dyke yield an isochron age of 172±7 Ma. The Hida gneiss in the Urushiyarna area seems to have been formed by the main stage of metamorphism and the deformation having been already finished before the intrusion of gabbro.
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Rb-Sr isochrons were obtained on the cretaceous acid volcanic rocks of Himeji district, Hyogo prefecture, Japan. The isochron of the Tenkadaiyama forInation, younger rocks in this area, yielded T=67.5±5. 9m. y. and (87Sr/86Sr) . =0.7105±0.0006by both whole rock specimens and separated fractions. On the contrary, it was determined two ages on the Aioi formation, older extrusive rocks. One age was obtained by the whole rock isochron which gave T=105±16m. y. and (87Sr/86 Sr) 1= 0. 7057±0.0008. The other was obtained by internal isochrons. Two rock specimens of the Aioi formation were separated for internal isochrons. One gave T=70±7m.y. and (87Sr/86Sr) =0.7079±0.0007, and the other gave the same age and higher initial strontium ratio, respectively. This indicates that the Aioi formation erupted at about100m. y. BP and was influenced at about70m. y. BP by the eruption of the Tenkadaiyama formation which caused re-equilibration of the Rb-Sr system in the mineral parts of the Aioi formation to give a younger internal isochron age.
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Abstract Based on the Rb‐Sr isotopic study of the granitoids in the Hengduan Mountains, the classification and geologic significance of whole‐ rock Rb‐ Sr isochrons as well as the factors influencing homogenization of the isotopic systems are discussed. Usually, there is no good linear correlation of isochrons for diorites and alkali‐rich intrusive rocks (including alkaline rock and alkalic granite). But by means of the external isochron of monominerals from the rocks, isochrons with good linear correlation as well as ages and Sr initial ratios with a high confidence can be obtained. In order to obtain a satisfactory isochron, the discriminant parameters should be calculated from the Rb / Sr ratios and estimated ages of samples. Only those that meet the requirements can be used as the Rb‐Sr isochrons. The quality of an isochron should be judged from three factors, i.e. correlation coefficient, MSWD and homogenization degree.
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Geochronology
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Six whole rock samples of the Tirodi gneiss belonging to the basal part of the Sausar Group rocks in the Dongri Buzurg area yield a good Rb-Sr isochron corresponding to an age of 1525 +- 70 Ma and an initial Sr ratio of 0.7148 +- 0.0033. Mineral isochrons for two of these rocks give concordant but younger ages at 860 Ma in good agreement with the previously reported K/Ar ages on minerals separated from the Sausar Group rocks. The 1525 Ma event is interpreted to mark the main phase of regional metamorphism of the Sausar Group of rocks including the granitization of the basal Tirodi unit. The deposition of the sediments of the Sausar Group and their basement must therefore be even older. The good agreement of the Rb/Sr and K/Ar mineral ages at about 860 Ma indicates the latest thermal overprint on the Sausar rocks. The regional metamorphism of the Tirodi gneiss at about 1500 Ma ago is also reflected in the Rb/Sr and K/Ar mineral ages of the Amgaon gneiss and Dongargarh granite to the east of the Tirodi gneiss. 6 references, 4 figures, 3 tables.
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The chemical Th-U-total Pb isochron age was detemined for zircon grains from the post-tectonic Mereb Granite in northern Ethiopia. Twenty-seven analyses from five zircon grains yield a well defined isochron of 545±24 Ma. One of the analyzed grains is chronologically zoned with a core having age range of 732-828 Ma. The core is interpreted as a xenocryst. The 545±24 CHIME zircon age, slightly younger than the previously reported Rb-Sr whole-rock isochron age of 633±62 Ma, dates emplacement of the Mereb Granite, and agrees well with the 541+10/-16 Ma U-Pb zircon age for the post-tectonic Mao granite in western Ethiopia.
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