Re–Os and U–Pb geochronology of porphyry Mo deposits in central Jilin Province: Mo ore-forming stages in northeast China
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AbstractCentral Jilin Province lies along the eastern edge of the Xing–Meng orogenic belt of northeast China. At least 10 Mo deposits have been discovered in this area, making it the second-richest concentration of Mo resources in China. To better understand the formation and distribution of porphyry Mo deposits in the area, we investigated the geological characteristics of the deposits and applied zircon U–Pb and molybdenite Re–Os isotope dating to constrain the age of mineralization. Our new geochronological data show the following: the Jidetun Mo deposit yields molybdenite Re–Os model ages of 164.6–167.1 Ma, an isochron age of 168 ± 2.5 Ma, and a weighted mean model age of 165.9 ± 1.2 Ma; the Houdaomu Mo deposit yields molybdenite Re–Os model ages of 167.4–167.7 Ma, an isochron age of 168 ± 13 Ma, and a weighted mean model age of 167.5 ± 1.2 Ma; and the Chang'anpu Mo deposit yields a zircon U–Pb age for granodiorite porphyry of 166.9 ± 1.5 Ma (N = 16). These new age data, combined with existing molybdenite Re–Os dates, show that intense porphyry Mo mineralization was coeval with magmatism during the Middle Jurassic (167.8 ± 0.4 Ma, r > 0.999). The geotectonic mechanisms responsible for Mo mineralization were probably related to subduction of the Palaeo-Pacific plate beneath the Eurasian continent. Combining published molybdenite Re–Os and zircon U–Pb ages for northeast China, the Mo deposits are shown to have been formed during multiple events coinciding with periods of magmatic activity. We identified three phases of mineralization, two of which had several stages: the Caledonian (485–480 Ma); the Indosinian comprising the Early–Middle Triassic (248–236 Ma) and Late Triassic (226–208 Ma) stages; and the Yanshanian phase comprising the Early–Middle Jurassic (202–165 Ma), Late Jurassic–early Early Cretaceous (154–129 Ma), and Early Cretaceous (114–111 Ma) stages. Although Mo deposits formed during each phase/stage, most of the mineralization occurred during the Early–Middle Jurassic.Keywords: Re–Os and U–Pb geochronologyporphyry molybdenum depositsmineralization periodscentral Jilin Provincenortheast China AcknowledgementsWe thank the staff of the National Research Centre of Geoanalysis, Chinese Academy of Geological Sciences, and the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China, for Re–Os and U–Pb zircon analysis dating. In particular, we express our gratitude to the reviewers for their critical reviews and excellent suggestions which provided for the improvement of this article. This work was jointly supported by the Project for Scientific and Technology Development (Grant No. 20100450), the National Natural Science Foundation of China (Grant No. 41172072), and the Chinese Geological Survey Programme (Grant No. [2010]01-26-06).Keywords:
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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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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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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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