The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiol...The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiolitic complexes.The Beshta-Kamenistaya orthogneisses and associated rocks of the nappes differ markedly from those of the Main Range zone.All of them were overthrust to the Main Range zone during the Bretonian orogeny.The age of their protolith and the metamorphism are still not defined.Using the zircon U-Pb LA-ICP-MS dating two age populations of zircons have been distinguished in the rocks of the Beshta-Kamenistaya intrusions.The age of the main population of zircons from orthogneisses is 426-300 Ma.Several age groups can be distinguished in this population.The main group yielded a Concordia age of386.9±1.4 Ma.There are also smaller peaks at 409-405,375-373,and 351 Ma.The oldest ages(426-395 Ma)were detected in the core parts of the complex crystals.We assume that the crystallization of the parental for orthogneisses rocks(tonalities)took place at 410-395 Ma,whereas the Concordia age of 386.9±1.4 Ma and a peak at 375-373 Ma correspond to the metamorphic event.The whole metamorphic cycle,including progressive and regressive stages,occurred between 395 and 370 Ma.Zircons,dated in the Beshta-Kamenistaya intrusion at350 Ma and younger,correspond to the Late Variscan orogeny.Zircons dated 3102-2769 Ma represent xenocrysts captured by the melts during their formation from the ancient rocks in the crystalline basement.展开更多
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文摘The Beshta-Kamenistaya intrusions are located in the Main Range structural zone of the Greater Caucasus.They are composed of tonalitic gneisses that genetically resemble granites of the tholeiitic series of the ophiolitic complexes.The Beshta-Kamenistaya orthogneisses and associated rocks of the nappes differ markedly from those of the Main Range zone.All of them were overthrust to the Main Range zone during the Bretonian orogeny.The age of their protolith and the metamorphism are still not defined.Using the zircon U-Pb LA-ICP-MS dating two age populations of zircons have been distinguished in the rocks of the Beshta-Kamenistaya intrusions.The age of the main population of zircons from orthogneisses is 426-300 Ma.Several age groups can be distinguished in this population.The main group yielded a Concordia age of386.9±1.4 Ma.There are also smaller peaks at 409-405,375-373,and 351 Ma.The oldest ages(426-395 Ma)were detected in the core parts of the complex crystals.We assume that the crystallization of the parental for orthogneisses rocks(tonalities)took place at 410-395 Ma,whereas the Concordia age of 386.9±1.4 Ma and a peak at 375-373 Ma correspond to the metamorphic event.The whole metamorphic cycle,including progressive and regressive stages,occurred between 395 and 370 Ma.Zircons,dated in the Beshta-Kamenistaya intrusion at350 Ma and younger,correspond to the Late Variscan orogeny.Zircons dated 3102-2769 Ma represent xenocrysts captured by the melts during their formation from the ancient rocks in the crystalline basement.
