Zircon SHRIMP U-Pb dating on plagiogranite from Kuerti ophiolite in Altay,North Xinjiang
Abstract Field observation, petrological and geochemical characteristics of plagiogranite from Kuerti ophiolite indicate a similar origin to those in shearing zones. It is derived from partial melting of amphibolite that is developed from gabbro within the ocean layer 3 shear zone by the low-angle shearing deformation during the oceanic crust migrating process. Zircon SHRIMP age of 372±19 Ma for the plagioganite from Kuerti ophiolite indicates that this ophiolite formed in the Devonian period and it also represented the time of extension of the Kuerti backarc basin that is relevant to the northwards subduction of the Paleo-Asian oceanic crust. Therefore, the northwards subduction of the Paleo-Asian Ocean beneath the Siberian Plate began in the early stage of the Late Paleozoic era.
SHRIMP U-Pb dating of detrital zircons from the Sanbagawa Belt,Kanto Mountains,Japanneed to revise the framework of the belt
Radiometric ages of detrital zircons in three samples of psammitic schists from the Sanbagawa Belt,Kanto Mountains,were obtained from the 238U/206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe(SHRIMP II）.Most of the zircon ages cluster around Cretaceous,with a few ages corresponding to older zircons.The origins of the detrital zircons are mainly Cretaceous igneous rocks.The ages of the youngest zircons in samples AM48p,SnbE,and AM29p indicate Late Cretaceous time,and they are 78.8±1.3Ma,91.4±1.4Ma,and 95.3±1.5Ma,respectively.The age difference between the youngest detrital zircon age and white mica K-Ar age is 13 Myr.The Sambagawa Belt is believed to be a metamorphosed phase of the Chichibu Belt,which is a Middle Jurassic to earliest Cretaceous accretionary complex;however,the results of this study suggest that the protolith of the Sanbagawa belt was accreted in Late Cretaceous,similar to the Shimanto belt that runs parallel to the Sambagawa and Chichibu belts.
Zircon ‘microvein’ in peralkaline granitic gneiss, western Ethiopia Origin, SHRIMP U-Pb geochr onology and trace element investigations
Azircon‘microvein’ composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians.U–Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, andMn) abundances of the ‘microvein’ and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the ‘microvein’ zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively,reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr.The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that darkgrey BSE domains were formed by dissolution–reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals.The bright and dark-grey BSE domains in Type-I zircon yield U–Pb ages of 779±69Ma and 780±35Ma, and similar domains in Type-II zircon dated at 778±49 Ma and 780±31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical,within analytical uncertainties, to the 776±12Ma zircon U–Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming ‘microvein’ and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatic–hydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of or thomagmatic or hydrothermal fluids containing fluorides as a major constituent,which expelled a considerable amount of trace elements,namely,Hf, U, Th,Y, and theREEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼0.5 to 1.0). Furthermore, the ca. 780–776Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.
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