ZIRCON ANATOMY FROM THE ROCKS ASSOCIATION OF THE OSTRIVSKY QUARRY (ROS-TIKYCH MEGABLOCK OF THE UKRAINIAN SHIELD)
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Abstract:
Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.Keywords:
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Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.
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Abstract Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron‐bright, U‐rich (800–4400 ppm) with low Th/U‐ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μ m‐thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south‐vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre‐Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW‐plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high‐pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.
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We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa–Shizuoka Tectonic Line using detrital zircon U–Pb dating and Raman carbonaceous material geothermometry, respectively. U–Pb dating of a psammitic rock yielded a youngest age of ∼100 Ma, which corresponds to the protolith age. Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of ∼350–380 °C. The protolith age is consistent with those of the Sanbagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K–Ar dating of the YMR reflect thermal resetting due to contact metamorphism.
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Abstract Detrital zircon U-Pb studies of mudstone provenance are rare but may preferentially fingerprint distal zircon sources. To examine this issue, Pierre Shale and Trinidad Sandstone deposited in a Late Cretaceous deltaic environment in the Raton Basin, Colorado (USA), were measured for detrital zircon U-Pb age by laser ablation–inductively coupled plasma–mass spectrometry. Two major detrital zircon age peaks at ca. 70 and 1690 Ma are found in both Pierre Shale and Trinidad Sandstone but in inversely varying proportions: 68% and 16%, respectively, for the finest zircon fraction (~15–35 μm) in the shale, and 25% and 32%, respectively, for the coarsest zircon fraction (~60–80 μm) in the sandstone. Proximal sources in the Sangre de Cristo Mountains, directly west of the Raton Basin, contain coarse-grained, ca. 1690 Ma zircon, whereas distal sources in Laramide uplifts and basins in Colorado, New Mexico, and Arizona contain fine-grained, ca. 70 Ma zircon. The results indicate that U-Pb zircon provenance of mudstone reflects availability of volcanic and other fine-grained source rocks rather than simply distal sources. U-Pb zircon provenance studies should routinely include mudstone units because these units may identify fine-grained zircon sources more reliably than sandstones alone.
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Micro-scale in situ REE analyses of zircon cores and rims have been carried out for eclogites from Xindian, Shuanghe, Huangzhen and jadeite quartzite from Shuanghe, Dabie Mountains. Combining with the CL internal structure and U-Pb dating, we studied the zircon REE composition change from the protolith inherited zircon to the metamorphic zircon during the ultra-high pressure metamorphism. The results show that the inherited core and metamorphic rim have quite different REE contents. It depends on how metamorphic zircon forms, overgrowth or recrystallization of protolith zircon. In general, the metamorphic zircon rims have lower REE contents compared with the protolith cores, especially in HREE. The metamorphic rims also have lower Th/U ratio than that of the cores. Detailed comparative study on REE and Th/U ratio of inherited zircon cores and metamorphic zircon rims may provide important information on forming process of metamorphic zircons and physico-chemical environment when it forms.
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Detrital zircon U-Pb and muscovite 40Ar/39Ar dating are useful tools for investigating sediment provenance and regional tectonic histories. However, the two types of data from same sample do not necessarily give consistent results. Here, we compare published detrital muscovite 40Ar/39Ar and zircon U-Pb ages of modern sands from the Yangtze River to reveal potential factors controlling differences in their provenance age signals. Detrital muscovite 40Ar/39Ar ages of the major tributaries and main trunk suggest that the Dadu River is a dominant sediment contributor to the lower Yangtze. However, detrital zircon data suggest that the Yalong, Dadu, and Min rivers are the most important sediment suppliers. This difference could be caused by combined effects of lower reaches dilution, laser spot location on zircons and difference in closure temperature and durability between muscovite and zircon. The bias caused by sediment laser spot targeting a core or rim of zircon and zircon reworking should be considered in provenance studies.
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