Early Palaeozoic Acid Magmatism in the Saxothuringian Belt: New Insights from a Geochemical and Isotopic Study of Orthogneisses and Metavolcanic Rocks from the Fichtelgebirge, SE Germany
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Orthogneisses and acid metavolcanic rocks from the Fichtelgebirge, NE Bavaria, Germany, are predominantly chemically evolved (monzo)granites and rhyodacites–dacites, respectively. The metavolcanic rocks are variably tectonized and include samples with anomalously low CaO, Na2O and MnO (<0.01 wt %) and high SiO2 (>80 wt %) and K2O (>6 wt %) concentrations, implying selective element mobility during post-eruptive events. Sm–Nd isotope data for samples from three main orthogneiss units (Wunsiedel, Selb, Waldershof) yield a composite Sm–Nd whole-rock isochron corresponding to an age of 560 ± 45 Ma. This age estimate is constrained, however, to be less than ∼560 Ma by the presence of detrital Cadomian zircons in the country rocks. Fifteen specimens from the Wunsiedel orthogneiss give an Rb–Sr whole-rock isochron of 480 ± 4 Ma with an initial 87Sr/86Sr ratio of 0.7095 ± 0.0007 (MSWD = 2.7). Rb–Sr isotope data from the Waldershof orthogneiss and the metavolcanic rocks suggest, however, that, in general, Sr isotopic equilibrium, if ever reached, was significantly modified during later events. Taking recent geochronological literature data into account, it now appears that Early Palaeozoic acid magmatism in the Fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the Early and Mid Ordovician and ended with the eruption of the volcanic successions during the Late Ordovician. K–Ar ages of the investigated rocks range from 316 to 298 Ma for muscovites and from 306 to 280 Ma for biotites, demonstrating thermal influences from Late Visean to Late Stephanian (325–290 Ma) granite intrusions. The involvement of dominantly crustal-derived melts is considered to account for the peraluminous character (A/CNK > 1.08), high initial 87Sr/86Sr (> 0.709) and negative εNd(500 Ma) (−2.9 to −6.4) of the intrusive and volcanic rocks. The generation of the melts is assumed to have taken place within an overall extensional geodynamic setting. The wider range in εNd(500 Ma) for the metavolcanic rocks (−3.8 to −6.4; orthogneisses: −2.9 to −4.0) suggests that differential admixture of a pelagic sedimentary component during emplacement may have occurred. Nd model ages range from 1.5 to 1.7 Ga and are consistent with those of granitoids from the Erzgebirge and the Lausitz but completely different from those of metavolcanic rocks from the Thüringer Wald. In addition, Nd model ages of the Early Palaeozoic granitoids in the Fichtelgebirge are consistent with those of the Late Carboniferous granitoids from the same area. This suggests similar source material for the pre-Variscan and the late Variscan Fichtelgebirge granitoids.Keywords:
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Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial 87 Sr/ 86 Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial 87 Sr/ 86 Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial 87 Sr/ 86 Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial 87 Sr/ 86 Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial 87 Sr/ 86 Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial 87 Sr/ 86 Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial 87 Sr/ 86 Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial 87 Sr/ 86 Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.
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Precise electron microprobe analysis makes it possible to determine the Th, U and Pb concentrations in an area less than 5 μm across in a single grain of monazite, zircon or xenotime; the detection limit of PbO at 2σ confidence level is 0.005–0.008 wt.% and the relative error is 3% for 0.2 wt.% concentration level. The subgrain analyses of monazite are plotted on the coordinates of PbO and ThO2* (ThO2 plus the equivalent of UO2), and those of zircon and xenotime on the coordinates of PbO and UO2* (UO2 plus the equivalent of ThO2). Data points are arrayed linearly and enable us to define an isochron which passes through the origin. The chemical Th-U-total Pb isochron ages coincide well with mineral and whole-rock ages isotopically determined for the same samples. The chemical Th-U-total Pb isochron method was applied to the age determination of monazite, zircon and xenotime from the Tsubonosawa paragneiss and the host Hikami granite in the South Kitakami terrane of Northeast Japan. The chemical ages show that (1) the sedimentation of the gneiss-protolith occurred soon after the emplacement of 500 Ma granitoids in the source region, where the 500 Ma granitoids were widespread together with Precambrian rocks possibly dated back to as old as Archean (3080 ± 180 Ma), (2) the gneiss-protolith was metamorphosed to the amphibolite facies grade about 430 ± 10 Ma ago, and (3) the gneiss was affected by multiple thermal events of 350, 260, 180 and 100 Ma. The 350 Ma event corresponds to the intrusion of the Hikami granite, and the 100 Ma one to the intrusion of the Cretaceous Kesengawa granite. The 260 and 180 Ma events may correspond in age to the main metamorphic event and subsequent igneous activity in the Hida terrane, central Japan. The chemical Th-U-total Pb isochron age in terms of the precise microprobe analysis of low-level Th, U and Pb will open a new vista on the Paleozoic-Mesozoic tectonics of the Japanese Islands.
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In the paper,the Wilson's Vital Cyclicing theory in the ocean was used to explain the magmatism in the south of Qinghai Tibet plateau,where magmatism underwent rift type magmatism-ocean type magmatism-island type magmatism-collision type magmatism-intracontinent collision type magmatism. They reveal the internal relations between the magmatic evolutionary process and plate tectonics.
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151 in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogeneity inδ18O values, with variation in different rocks from -8.5‰ to +9.7‰ and within one sample from 2‰ to 12‰. No measurable difference inδ18O was observed between protolith magmatic (detrital) zircons and metamorphic recrystallized zircons within analytical uncertainties from the ion micro-probe measurements. This indicates that the metamorphic zircons have inherited the oxygen isotopic compositions of protolith zircons despite the HP to UHP metamorphism. According to their protolith ages from zircon U-Pb in situ dating by the same ion micro-probe, two groups of oxygen isotope composition are recognized, with one having δ18O values of 6‰-7‰ for old protolith of 1.9-2.5 Ga ages and the other 0‰-2‰ for young protolith of 0.7-0.8 Ga ages. The latter anomalously lowδ18O values of zircons indicate that the magma has had the obvious involvement of meteoric water when forming the young protolith of high-grade metamorphic rocks. This may be correlated with the snowball Earth event occurring in South China and the world elsewhere during the Neoproterozoic.
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