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Granitic rocks forming in the syn- to post-orogenic stages can trace the compositional and structural complexity of the crust beneath an orogenic belt. The Qinling orogenic belt undertook multiple stages of tectonics and magmatism, resulting in the multifaceted evolution and compositional diversity of the crust. In the present study, the Guangtoushan and Miba plutons in South Qinling were chosen to reveal the crustal heterogeneity in study area via isotopic geochemistry and zircon geochronology. The Guangtoushan pluton was emplaced between ~215 Ma and ~202 Ma and the Miba pluton formed at ~213 Ma, as constrained by zircon U-Pb isotopic dating. Granitic rocks of the Miba pluton are characterized by amphibole bearing and homogeneous composition, with relatively depleted Sr-Nd isotopic compositions (initial 87Sr/86Sr values of 0.7060 to 0.7084 and initial εNd values of −5.4 to −9.5) and high Pb isotopic values. The Guangtoushan pluton contains muscovite and complex inherited zircon grains and has variable Sr-Nd isotopic composition (initial 87Sr/86Sr values of 0.7050 to 0.7091 and initial εNd values of −4.5 to −12.9) and low Pb isotopic values. Felsic magmas of the Guangtoushan pluton should be derived mainly from meta-sedimentary rocks beneath South Qinling, while the Miba pluton originated primarily from partial melting of meta-igneous rocks. The compositional diversity recorded in the Early Mesozoic plutons was caused by the heterogeneous crust, and partial melting was induced by heating of the up-welling asthenosphere in a post-collision setting. [ABSTRACT FROM AUTHOR]

Sediments or clastic rocks can record the evolution history of basins, orogenic processes, crustal uplift and erosion, and even paleo-environments. Detrital minerals such as zircon, garnet, and apatite are useful media for studies of sedimentary sources and basin evolution. Detrital zircon has been widely taken as an indicator for provenances and tectonic evolution of geological terrenes via age distribution patterns. Apatite can remain stable during erosion and transportation and is also considered as an ideal object for source tracing. This mineral normally contains high Sr and negligible Rb. Its Sr isotopic composition can remain almost unchanged after crystallization. Unlike isotopic ages of detrital minerals, Apatite isotopic compositions have been less frequently used for tracing the provenance of sedimentary rocks in the last few decades. In the present study, we report on the Sr isotopic composition of individual apatite grains of Neogene and Jurassic conglomerates from the Hui-Cheng basin in the South Qinling orogenic belt, obtained via thermal ionization mass spectrometry. Detrital apatite grains of Jurassic rocks have narrow ranges of 87Sr/86Sr values from 0.7076 to 0.7100, but those of Neogene rocks gave variant 87Sr/86Sr values from 0.7055 to 0.7534, providing distinct evidence for complex sources of Neogene sedimentary rocks. Analytical results show that the distribution patterns of 87Sr/86Sr values of single-grain detrital apatite fit the distribution patterns of detrital zircon U-Pb from the isotopic ages very well. Detrital apatite Sr isotopic composition can provide essential information for tracing the origins and evolution of sedimentary sources. [ABSTRACT FROM AUTHOR]

The Ghohroud granitoids (GG), containing mafic microgranular enclaves (MMEs) are located in the central part of the Urumieh‐Dokhtar Magmatic Arc (UDMA) in central Iran. They are associated with the subduction‐related magmatism in the Alpine‐Himalayan orogenic belt. The GG are comprised of a variety of intermediate and felsic rocks, including tonalite, granodiorite, granite, diorite porphyry and monzodiorite. The MMEs are gabbroic diorite and tonalite in composition and characterized by a fine‐grained hypidiomorphic microgranular texture with occasional chilled margins. They show rounded, sharp or irregular contact with the host granitoids. The occurrences of quartz, K‐feldspar and corroded plagioclase indicate that MMEs are the products of mixing between mantle and crust‐derived magmas. New ages of zircon U–Pb dating reveal that the GG in the Kashan area emplaced at ca. 19–17 Ma (Burdigalian). All the samples of MMEs and granitoid host rocks in this study are metaluminous and calc‐alkaline with I‐type affinities. They are enriched in light rare earth elements (LREEs) and show slight negative Eu anomalies (Eu/Eu* = 0.36–0.95). These features in a combination with the relative depletion in Nb, Ta, Ti and P, indicate the granitoids and MMEs are closely associated with subduction‐related magmas at an active continental margin. The host rocks yield relatively homogeneous isotopic compositions of initial 87Sr/86Sr ratios ranging from 0.706036 to 0.707055, εNd(t) values varying from −2.25 to 0.8, and the Nd model ages (TDM) vary in a limited range of 0.70–0.96 Ga. The MMEs show similar initial 87Sr/86Sr ratios (0.706420–0.707366), εNd(t) values (−1.32 to −0.27), TDM (0.68–1.09 Ga) and Pb isotopic compositions with host granitoids, which imply they attained isotopic equilibration during magma mingling and mixing. In combination with the petrographic, chemical and isotopic results, we suggest that the origin of MMEs and their host rocks were related to the interaction between crust‐derived melts and mantle‐derived mafic magmas. The magma‐mixing event possibly occurred during the transition from subduction to collision in the UDMA along with the closure of the Neotethyan ocean. [ABSTRACT FROM AUTHOR]

Abstract: The Sm–Nd isotopic system, one of the important techniques for dating and geochemical tracing, has been widely used in geosciences. Precise and accurate measurements of Nd isotopic ratios are more difficult to achieve for microsamples that contain less than 50ngNd, when routine Nd+ technique is applied on thermal ionization mass spectrometer (TIMS). Compared to Nd+ technique, the NdO+ technique, owing to more efficient ionization, is a better choice for high precision measurements of Nd isotopic ratios of micro- or low-content samples. This study presents measurement results of sub-nanogram size samples of standard reference solution and rock-powder using NdO+ technique on the latest TIMS (GV product IsoProbe-T). Sixteen rock-powder references and four reference solutions were measured both by NdO+ and Nd+ techniques for an evaluation of accuracy and precision of measurements obtained by the NdO+ technique. Results show that analytical precision and reproducibility of NdO+ measurements of sub-nanogram Nd is comparable with those obtained by the Nd+ measurements of samples containing 200ngNd. Internal precision of the 143Nd/144Nd ratios achieved with both techniques is better than 0.003%. Our results show that the microsample NdO+ technique is a very promising method for high resolution Sm–Nd geochronology and isotopic tracing. [Copyright &y& Elsevier]

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The Moldanubian zone of the Black Forest, Germany, part of the internal zone of the central European Variscan belt, exposes high-pressure metamorphic rocks as small eclogite lenses in host gneisses. We report zircon U-Pb and PbPb ages and garnet Sm-Nd and Pb-Pb ages for five eclogites. The less mobile trace element contents of these eclogites are similar to those of mid-ocean ridge basalts but have moderate to highly fractionated concentrations of the rare earth elements, enrichments in the incompatible elements, and negative anomalies in the high-field strength elements. Initial ∈[sub Nd] values range from -3 to +1.8, and initial[sup 87]Sr/[sup 86]Sr ratios range from 0.7040 to 0.7062. These characteristics of the eclogites suggest an origin from mantle-derived magmas contaminated either by evolved crustal material during intrusion or altered by fluid before or during eclogitization. A combination of zircon U-Pb and Pb-Pb evaporation dates of the eclogites constrains a magmatic episode between ∼486 and ∼445 Ma. This episode was contemporary with the early Paleozoic mafic to ultramafic magmatism in the eastern part of the Variscan belt and probably represents an early Paleozoic rift-stage product. Garnets subjected to the stepwise acid-leaching method from three eclogites yield Sm-Nd isochron ages of about 348-340 Ma, similar to the dates previously reported. The same garnets give less precise Devonian to Early Carboniferous Pb-Pb isochron ages of 406 ± 31 Ma, 386 ± 21 Ma, and 353 ± 89 Ma, which probably date to an earlier stage of the Variscan convergence. [ABSTRACT FROM AUTHOR]