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The Saveh magmatic complex is a composite intrusion of diorite to tonalite. It is part of the Urumieh-Dokhtar magmatic arc in central Iran. Zircon U Pb dating indicates that emplacement and cooling occurred 37–40 Ma ago in late Eocene (Bartonian) time. Saveh igneous rocks define a continuous mafic-felsic continuum in terms of SiO 2 (55.4–70.8 wt%), Al 2 O 3 (14.2–19.8 wt%) and CaO (6.4–17.1 wt%) contents. The initial values of 87 Sr/ 86 Sr and ɛ Nd (t) are 0.7048 to 0.7062 and + 2.6 to +3.4 (Mean = +2.9). Nd model ages (T DM  = 555 to 616 Ma) are consistent with derivation from Iranian continental crust and subcontinental lithospheric mantle of Cadomian age (500–600 Ma). Saveh igneous rocks are enriched in K, Cs and Pb and depleted in P, Nb and Ta, indicating that the magma source was moderately affected by slab-derived fluids, as expected for the Eocene tectonic environment. The continuum of rock types from diorite to graphic eutectic tonalite indicates that magmatic evolution was controlled by the F (FeO + MgO + MnO)-Anorthite (An)-Orthoclase (Or) cotectic with minor contamination by supra-crustal components. Similar initial ratios of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd for all of the rocks and the slightly younger age of tonalitic rocks (2–3 Ma younger) further indicate that the crystallization of quartz and plagioclase with graphic texture occurred in the final stage of magmatic differentiation. Saveh magmas reflected partial melting of Iranian subcontinental lithospheric mantle due to extension above the Neo-Tethys subduction zone. [ABSTRACT FROM AUTHOR]

Dioritic--trondhjemitic gneisses of Neoarchean age are widely distributed throughout the Gualanyu--Shangying area in Eastern Hebei, North China Craton. These orthogneisses consist of dioritic, trondhjemitic, and quartz-rich trondhjemitic gneisses, which were subjected to amphibolite- to granulite-facies metamorphism. LA-ICP-MS zircon U--Pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 ± 23 to 2513 ± 8 Ma, reflecting an important Neoarchean dioritic--trondhjemitic magmatic event followed by sequential Proterozoic tectonothermal events between 2472 ± 60 and 2307 ± 50 Ma. Dioritic and trondhjemitic gneisses exhibit similar zircon ɛHf(t) values of +1.5 to +5.2 and +2.9 to +5.3, respectively, indicating their depleted mantle affinity. Geochemical analysis reveals that the dioritic gneisses have low SiO2 contents (55.4-63.0%), various MgO contents (2.07-4.78%) and Mg# (36.07-63.56), and high (La/Yb)N ratios (5.45-15.19). In contrast, the trondhjemitic gneisses exhibit higher SiO2 contents (67.4-73.8%) and (La/Yb)N ratios (12.86-76.73), and lower MgO contents (0.66-2.67%), although their Mg# (36.29-61.41) are similar to those of dioritic gneisses. These geochemical and zircon Lu--Hf isotopic features, together with geochemical modeling, suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids, and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas. Quartz-rich trondhjemitic gneisses exhibit extremely high SiO2 contents (77.4-84.6%) with zircon ɛHf(t) values ranging from -3.4 to +4.1, suggesting that their precursors formed after the partial melting of a mixed magmatic source. It is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids, and a felsic member derived from the partial melting of ancient crustal material. These new data, combined with previous studies of contemporaneous metavolcanic rocks in Gualanyu-Shangying area and surrounding areas, suggest that the magmatic precursors of these Neoarchean dioritic--trondhjemitic gneisses formed in a subduction-related tectonic environment at an E-W strike convergent plate margin. Furthermore, the Eastern Block of the North China Craton had a major lateral accretion of crustal growth in the north margin in the Neoarchean, whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the Eastern Block. [ABSTRACT FROM AUTHOR]

[Display omitted] • Newly ca. 880 Ma gabbro-diorite and gabbro are identified in the Bikou Terrane. • Both the gabbro-diorite and gabbro were formed in a subduction-related tectonic setting. • The Bikou Terrane was a part of the subduction-related system during the early Neoproterozoic. The Neoproterozoic mafic intrusions in the Bikou Terrane along the northwestern margin of the Yangtze Block record crucial information on evaluating the tectonic evolution of the Yangtze Block, South China. However, their petrogenesis and tectonic significance are still poorly constrained. In this paper, we present the results of a comprehensive study of zircon U–Pb geochronology and whole-rock geochemistry from the gabbro-diorites and gabbros of the Bikou Terrane. Zircon U–Pb dating results indicate that these gabbro-diorite and gabbro samples were formed coevally at ca. 880 Ma. The gabbro-diorite samples are calc-alkaline in composition and have intermediate contents of SiO 2 , Mg#, Ni, and Cr. They are enriched in LREEs, and depleted in HFSEs (i.e., Nb, Ta, Zr, and Hf). They have relatively high ratios of Th/Yb, and low ratios of Nb/Yb, Nb/La, Th/Nd, Th/Zr, and U/Th, indicating that they were derived from partial melting of a mantle wedge source metasomatized by slab-derived melts. The gabbro samples are commonly classified as sub-alkaline tholeiitic rocks, and have constantly low to medium values of MgO, Mg#, Cr, and Ni, with distinct spikes at Th, U, Nb and Ta concentrations. They show an E-MORB-like enriched LREEs, low ratios of Nb/La, Ta/Yb, Th/La, and Nb/Zr, and medium ratios of Nb/Yb and Th/Yb, implying that they were most probably originated from the interaction between the upwelling enriched asthenospheric mantle and the mantle wedge peridotite. This mechanism may be triggered by the slab rollback process in a subduction-related environment. Our new investigations, along with available observations from other scholars, support the persistent subduction-related system along the northwestern margin of the Yangtze Block during the early Neoproterozoic. [ABSTRACT FROM AUTHOR]