Petrogenesis of the Late Mesozoic gabbros in the eastern Jiangnan Orogen, South China: Characteristics of the lithospheric mantle
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As we know, mantle‐derived mafic rocks can be used to effectively trace the nature of the mantle source and constrain the regional tectonic setting and evolution. Recently, a Late Mesozoic mafic intrusive body (Huangmao pluton) has been found in the eastern Jiangnan Orogen (EJO). In this study, we investigate this pluton using petrography, whole‐rock geochemistry, Sr–Nd–Pb isotopes, and geochronology. The Huangmao pluton is primarily composed of a fine‐grained gabbro‐diabase, which is characterized by consistent SiO 2 , MgO, Fe 2 O 3 T , and TiO 2 contents. These gabbros exhibit continental crust‐like trace element signatures, i.e., enriched in large‐ion lithophile elements (LILE), Pb, and light rare earth elements (LREE), but depleted in high‐field‐strength elements (HFSE). In addition, the Huangmao gabbros have moderate initial 87 Sr/ 86 Sr ratio (0.7065–0.7080) and slightly negative to weakly positive ε Nd ( t ) value (−2.06 to +0.84). Finally, U–Pb zircon dating confirms that the Huangmao pluton was emplaced in the Late Jurassic (152.6 ± 3.1 Ma). Through an integrated geochemical analysis, these Late Mesozoic gabbros were genetically originated from an enriched lithospheric mantle source, which formed by a metasomatic reaction between mantle peridotite and sediment‐derived felsic melt. Consequently, the lithologies of the metasomatite were predominantly pyroxenites with a small amount of pyroxene‐rich peridotites. Considering the large distance between the modern trench and the EJO, the flat‐slab subduction and rollback model of the palaeo‐Pacific Plate is the most reasonable interpretation of the geodynamic setting that resulted in Late Mesozoic magmatism in the EJO.Keywords:
Petrogenesis
Fractional crystallization (geology)
Felsic
Peridotite
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Felsic
Petrogenesis
Hornblende
Fractional crystallization (geology)
Igneous differentiation
Pyroxene
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Petrogenesis
Felsic
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Felsic
Petrogenesis
Large igneous province
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The Xiangpishan complex in the Zongwulong‐Qinghainanshan Tectonic Belt, Northeast (NE) Tibetan Plateau, is a composite concentric pluton consisting of a felsic core (granodiorite) surrounded by quartz diorite in the middle to diorite and minor gabbro at the margin with locally less volume of monzogranite. Many dioritic enclaves are unevenly distributed within host granitoids. We evaluate the petrogenesis of the complex by geochemical, geochronological, and Hf isotopic data in tandem with regional data in the belt. The results show that felsic rocks were emplaced during the Late Permian (ca. 262–256 Ma); whilst gabbros yielded a younger age of ca. 249 Ma. As the hybrid phase from mixing between felsic and mafic magmas, enclaves and diorites have analogous ages (ca. 257–254 Ma) to both. Zircons from gabbro and enclaves are marked by higher ε Hf ( t ) value up to +1.82; whilst granodiorites have lower ε Hf ( t ) value of −5.48, consistent with hybrid diorites possessing intermediate ε Hf ( t ) values of −3.14 to 0.34. Furthermore, the quantitative calculation from Mass Balances Modelling suggests that the mass of mafic magma (ca. 67%–79%) is involved to achieve the hybridization. Geochemically, these rocks crystallized from calc‐alkaline magma with different sources, but demonstrated consistent arc‐like signatures, as they are enriched in large‐ion lithophile elements and light rare earth element (LREE), and depleted in high‐ field‐strength elements and heavy rare earth element (HREE). Besides, they present negative Nb–Ta anomalies together with significant P and Ti troughs. Finally, an evolutional model has been proposed that the asthenosphere‐lithosphere interaction played an important role during the emplacement of the complex, where the limited volumes of mantle‐derived melt act as the suppliers of heat and mass (mainly volatile components) to induce partial melting of the juvenile mafic lower crust and mixed (or mingled) with the produced crust‐derived magma during the oceanic subduction, which led to the generation of diorites as well as mafic microgranular enclaves.
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Both felsic and mafic intrusions occur in mantle peridotites of the New Caledonia ophiolite (Peridotite Nappe) and record important information regarding the magmatic evolution during subduction initiation. A systematic petrological, geochronological and geochemical study on gabbro/diorite samples from the Peridotite Nappe is performed to explore their petrogenesis. The gabbro is mainly composed of plagioclase, clinopyroxene and amphibole. Most clinopyroxene have been altered to hornblende or actinolite, whereas some amphibole with high Al2O3 (>8 wt%) and low SiO2 (<46 wt%) contents are of a magmatic origin. This suggests that the gabbro crystallized from hydrous melts. The gabbro shows depleted Sr-Nd-Hf isotope compositions, suggesting that the parental magma of the gabbro could have been derived from the depleted MORB-type mantle (DMM) asthenosphere. The gabbro zircon have δ18O values of +4.74-+6.63‰, which are slightly higher than the mantle-like δ18O values of +5.3 ± 0.6‰, indicating the involvement of subduction-related fluids in their genesis. The diorite mainly consists of albite (An1-4) and tremolite. The felsic dikes within the New Caledonia ophiolite, including granitoids, high-Mg felsic dikes and diorites, are oceanic plagiogranites. Extremely low TiO2 contents, roughly negative correlation of Yb and SiO2 contents, REE patterns of plagiogranites and mafic rocks, and compositions of zircon suggest that the plagiogranites were not products of fractional crystallization of MORB but were generated by hydrous melting of mafic protoliths. The diorite zircon show core-rim textures. Both rims and cores yield identical U-Pb ages of 54.6 ± 0.9 Ma vs 54.6 ± 0.6 Ma, and no old inherited cores were identified based on significantly older dates. Our studies show zircon U-Pb ages of ~55 Ma for the gabbro/diorite and titanite U-Pb age of ~53 Ma for the gabbro, which are older than the New Caledonia CE-boninites (51–47 Ma). Therefore, the magmatic evolution of the New Caledonia ophiolite during subduction initiation has been reconstructed.
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Peridotite
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In this paper,two representative samples of Maixie pluton were selected for LA-ICP-MS zircon U-Pb dating,yielding U-Pb ages of(434±2)Ma and(437±2)Ma,which indicates that the Maixie granites were formed in Caledonian orogen.Whole-rock geochemical data indicate that the rocks belong to high-K calc-alkaline series,they are typical metaluminous I-type granite,with some features similar to those of the high Ba-Sr granite,i.e.,high content of Ba and Sr and high ratios of Sr/Y and La/Yb,but low content of Y(19 μg/g) and Yb(1.8 μg/g) and low ratios of K/Rb and Rb/Sr(average 0.37).Besides,the rocks show negative Eu anomaly,and are depleted in high field strength elements,such as Nb,P,Ti and so on.The Hf isotopic compositions of zircons show that the rocks have eHf(t) values of-35.32 to-1.50 and T2DM(Hf) model ages ranging from 3.63 to 1.51 Ga,implying that the magma was derived from Paleo-or Meso-Proterozoic rocks,with involvement of some jurenile mantle in petrogenesis.Combined with the geochemical composition,the inhomogenous zircon Hf isotopic compositions as well as the appearance of common mafic microgranular enclaves,we suggest that the Maixie pluton were formed by magma mixing between the mafic magmas and felsic magmas generated by partial melting of overlying lower crust inducing mafic magmas.
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