Details of the collision between the Cathaysia and Yangtze blocks have long been debated. Detailed geological, geochemical, and C and O isotopic studies were carried out on the plagioclase amphibolite and marble in the Chencai area, Zhuji city, Zhejiang Province. The plagioclase amphibolite yields U–Pb age of 435 ± 11 Ma, which represents the metamorphic age, and the youngest age indicates that the plagioclase amphibolite formed after 463 Ma. The marble in the Chencai area yields U–Pb age of 431 ± 7 Ma and represents the metamorphic age, and the youngest inherited zircon indicates that the marble formed after 457 Ma. Combined with the two ages, we believed that they were both formed at early Paleozoic (after 457 Ma) and suffered metamorphism during the Caledonian (431–435 Ma). Meanwhile, REE and trace‐element characteristics show that the plagioclase amphibolite originated from the mantle source with the characteristics of OIB. The marble C and O isotopic characteristics indicated that the protolith of marble may be marine carbonate. The occurrence of the oceanic island seamount indicates that the Chencai Complex should be a subduction–accretion complex (CSAC), which formed at early Paleozoic. Given the presence of Caledonian granulite and garnet amphibolite in the Longyou area, amalgamation of the Cathaysia and Yangtze blocks was completed after 457 Ma. The CSAC is the product of the Yangtze–Cathaysia collision, with the collision causing widespread amphibolite‐facies metamorphism.
A number of Late Mesozoic (ca. 130 Ma) A‐type granitic plutons have been identified in Southeast (SE) China. The Lingshan pluton, located in the Gan‐Hang Belt, is a typical one which is mainly composed of coarse‐grained biotite monzogranite in the pluton centre and medium‐ to coarse‐grained syenogranite on the margin. Zircons from three representative samples show U–Pb ages of 132.0 ± 1.3, 134.4 ± 2.1, and 133.0 ± 1.9 Ma, respectively, indicating that the Lingshan pluton were formed in the Early Cretaceous. The Lingshan granites display typical A‐type affinities, with high SiO2, K2O, Na2O + K2O, rare earth element (REE) and high field strength element (HFSE) contents, high Ga/Al and Fe# [FeOT/(FeOT + MgO)] ratios and Zr + Nb + Ce + Y contents, and apparent depletion of Ba, Sr, Ti, and Eu. The calculated zircon saturation temperatures (TZr) are high, ranging from 841 to 966 °C (mean of 886 °C). The εHf (t) values and TDM2 (Hf) of the Lingshan granites mainly vary from −3.0 to 0.2 and 1.18 to 1.38 Ga, respectively. Consistently, the εNd(t) values vary from −3.9 to −2.8 and TDM2 (Nd) range from 1.16 to 1.26 Ga. These isotopic data indicate that the Lingshan granites were likely to be generated by partial melting of Mesoproterozoic metamorphic basement rocks with the involvement of a certain amount of mantle‐derived materials. Subsequent fractional crystallization processes were also suggested during subsequent magma evolution. We propose that the ca. 130 Ma Lingshan granites together with other coetaneous A‐type granitic rocks from adjacent region indicate a back‐arc extensional setting in SE China during Early Cretaceous. The extension event might be caused by the rollback of paleo‐Pacific Plate which results in upwelling of the asthenosphere and subsequently induces a high degree partial melting of Mesoproterozoic basement sedimentary material, to generate the A‐type granitic plutons along the Gan‐Hang Belt.
Hong Kong is located at the southern margin of a Mesozoic igneous belt in Southeast China, where voluminous magmatism formed during the Early Jurassic to Early Cretaceous. The final phase of volcanism formed the rhyolitic High Island Formation, which shows geochemistry similar to A‐type granite. For example, these rocks have high content of SiO2 and alkali (e.g., high K2O + Na2O, with K2O/Na2O ratios greater than 1.0) and are characterized by weakly enrichment in high‐field‐strength elements (HFSEs) and rare‐earth elements (REEs) (except for Eu) and extreme depletion of Ba, Sr, P, Ti, and Eu, with high 10,000× Ga/Al ratios. Zircon U–Pb dating for three porphyroclastic rhyolite samples from the High Island Formation yielded weighted mean 206Pb/238U ages of 140.0 ± 0.8, 139.8 ± 0.7, and 139.1 ± 1.1 Ma. These zircons have εHf(t) values of −9.0 to +0.4, with two‐stage Hf model ages (TDM2) of 1,763–1,172 Ma, indicating a magma source that involved melting of predominantly Palaeoproterozoic to Mesoproterozoic continental crust with a minor juvenile mantle component. The ~140 Ma A‐type magmas in Hong Kong area were probably formed in response to rollback and/or break‐off of the Palaeo‐Pacific Plate. Previous studies indicate that the tectonic transformation from the Palaeo‐Tethys to Palaeo‐Pacific tectonic domain occurred at >165 Ma. Hong Kong underwent a repeated slab subduction and rollback process involving (a) Palaeo‐Pacific Plate subduction formed I‐type granites and continental margin arc volcanic rocks at 165–161 Ma; (b) slab rollback lead to the formation of A‐type granites at 161–159 Ma with minor magmatism formation during the rollback stage at ~159–148 Ma; (c) renewed slab subduction formed continental margin arc rocks at 148–141 Ma; and (d) renewed slab rollback resulted in the formation of A‐type volcanic rocks at 141–139 Ma (e.g., High Island Formation).
The origins of early Paleozoic orogen in South China have two different models: subduction model and intra‐continental model. Here we report two new identification of ~440 Ma arc‐related ultramafic intrusions in Tingzifan (TZF) and Fomuting (FMT) along Jiangshan‐Shaoxing fault (JSF) in South China, respectively. The Silurian ultramafic intrusions are composed of olivine pyroxenite, the SiO2, MgO and TiO2 contents of olivine pyroxenites are 39.67–41.25 wt%, 28.98–31.38 wt% and 0.23–0.51 wt%, respectively. The geochemical compositions of the olivines, clinopyroxenes and hornblendes suggest an arc‐related environment for these intrusions. As for the whole‐rock trace elements, the ultramafic intrusions contain low total rare earth element (REE) contents (27.59–34.26 μg/g) and high field strength elements (HFSEs), such as Nb, Zr, Hf, Ti, and are systematically enriched in large ion lithophile elements and light rare earth elements (LREEs). Trace element compositions share most features of Alaskan‐type ultramafic‐mafic intrusions. Isotopically, the TZF and FMT ultramafic intrusions are characterized by negative Zircons εHf(t) values (0.38–7.54). Combined with their whole‐rock and mineral chemistry as well as zircon Hf isotope, we suggest that the Alaskan‐type TZF and FMT pyroxenite were formed at the root of the continental arc by underplating and fractional crystallization of mafic magma which derived from subduction metasomatized mantle source. Thus, we proposed that the early Paleozoic ultramafic–mafic along Jiangshan‐Shaoxing fault were most likely related to early Paleozoic subduction of the Paleo‐South China Ocean between Cathaysia and Yangtze blocks, arguing that the origins of early Paleozoic orogen in the South China Block is a typical subduction‐accretionary collisional‐type orogenic belt rather than an intraplate belt.
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