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The Neoproterozoic magmatism in the northern margin of the Yangtze Block: Insights from Neoproterozoic (950–706 Ma) gabbroic-granitoid rocks of the Hannan Complex

2019; Elsevier BV; Volume: 333; Linguagem: Inglês

10.1016/j.precamres.2019.105442

1872-7433

Wenhao Ao, Yan Zhao, Yukun Zhang, Mingguo Zhai, Hong Zhang, Ruiying Zhang, Qian Wang, Yong Sun,

earthquake and tectonic studies

The Hannan Complex, located in the northwestern margin of the Yangtze Block, South China, consists of numerous mafic-ultramafic and felsic plutons formed at ca. 950–700 Ma. However, their geotectonic setting and petrogenesis keep in controversy. This study presents whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf composition and whole-rock Hf isotope of gabbroic-granitoid rocks of the Hannan Complex, aiming to clarify their spatial-temporal distribution and discuss their geological significance. The Dahanshan gabbro in the Hannan region yields zircon U-Pb age of 793–779 Ma. The positive zircon εHf(t) values (+8.0–+10.4) and positive whole-rock εHf(t) values (+7.0–+12.6) suggest a relatively depleted mantle source of the gabbro. However, Hf model ages and whole-rock geochemical data suggest addition of enriched components to magma source. Thus, the magmas derived from decompression melting of MORB-like mantle and flux-induced melting of metasomatized lithospheric mantle wedge both contributed to generation of Dahanshan gabbro in a back-arc setting. The Hongmiao granite pluton, having crystallization age of 802–791 Ma, is characterized by aluminous A-type granite signatures and showing A2-type granite affinities. Taking negative εHf(t) values (–24.5 to −3.3) of zircons into account, the Hongmiao granite was generated by partial melting of pre-existing crustal materials in an extensional enviroment. The Xishenba trondhjemitic biotite granite formed at ca. 890 Ma. It belongs to metaluminous to peraluminous calc-alkaline I-type granitoid rock series with negative zircon εHf (t) values (–19.4 to −6.2), implying a magma source of pre-existing crustal materials in an arc setting. Thus, combined with previously published data, it can be concluded that the gabbroic-granitoid rocks of the Hannan Complex were generated in subduction-related arc-back-arc setting during Neoproterozoic (950–706 Ma). The Neoproterozoic (ca. 950–706 Ma) magmatism represents a significant crustal growth event in the northern margin of the Yangtze Block, simultaneously, accompanied by certain degree of crustal reworking. According to temporal-spatial distribution of different rock types, the formation of the Neoproterozoic (ca. 950–706 Ma) Hannan Complex can be subdivided into four stages: ca.950–890 Ma and ca.890–840 Ma in a continental arc setting and a continuous continental arc setting, respectively; whereas, ca.840–765 Ma and ca.765–706 Ma in a back-arc setting and a compression-extension tectonic setting, respectively.