Abstract
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.
Published Version
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