资源环境科技发展态势分析平台

Bimodal (mafic and felsic) rock suites are common in intracontinental orogens, but there is no consensus about their origin. This paper presents a combined study of zircon U-Pb ages and Hf-O isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes, as well as phenocryst major elements for Meso-zoic bimodal volcanics from the Middle-Lower Yangtze Valley in South China. The results indicate the presence of a common crustal component in the magma sources of both mafic and felsic volcanics, leading to proposition of a new mechanism for the origin of bimodal magmatism. Zircon U-Pb dating yields consistent ages of 134-125 Ma for the bimodal volcanism. Lithochemically, the mafic volcanics are mostly basaltic to trachybasaltic whereas the felsic volcanics are mostly trachytic to rhyolitic. Geochemically, the mafic rocks are characterized by: (1) arc-like trace element distribution patterns in the primitive mantle-normalized spidergram; (2) moderate enrichment in whole-rock Sr-Nd-Hf isotope compositions, with (Sr-87/Sr-86)(i) ratios around 0.7066, epsilon(Nd)(t) values of -5.47 to -5.30 and epsilon(Hf)(t) values of -3.01 to -2.73; and (3) highly variable zircon epsilon(Hf)(t) values of -20.5 to 7.9. These geochemical features indicate that the mafic volcanics would originate from ultramafic metasomatites that were generated by reaction of the mantle wedge peridotite with altered oceanic basalt-derived aqueous solutions and terrigenous sediment-derived hydrous felsic melts. The felsic volcanics show peraluminous lithochemistry and systematically more enrichment in whole-rock Sr-Nd-Hf isotope compositions than the mafic volcanics. Zircons from the felsic volcanics exhibit similar epsilon(Hf)(t) values to those of the mafic rocks and have variably high delta O-18 values of 6.3-9.2 parts per thousand. These felsic volcanics cannot be produced by differentiation of the mafic magmas or partial melting of the local crust. Instead, they would be derived from partial melting of terrigenous sediment at a lower lithosphere depth. There are similar zircon Hf isotope compositions between the mafic and felsic volcanics, suggesting that their magma sources contain a common crustal component, which is most likely terrigenous sediment. All these observations, together with the tectonic setting of the target region, indicate that the magma sources of both mafic and felsic volcanics were formed during subduction of the Cathaysia oceanic slab in the Precambrian and were stored in the fossil orogenic root for hundreds of millions of years. These two sources underwent reactivation in the Mesozoic for simultaneous partial melting to generate the bimodal magmatism. As such, this study reveals a new mechanism for bimodal magmatism in an intra-continental setting, emphasizing the presence of a common crustal component in the sources of bimodal rock suites.