Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

• Two groups of andesites in the Ningwu basin show distinct geochemical features.
• Primary melts of these andesites are of K-rich basaltic to andesitic composition.
• The andesites were derived from partial melting of fertile and enriched mantle sources.
• The mantle sources were enriched with melt-mobile components from subducted oceanic sediment.
• The sediment was added to the mantle sources through the Neoproterozoic oceanic subduction.

An integrated study of zircon U–Pb ages, Hf–O isotopes, whole-rock major-trace elements and Sr–Nd–Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle–Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1–3.7 wt.% and low Mg# values of 1.6–51.2, high K2O contents of 2.0–9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr–Nd–Hf–Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050–0.7082, negative εNd(t) values of − 8.1 to − 1.8, and negative εHf(t) values of − 6.7 to 0.4. Zircon U–Pb dating yields consistent ages of 128 ± 2 to 133 ± 2 Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7–7.8‰ and variable εHf(t) values of − 5.8 to 0.7. All andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle–Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical composition of andesites is primarily governed by their mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesites but they are not the governing factors for their trace element and radiogenic isotope compositions.

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