Geochronology, geochemistry and petrogenesis of the Dengfuxian lamprophyres: Implications for the early Cretaceous tectonic evolution of the South China Block

Abstract

The middle–late Mesozoic tectonic evolution of the South China Block is not well constrained due to limited studies of synchronous mafic rocks in this area. The Dengfuxian area is located at the central to northern parts of the Nanling Range in the South China Block with many lamprophyre dikes seen cross-cutting the late Jurassic granites. This contribution is the first to report the whole-rock geochemistry, zircon U–Pb age and Sr–Nd–Hf isotope signature of these lamprophyres; provide a geochronological and petrogenetic studies that sheds new light on regional geodynamic evolution. The Dengfuxian lamprophyre dikes have variable contents of major oxides like SiO2 (58.45–65.19 %), Al2O3 (12.18–15.73 %), Fe2O3T (6.31–6.93 %), MgO (2.48–4.14 %), CaO (4.21–5.19 %), Na2O (1.08–3.02 %), K2O (2.78–4.00 %), TiO2 (0.59–1.00 %), MnO (0.15–0.33 %) and P2O5 (0.28–1.22 %). Identified as high-K calc-alkaline rock series, the lamprophyres are enriched in large ion lithophile elements (e.g., Rb, U, K and Pb) but depleted in Ba, Sr and high field strength elements (e.g., Nb, Ta, Zr and Hf). The HREEs are relative depleted compared to LREEs, with weak Ce anomalies (Ce/Ce* = 0.99–1.06) but marked negative Eu anomalies (Eu/Eu* = 0.61–0.79). A total of 30 magmatic zircon grains from the studied lamprophyre yields weighted mean U–Pb ages of 141–143 Ma, suggestive of an early Cretaceous emplacement. The Negative εNd(t) (–4.3 to –6.0) and εHf(t) (–4.1 to –10.9) values, old Nd depleted mantle model ages (TDM = 1231–1628 Ma) and Hf single stage model ages (TDM = 998–1243 Ma), with high initial 87Sr/86Sr (0.71057–0.71322) indicate that the parent magma was sourced from a subduction-modified enriched mantle. Together with integrated major and trace elements studies, it can be inferred that the parent magma of the Dengfuxian lamprophyres originated from low degree (1–3 %) partial melting of an enriched mantle source (garnet peridotite) that was altered by subduction sediments in a progressive intra-plate extension environment. The magma experienced significant fractional crystallization of olivine, clinopyroxene, plagioclase, Fe-Ti oxides and apatite with little crustal contamination. A two-stage geodynamic model is presented to constrain the tectonic evolution of the South China Block during the early Cretaceous.