Spatio-temporal framework of the Jurassic mafic rocks in South China: Implications for two stages of intra-continental extension

Abstract

During Late Mesozoic, the Paleo-Tethyan and Paleo-Pacific dynamic domains had profound influences on the Southeastern (SE) South China Block (SCB). The SE SCB was in an active continental margin associated with the Paleo-Pacific domain during the Cretaceous, and the Cretaceous mafic rocks show a spatial geochemical migration from the interior (OIB and OIB-like) to coastal province (arc). However, the spatio-temporal framework of the Jurassic mafic rocks in the SE SCB, which is critical for unravelling the tectonic transition between these two domains and the linkage of the Jurassic magmatism with these two domains, remain unclear. Thus, this study presents petrological, zircon U-Pb geochronological and Hf-O isotopic, and whole-rock elemental and Sr-Nd-Pb isotopic analyses for the gabbros collected from the SE SCB. Our new results, combined with previously published data, indicate that the Jurassic mafic rocks (ca. 197–146 Ma) can be divided into three groups (Groups A, B and C) according to their geochemical characteristics. Group A (ca. 197–175 Ma and ca. 168 Ma), reflecting asthenospheric melts, has the highest (Nb/U)n and (Ta/La)n ratios with OIB geochemical signatures. While Group C (ca. 178–171 Ma and ca. 161–146 Ma) has the lowest (Nb/U)n and (Ta/La)n ratios with ‘arc-like’ affinities. Group B (ca. 197–174 Ma and ca. 166–158 Ma), with OIB-like characteristics, falls into a trajectory between Groups A and C. Groups B and C record asthenosphere-lithosphere interaction. Groups A and C occur in the interior of the SE SCB, whereas Group B appears throughout the SE SCB without any spatio-temporal geochemical migrations. In addition, the Jurassic mafic rocks have higher Nb/La, εNd(t), (206Pb/204Pb)i and εHf(t) values than the Cretaceous arc mafic rocks. The Early-Middle Jurassic mafic rocks (ca. 197–171 Ma) are characterized by a WNW-trending belt, whereas the Late Jurassic mafic rocks (ca. 168–146 Ma) mainly occur as a NE-trending belt. The geochemical and spatial signatures indicate that the Jurassic mafic rocks (especially for Group C) are not true arc rocks and formed in an intra-continental extensional setting, but instead stress release from the NE-trending compression (ca. 197–171 Ma) and from far-field effects of the Paleo-Pacific subduction system (ca. 168–146 Ma), respectively. The Paleo-Pacific system did not have a dominating influence on the SE SCB until after the Middle-Late Jurassic.