Early Jurassic high εNd(t)-εHf(t) granites in the Southeastern South China Block: Early Jurassic crustal growth or crustal reworking?

Abstract

Granites that have high εNd(t) and zircon εHf(t) (here phrased as ‘HNHGs’) can offer significant insights into the processes of crustal growth and reworking. This study investigates Early Jurassic HNHGs in the Southeastern (SE) South China Block (SCB) to determine whether they reflect crustal growth or reworking processes, which has implication for understanding the tectonic setting of the SE SCB. We presented new zircon geochronological and Hf-O isotopic, and geochemical analyses on these HNHGs to assess the fundamental crustal processes. Zircon U-Pb geochronological analyses yield the crystallization ages of 192.9 ± 0.8 Ma, 186.7 ± 2.5 Ma and 188.6 ± 1.3 Ma for the Wengong, Gaokeng and Chabei granites, respectively. These HNHGs display high SiO2 (69.75–77.50 wt%), low MgO (0.07–0.83 wt%) and Rb/Sr (0.74–14.3), belonging to unfractionated granites. They have low (87Sr/86Sr)i and zircon δ18O, high εNd(t) and zircon εHf(t), originating from partial melting of a juvenile crust. The HNHGs show a spatial distribution that overlaps with Early Paleozoic lithospheric mantle-derived mafic igneous rocks in the SE SCB. Isotope modeling results imply that the juvenile crust was likely a consequence of mixing among depleted mantle, subordinate enriched lithospheric mantle and SCB basement rocks during Early Paleozoic. These HNHGs record crustal reworking, rather than crustal growth during Early Jurassic, and formed in a continental extension setting.