Petrogenesis of Mid-Neoproterozoic low δ 18 O and high Ba–Sr granitoids in the western margin of the South China Block

Mid-Neoproterozoic granitoids from the western margin of the Yangtze Block provide crucial insights into crust–mantle interactions and the crustal growth processes of the South China Block during Rodinia supercontinent cycles. Zircon U–Pb dating shows that the Datian diorite and granite were emplaced contemporaneously at c. 770–760 Ma. The Datian diorites show variable SiO 2 contents (56.60–67.03%), high magnesium numbers (49.32–55.18), medium Na 2 O + K 2 O values (5.02–6.46), A/CNK (1.02–1.16) shows peraluminous and high Cr and Ni concentrations. By contrast, the granites show high SiO 2 (70.91–80.73%) and low MgO (0.11–0.67%) contents, with a range of Na 2 O + K 2 O contents of 4.72–8.52%, indicating a subalkaline series, whereas the A/CNK values (0.86–1.22) indicate metaluminous to peraluminous rocks. Both the diorite and granite are enriched in large ion lithophile elements, but are depleted in high field strength elements, giving them typical arc affinities. The diorites display moderate positive ε Hf ( t ) values (+2.3 to +6.6) and mantle-like δ 18 O values (4.95–6.30‰). The granites have similar ε Hf ( t ) values (+1.83 to +7.15), but low δ 18 O values (0.59–5.90‰). More importantly, the diorites have high Ba–Sr (Ba + Sr = 898–1720 ppm) signatures and are characterized by amphibole-rich lithologies. These geochemical and isotopic signatures suggest that the parental magmas of the diorites were most likely generated by partial melting of a mantle wedge metasomatized by a subducted oceanic slab. The high Ba–Sr melts subsequently differentiated to generate the Datian diorite in a continental arc setting. The low δ 18 O granites were formed through partial melting of a juvenile lower gabbroic crust that had been affected by high-temperature hydrothermal alteration. The high Ba–Sr granitoids share a geochemical affinity with Archean sanukitoids, forming via amphibole-dominated fractional crystallization of a fluid-metasomatized mantle. The development of mid-Neoproterozoic continental arcs along the margin of the Yangtze Block suggests that South China occupied a peripheral location in Rodinia.