Mesoproterozoic rift setting of SW Hainan: Evidence from the gneissic granites and metasedimentary rocks

Abstract

The Mesoproterozoic Baoban Complex, southwest Hainan Island, has played a crucial role in deciphering Nuna (Columbia) supercontinent reconstructions. Metasedimentary rocks in the Baoban Complex are intruded by Mesoproterozoic gneissic granites and mafic rocks. Our new geochemical and geochronological data show that these gneissic granites are monzogranites and granodiorites and crystallized at ∼1430 Ma. Geochemically, they are subdivided into two groups with I- and A-type affinities. The groups show broadly similar εNd (t) values ranging from −2.4 to +0.9, zircon εHf (t) from −5.4 to +7.8 and δ18O values from +5.0 to +8.6‰, suggesting derivation from the same source involving juvenile mafic melt and ‘‘ancient” continental crust components. Metasedimentary rocks have high La/Sc (1.4–5.8) but low Sc/Th (0.6–1.6) and Co/Th (0.3–1.1) ratios, as well as negative εNd (t) of −4.5 to −4.1, indicative of an “ancient” felsic igneous source with a recycled sedimentary component. Detrital zircon grains from the metasedimentary rocks yield three primary age-peaks of ∼1780 Ma, ∼1610 Ma and ∼1440 Ma, with the depositional age synchronous with those of the gneissic granites and metabasites (∼1430 Ma). Our geochemical and geochronological data, along with regional geological constraints, suggest a rift setting. The age populations and hafnium compositions of detrital zircons from the metasedimentary rocks in the Baoban Complex, southwest Hainan are comparable to those igneous zircons from East Antarctica, pointing out a probable affinity between southwest Hainan and East Antarctica in the Nuna supercontinent.