Detrital zircon ages and heavy mineral composition along the Gulf of Tonkin - Implication for sand provenance in the Yinggehai-Song Hong and Qiongdongnan basins

Neogene reservoir sand in the Yinggehai-Song Hong (YGH-SHB) and Qiongdongnan basins (QDNB) was derived from the Red River, Hainan Island and the Vietnamese Truong Son Belt. We investigate the sand dispersal pathways through comparisons of detrital zircon geochronology and heavy mineral analyses of modern Vietnamese river sand, Holocene to Recent near-coastal sand from Hainan Island and Vietnam and offshore Neogene sand, and we evaluate the methodologies. Eleven distinct zircon age clusters characterize the Gulf of Tonkin provenances. Red River sand contains the most complex age pattern with similar to 35Ma, similar to 88Ma,similar to 155Ma, similar to 250Ma, similar to 440Ma peaks and broader pre-Cambrian clusters. The similar to 88Ma peak uniquely occurs in Red River sand. Northern Truong Son Belt sand contains similar to 35Ma, similar to 250Ma, similar to 440Ma peaks and pre-Cambrian clusters comparable to the Red River. The lack of Cretaceous zircons and a high proportion of similar to 250Ma relative to similar to 440Ma zircons distinguishes the northern Truong Son Belt from the Red River. The southern Truong Son Belt is characterized by low proportions of similar to 250Ma relative to similar to 440Ma zircons and the absence of Cretaceous and Cenozoic zircons. Hainan Island sand is identified by the presence of similar to 100 Ma zircons and the absence of Cenozoic and similar to 88 Ma zircons. The zircon age patterns suggest simple, although dynamic, sand dispersal pathways with mixed sand sources over much of the basins likely resulting from sediment reworking and downstream river merging induced by Neogene sea-level fluctuations. Sand in the western YGH-SHB was mainly sourced from the Truong Son Belt, the Red River sourced the northern and central YGH-SHB and QDNB, and Hainan sand was primarily deposited along the island margin. Heavy mineral compositions in the Gulf of Tonkin primarily reflect sediment compositional maturity. Epidote, amphibole, pyroxene and garnet indicate immaturity, whereas a high zircon content tends to indicate high maturity. The use of heavy minerals as provenance indicator should proceed with the utmost caution.