ABSTRACT: Cenozoic Magmatism in the South China Basin: Decompression Melting and Implications of an Enriched Mantle Source
Martin F. J. Flower, Kan Tu, Ming Zhang, Guanghong Xie
A widespread eposide of interplate volcanism followed the cessation of seafloor spreading in the South China Basin (SCB), affecting the South China Sea, and fringing areas of southern China and Indochina. Geochemical data for basalts from South China Sea islands and seamounts, Hainan Island, and Taiwan define an enriched (Dupal-like) mantle domain yielding oceanic island basalt (OIB) suites with ^Dgr7/4Pb = 2-13, ^Dgr8/4Pb = 45-73, 87Sr/86Sr > ^sim0.70325, Th/Ta > 2, and Th/Ba > 0.02.
Opening of the SCB resulted from disaggregation of the South China block in response to the Indo-Eurasian collision, a process involving at least one seafloor spreading episode, terminated by collision of microcontinents with the Philippines and Borneo. The lack of precursive flood basalt suggests that active mantle upwelling was not involved and that melting was a passive effect of lithosphere stretching. However, while mantle decompression at ambient stretching factors (^sim1.7-2.5) appears to permit melting on the observed scale, the enriched source may preclude such a simple mantle dynamic.
Three alternatives are considered: (1) passive melting of a "mature" metasomatised boundary layer, (2) active melting of thermally eroded subcontinental lithosphere (deep enrichment) or metasomatised boundary layer (shallow enrichment), and (3) relict diapirs of pre-SCB and/or Java trench subduction slabs (intermediate/deep enrichment).
These models are evaluated in terms of chemical and isotopic mass balances associated with the generation and movement of small melt fractions in depleted, nondepleted, and enriched mantle.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990