TRACE ELEMENT COMPOSITION OF SOUTH CHINA SEA OCEANIC CRUST: INSIGHTS INTO SUBDUCTION RECYCLING

Abstract:

The composition of lavas in volcanic arcs reflect contributions from both the downgoing slab and the mantle wedge. The significance of slab-derived source contributors may be constrained through characterizing the elemental compositions of those materials that reach trenches for comparisons to arc volcanic outputs. While the roles of marine sediments and oceanic crust in subduction recycling have been characterized at a global scale for the major ocean basins [1,2,3,4], comprehensive subduction recycling studies at finer scales have largely not been performed. The South China Sea (SCS) is a marginal marine basin within which subduction of local ocean crust and sediment occurs only at the Manila trench, feeding the Luzon volcanic arc. International Ocean Discovery Program Expeditions 349, 367 and 368 to the SCS recovered igneous basement at six locations (U1431, U1433, U1434, U1500, U1502 and U1504). We have analyzed representative unaltered and altered basaltic rock samples from these SCS sites to develop a compositional profile for oceanic crust across the basin.

Rare earth element and mobile element (Rb, Cs, K, Ba) systematics of basement samples from Exp. 367 and 368 (U1500 & U1502), closer to the continental margin, indicate more extensively altered basalt relative to samples recovered during Exp. 349 (Sites U1431, U1433, U1434). Hole U1500 samples show elevated contents of mobile alkalies Rb, Cs, and K, and evidence for Ce anomalies, with normalized REE abundance patterns similar to those in U1434 basalts. Hole U1502 basalts are highly altered, with evidence for the redistribution of less-mobile species, such as Zn and Sr. Schistose mafic rocks from Hole U1504 show distinctively enriched LREE patterns relative to all other Holes, and are prominently enriched in both mobile elements and key immobile species (Th). The compositional variations among recovered SCS mafic rocks suggest the potential for changes in the materials subducted along strike at the Manila trench, and possible related variations in the down-going plate contributions to Luzon arc lavas.

[1] Plank & Langmuir, Nature, 1993; [2] Plank & Langmuir, Chem Geol. 1998; [3] Plank, Treatise on Geochemistry (2014); [4] Kelley et al. GCubed, 2003