Geochemical characteristics of basaltic volcanism within back-arc basins

Abstract

Summary: Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB and basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.