Boninitic and low-Ti subduction-related lavas from intraoceanic arc-backarc systems and low-Ti ophiolites: a reappraisal of their petrogenesis and original tectonic setting

Abstract

A comprehensive review of boninitic and other low-Ti subduction-related lavas from modern intraoceanic arc-backarc systems, and of low-Ti ophiolites shows a complete compositional range from boninite to island arc basalts. Transitional low-Ti types distinguishable from the main magma types are well represented and defined here as (1) transitional boninites (mainly from low-Ti ophiolites) and (2) low-Ti island arc basalts. Major-compatible and incompatible elements are clearly decoupled in all these low-Ti subduction-related magmas. In general, their petrogenesis reflects a high degree of hydrous but not necessarily water-saturated melting (at pressures lower than 15–20 kbar) of severely depleted mantle sources enriched in LILE and H2O which are residual after the earlier extraction of basalt (s). As the residues after the extraction of low-Ti subduction-related magmas very likely have a dunitic-clinopyroxenefree harzburgitic composition, it is proposed that the CaO/Al2O3 ratios of the primary melts reflect those of their mantle sources, particularly for the boninites and transitional boninites. In this model, the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene-bearing to clinopyroxene-free harzburgite, whereas those of transitional boninites-low-Ti island arc basalts would have clinopyroxene-bearing harzburgite-clinopyroxene-poor lherzolite compositions. The metasomatizing components involved in the genesis of low-Ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites. The common eruptive setting indicates that these low-Ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime. The petrogenesis of Tertiary boninites and transitional boninites and the comparable magma types of the low-Ti ophiolites require at least these components: (1) A variably depleted mantle source, probably a subarc mantle residual after island arc basalt extraction; (2) a hydrous fluid, probably derived from dehydration of the subducted, altered oceanic crust, which carries most of Sr, Ba, K and Rb in the subarc source and (3) a LREE ± Zr and Nb-enriched component with a low 143Nd/144Nd ratio, probably derived from an “ocean island-type” source. It is proposed that this third component also carried significant amounts of Rb (and K) which totally obliterated the Ba spike typical of island arc tholeiites (IATs) and produced the distinctive Rb spike typical of the boninite-transitional boninite magmas. This component is hypothetically identified in liquids produced by partial melting of small scale, large magnitude K richerite ± phlogopite-bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of Crawford et al. (1981) for the genesis boninite-transitional boninite magmas. Geochemical, thermal and geological constraints are best explained in a model which restricts the genesis of boninite-transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and/or early interarc basin structures connected with hot rising asthenospheric mantle diapirs.