Parallels in the origin of the geochemical signatures of island arc volcanics and continental potassic igneous rocks: The role of residual titanates

The extent of control of Ti, Nb and Ta concentrations by residual titanate minerals in island arc and subcontinental mantle enrichment processes is assessed with reference to recent reviews of the geochemistry of Indonesian Sunda island arc volcanics and of continental ultrapotassic rocks, and a reinterpretation of experimental results on TiO₂ saturation levels in arc volcanic compositions. It is considered likely that the low-Ti-Nb signature of potassic eastern Sunda arc rocks originates in at least two distinct mantle sources, both of which contain residual titanates during partial melting. A low-Nb-Ta component, which is also characteristic of island arcs in which potassic magma types are absent, is probably due to melting of the rutile-bearing hybridized product of reaction between silicic partial melts of subducted oceanic crust and the peridotite mantle wedge immediately above the subduction zone. A low-Ti component containing moderate amounts of Nb is assigned to K-rich low-degree partial melts of peridotite which have solidified at shallower levels, creating a vein system with incompatible-element-enriched composition. The source of potassic arc volcanics is thus a composite of peridotite wedge plus hybridized mantle and K-rich veins. Five plausible models with two residual titanates are presented, in which the second residual titanate must be active either during melting of the peridotite wedge or K-rich veins, or during melting of peridotite at deeper levels producing the K-rich melt which later crystallised as veins. Titanate saturation in basic melt compositions is promoted by a combination of the effects of high pressure, low temperature assisted by high H₂O content, high f_O2 and high contents of incompatible elements. The effects of high pressures and high f_O2 of reducing TiO₂ contents of melts in equilibrium with titanate minerals have probably been underestimated in previous studies. Continental mantle enrichment events may be analogous to the K-rich sub-arc component due to comparably low heat flow enabling H₂O assisted low-degree melting of mantle material in conditions which do not occur in other tectonic environments. Continental ultrapotassic rocks show a gradation of HFSE abundances which overlap with island arc potassic rock characteristics, implying a continuum of varying environments and physicochemical conditions such as H₂O content, f_O2 and source mineralogy. These variables may also determine which, if any, titanate mineral is present in individual sub-continental regions.