Contemporaneous alkaline and subalkaline intraplate magmatism in the Dunedin Volcanic Group, NZ, caused by mantle heterogeneity

Abstract

ABSTRACT Monogenetic volcanism in the Maniototo Basin in the Dunedin Volcanic Group was unusual because the eroded lavas, plugs and dikes comprise spatially and temporally restricted silica-saturated and silica-undersaturated magmas. 40Ar/39Ar dating coupled with existing data and field relationships indicate that volcanism was focused at ∼11 Ma. Major and trace element data show the volcanic rocks to have mafic compositions and little evidence for fractionation and/or crustal contamination, but to have been variably altered. The volcanic rocks comprise an alkaline group and an alkaline to subalkaline transitional group, with one intermediate unit. With the exception of one flow, the transitional group has more radiogenic 87Sr/86Sr and less radiogenic 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb than the alkaline group, as well as smaller Ti and K primitive mantle-normalised anomalies. Overall, the different magma compositions cannot be related to varying degrees of melting of equivalent mantle sources. Trace element ratios indicate that residual Ti and K-bearing mantle phases (amphibole, phlogopite) probably played a major role in the source of alkaline group but a smaller role in the transitional group. Therefore, the varying compositions of the Maniototo volcanic rocks relates to melting of a heterogeneous lithospheric mantle, or/and varying degrees of interaction with mantle hydrous metasomes as the magmas ascended.