K‐Ar ages of basanitic dikes, Awatere Valley, Marlborough, New Zealand

Twenty‐one basanites from a regional dike swarm in the Awatere River, Winterton River and Gladstone Stream, Awatere Valley area, Marlborough, have been analysed (major and trace elements) and dated by the K‐Ar method. K‐Ar ages of the dikes range between 100 and 60 Ma. The younger age extends the previously determined age range of igneous activity in the Awatere Valley (fossil age of volcanic activity; K‐Ar and fission‐track ages of volcanic and plutonic rocks) of 103–90 Ma for another 30 Ma into early Tertiary time. Considering only phenocryst‐free or phenocryst‐poor dikes, SiO2, total alkalis, Ba, Ce, Y, Zr, and Differentiation Index (D.I.) increase with decreasing age, and MgO, CaO, Cr, Cu, Ni, Sc, and Mg number decrease. This variation reflects a time‐dependent evolution of magma composition controlled by early‐stage olivine/pyroxene fractionation, gradually increasing sodic enrichment of plagioclase, and later crystallisation of K‐feldspar.