Geochemical studies on the Hohonu Batholith, of the West Coast, South Island, New Zealand, have recognised two distinct but chemically related suites of mid-Cretaceous granitoids. The suites are characterised by restricted radiogenic isotopic compositions (Sr(i) = 0.7062 to 0.7085; εNd(i) = -4.4 to -6.1), and represent melting of a mafic lithosphere source followed by interaction with Ordovician metasediments. The two suites (Te Kinga Suite and Deutgam Suite) are distinguished by contrasting contents of Al₂O₃, Na₂O, Sr, Ba, Eu and HREE, attributable to different residual assemblages controlled by differing H₂O contents during melting of a metabasaltic source. The relatively mafic, metaluminous, I-type Deutgam Suite represents magmas derived by dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. In contrast, the peraluminous, high silica compositions of the Te Kinga Suite were produced by melting at higher H₂O contents, reducing the stability of plagioclase and resulting in a melt in equilibrium with a plagioclase-free eclogitic (garnet + amphibole) residue. Residual plagioclase during generation of the Deutgam Suite resulted in lower Al₂O₃, Na₂O, Sr, Ba and Eu contents, whereas residual garnet during generation of the Te Kinga suite resulted in depleted HREE contents. The mid-Cretaceous granitoids of the Hohonu Batholith were generated during a period of rapid tectonic transition from crustal thickening during collision to crustal thinning and core complex formation during extension.