Chatham Schist

ABSTRACT New structural, petrographic, whole rock chemical and mineral chemical data from the Chatham Schist, the easternmost part of the Haast Schist, are presented. In combination with existing geochronological data, these bring knowledge of the basic geological framework of the Chatham Schist to the same standard as the rest of the Haast Schist. Chatham Schist is dominated by greyschist whose protoliths are correlated with the Permian‐Triassic Rakaia Terrane of the Torlesse Composite Terrane. Graded bedding can be recognised in a few places in the northern Chatham Schist, but no fossils have been found. Metamorphic grade on Chatham Island is pumpellyite‐actinolite facies and textural grade varies from TZ2A‐3. Transitional greenschist‐amphibolite facies, garnet‐biotite zone Chatham Schist has been sampled in a xenolith from a Pliocene volcano and in a Southeast Chatham Terrace dredge. The age of Chatham Schist prograde metamorphism is Middle to Late Jurassic. At least two phases of penetrative deformation, and a third, brittle faulting phase, are recognised. Chatham Schist was rapidly exhumed in the late Early Cretaceous, possibly as a result of collision of the Hikurangi Plateau against the Gondwana supercontinent. Detrital garnets and spinels now found on Chatham Islands beaches were not sourced from Chatham Schist.