The Chrystalls Beach‐Brighton block, southeast Otago, New Zealand: Petrography, geochemistry, and terrane correlation

Abstract The Chrystalls Beach‐Brighton coastal block in southeast Otago has commonly has been placed in Caples Terrane, but has recently been described as a geochemically anomalous area of uncertain terrane affinity. Data points on discriminant diagrams occupy fields centred between those for type Caples Group and Torlesse Terrane, overlapping both. The psammites average 71.9% SiO 2 , closely comparable to Torlesse Terrane psammites, in contrast to the majority of type Caples Group psammites (av. 64.3%) and Waipapa Terrane psammites (64.4%). QFL plots show the Chrystalls Beach psammites as a petrofacies distinct from those described hitherto for Torlesse Terrane (lithic feldsarenites) and Caples Group and Murihiku Terrane (volcarenites). Phosphatic nodules in melange zones associated with metabasites and cherts in the Chrystalls Beach Complex contain Middle Triassic radiolarians. Middle‐Late Triassic tube fossils Torlessia sp. and Titahia corrugata Webby occur in the Chrystalls Beach Complex as in the Torlesse Terrane, but are unknown in type Caples Group sediments in which the only dated fossils are Permian. Both the trench or trench‐slope Chrystalls Beach‐Brighton psammites and the late Middle to early Late Triassic Kaihikuan sediments of the Murihiku Terrane were derived from regions of largely felsic volcanism with underlying granitoids. The geochemical match is imperfect and the sedimentary facies are different. The Chrystalls Beach‐Brighton block is unlikely to be a tectonically introduced and atypical part of the Torlesse Terrane. It may be: (1) an atypical and geochemically more evolved part of the Caples Terrane, younger than dated rocks preserved in the type area, or (2) a separate terrane fragment with a different history from its neighbours. A suggested correlation with the North Island Waipapa Terrane invites questions as to the true terrane affinity of the rocks concerned and of Caples‐Waipapa relationships in general.