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

The Chrystalls Beach-Brighten coastal block in southeast Otago has commonly has been placed in Caples Terrane, but has recently been described as a ge ochemically anomalous area of uncertain terrane affinity. Data points on di scriminant diagrams occupy fields centred between those for type Caples Gro up and Torlesse Terrane, overlapping both. The psammites average 71.9% SiO2 , closely comparable to Torlesse Terrane psammites, in contrast to the majo rity of type Caples Group psammites (av. 64.3%) and Waipapa Terrane psammit es (64.4%). QFL plots show the Chrystalls Beach psammites as a petrofacies distinct from those described hitherto for Torlesse Terrane (lithic feldsar enites) 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. Middl e-Late Triassic tube fossils Torlessia sp. and Titahia corrugata Webby occu r in the Chrystalls Beach Complex as in the Torlesse Terrane, but are unkno wn in type Caples Group sediments in which the only dated fossils are Permi an. Both the trench or trench-slope Chrystalls Beach-Brighten psammites and the late Middle to early Late Triassic Kaihikuan sediments of the Murihiku Ter rane were derived from regions of largely felsic volcanism with underlying granitoids. The geochemical match is imperfect and the sedimentary facies a re different. The Chrystalls Beach-Brighten block is unlikely to be a tecto nically introduced and atypical part of the Torlesse Terrane. It may be: (1 ) an atypical and geochemically more evolved part of the Caples Terrane, yo unger than dated rocks preserved in the type area, or (2) a separate terran e fragment with a different history from its neighbours. A suggested correl ation with the North Island Waipapa Terrane invites questions as to the tru e terrane affinity of the rocks concerned and of Caples-Waipapa relationshi ps in general.