The stratigraphic and structural position and age of the Croisilles Melange, east Nelson, New Zealand

Abstract

Abstract In Croisilles Harbour area, sheared serpentinite and enclosed blocks of ophiolitic and sedimentary rock are interpreted as forming part of a tectonic unit, the Croisilles Melange. The melange separates poorly dated sedimentary rocks of the Rai Sandstone in the west from undifferentiated Pelorus Group in the east. A small number of gastropods previously used to assign an Early Permian age to the Croisilles rocks date the sediments in a single block and not the age of the melange or its emplacement. The gastropods show affinities with Mourlonia (Mourlonia) strzeleckiana (Morris) of Late Permian age. Radiometric dating (U-Pb, K-Ar) shows that the ophiolitic blocks in the melange are of Early Permian age. Thus the melange is derived from an ophiolite of similar age to the nearby Dun Mountain Ophiolite Belt and from an Upper. Permian sedimentary sequence. Lithology and whole rock chemistry of the Mourlonia-bearing block suggest that the source cannot be readily attributed to any of the structural or s...