Geochemistry of the Willsher Group, southeast Otago, New Zealand, and comparison with the Murihiku and Dun Mountain-Maitai terranes

Triassic Willsher Group sandstones and siltstones in the Kaka Point Structural Belt of southeast Otago have geochemical provenance signatures and petrographic characteristics indicative of derivation from a moderately evolved, partially dissected, medium-K volcanic arc, probably of continental island arc nature. Pre-Etalian and Etalian sandstones are andesitic to dacitic in composition, whereas those in the Kaihikuan Stage are more felsic. These features are demonstrated by increased silica contents, immobile element ratios, and rare earth element parameters including LaN/YbN and LaN/SmN ratios. Significant negative Eu anomalies appear at the top of the Willsher succession. Comparison with the chemistry of coeval and older sandstones from the adjoining Dun Mountain-Maitai Terrane shows the Willsher Group sediments are substantially more felsic, and close linkage between the two terranes can be ruled out. Comparison with coeval intervals in the adjacent Murihiku Terrane, including new rare earth element data presented and discussed here, shows that pre-Etalian Willsher sandstones are more felsic, but that Etalian and Kaihikuan equivalents are similar in composition. In both the Willsher Group and the Murihiku Terrane, sandstones become more felsic stratigraphically upward, although the Willsher Group shows more variability within age groups, probably due to local variations in source proportions. These features and others previously described suggest the Kaka Point Structural Belt should be regarded as a separate subterrane of a greater Murihiku Terrane. (author). 38 refs., 10 figs., 4 tabs