Abstract
A sodalite tinguaite dredged from the bed of the Clutha River, at the Clyde Dam site in Central otago, shares mineralogical, geochemical, and hand specimen characteristics with a sodalite tinguaite collected from near the Twirligig, Burke River, south Westland. Tinguaites have a restricted geographic distribution, occurring as intrusions into the host Haast Schists in the Haast-Burke River area of the oligocene-Miocene Alpine Dike Swarm. A second boulder from Clyde, with the mineralogy albite-magnesioriebeckite-aegirine is interpreted as a fenite, a rock type that also crops out extensively in the Haast-Burke River area. Fenites occur at the margins of evolved dike rocks and are produced by metasomatic alteration of the country rock.
The occurrences of the tinguaite and fenite at Clyde require that the position of the Main Divide originally lay west of the Burke River, enabling rocks exposed in that catchment to have been transported into the Clutha drainage system. The 25 km eastward migration of the Main Divide required to explain the tinguaite and fenite distribution was achieved through the Haast River capturing successively the Clarke, landsborough, Burke, and Wills Rivers in response to erosion associated with deformation on the Pacific-Australian plate boundary.