Xonotlite and rodingites from Wairere, New Zealand

A LENSOID mass of serpentinite is continuously exposed over some 650 m along the trace of the NNE.-trending Waipa Fault at Wairere (38~ 33" S., 175 ~ 2' E.) south of Te Kuiti (Fleming, 1948; O'Brien, I97O). Extensive quarrying of the deposit has revealed the presence of numerous highly-altered xenoliths of gabbro ranging in size from hand specimens to about 12o m a and exhibiting strongly grooved and slickensided surfaces. The original composition of the majority of specimens examined was close to that of a eucrite but hydrothermal alteration has not only destroyed the original texture of the rock but also resulted in saussuritization and albitization of the feldspars. About the margins of several xenoliths, rodingites have formed and veins and layers of xonotlite occur both within and around many of the gabbros (fig. I). Dr. P. Marshall, in Bell, Clarke, and Marshall (I 911), proposed the name 'rodingite' for certain 'grossularite-diallage' phases associated with the Dun Mountain massif. His definition was based upon the rock occurring at the type locality in the upper reaches of the Roding River, Nelson. Lauder (I965, p. 485) redefined rodingite to include ' . . . all leucocratic vein and dike rocks in ultrabasic terrains that contain considerable amounts of grossular, diopside and serpentinite and may contain a greater or less amount of prehnite, vesuvianite and perhaps other minerals'. Considerable mineralogical variation exists in rodingites and Lauder's definition is felt to be too restrictive. If a strictly mineralogical and textural definition is required the criteria given by Marshall are probably sufficient but for the purposes of this discussion the term has been extended to include any metasomatic rock, of basic or ultrabasic parentage, consisting of clinopyroxene, which may be derived from the parent, garnet derived wholly by metasomatic process, and possibly other metasomatic products such as prehnite, serpentine, chlorite, and xonotlite. Specimen numbers quoted in the text are those of the University of Auckland Geology Department rock and thin section collection.