In southern New Zealand, minerals of the axinite group are widespread in vein assemblages in regionally metamorphosed rocks of the prehnite-pumpellyite, pumpellyite-actinolite, and chlorite zone greenschisfta cies.F e, Mg-axinites, approachinge nd-memberf erroaxinite in composition, along with quartz and often prehnite, pumpellyite, iron-rich epidote, and chlorite fll veins in spilitized volcanic and greywacke lithologies. Tinzenite and more cofilmonly manganaxinites occur in quartz veins in nearby femrginous and manganiferous cherts. Chemical analyses of vein axinites are presented, as well as analyses of four porphyroblastic ferroan manganaxinites which occur as rock-forming minerals at widely separated localities.

Compositional variability in published axinite analyses along with those of this study can be attributed partly to formation temperatures. A low-temperature miscibility gap may exist in the axinite group. Tinzsnile or manganaxinite and ferroaxinite are stable in low-grade metamorphic rocks of appropriate compositions, whereas ferroan manganaxinites and manganonn ferroaxinites occur in some pegmatites, skarns, and regionally metamorphosed rocks which equilibrated at more elevated temperatures.

For a wide lange of burk compositionsi n biotite and garnet zone greenschisfta cies and amphibolite facies rocks of southern New Zealand, tourmaline is the only observed borosilicate phase. At metamorphic conditions typical of these grades, axinite minerals would be restricted to relatively Ca-rich lithologies by a reaction of the form:
3 ferroaxinite + 2 chlorite + 2 albite + 5 quartz = 2 tourmaline + 4 epidote + 2 actinolite + 5 water
This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.