Petrography and origin of metasedimentary xenoliths in lavas from Tongariro Volcanic Centre

Abstract

Abundant xenoliths of metasedimentary origin in lavas of Tongariro Volcanic Centre (New Zealand) are of three main types. Quartz-rich xenoliths are conspicuous in most lavas and include rare garnet-bearing schists and more abundant metaquartzites. The latter represent restites of garnet- schist modified by extraction of granitic partial melt. Feldspar-rich xenoliths occur in subequal abundance to quartz-rich xenoliths and include biotite-schists and pyroxene-hornfelses. The latter represent restites after extraction of partial melt from biotite-schist. Uncommon (but widespread) spinel-rich xenoliths also occur. All of the above xenolith types were derived from Torlesse terrane greywacke, regionally metamorphosed to gneiss. They represent the quartzose, feldspathic, and micaceous layers of gneiss, thermally disaggregated following immersion in host lava. The dominance of restite assemblages (i.e., metaquartzites and pyroxene-hornfelses) testifies to the widespread production, extraction, and assimilation of granitic partial melts of xenoliths with host magmas. © Crown 1987.