Metasomatism in the Wairere Serpentinite, King Country, New Zealand

Abstract

SummaryMetasomatic zones formed at the borders of various mafic inclusions in the Wairere Serpentinite, central North Island, New Zealand, show eucritic and hornblende gabbros grading through distinct zones of altered gabbro (hydrogarnet-free) to coarse and finegrained rodingites (hydrogarnet-bearing). The hydrated, calcium-rich, aluminium-silicate mineral assemblages of each reaction zone change between the zones, with one mineral phase being altered to another at each replacement front. Initial alteration of the gabbros albitized, prehnitized, and epidotized plagioclase and chloritized hornblende and clinopyroxene. Continuing metasomatism garnetized prehnite and generated secondary amphibole and pyroxene while the original host harzburgite was highly serpentinized and, adjacent to the inclusions, the serpentinite was chloritized so as to form a further distinct zone between peridotite and gabbro. These mineral replacement reactions are reflected in the bulk chemistry of the reaction zones, which show that increasing alteration of the gabbro is accompanied by a marked increase in Ca and H2O, a possible small gain in Mg, and depletion in Si, Al, and alkalis. These trends are balanced in the host peridotite by Si enrichment and Ca loss upon serpentinization. Clinopyroxenes and hornblendes of the zones show an increase in Ca and Mg and a decrease in Si with increasing metasomatic alteration. Garnets also decrease in Si but chlorites show an increase in both Si and Mg. Additional minerals analysed from both primary and altered rocks include olivine, orthopyroxene, biotite, plagioclase, pectolite, rosenhahnite, sphene, and chrome spinel. The zones are regarded as the result of reaction between the inclusions and Ca- and Al-rich solutions derived, in turn, by action of alkaline, CO2-poor solutions, which percolated through fractures in the original host peridotite and, in the process of serpentinization, released Mg and enhanced Ca and Al levels. At the contact of the mafic inclusions, chemical gradients were generated down which various chemical species diffused to different levels forming the zonation. The reactions probably took place in the vicinity of 260 to 350 °C, 2 to 3 kb, and at constant volume. Cataclasis and brecciation of the outer zones indicate more than one metasomatic episode.