Metasomatism in the Wairere Serpentinite, King Country, New Zealand

Abstract

Metasomatic 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 fine-grained 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 meta-somatism garnetized prehnite and generated secondary amphibole and pyroxene while the original host harz-burgite 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 H20, a possible small gain in Mg, and depletion in Si, A1, and alkalis. These trends are balanced in the host peridotite by Si enrichment and Ca loss upon serpentinization. Clinopyroxenes and horn-blendes 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, ortho-pyroxene, biotite, plagioclase, pectolite, rosenhahnite, sphene, and chrome spinel. The zones are regarded as the result of reaction between the inclusions and Ca-and AI-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 AI 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 z6o to 35 ~ ~ 2 to 3 kb, and at constant volume. Cataclasis and brecciation of the outer zones indicate more than one metasomatic episode. THE Wairere Serpentinite has been described by O'Brien and Rodgers 0973a; 1974). Ca meta-Copyright the Mineralogical Society somatism at the contacts between serpentinite and included eucritic gabbro pods was briefly reported on by the same authors (I973b). This account is a far more comprehensive study of the metasomatic rocks and minerals and is based on samples collected from the active quarry in I973-4. Fig. i shows the main serpentinite quarry area as it was in those years (cf. O'Brien and Rodgers, :973a, fig. 2). Localities B, J, K, and L are eucritic-gabbro pods, which were partially quarried out and stood, in situ, in sheared serpentinite. C and G were similar gabbros that had been totally blasted but lay on the quarry floo...

Interplay of irreversible reactions and deformation: a case of hydrofracturing in the rodingite–serpentinite system

“…Therefore, the reaction (R4) is the most appropriate reaction describing rodingite formation. This result supports the theory that the infiltration of Ca-rich fluids leads to the formation of rodingite (e.g., O'Brien and Rodgers 1973; Leach and Rodgers 1978;Wares and Martin 1980;Schandl et al 1989;Python et al 2011) rather than loss of SiO 2 from the protolith and the resultant enrichment of CaO in rodingite (e.g., Coleman, 1967). Serpentinization can produce Ca-rich fluid (e.g., Honnorez and Kirst 1975;Evans et al 1979), and present-day serpentinization accompanies Ca 2+ -OH − -type waters (Barnes and O'Neill 1978), thus validating our conclusion.…”

“…Rodingite can be formed from various protoliths such as gabbroic rocks (Coleman 1967;O'Brien and Rodgers 1973;Honnorez and Kirst 1975;Leach and Rodgers 1978;Muraoka 1985;Kobayashi and Kaneda 2010), basaltic rocks (Coleman 1967;Evans et al 1979;Barriga and Fyfe 1983) 1980). In the study area, gabbro (now metagabbro) is the only possible protolith of rodingite as no other rock types are present as dykes in the serpentinite.…”

Rodingite with Ti- and Cr-rich vesuvianite from the Sartuohai chromium deposit, Xinjiang, China

“…Rodingites have a genetical relationship closely with gabbro, basalt, granite and other rocks by metasomatism adjacent to or in peridotite that underwent serpentinization (e.g., Leach and Rodgers, 1978;Kobayashi and Shoji, 1988). Although it is quite difficult to estimate the parent rock of rodingite, the massive rodingite from the Sartuohai chromium deposit should have been a gabbroic rock according to the texture of rodingite (Figs.…”

The geochemistry, origin, and tectonic significance of rodingites from the Dun Mountain Ultramafics, D'Urville Island, New Zealand

“…However, most authors invoke metasomatism concomitant and complementary to serpentinisation of the ultramafic rocks with which rodingites are invariably associated (e.g., Leach & Rodgers 1978). Grange (1927) interpreted rodingites in the type locality near Roding River in the Nelson district, New Zealand, as altered gabbros consisting of hydrated calcium aluminosilicates and relict diopside.…”