Petrochemistry of ignimbrites from the central North Island and Coromandel, New Zealand

Abstract

Twenty one different ignimbritic eruptives have been studied from the Taupo Volcanic Zone and the Kaimai-Coromandel area. These include welded and unwelded pumice breccias, lenticulites and intensely welded ignimbrites. The range of silica content exhibited by the rocks is from 63-76%, most being in the range 70-73%. The more basic rocks are the most crystal-rich. The important mineral species found in the ignimbrites are: Ti-magnetite-ilmenite-p1agioclase-hypersthene-augite± calcic hornblende± biotite± sanidine± pyrrhotite (+ apatite + zircon) Epidote, gedrite, actinolite, haematite, olivine, pure magnetite are rare accessories of restricted occurrence. Eruptives from the Okataina volcanic centre generally contain cummingtonite. The use of a variety of geothermometers shows that the mineral assemblage is temperature dependent and the ignimbrites can be divided into three main groups on the basis of the Fe-Mg assemblage: (i) hypersthene + augite (>820°C) (ii) hypersthene + augite + calcic hornblende (<820°C) (iii) hypersthene + augite + calcic hornblende + biotite (<765°C) A fourth assemblage noted in one sample, which is inferred to represent a mixed temperature assemblage contains: hypersthene + augite + olivine (forsterite) + biotite Biotite-sanidine-bearing ignimbrites typically have quench temperatures below 765°C. Estimates of total pressure from the phenocryst assemblage suggest that ignimbrites are erupted from magma chambers at 10-15 km. Gravity data suggests depths may be as shallow as 5-6 km. Estimates of the fugacity of gas species in the system H-O-S have been made and the range of values obtained are (as –log f gas): fO2 = 10 – 16 fH2 = -1.8 - -0.6 fS2 = 1.8 – 4.3 fSO2 = 0.5 – 3.7 fSO3 = 6.9 – 11.1 fH2S = -1.8 – 0.5 It is possible to demonstrate variation in oxygen fugacity and change in oxygen "buffer" assemblages with falling temperature and increasing silica activity from the mineral assemblages. PH2O estimates show that it ranged from at least half PTOTAL to equal PTOTAL at the time of eruption (2-5 Kbars PH2O). The pumiceous eruptives appear to have lower PH2O than the intensely welded eruptives. Estimates of temperature and pressure indicate ignimbrite melts formed at temperatures in excess of 1000°C at pressures greater than 3-5 Kbars. It is considered that ignimbrite magmas formed from crustal fusion of basement greywacke (or their metamorphic equivalents), at depths of 20-25 km. Initial melts had a low water content and these migrated from the zone of fusion to shallow magma chambers at depths of 6-15 km. The "dry" melts became water-saturated by reaction with "wet" country rock. The heat for crustal fusion is considered to come from basic magas (basalt-andesite) derived by partial melting of quartz eclogite on the Benioff zone underlying the Taupo region, which are intruded into the lower crust at 1300-1400°C. Estimates of the volumes of basic magma required to melt greywacke suggest that a given volume of basic magma could produce 1.5-3 times its equivalent volume of greywacke melt.