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Fluid evolution during metamorphism of the Otago Schist, New Zealand: (I) Evidence from fluid inclusions
Author(s) -
Martin Smith,
Yardley
Publication year - 1999
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.1999.00189.x
Subject(s) - terrane , fluid inclusions , geology , metamorphism , schist , greenschist , geochemistry , metamorphic facies , inclusion (mineral) , mineralogy , metamorphic rock , quartz , facies , geomorphology , tectonics , paleontology , structural basin
Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350 °C, with modal values for individual samples ranging from 163 to 229 °C, but coexisting, low‐salinity inclusions exhibiting metastable ice melting show a narrower range of T h from 86 to 170 °C with modes from 116 to 141 °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400 °C and 4.1–6.0 kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea‐water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen‐bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea‐water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades.