Oxygen and hydrogen isotope compositions as indicators of granite genesis in the New England Batholith, Australia

J. R. O'Neil*, S. E. Shaw, R. H. Flood

*Corresponding author for this work

Research output: Contribution to journalArticle

136 Citations (Scopus)

Abstract

Oxygen and hydrogen isotope studies of a number of granite suites and mineral separates from the New England Batholith indicate that δO18 can be used to discriminate the major granite protoliths. The granite suites previously subdivided on the basis of mineralogical and geochemical criteria into S-type (sedimentary) and I-type (igneous) have δO18 values consistently higher in the S-type granites (10.4-12.5) than in the spatially related I-type plutons (7.7-9.9). There appears to be a systematic variation in δO18 from the most S-type to the most I-type granites, the dividing point between the two occuring at δO18 equal to 10. A group of leucocratic granites that form about half of the batholith and difficult to classify mineralogically and geochemically is found to have low δO18 values (6.4-8.1), suggesting an affinity to the most I-type granites. A single leucogranite pluton with minor muscovite has a δO18 of 9.6 which is significantly higher than other leucogranites indicating a different origin perhaps involving amphibole fractionation. The behavior of δD in the plutonic rocks is much less systematic than δO18. Excluding samples collected adjacent to major faults, the δD values show a rough positive correlation with water content similar to, but less pronounced than, the trend previously observed in the Berridale Batholith, southeastern Australia. This relation is considered to reflect an interaction between meteoric water and the granites, the largest effect being observed in samples with the least amount of water. Of note is the generally lower δD values of the upper Paleozoic New England Batholith compared with the Silurian Berridale Batholith. This difference may be related to a near equatorial paleolatitude of 22 °S in the Silurian and near polar paleolatitudes in the late Carboniferous that have been inferred for these regions. Granite samples collected from near major faults, and one ignimbrite sample of rhyodacite composition, have very low δD values (less than -120) suggesting a much greater degree of interaction with meteoric water.

Original languageEnglish
Pages (from-to)313-328
Number of pages16
JournalContributions to Mineralogy and Petrology
Volume62
Issue number3
DOIs
Publication statusPublished - Jan 1977

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