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

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.