Nd isotopic characteristics of S- and I-type granites

Abstract

The initial Nd and Sr isotopic composition has been determined in S- and I-type granites from the Paleozoic Berridale and Kosciusko Batholiths of southeast Australia. The Nd and Sr isotopic variations form a strongly covariant array with S-types granites having a relatively restricted range in ε Nd values from −6.1 to −9.8 but a large range in initial 87Sr 86Sr of from 0.7094 to 0.7184. These characteristics are indicative of an ∼1400-m.y. sedimentary or metasedimentary source for S-types. I-types have variable initial Nd ranging from +0.4 to −8.9, and a more limited range in initial 87Sr 86Sr of from 0.70453 to 0.7119. These isotopic characteristics are consistent with a two-component mixing model whereby a depleted mantle-like component (DMC) with ε Nd = +6 and 87Sr 86Sr= 0.703 , is mixed with a crustal component (CC) having ε Nd = −9 and 87Sr 86Sr= 0.720 . Although this two-component mixing model satisfies the isotopic constraints the source rock chemistry of the I-types is not compatible with the large proportion (up to 50%) of sedimentary material implied by the isotopic data. This indicates that more than two components are required to account for both the isotopic and chemical data. Both the chemical and isotopic data are consistent with I-type granites having been formed from source rocks of predominantly mantle derivation and obtained progressively from the mantle over a period of 1000 m.y. prior to granite formation.