I- and S-type granites in the Lachlan Fold Belt

Abstract

Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe3+/Fe2+, and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks.S-type granites have higher δ18O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks.Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

Genesis and emplacement of felsic Variscan plutons within a deep crustal lineation, the Penacova-Régua-Verín fault: An integrated geophysics and geochemical study (NW Iberian Peninsula)

“…The major and trace element trends in the studied granites are similar to those observed by Chappell and White (1992) in fractionated I-type granites from SE Australia that is, an increase in Th, Y and HREE contents. The decrease in P 2 O 5 shown in studied granites is also an important criterion for distinguishing I-type granites because apatite reaches saturation in metaluminous and weakly peraluminous magmas (Chappell and White, 1992).…”

“…The low normative corundum values (b0.7% in VPAG + AFG and close to 1% in PSG) are consistent with their being I-type (Chappell and White 1992;Villaseca et al, 1998a) as seen in the A-B diagram (Fig. 5).…”

“…The decrease in P 2 O 5 shown in studied granites is also an important criterion for distinguishing I-type granites because apatite reaches saturation in metaluminous and weakly peraluminous magmas (Chappell and White, 1992). The same geochemical trends were observed for I-type granites in other regions of the Iberian Peninsula, namely in the Spanish Central System (Villaseca et al, 1998b).…”

“…xenotime), thus those elements increase in the residual melt (Chappell, 1999). The behaviour of phosphorous can be explained in terms of the strong control of the melt peraluminousity on apatite solubility (Chappell and White, 1992;Pichavant et al, 1992). The depletion in P during differentiation of these biotite granites is due to two causes: i) the low P content in parental I-type granitic melts due to the low apatite solubility and ii) the progressive apatite (and monazite, in lesser amounts) crystallization.…”

Syn-orogenic high-temperature crustal melting: Geochronological and Nd–Sr–Pb isotope constraints from basement-derived granites (Central Damara Orogen, Namibia)

“…Slightly peraluminous granites like the grey granites can either form from metasedimentary or meta-igneous sources (Chappell et al, 1998;Chappell and White, 1992 Miller, 1985). Similarly, Sr (~150 ppm) and Ba (> 500 ppm) abundances are also not characteristic for a pelitic parent (Sr:…”