High‐ and Low‐Temperature I‐type Granites

Abstract

: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H2O.

Zircon REE patterns and geochemical characteristics of Paleoproterozoic anatectic granite in the northern Tarim Craton, NW China: Implications for the reconstruction of the Columbia supercontinent

“…Although muscovite is one of the major minerals (∼10 vol.%), the granite shows a weakly peraluminous character with A/CNK ratios of 0.97-1.13. These relatively low A/CNK ratios favor a petrogenetic interpretation of I-type granite, produced by partial melting of an igneous source (Chappell and White, 1992;Chappell et al, 1998). In contrast to zircons from the Neoarchean TTG rocks (Fig.…”

Granitoid Types Related to Cretaceous Plutonic Au‐Quartz Vein and Cu‐Fe Skarn Deposits, Kitakami Mountains, Japan

“…These rocks are dominated by quartz diorite to granodiorite composition, but also include minor gabbro and diorite (i.e., gabbroids) and rare granite. The plutonic rocks occur as isolated plutons, the largest one being the 615 km 2 Tono pluton (Geological Survey of Japan, 1974), which could be a typical hightemperature I-type (Chappell et al, 1998). The plutons essentially belong to the magnetite series because they are S. ISHIHARA and H. MURAKAMI 282 RESOURCE GEOLOGY : Tsuchiya and Kanisawa (1994) and Tsuchiya et al (2000).…”

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:…”