The Cornubian Batholith

An example of magmatic fractionation on a crustal scale

Bruce W. Chappell*, Rick Hine

*Corresponding author for this work

    Research output: Contribution to journalArticle

    48 Citations (Scopus)

    Abstract

    The Cornubian Batholith comprises six major and several smaller bodies of S-type granite in southwestern England. These late-Variscan granites comprise two-mica granites, and much less abundant Li-mica granites that are restricted to one of the major bodies (St Austell) and smaller bodies. Some of these intrusive rocks are associated with major Sn mineralization. This paper is concerned with the geochemistry of the two-mica granites, which are felsic, strongly peraluminous, and have a high total alkali content and low Na:K. Rocks with very similar compositions to these granites occur elsewhere, including the Variscan granites of continental Europe, and in southeastern Australia. In detail all of the major plutons of this batholith have distinctive compositions, except for Bodmin Moor and Cammenellis which cannot be discriminated from each other compositionally. A comparison with experimental data shows that the granites attained their major element composition under conditions of crystal liquid equilibrium, with the final melt being saturated in H2O, at temperatures close to 770°C and pressures about 50 MPa. That temperature estimate is in good agreement with values obtained from zircon saturation thermometry. The specific minimum-temperuture composition excludes the possibility of widespread transfer of elements during hydrothermal alteration. Minor elements that are relatively very abundant are Li, B, Cs and U, while F, Ga, Ge, Rb, Sn, Ta, W and Tl are quite abundant and P is high for felsic rocks. Sr, Ba, and the trace transition metals Sc to Zn, are low, but not as low as they commonly are in very felsic granites. These trace element abundances, and the H2O-saturation, resulted from the fractional crystallization of a melt derived by the partial melting of feldspathic greywackes in the crust. The Cornubian granites have compositions very similar to the more felsic rocks of the Koetong Suite of southeastern Australia, where a full range of granites formed at the various stages of magmatic fractionation postulated for the Cornubian granites can be observed. The operation of fractional crystallization in the Cornubian granites is confirmed by the high P abundances in the feldspars, with P contents of the plagioclase crystals correlating with Ab-content. Most of the granites represent solidified melt compositions but within the Dartmoor pluton there is a significant component of granites that are cumulative, shown by their higher Ca contents. The Cornubian plutons define areas of high heat flow, of a magnitude which requires that fractionated magmas were transported laterally from their sources and concentrated in the exposed plutons. The generation of these granite plutons therefore involved magmatic fractionation during the stages of partial melting, removal of unmelted material from that melt, and fractional crystallization. During the later stages of those processes, movement of those magmas occurred on a crustal scale.

    Original languageEnglish
    Pages (from-to)203-244
    Number of pages42
    JournalResource Geology
    Volume56
    Issue number3
    DOIs
    Publication statusPublished - 1 Sep 2006

    Keywords

    • Fractional crystallization
    • High heat flow
    • Peraluminous
    • Phosphorus in feldspars
    • S-type
    • Tin mineralization
    • Two-mica granite

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