Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon
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ORIGINAL PAPER
Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon A. J. Miles • C. M. Graham • C. J. Hawkesworth M. R. Gillespie • R. W. Hinton • Edinburgh Ion Microprobe Facility (EIMF)
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Received: 16 July 2012 / Accepted: 9 February 2013 / Published online: 9 March 2013 Springer-Verlag Berlin Heidelberg 2013
Abstract Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of [11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4–6 km. The
Communicated by J. Blundy.
Electronic supplementary material The online version of this article (doi:10.1007/s00410-013-0862-9) contains supplementary material, which is available to authorized users. A. J. Miles (&) C. M. Graham R. W. Hinton Edinburgh Ion Microprobe Facility (EIMF) School of GeoSciences, University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh EH9 3JW, UK e-mail: [email protected] C. M. Graham e-mail: [email protected] R. W. Hinton e-mail: [email protected]
crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H2O-rich (*6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H2O-saturated liquidus at *4 km. H2O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently ‘hot’ and ‘cold’ granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons. Keywords Accessory minerals Trace elements Hot zone Granite emplacement Magma ascent Incremental assembly
Edinburgh Ion Microprobe Facility (EIMF) e-mail: [email protected] C. J. Hawkesworth Department of Earth Sciences, University of St Andrews, North Street, St Andrews KY16 9AL, UK e-mail: [email protected] M. R. Gillespie British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK e-mail: [email protected]
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