Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon

• Published in: Contributions to mineralogy and petrology Vol. 166; no. 1; pp. 1 - 19
• Main Authors: Miles, A. J., Graham, C. M., Hawkesworth, C. J., Gillespie, M. R., Hinton, R. W.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.07.2013; Springer; Springer Nature B.V
• Subjects: Crystals; Earth and Environmental Science; Earth Sciences; Geology; Halides; Igneous rocks; Magma; Metal concentrations; Mineral Resources; Mineralogy; Minerals; Original Paper; Petrology; Phosphate minerals; Phosphate rock; Trace elements; Zirconium; United Kingdom > UK; Scotland; Hot zone; Accessory minerals; Trace elements; Granite emplacement; Magma ascent; Incremental assembly
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-013-0862-9

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 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 H 2 O-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 H 2 O-saturated liquidus at ~4 km. H 2 O 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.