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Journal of Petrology Advance Access published online on April 15, 2005
Journal of Petrology, doi:10.1093/petrology/egi031
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received September 25, 2003
Accepted March 1, 2005

Article

Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth

D. GAGNEVIN 1*, J. S. DALY 1, G. POLI 2, and D. MORGAN 3

1 DEPARTMENT OF GEOLOGY, UNIVERSITY COLLEGE DUBLIN, BELFIELD, DUBLIN 4, IRELAND
2 DEPARTMENT OF EARTH-SCIENCES, PIAZZA UNIVERSITÁ, 06100 PERUGIA, ITALY
3 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK

* To whom correspondence should be addressed.
D. GAGNEVIN, E-mail: Damien.Gagnevin{at}ucd.ie


  Abstract

K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87Sr/86Sr ratios (ISr) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high ISr-Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of ISr, but a larger range of Rb/Sr. Lower ISr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high ISr silicic melts (as seen in the inner cores) and (2) later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.

Keywords: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis.
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