The Open UniversitySkip to content
 

Differentiation of peraluminous leucogranites “en route” to the surface

Tartèse, Romain and Boulvais, Philippe (2010). Differentiation of peraluminous leucogranites “en route” to the surface. Lithos, 114(3-4) pp. 353–368.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1016/j.lithos.2009.09.011
Google Scholar: Look up in Google Scholar

Abstract

Leucogranites display petrological and geochemical heterogeneities that can be related to either primary processes in the source of melts or to secondary processes such as fractional crystallization. In the Armorican Massif, a Hercynian domain in western France, syntectonic leucogranites were emplaced along major shear zones, the so-called South Armorican Shear Zone. The Lizio and Questembert massifs are derived from a similar metasedimentary source (εNd(T) between − 4 and − 6) and have similar magmatic volumes. They display, however, distinct depths of emplacement, with Questembert being emplaced ca. 5–10 km shallower than Lizio. The shallower emplacement of Questembert is recorded by contact metamorphic assemblages and the levels of intrusion in correlated sedimentary sequences. The Questembert magmas are more differentiated than the Lizio ones, as demonstrated by a lower amount of modal biotite, a higher SiO2 content (72.8–74.2 wt.% vs. 71.7–73.3 wt.%) and a more pronounced peraluminous character. The high-SiO2 Questembert magmas can be derived from the low-SiO2 Lizio ones by 15–20 wt.% fractionation of the assemblage alkali feldspar + plagioclase + biotite. Fractionation of zircon and monazite, hosted in biotite, is indicated by a systematic decrease in Zr and Th + LREE contents. The magmatic evolution is well recorded by the oxygen isotope evolution of whole rocks and mineral separates. The Questembert massif records evidence of a two stage fluid–rock interaction. A high-T stage corresponds to the magmatic fluid exsolution, and is recorded in the most evolved samples: appearance of tourmaline in the matrix, changes in the composition of muscovite, increase in whole rock Be, Sn, Rb and W contents, fractionation of K/Rb, Zr/Hf and Nb/Ta ratios, and fractionation of REE patterns. A second post-solidus alteration is indicated by oxygen isotope disequilibrium between quartz and feldspar in the Questembert massif and requires the circulation of low-δ18O fluids, likely derived from the surface. We propose that the distinct petrological, mineralogical and geochemical evolutions of the Questembert and Lizio magmas are related to processes acting during the ascent of magmas through the continental crust. Questembert is more differentiated than Lizio because it covered a greater distance during its vertical migration, enhancing crystal fractionation from melts.

Item Type: Journal Article
Copyright Holders: 2009 Elsevier B.V.
ISSN: 0024-4937
Keywords: geochemistry; leucogranites; differentiation; South Armorican shear zone
Academic Unit/Department: Science > Physical Sciences
Item ID: 31680
Depositing User: Romain Tartèse
Date Deposited: 20 Jan 2012 11:29
Last Modified: 23 Oct 2012 14:29
URI: http://oro.open.ac.uk/id/eprint/31680
Share this page:

Actions (login may be required)

View Item
Report issue / request change

Policies | Disclaimer

© The Open University   + 44 (0)870 333 4340   general-enquiries@open.ac.uk