Journal of Petrology
Gianfranco Di Vincenzo, Paul A. M. Andriessen, Claudio Ghezzo
ABSTRACT
Detailed petrographic and geochemical data and Sr and Nd isotope compositions
of enclaves and host-granite are reported for one of the largest strongly peraluminous
cordierite-bearing intrusions of the Hercynian Sardinia-Corsica Batholith: the
San Basilio Granite. Compared to other peraluminous series, the San Basilio
Granite has a "non-minimum melt" composition and shows variations primarily
due to fractionation of early-crystallised plagioclase, quartz and biotite.
Crystallisation age is constrained at about 305 Ma, by Rb-Sr whole-rock age
[305 ± 23 Ma with (87Sr/86Sr)i = 0.71105 ± 0.00041], and occurred
during late Hercynian tectonic events. eNd(305 Ma) values range from -7.8 to
-7.5.
The San Basilio Granite contains both magmatic and metamorphic enclaves. Magmatic
enclaves, similar to mafic microgranular enclaves common in calcalkaline granitoids,
are tonalitic in composition and show a variation in silica content from 60.3
to 67.7 wt % correlating with a variation in (87Sr/86Sr)(305 Ma) and eNd(305
Ma) from 0.7092 to 0.7109 and from -6.6 to -7.4, respectively. Together with
petrographic and other geochemical data, the Sr and Nd isotope data record different
stages in a complex homogenization process of an unrelated mafic magma with
a crustal melt. A process of simple mixing may account for the variations of
non-alkali elements and to some extent of Sr and Nd isotopes, whereas the distribution
of alkali elements require diffusion-controlled mass transfer.
Petrographic and mineralogical data on metamorphic enclaves and geochemical
modelling for trace elements in granite indicate melt generation by high-degree
partial melting involving biotite breakdown of a dominantly quartzofeldspathic
protolith at about T > 750°ree; - 800°ree; C and P > 6 kbar leaving a
granulite facies garnet-bearing residue, followed by emplacement at about 3
kbar.
eNd(305 Ma) values of the granite fall within the range defined by the pre-existing
metamorphic rocks but (87Sr/86Sr)(305 Ma) ratios are lower, indicating involvement
of at least two distinct components: a dominant crustal component and a minor
well-mixed mafic end-member. These data point to a decoupling between the Sr-Nd
isotope systematics and major and trace element compositions, suggesting that
the effect of the mafic component was minor on granite major and trace element
concentrations, but significant on Sr and Nd isotopes. The study of the magmatic
enclaves and the isotopic evidence demonstrate that unrelated mafic magmas,
probably derived from the mantle, had a close spatial and temporal association
with the production of "non-minimum melt" strongly peraluminous granites, and
support the proposal that heat from the mafic magma contributed to crustal melting.
KEY WORDS:
cordierite-bearing granite; enclaves; felsic-mafic interaction;
Sardinia-Corsica Batholith; Sr and Nd isotopes.
Return to header
Current Contents
................
J. Petrology Editor's Page
This page maintained by P Stuart and
J Rosenbaum
©1996 last updated 15 May 1996