Accessory mineral growth histories : implications for granitoid petrogenesis
Accessory minerals in granitoids are major repositories of several
geochemically-important trace elements and isotopes and in order to quantify the influence
that they have over granitoid petrogenesisi t is necessaryto characterizef ully their behaviour.
In particular it is necessary to understand accessory mineral/melt partitioning of trace
elements and within grain elemental diffusivities, the latter is of relevance when assessing the
state of isotopic equilibration between a refractory accessory phase and a contacting melt.
In this study the backscattered electron (BSE) imaging technique, coupled with
quantitative electron microprobe analysis indicate that granitoid zircons and titanites (mainly
taken from Caledonian intrusive complexes) are commonly compositionally zoned. The
zoning textures observed in these minerals, namely crystal face-parallel zoning, non-planar
compositional zoning (included here are subhedral and anhedral core structures) and
compositional sector zoning, indicate that the kinetic factors of crystal growth, ie. within
magma elemental diffusion rates, crystal growth rates, interface kinetics and dissolution
kinetics, are largely responsible for the patterns of compositional zoning that have been
observed. This fording is in marked contrast to other studies which have assumed that
kinetics are not important in crystallizing plutonic granitoid magmas.
Accessory mineral growth histories have been studied in a few well constrained samples
from the Caledonian-age Strontian Complex of NW Scotland. The zircons from the central
intrusion of this composite pluton have abundant inherited cores. The cores contain a variety
of zoning structures and have a wide range of composition, which are taken to indicate that
the cores had a wide variety of ultimate sources. Titanites from both the outer and inner
intrusions have compositional sector zoning and the range of composition shown by the
titanites is largely due to this fact. In this pluton magma composition appears to have little
influence on titanite chemistry. The REE abundances in both parts to the intrusion are largely
controlled by the accessory phases and each of these phases have very different
rock-normalised REE distribution patterns. These abundance patterns are dependent on the
relative partition coefficients that each phase has for the REE, the accessory mineral
assemblagep resenta nd the crystallization order of that assemblage.
The zircons from the central acid member of the Strontian Complex, which are known to
have substantial U-Pb inheritance were extracted and analysed for their Sm-Nd isotopic
composition. The results apparently indicate that refractory zircons can also preserve their
Sm-Nd isotopic composition, a phenomenon not previously reported. That is diffusion of
Sm and Nd (and presumably the other REE) within refractory zircon at elevated temperatures
appears to be sufficiently slow that complete isotopic equilibration between a zircon and a
contacting melt may not always occur. Such disequilibrium potentially enables granitoid
magma provenance to be studied with much greater resolution than hitherto possible.