We just got a manuscript accepted in Geology.
The paper (A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites) presents a first-order U-Pb and trace element comparison of suspected antecryst zircon populations that are included within the megacrystic rapakivi texture -forming alkali feldspar ovoids to their respective groundmass zircon populations.
We utilized a simple (but also rather tedious) microdrilling approach to extract zircon from within the rapakivi ovoids and were able to show that the populations show dissimilar geochronological and compositional characteristics.
The paper will appear in the March 2016 issue...
Abstract
Entrainment of antecrystic zircon in early saturated main silicate
phases has been recognized as a complicating factor in high-precision
U-Pb geochronology of Phanerozoic granitic systems, but has not
been demonstrated for Precambrian rocks. We report U-Pb ages
(secondary ion mass spectrometry) and trace element (laser ablation–inductively
coupled plasma–mass spectrometry) compositions
of zircon from three main ferroan granite types of the late Paleoproterozoic
Wiborg rapakivi granite batholith of southeastern Finland
and vicinity. Zircon was extracted from the interior parts of megacrystic
(diameters as large as 15 cm) rapakivi-textured alkali feldspar
ovoids using a microdrilling technique. Compared to zircon in ovoid
groundmass, zircon in the ovoids has higher incompatible element
values (e.g., uranium and rare earth elements) and, for two of the
three samples examined, slightly older (by ∼1 m.y.) average U-Pb ages.
These observations suggest that the ovoid material of the rapakivi
granites entrained zircon from earlier magmas that were compositionally
different than the final magmatic host of the ovoids. Groundmass
zircon implies similar average U-Pb ages for all the studied samples,
ca. 1628 Ma; this is regarded as the final crystallization age of the
granites. These observations show that it is viable to measure statistically
valid age differences between antecrysts and groundmass
of Proterozoic granites. Overall, our sampling technique provides
enhanced textural control of antecrystic zircon in a wide range of
sample materials.
