Staff directory
Jean-Paul Liégeois
Earth Sciences
Geodynamics and mineral resources
Geodynamics and mineral resources
Publication details
Stern, R.J., Ali, K.A., Liégeois, J.P., Johnson, P., Wiescek, F. & Kattan, F. 2010. ‘Distribution and Significance of pre-Neoproterozoic zircons in juvenile Neoproterozoic igneous rocks of the Arabian-Nubian shield’. American Journal of Science 310: 791–811. DOI: 10.2475/09.2010.02. I.F. 2.270.
Article in a scientific Journal / Article in a Journal
Igneous rocks of the Arabian-Nubian Shield (ANS) have lithologic
associations (ophiolites, calc-alkaline igneous rocks, immature sediments) and radiogenic
isotopic compositions consistent with formation as juvenile continental crust as a
result of accreting intraoceanic arc systems during 880 to 630 Ma, with crustal
differentiation continuing until 570 Ma. ANS igneous rocks locally contain zircons
with ages that are much older than this, leading some researchers to infer the presence
of pre-Neoproterozoic crust at depth in spite of Nd isotopic evidence that ANS crust is
overwhelmingly juvenile. The ANS is flanked by pre-Neoproterozoic crust but geochronology
and isotopic compositions readily identify such tracts. We have compiled
U-Pb zircon ages for 302 samples of ANS igneous rocks that have been analyzed for the
age of individual zircons (2372 ages) and find that a significant proportion (5%) of
these have ages older than 880 Ma (zircon xenocrysts). Zircon xenocrysts are more
common in volcanic than plutonic rocks and mafic relative to felsic igneous rocks. Four
explanations are considered: 1) contamination during sample processing; 2) involvement
of pre-Neoproterozoic crust; 3) incorporation of detrital zircons from sediments;
and 4) inheritance from a mantle source. Possibilities 1 and 2 are discounted, and we
conclude that the presence of pre-880 Ma zircon xenocrysts in ANS igneous rocks with
mantle-like isotopic compositions indicates either incorporation of sediments or
inheritance from the mantle source region, or both.