Distribution and significance of pre-neoproterozoic zircons in juvenile neoproterozoic igneous rocks of the arabian-nubian shield

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.