Shrimp dating zircon
This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia.SHRIMP U-Pb dating of magmatic zircons from a granodiorite (115) has yielded a concordant 206Pb/238U age of 488.5×4.1Ma which is corresponding to the Late (Furongian) Cambrian.Zircons from the Khovd complex intrusions indicate different SHRIMP U-Pb ages: The best estimate of the crystallization age of sample 1, based on the weighted mean 206Pb/238U ratio is 465.5×4.6Ma. Two zircons from the sample 1 indicate largely different SHRIMP U-Pb ages: 204×3 Ma and 218×2Ma (Late Triassic).
Five samples were collected from two complexes in the Khovd province which is located in the western part of Mongolia.Zircons for the SHRIMP geochronology were separated from one granodiorite (115) sample of the Togtokh's shil and four different phases' granite samples (1, 039, 068, and 085) of the Khovd intrusive complexes.A total of 15 spot analyses were made on twelve zircons of sample 039 and one of them was rejected and yields a weighted mean 206Pb/238U age of 398.8×2.8Ma which correspond to the Early Devonian. The fourteen analyzed spots of 10 zircon grains of sample 085 and yield a weighted mean 206Pb/238U age of 422.8×5Ma which correspond to the Late (Ludlow) Silurian.The best estimate of the crystallization age of sample 068, based on the weighted mean 206Pb/238U ratio is 284.2×1.7 Ma. For this reason, we suggest that the Togtokh's shil and Khovd complexes' intrusions were emplaced during the Paleozoic ages.
Zircons from all samples are colorless, transparent to untransparent, and subhedral to euhedral, stubby or equant and prismatic to elongate in shape.Most of the zircon Th/U ratios are mostly bigger than 0.3, indicating it is of magmatic origin.