Zircon SHRIMP U-Pb ages and Hf isotope characterestics of the zircons from tuff in Ordovician Majiagou Formation in the central—eastern Ordos Basin and their geological significance
Objectives:The Ordovician Majiagou Formation has become the most popular carbonate formation in the Ordos Basin due to its potential for oil and gas exploration,but its age attribution remains controversial.In the Ordos Basin,two layers of tuff develop in the upper part of the 5th Member of the Ordovician Majiagou Formation(hereinafter referred to as the"Ma-5"Member),which are commonly used as regional marker beds for stratigraphic division.Studying the zircon ages of these tuffs can establish the chronological stratigraphic framework of the Majiagou Formation and determine the tectonic setting during its depositional period.Methods:We conducted zircon SHRIMP U-Pb dating and in-situ Hf isotopic characteristic analyses on the tuffs of the Ma-5 Member from the Well S148 and the Well T56 in the Ordos Basin.Results:The zircon SHRIMP U-Pb dating result of the tuff shows that the 206Pb/238U ages of the tuff sample from the Well T56 ranges from 442.3 Ma to 474.3 Ma,with a weighted average age of 460.9±2.9 Ma,and those of the tuff sample from the Well S148 ranges from 442.0 to 471.6 Ma,with a weighted average age of 458.4±2.4 Ma.The measured n(176Hf)/n(177Hf)ratios of in-situ zircon Hf isotopes range from 0.282687 to 0.280820,andεHf(t)values vary from 6.67 to 11.32,with two-stage model ages ranging from 720 to 1024 Ma.Conclusions:Based on the zircon age results from the tuff of the Ma-5 Member,the depositional period of the Ma-5 Member has been constrained to the Middle Ordovician Darriwilian Stage to the Upper Ordovician Sandbian Stage.A comparison of the U-Pb ages and Hf isotope characteristics of zircons from the tuffs at the periphery of the Ordos Basin suggests that the volcanic tuffs of the Ma-5 Member may have originated from the North Qinling and/or North Qilian orogenic belts in the southwestern part of the basin.Furthermore,it is inferred that prior to the emplacement of the tuffs,the North Qinling and/or North Qilian Ocean initiated a northward subduction and entered a stage of ocean—land transition,indicating the transformation of the western and southern margins of the basin into an active continental margin during the Late Middle Ordovician.
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维普
