Age and petrogenetic implication of the Early Mesozoic dykes in the southwestern Ordos Basin

The Early Mesozoic dykes in Longmen area, the southwestern Ordos Basin are infrequent records of magmatism, which provides an ideal object for discussing the tectonic setting of the Early Mesozoic in the Ordos Basin.Petrography, petrogeochemistry and zircon U-Pb dating as well as Lu-Hf isotope for the Early Mesozoic dykes in the Longmen area of the southwestern Ordos Basin are studied.The result shows that these dykes are calc-alkaline to high poassium calc-alkaline dioritic porphyrite and monzonitic porphyrite characterized by low Mg^#, Cr, Ni and Co contents.They are relatively rich in REE and LILE, poor in HFSE and depleted in Nb, Ta, Ti and P.However, the LREE/HREE ratios of monzonitic porphyrite are slightly lower and with much lower Yb, Y and higher Sr, Sr/Y ratios than those of the dioritic porphyrite, indicating that the former was formed by partial melting of deeper crustal materials under relatively high pressure, while the later was the product of melting of shallow crust material under lower pressure.Both of the dioritic and monzonitic porphyrite yield 242 Ma, consisting with those magmatism during the initial compression after the Early Mesozoic ocean basin closure in the Qinling orogenic belt.Their zircon ε_Hf(t)=-33.89~-9.73, t_DM2=1564~2780 Ma, which implies that the tectonic and magmatic activity occurred in Qinling orogenic belt since the Early Mesozoic has affected the southwestern Ordos Basin.All suggested that these dykes are resulted from the partial melting of Neoarchean and Paleoproterozoic basement of the Ordos Basin at different depths under the background of northward subduction of the Mianlue ocean basin in Qinling orogenic belt, which were roughly corresponding to the early Mesozoic tectonic transition from compression to extension in the Qinling orogenic belt.Consequently, the Ordos Basin has entered into a tectonic setting from the compression to the extension at the initiation of Chang 7 Member deposition during the Late Triassic.