Petrogenesis of the Early Cretaceous trachy-dacite from Mashan in the Jiaolai Basin, North China Craton.

Extensional basin, extensional fault and coeval magmatism represent the most important responses to lithosphere thinning, that are formed by extension of upper crust and partial melting of lower crust or mantle. Volcanic rocks can document evolution processes of magmas that derived from deep crustal/mantle or crust mantle interactions. Petrogenesis of the volcanic rocks in the Jiaolai Basin, however, is debated, and therefore has hampered understanding of the Cretaceous geodynamic of the Jiaolai Basin. In present study, we conducted a study on petrology, geochronology, and geochemistry of trachy-dacite in Mashan area in the eastern North China Craton (NCC) LA-ICP-MS zircon U-Pb geochronology documented that the trachy-dacite erupted at 119. 3 +/- 1. 6Ma. The dominant negative epsilon(Hf)(t) values, ranging from -24. 6 to -11. 5, and t(DM)(2) between 2700Ma and 1200Ma, indicate that they were derived from ancient lower crust or lithospheric mantle involved components of the ancient crust. These trachy-dacite rocks in the Jiaolai basin display SiO2 =65. 73% similar to 66. 08%, K2O = 3. 13% similar to 4. 03%, and total alkali ranging from 7. 56% to 8. 15%, indicating their affinity of high-K calc-alkaline to shoshonitic in origin. They are characterized by high -moderate LREE/HREE and (La/Yb), ratios, without significant Eu anomalies (delta Eu = 0. 94 similar to 1. 02), negative Nb-Ta anomalies, and high concentration of LIFE than HFSE, demonstrating that they can be derived from partial melting of lower crust and enriched mantle. We therefore proposed that the coeval Cretaceous volcanic rocks exposed in the Jiaolai Basin were derived from partial melting of the ancient lower crust that were contaminated by enriched lithospheric mantle, and fractional crystallization. The Jiaolai Basin and coeval volcanic rocks, corresponding to NCC lithosphere thinning, were products of slab rollback of subduction of the Paleo-Pacific plate during Early Cretaceous.