Petrology of the Mesozoic magmatic rocks in Ningwu area: Insights from in-situ zircon Hf isotope and Nd-Sr isotopes, EastChina

The Early Cretaceous magmatic rocks in Ningwu basin are composed of basic to intermediate volcanic and andesitic porphyry rocks. Based on detailed study of the zircon U-Pb age, petrological, mineralogical and geochemical data, the characteristics of the magma system, the petrogenesis of different rock types and the nature of the mantle source were discussed to provide new constraints on the origin and tectonic setting of the Mesozoic Ningwu basin. The zircons from pyroxene diorite porphyry yield a U-Pb age of 128.9Ma. Longwangshan volcanic rocks are rich in K, Si and poor in Ca, Mg, whereas Dawangshan volcanic rocks and andesite porphyry are rich in Na. The volcanic rocks and andesitic porphyry are all enriched in LILE and have a slightly depletion of HFSE with negative Ti, Nb, Ta anomaly. Sodic Dawangshan volcanic rocks have slightly lower I-Sr (0.70544 similar to 0.70768), higher Sr content and Mg#. Phenocrysts of plagioclase in pyroxene diorite porphyry display disequilibrium and in-situ zircon Hf isotopes epsilon(Hf)(t)=-6.2 similar to+0.6, indicating magma mixing process. I-Sr(t)=0.70525 similar to 0.70774, epsilon(Nd)(t)=-8.2 similar to-4.1 of the Cretaceous volcanic and andesitic porphyry rocks in Ningwu. The K-rich and Na-rich rocks were generated by melting of enriched lithospheric mantle then the melts mixed with lower crust Kongling grey gneiss and metabasite respectively. The mantle source of the K-rich and Na-rich rocks contain rutile and clinopyroxene respectively. They may form in the stretching environment with lithospheric mantle upwelling and small extent of partial melting, then the partial melts quickly underplayed and heated the grey gneiss and amphibolite base, mixing with its melts.