Geochronology, geochemistry and Sr–Nd–Hf isotopes of mafic dikes in the Huicheng Basin: Constraints on intracontinental extension of the Qinling orogen

• First describe of mafic dikes (∼107 Ma) in Lower Cretaceous sandstones in the Huicheng Basin of the Qinling orogen.
• A partial melting of an enriched lithospheric mantle source is inferred.
• Significance for an Early Cretaceous intracontinental extensional setting is stressed.

The diabase dikes intruded the Lower Cretaceous sandstone in the Huicheng Basin are key to understanding the Mesozoic tectonic evolution of the Qinling orogen. LA-ICP-MS zircon U–Pb dating yields a crystallization age of 107 ± 1 Ma for them. The dikes have low contents of SiO2 (42.46–50.16 wt.%), MgO (3.47–5.59 wt.%) with low Mg# of 49–59, and TiO2 (1.35–1.63 wt.%), high TFe2O3 (8.15–9.36 wt.%), Al2O3 (14.75–17.23 wt.%) and K2O (0.87–3.61 wt.%). Their Ni and Cr contents are in range of 16.8–111 and 45.7–315 ppm, respectively. They are significantly enriched in light rare earth elements and large ion lithophile elements (e.g., Cs, Pb and Ba), and depleted in Rb, K, P, and Ti. The dikes show relatively high whole-rock initial 87Sr/86Sr ratios (0.7071–0.7079) and negative εNd (t) values (−1.5 to −4.8) with single-stage Nd model ages of 941–1186 Ma. In addition, they have low zircon εHf (t) values (−8.6 to +3.5) with single-stage Hf model ages of 674–1117 Ma. Both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution. They were derived from partial melting of an enriched lithospheric mantle source. Together with regional geological data, these results suggest that the Qinling orogen experienced an intracontinental extension during the late Early Cretaceous.