Petrogenesis of Early Cretaceous alkaline dolerite dykes in the Dabie Orogen, China: Constraints on the timing of lithospheric thinning

The Dabie Orogen in central China has long been proposed to transform from Triassic compression related to northward deep subduction to Late Jurassic to Early Cretaceous extension involving marked crustal thinning and doming processes. Zircon U-Pb ages, mineral chemistry, and whole-rock geochemistry of alkaline dykes intruding Early Cretaceous intermediate-mafic-ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension. These dykes of dominantly dolerites with minor monzodiorites have zircon U-Pb ages of 128.3-129.7 Ma and are characterized by Sr-Nd isotopic ratios around the enriched mantle I. The dolerites can be divided into two groups, with the first group having lower incompatible elements but more enriched Sr-Nd isotopes than the second group. Equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation. Systematic normative minerals, trace elements and isotopic contrasts of Early Cretaceous gabbros (125-130 Ma), dolerite dykes of this study, and Early Cretaceous mafic dykes (111-115 Ma) indicate that the source regions for the dolerites are deeper than those for the host gabbros. Therefore, the lithospheric mantle thickening probably continued until the formation of the dolerites, and the lithospheric mantle thinning occurred significantly later than previously proposed. The thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125-130 Ma but changed to less than that for the source region of tholeiitic basalts before 115 Ma, suggesting a minimum decrease of ~ 20 km within ~ 10 m.y. This rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 Ma.