Geochronology and geochemistry of the Heihe mafic pillow lavas in the Qinling Mountains, China

The Qinling Mountains in central China are the joint orogenic zone between the Sino-Korean (or North China) and the Yangtze craton blocks. The age and genesis of the Danfeng mafic volcanics in the north of the Shangzhou-Danfeng fault zone, i.e. the main suture zone in the Qinling orogenic belt, have been controverted for a long time because their age is closely related to the converged time of two blocks. The ages and the geochemical data of the Heihe pillow lavas for the Danfeng mafic volcanics in the Heihe River area in the Qinling orogen are reported in this paper. The obtained isochron age by the Sm-Nd isotopic data of the 13 whole-rock samples for the mafic pillow lavas is 963±130 (2σ) Ma, corresponding to INd = 0.51173±16 (2σ),ɛNd(T)= +6.6, MSWD0.57. However, the Rb-Sr isotopic analytical results for the same samples as the Sm-Nd whole-rock ones are disperse. For the Sm-Nd isotopic systems were interfered during the later geological functions, the Sm-Nd isochron age for the whole-rock sample (Q9511WR) and the mineral phenocrystal samples: amphiboles (Hb) and plagioclases (Plag) presents the better uncertainty, whereas isochron ages of 930 Ma and 437 Ma are given if the WR-Plag and WR-Hb are calculated respectively, and their Rb-Sr isochron age is 268±47(2σ) Ma, Isr = 0.70475±11 (2σ), MSWD0.96. The major and trace elements for the lavas show that they were formed in the quasi-N-MORB setting.