Geochemistry and petrogenetic implications of a Late Devonian mafic–ultramafic intrusion at the southern margin of the Central Asian Orogenic Belt

The Heishan mafic–ultramafic intrusion was emplaced into Neoproterozoic metamorphic strata in the eastern Beishan Fold Belt at the southern margin of the Central Asian Orogenic Belt. The dominant rocks of the intrusion are harzburgite and lherzolite. Olivine gabbro-norite and gabbro dyke occur only along the southern margins of the intrusion. The rocks of the Heishan intrusion are characterized by enrichments of large ion lithophile elements, strong negative Nb (Ta) anomalies and positive K and Pb anomalies. These features are perfectly consistent with those of the Devonian volcanics in the Beishan Fold Belt and indicate subduction-related magmatism. The Heishan intrusive rocks have restricted εNd(t) (+ 0.34 to + 3.95) and a large range of (87Sr/86Sr)t ratios (0.7041 to 0.7087). Their high (207Pb/204Pb)t (15.55–15.64) and (208Pb/204Pb)t (37.71–38.29) values are comparable with those of the volcanics along the Pacific margins of the Americas. Zircon ID-TIMS U–Pb age data (356.4 ± 0.6 Ma and 366.6 ± 0.6 Ma) and SHRIMP U–Pb age data (358 ± 5 Ma and 357 ± 4 Ma) indicate that the intrusion was emplaced in the Late Devonian. Our calculations indicate that the parental magma of the Heishan intrusion had a high-Mg basaltic composition with ~ 11.3 wt.% MgO and ~ 10.0 wt.% FeOT. It is proposed that the high-Mg basaltic magma was generated from partial melting of the asthenosphere and mantle wedge triggered by upwelling of asthenosphere due to slab break-off in an active continental margin.