Petrogenesis and tectonic significance of the Early Devonian lamprophyres and diorites in the Alxa Block, NW China

Abstract The North Qilian orogenic belt (NQOB) has been defined as a subduction-collision zone between the Alxa Block and the Qilian Block during the Early Paleozoic. To constrain the post-collisional tectonic evolution of the NQOB, analyses of zircon U-Pb-Hf isotopes, whole-rock major, trace element and Sr-Nd-Pb isotope compositions of the newly discovered Early Devonian lamprophyres and diorites dikes from the Longshoushan area in southwestern margin of the Alxa Block were conducted. Zircon U-Pb dating yields emplacement ages of 400 ± 4 Ma and 403 ± 6 Ma for two lamprophyre dikes and 391 ± 3 Ma for two diorite dikes. The lamprophyres dikes are shoshonitic-high-K (calc-alkaline) in nature, and are characterized by SiO2 contents of 53.6–56.3 wt %, (87Sr/86Sr)i ratios of 0.7064 to 0.7072, eNd(t) values of 0.1 to 1.0, and zircon eHf(t) values of -8.0 to -2.9. The diorite dikes are high-K (calc-alkaline), and are characterized by MgO contents of 6.32 –6.98 wt %, (87Sr/86Sr)i ratios of 0.7089–0.7137, eNd(t) values of -3.8 to -3.5, and zircon eHf(t) values of -9.9–0.4. Both the lamprophyre and diorite dikes show parallel enrichments in LREEs and LILEs and depletions in HREEs and HFSEs and have similar ratios of (206Pb/204Pb)i (17.587 to 18.133), (207Pb/204Pb)i (15.518 to 15.584) and (208Pb/204Pb)i (37.676 to 38.058). Geochemical and isotopic data suggest that the lamprophyre and diorite dikes were derived from low-degrees melts of amphibole- and phlogopite-bearing lherzolite and phlogopite-bearing lherzolite, respectively, in the spinel-garnet transition facies. Their parental magmas both experienced extensive fractional crystallization in a deep magma chamber and negligible crustal contamination during their ascent. Regarding the Palaeozoic tectonic development of the North Qilian orogenic belt, we propose that the Early Devonian lamprophyres and diorites possibly are related to North Qilian orogen unrooting and collapse and marking the end of the North Qilian orogenic events.