Geochemistry and zircon ages of the Yushigou diabase in the Longshoushan area, Alxa Block: implications for crust-mantle interaction and tectonic evolution

摘要

The North Qilian orogenic belt in North China has been defined as a subduction-collision zone between the Alxa Block and the Qilian Block. We present petrography, zircon U-Pb geochronology, major- and trace-element geochemistry, and Sr-Nd-Pb-Hf isotope analysis for the Yushigou diabase from the Longshoushan area, which is located SW of the Alxa Block, aiming to understand its petrogenetic link to subduction processes. The Yushigou diabase belongs to the tholeiite series, and shows enrichment in light rare earth and large-ion lithophile elements, and a depletion in heavy rare earth and high-field-strength elements. Laser ablation - inductively coupled plasma - mass spectrometry U-Pb zircon dating yielded an emplacement age of 414 +/- 9 Ma, with an epsilon(Hf)(t) value in the range of -10.3 to 1.8. The whole-rock initial Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 ratios of the diabase range over 16.811-17.157, 15.331-15.422 and 37.768-37.895, respectively. The (Sr-87/Sr-86)(i) ratios vary between 0.7086 and 0.7106, and epsilon(Nd)(t) values vary between -14.4 and -13.4, which are significantly higher than the epsilon(Hf)(t) value (Nd-Hf decoupling). An interpretation of the elemental and isotopic data suggests that the Yushigou diabase was derived from partial melting of an enriched mantle I (EM-I) -type lithospheric mantle in the spinel-garnet transitional zone. Based on the geochemical features and previous regional geological data, we propose that the Silurian magmatism was most likely triggered by slab break-off after the closure of the North Qilian Ocean, and ancient continental materials from the subduction slab metasomatized the overlying lithospheric mantle during exhumation.