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

Abstract

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 ϵHf(t) value in the range of −10.3 to 1.8. The whole-rock initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the diabase range over 16.811–17.157, 15.331–15.422 and 37.768–37.895, respectively. The (87Sr/86Sr)i ratios vary between 0.7086 and 0.7106, and ϵNd(t) values vary between −14.4 and −13.4, which are significantly higher than the ϵHf(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.

Late Permian High-Ti Basalt in Western Guangxi, SW China and Its Link With the Emeishan Large Igneous Province: Geochronological and Geochemical Perspectives

“…The Lu-Hf and Sm-Nd isotopic systems have similar geochemical characteristics, and hence the εHf(t) and εNd(t) values show strongly positive correlation . Nd-Hf decoupling of magmatic rocks are commonly related to residual garnet or zircon enrichment in the magma source region (Patchett et al, 2004;Vervoort et al, 2000;Zeng et al, 2021a), and/or crustal assimilation (Patchett et al, 1984;Carpentier et al, 2009). From Figure 8B, it is shown that the Tongdeng samples .…”

“…FIGURE 8 | (A) ( 87 Sr/ 86 Sr) i -εNd(t) and (B) εHf(t)-εNd(t) (base map afterZeng et al, 2021a;Zeng et al, 2021b) diagrams for the Longlin basalt samples. MORB, mid-ocean-ridge basalt; EM I and EM II, enriched mantle; HIMU, LCC/MCC, middle to lower continental crust; UC, upper continental crust; IMM, Indian MORB; PMM, Pacific MORB.…”