Triassic depleted lithospheric mantle underneath the Paleozoic Chinese Altai orogen: Evidence from MORB-like basalts

Abstract

Whole-rock geochemical and Sr–Nd–Pb–Hf isotopic analyses were conducted on the Triassic Ashele post-orogenic basalts from the Chinese Altai. The tholeiitic Ashele basalts possess moderate Na2O (2.46–4.52 wt%) but extremely low K2O (≤0.06 wt%) contents, and are depleted in light rare earth elements (LREE). Their (La/Sm)N and (La/Yb)N ratios range from 0.78 to 0.88 and from 0.72 to 0.84, respectively, which, together with their low initial 87Sr/86Sr (0.7048–0.7052), high εNd(t) (+4.9 to +5.6) and εHf(t) (+14.3 to +16.3) values, indicate that the magmas were derived from a depleted mantle source. Their 206Pb/204Pb and 207Pb/204Pb ratios (17.919–18.125 and 15.507–15.522, respectively) also support this interpretation. On the other hand, the Ashele basalts are enriched in Cs, Th and U, but depleted in Rb, Ba, Nb and Ta, possibly due to slight mantle metasomatism by subduction-related fluids. Because there was no record for large-scale thermal activity in the deep mantle during the Triassic, and geological evidence demonstrates reactivation of the Erqis fault, we propose that such a tectonic reactivation affected the lithospheric mantle and resulted in its partial melting and subsequent generation of the Ashele basalts. This study shows that a depleted lithospheric mantle, not an ancient continental lithosphere, was underneath the Chinese Altai in the Triassic, which was possibly accreted during the Paleozoic orogeny. Therefore, the lithospheric mantle in this area today was possibly evolved from such a depleted mantle modified by consecutive partial melting after the Paleozoic orogeny.