Heishan mafic–ultramafic rocks in the Qimantag area of Eastern Kunlun, NW China: Remnants of an early Paleozoic incipient island arc

The Heishan mafic–ultramafic complex in the Qimantag Mountains of the Eastern Kunlun Orogen, consists of olivine pyroxenites, pyroxenites, gabbros, and massive basalts. Zircons from the gabbros have magmatic sector zoning, with Th/U ratios of 0.5–2.3, and yield a SHRIMP age of 486 ± 6 Ma (MSWD = 0.31), which is taken as the time of magma crystallization. Spinel, pyroxene, and plagioclase mineral compositions indicate that the rocks formed in an island arc environment. The cumulates have low (La/Yb)N ratios (0.6–0.8) and depleted LREE patterns, high abundances of Cr (3031–1185 ppm), Co (119–50 ppm), and Ni (953–291 ppm), and εNd(t) values of about + 8.0. The massive basalts have somewhat higher contents of TiO2 (1.0%–1.3%), Na2O (2.6%–2.8%), total REE and Mg#s (55–63) than those of the cumulates. The (La/Yb)N ratios of the basalts range from 0.8 to 1.2 and the chondrite-normalized REE patterns are flat. εNd(t) values of the basalts range from + 7.7 to + 9.4. The compositions of the cumulates and volcanic rocks, especially their similar Nd isotopes, suggest that the rocks were derived from similar depleted mantle sources. The primary magmas were probably generated in an initial arc environment. We conclude that the Heishan mafic–ultramafic complex in the Qimantag area preserves a record of Proto-Tethyan oceanic lithosphere formed in the early Paleozoic. It possibly marks a suture between the Qaidam and North Kunlun terranes in the western segment of the Eastern Kunlun Orogen.