Geochronology and geochemistry of volcanic rocks from the Tanjianshan Group, NW China: Implications for the early Palaeozoic tectonic evolution of the North Qaidam Orogen

Abstract

AbstractThe widely exposed Tanjianshan volcanic rocks in the North Qaidam Orogen (NW China) encode abundant information about early Palaeozoic magma origin and tectonic evolution. This research utilizes petrological, geochemical, and geochronological methods to investigate the last volcanic cycle (Formation d) of the Tangjianshan Group, aiming to reveal the tectonic evolutionary processes of the North Qaidam Orogen. Formation d can be further divided into two major volcanic sequences, that is, Formation d‐1/d‐4 and Formation d‐3. Formation d‐1/d‐4 is a set of intermediate–basic volcaniclastic rocks intercalated with volcanic lava and contemporary hypabyssal diabase–gabbro intrusive rocks. The rocks are characterized by high average concentrations of Al2O3, K2O, Na2O, and P2O5, pronounced differentiation of light (LREE) and heavy rare earth elements (HREE) and enrichment of LREEs and light‐ion lithophile elements (LILE), showing continental arc affiliation and enriched mid‐ocean‐ridge basalt (E‐MORB) characteristics. They have positive zircon εHf (t) values (7.5 to 16.1) and variable whole‐rock εNd (t) values (−2.7 to 7.0), with crustal Hf model ages (TDMC) and two‐stage Nd model ages (T2DM) ranging from 428 to 970 Ma and from 618 to 1,410 Ma, respectively. The magma may have originated from an enriched mantle source and experienced crustal contamination in a continental back‐arc basin. In contrast, Formation d‐3 is composed of volcaniclastic rocks intercalated with layered basaltic andesite and basalts. It has higher average concentrations of CaO, FeOT, and MgO, lower ΣREE and flatter REE patterns, lower average 87Sr/86Sr ratios (0.70557), higher average 143Nd/144Nd ratios (0.51285), and exclusively positive εNd (t) values (1.3 to 5.0), consistent with normal mid‐ocean‐ridge basalt (N‐MORB) influence. The magma may have been derived from a normal mantle source without significant modification, which probably formed in a rift‐related tectonic setting or even a mid‐ocean ridge that developed within a back‐arc basin. Magmatic zircons from Formation d‐1/d‐4 yield U–Pb ages mostly at 460–440 Ma, whereas zircons modified by continental exhumation‐related orogenic fluids are mainly dated at 430–420 Ma. Magmatic zircons from Formation d‐3 mainly have U–Pb ages of 440–430 Ma. Integrating the magma genesis and rock affiliation interpretations of this study with geological information for the study area and adjacent regions, a four‐stage tectonic evolutionary model is proposed for the early Palaeozoic North Qaidam Orogen: (a) oceanic subduction with arc magmatism (520–460 Ma), (b) continental collision with back‐arc magmatism (Formation d‐1/d‐4) (460–440 Ma), (c) mid‐ocean ridge extension with rift‐related magmatism (Formation d‐3) (440–430 Ma), and (d) continental exhumation with orogenic fluid modification (430–420 Ma).