Origins of the meta‐mafic rocks in the southern Dunhuang Block (NW China): Implication for tectonic framework of the southernmost Central Asian Orogenic Belt

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world with prominent juvenile crust addition, where the southernmost margin of the CAOB located is one hot topic in the international studies. The Dunhuang Block located to the south of the Beishan Orogen is a key position for outlining the tectonic framework of the south margin of the CAOB. However, at present, the tectonic attribution of the Dunhuang Block is still controversial. In this paper, we focus on the origins and protoliths of the meta‐mafic rocks from the Hongliuxia complex in the southern Dunhuang Block for determining the tectonic attribution of the Dunhuang Block. In the field, the meta‐mafic rocks are mainly exposed as tectonic blocks within the metasedimentary rocks displaying the “block‐in‐matrix” fabrics or as tectonic slices juxtaposed by faults. Geochemically, the meta‐mafic rocks could be divided into three groups based on the rare earth elements (REE) and trace elements: the group 1 samples are depleted in the LREE relative to HREE and show flat HREE patterns, similar to the mid‐oceanic ridge basalt; the group 2 samples show flat REE patterns without obvious differentiation between the LREE and HREE, like the oceanic plateau basalt; the group 3 samples are enriched in LREE relative to LREE, and have negative Nb, Ta, and Ti anomalies, similar to the island arc basalt. Conclusively, the protoliths of the meta‐mafic rocks dominantly belong to the oceanic plate components. They were metamorphosed during Early Silurian to Middle Devonian in different depths of the subduction zone documented in previous studies. So, we suggest the Hongliuxia complex is an exhumed Palaeozoic subduction–accretion complex (SAC) formed during the oceanic plate subduction. This result confirms the Palaeozoic subduction event occurred in the Dunhuang region and indicates that the Dunhuang Block is a Palaeozoic Orogen rather than a part of Precambrian continent block. Considering the regional geology context, we speculate the Dunhuang Orogen belongs to the southernmost margin of the CAOB.

Diverse P‐T‐t Paths Reveal High‐Grade Metamorphosed Forearc Complexes in NW China

“…The metapelite (DH53), which has the protolith properties of a sedimentary Upper Continental Crust, is possibly a fragment of accreted material in a forearc that was sourced from a nearby continent. Moreover, many high‐grade metabasic blocks from the Hongliuxia mélange in southern Dunhuang where the Devonian eclogitic block was collected (H. Y. C. Wang, Chen, et al., 2017), show various affinities to IAB, E‐MORB, and normal mid‐ocean ridge basalt (N‐MORB) (Figure 6) (Shi et al., 2021). Such a major juxtaposition of rocks with widely different Mesoarchean to Paleozoic ages, and of diverse, widely different origins suggests that the Dunhuang Complex was derived from a diverse accreted forearc.…”

“…(f) Chondrite-normalized rare earth element (REE) diagram and chondrite concentrations from Boynton (1983). Data of metabasic rocks from southern Dunhuang are from Shi et al (2021).…”