Geochronological and geochemical evidence for a Late Ordovician to Silurian arc–back-arc system in the northern Great Xing’an Range, NE China

The early Paleozoic tectonic evolution of the Xing'an–Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block, i.e., the Xinlin–Xiguitu Ocean and the Nenjiang Ocean. However, the early development of the Nenjiang Ocean remains unclear. Here, we present zircon U–Pb geochronology and whole-rock elemental and Sr–Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area. LA-ICP-MS zircon U–Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443–436 Ma and 452–451 Ma, respectively. The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs, and depleted in HFSEs, with relatively low U/Th ratios of 0.18–0.36 and ε_Nd(t) values of -1.6 to +0.5. These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts, consistent with a back-arc basin setting. By contrast, the andesitic tuffs are characterized by high MgO (>5 wt.%), Cr (138–200 ppm), and Ni (65–110 ppm) contents, and can be termed as high-Mg andesites. Their low Sr/Y ratios of 15.98–17.15 and U/Th values of 0.24–0.25 and moderate (La/Sm)_n values of 3.07–3.26 are similar to those from the Setouchi Volcanic Belt (SW Japan), and are thought to be derived from partial melting of subducted sediments, and subsequent melt-mantle interaction. The Duobaoshan basalts have high Nb (8.44–10.30 ppm) and TiO₂ contents (1.17–1.60 wt.%), typical of Nb-enriched basalts. They are slightly younger than regional adakitic rocks and have positive ε_Nd(t) values of +5.2 to +5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts. Synthesized with coeval arc-related igneous rocks from the southeastern Xing'an Block, we propose that the Duobaoshan high-Mg andesitic tuffs and Nbenriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi–Duobaoshan arc belt, and they were formed by the northwestern subduction of the Nenjiang Ocean. Such a subduction beneath the integrated Xing'an–Erguna Block also gave rise to the East Ujimqin–Xinglong igneous belt in a continental back-arc basin setting. Our new data support an early Paleozoic arc–back-arc model in the northern Great Xing'an Range.