Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve the understanding of plate tectonic processes and the history of continental growth. Here, we report on newly recognized trondhjemite in the north of Diyanmiao ophiolite belt in North China. The trondhjemite was found along the Erenhot-Hegenshan suture zone. U-Pb zircon dating revealed that the trondhjemite crystallized at 309 +/- 2.1 Ma. The trondhjemite had a high amount of SiO2 (68.94-76.45 wt %), Al2O3 (13.37-15.90 wt %), and Sr (232-601 ppm); and a low amount of K2O (1.57-2.70 wt %), Y (6.91-9.39 ppm), Ni (1.10-4.19 ppm), and Cr (1.55-13.50 ppm). The Na2O/K2O ratios were 1.90-4.37. There was a lack of negative Eu anomalies. It was relatively enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr; was depleted in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P; and had low total rare-earth element (REE) contents (27.73-49.63 ppm) with distinct REE fractionation (chondrite-normalized (La/Yb)(N) of 5.76-10.52), which was similar to adakitic rocks formed by partial melting of subducted oceanic crust. The trondhjemite, together with Diyanmiao ophiolite (335.6 Ma), may have formed during the stages of intra-oceanic subduction, suggesting that in the Early Carboniferous-Late Carboniferous, the southern Paleo-Asian Ocean was in its subduction stage.