MAGMATIC RECORDS OF SOUTHWARD SUBDUCTION OF THE BANGONG–NUJIANG OCEAN LITHOSPHERE DURING THE EARLY CRETACEOUS IN CENTRAL TIBET

It is hotly debated whether the Early Cretaceous magmatism in the central and northern Lhasa subterranes in Tibet was generated by low-angle northward subduction of the Neo-Tethyan Ocean or by southward subduction of the Bangong–Nujiang Ocean. We present zircon U–Pb age and Hf isotope, whole-rock major and trace element, and Sr–Nd–Pb isotope data of the basalts and andesites from Yanhu on the northern Lhasa subterrane to address this question. Three andesite samples are dated at 131 Ma, 121 Ma, and 116 Ma, respectively. These volcanic rocks can be classified into medium-K and high-K calc-alkaline series.

The medium-K basalts are characterized by low K₂O (0.5–0.9 wt.%), MgO (4.5–7.0 wt.%) and strong depletions in Nb, Ta and Ti. The medium-K Mg-rich andesites have high MgO (3.5–6.1 wt.%), suggesting a mantle source. All medium-K samples show similar isotope signatures with positive Ɛ_Nd(t) values of+2.3 to +4.7, (²⁰⁶Pb/²⁰⁴Pb)i of 18.34–18.48, and zircon Ɛ_Hf(t) ranging from +2.4 to +17.0 (excepts three spots of –15.7, –13.3 and –7.1). These rocks may have been derived from the partial melting of mantle wedge source metasomatized by slab/sediment-derived fluids.

The high-K basalts have high K₂O (1.8–2.6 wt.%). They are enriched in light rare-earth elements (La/Yb = 4.9–13.2) when compared with the medium-K basalts (La/Yb = 3.4–5.1) and have strong incompatible element enrichment, suggesting a low degree of partial melting. They share the same isotope features with medium-K basalts, indicate that they also originated from the mantle wedge. One trachyandesite sample has low MgO (2.3 wt.%) and can be generated by 30% fractionation of clinopyroxene from the high-K calc-alkaline basalts.

In summary, the Yanhu volcanic rocks are arc rocks related to a subduction setting. The Early Cretaceous magmatism in the central and northern Lhasa subterranes is nowadays about 100–300 km to the Indus–Yarlung Zangbo suture zone (remnants of the Neo-Tethyan ocean). They are generally interpreted as the products of low-angle northward subduction of the Neo-Tethyan Ocean. However, this interpretation has been challenged because of north-south shortening of Lhasa Terrane (probably >300 km since the Early Cretaceous) and identification of the Early Cretaceous arc in the southern Lhasa subterrane. We are keeping on working on this topic.