Middle–late Permian mantle plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean region, Tibet: Evidence from mafic rocks

Abstract

The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean and was located on the Qinghai–Tibet Plateau. Doubts exist as to the internal dynamics and evolution of the Sumdo Paleo-Tethys Ocean region during the Permian, owing to insufficient knowledge regarding the remnants of oceanic crust. Here, we report results for petrology, laser ablation–inductively coupled plasma–mass spectrometry zircon UPb dating, whole-rock geochemistry, and zircon Hf and whole-rock SrNd isotopes of mafic rocks in the Sumdo area. Geochemical characteristics of these mafic rocks allow them to be divided into two groups, A and B. Group A rocks are characterized by high SiO2 and TiO2 contents, low Al2O3 contents, and Na2O > K2O. Their chondrite-normalized rare-earth-element (REE) and primitive-mantle-normalized trace-element patterns are similar to those of N-MORB. The samples exhibit enrichment in U and depletion in Th, depletion in light REEs, and high 143Nd/144Nd values. Group B rocks have high SiO2, TiO2, and Al2O3 contents. Their chondrite-normalized REE patterns and primitive-mantle-normalized trace-element patterns are similar to those of E-MORB. Group B rocks are enriched in Nb and Ta, depleted in Th and U, and show high 87Sr/86Sr and low 143Nd/144Nd values. Zircon UPb dating of five meta-gabbro rock samples yielded ages of 268.9 ± 1.6 Ma (STN66), 273.8 ± 2.2 Ma (STN79), 265.1 ± 4.1 Ma (STE26), 258.7 ± 4.4 Ma (STE89), and 261.0 ± 3.1 and 263.7 ± 5.4 Ma (STE41). STN79 shows N-MORB geochemical characteristics and has εHf(t) values of +12.4 to +14.6, whereas STE41 displays E-MORB geochemical characteristics with εHf(t) values of −2.6 to +0.4. The Sumdo mafic rocks were not contaminated by subducted sediments and/or slab-derived fluids, and formed in a mid-ocean ridge setting. Combining results of previous studies with our new geochemical data for mafic rocks in the Sumdo area, we infer the occurrence of plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean during the middle–late Permian. Mixing of mantle plume/hotspot melt and depleted asthenospheric mantle explains the origin of basaltic magma that formed rocks with the observed geochemical and isotopic characteristics in the Sumdo area.