Origin and tectonic implications of boninite dikes in the Shiquanhe ophiolite, western Bangong Suture, Tibet

The Shiquanhe ophiolite, hosting newly discovered boninite dikes, is located in the western part of the Shiquanhe-Jiali ophiolite sub-belt (SJO sub-belt) in the Meso-Tethyan Bangong-Nujiang suture zone (BNSZ), Tibet. The boninite dike samples are plagioclase-free, have a porphyritic texture with pyroxene phenocrysts, and a microcrystalline groundmass. Characteristically they have high MgO (7.24-8.59 wt%), SiO2 (54.02-61.49 wt %), low TiO2 (0.15-0.24 wt%), CaO/Al2O3 (0.42-0.58) and are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). They display slightly concave spoon-shaped REE pattern and positive anomalies in Zr and Hf as well as negative Nb, Ta, and Ti anomalies which are similar to the typical low-Ca boninite in Cape Vogel, Papua New Guinea and elsewhere. These characteristics indicate that the Shiquanhe boninite originated from a depleted mantle source replenished by subducted sediments after the extraction of tholeiitic melts. The mantle source of the Shiquanhe boninites is harzburgite, and its melting degree is estimated to be approximately 20-35%. About 87.5% of melts from the depleted harzburgite mixed with about 12.5% of molten subducted sediments, forming the Shiquanhe boninite dikes in a forearc environment. This study on newly discovered boninite dikes further provides crucial evidence of island arc affinity of SJO in the Bangong Meso-Tethys. It gives another example of the relatively rare examples of forearc ophiolites in the Precambrian to Phanerozoic orogenic belts around the Earth.

Automated summary

The Shiquanhe boninite dikes, located in the Meso-Tethyan Bangong-Nujiang suture zone in Tibet, are characterized by high MgO and enrichment in light rare earth elements, indicating their origin from a depleted mantle source replenished by subducted sediments. These newly discovered boninite dikes provide crucial evidence of island arc affinity for the Shiquanhe ophiolite, showcasing their rarity in orogenic belts on Earth.