Geochronology and geochemistry of the Hatu ophiolite in the East Kunlun Orogenic Belt, northern Tibetan Plateau: Constraints on the Proto-Tethys Ocean

The Early Paleozoic ophiolites exposed within the Aqikekulehu-Kunzhong ophiolitic mélange zone of the East Kunlun Orogenic Belt (E-KOB) in the northern Tibetan Plateau offer vital clues regarding the tectonic evolution of Proto-Tethys Ocean and subduction-accretionary process of the E-KOB. The Hatu ophiolitic mélange, contains a series of ophiolitic blocks and matrix (meta-) clastic rocks. Detailed field investigations reveal that the ophiolitic blocks, termed as the Hatu ophiolite, consist of gabbro, diabase and basalt. All of the mafic rocks are featured by low total REE (22.61–45.92 ppm), the flat chondrite-normalized REE patterns, slight LREEs enrichment with (La/Yb)_N = 1.07–1.28. The positive zircon ε_Hf(t) values of the gabbro and basalt range from + 11.72 to + 14.29 and from + 4.10 to + 10.93, respectively. Geochemical data indicates that all mafic rocks exhibit normal mid-ocean ridge basalt (N-MORB) properties, with their parental magmas originating from a depleted mantle source. Zircon U-Pb dating results show that two gabbro samples yield consistent weighted mean ages of 421 ± 2.9 Ma and 426 ± 4.9 Ma, while the weighted mean age of basalt is 426 ± 5.9 Ma, indicating the magmatic component of the ophiolite was formed at 426–421 Ma. The comprehensive evidence, encompassing integrated geological, geochemical and chronological data, suggests that the gabbro, diabase and basalt blocks of Hatu ophiolite are most likely the Early Paleozoic oceanic fragments having N-MOR affinities. The new evidence indicates that the Proto-Tethys Ocean in the E-KOB experienced mid-ocean ridge spreading during the Silurian.