Late Paleozoic to Early Mesozoic Evolution of Neo-Tethys: Geochemical Evidence from Early Triassic Mafic Intrusive Rocks in the Tethyan Himalaya

Abstract

Here we report geochronological, geochemical, and Sr-Nd-Hf isotopic data of a basic intrusion exposed in the Jiacha area within the Tethyan Himalaya, southern Tibet. The Jiacha dikes are composed mainly of mafic rocks (gabbros). In zircon U-Pb dating of Jiacha dikes samples, the weighted mean ages of magma emplacement were determined to be 241.5±4.2 to 245.5±3.3 Ma, showing a Middle Triassic magmatic event in the eastern part of the Tethyan Himalaya. The Jiacha dikes have relatively low MgO and total alkali (K2O+Na2O) contents but high TiO2 contents, exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in light REEs and no obvious Eu anomalies, and show enrichment in high field strength elements and depletion in large-ion lithophile elements. Their initial 87Sr/86Sr ratios vary in the range 0.704250–0.704321, with a positive εNd(t = 243 Ma) of +3.07 to +3.67, and their initial 176Hf/177Hf ratios vary in the range 0.282872–0.283044, with a positive εHf(t = 243 Ma) of +8.53 to +14.45. We propose that the Jiacha dikes were derived from spinel lherzolites in the mantle with no crustal contamination and underwent fractional crystallization of clinopyroxene. Considering previous studies, we propose that Jiacha dikes represent oceanic crustal products of the early evolution of the Neo-Tethys, indicating that the ocean had already begun to open in the Middle Triassic.