The discovery of the amphibolite in the Kalamaili ophiolitic mélange formed in mid-oceanic ridge setting

The amphibolite, which is lenticular in shape and is wrapped in metamorphic peridotites, was discovered in the Kalamaili ophiolitic mélange belt, East Junggar, Xinjiang. LA-ICP-MS zircon U-Pb dating of the amphibolite yielded an age of (402.7±4.9) Ma, indicating that it was formed in Early Devonian. Petrographic, whole-rock chemical and Sr-Nd isotopic data suggest that the source rocks of the amphibolites were probably the meta-gabbro. The rock samples have SiO₂, MgO and Mg# values of 47.58%-50.57%, 8.04%-9.12% and 63.89-67.64, respectively. They also have Ni and Cr values of 68.9×10^-6-99.2×10^-6 and 306×10^-6-398×10^-6. All these geochemical data are similar to the average value of MORB. The REE of the rocks show weak depletion to flattening pattern. The features of the trace elements of the rocks are similar to those of the N-MORB, suggesting no obvious depletion of Nb, Ta, Zr, Hf and Ti. The samples have high ε_Nd (t) values varying from 8.7 to 9.2, suggesting that the original magma of the rocks was derived from the depleted mantle. On the TiO₂ versus FeO_T/MgO and Th versus Ta versus Hf/3 diagrams, all samples fall in the MORB field. Combined with the geological background, the authors hold that the amphibolite was probably formed in a midoceanic ridge setting and represented the lithosphere fragments of the dismembered ancient ocean basin, being a part of the Kalamaili ophiolitic mélange. This research reveals that the opening time of the Kalamaili ocean basin was the Early Devonian at least.