Titanomagnetites and ilmenites from the Early Cenozoic basalts and limburgites of the Northern Tien Shan

Abstract

Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO2/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.