Miocene tearing of Himalayan lithospheric mantle: Evidence from mantle-derived silicocarbonatites from the Cona rift

The extensive production of Miocene high-to ultrahigh-K basaltic rocks and high-Sr/Y granites across southern Tibet might be related to mantle geodynamics, but there is a lack of direct petrological evidence for the exact mechanism involved in this igneous activity. This study reports a study of silicocarbonatite dikes that intrude sedimentary rocks of the Tethyan Himalayan Sequence, with the aim of elucidating the geodynamic mechanism that led to this magmatism. These dikes are composed mainly of ferro-and magnesio-dolomite (55-65 vol%) and quartz (25-30 vol%), with accessory minerals of anatase, titanite, chrome spinel, apatite, monazite, pyrite, and zircon. The carbon (delta 13C =-6.3%o to-6.0%o) and initial Nd [epsilon Nd(18.4 Ma) = +0.2 to +0.4] isotopic compo-sitions, as well as the relatively high whole-rock contents of Cr (218-956 ppm), Ni (84.1-974 ppm), and Co (28.1-96.3 ppm), indicate that these carbonatitic magmas were sourced from a mantle region. Sensitive high -resolution ion microprobe zircon U-Pb analysis results showed that these carbonatitic igneous rocks have inherited single-grain 206Pb/238U ages of 2394.2-28.5 Ma and a weighted mean 206Pb /238U age for the ten youngest zircon grains of 18.4 +/- 0.2 Ma, which represents the age of magmatic crystallization. Low-viscosity carbonatitic magma is inferred to have ascended rapidly, mixing with high-viscosity silicate melts at mid-dle-lower crustal levels, ultimately forming silicocarbonatitic (SiO2 = 30.17-37.67 wt%) dikes that intruded shales and sandstones of the Tethyan Himalayan Sequence. Tearing of subducted Indian lithosphere might have occurred beneath the eastern Tibet-Himalayan Orogen, allowing magma to upwell and generate linearly distributed Miocene igneous rocks along the N-S-trending Cona rift.