Trace elements in the black Pl bearing xenoliths in Quaternary basalts from Baikal rift. Implications to the origin of granites

It is suggested that in the genesis of the large volumes of the granites of Angara-Vitim Batholiths and similar huge granite massifs the important role play the plume magmas which were the major supply of the heart. But they also could directly participate in the magma generation providing volatiles and alkalis and other components. But of course the major melting components were granulites and lower crust gneisses.

We studied major elements and the geochemistry of the Pl- bearing pyroxenites captured by the Quaternary alkaline basalts from Baikal rift – using EPMA, SEM and LAM ICP methods from Vitim plateau, Tunka valley (Karierny volcano) (Ashchepkov et al., 2024) mainly compositions oof amphiboles, Pyroxenes, plagioclases and K-Fspars as well as micas and more rare garnets.

 Studied amphiboles mainly reveal  inclined spectra (La/Yb_n ~5-10) of pyroxenes and higher for amphiboles (~ 15 ) having higher REE and elevated LILE. Single – grain The Pls have flatter REE patterns with the peaks in Ba the K- alkali feldspars have lower REE levels and higher Ba, LILE Sr peaks. Much lower REE and higher Ba, alkalis reveal micas.

Less inclined spectra of minerals from Karierny volcano 13.5 Ma indicate a less deep of origin of the parent melts.

Thermobarometry of Vitim pyroxenites gives 5-12 kbar and for amphiboles, 8-5 kbar is somewhat shallower. In Tunka valley 10-4 kbar and 7-4 in amphiboles. This corresponds to the lower thickness of the crust in Tunka and lower garnet influence.

 

On the whole, amphiboles give closer TRE spectra to alkaline granitoids. But all of them have a minimum Pb, which indicates fractionation processes, and granitoids (Tsygankov et al., 2014-2017), on the contrary, have a Pb peak, which indicates partial melting. In addition, in syenites, the concave part of the HREE spectrum indicates low-temperature garnet in restites.

the Vitim Plateau imply the participation of garnet in the processes of partial melting.

 

The genesis of the alkaline granites and syenites (Tsygankov et al., 2014) suggest the fractional and possibly disequilibrium melting of the K -FSp and micas and partly butch melting of Ampx-Pl of the granulite and gneisses possibly in different levels as well the direct infiltration of alkaline basalt are also suggested. These magmas are more typical for the regions with the thicker crust. The major factor was the plume melts impact.

For the Ca- alkaline granitoids the batch melting is more realistic model with the higher participation of the Pl and amphiboles in magma genesis. And they corresponds to the central parts of the Transbaikal with the relatively thinner crust. These means also the more higher melting of crust material  under plume influence. Supported by RNF grant 23-17-00030