LATE MESOZOIC MAGMATISM IN THE DZHIDA SECTOR OF THE WESTERN TRANSBAIKALIA RIFT ZONE: EVOLUTIONARY STAGES, ASSOCIATIONS, AND SOURCES

Abstract

Magmatic activity occurred in the Dzhida sector of the Western Transbaikalia rift zone from the Late Jurassic throughout the Early Cretaceous. The time interval of 159-100 Ma was marked by at least six pulses of magmatic activity, which are recognized as stages, each of which was responsible for a certain type of magmatic associations with its distinct compositions of magmatic rocks. The magmatic products are dominated by mafic rocks of elevated alkalinity with subordinate amounts of phonotephrite, subalkaline-alkaline trachyte, nepheline syenite, trachyhryodacite, trachyrhyolite, and comendite. The composition of the volcanic rocks shows systematic variations with time. For example, sialic rocks were produced during early stages and, along with mafic volcanics, compose bimodal associations. This style of magmatism took place in the Late Jurassic (trachybasalts and trachyte-trachydacites of the Ichetui Formation), late in the Late Jurassic-early in the Early Cretaceous (contrasting basalt-trachydacite association with comendites), early in the Early Cretaceous (rare occurrences of trachyrhyolites and their lava breccia in association with subalkaline olivine basalts), and in the mid-Early Cretaceous (syenite-nepheline syenite association). The rock associations produced late in the Early Cretaceous and early in the Late Cretaceous contained no sialic derivatives. Simultaneously, the volumes of magmatic products decreased, the basalts became progressively more alkaline, and the rocks were depleted in rare lithophile elements. The variations in the rock compositions with time are thought to have been caused by changes in the composition of their mantle sources from the enriched mantle (EMII) toward the depleted mantle (DM).