U-Pb isotopic age, composition, and sources of the plagiogranites of the Kalba range, Eastern Kazakhstan

Abstract

Of special importance in the evolution of the Paleoasian ocean is the Carboniferous‐Permian stage, which reflects the closure of the oceanic basin and formation of the structure of the Central Asian fold belt (CAFB) in its present-day form [1]. This is also the time (320‐280 Ma) when the largest Angara‐Vitim granitoid batholith and conjugate synplutonic basite dikes in Transbaikalia, granitoid intrusions and bimodal basalt‐rhyolite series of the Gobi‐Tien Shan zone of South Mongolia, and the Kalba‐Narym and ZharmaSaur batholithic bodies and preceding subalkaline gabbroids, picritoids, high-Al plagiogranitoids, and paleovolcanic structures of the central type in Eastern Kazakhstan were formed. Unlike the Permian‐Triassic stage, which was proved to be related to the Siberian superplume [2], the geodynamic nature of the Carboniferous‐Permian magmatism of the CAFB remains controversial. The plume model was proposed for the Angara‐Vitim batholith [3], and break-off of lithospheric plates in the collision zone between the Kazakhstan and Siberian paleocontinents was suggested for the batholithic belts of Eastern Kazakhstan [4]. These models believe that the collision-related mantle was supplied either with autonomous plumes or with asthenospheric fingers produced by mantle extinction at the divergent margins of lithospheric plates. The solution of this problem consists in studying key magmatic complexes (composition, structure, age), with a special emphasis on the sources of magmatic melts. One such key complex is the plagiogranite of the Kalba Range traditionally distinguished in the Kunush Complex. One group of researchers suggested an Early Carboniferous age and island arc affinity [5], while others consider them as collision-related Late Carboniferous rocks [6, 7], or as products of the Tarim plume [8]. The analysis of the tectonic position of the magmatic complexes of Great Altai (according to N.A. Eliseev) and their correlation with sources [8, 9] sheds no light on this controversary. This article is aimed at accurate dating of the plagiogranites of the Kalba‐Narym zone for refining their position in the magmatic scheme, as well as for estimating the formation conditions and sources of plagiogranite magmas based on their composition and geochemical modeling of the protolith‐melt system. We studied the plagiogranites from the Zhilandy and Tochka massifs (Fig. 1). The Zhilandy Massif forms an isometric body, with medium-grained biotite granites in the central part and fine-grained porphyritic varieties and plagiogranite porphyry dikes in the marginal parts. The plagiogranites consist of large biotite, short-prismatic zoned andesine‐oligoclase, and single grains of orthoclase‐microperthite and green hornblende. The Tochka Massif is made up of NW-extended minor bodies and dikes of fine-grained porphyritic biotite plagiogranites and plagiogranite porphyries. The plagiogranites of the Tochka Massif differ macroscopically from the Zhilandy rocks owing to superimposed processes of mylonitization; however, microscopically, they have similar petrographic and mineral composition. The dikes of plagiogranite porphyries of both the massifs contain phenocrysts of short-prismatic zoned plagioclase, quartz, and biotite embedded in the microcrystalline groundmass. In both cases, the plagiogranites intrude black shales of the Takyr Formation ( D 3 –C 1 ) and are cut by the Early Permian granitoids of the Kalba batholithic belt [5‐7, 10].