Tonalites and plagiogranites of the Char suture-shear zone in East Kazakhstan: Implications for the Kazakhstan-Siberia collision

The paper presents first U–Pb zircon ages and geochemical data from Carboniferous granitoids (tonalites and plagiogranites) of the Char suture–shear zone in East Kazakhstan, which is located in the north-western Central Asian Orogenic Belt (CAOB). The study included analysis of geological setting, major and trace elements, and rock petrogenesis. The Char tonalites and plagiogranites occur as NW-striking linear chains inside Visean serpentinite mélange. Petrographycally, the tonalites show signs of syntectonic deformation, and the plagiogranites are less deformed suggesting their later intrusion. The tonalites yielded a LA-ICP-MS zircon age of ca. 323 Ma, i.e. exactly at the boundary between the early and late Carboniferous. Compositionally, the tonalites and plagiogranites are characterized, respectively, by high SiO2 (67–70 and 73–74 wt.%) and Al2O3 (17–19 and 14–15 wt.%), Sr/Y > 40 and low Yb = 0.2–0.5 ppm. Their multi-element patterns show clear Nb-Ta negative anomalies. The low Nb/Ta ratios (7–15) and Zr (114–191 ppm) suggest a MORB-type protolith (amphibolite) with subchondritic Nb/Ta (8–17) and low Zr (1–72 ppm). The low contents of K and Rb suggest weak assimilation of the melts by island arc felsic crust. The subchondritic Nb/Ta ratios exclude their derivation by the melting of subducted/dehydrated MORB. We argue that the Char high-Al tonalites and plagiogranites formed by the melting of hydrated MORB at the base of the mafic lower crust at pressures of 10–15 kbar. The occurrences of the Char tonalites and plagiogranites inside the Visean serpentinite mélange overlapped by Serpukhovian conglomerates, their alignment parallel to deformation zones, and their geochemical features suggest their origin by the melting of mafic lower crust in relation to the collision of the Siberian and Kazakhstan continents.