Metamorphism and K-Ar white mica ages of pelitic schists in the Makbal Complex, Kyrgyz Northern Tien-Shan

Abstract

Garnet-free pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244) of the Neldy Formation in the Makbal HP-UHP Metamorphic Complex, Kyrgyz Northern Tien-Shan are petrologically and geochronologically described. The pelitic schist (KG1251) consists mainly of phengite, chlorite and quartz, with small amounts of albite, titanite, calcite, rutile and carbonaceous matter. The peak metamorphic conditions are roughly constrained as T <630 °C and P = 0.9–1.7 GPa. A K-Ar age for schistosity-forming phengite is obtained as 524 ± 13 Ma, and it is almost identical to the previously reported peak metamorphic ages (ca. 500 Ma) of eclogites and garnet-chloritoid-talc schists in the Makbal Complex. The pelitic schist (KG1244) consists mainly of white mica (phengite-core and muscovite-rim), chlorite and quartz, with minor amounts of garnet, chloritoid, albite, tourmaline, zircon, monazite, titanite, rutile, calcite and carbonaceous matter. The peak metamorphic conditions are estimated as T = 485–545 °C and P = 1.2–1.5 GPa (high-P/T metamorphism), followed by a low-P/T metamorphism of T = ca. 500 °C and P >0.3 GPa, which is probably caused by contact metamorphism of granitic intrusions. The K-Ar white mica age of 474 ± 12 Ma obtained by the present study is similar to the ages of the Ordovician granitic intrusions. The peak metamorphic conditions of the Neldy pelitic schists (KG1251 and KG1244) are significantly lower in pressure than those of the previously reported eclogites and garnet-chloritoid-talc schists located in the lower tectono-structural levels of the Makbal Complex.