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We present results of geochemical and isotope (Rb--Sr, Sm--Nd, and Lu--Hf) studies of the early Paleozoic plagiogranitoid associations in the south of the Lake Zone in Western Mongolia, which formed at the island-arc and accretion--collision stages of the regional evolution. According to the petrogeochemical composition, the early Paleozoic plagiogranitoid associations of the island-arc (Tugrug, Hatan-Hunga, Udzur-Hunga, and Bayasgalant plutons, 531--517 Ma) and accretion--collision (Tugrug, Mandalt, and Dut Uul plutons, 504--481 Ma) stages are high- and low-alumina rocks. The recognized types of plagiogranitoids, with regard to their trace-element composition, indicate that their parental melts were generated from MORB-type metabasites at ≥10--12 kbar, in equilibrium with garnetcontaining restite, and at ≤8 kbar, in equilibrium with plagioclase-containing restite. The Sr--Nd isotope data on the rocks and the Lu--Hf isotope parameters of their magmatic zircons show two groups of plagiogranitoids, with different sources of melts. The first group includes plagiogranitoid associations of most plutons (Tugrug, Udzur-Hunga, Hatan-Hunga, Bayasgalant, and Dut Uul) with isotope parameters (εNd = 8.5--4.6, (87Sr/86Sr)0 = 0.7034--0.7036, and εHf = 14.7--11.9) indicating the juvenile nature of their sources. The second group includes plagiogranitoids of the Mandalt pluton; their isotope parameters (εNd = 1.4--0.2, (87Sr/86Sr)0 = 0.7053, and εHf = 7.2--5.4) indicate that the parental melts were generated mostly from enriched-mantle metabasites. The Hf isotope data on inherited and xenogenic zircons (664-519 Ma) from the early Paleozoic plagiogranitoid associations of the southern Lake Zone permit us to separate these rocks into three groups according to their εHf values (14.5-12.8, 2.9, and 10.-6.7). The Hf isotope parameters of magmatic and inherited zircons, with regard to their age, indicate that the source of the parental melts lacked rocks with a long crustal history, such as the early Precambrian associations of the Dzavhan microcontinent. [ABSTRACT FROM AUTHOR]