Geochemistry, Sm-Nd, Rb-Sr, and lu-Hf isotopes, sources, and conditions of formation of early Paleozoic plagiogranitoids in the south of the lake zone in Western Mongolia

We present results of geochemical and isotope (Rb-Sr, Stn-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 garnet-containing 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 (epsilon(Nd )= 8.5-4.6, (Sr-87/Sr-86), = 0.7034-0.7036, and epsilon(Hf) = 14.7-11.9) indicating the juvenile nature of their sources. The second group includes plagiogranitoids of the Mandalt pluton; their isotope parameters (epsilon(Nd) = 1.4-0.2, (Sr-87/(86) Sr)(0) = 0.7053, and e(Hf) = 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 epsilon(Hf) values (14.5-12.8, 2.9, and 10.6-6.7). 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.