Abstract
We present data on the geochemical and Sr-Nd isotope compositions of rocks and on the Lu-Hf isotope composition of magmatic and xenogenic zircons from granitoids and gabbroids of the late Neoproterozoic island arc structure of the Lake Zone. Plagiogranitoids, gabbroids, and quartz diorites (559-542 Ma) formed at the late Neoproterozoic subduction stage of magmatism, and two-feld-spathic granites (similar to 483 Ma) mark Cambrian-Ordovician accretion-collision processes. We have established that the volcanic rocks of the late Neoproterozoic island arc and/or its oceanic base, which formed from the depleted mantle, were the mafic source of plagiogranitoids. This is proved by the overlapping positive epsilon(Nd) values of plagiogranitoids and the host volcanic rocks and by the commensurate epsilon(Nd) values of magmatic zircons from the plagiogranitoids and depleted mantle. The lower epsilon(Nd) values of gabbro and quartz diorites from the Tavan Hayrhan and Shuthuyn plutons, the lower epsilon(Nd) values of zircons from these rocks, and the high (Sr-87/Sr-86)(0) ratios and K2O, Rb, and Th contents point to the generation of these rocks from a less depleted mantle source, namely, mantle wedge peridotites. The isotope composition of the latter changed at the previous subduction stage under the impact of fluids and with the contribution of subducted sediments. The least radiogenic Hf isotope composition of magmatic and xenogenic zircons from Ordovician accretion-collisional two-feldspathic granites of the Ih Zamiin pluton suggests their formation through the melting of the late Neoproterozoic-Cambrian island arc crust with the contribution of more differentiated crustal sources enriched in Th, Nb, and LREE and characterized by low epsilon(Nd) values. The age of xenogenic zircons (<= 716 Ma) in the studied granitoids and gabbroids and their similarity in Hf isotope composition to magmatic zircons from the same rocks confirm the formation of the late Neoproterozoic island arc of the Lake Zone in an intraoceanic setting far from ancient continental sources similar to the Dzavhan microcontinent.
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