Granitoids of the Yugodzyr ore district (south-eastern Mongolia) and associated W-Mo mineralization

On the basis of complex geochronological studies, Late Paleozoic (282 +/- 6 Ma), Early Mesozoic (200 Ma), and Late Mesozoic (151-165 Ma) granitoids we re distinguished among granitoids of the Yugodzyr ore district. Ore-bearing Li-F granitoids, enriched in Li, Rb, F, Nb, Ta, Sn, and Hf and impoverishe d in Sr, Ba, Zr, and light REE, were formed twice (about 280 Ma and 150 Ma) . The age of Mo-W greisens of the Yugodzyr ore deposit is the same as the a ge of the parent granitoids. The Nd and Sr isotope composition of granitoid s of the Yugodzyr ore district testifies to the formation of their magmas a t the expense of the substrate of the Caledonian continental crust, the Sm/ Nd ratio of which is close to the average Sm/Nd ratio of the continental cr ust. The average Sm-Nd model age of this crust (950 Ma) is more ancient tha n the age of the real separation of the crust from the depleted mantle (600 Ma). This can be explained by the participation in processes of the crust formation not only of basites of the primary oceanic crust of the Caledonid es, but also of a more ancient component: pelites reflecting the average co mposition of erosion areas at margins of the Caledonian sea basins. Sources of magmas of the Late Paleozoic Li-F granitoids with elevated Sr-87/Sr-86 ratios were enriched in mica, which defined geochemical and isotope specifi c features of the granitoids. Younger granitoids were formed from sources m ore impoverished in mica. The Caledonian continental crust served as a sour ce of ores as well as of granites, synchronous with the ores. As compared t o the initial (parent) granites, the Late Paleozoic Li-F amazonite granites (the Yugodzyr massif) and Late Mesozoic ore-bearing granites (Yugodzyr Mo- W deposit) are characterized by an elevated (Sr-87/Sr-86)(0) ratio and appr oximate epsilon(Nd) Value. This is explained by a very high differentiation of the initial magmas, which led to the stratification of the magmatic cha mber with an approximately tenfold Rb/Sr ratio growth in the residual chamb er. Within no more than 5-7 years, the (Sr-87/Sr-86)(0) ratio in various zo nes of such a chamber could reach values really observed in the late differ entiates of the studied granites. Here, the Sm/Nd ratio only slightly chang es in such a chamber. The granitoids of the Yugodzyr ore district were form ed in an intraplate environment: the Late Paleozoic granitoids-in the proce sses of continental riftogenesis; the Late Mesozoic-at the expense of an ac tive mantle hot spot. For both granitoid types, asthenospheric diapirs serv ed as sources of an elevated heat flow in the Caledonian continental crust of the Ulanul block, a terrain in the South-Mongolian Hercinides. The crust itself was a source of granitoid magmas. The asthenospheric diapirs could generate a thermostatic system within a period of several thousand years, q uite sufficient for changing the (Sr-87/Sr-86)(0) ratio in differentiates o f granite magmas with considerably variable Rb/Sr ratios.