Shoshonitic liquid line of descent from diorite to granite: The Late Precambrian post-collisional Tismana pluton (South Carpathians, Romania)

The post-collision late-kinematic Tismana pluton belongs to the shoshonitic series. It is part of a Late Precambrian basement within the Alpine Danubi an nappes of the South Carpathians (Romania). This pluton displays an excep tionally complete range of compositions from ultramafic to felsic rocks (gr anites). Widespread mingling/mixing relationships at all scales give rise t o a variety of facies. A liquid line of descent from the diorites to the gr anites is reconstructed by considering the variation in major and trace ele ments (REE, Sr, Rb, Ba, Nb, Zr, Hf, Zn, V, Co, Cr, U, Th, Ga, Pb) from 33 s elected samples as well as mineral/melt equilibrium relationships. The firs t step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock foun d in the massif. A possible contamination by lower crustal mafic component takes place at this stage. The second step marks the appearance of apatite and Fe-Ti oxide minerals as liquidus phases, and the third step, saturation of zircon. Mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating (endo-h ybridisation). As depicted by Nd and Sr isotopes, fractional crystallizatio n was combined to an important early contamination by a mafic lower crust i n a deep-seated magma chamber and to a later and mild contamination by a fe lsic medium crust in an intermediate chamber. The mingling essentially occu rred during the final emplacement in the high-level magma chamber. The monz odioritic parent magma, identified by major and trace element modelling, is shown by Sr and Nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it, The necessarily recent enrichment in K2O and associated elements of the Lithospheric mantle is lik ely to be related to the preceding Pan-African subduction period. The parti al melting of this newly formed deep source has to be linked to a major cha nge in the thermal state of the plate. (C) 1998 Elsevier Science B.V. All r ights reserved.