Post-collisional melting of crustal sources: constraints from geochronology, petrology and Sr, Nd isotope geochemistry of the Variscan Sichevita and Poniasca granitoid plutons (South Carpathians, Romania)

摘要

The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite diorite and geochemical analyses (major and trace elements, Sr-Nd isotopes) of representative rock types from the two intrusions grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U-Pb zircon data yielded 311 ±2 Ma for the intrusion of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline. Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks corresponding to several magma batches: the Poniasca biotite diorite (P_1) shows a clear crustal character while the Poniasca granite (P_2) is more juvenile. Conversely, Sichevita biotite diorite (S_1), and a granite (S_2~*) are more juvenile than the other Sichevita granites (S_2). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within P_1 and S_1 groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita-Po-niasca plutons took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago, especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous-early Permian times) being known in the Getic nappe. The geodynamical environment of Sichevita-Poniasca was typically post-collisional of the Variscan orogenic phase.