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)

Abstract

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 (P₁) shows a clear crustal character while the Poniasca granite (P₂) is more juvenile. Conversely, Sichevita biotite diorite (S₁), and a granite (S₂*) are more juvenile than the other Sichevita granites (S₂). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within P₁ and S₁ 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–Poniasca 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.

Appendix: Methods

Zircon grains were hand selected and mounted in epoxy resin, together with chips of the TEMORA (Middledale Gabbroic Diorite, New South Wales, Australia, age= 417 Ma, Black et al. 2003) and 91500 (Geostandard zircon, age= 1,065 Ma, Wiedenbeck et al. 1995) reference zircons. The grains were sectioned approximately in half and polished. Each analysis consisted of five scans through the mass range; the spot diameter was about 18 μm and the primary beam intensity about 4 nA. The data were reduced in a manner similar to that described by Williams (1998) and references therein), using the SQUID Excel Macro of Ludwig (2000). The Pb/U ratios were normalised relative to a value of 0.0668 for the ²⁰⁶Pb/²³⁸U ratio of the TEMORA zircon, equivalent to an age of 416.75 Ma (Black and Kamo 2003). Uncertainties given for individual analyses (ratios and ages) in Table 1 are at the one σ level, whereas uncertainties in calculated concordia ages are reported at the 2σ level.

Whole-rock analyses were performed by XRF on an ARL 9400 XP spectrometer. The major elements were analysed on lithium tetra- and metaborate glass discs (FLUORE-X65®), with matrix corrections following the Traill-Lachance algorithm. Trace elements (Sr, Rb, Nb, Ni, Zn, and Cu) were measured on pressed pellets and corrected for matrix effects by Compton peak monitoring.

Selected samples were analysed for REE, Y, U, Th, Zr, Hf, Nb, Ba, Ta and Ga by ICP-MS on a VG Elemental Plasma Quad PQ2 after alkali fusion, following the method described in Vander Auwera et al. (1998).

Sr and Nd isotopic compositions were made at the Université Libre de Bruxelles on a Micromass GV Sector 54 multicollector mass spectrometer. The average ⁸⁷Sr/⁸⁶Sr ratio of the NBS SRM987 standard and ¹⁴³Nd/¹⁴⁴Nd ratio of the Rennes Nd standard during the period of analyses were 0.710271 ± 10 (2σ_m on 8 measurements) and 0.511971 ± 9 (on 12 measurements), respectively. Sample ratios have been standardised to a value of 0.710250 for NBS987 and to 0.511963 for the Merck standard (corresponding to a La Jolla value of 0.511858).