Bernard Platevoet

Aluminous subsolvus anorogenic granite genesis in the light of Nd isotopic heterogeneity

The problem of the genesis of anorogenic granites is far from being resolved, and this may result from their petrogenetic variability. Several kinds of A-type granites are indeed present in many anorogenic provinces: a peralkaline and an aluminous trend commonly occur, and the latter often comprises hypersolvus and subsolvus granites. As an example of the aluminous subsolvus group, we have studied major, trace elements and NdSr isotopes of the Tana granite (SW Corsica, France), along with the coeval mafic and intermediate rocks. Petrological and trace-element constraints suggest that the associated mafic rocks are not representative of the magmatic precursor of the granite. Rather, the intermediate rocks (monzonites) are more akin to the parental magma that yielded the granite through fractional crystallization. The aluminous subsolvus anorogenic granite and related monzonite are characterized by variable Nd isotopic compositions (ϵNdi from −4.7 to −1.3 and from −2 to −0.1, respectively), while Sr isotopes give good isochrons. It is argued that the Nd isotopic heterogeneity was not acquired at a late stage in the upper crust by contamination from partial crustal melts or F-rich late- to post-magmatic fluids. Similarly, the lack of correlation between trace-element concentrations and Nd isotopes in a single granitic body precludes a process of crustal contamination of mantle melts. Instead, it is inferred that these low and very heterogeneous ϵNdis were probably inherited from the magma source region, in the lower crust. The preservation of this heterogeneity during magma ascent suggests that only limited mechanical and chemical mixing occurred in magma chambers. It is more likely that separate batches of low-viscosity A-type magmas ascended individually, mainly through fracture propagation in an extensional regime. The possible derivation of granites from monzonites through fractional crystallization, together with the examination of the Corsican lower-crust composition, and already published dehydration melting experiments suggest that the aluminous subsolvus trend of A-type granites may be generated through the partial melting of metamorphosed, amphibole-bearing and K2O-rich mafic cumulates that have not undergone a previous partial melting event.

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

Post-collisional potassic-rich volcanism of Gölcük Volcano in the Isparta area of southwestern Turkey consists of two groups: (i) extracaldera lavas, corresponding mainly to Pliocene activity; and (ii) intracaldera lavas and pyroclastics (ignimbrite flows and ash/pumice fall deposits) formed during the Quaternary. Extracaldera volcanic rocks mainly comprise lamprophyre (minette), basaltic trachyandesite, trachyandesite, and trachyte. A close relationship exists between the silica content and phenocryst type in the extracaldera volcanics such that trachyte–trachyandesites with SiO2 < 57 wt% and basaltic trachyandesites are characterized mainly by mafic phenocryst phases (e.g. pyroxene, amphibole, biotite–phlogopite). These features suggest suppression of plagioclase crystallization under high H2O pressure conditions. Intracaldera volcanics are composed of tephriphonolitic dikes, remnants of lava flows and domes at the caldera rim, and a trachytic lava dome on the caldera floor. The Gölcük flows and pyroclastics are mainly characterized by strong incompatible element enrichment in large ion lithophile elements (LILEs; e.g. Cs, Ba, U, and Th) relative to K, Rb, and high-field strength elements (e.g. Nb, Ta, and Ti). We conclude that the Gölcük lavas were derived from a metasomatized lithospheric mantle source containing phlogopite–amphibole garnet peridotite; the latter resulted from metasomatism by a hydrous fluid phase related to subducted sediments and oceanic crust. The parental magma for the extracaldera volcanics was lamprophyric, and that for the intracaldera volcanics was basanitic. All the geological and geochemical data show that the alkaline Gölcük lavas display a gradual decrease in silica content with decreasing eruption age, indicating that, in the Isparta volcanic province, the asthenospheric melt component became more important over time. In the extracaldera volcanics, 87Sr/86Sr isotope ratios of the evolved trachyte–trachyandesites range between 0.70366 and 0.70504, whereas these ratios are lower in the less evolved basaltic trachyandesite and lamprophyres, varying in narrow ranges around 0.70365 and between 0.70374 and 0.70453, respectively. The 143Nd/144Nd values lie between 0.51264 and 0.51273 in trachyandesites, 0.51267 and 0.51273 in basaltic trachyandesites, and 0.51270 and 0.51274 in lamprophyres. In the intracaldera lavas, the 87Sr/86Sr isotope ratio is 0.70361 in the tephriphonolite and 0.70388 in the intracaldera trachytic lava dome. The 143Nd/144Nd isotope ratio is 0.51274 in the analysed tephriphonolitic flow and 0.51271 in the intracaldera trachytic lava dome; these values are higher than that of trachyte–trachyandesites of the extracaldera volcanics. The Sr–Pb isotopic signatures indicate that crustal contamination was significant for the evolved extracaldera volcanics, but was negligible for the intracaldera volcanics. The εNd values of the Gölcük volcanics range between 0 and 2.0. The low Sr isotope ratios and positive εNd values are characteristic features of a depleted mantle source. The isotopically depleted and incompatible enriched nature of the Gölcük lavas point to recent enrichment processes prior to partial melting of the mantle source. Conversely, their radiogenic lead isotope compositions (206Pb/204Pb = 19.19–19.54, 207Pb/204Pb = 15.64–15.67, 208Pb/204Pb = 39.12–39.49) indicate an enriched mantle source region.

The size-isotopic evolution connection among layered mafic intrusions: Clues from a Sr-Nd isotopic study of a small complex

Several theoretical and experimental works have focused on the processes occurring in continental mafic magma chambers. In contrast, systematic isotopic studies of natural remnants of these latter remain scarce, although they can give fundamental constraints for theoretical studies. This is especially true if different layered complex with contrasting characteristics (e.g., different size) are compared. For this reason, we present the results of a Sr-Nd isotopic profile across a small layered mafic intrusion of Permian age exposed near Fozzano (SW Corsica). In the main zone of the layered section, decreasing 87Sr/86Sri and increasing 143Nd/144Ndi are observed from less evolved (bottom) to more evolved (top) rocks. This peculiar pattern precludes assimilation and fractional crystallization (AFC) as a dominant mechanism in the petrogenesis of this body. Instead, we interpret this trend as reflecting the dilution of an early stage contaminated magma by several reinjections of fresh basalt in the chamber. In agreement with mineralogical and structural data, every cyclic unit is interpreted as a new magmatic input. On the basis of rough refill and fractional crystallization (RFC) calculations, the average volume for each reinjection is estimated to have been about 0.04 km3. The cumulative volume of these injections would amount to about 75% of the total volume of the layered complex. This implies that reinjections were accompanied by an important increase of the volume of the chamber or by magma withdrawal by surface eruptions. The RFC mechanism documented within this small layered body contrasts with the isotopic pattern observed between several intrusions at the regional scale in SW Corsica, and within large continental mafic magma chambers elsewhere. In these cases the isotopic evolution is dominated by AFC processes, and there is no clear isotopic evidence for reinjections, unless major influx of fresh magma occurred. It is suggested that there is a close relationship between the assimilation rate and the magma chamber volume. Small magma chambers are quickly isolated from their country rocks and better preserve the subtle isotopic signature of reinjection processes.

Unusual Ti and Zr aegirine-augite and potassic magnesio-arfvedsonite in the peralkaline potassic oversaturated Buhovo-Seslavtzi complex, Bulgaria

In Stara planina Mts., Bulgaria the Buhovo-Seslavtzi pluton is built up of potassic monzonites and quartzsyenites, which contain calcic pyroxenes and amphiboles, plagioclase and potassium feldspar. The plutonic rocks are accompanied by dykes of different composition: lamprophyres, syenite-aplites, syenite-porphyries, bostonite-porphyries and peralkaline syenite and granite porphyries. In the dykes of bostonite porphyries and peralkaline syenite and granite porphyries calcic pyroxenes and amphiboles are replaced by alkaline mafic phases: aegirine-augites, sodic-calcic and sodic amphiboles. Aegirine-augites are Ti-rich up to 5.95 wt. % TiO 2 and Zr-rich up to 0.83 wt. % ZrO 2 . Ti and Zr present antipathetic relationships in aegirine-augite, as their repartition depends on the agpaicity. Ti enters pyroxene structure under strongly agpaitic conditions, whereas Zr enters aegirine-augite in mildly agpaitic conditions. Composition of sodic-calcic and sodic amphiboles evolves through ferrowinchite, ferrorichterite, richterite toward magnesio-arfvedsonite. Evolution ends with crystallization of potassic magnesio-arfvedsonite, which contains up to 4.67 wt. % K 2 O, as K totally fills [A] site. Pyroxenes and amphiboles show persistent magnesian character during the evolution toward sodic-calcic and sodic varieties. This trend is not common for silica-oversaturated rocks, and indicates high fO 2 conditions and crystallization between NNO and HM buffers. The high activities of alkalis and silica are considered as the most important factors leading to the formation of these peculiar aegirine-augite and potassic magnesio-arfvedsonite.

In situ differentiation and evolution of potassic syenites from Svidnya, Bulgaria

Potassic syenites from Svidnya, Bulgaria crop out as small isolated bodies as the primary for this intrusion liquid has basic to intermediate composition. The evolution in a closed magma chamber created plutonic rocks ranging from basic (melasyenite) to acid (granite) and from metaluminous to peralkaline. The most mafic varieties show cumulative textures typical for orthocumulates with cumulus phases clinopyroxene, biotite, apatite and potassium feldspar as gravitational settling is a viable process for separation of particles in the bottom parts of magma chamber. In the middle stratigraphic level of biggest body modal igneous layering with development of dark (clinopyroxene + amphibole) and light (potassium feldspar) laminas was observed. Oscillatory crystallization around eutectic point resulted in cyclic separation of mafic and felsic phases in repetitive layers. Fractionation of Ca- and Al-rich phases—clinopyroxene, biotie and potassium feldspar created peralkaline residual liquid strongly enriched in HFS elements.