Valeria Caironi

Petrogenesis of contrasting hercynian granitoids from the Calabrian Arc, southern Italy

Abstract The granitoids of the southern Calabrian Arc have been investigated for whole-rock and phase chemistry, zircon typology, REE, Sr and Nd isotopes. Two distinct granitoid associations, which are related in time and partly in space, are present: a calc-alkaline one and a peraluminous one. The prevailing calc-alkaline association is compositionally expanded (SiO2=48–70%) and biotite dominated, with tonalites and granodiorites as predominant rock types. The peraluminous association is compositionally restricted (SiO2=67–76%) and contains two-mica ± Al-silicates. Distinct peraluminous typologies occur also as core facies within the calc-alkaline types. All granitoids are ilmenite-bearing. The Cittanova (CN), Villa S. Giovanni (VSG) and Capo Rasocolmo (CR) peraluminous granites display zircon typology, REE patterns, ϵSrt- (+51 to +113) and ϵNdt-values (−8.5 to −4.6) at 290 Ma, suggesting a dominantly quartzofeldspathic metasedimentary source. In more detail, the CN types and the CR-VSG types require heterogeneous and different sources. The calc-alkaline granitoids display very variable REE patterns (CeN=25–227 and YbN=3.5–18.5 in the tonalites-granodiorites) and variable age-corrected ϵNdt-values (−8.5 to −0.25), whereas Sr values vary little (+82 to +93). Thus, in terms of ϵNdtϵSrt covariation, the data points define a vertical array, which is inconsistent with a model involving crustal contamination by mantle derivatives. A more viable mechanism seems to be the melting of hydrous and heterogeneous mafic lower crust (and/or basic underplate), producing distinct magma batches evolving independently. Crustal contamination, mingling and fractionation processes may all have contributed to the observed geochemical variations within the granitoids. The peraluminous granodiorites occurring within the Serre and Capo Vaticano multipulse calc-alkaline plutons exhibit isotopic ratios (ϵNdt = −6.11 to +0.33 and ϵSrt = +93 to +97) which are similar to those of the calc-alkaline host rocks, suggesting a possible genetic link. Geologic and geochemical data indicate a continental collision setting for this plutonic activity. The magmas were produced during late-thickening to exhumation phases following collision and moderate crustal overthickening, and were emplaced subsequently during a short time span. A mantle contribution should have favoured melting, which affected various sectors of the continental crust.

Chemical variations of zircon compared with morphological evolution during magmatic crystallization an example from the Valle del Cervo Pluton (Western Alps)

This study correlates the main chemical variations of zircon with its morphological evolution within a suite of coeval rocks (monzonite, syenite and granite from the Tertiary Valle del Cervo Pluton, Western Alps), by coupling microprobe analyses and the study of crystal typology. Zircon concentrates from 8 different rock samples were first studied with the typology method; then, crystals for microprobe analysis (SiO 2 , ZrO 2 , HfO 2 , Y 2 O 3 , P 2 O 5 , ThO 2 , UO 2 ) were hand-picked from 4 selected rock samples. The typologic characters of the studied populations are typical of zircons from shoshonitic rocks. Monzonite and granite zircons are characterized by dominant {100} and {101} forms; the development of the {110} prism slightly increases in the granite samples. Microprobe data evidence that the bulk composition of the studied zircons corresponds to the composition of the main magmatic growth stage in zircons from calkalkaline and K-calkalkaline rocks from literature. All the zircons of the Valle del Cervo pluton exhibit large scale chemical zoning from the inner to the outer portion within single crystals, as well as chemical variations within each rock type. However, from the least to the most differentiated rock types the evolution trends are not continuous. The monzonite zircons mainly show HfO 2 enrichments; the syenite zircons do not show any particular trend, whereas the granite crystals show a late enrichment in UO 2 and, to a lesser extent, in Y 2 O 3 and ThO 2 . These different behaviours, together with typology data, support the hypothesis that syenite and granite are genetically linked, but they are not produced by fractionation of the same magma that generated the monzonite. The higher HfO 2 contents in the monzonite crystals with respect to the syenite and granite ones suggest a larger crustal contribution in the genesis of the monzonite, as suggested by the published isotopic data. Furthermore, in the monzonite crystals the wide variation range in HfO 2 contents and the occurrence of oscillatory zoning (observed in BSE images) could be due to changes in Hf concentration in the magma during zircon crystallization (assimilation or mixing processes). The different patterns of chemical variations in the studied crystals may be explained with the influence of many interacting factors: occurrence of other minerals, which compete with zircon for the same trace elements, and time of their crystallization with respect to zircon; chemical evolution of the magma (through FC, AFC or mixing processes) during zircon crystallization; local disequilibrium phenomena at the crystal surface during zircon growth. As a consequence, the influence of zircon fractionation on the geochemical evolution of magmas cannot be simply modelled by using an “ideal” zircon composition.

U–Pb zircon geochronology of volcanic deposits from the Permian basin of the Orobic Alps (Southern Alps, Lombardy): chronostratigraphic and geological implications

U–Pb zircon ages from volcanic rocks of Early Permian age (Southern Alps, Lombardy), associated with fault-controlled transtensional continental basins, were determined with the laser ablation (LA)-ICP-MS technique. Four samples were collected at the base and at the top of the up to 1000 m thick volcaniclastic unit of the Cabianca Volcanite. This unit pre-dates the development of a sedimentary succession that still contains, at different stratigraphic levels, volcanic intercalations. Age results from a tuff in the basal part of the unit constrain the onset of the volcanic activity to 280 ± 2.5 Ma. Ignimbritic samples from the upper part of the unit show a large scatter in the age distribution. This is interpreted as the occurrence of antecrystic and autocrystic zircons. The youngest autocrystic zircons ( c . 270 Ma) are thus interpreted as better constraining the eruption age, constraining the duration of the volcanic activity in the Orobic Basin to about 10 Ma. The new geochronological results compared with those of other Early Permian basins of the Southern Alps reveal important differences that may reflect (1) a real time-transgressive beginning and end of the volcanic activity or (2) the complex mixing of antecrystic and autocrystic zircon populations in the analysed samples.

Typology of detrital zircon as a key to unravelling provenance in rift siliciclastic sequences (Permo-Carboniferous of Spiti, N India)

AbstractDetrital zircon populations from Carboniferous to Permian sandstones from the Lozar Section of Spiti, northern India, were analyzed with the typology method in order to obtain complementary information on the source areas of the sediments. Zircon grains were subdivided into several groups and subgroups, according to degree of abrasion and morphological features.First appearance of detrital zircons with distinct typologic signature within successive stratigraphic intervals provided useful data about the tectono-magmatic evolution of the northern Indian margin during Late Paleozoic rifting of Gondwana and initial opening of Neotethys. The base of the studied sequence (Lower Carboniferous Thabo Fm.) is characterized by a largely cratonic provenance, seemingly from the Indian Shield to the South. In the Upper Carboniferous Chichong Fm., first occurrence of typical zircons from anatectic granites and increasing abundance of granitoid detritus suggest rapid uplift and unroofing of anatectic rocks of pro...

Sr, Nd isotope evidence for an enriched mantle component in the origins of the Hercynian gabbro-granite series of the “Serie dei Laghi” (Southern Alps, NW Italy)

In the “Serie dei Laghi” of the Southwestern Alps, large Permian granite plutons and mafic-intermediate stocks and dykes (the “Appinite suite”) were emplaced in a post-collision environment. The magmatic rocks of the Serie dei Laghi are metaluminous and exhibit petrographic and geochemical characters typical of medium-K, calc-alkaline series. Granites and leucocratic Appinites show LREE and LILE enriched patterns and negative Nb, P and Ti spikes typical of calcalkaline series. In the mafic Appinite samples, the above characteristics become progressively less pronounced, as their acidity decreases. The gabbro-noritic Appinites (Mg# = 67–75, Ni and Cr contents up to 163 ppm and 882 ppm, respectively) are the most primitive of the Permian magmatic rocks reported to date in the Southern Alps east of the Ivrea Verbano Zone. On the basis of their geochemical and isotope patterns, they can, with a reasonable degree of confidence, be considered mantle derivatives that underwent very little, if any, crustal contamination. Their overall geochemical and isotope characteristics (ϵNd = −0.06/-2.45, (87Sr/86Sr)280 = 0.7044–0.7072) suggest an enriched mantle as the source of the Serie dei Laghi magmatic series. The strong overall correlation of the whole rock series in the Nd and Sr covariation diagrams indicates the prominence of mantlecrust interactions in the evolution of the Permian plutonic rocks from the Serie dei Laghi. Moreover, the correlation between isotope ratios and SiO2 suggests that the magmas evolved in crustal chambers. The entire Permian intrusive series was generated through complex crust-mantle interaction mechanisms which began with a mantle-derived magma isotopically similar to gabbro-noritic Appinites. The following picture can thus be envisioned: a basaltic magma rising from the mantle resided in a magma chamber at the base of the crust and underwent combined assimilation and crystal fractionation (AFC); the amount of assimilated crust at this stage was about 10%. A late-orogenic transtensive regime allowed small magma batches to rise from the chamber directly to the surface without further crustal exchange (Appinites). Continued magma input, crustal assimilation, crystallization and tapping extended the duration of the magma chamber up until the post-orogenic stage. Crustal extension enabled the formation of shallower magma chambers in which large volumes of hybrid magmas (the Alzo-Roccapietra, Montorfano and Baveno-Mottarone plutons) resided and further assimilated crustal material (20–25%).