Roman Region Research Articles

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).

Crystal Chemistry and Site Configuration of the Clinopyroxene from Leucite-bearing Rocks and Related Genetic Significance: the Sabatini Lavas, Roman Region, Italy

Crystal Chemistry and Site Configuration of the Clinopyroxene from Leucite-bearing Rocks and Related Genetic Significance: the Sabatini Lavas, Roman Region, Italy

The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region

Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.

Ancient subcontinental mantle: A source for K-rich orogenic volcanics

Active island-arc volcanoes west of the arc/continent collision between the Sunda arc and northwest Australia are erupting mafic volcanics that range from calc-alkaline basalts through shoshonitic trachybasalts to leucitites. These mafic arc volcanics display a threefold enrichment in K, Rb, Sr, Ba, La, and Nb contents along the arc from Bali to Flores, increasing toward the collision zone. The enrichment is very well correlated with increasing 87Sr/86Sr and decreasing 143Nd/144Nd values. The geochemical and isotopic data imply that the arc volcanics are being generated from two source materials; one yielding material poor in K-group elements, with relatively low 87Sr/86Sr ( 0.707) values and low 143Nd/144Nd values. The K-rich material appearing in the arc volcanoes is being derived from ancient subcontinental mantle involved in the collision zone. Eastern Sunda K-rich mafic volcanism first appeared after the collision began. Before the collision, ancient northwest Australian mantle erupted K-rich, diamond-bearing ultramafics with high 87Sr/86Sr values and low 143Nd/144Nd values. These form part of a typical ultrapotassic continental volcanic association, yet they have Ba/La and Ba/Nb values characteristic of arc volcanism, showing their parent mantle to be geochemically and isotopically capable of supplying the K-rich material appearing in the eastern Sunda arc volcanoes. Terranes possibly underlain by similar “Gondwanaland” mantles surround the collision zone, which is the site of large-scale mixing between oceanic and continental lithospheres. Other petrogenetic theories seem incapable of accounting for the regional variations in eastern Sunda arc geochemistry. Striking geochemical and isotopic resemblances between eastern Sunda leucitites and K-rich orogenic volcanics elsewhere in the world, such as those of the Roman region, imply similar origins involving K-rich subcontinental mantle material. Where a “K-h” correlation occurs, it might only be a signal that subduction of a lithospheric slab beneath a continental margin is inducing melting in old mantle, deep in the continental lithosphere.

The white trachytic tuff of Roccamonfina Volcano (Roman Region, Italy)

The White Trachytic Tuff (WTT) is a compositionally-zoned, trachytic, pyroclastic-flow deposit which erupted from Roccamonfina volcano about 300,000 years ago. It was principally emplaced as unwelded, pumice-rich flow units with an estimated volume of 10 km3. These now cover the flanks of the volcano on all sides except the west, behind the highest rim of Roccamonfina's summit caldera; the caldera was probably in existence prior to the WTT activity. Eruption of the WTT generally initiated the leucite-free, second stage of Roccamonfina's development, following a long history of leucite-bearing volcanism, but minor leucite-bearing lavas and pyroclastics overlie the WTT as well. The WTT was in turn followed by progressively more basic, leucite-free magmas (latite, trachybasalt, and basalt). During the course of the eruption, the WTT evolved from white, crystal-poor pumices containing 66% SiO2 and 1.2% CaO, to grey pumices containing higher crystal contents, 60% SiO2, and 3% CaO. Early pumices are also relatively enriched in Mn, Na, Zn, Ga, Rb, Y, Zr, Nb, Cs, La, Ce, Yb, Lu, W, Hf, Th, and U, and depleted in Ti, Fe, Mg, K, Sc, V, Cr, Co, Sr, Ba, Nd, Sm, Eu, Tb, Dy, and Ta. The pumices are essentially bimodal in composition, with several minor intermediate types including megascopic, physical mixtures of the white and grey varieties. Certain WTT pumices, including all analyzed intra-caldera samples, are relatively enriched in Pb, Th, Zr, Rb, Ga, Zn, and Cs compared to the rest of the suite. These enrichments may reflect local assimilation of carbonates or more complex exchange processes at the magma chamber margin. All WTT pumices contain the phases sanidine, plagioclase, clinopyroxene, biotite, titanomagnetite, sphene, and apatite; grey varieties also contain magnesian olivine crystals which are probably xenocrysts. The white, crystal-poor types show relatively simple mineralogies with little compositional variability or zoning among crystals of a single phase. Other pumice types, and dark, trachyandesitic inclusions separated from white pumices, show a large compositional spectrum of individually homogeneous crystals. These compositionally diverse crystals and inclusions are interpreted as a result of widespread mixing between the trachytic magmas and more basic magmas prior to or during the WTT eruption. Major-element crystal-fractionation models can successfully derive the early trachytes from the late trachytes by 50–85% separation of a syenitic assemblage of all phases. The predicted phase proportions and compositions closely match cumulate syenite nodules found at Roccamonfina. Trace element models are permissive of syenitic fractionation within the large uncertainties allowed by published partition coefficients.

Significance of the pyroxene chemistry from leucite-bearing and related assemblages

The mineral chemistry of the pyroxene from representative suites of leucite-bearing and related assemblages occurring in south-west Uganda, Roman Region, Australia and Antarctica is considered in terms of some serial characteristics of the host rocks. The pyroxene compositions show distinct and systematic chemical variations, which are evaluated in terms of the ratios Ti/Al and (2−Si)/Al=Td, calculated from the structural formula. Both ratios generally increase from early- to late-crystallized pyroxene compositions but the variations are typical of individual rock series and depend on the level of alkalinity of the host rocks, i.e. (Na+K)/Al and the pressure under which the pyroxene crystallized. Low Ti/Al and Td values result largely from the influence of pressure, while Ti/Al>0.5 and Td>1.0 reflect the chemistry of the host rock, particulary in terms of high (Na+K)/Al ratio. Some experimental data bearing on the genetic interpretation of the pyroxene chemistry from the above assemblages are reappraised.

Geochemistry of micas from the Finero spinel-lherzolite, Italian Alps

The Finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the Ivrea Zone (Italian Alps) for its abundant coarse phlogopite. An average composition (SiO2 39.9, TiO2 0.97, Al2O3 16.0, Cr2O3 1.16, FeO 2.73, MgO 24.5, NiO 0.16, BaO 0.31, Na2O 0.58, K2O 8.7, Rb2O 0.056, Cl 0.03, F 0.10 wt.%) is similar in Fe, Cr, Ni, Ba and F/Cl to primary-textured micas from coarse garnet-lherzolite xenoliths from S. Africa, but is higher in Ti, Na, Rb, and Al, and lower in halogens. The distinct values of Ti and Fe for five specimens of Finero peridotites demonstrate local spatial variation. The overall ranges of TiO2 (0.5–1.7) and FeO (2.3–3.6) fall within the range for secondary-textured micas in peridotite xenoliths from S. Africa. The Finero micas are lower in both K/Rb and K/Ba than the primary and secondary micas from S. Africa, and their mean values of K/Rb (110–220) and K/Ba (15–39) are lower than for almost all bulk rocks, but fit well with the ranges of 109–180 and 12–49 for the high-K lavas of the Roman region.

Petrology of clinopyroxenite ejecta from Somma-Vesuvius and their genetic implications

A suite of clinopyroxenite nodules, megacrysis and associated lavas from Somma-Vesuvius, Italy, has been investigated to establish its possible genetic relationships with the leucitebearing lavas of the Roman Region. The clinopyroxenites are essentially composed of clinopyroxene + mica and subordinate olivine, plagioclase, spinels, apatite and glass. The megacrysts are clinopyroxene fragments. The associated lavas are leucite-tephrites and a tephritic leucitite.

Petrogenesis of leucite-bearing lavas in the Roman volcanic Region, Italy

The petrogenesis of leucite-bearing lavas from the Roman Region, Italy, is considered in the light of the petrochemistry and mineralogy of the Sabatini lavas, believed to be transitional between the mafic Alban Hills leucitites and the differentiated Vico and Vulsini suites.

Isotopic evidence for the derivation of some Roman region volcanics from anomalously enriched mantle

Strontium isotope data are reported for primitive lavas (leucitites, tephritic leucitites, K-rich basalts, and related types) from the Roccamonfina volcano. A strong positive correlation is found between 87Sr/86Sr and the abundances of K, Rb, Sr, Ba and Zr. It is argued that the present contents of these elements in the lavas are not far removed from their concentrations in their parental primary magmas. Models involving disequilibrium and equilibrium melting of respectively homogeneous and heterogeneous source rocks are discussed. It is concluded that a heterogeneous source highly enriched in incompatible elements at some stage in the past (? 300 m.y.) is indicated.

Petrogenesis of Potassium-rich Lavas from the Roccamonfina Volcano, Roman Region, Italy

Petrogenesis of Potassium-rich Lavas from the Roccamonfina Volcano, Roman Region, Italy

Latiumite (sulphatic potassium-calcium-aluminium silicate), a new mineral from Albano, Latium, Italy

The ejected blocks found in the 'peperino' of the Alban Hills have long attracted the attention of mineralogists on account of their variety in content of well-crystallized minerals. It may be recalled that this Roman region provides the earliest and finest examples of the white octahedral haiiyne, at first referred to a separate species (berzeline), and that this mineral with leueite, yellow garnet, wollastonite, green clino-pyroxene, and melilite form characteristic assemblages in the tufts from Albano, Frascati, and other places.