Ophiolite Tectonics,Rock Magnetism and Palaeomagnetism,Cyprus

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron–titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E–W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7–15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young (<12 Ma) remanences probably acquired (or re-acquired) during uplift of the Troodos terrane. In the cover of pre-Messinian deformed limestone (>8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in present-day coordinates, Troodos rocks moved ~1,000 km South; then ~4,500 km East and finally ~900 km North at an approximate rate of 75 km/Ma [1 km/Ma = 1 mm/a]. This true motion path commenced ~88 Ma ago and rates of motion since 65 Ma may be too high due to the limited precision of strain-corrections of the ChRM orientations in limestone. This true motion path is compatible with the eastward and then northward rotation of Africa relative to Europe although other workers show relative motion paths.     

Extreme tectonic rotations within an eastern Mediterranean ophiolite (Baër-Bassit, Syria)

Palaeomagnetic results from 27 sites at five localities within the dismembered Baër-Bassit ophiolite of northern Syria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of the Arabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within one of these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressive increase in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation (exceeding 200°). Although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differential thrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodos ophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partially rotated Baër-Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the Late Tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slip fault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed in the study are therefore of composite origin, and reflect the complex development of structural fabrics within the ophiolite.     

Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO₂ content and a deeper lava sequence with high TiO₂ content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava-sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO₂ content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.     

Chert–clastic sequence in the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan

Ocean plate stratigraphy (OPS) within an ancient accretionary complex provides important information for understanding the history of an oceanic plate from its origin at a mid‐ocean ridge to its subduction at a trench. Here, we report a recently discovered chert–clastic sequence (CCS) that comprises a continuous succession from pelagic sediments to terrigenous clastics and which constitutes part of the OPS in the Akataki Complex within the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan. As well as describing this sequence, we present U–Pb ages of detrital zircons from terrigenous clastic rocks in the CCS, results for which show that the youngest single grain and youngest cluster ages belong to the Santonian–Campanian and are younger than the radiolarian age from the underlying pelagic sedimentary rock (late Albian–Cenomanian). Thus, the CCS records the movement history of the oceanic plate from pelagic sedimentation (until the late Albian–Cenomanian) to a terrigenous sediment supply (Santonian–Campanian).     

Oxygen and hydrogen isotope studies of serpentinization in the Troodos ophiolite complex,Cyprus

Oxygen and hydrogen isotopic compositions were measured on 12 serpentine and 2 actinolite samples from the Troodos ophiolite complex, Cyprus. The single analyzed antigorite(δD= ?60, δ¹⁸O= 7.1) is isotopically similar to all previously analyzed antigorites from ultramafic bodies. However, although their D/H ratios are relatively “normal”(δD= ?70to?92), the δ¹⁸O values of most of the Troodos lizardite-chrysotile serpentines (+12.6 to +14.1) are much higher than the 2.0–9.3‰ range typically found in such serpentines. Such high δ¹⁸O values have previously been found only in the serpentine-like mineraloids termed “deweylites”, which apparently formed at Earth-surface temperatures, and in a single sample from Vourinos, Greece that is in contact with high-¹⁸O limestone. The Troodos lizardite-chrysotile samplescannot have formed by reaction with heated ocean waters, but instead must have formed in contact with large amounts of some type of meteoric, metamorphic, or formation water, either (1) at very low temperatures in a near-surface environment, or (2) at about 100°C from waters that were abnormally enriched in¹⁸O(δ¹⁸O ≈ +4 to +8). The latter possibility seems most plausible inasmuch as extensive evaporites were deposited throughout the Mediterranean Sea during the late Miocene, and this would have been accompanied by strong¹⁸O enrichments of the local meteoric waters. Heated ocean waters, however, probably were responsible for the formation of the actinolitic amphiboles(δ¹⁸O= 4.6 to 5.5; δD= ?51to?46) from the gabbro and ultramafic zones in the Troodos complex. The amphiboles must have formed at considerably higher temperatures and at an earlier stage than the lizardite-chrysotile serpentinization.     

The palaeorotation of the troodos microplate,cyprus, in the late mesozoic-early cenozoic plate tectonic framework of the Eastern Mediterranean

The Troodos ophiolite formed during the Late Cretaceous (Turonian) in a spreading setting above a northerly-dipping subduction zone. Palaeomagnetic data establish that the ophiolite experienced a 90° anticlockwise palaeorotation that occurred during the Late Cretaceous-Early Eocene interval During this period, the ophiolite was stranded adjacent to a generally south-facing continental margin, the remnants of which are preserved in south-west Cyprus (Mamonia Complex), southern Cyprus (Moni melange) and in northern Cyprus (Kyrenia Range). A review of field evidence from these Mesozoic basin margin units shows that strike-slip played a critical role in their Late Cretaceous-Early Tertiary deformation while magnetic and gravity surveys support the existence of major lineaments preserved at depth beneath these areas. Thus, in agreement with available palaeomagnetic data from mainland Turkey and Africa, the rotated crustal unit is believed to be relatively small with its boundaries probably located in the vicinity of Cyprus. In the favoured palaeorotation model, a small supra-subduction zone oceanic crustal unit was rotated about a pole located close to the exposed ophiolite complex. Fragments of the northern continental margin became attached to the rotating microplate along strike-slip lineaments and were then carried southward to their present position. In the light of the regional tectonic setting, it was probably the oblique consumption of crust beneath the ophiolite and/or a collision outside the present area of Cyprus that provided the necessary driving force for the palaeorotation.     

Chilling statistics indicate an ocean-floor spreading origin for the Troodos complex,Cyprus

Different models for the generation of ophiolite complexes lead to differing predictions of the nature, extent and consistency of one way chilling (see text) of dykes in the sheeted unit of such complexes. Measurements of the degree of one way chilling were made on a number of transects of the Diabase (sheeted) unit of the Troodos complex. Statistical analysis of the results strongly favours an ocean-floor spreading model over the other models considered for the generation of the complex, with the spreading axis lying to the west of the complex in its present orientation. In addition, the analysis shows that the stratigraphically central portion of the sheeted unit must be composed entirely of dykes. This method can be used to determine the origin of other ophiolite complexes that have sheeted dyke units.     

The structure of the Campanian Plain and the activity of the Neapolitan volcanoes (Italy)

The central Campanian Plain is dominated by the structural depression of Acerra whose origin is tectonic, but may have been enlarged and further depressed after the eruption of the Campanian Ignimbrite (42-25 ka). The deposits of the Campanian Ignimbrite are possibly the results of multiple eruptions with huge pyroclastic deposits that covered all the Campanian Plain.The more recent activity of Vesuvius, Campi Flegrei and Procida occurred on the borders of Acerra depression and resulted from a reactivation of regional faults after the Campanian Ignimbrite cycle. The activity of Vesuvius produced the building of a stratovolcano mostly by effusive and plinian explosive eruptions. The Campi Flegrei area, on the contrary, was dominated by the eruption of the Neapolitan Yellow Tuff at 12 ka that produced a caldera collapse of the Gulf of Pozzuoli. The caldera formation controlled the emplacement of the recent activity of Campi Flegrei and the new volcanoes were formed only within the caldera or along its rim.     

Geology, magmatic affinity and geotectonic environment of some Caledonian ophiolites in Norway

Ophiolites within the Norwegian Caledonides were generated during at least two distinct periods, i.e. Tremadocian-Arenigian and Ashgillian. The older generation show a long-lived magmatic development (ca 500-470 Ma) with rocks that range from MORB, IAT, boninites, calc-alkaline to alkaline basalt in geochemical affinity. This development is compatible with generation in a subduction-influenced environment, and a plausible modern equivalent may be the ensimatic arc-basin evolution in the western Pacific.The metabasalts of the younger (Ashgill, 443±3 Ma) ophiolite occurrence are characterized by a dominance of N- to E-MORB compositions with subduction zone influence. The volvanics of the complex are intercalated with, and overlain by, continentally-derived sediments. Calc-alkaline as well as alkaline lavas and volcanic-lastics apparantly define late magmatic products. Associated with the youngest ophiolite is a sedimentary melange with olistoliths of mature island arc-derived volcanics, as well as clasts with MORB-IAT affinity, chert and sandstone, the latter probably derived from an accretionary prism. An Andaman Sea model appears most applicable to the youngest ophiolite/associated sediments assemblage.Contemporaneously with the renewed, Ashgillian spreading episode, widespread deposition of clastic sequences of Ashgillian age took place unconformably on the Lower Ordovician ophiolite complexes in fault controlled basins.     

U–Pb zircon geochronology of the Daraban leucogranite,Mawat ophiolite,Northeastern Iraq: A record of the subduction to collision history for the Arabia–Eurasia plates

The Mawat ophiolite is part of the Mesozoic Neo‐Tethyan ophiolite belt of the Middle East and is located in the Zagros Imbricate Zone of Iraq. It represents fossil fragments of the Neo‐Tethyan oceanic lithosphere within the Alpine collisional system between the Arabian and Eurasia Plates. The first U–Pb zircon dating of the Daraban leucogranite from the Mawat ophiolite provides a ²⁰⁷Pb–²⁰⁶Pb age of 96.8 ± 6.0 Ma. The age is 59.0 ± 6.0 m.y. older than the previously published age of the Daraban leucogranite obtained by ⁴⁰Ar–³⁹Ar muscovite dating method. The U–Pb dating of magmatic zircons collected from the Daraban leucogranite, which intrudes into the Mawat ophiolite, reveals that melting of the pelagic sediment beneath the hot Zagros proto‐ophiolite in an intra‐oceanic arc environment led to anatexis at the subduction front and the generation of granitic melts at 96.8 ± 6.0 Ma, which were emplaced in the overlaying mantle wedge. This process was a response to the initial formation of the Neo‐Tethys ophiolite above a northeast‐dipping intra‐oceanic subduction zone at 96.8 ± 6.0 Ma. Published ⁴⁰Ar–³⁹Ar muscovite dating from the same leucogranite dike yields plateau ages of 37.7 ± 0.3 Ma, reflecting that the age was reset during the Arabia–Eurasia continental collision. Therefore, the bimodal age populations from the granitic intrusion in the Mawat ophiolite preserve a record of the subduction to the collision cycle of the Zagros Orogenic Belt. The 59.0 ± 6.0 m.y. age difference from the Daraban leucogranite represents the duration of the subduction‐collision cycle of the Zagros Orogenic Belt in the Kurdistan region of Iraq and the time span for the closure of the Neo‐Tethys Ocean along the northern margin of the Arabian plate.     

Bay of Islands ophiolite suite,Newfoundland: Petrologic and geochemical characteristics with emphasis on rare earth element geochemistry

Characteristic geochemical features of the ophiolite suite from the Bay of Islands Complex have been determined by major and trace element analyses of 13 rocks. Based on elements, such as rare earth elements (REE), whose abundances are relatively immobile during alteration and metamorphism, we find that (1) the pillow lavas and diabases are relatively depleted in light REE similar to most tholeiites occurring along spreading oceanic ridges, in back-arc basins and comprising the early phases of volcanism in island arcs; (2) the gabbros, composed of cumulate plagioclase and olivine with poikilitic clinopyroxene, have REE contents consistent with formation as cumulates precipitated from magmas represented by the overlying pillow lavas and diabases; (3) as in most harzburgites from ophiolites, the Bay of Islands harzburgite and dunite have relative REE abundances inconsistent with a genetic relationship to the overlying basic rocks — this inconsistency may be primary or it may result from late-stage alteration, contamination and/or metamorphism; (4) some Bay of Islands lherzolites have major and trace element abundances expected in the mantle source of the overlying basic rocks. Overall, the geochemical features of this Bay of Islands ophiolite suite are similar to those from Troodos and Vourinos, but these data are not sufficient to distinguish between different tectonic environments such as deep ocean ridge, small ocean basin or young island arc.     

Ophiolite detachment related to syn-collision thrusting

Many major ophiolite bodies can best be explained by detachment and initiation of subduction at a spreading axis in a narrow oceanic basin bordered on the external side by a passive continental foreland margin, followed by subduction of the remnant ocean basin and syn-collision emplacement of the ophiolite and overlying arc system onto the foreland. Evidence from Burma and the Philippines suggests that detachment and subduction at a spreading axis were related to regional compressive stress within an earlier collision belt on the internal side of the ophiolite. In Burma, detachment of a Jurassic ophiolite was in response to foreland thrusting in a Triassic collision belt to the east, while in the western Philippines, detachment of a Palaeocene ophiolite can most easily be explained as a response to back-thrusting in a late Cretaceous collision belt in Mindanao.     

High-resolution terrestrial carbon isotope and planktic foraminiferal records of the Upper Cenomanian to the Lower Campanian in the Northwest Pacific

Making Upper Cretaceous biostratigraphic correlations between the Northwest Pacific and Tethyan–Atlantic sections have been difficult because of rare frequencies of age-diagnostic macro- and microfossils in the sequences in the Northwest Pacific region. In order to correlate these sections precisely, an integrated planktic foraminiferal and bulk wood carbon-isotope stratigraphy from the upper Cenomanian to the lower Campanian succession (the middle–upper part of the Yezo Group) of Hokkaido, northern Japan is established with an average resolution of 50 k.y. The δ¹³C curves from bulk wood of the Yezo Group and from bulk carbonate of English Chalk show remarkably similar patterns of isotopic fluctuation, allowing the correlation of 22 carbon isotopic events between these sections. This high-resolution correlation greatly improves the previous micro- and macrofossil biostratigraphic schemes in the Northwest Pacific region, and reveals that global events, such as the oxygen depletion at the OAE 2 horizon, the constant decrease in pCO₂ during the Late Cretaceous, and the eustatic sea-level falls in the late middle Turonian, Santonian/Campanian Boundary and early Campanian, are recorded in the Upper Cretaceous sequence of the Northwest Pacific.     

An island arc origin for the Canyon Mountain ophiolite complex,eastern Oregon,U.S.A.

The Canyon Mountain ophiolite, Oregon, is exceptional in lacking sheeted dikes, basaltic pillow lavas, and sediments that are characteristic of many other ophiolites. Instead, the uppermost portion of the complex consists of a significant volume of plagiogranites, which, in addition to minor basalts, intrude a large section of keratophyres believed to be of volcanic origin. The trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite. It is suggested that the plagiogranites, basalts, and keratophyres comprise a sill complex. Both the plagiogranites and the keratophyres are similar, respectively, to low-K₂O plutonic and extrusive rocks of island arcs. The mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation, related to magmatism and diapirism above a Benioff zone. Radiometric age dates of plagiogranites confine the minimum age of the complex to the Early Permian. The Canyon Mountain ophiolite may thus be correlative with other fragments of a Lower Permian arc terrane throughout northeastern Oregon which were chaotically mixed during renewed subduction in middle to late Triassic time.     

Comment on “the troodos ophiolitic complex was probably formed in an island arc”, by A. Miyashiro

The use of bulk chemical analyses from the Troodos complex to determine the chemical affinity of the ophiolites is unjustified since these rocks exhibit conspicuous alteration in thin section. Very similar rocks on mainland Greece can be shown, on mineralogical grounds, to have been mainly tholeiitic originally, with a subordinate alkaline trend. This is compatible with their origin at an oceanic ridge. Regional geological evidence provides strong support for this origin. However, bulk chemical analyses from them could be used, like those from Troodos, to argue against a mid-ocean ridge origin. This suggests that the use of bulk chemical analyses from these rocks is very unreliable. In the presence of other, more reliable data, inferences from bulk chemistry can be rejected. The Troodos ophiolites are very similar, in both petrology and geological position, to those in Greece and probably originated in the same way.     

Origin of the troodos and other ophiolites: A reply to hynes

Geologic and tectonic evidence on the origin of the Troodos ophiolitic complex is ambiguous, being compatible with its creation in an island arc or a continent as well as in a mid-oceanic ridge. However, there is decisive petrologic and chemical evidence against its origin in a mid-oceanic ridge (and in a marginal sea). A large proportion (about half) of volcanic rocks in the sheeted complex and lower pillow lavas of Troodos have SiO₂ > 52.5% and FeO*/MgO > 2.0 in contrast to mid-oceanic ridge volcanics which are almost entirely basic (SiO₂ < 52.5%) with FeO*/MgO < 2.0. (FeO* means total iron as FeO.) A considerable proportion of volcanic rocks in Troodos belong to the calc-alkalic series. These facts indicate that the Troodos massif was formed probably in an island arc or a continental region.In order to rebut the above chemical evidence, Hynes resorts to an assumption of thorough compositional change of the Troodos volcanic rocks by metasomatism. However, this assumption is not plausible.     

Was there a wind-driven Tethys circumglobal current in the Late Cretaceous?

The problem of the existence of a Tethys circumglobal current (TCC) in the Late Cretaceous continental geometry (Campanian) is addressed. Within an ocean model which is expected to strongly overestimate the wind-driven TCC volume transport, a relatively weak TCC is found for the reconstructed Campanian paleogeography used. As a measure of the strength of the TCC, a circumglobal flow index is introduced. This index is based on volume fluxes through meridional sections within the equatorial part of the domain. The impact of changes in the paleogeography on the TCC is considered by computing steady flows for different shapes of Eurasia. The results on the extent of Eurasia such that a strong TCC appears provide support that no strong wind-driven TCC has existed during the Campanian.     

Os isotope dating and growth hiatuses of Co-rich crust from central Pacific

Up to now, accurate determination of the growth age and hiatuses of the Co-rich crust is still a difficult work, which constrains the researches on the genesis, growth process, controlling factors, regional tectonics, paleo-oceanographic background, etc. of the Co-rich crust. This paper describes our work in determining the initial growth age of the Co-rich crust to be of the late Cretaceous Campanian Stage (about 75-80 Ma), by selecting the Co-rich crust with clear multi-layer structures in a central Pacific seamount for layer-by-layer sample analysis and using a number of chronological methods, such as Co flux dating, dating by correlation with 187Os/188Os evolution curves of seawater, and stratigraphic divi- sion by calcareous nannofossils. We have also discovered growth hiatuses with different time intervals in the early Paleocene, middle Eocene, late Eocene and early-middle Miocene, respectively. These re- sults have provided an important age background for further researches on the Co-rich crust growth process and the paleo-oceanographic environment evolution thereby revealed in the said region.     

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km²; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area. Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.