Ophiolitic dunites offer critical insights into mantle and crustal processes within oceanic lithosphere. This study presents an integrated petrological and geochemical investigation of dunites from the New Caledonia ophiolite (Peridotite Nappe), a forearc lithospheric remnant. Two distinct dunite types with contrasting origins are identified. Type I dunites are crustal cumulates formed by fractional crystallization of primitive magmas. They occur at the base of the crustal ultramafic sequence with sharp contacts against pyroxenite layers, display a medium-to coarse-grained granular texture characterized by olivine grains forming triple junctions (similar to 120 degrees), and contain olivine with low Fo and spinel with low Cr#. Their mineral compositions follow fractional crystallization trends. Type II dunites are mantle-derived replacive bodies formed by melt-rock reaction between silica-undersaturated melts and harzburgite. They occur as veins or lenses within harzburgites with irregular contacts, exhibit granular to porphyroclastic textures with spinel forming "holly-leaf" or vermicular shapes commonly intergrown with orthopyroxene, and are characterized by extremely high olivine Fo and spinel Cr#. Geochemical data indicate that the two types likely formed during separate magmatic events. Furthermore, postcumulus infiltration of boninite-like melts into Type I dunites is evidenced by clinopyroxene with higher Mg# than coexisting olivine. The coexistence of these two dunite types underscores multiple magmatic processes in the formation of the New Caledonia ophiolite, involving both melt migration and crystal accumulation from depleted mantle-derived magmas.
This paper describes a four-day field trip across the Alpine Corsica, the southern branch of the Alpine collisional belt made up of oceanic and continental units derived from the closure of the Ligurian-Piedmont Oceanic Basin, and the subsequent collision between the continental margins of Adria and Europe plates. On the first day, the relationships between the main tectonic units, i.e., the Upper Units, the Schistes Lustr & eacute;s Complex and the Lower Units, can be observed along an east-west transect from Bastia to Ile-Rousse. On the second day, the features of the Upper Units can be observed in the Balagne area, where an ophiolite sequence from the Middle to Late Jurassic period, along with its Late Jurassic to Late Cretaceous sedimentary cover, is clearly visible. This sequence, belonging to the Balagne Nappe, is characterized by the repeated occurrence of coarse-grained deposits supplied from the Europe continental margin, which makes it unique in the Alpine-Apennine belt. On the third day, the oceanic units of the Schistes Lustr & eacute;s Complex in the Golo Valley will be examined. This complex comprises several ophiolite sequences that were accreted to the Alpine wedge and deformed under conditions of pressure and temperature ranging from blueschist to eclogite facies. On the fourth day, the Lower Units in the Corte, Popolasca and Ponte Leccia areas are visited. These units represent slices of the thinned continental margin of the Europe plate that were involved in subduction prior to continental collision. Overall, this paper provides a comprehensive overview of the stratigraphic, deformational, and metamorphic features of Corsica, which is one of the most significant fragments of the Alpine collisional belt in the western Mediterranean region.
Chrome-spinels are considered as valuable provenance indicators in heavy mineral analysis. Their importance is emphasized by the fact that they are ultra-stable during weathering, transport and diagenesis. However, some earlier publications have inferred that high-Allow-Cr spinels are less resistant which may have a strong influence on provenance studies. In the Western Carpathians and Eastern Alps, there is a remarkable contrast between Late Jurassic to mid-Cretaceous ophiolite-bearing sediments (so-called exotics) derived from the Neotethys that are depleted in high-Allow-Cr spinels and primary ophiolitic occurrences of Meliata Unit (also a part of Neotethys) which display the entire spectra of the Cr-spinel chemistry. It was unknown, however, whether this difference was primary or secondary, caused by later removal of high-Al/low-Cr spinels. Therefore, Cr-spinels from the Upper Cretaceous ophiolite-bearing conglomerates in the area of the Doblina Ice Cave derived from the Meliata Unit were analyzed to test their stability during weathering, transport and diagenesis. Parallel analysis of spinels from sandy and pebble material showed that the spinels from both fractions cover all the chemistry spectrum from high-Cr/low-Al to low-Cr/high-Al types, without any depletion. Good preservation of all sorts of Cr-spinels in the sandy material indicates that the depletion of Cr-spinel assemblages that was recorded in the Late Jurassic to mid-Cretaceous exotics all over the Alpine-Carpathian area appears to be primary. This was caused by lack of lherzolinic (MORE) ophiolites in the source area. Although the Meliata unit had formed a part of the same Neotethyan oceanic system, the analyzed ophiolites which include lherzolitic (MORB) types could not have been the source of Cr-spinels for the exotics-bearing flysches.
This paper presents a complete stratigraphic, tectonic, and structural dataset of the Internal Ligurian Units outcropping in the Antessio area (Northern Apennines), based on new geological mapping and micro-and mesoscale analyses. A stack of three Internal Ligurian Units, namely, from top to the bottom, the Gottero, Bracco-Val Graveglia and Colli-Tavarone Units, has been recognized. The general stratigraphic characteristics of these units indicate that they are derived from dismembering of a sequence derived from the Ligurian-Piedmont oceanic basin. This sequence includes Middle to Late Jurassic ophiolites, Late Jurassic to Late Cretaceous pelagic deposits, Late Cretaceous to Early Tertiary turbidites and Early Tertiary mass-transport deposits. This sequence is interpreted to have developed during the progressive trench-ward motion of a segment pertaining to the inner part of the Ligurian-Piedmont oceanic basin. In the Antessio area, the deformation history of the Internal Ligurian Units includes two deformation phases, whose features support the involvement of these units in a subductionrelated accretionary wedge zone by coherent underplating followed by exhumation. This picture suggests a close correlation between the Internal Ligurian Units of the Antessio area and those of the Bracco Massif, as well as those of the Graveglia and Vara valleys.
Microfossils are a group of small size fossil organisms commonly used by geologists in relative age determination and paleoenvironmental reconstructions. More recently, micropaleontology has found applications as an archaeometric tool in the field of archaeology and architecture providing a minimally invasive method for the study of artifacts and building phases. The purpose of this paper is to apply micropaleontological analyses of calcareous nannofossils and calpionellids assemblages to establish the provenance of carbonate lithotypes widely used as ashlars in religious buildings of Prato. The monumental architecture of Prato extensively used a local limestone, the Pietra Alberese. It is a grey marly limestone, which upon exposure to atmospheric agents, becomes whitish or it takes on a brownish colour. This brownish chromatic alteration is found distributed both randomly in isolated ashlars but also appears in linear patterns, particularly in religious buildings. Materials taken from the fa & ccedil;ade of San Francesco Church in Prato document that the different pattern of alteration arises from the use of two different calcareous lithotypes, namely Pietra Alberese (Eocene) and Calpionella Limestone (Lower Cretaceous). The Pietra Alberese sometimes suffers an ochre colour of alteration, powder to the touch and the altered ashlars show a random distribution on San Francesco Church fa & ccedil;ade, while Calpionella Limestone regularly suffers of ochre colour of alteration, not powdery to the touch, and is distributed according to a linear pattern.
