Datolite, ideally CaB[(OH/SiO4)], from hydrothermal veins crosscutting pillow basalt in 10 different localities of the Northern Apennine ophiolites was investigated with regard to mineral chemistry and fluid inclusion microthermometry. Bulk analyses of datolite crystals show REE contents below chondritic, except for La and Ce. With respect to host rock, datolite is occasionally enriched in La, Rb, Cs, Be, and shows relatively high contents of chalcophile elements (Cu, Zn, Pb, Ni) when occurring in contact with sulfide-mineralized basalt. Volatiles escaped during the decomposition in the temperature range 600 and 700 °C. The main component is water. The temperature maximum of water release is different and frequently with a shoulder or a second maximum. Together with water, sulfur species as H2S and SO2 and traces of boron species escaped. The CO2 release by the decomposition especially of datolite from Castellaro and Cinghi has a maximum in the range of 500-580 °C and is different from the decomposition of calcite. Together with CO2 a boron species escaped. Chlorine does not detect. Two-phase (L+V) fluid inclusions texturally identifiable as primary and secondary were observed, yielding average homogenization-temperatures of 236 and 173 °C, respectively. Fluid inclusion cooling data yield calculated salinity in the range of 10-16 wt% NaCl equivalents, thus relatively higher compared with seawater. The results are compatible with those reported for fluids formed under diagenetic conditions, but differ from those observed in seafloor hydrothermal systems and/or emanating from magmas. Distribution of trace elements between datolite and host basalt indicates enrichment with respect to the host rock limited to a few elements such as La, Rb, Cs, Be, Ni, Cu, Zn and Pb. The lithophile elements can be hosted in the datolite lattice, whereas the chalcophile metals and Ni are probably carried in sub-microscopic inclusions. 相似文献
The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al2O3 and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.
Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al2O3 and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.
The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics. 相似文献
The eastern end of the Haylayn massif exposes a complex paleoridge structure interpreted as the tip of a northwestward propagating segment (Nicolas et al., this issue). The area, revisited from a petrostructural and geochemical viewpoint, offers the most documented exposures of the association of olivine gabbros and gabbronorites in Oman (Juteau et al., 1988). Gabbronorites were injected while the main gabbro unit was deforming in the magmatic state. Both units do not differ chemically, except for the SiO2 enrichment of the orthopyroxene-rich gabbros relative to olivine-gabbro. In addition, they display the same trace element signature, which implies the same parent magma for both units. The extension of the stability field of orthopyroxene is assigned to increase of oxygen fugacity due to hydration. The source of hydration is the ridge axis hydrothermal circulation, suggesting hydrothermal/magma interaction at temperatures above the gabbro solidus. The distribution of gabbronorites at the scale of the entire ophiolite suggests a relation with ridge tectonics where high-T conditions of hydrothermal-magmatic interaction are met. Such conditions are met when propagating segments rotate the structures of the dying magma chamber. 相似文献
The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ?Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ?Ma), to orogenic collapse (<14 ?Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ?Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ?Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ?km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ?km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ?km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening. 相似文献