Evolution of sulfide mineralization in ferrocarbonatite, Swartbooisdrif, northwestern Namibia: Constraints from mineral compositions and sulfur isotopes

The Mesoproterozoic (ca. 1140–1120 Ma) ferrocarbonatite dykes of Swartbooisdrif, northwestern Namibia, transect the anorthositic rocks of the Kunene Intrusive Complex. The ferrocarbonatite is mainly composed of ankerite and magnetite, and is highly contaminated by fragmented and fenitized wallrock anorthosite (albite, biotite and sodalite), with Fe–Cu–Ni sulfides (assemblage: pyrite, chalcopyrite, millerite, polydymite, fletcherite) being widespread minor components. In order to constrain the compositional evolution and to reconstruct the T–f(O2) conditions during progressive crystallization and subsolidus re-equilibration of the ferrocarbonatites, a detailed study of the mineral chemistry and sulfur isotope signatures of the sulfide mineralization has been carried out. Owing to the finely intergrown nature of the sulfide assemblages, sulfur isotope analysis has been performed using an in situ laser-microprobe technique. The δ34S values of early carbonatite-hosted pyrite range from 3.8 to 5.1‰. Similar δ34S values were obtained for metasomatically formed pyrite from the fenitized anorthosites (+3.3 to +3.4‰). Distinctly lower, negative δ34S values have been determined for pyrite (−2.4 to −2.1‰) and chalcopyrite (−3.3 to 0.0‰) in late sulfide–oxide veins, with the lowest values obtained for sulfides from samples that contain additional secondary barite. This overall isotopic trend is interpreted in terms of changing proportions of oxidized and reduced sulfur species in the carbonatite-derived fluids, and is supported by calculations of phase equilibria in the system Cu–Fe–S–O–H and modeling of isotopic mass-balance. During the early stage of mineralization, the oxygen fugacity was presumably low, with reduced sulfur species (H2S, HS−) dominant in the fluids, promoting the precipitation of pyrite and the metasomatic formation of nearly pure, SO4-free sodalite. A subsequent increase in f(O2) led to a systematic increase in the ∑SO4/∑H2S ratio of the fluid. This resulted in a depletion of 34S in the residual reduced sulfur, which is manifested by the negative δ34S values of the late sulfide mineralization

Evolution of sulphide mineralization in ferrocarbonatite, NW Namibia: Constraints from sulphur isotopes

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Early exhumation of high-pressure rocks in extrusion wedges: Cycladic blueschist unit in the eastern Aegean, Greece, and Turkey

Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼3040 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rock

Origin and geodynamic evolution of late Cenozoic potassium-rich volcanism in the Isparta area, southwestern Turkey

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