Igneous petrology and mineralogy Sulfur-enriched apatite and magmatic anhydrite in the epidote-bearing subvolcanic rocks, the Middle Urals, Russia

Avdonina I.S., Pribavkin S.V.

Institute of Geology and Geochemistry, Yekaterinburg, Russia

Iavdonina@gmail.Com

The paragenesis of epidote, Mg-rich amphibole, plagioclase (An_20-30), biotite, titanite, apatite and anhydrite as phenocrysts in the trachyandesites and trachydacites plays a significant role in a determination of fluid regime and the conditions of wet melt crystallization.

The object of research is located in the Middle Urals. It is the area of maximal compression and bend, which caused by effect of Ufim jut of East-European platform on late orogen stage. The reconstruction of magmatism of the Middle Urals segment according to V.N. Puchkov [3] shows that epidote-bearing dykes correspond to the Devonian island arc system, which is situated to the east of Tagil Arc. This arc system, apparently, was founded not only on the epioceanic island-arc basement, but also on the continental terrain. The radiological age of the epidote-bearing porphyries was determined by U-Pb (zircon) and corresponds to 388±2 Ma The data obtained indicate, that intrusion of the dykes was on the boundary of the Early and Middle Devonian [2].

Phenocrysts of apatite correspond to (CaOH)-apatite and (CaF)-apatite. The feature of these apatite is an elevated content of sulfate component (up to 1,5 wt.%). In the consequence of investigation of representative apatite samples we have observed a strong SO₃/ P₂O₅, SO₃/SiO₂ and SO₃/Na₂O isomorphism. Therefore the sulfur component enters into the structure of apatite. The phenocrysts from trachyandasites are more sulfur-enriched than apatite from trachydacites. The strong relation of sulfur decreasing from cores to rims in phenocrysts have also been observed. In addition the content of sulfur decrease from the early apatite generation to the late generation. The crystals which enclosed in amphibole, epidote and plagioclase are characterized by SO₃ (0,6 wt.%-1,5 wt%) content. That higher than in the grains from groundmass. This fact can indicate that fluid sulfur took part in the melt crystallization on the early stage.

The following petrography observation suggests that anhydrite phenocrysts compose 0,3-1,5 vol.% of rocks. Anhydrite occurs as primary inclusions within apatite (demonstrating that the melt was actually anhydrite-saturated). These crystals are prismatic with length up to 0,2 mm. But the bulk of phenocrysts, located in the groundmass, and represented by rectangular pseudomorphs. They consist of grossular, albite, chlorite and apatite. Often these pseudomorphs are leached. Bearing in mind such great content of anhydrite phenocrysts we can make the assumption about high sulfur content (on the order of 1000-3000 ppm S) in the melt.

Despite careful examination, we could not find any traces of magmatic sulfides in neither as inclusions within phenocryst phases, nor as oxidized remains in the matrix. Most of the sulfur present in this trahyandasites was therefore dissolved as sulfate, suggesting high fO₂. High magmatic oxygen fugacities are suggested also by the assemblage quartz + magnetite + titanite +Mg-rich hornblende [4], which requires a minimum of logfO₂≥NNO+ 1 at the reconstructed crystallization temperature of 770±50 ^oC. Relatively reduced magmas (logfO₂ <NNO+ 0.5) are characterized by the predominance of S^2- and become saturated with sulfides (pyrrhotite or immiscible sulfide melts) [1].

The obtained data suggest that trahyandesite-dacite melt, formed in above-subduction geodynamic environment in the Middle Urals in the mid Devonian, was sulfate-enriched.

The article was supported by the Russian Foundation for Basic Research № 09-05-00911-а, programs of the Ural Division, Russian Academy of Science 09-T-5-1011 and 09-T-5-1019.

References:

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2. Pribavkin SV, Borodina NS, Gmyra VG (2007) New data about composition and age of dacites with muscovite, garnet, epidote phenocrysts (the Central Ural). Yearbook-2006, Inst Geol Geochim: 138-144

3. Puchkov VN (2000) Paleogeodinamic of the South and Central Ural. Ufa: Dauria. 145

4. Wones, D.R., 1989. Significance of the assemblage titanite + magnetite+ quartz in granitic rocks. Am. Mineral. 74, 744–749.