Blueschist metamorphism of metabasite dykes in the serpentinites of the Frido Unit, Pollino Massif
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Abstract:
Metamorphic evolution of metabasite dykes included in tectonic slices of serpentinites in ophiolitic rocks from Frido Unit (Pollino Massif) is reported.Petrological analysis via PT pseudo-sections calculation using Theriak-Domino software have been done for the system Na2O-CaOK2O-MgO-FeO-Al2O3SiO2-H2O (NCKMFASH) and showed that the pressure and temperature conditions of blueschist metamorphism are estimated to 8-12 kbar and 300-400 °C. The minerals in the metabasite dykes are chemically homogeneous, suggesting that they are almost completely equilibrated. The retrograde metamorphic evolution of the metabasites from high- to intermediate-pressure conditions is documented by the occurrence of minerals belonging to pumpellyite facies assemblages.High-pressure metamorphism in blueschist facies condition is related to the subduction of Frido Unit in low gradient conditions. The use of pseudosections stepwise controlled by petrographic observations allowed to estimate the P-T conditions in metabasite dykes.Keywords:
Blueschist
Massif
The Alpine metamorphic evolution of the Cycladic Crystalline Complex of the Aegean aera of Greece is characterized by an Eocene eclogite-to-blueschist facies metamorphism and an Oligocene/ Miocene greenschist-to-amphibolite facies metamorphism. High pressure assemblages are well preserved in the north of Sifnos island (Blueschist Unit), whereas the rest of the island is dominated by greenschist-facies rocks (Greenschist Unit). However, rocks from Sifnos island preserve a complex record of both metamorphic events at all scales. In this paper we combine structural and petrological observations with major and trace geochemistry and stable-isotope data to characterize (a) the fluid involvement during highpressure and greenschist-facies metamorphism and (b) the nature of the retrogression from blueschist to greenschist facies rocks.
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Blueschist
Greenschist
Felsic
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Abstract Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐ P nappe stack consisting of belts in which high‐ P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.
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Glaucophane
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Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite–actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P–T conditions are estimated to be within 200–350°C and 5–7 kbar. These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P–T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramafic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.
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Amphibole
Glaucophane
Ultramafic rock
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Abstract Deerite has been found in layers of Fe-rich metasediments, metamorphosed under conditions of blueschist to eclogite facies ( T = ca. 480°C). Geochemical comparisons with other deerites described in the literature show that the Ile de Groix deerites most closely resemble the Mn-poor, alpine type, and Sifnos deerites. The P-T conditions of the Ile de Groix metamorphism do not conform with the stability field of deerite. In the assemblages examined here deerite seems to be a relic from earlier stages of prograde metamorphism.
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Crossitic amphibole and its possible tectonic significance in the Richmond Area, southeastern Quebec
Crossitic amphibole suggesting blueschist-type metamorphism has been found in the Richmond area, southeastern Quebec. Prehnite facies metamorphism to the northeast of Richmond and greenschist facies metamorphism to the south along with the blueschist-type metamorphism and geophysical data suggest that the Richmond area may represent a partially eroded Ordovician subduction zone.
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Amphibole
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