Tourmalinites in the metamorphic complex of the Svratka Unit (Bohemian Massif): a study of compositional growth of tourmaline and genetic relations

Abstract

<![CDATA[ Tourmalinites (rock with an association Tu + Qtz + Ms &plusmn; Grt &plusmn; Bt &plusmn; Ky &plusmn; Sil &plusmn; Pl &plusmn; Kfs) from the Svratka Unit form stratiform layers hosted in mica schists. The chemical composition of tourmaline from tourmalinites varies from Al-rich schorl to dravite. The tourmaline usually exhibits three compositional domains, which are, from centre to the rim: a chemically inhomogeneous brecciated core (zone I), a volumetrically minor internal rim zone II, and a dominant outermost zone III. &#060;br&#062;The compositional variability of tourmaline in all the zones is controlled by the (&#060;sup&#062;X&#060;/sup&#062;&#060;span style=&#034;border:solid 1px black;font-size:0.5em;width:1em;position:relative;top:-1px;margin:2px&#034;&#062;&nbsp;&#060;/span&#062;&#060;sup&#062;Y&#060;/sup&#062;Al&#060;sup&#062; W&#060;/sup&#062;OH) (&#060;sup&#062;X&#060;/sup&#062;Na&#060;sup&#062; Y &#060;/sup&#062;Mg&#060;sup&#062; W&#060;/sup&#062;F)&#060;sub&#062;-1&#060;/sub&#062; and &#060;sup&#062;Y&#060;/sup&#062;Fe&#060;sup&#062;Y&#060;/sup&#062;Mg&#060;sub&#062;-1&#060;/sub&#062; substitutions. The tourmaline of the zone I corresponds to highly vacanced X-site, Al-rich schorl with lower F (up to 0.43 apfu), which is interpreted as an older, low-temperature hydrothermal tourmaline. Tourmaline of the zone II corresponds to dravite rich in F (reaching up to 0.66 apfu) crystallizing during the prograde metamorphism. Lastly, the Al-rich schorl-dravite of the zone III, grew most likely during retrograde metamorphism. &#060;br&#062;The tourmaline from the host mica schists has a similar chemical composition. The central dravite part corresponds to the zone II, and the predominant schorl-dravite rim to the zone III, in tourmalines from tourmalinites. Overall, the dravite exhibits compositional characteristics of the prograde, amphibolite-facies metamorphic event characterised by the mineral assemblage Qtz + Ms + Bt + Tu &plusmn; Ky &plusmn; St &plusmn; Grt. The second generation of tourmaline (schorl-dravite), as well as the rim of garnets present in both the mica-schists and tourmalinites, formed during exhumation of the Svratka Unit accompanied by decreasing pressure and temperature. In the mica schists, this event resulted in decompression breakdown of staurolite, according to the reaction St + Ms + Qtz = Grt + Sil + Bt + H&#060;sub&#062;2&#060;/sub&#062;O. Breakdown of muscovite could have released B and F used for tourmaline formation. The P-T conditions of this retrograde metamorphism were calculated at 600-640 &deg;C and 5-6 kbar. &#060;br&#062;Tourmalinites are interpreted as a part of a metamorphosed volcano-sedimentary complex primarily rich in F and B, however, the derivation of all the F- and B-rich fluids from the neighbouring migmatites and metagranites is unlikely. Similarities in the chemical composition of the tourmalinites and the mica schists suggest a similar protolith to both rock types. The variation in most of the elements reflects the mineral composition associated with...

Tourmalines as a Tool in Provenance Studies of Terrigenous Material in Extra-Carpathian Albian (Uppermost Lower Cretaceous) Sands of Miechów Synclinorium, Southern Poland

“…The tourmalines from the Miechów Segment contain significantly less magnesium (on average around 5-8 wt% MgO) than tourmaline from the Vrbno Units (Table 3), ultimately eliminating the tourmalines from the Vrbno units as a major source of detrital material deposited in the Miechów area during the late Albian. The dravite dominant tourmaline from the Albian sands (Table 2) show strong similarity to tourmaline from the Jegłowa crystalline schists unit [35] and to the tourmaline described in rocks from the Svratka Unit [34] (Figure 10, Table 2). Both mentioned tectonostratigraphic units are in the eastern part of Bohemian Massif.…”

“…As a common mineral in detrital material, tourmaline is frequently used as a tool in provenance studies [34,35,44,55,60,[64][65][66][67][68]. Henry and Guidotti, [69] created a ternary Fe-Al-Mg diagram with discrimination fields reflecting the composition of tourmaline crystallized in different rock types.…”

“…The crystalline rocks, in which dravite dominates over schorl, as in tourmaline separated from Albian sands and sandstones ( Figure 10), are relatively scarce. These include pegmatites of the Szklary and Gogołów-Jordanów Massifs [35], granites and pegmatites in Tatra Mountains [35,125], mica schists of the Svratka Unit in the Czech Republic [34], quartz-sericite shales from Jegłowa in the eastern Sudetes [54] or some pegmatites [126] and marbles from the Bohemian Massif [60]. [17,18,22,55,58,[126][127][128][129][130].…”

“…Tourmaline-supergroup minerals (for simplicity, the short term tourmaline will be used in this paper) is the most common borosilicate mineral found in nature. It occurs mostly as an accessory mineral, seldom being a rock-forming mineral with the exception of rare rocks like metamorphic tourmalinites [33,34]. Tourmaline minerals found in igneous rocks are most often enriched in Fe and/or Mg and are therefore classified as minerals of the schorl-dravite series [35,36].…”

“…The very high resistance of tourmaline to both chemical and physical weathering [16], the prevalence of occurrence in igneous and metamorphic rocks and the highly diverse chemical compositions makes this mineral group an excellent tool for determining source areas for detrital material deposited in siliciclastic sedimentary basins [68][69][70]. The applicability of this tool in provenance analysis is also dependent on mineralogical data for tourmaline found in rocks from hypothetical source areas [34,35,41,44,54,60,67,71].…”