First discovery of MgTi-rich dumortierite in association with quartz, kyanite and corundum (Belomorian eclogite province)

Dokukina K.A. ¹, Konilov A.N. ¹, Van K.V. ²

1 Geological Institute ras, Moscow, Russia; 2 Institute of Experimental Mineralogy ras, Chernogolovka, Russia

dokukina@mail.ru

Dumortierite is a aluninium borosilicate with the idealized formula [∼(Al,□)Al6(BO3)Si3O16(O,OH)2] and is most abundant in pegmatites, aluminous metamorphic rocks, and metasomatic rocks. Dumortierite was described as a mineral from leucogranites and in pneumatolytic-hydrothermal systems.

For the first time a red dumortierite was found in felsic vein within Salma eclogites which formed on the Archean oceanic crust [1]. The Salma dumortierite-bearing rock consists of 40-50 % of quartz, 10-15 % of K-feldspar and 50-35 % of a composite polymineral pseudomorphose after hypothetic primary white mica. The pseudomorphoses consist of generally muscovite and/or biotite with kyanite, potassic feldspar, rutile, plagioclase and corundum (Fig.a). The dumortierite in the pseudomorphose in all cases has a red colour (Fig.b) and characterized by significant admixtures of magnesium and titanium. The same MgTi-rich dumontereite have been observed in other HP and UHP complexes (for exsample the Dora Moira Massif, Western Alps and [2, 3].

The probable relics of the primary phengite are characterized by elevated content of magnesium, titanium and sodium content. Coronas of feldspars develop around polymineral pseudomorphoses at boundary with large quartz monocrystalls. Potassium feldspar sequentially gives place to albite, and albite gives place to Ca-Na plagioclase from quartz rim to centre of pseudomorphose. These feldspar coronas probably initially formed at the primary white mica dehydration and the reaction between primary mica and quartz with feldspar and kyanite forming. Kyanite needles and plates occupied cleavage planes of the white mica. Feldspar coronas become a natural container that limited of a silica inflow to white mica area. At change of PT-conditions metamorphic reaction with forming of new minerals (mew muscovite, potassium feldspar, biotite and plagioclase, corundum and MgTi-rich dumortierite) happened only within these natural ampoules.

The dumortierite-bearing rock initially probably was presented quartz-white mica association. It composition is felsic (SiO2 69.96 wt.%), high-magnesian (#Mg 0.78) at low content of magnesium and iron (MgO 1.65, FeO 0.93 wt.%), high content of alkaline (Na2O 2.39, K2O 4.78 wt. %) and aluminum (Al2O3 17.0 wt.%) and characterized by low content of other main elements (CaO 0.96, TiO2 0.26, MnO 0.007, F 0.016, LOI 1.86 wt.%), high barium (1277 ppm) and elevated Rb and Sr. Rock has LREE-enriched pattern (LaN/LuN = 20, LuN/SmN = 0.16) with a negative europium anomaly (Eu/Eu* 0.76).

The studied rock probably can represent (1) hydrothermal quartz-white mica vein (or layer) that was formed as result of dehydration a boron-saturated oceanic sedimentary rocks at burial them to a subduction zone; (2) product of melting (remelting?) of a boron-saturated aluminous-siliceous sedimentary layers.

Fig. (a) BSE image of dumortierite-bearing rock. The white box outlines the area shown enlarged in (b).(b) plane-polarized light image of the red dumortierite within polymineral pseudomorphose after hypothetic primary white mica.

References:

1. Mints M.V., Belousova E.A., Konilov A.N., Natapov L.M., Shchipansky A.A., Griffin W.L., O’Reilly S.Y., Dokukina K.A., Kaulina T.V. (2010) Mesoarchean Subduction Processes: 2.87 Ga eclogites from the Kola Peninsula, Russia. Geology, 38, p. 739-742. doi: 10.1130/G31219.1

2. Schertl, H.-P., Schreyer, W., Chopin, C. (1991): The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps: detailed petrography, mineral chemistry and PTpath. Contrib. Mineral. Petrol., 108, p. 1-21.

3. Ferraris, G., Ivaldi, G., Chopin, C. (1995): Magnesiumdumortierite, a new mineral from very-high-pressure rocks (Western Alps). Part I: Crystal structure. Eur. J. Mineral., 7, p. 167-174.