Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Archean high-pressure anatexis of continental rocks of Belomorian eclogite province

K.A. Dokukina

Geological Institute of the RAS, Moscow, Russia

The Lomonosov Moscow State University, Moscow, Russia

ksdokukina@gmail.com

In limit of South-Kola active continental margin along N-E boundary of Belomorian accretionary orogen the eclogite bodies are which were formed as result of Meso-Neoarchean subduction of oceanic and continental complexes (Salma and Gridino association) [Mints et al., 2010]. Compositional and structural features of the Salma eclogites suggest that the protolith was oceanic crust with age of 2.89-2.82 Ga. The high-pressure rocks in the Gridino area developed in a continental crust of TTG composition and are especially evident in mafic enclaves and dykes with age of 2.87-2.82 Ga. We investigated Gridino high-pressure rocks and drew a conclusion about Archean age of eclogite metamorphism (not younger 2.7 Ga ago) [Dokukina, Konilov, 2011]. The event with age of 2.7 Ga corresponds to post-eclogite decompression high-pressure granulite-facies metamorphism. At this time partial melting of composite continental matter was, that included different felsite (TTG gneiss, granites and migmatites) and basite rocks (metagabbroid dykes and mafic pods).

Excellent example of such partial melting is on the Vargas Cape. Vargas Cape (VGS-84: N 65º56’, E 34º40’) is located 3 km north of Gridino. The area is underlain by migmatized tonalite gneisses intercalated with amphibole gneisses, which contain numerous amphibolite and eclogite bodies of different size (from a few centimeters to a few meters in size) and shape (from equant to strongly flattened) and dykes of ferriferous metagabbro, deformed together with the host gneisses. Nearly all the felsic and mafic rocks underwent partial melting during post-eclogite decompression (at decrease pressure from 20-17 to 12.3-10.9 kbar).

The initial stage of melting is characterized by the formation of a phengite-bearing leucosome. Numerous thin veins (from few to tens centimeters in size) penetrate gneiss and mafic rocks and contain restite bodies and unmelted tonalite and mafic fragments. Petrological studies of the leucosome showed relicts of previous high-pressure igneous conditions: Ba-bearing phengite (3.15-3.2 cations Si per 11 atoms O), K-feldspar and K-Ba feldspar, myrmekite and near-solidus symplectitic intergrowths of clinozoisite, phengite and quartz. The clinozoisite-quartz symplectite probably was restite of melting of mafic rocks. The phengite geobarometer [Caddick and Thompson, 2008] yields high-pressure conditions of leucosome crystallization: 16-25 kbar and 650-800 °C. Evidence of changing eclogitic to granulite conditions of decompression are: biotite replaces phengite; grossular garnet and clinopyroxene replace clinozoisite-quartz symplectite; plagioclase breaks down, with antiperthite forming. The Bt-Grt and Grt-Cpx geothermometers and Grt-Cpx-Pl-Qtz geobarometer indicate high-pressure granulite conditions: 750-800 ºC and 10.9-12.3 kbar. The leucosome is K-rich granite (Na₂O 2.18-3.2, K₂O 3.8-4.9 wt. %) with a high content of silica (SiO₂ 69.8-77 wt. %), anomalously high Ba (1548-3533 ppm), a low content of the remaining trace elements, LREE-enriсhed (La_N/Lu_N = 6.7-68.9, Lu_N/Sm_N = 0.06-0.82) or W-shaped (La_N/Lu_N = 2.97-3.27, Lu_N/Sm_N = 1.43-2.26) REE pattern with a positive europium anomaly (Eu/Eu* = 1.1-12.4), and very low total REE of 6-29 ppm (Fig. 1). These properties are evidence of eutectic nature of leucosome [for example, Skjerlie, Johnston, 1996]. Zircon grains from the phengite leucosome were dated by conventional U–Pb and SHRIMP II methods and yielded an age of about 2.71 Ga [Dokukina, Konilov, 2011].

An advanced stage of melting produced relatively large granite bodies and veins (from tens centimeters to tens meters in size) that crosscut foliation in all the metamorphic rocks. The granite bodies also have a K-rich composition (Na2O 2.76-3.9, K2O 3.1-4.91 wt. %), but with a normal content of silica (SiO₂ 66.6-74.5 wt. %), understated content of barium (429-858 ppm), LREE-enriched REE pattern (La_N/Lu_N = 9.7-55, Lu_N/Sm_N = 0.13-0.46), negative europium anomaly (0.3-0.6), and total REE of 110-300 ppm (Fig. 1). Zircon grains from a granite vein were analyzed by LA-ICP-MS, yielding an age of 2721 ± 19 Ma. This age coincides within error with a concordant age of 2713±6 Ma for zircon from a sample of phengite-bearing leucosome (unpublished data of L.M. Natapov and E.A. Belousova).

Partial melting bands and phengite-bearing leucosome forming developed only along boundaries between felsite and mafic rocks. The leucosome strong penetrate the metagabbro dykes and mafic pods. Apparently the boundaries between the gneiss and mafic rocks were favorable for a fluid migration, which promotes a partial melting of the tonalite gneiss and got involved the mafic rock in this melting.

This work was supported by grant RFBR 12-05-00856-a.

Figure 1. Chondrite-normalized REE patterns for studied rocks (locality Vargas Cape). Chondrite values are from Sun and McDonough (1989).

References:

Mints M.V., Konilov A.N., Dokukina K.A., Kaulina T.V., Belousova E.A., Natapov L.M. Griffin, W.L. O’Reilly S.Y. The Belomorian Eclogite Province: unique evidence of Meso-Neoarchaean subduction and collision. Doklady Earth Sciences. 2010. V. 434. № 2. P. 1311-1316.

Caddick M.J., Thompson A.B. Quantifying the tectono-metamorphic evolution of pelitic rocks from a wide range of tectonic settings: mineral compositions in equilibrium. Contributions to Mineralogy and Petrology. 2008. V. 156. P. 177–195.

Dokukina K.A., Konilov A.N. Metamorphic evolution of the Gridino mafic dyke swarm (Belomorian eclogite province, Russia) In: (Dobrzhinetskaya L., Cuthbert S., Faryad W., Wallis S., Eds.) Ultrahigh Pressure Metamorphism: 25 years after the discovery of Coesite and Diamond. 2011. Elsevier, p. 591-634.

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Sun S.S., McDonough W.F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: (A.D. Saunders and M.J. Norry, Eds.). “Magmatism in the Ocean Basins” Geol. Soc. Lond. Spec. Publ., 1989. V. 42. P. 313-345.