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Open AccessArticle

Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia

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Research Laboratory of Industrial and Ore Mineralogy, Cherepovets State University, 5 Lunacharsky Avenue, 162600 Cherepovets, Russia
2
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Science, 3 Avenue “Prospekt Koptyuga”, 630090 Novosibirsk, Russia
3
Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 0E8, Canada
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(1), 14; https://doi.org/10.3390/min10010014
Received: 9 October 2019 / Revised: 17 December 2019 / Accepted: 18 December 2019 / Published: 23 December 2019
(This article belongs to the Special Issue Feature Papers in Mineral Geochemistry and Geochronology 2019)
Dunite–harzburgite–olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84−89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30–35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo≤89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula. View Full-Text
Keywords: ultramafic complexes; subvolcanic intrusions; crystallization of highly magnesian magma; komatiites; picrites; Paleoproterozoic rocks; the Chapesvara complex; serpentinite belt; Kola Peninsula; Fennoscandian Shield; Russia ultramafic complexes; subvolcanic intrusions; crystallization of highly magnesian magma; komatiites; picrites; Paleoproterozoic rocks; the Chapesvara complex; serpentinite belt; Kola Peninsula; Fennoscandian Shield; Russia
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Barkov, A.Y.; Korolyuk, V.N.; Barkova, L.P.; Martin, R.F. Double-Front Crystallization in the Chapesvara Ultramafic Subvolcanic Complex, Serpentinite Belt, Kola Peninsula, Russia. Minerals 2020, 10, 14.

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