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High-alkaline and ultrapotassic volcanic rocks: occurrence in geological complexes of various geodynamic settings Gushchin A.V. Federal State Unitary Enterprise “Institute of Mineralogy, Geochemistry and Crystallochemistry of Rare Elements” (FGUP “IMGRE”), Moscow, Russia
Analysis of volcanic rock series geochemistry shows that alkalinity and potassic potential are primary factors, determining levels of major and minor elements concentrations over a wide range of silica contents. High-alkaline, peralkaline and ultrapotassic volcanic rocks in contrast to non-alkaline series as a rule are strongly enriched by such elements as Ti, P, Rb, Ba, Sr, Zr, Hf, Nb, Ta, Th, Y and light REE. To high-alkaline, peralkaline and ultrapotassic rocks belong such economically important associations as kimberlites, lamproites and carbonatites. To high-alkaline in a broad sense we attribute following kinds of volcanics: a) alkaline but non-peralkaline (ANP) rocks with high levels of alkali concentrations on total alkali-silica diagram slightly modified after [Magmaticheskie…, 1987] and with agpaitic index (AI) <1; b) alkaline and simultaneously peralkaline (AP) - similar (to above mentioned) rocks with AI > 1; c) non-alkaline but peralkaline (NAP) rocks with moderate content of total alkali and AI > 1. Evident distinctions in distributions of the three high-alkaline volcanic groups are revealed (table 1).
Table 1. Frequency distribution of three groups of high-alkaline volcanic rocks in thirteen geodynamic settings arranged approximately in Wilson cycle suite (in percents of total amount of whole volcanic rock analyses)
Abbreviations in the first line correspond to following geodynamic settings: hot spot: CHSP – continental, OHSP – oceanic; CRIFT - continental rifts; TRAPS – trap provinces; OCP – oceanic plateau; MOR - mid-oceanic ridges; BAB back-arc basins; island arcs: PIA – primitive, DIA – developed, MIA – mature; active continental margin zones: AMF – frontal, AMB – rear (back); COLL – collision belts, N - number of analysis used for estimations of high-alkaline rocks percents. *See explanations of these symbols in the text above.
As can be seen high-alkaline volcanic rocks are quite frequent in continental and oceanic hotspots and continental rifts. Any representatives of such rocks in MOR and BAB settings are absent. Scarcely appear in developed and mature island arcs, as well as in continental margin fronts. They begin really seen in rear zones of continental margins and in collision belts. The most abundant high-alkaline rocks of CHSP,OHSP, CRIFT, OCP and AMB settings belong to ANP group in narrow basanitic interval of silica (42,5-47,5 % SiO2). The rocks of AP and NAP groups (mainly lamproites of intermediate silica concentrations focused at 52,5-57,5 % SiO2) are most abundant in CHSP setting. In contrast to previous mentioned the high-alkaline volcanics of suprasubduction settings belong dominantly to acid compositions. Among these are peralkaline rhyolites (pantellerites) of New Zeland arc (Mayor Island), as well as ANP rhyolites of AMF and COLL settings.
The ultrapotassic volcanics (in accordance with modified diagram after Peccerillo and Taylor, 1976), are distinguished among high-alkaline and non-alkaline rocks (Table 2).
Table 2. Frequency distribution of ultrapotassic volcanic rocks from thirteen geodynamic settings (in percents of total amount of whole volcanic rock analyses)
*See explanations of these symbols in the text above. NA – non-alkaline ultrapotassic rocks.
Comparison of data placed in two table need to explain the great contrasts accompanying distribution of high-alkaline and ultrapotassic volcanic rocks, which is clearly dependent from geodynamic position. First of all is evident the diversity of three groups of settings: 1) intraplate environments (CHSP, OHSP, CRIFT, TRAPS, OCP), 2) spreading zones (MOR, BAB) and 3) supra-subduction belts (PIA, DIA, MIA, AMF, AMB and COLL). Main reasons determining observed contrasts and differences are following: Various deeps of magma origin zones with contrasts of pressure and temperature define at least two kinds of high-alkaline and ultrapotassic magmatism at plume and supra-subduction environments. Deep, intermediate and shallow levels of magma generation control composition and dimensions of magmatic products. Interaction of local (alkaline) and mass (tholeitic, andesitic) magma types determine possibility, place and time of alkaline manifestations. As a rule alkaline magmas appear in significant mass at grate distance from simultaneous spreading zones [Gushchin, 2005]. Alkaline magmatic provinces (Maimecha-Kotuy and others) have the best conditions for manifestation in distal zones, on peripherie of large basaltic fieldes (or large igneous provinces - LIPs). Differences in composition of melting materials in global (recycling), regional (plate segmentation) and local (block structure) scales are some of most important reason, triggering the initial processes of alkaline magmatism. In global scale the deeply buried slabs of oceanic lithosphere as source of alkaline magmas are considered. Conditions of solidification in magma chambers and conduits define the temperature, dimensions and velocity of magma uplift. These factors control processes of hybridism and magma mixing, which are far unfavorable for maintain the alkaline rarity, especially in the case of such local kimberlite and lamproitic bodies. Geodynamic contrasts of fluid composition (mainly water in supra-subduction zones) strongly determine the compositions and volumes of magmatic chambers and possibilities of interactions. Stages of earth crust, lithosphere and mantle evolution define conditions of metasomatic transformation in the mantle and subsequent melting forming high-alkaline and carbonatite manifestations [Kogarko et al., 2001]. All these factors surely are in narrow interrelations. They determine bimodality and gaps in series of high-alkaline and ultrapotassic volcanism, their existence or absence and frequency distribution patterns.
References: Gushchin A.V. The non-subduction high-alkaline oceanic volcanics (geodynamic position, geochemical types, associations and evolution) / Applied geochemistry. Issue 7. Mineralogy, geochemistry and genetic types of mineral deposites. Book 1. Mineralogy and geochemistry. Ed. in chief E.K.Burenkov & A.A.Kremenetsky / Annual of scientific articles. (Минералогия, геохимия и генетические типы месторождений) – M.: IMGRE, 2005. 360 p. P. 251-275. Kogarko L.N., Kurat G., Ntaflos T. Carbonate metasomatism of the oceanic mantle beneath Fernando de Noronha Island, Brasil // Contrib. Mineral. Petrol. 2001. Vol. 140. P. 577-587. Magmaticheskie gornye porody. T. 3-5. (Igneous rocks. Vol. 3-5). Bogatikov O.A. (ed.). 1985-1987. Moscow: Nauka. Peccerillo A. and Taylor S.R. Geochemistry of Eocene calc-alkaline rocks from Katsamonu area, Northern Turkey // Contrib. Miner. Petrol. 1976. Vol. 58. P. 63-82. |