Mantle Depth Facies of High-Pressure Pyroxene in the Kola Region

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1 ISSN X, Doklady Earth Sciences, 2009, Vol. 424, No. 1, pp Pleiades Publishing, Ltd., Original Russian Text D.R. Zozulya, P. Peltonen, H. O Brien, M. Lehtonen, 2009, published in Doklady Akademii Nauk, 2009, Vol. 424, No. 4, pp GEOLOGY Mantle Depth Facies of High-Pressure Pyroxene in the Kola Region D. R. Zozulya a, P. Peltonen b, H. O Brien b, and M. Lehtonen b Presented by Academician N.V. Sobolev February 12, 2008 DOI: /S X Received February 20, 2008 The Kola region is a classic area of alkaline and carbonatite magmatism. It is considered to be permissive for diamond-bearing magmatism, in agreement with the general prerequisite of kimberlite localization their occurrence in platforms with a Precambrian basement and km thick lithosphere. It is known that this rock is mainly derived from the mantle. The major goal of this study is to clarify magma-forming mantle depth facies of alkaline rocks, carbonatite, and kimberlite in the Kola province. The P T conditions of crystallization of Cr-diopside were estimated from its chemical composition. Being either deep-seated xenocrysts or constituents of mantle xenoliths in kimberlites, the high-pressure mineral Cr-diopside bears information on the depths of magma generation in lithosphere mantle and heat flow of the region. The thermodynamic properties of Cr-diopside allow us to specify the prospects of this region and its separated localities for the occurrence of diamondiferous kimberlite and lamproite bodies. The grains of the mineral collected in the process of panning the Quaternary sediments of the Kola region were used for complete regional characteristic. The igneous minerals from different levels of the mantle can be present in the examined collection of Crdiopsides (mainly from the Quaternary alluvial, nearshore marine and glacial sediments) of the southern (Zarechensky, Yermakovsky, Varzuga, Ondomozero, and Pulon ga exploration areas), central (Lovozero area), and northern (Sredny and Ribachy Peninsulas, Murmansk and Iokan ga areas) parts of the region (Fig. 1). Kimberlite of the Zimnii Bereg [1] and Tersk districts [2], northern Finland [3], and permissive areas a Geological Institute, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, Russia b Geological Survey of Finland, P.O.B. 96, FI-02151, Espoo, Finland; zozulya@geoksc.apatity.ru of the Kola Peninsula [4, 5] can be the deepest source of chrome diopside. Xenocrysts and xenoliths of ultramafic and eclogitic assemblages from numerous dikes and explosion pipes composed of alkaline ultramafic rocks (monticellite kimberlite, alkali picrite, olivine melilitite, monchikite, damkiernite, melanephelenite) abundant in the northern Kandalaksha Graben, Khibiny, and Lovozero nepheline syenite massifs, and on the shore of the Barents Sea [6 8], as well as igneous alkaline ultramafic rocks (Kovdor, Afrikanda, Salmagore, Vuorijarvi, and Seblyavr massifs) are derived from shallower sources. The southern and central districts are located within the area of the action of the Scandinavian glacier (Fig. 1), which transported clastic material to the east [9]. The northern district is located within fans of the same glacier that transported material to the north and northeast [9]. Glacial deposits, which are intermediate collectors for the sampled rock facies, are the most abundant loose Tuloma River Kandalaksha Gulf Barents Sea Pokoi River Varzuga River 0 40 km Iokanga River Fig. 1. Areas of loose sediment panning and location of (1) nepheline syenite and (2) alkaline ultramafic intrusions; and (3) explosion pipes in the Kola region; (4) directions of transportation of clastic material by the late Valdaian Glacier Narrow entrance to the White Sea

2 MANTLE DEPTH FACIES OF HIGH-PRESSURE PYROXENE 53 Representative chemical analyses of Cr-diopside in the Kola region, wt % Component Ultramafic assemblages from the field of diamond stability PL-27-1 PL-27-2 PL-27-3 PL-23-1 PL-23-3 PL-23-4 PL PL SH-2 P-256 SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO K 2 O Na 2 O NiO Total Ca/(Ca + Mg) mg# P, kbar Component Ultramafic assemblages from the field of graphite stability Eclogitic assemblage PL-20 PL-23-1 PL-23-5 PL-23-6 PL PL-27-4 PL-23-8 PL PL PL-3-5 SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO K 2 O Na 2 O NiO Total Ca/(Ca + Mg) mg# P, kbar Note: The P T parameters of crystallization of Cr-diopside from ultramafic assemblage were estimated from the geothermobarometer [12]. sediments in all the districts studied. It is established [10] that most clastic material (75 92%) of the moraine is proximal (0 5 km) and the content of parental rocks at a distance of km from the primary source rarely exceeds 15%. Thus, the portion of material that came from some distance away is insignificant and sampling areals are dominated by local material. To resolve the problem, Cr-diopside from the deepest ultramafic assemblages was selected. The known P T parameters of crystallization of the deep-seated xenoliths from eclogitic and ultramafic assemblages of dikes and explosion pipes in the Kola region range from 10 to 15 kbar and C [6, 7]. Pale to bright green (emerald) Cr-diopside occurs as slightly rounded grains or fragments of grains. Grain size is mm. The pyroxene composition was determined at the Geological Survey of Finland in Espoo on a Cameca

3 54 ZOZULYA et al. Depth, km ( ) n = 240 n = 42 n = (b) (c) P, kbar Camebax SX50 microprobe and at the Geological Institute of the Kola Scientific Center, Russian Academy of Sciences, in Apatity, on a Cameca MS-46 microprobe operating at 25 kv and 48 na with a beam diameter of 1 µm. We analyzed 550 grains. The compositional variations of pyroxenes are as follows (wt %): MgO, Cr 2 O 3, Al 2 O 3, Na 2 O, and <0.45 TiO 2. The wide range of compositions is likely caused by the presence of Cr-diopside belonging to eclogitic and various ultramafic assemblages in the analyzed collection. By composition, all the examined Cr-diopsides are divided into two groups. The first group is characterized by an elevated content of Na 2 O and Al 2 O 3 and low MgO and CaO, which corresponds to the aluminous eclogitic assemblage from deep alkaline ultramafic xenoliths of the Kola Peninsula. The second group with elevated MgO, CaO, and Cr 2 O 3, and low Na 2 O and Al 2 O 3 contents corresponds to the assemblage of diamondiferous ultramafic rocks from the kimberlite explosion pipes of the Kola Peninsula and Arkhangel sk province and can contain material from diamond-free diatrem and intrusive alkaline ultramafic rocks of the region. The most complete and representative analyses of Cr-diopsides from the ultramafic and eclogitic assemblage are shown in the table. The following criteria were chosen to refer Cr-diopside to the ultramafic assemblage (according to [11]) (wt %): Al 2 O 3 < 4, Cr 2 O 3 > 0.5, Na 2 O < 2, and MgO > 15. It should be noted that in samples with Crdiopside from the ultramafic assemblage, there are indicator minerals of kimberlite including pyropes of the potentially diamond-bearing lherzolitic and harzburgitic assemblages, Cr-spinels, and olivines [11]. The pressure and temperature of crystallization of Cr-diopsides from ultramafic assemblages were determined with a clinopyroxene thermobarometer [12], which estimates clinopyroxene crystallization conditions from its composition. The applied Cr-n-Cpx barometer is based on Cr partitioning between clinopyroxene and garnet. The determination of temperature is related to the partition of the enstatite component in clinopyroxene. The thermobarometer was calibrated on the basis of experimental data on the synthesis of clinopyroxene at 850 C and a pressure of 0 60 kbar. The reproducibility of this method is ±30 ë and ±2.3 kbar. To provide the best accuracy, we used the most complete and high-quality chemical analyses, where sites T and (M1 + M2) are occupied for more than The calculated P T parameters of pyroxene formation of the ultramafic assemblage in the Kola region satisfying the aforementioned conditions (327 analyses total) are plotted on the diagram of diamond and graph- Fig. 2. Calculated P T parameters of Cr-diopside formation of the ultramafic assemblage from loose sediments in the Kola region: (a) southeastern (open squares) and southwestern (solid squares) parts; (b) central part; (c) northern part.

4 MANTLE DEPTH FACIES OF HIGH-PRESSURE PYROXENE 55 ite stability at kbar ( km) and C (Fig. 2). Thus, the shallowest Cr-diopsides from intrusive carbonatite, alkaline ultramafic rocks, and eclogite crystallized at the crust level were not considered in the subsequent analysis. Most Cr-diopsides from the southeastern region fall in the field of graphite stability (Fig. 2a). This is due to the selection is dominated by Cr-diopsides from the relatively shallower igneous complexes of the region (carbonatite alkaline ultramafic dikes and explosion pipes with deep-seated ultramafic xenoliths). The parameters of their crystallization vary from 20 to 45 kbar and to C (Fig. 2a). However, about 15% of samples fell in the field of diamond stability and crystallized at kbar and C. The Cr-diopsides from the southwestern region that crystallized at 45 to 65 kbar and 850 to C substantially fell into the field of diamond stability (Fig. 2a). The following parameters of crystallization are characteristic of Cr-diopsides from the central region: kbar and 1150 C, with about 20% of samples being derived from diamond depth facies (Fig. 2b). Cr-diopsides of the northern Kola region were formed within a wider range of temperature ( C) and narrower range of pressure (20 45 kbar); pyroxenes of the diamond depth facies are absent (Fig. 2c). Data from the table demonstrate that Cr-diopsides of various depth facies are different in chemical composition. Pyroxenes from the field of diamond stability are characterized by low contents (wt %) of TiO , Al 2 O , FeO , and N 2 O ; elevated mg# ; and narrow variation of the enstatite component (Ca/(Ca + Mg) = ). Pyroxenes of close composition were studied both from intergrowth with diamond and as inclusions in diamonds from kimberlites of the Yakutia and Arkhangel sk provinces [1, 13]. Pyroxenes from the field of graphite stability are distinguished by elevated contents (wt %) of TiO , Al 2 O , FeO , and N 2 O ; low mg# ; and a slightly elevated enstatite component (Ca/(Ca + Mg) = ). Cr-diopsides of such composition are characteristic of deep-seated xenoliths and xenocrysts from alkaline ultramafic dikes and explosion pipes of the Kola region [6, 7]. Two model geotherms (according to [14]) calculated for low heat flow, optimal diamondiferous kimberlite magmatism, and elevated are plotted in the diagrams (Fig. 2). Two different pyroxene geotherms can be recognized for the southern Kola region (Fig. 2a). Cr-diopsides from the field of graphite stability correspond to elevated heat flow (38 44 mw/m 2 ), whereas most Crdiopsides of the diamond depth facies correspond to low heat flow (35 38 mw/m 2 ). Thus, in the studied collection, there are Cr-diopsides from three areas of deepseated igneous rocks of the southern region with different thermal properties of the mantle: the central southern part (northern side of the Kadalaksha Graben) dominated by deep-seated alkaline ultramafic dike diatrem rocks is characterized by a heated lithosphere; the southwestern and southeastern parts distinguished by a cold lithosphere are permissive for the deepest diamond-bearing kimberlite magmatism. The pyroxene geotherm common to Cr-diopsides of diamond and graphite depth facies indicating a slightly elevated heat flow of mw/m 2 is found in the central Kola region (Fig. 2b). In the northern region (Fig. 2c), along with the usual heat flow of mw/m 2, a pyroxene geotherm with a maximum heat flow of mw/m 2 is found that appears to be caused by spatial closeness to the Barents rift system [15]. Thus, this study showed for the first time that in the Kola region along with shallower alkaline ultramafic rocks, there are rocks formed at great depths of the lithosphere mantle (down to 200 km in the southeastern and southwestern region; down to 170 km, in the central region; and down to 140 km, in the northern region). Some of them (in the northern and central region) can belong to the diamond depth facies. The heat flow tends to increase (from 35 to 50 mw/m 2 ) from the southern region to the central and northern regions. ACKNOWLEDGMENTS We are grateful to researchers of the Geological Institute of Kola Scientific Center, Russian Academy of Sciences, O.P. Korsakova, V.Yu. Kalachev, I.V. Chikirev, and V.I. Basalaeva for their assistance in field and laboratory investigation. This study was supported by the Department of Earth Sciences, Russian Academy of Sciences (Program 4), and the Russian Foundation for Basic Research (project no ). REFERENCES 1. 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