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Lamproites from the Kostomuksha area, Karelia: new geochemical, Nd-Sr-O isotopic and mineralogical data
Kargin A.V., Nosova A.A., Kononova V.A., Larionova Y.O.
The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences (IGEM RAS), Moscow, Russia;
New whole-rock major and trace elements, Sr, Nd, O, C isotopic and mineralogical data on the dykes and lens-like lamproitic bodies from the Kostomuksha area, in the northeast European Platform, are reported. The lamproites are subdivided into two main types, Cpx-Phl-Ol and Phl-Ol, which show different mineralogical and geochemical features and originated from mantle sources with differences in depth of melting and composition.
Lamproites are rare potassium- and Mg-rich igneous rocks. They were found in different tectonic settings: in post-orogenic (SE Spain, central Italy, NW Turkey) and in intraplate (India, Antarctica, Australia and others) environments. Young, mainly Cenozoic lamproites are widely distributed, whereas occurrences of ancient Proterozoic lamproites are few in number. The most famous ancient lamproites such as Argyle (Australia) and Krishna (India) are around 1200 Ma in age, originated in intraplate setting, and have high diamond productivity.
The Kostomuksha- Lentiira-Kuhmo lamproitic field located in the northeast European Platform is another example of 1200 Ma-old diamondiferous lamproite. In spite of the detailed study of these lamproites for the last two decades by Russian and Finnish petrologists (Nikitina et al., 1997; Belaytcky et al., 1997; Antonov, Ulianov, 2008; O'Brien, Tyni, 1999; O’Brien et al., 2007; Lehtonen, O’Brien, 2009), many questions concerning their origin are still debatable.
In this work, we report new whole-rock major and trace elements, Sr, Nd, O, C isotopic and mineralogical data on the dykes and lens-like lamproitic bodies from the Kostomuksha area.
Our study showed that the lamproites are subdivided into two main types, Cpx-Phl-Ol and Phl-Ol, which show different mineralogical and geochemical features.
Cpx-Phl-Ol-type lamproite consists of two generations of olivine, as well as phlogopite, subordinate clinopyroxene and minor apatite, spinel, magnetite, and perovskite. In some samples perovskite is the predominant mineral. Olivine is completely replaced by serpentine, sometimes by talk. Phlogopite occurs as phenocrysts, as well as poikilitic crystals and flakes in the groundmass. In addition, phlogopite is observed as inclusions in olivine phenocrysts. Chemical features of mica are typical of those in lamproites, with Mg# variations from 0.63 to 0.82. Clinopyroxene forms unzoned diopside crystals.
Phl-Ol-type lamproite consists of two generations of olivine and phlogopite. The minor phases are apatite, spinel, and magnetite. Mg# of the phlogopite varies from 0.66 to 0.86. In some samples phlogopite crystals are zoned, with core-to-rim increase in Fe and Al contents.
Cpx-Phl-Ol-type lamproite has higher MgO (25.58-28.79 wt. %), and lower Al2O3 (average 7.39 wt. %), CaO (average 7.48 wt. %), P2O5 (average 0.32 wt. %) contents than the Phl-Ol-type lamproite (13.02-26.28; 9.23; 11.08; 1.29 wt. % respectively). Based on these petrochemical features we can assume that Cpx-Phl-Ol-type lamproite has a more primitive nature. Variations of Al2O3 vs CaO in binary diagram form two distinct trends that show predominantly clinopyroxene fractionation for the Cpx-Phl-Ol-type lamproite and mainly phlogopite fractionation for the Phl-Ol-type lamproite.
Trace elements distribution in the Kostomuksha lamproite samples supports this conclusion. The Cpx-Phl-Ol-type lamproite have higher Cr (average 1300 ppm) and Ni (average 960 ppm) abundances than the Phl-Ol-type lamproite (900 and 800 ppm respectively). The average concentrations of Sr and Ba in the Cpx-Phl-Ol type lamproite are 475 ppm and 1300 ppm, respectively, whereas those in the Phl-Ol-type lamproite are 1015 ppm and 1700 ppm, respectively. The main geochemical difference between these two types is their LREE/HREE ratios. The (La/Yb)n ratios are significantly higher in the Cpx-Phl-Ol type lamproite (240-600) than in the Phl-Ol type lamproite (140-270). The mantle-normalized trace element pattern (Fig. 1) demonstrates that the Phl-Ol-type lamproites are more enriched in LILE, HFSE and HREE than the Cpx-Phl-Ol type lamproites.
The geochemical differences between these types cannot be explained only by difference in fractionation paths, but presumably indicate differences in depth of melting and source composition.
The Sr-Nd isotopic composition of the Kostomuksha lamproites is similar to those of analogous rocks from other localities (Leucite Hills, Gaussberg). The lamproites have low 87Sr/86Sr ratios that vary from 0.7034 to 0.7067. Note that samples with higher Sr concentrations have lower 87Sr/86Sr ratios, indicating some crustal contamination. Values of εNd range from -8.4 to -6.9, with Nd model ages of 2.0 Ga. These model ages suggest that old metasomatic event associated with LREE enrichment of the mantle source occurred simultaneously with 2.0-Ga-old plume magmatism in the Karelian Craton.
The δ13C of the carbonates from lamproites fall within a relatively narrow range from – 4.4 to -6.8 ‰. On the contrary, the δ18OSMOW values of the carbonates vary widely from +13.3 to +18.6 ‰. Strong correlations between δ18O of carbonates and MgO, Cr, Ni concentrations in host lamproites indicate the primary nature of carbonates (as well as C isotopic composition) and the existence of process that controlled O isotopic composition of carbonate during fractionation.
Based on compilation of previous datasets plus new data, we proposed a new model for the petrogenesis of the Kostomuksha lamproites.
Fig. 1. Primitive mantle-normalized incompatible element diagrams for the Kostomuksha lamproites. Primitive mantle values are (McDonough, Sun, 1995).
This study was financially supported by RFBR grant 05-10-00901
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