Lithospheric roots and asthenospheric upwarps of the NE Baltic Shield:
spatial controls for kimberlitic and alkaline magmatism
Zozulya D.R.*, Peltonen P.**, O’Brien H.***, Lehtonen M.***
* Geological Institute, Kola Science Centre, Apatity, Russia; ** Northland Exploration Oy, Espoo, Finland; *** Geological Survey of Finland, Espoo, Finland.
The NE Baltic Shield can be considered as prospective on diamondiferous magmatism at a whole. The more general structural-tectonic condition is met for it – confinement to the platform with Precambrian basement. This part of the Shield represents the oldest continental crust with cool lithosphere (<40 mWt/m2) of 170-240 km thickness. The petrological and mineralogical criteria for kimberlitic magmatism are also fulfilled. The region is characterized with abundant dykes and explosive pipes of alkaline and alkaline-ultramafic composition. The numerous indicator kimberlitic minerals and diamonds are recovered from the Quaternary sediments of the region. Based on their findings the inferred kimberlitic fields are predicted in the region (Makeevka, Pyalitsa, Pulonga, Snezhnitsa fields in the south-east, Zarechensk field in the south-west (Zozulya et al., 2008)). In the southern part of region the two Ermakovsky low-grade kimberlitic pipes are found (Kalinkin et al., 1993).
More than 600 pyrope and chrome diopside grains recovered from the Quaternary sediments (till, alluvium, coastal sediments) of the southern, central and northern parts of the Kola craton were analyzed in order to determine their P-T parameters. Representing either mantle xenocrysts or constituents of mantle xenoliths, the pyropes and chrome diopsides contain valuable information on the composition of the lithospheric mantle and its thermal properties.
It is established (Hirvas, 1977) that in the glaciated areas the main portion (75-90%) of the transported detrital material has rather local (0-5 km) than distal sources. So the samples studied mostly characterize the closest areas. As the sediments could contain the shallower pyropes and diopsides from crustal eclogites and eclogitic xenoliths and xenocrysts from non-kimberlitic dykes and pipes the thorough discrimination of minerals originally derived from the deep lithospheric peridotites was done (G9 (lherzolitic) and G10 (harzburgitic) pyropes; chrome diopsides with Al2O3<4.5 wt%, Cr2O3>0.5 wt%, Na2O<2 wt% and MgO>15 wt%). Ni-thermometry of Ryan et al. (1996) on pyropes gives a range of temperatures between 650-1250œC, corresponding to a sampling interval of ca. 75-190 km. From the distribution of the different pyrope groups and their trace element compositions, stratified structure of the SE Kola craton lithospheric mantle is inferred: G10-pyropes are absent in the shallow mantle horizon (75-110 km) which is the main source of G9-pyropes, while deeper mantle horizon between 110 and 190 km has contributed abundant G10-pyropes to magmas. Ca. 20 % of these pyropes are derived from the stability field of diamond, i.e. from the depth of 130-190 km. The stratified structure is established also for major part of Karelian craton with Kaavi-Kuopio and Kuhmo kimberlitic and lamproitic fields (Lehtonen et al., 2004; Peltonen & BrØgman, 2006; Peltonen et al., 2008).
P-T values for peridotitic chrome diopsides using the single-grain thermobarometer of Nimis & Taylor (2000) imply that most of grains from SE Kola fall into the graphite stability field within 20-45 kbar and 700-1300 œC. These grains apparently originated from non-diamondiferous ultramafic xenoliths in alkaline-ultramafic dykes of the region and located to the west from sampled area. Nevertheless, ca. 15% of SE Kola diopsides yield values of 40-60 kbar and 700-1100 œC and have been derived from the stability field of diamond. Diopsides from SW Kola yield P-T estimates within of 45-65 kbar and 850-1100œC, and mostly fall into diamond stability field. Diopsides from the central Kola yield P-T values of 20-55 kbar and 700-1150œC, ca. 20% of them being derived from the diamond field. Diopsides from the northern Kola region, in turn, yield P-T values of 20-45 kbar and 600-1300œC, them all falling within the stability field of graphite. The maximum depth of xenocryst sampling varies from up to 200 km in the south-eastern and south-western Kola, to 170 km in central Kola, and down to 140 km in the northern Kola region.
The P-T values for chrome diopsides thus imply significant regional variations in the heat flow. Within the southern part of the Kola, adjacent to the Kandalaksha graben, the chrome diopside data is consistent with the 38-46 mW/m2 model geotherm of Pollack & Chapman (1977). Importantly, towards east and west, away from Kandalaksha graben, the lithosphere appears to become thicker and the heat flow corresponds to the cool cratonic model geotherm of 35-38 mW/m2. The central Kola, in turn, is characterized by more elevated heat flow of ca. 38-44 mW/m2. The highest heat flow values (up to 50 mW/m2), are observed in the northernmost Kola craton, adjacent to the Barents rift system.
Pyropes and chromites from Ermakovsky kimberlite (southern Kola craton) show a range of temperatures between 650-1000œC, corresponding to a sampling interval of ca. 75–150 km. This is in accordance with the low diamond grades determined for these pipes. The data show a different lithosphere mantle composition beneath area; one that is relatively uniform in its harzburgite-lherzolite distribution. The similar lithospheric composition is observed for the NE segment of Karelian craton with Kuusamo kimberlitic field (Peltonen et al., 2008). This could be explained by the spatial proximity of the Ermakovsky area to the Kandalaksha graben of the Belomorian rift system. Paleozoic reactivation of the rift may have altered the stratified Archean lithospheric mantle. Additionally, melt metasomatism is confirmed for this mantle, based on higher TiO2 contents in pyropes (average value 0.21 wt%) relative to those from the south-eastern area (average value 0.11 wt%).
Based on the whole data on xenocryst thermobarometry from Karelia and Kola cratons the follow structure of the lithosphere could be outlined along the ca. 1000 km transect “Karelian craton-Kandalaksha graben-Kola craton” (Fig. 1). For the major part of Karelia craton (Kaavi-Kuopio and Kuhmo areas) and for the SE part of the Kola craton the deep lithospheric roots are inferred, up to 240 and 200 km, correspondingly. The lithospheric mantle here is stratified: the shallow horizon (75-110 km) is of mainly werhlitic (Kaavi-Kuopio) or lherzolitic (Kuhmo and SE Kola) content; the middle horizon (110-190 km) is of lherzolitic-harzburgitic content; and the deepest horizon (190-240 km) is of lherzolitic (Kaavi-Kuopio) or lherzolitic-harzburgitic (Kuhmo) content and presented only in Karelian craton. The roots are of Archean age (Peltonen & BrØgman, 2006) and cratonic geotherm is cool (34-38 mW/m2). These areas are most promising for diamondiferous kimberlitic magmatism.
Fig. 1. Simplified geological cross-section of the lithosphere along the “Karelian craton - Kandalaksha graben - Kola craton” transect with kimberlitic occurrences shown.
Asthenosphere uplift is observed in the NE part of Karelia craton (Kuusamo area) and in southern Kola craton (Ermakovsky area, adjacent to Kandalaksha graben) up to 180 and 140-150 km, correspondingly. The lithospheric mantle of this segment is of homogeneous lherzolitic-harzburgitic composition and characterized by elevated heat flow values (38-46 mW/m2). Apparently, the regeneration of mantle occurred here during 1.2 and 0.36 Ga rifting. This area is characterized with the abundant occurrences of rift-related alkaline and ultrabasic-alkaline dykes and pipes and has low/no diamond potential.
The data obtained indicate also on the shallower thickness of the northern part of the Kola craton down to 140 km. This area is also characterized by the elevated heat flow values (up to 38-50 mW/m2) and by the presence of non-diamondiferous alkaline-ultrabasic rocks.
The study is supported by the Department of Earth Science RAS (Priority programs 6 & 8).
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