Regional peculiarities of lamproite and kimberlite magmatism of the southwestern Siberian platform
Egorov K.N., Kiselev A.I., Minayeva Yu.A.
Institute of the Earth’s crust SB RAS, Irkutsk, Russia
Within the southwestern Siberian platform three stages of lamproite and kimberlite intrusions are distinguished: Mesoproterozoic (Middle Riphean ~1200 Ma), Neoproterozoic (Vend ~ 630 Ma) and Middle Paleozoic (Devonian ~370 Ma). Diamond potential of potassic mantle rocks within the southwestern Siberian platform was previously determined only for Middle Riphean lamproites of the Ingashi field. In 2008 the executives of branch office “Siberian diamonds” after thermochemical decomposition of 25 kg sample of -0.5 mm class and 0,1-0,5 mm in size recovered 10 microdiamonds of 0.1-0.5 mm in size from one Middle Paleozoic dyke of lamproitoids of the Biryusa River basin. All crystals are transparent of intensive yellow, greenish-yellow color. By crystallographic form the diamonds belong to combinative crystals morphology of which is complicated by combination of hexaoctahedron, tetrahexahedron and hexahedron faces. The presence of “ancient” Precambrian and Phanerozoic types of diamonds in the Biryusa and Ingashet Rivers placers confirms the existence of two (Middle Riphean and Middle Paleozoic) epochs of tectono-magmatic activity following by formation of diamondiferous lamproite (kimberlite) bodies within the Siberian platform southern boundary (Egorov et al., 2009).
Mesoproterozoic stage (Middle Riphean)
Lamproites of Prisayan’ye (emplacement age 1268±12 Ma) are the sole occurrences of Middle Riphean diamondiferous magmatism within the Siberian platform. By classification (Bogatikov et al., 1991) lamproites belong to phlogopite-olivine ultrabasic varieties with titanium oxide content from 0.92 to 3.89 mass %. The rocks contain a series of typomorphic minerals typical of ultrabasic lamproites: tetraferriphlogopite (TiO2 6-8 mass.%, Al2O3 5-7 mass %), priderite, Nb-rutile (Nb2Î5 up to 1.61 mass %), Mn-ilmenite (MnÎ 2-3 mass %, admixture of Nb2Î5 up to 3.5 mass %), F-Sr-apatite (SrÎ 1.5-12 ìàss %, F 2-4 ìàss %), La-Ce rhabdophanite (La2Î3 20-22 ìàss %, Ce2Î3 34-36 ìàss %), armolkolite (Cr2Î3 up to 0.5 ìàss %). Diamonds, pyropes of ultrabasic and eclogite (including diamondiferous) parageneses are identified among high-pressure accessory minerals. In addition, chromspinelids (Cr2O3 up to 65.2 ìàss %) and chromdiopside (Cr2O3 up to 3.6 ìàss %) are present in lamproites; picroilmenite is not found.
Lamproites are characterized by relatively high concentrations of LILE and negative anomalies of Nb, Ta within spectra of rare elements. Isotopic composition of lamproites (eNd = -9.9 ÷ -3.8; 87Sr/86Sr(t) = 0.7044 ÷ 0.7061) suggests the formation of these rocks in enriched mantle of 1 type (EM-1). TNd(DM) model age of enrichment of the lamproites mantle source is equal to 2.1-2.0 Ga.
Neoproterozoic stage (Vend)
Within Prisayan’ye various potassic alkaline ultrabasic rocks of the Zima complex (650-630 Ma) are common. Micaceous kimberlites found in the Yarma River basin compose of single dykes or associate with picrites in composite dyke bodies.
Micaceous kimberlite from the Bushkanaiskaya dyke is composed of porphyric segregations of altered olivine, phlogopite, calcic diopside and chromdiopside (Cr2O3 1.5-2.5 ìàss %). The rock ground mass is represented by pseudomorphs after olivine of II generation, laths of phlogopite, microlites of diopside and serpentine-carbonate matrix. Spinellids form a wide range of composition: from Ti-bearing alumomagnesiochromites to Ti-magnetites. High content in the kimberlite ground mass of Cr-spinellids (Cr2O3 45-55 ìàss %) and small amount of ulvospinels and Ti-magnetites suggests its prospective diamond potential (Arkhangel’skaya…, 1999). Rare grains of picroilmenite and manganoilmenite with abnormal high content of Mn oxide (up to 17 mass %) are observed.
Micaceous kimberlite from the Bushkanaiskaya dyke falls in low-Ti petrogeochemical type (0.6-0.7 ìàss % TiO2) (Bogatikov et al., 2007). The kimberlite is characterized by relatively low contents of Zr (100.68 ppm), Nb (37.85), Ce (26.61), La (17.26), Y (6.65) and low total of REE (64.37 ppm). It has relatively high values of Zr/Nb (2.66), Ba/Nb (9.81), Rb/Nb (0.87), Ba/La (21.51), Ba/Th (531.11) indicator ratios and low values of La/Yb (23.15) and Ce/Y (4.0). Negative anomalies of Th, U, Ce and maxima of Ba, Pb, Zr, as well as slightly differentiated gentle slope of HFSE and REE distribution spectrum are typical of it. The kimberlite isotopic composition (-9.0 eNd and 0.7050 87Sr/86Sr(t)) shows that its mantle source corresponds to enriched mantle of 1 type (EM-1). TNd(DM) model age of enrichment of the kimberlite mantle source is 2.1 Ga.
Paleozoic stage (Devonian)
Within the Prisayan trough (Biryusa River basin) the dyke bodies composed of lamproite-like potassic rocks are found; the age of one rock is 370 Ma (Rb-Sr dating). Petrographic mineralogical and petrogeochemical peculiarities of potassic mantle-derived magmatites allow to falling them in lamproites of basic composition according to classification (Bogatikov et al., 1991).
Lamproitoids are composed of phenocrysts of chromous augite; chrome-diopside (Cr2O3 up to 1.2-2 ìàss %, Na2O 0.6-1.8 ìàss %), altered olivine and more rarely Ti-Ba-phlogopite. Groundmass is represented by subcalcic salite, psedomorphs after olivine, sanidine, ferruginous phlogopite and devitrified glass. Spinellids with composition from alumomagnesian chromspinellids to Ti-ferrichromites are predominant among oxidized ore minerals, Ti-magnetites and ilmenites with MnO content up to 4 mass % are less common. Microdiamonds, single grains of pyropes of lherzolite paragenesis (Cr2Î3 no more than 3 ìàss %), pyropes-almandines, garnets of goldmandite-uvarovite series, chromgrossulars, chromspinellids (Cr2Î3 40-59 ìass %) are found among accessory minerals.
Petrochemical specific character of lamproitoids is expressed in relatively low Ti content (TiO2 0.55 ìàss %), moderate (MgO/MgO+FeOtotal) ratio (MgO 12.88 ìàss %), relatively high Al (Al2O3 13.41 mass %) and K contents (K2O 3.4 ìàss %) and high Cr content (up to 1330 ppm). Lamproitoids are characterized by relatively high Ba (4413.84 ppm), moderate La, Ce and low Nb (67.02 ppm), Ta (5.76 ppm) and Zr (53.25 ppm) concentrations. Distribution peculiarities of REE (negative anomalies of Th, U, Nb and to a lesser degree of Ta) in lamproitoid suggests the derivation of primary melts from lithosphere with traces of crustal contamination.
The available isotopic data (εNd = -8.39, 87Sr/86Sr(t) = 0.7086) suggests that the lamproitoid mantle source corresponds to enriched mantle of EM-1 type. Relatively high 87Sr/86Sr(t) value in lamproitoid can be related to assimilation by melt of crustal material with high 87Sr/86Sr ratio. TNd(DM) model age of the mantle source enrichment of Middle Paleozoic lamproitoids is 1.8 Ga, i.e. is typical of model ages of EM1- type sources (AR3-PR1) (Arkhangel’skaya…, 1999).
Thus, within the southwestern Siberian platform three stages (Middle Riphean, Vend, and Devonian) of occurrence of potassic (including diamondiferous) mantle magmatism are distinguished. TNd(DM) model age of the mantle source enrichment of diamondiferous lamproites and lamproitoids along with micaceous kimberlites varies from 2.1 to 1.8 Ga. This temporal interval corresponds to collision events associated to accretion of terranes and formation of Siberian craton in its recent structure. At final stage of the collision system development the tectonically thickened lithosphere has become gravitationally unstable. This event was followed by destruction (delamination) of lower lithospheric mantle and foundering of their blocks into asthenosphere.
Ascent of hotter asthenosphere in the places of lithospheric thinning contributed to increase of the degassing of fluids and their matasomatic influence and enrichment of incoherent elements of the lower lithospheric mantle. Delaminated blocks of mantle lithosphere as well as fragments of ancient subducted crust could be the additional sources of enrichment (Bogatikov et al., 2007 a.o.). As a result, in the range of 2.1-1.8 Ga the metasomatized areas capable to generate various potassic magmatites were formed during collision hummocking of microcontinents and delamination of lithosphere. Break in time between intrusion of lamproites, kimberlites of the Prisayan region and metasomatic enrichment of their mantle sources is enough great : from 0.8 to 1.4 Ga. Such significant temporal interval created favorable conditions for enrichment of the source in fluids and/or crustal material.
It should be noted that diamondiferous lamproites, lamproitoids and kimberlites of the southwestern Siberian platform have similar isotopic geochemical signatures (negative values of eNd, ancient model age, source of EM1-type) significantly different from micaceous picrites and alkaline rocks of the Zima complex of Prisayan’ye. Spatial coincidence of diamondiferous lamproites dykes of the Ingashi field, numerous dispersion haloes of indicator minerals of diamond along with dykes of micaceous kimberlites grouped in single N-W-trending linear zone considerably increases the prospects for primary diamond potential within Eastern Prisayan’ye.
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