2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Rock-Forming Minerals of Paleozoic Alkaline-Ultrabasic Dikes within the Spitsbergen Archipelago 

Marina Yu.Burnaeva

FGUP “VNIIOkeangeologia named after I.S.Gramberg”, St.Petersburg, Russia

burnaevam@mail.ru

The dikes have been studied in detail to reveal their association with a kimberlite formation. Data on the composition of rocks and on clinopyroxene characteristics were reported at a school “Alkaline Magmatism of the Earth” in 2008 and 2009, respectively.

Mineral facies in amounts of 62 were defined under the study. The main rock-forming minerals of the dikes are olivine, clinopyroxene and mica; the olivine being about entirely changed: serpentinized, chlorized, carbonatized. Slightly changed grains are typical of a mineral from xenolite. More frequently these grains are colorless showing refractive indices (Ng’=1.690, Np’=1.651) that allows this mineral to be referred to 90% forsterite, but observed is a variety of yellowish mineral which is more ferrugeneous (81% Fo). Microprobe analysis recorded isomorphic admixture of nickel (0-0.28 mass.% NiO) and calcium (0-0.25 mass.% CaO) in the mineral, manganese was once recorded (0.08 mass.% MnO). 

Clinopyroxene and mica bear more comprehensive information for the dikes.

All the clinopyroxenes studied mainly refer to Ca-Mg-Fe varieties. Three mineral varieties: diopside, augite and fassaite were discerned among them by crystallochemical formulae. Three mineral generations were ascertained.  The first one is a high-chromous diopside and augite (up to chromodiopside and chromoaugite). In most cases this mineral consists of natrium contained in cosmochlorite and jadeite minerals. These characteristics of pyroxene of generation 1 result from its formation at great depths under high temperature and pressure. Pyroxene of generation 2 is presented by augite often with increased content of chromium, occasionally of natrium, by diopside and diopside enriched in titanium. This pyroxene shows a zonal pattern. Clinopyroxene of generation 3 is presented by diopside, augite, often with increased content of titanium (up to titanaugite) and by fassaite; subsilic varieties of the mineral appear. All the compositions are recorded to be divided into two groups, suggesting two basic stages in rock formation: abyssal and hypabyssal ones.

In single cases, recorded in the dikes are aegirine and orthopyroxene – enstatite (Mg#=88-92%). 

The orthopyroxene is supposed to have been released under xenolith desintegration. The microprone analysis shows the orthopyroxene to involve up to 4.4% Al₂O₃, up to 7.4%  FeOt, up to 1.6 CaO. Most of the grains contain Cr₂O₃ (up to 0.5%). By the positive value of discriminant function of D(x)=+0.506-0.1008Al^IV+0.009Al^VI+0.00515Mg-0.0419Ca+0.064Na, enstatite from the dikes refer to ultrabasite type and enters the field of compositions of enstatite from peridotite of ultrabasite formation (Dobretsov et al., 1971).

Mica is abundant in the dikes. For the most part, it is trioctahedral iron-magnesium mica of biotite-phlogopite series, less frequently – dioctahedral mica – muscovite and ganterite? formed under final phases of rock crystallization.

Iron-magnesium mica is observed in every variety of the dikes. A brown-colored mineral showing direct pattern of pleochroism  is presented by three generations: 1 - megacrysts (up to 2 cm); 2 - large often poicilitic microlites,  tabular grains of mica from amygdules; 3 - small microlites and xenomorphic grains. The total content of mineral is from 2 to 18%.

The composition of mica has been studied by microprobe analysis. Variations in the composition are given in the table. The mica studied is characterized by high contents of titanium and in some samples by great amounts of barium and phosphorus admixture.

Titanium is important to clear up what genesis the mica is of. A diagnostic diagram for TiO₂ – MgO by Yu.V.Malyshonok (1993) is used to compare mica of different genesis. In the diagram, compositions of the dikes mainly enter the fields of titaniferous and titanous mica partly typical of lamprophyres and lamproites (fig. 1).

Mica of generations 2 and 3 shows a zonal pattern. There, contents of SiO₂, FeOt, , MnO, K₂O and TiO2 increase and MgO, Al₂O₃ decrease from center to edge.  Occasionally, the content of Na₂O is increased in edge zones. No zonal pattern is observed in megacrysts (generation 1).

Similarly to the clinopyroxenes, all the compositions of mica from the dikes are divided into two groups by the nature of interaction between mineral-forming elements, belonging to the abyssal and hypabyssal stages of the dike formation. For mica of the first group, the increase in silica content is responsible for the increase in that of magnesia; the content of aluminia is nearly stable while variations in the content of ferrum. For mica of the second group, the increase in MgO is caused by the decrease in SiO₂ and the increase in  ferrum is followed by the decrease in Al₂O₃. As a whole, the compositions evolve towards the decrease in magnesia content due to the increase in the content of ferrum, some increase in the content of titanium and the decrease in the content of aluminia. A similar trend is called Aldansky, it is typical of collisional lamproites (Bogatikov et al., 1991).

In mica from kimberlite and lamproite as compared to the studied compositions of mica from the dikes, the content of SiO₂ and MgO is clearly higher, but that of Al₂O₃, FeOt, CaO is lower.  The content of BaO in barium-containing mica from the dikes reach 4.3 mass %, but mica from kimberlite and lamproite show values under 1.5%.

In kimberlite and similar rocks, special attention is always paid to chrome-spinel. In the dikes, chrome-spinelide is presented mainly by picotite, chrompicotite and subferrichrompicotite. Al₂O₃ (18.2-55.5/39.8), TiO₂ (0-2.7/0.2), FeO (5.3-13.8/9.8), Fe₂O₃ (0-8.6/4.1), MgO (14.2-21.7/18.2), Cr₂O₃ (8.3-49.8/26,0) were recorded (mass% from-to/average) in compositions of chrome-spinelide (n=23) by microprone X-ray spectrographic analysis.

Ilmenite in the dikes is rarely observed as single black-colored grains of tabular or fragmental shapes, 0.2-0.3 mm in size. Under reflected light the grains are about homogeneous; rather thin rare hematite plates being observed only occasionally. No magnesium was recorded by the microprone analysis in the mineral composition but manganese admixture is present. In ilmenite grain included in clinopyroxene from augitite dike, a pyrophanite component is 38% (17.5 mass % in composition). This grain has probably been crystallized from a melted inclusion in pyroxene.

In microsections and grinding samples, recorded were single fragmental occasionally rounded grains of garnet. The mineral is most commonly of pink (almandine) or reddish-brown color (andradite). Pyrope was recorded in two samples (grains of violet-red color, with refractive indices < 1.740) and studied by microprone analyzer. The results were processed according to a scheme presented by D.Schulze (2003). Compositions of the grains enter the field of mantle garnets; 11 and 6 of them corresponding to eclogite and lherzolite ones, respectively (among them 2 show TiO₂ > 0,5 mass %), that is typical of garnet from megacrysts.

During the study into material composition of the dikes, distinguished were minerals typical of abyssal conditions: properly magmatic, autometasomatic and hydrothermal stages. For olivine, clinopyroxene, cromo-spinelide and magnesium-ferruginous mica, a few generations were recorded, showing the abyssal and hypabyssal stages of rock formation. Distinguished is a zonal pattern formed due to nonequilibrium they have been crystallized under. Among the minerals there are varieties characteristic of alkaline and alkaline-basaltic rocks – astrophyllite, baddeleyite, aegirine, pyroxene with abundant titanium and oligoclase, iron-magnesium mica containing barium and considerable amounts of titanium, arfvedsonite and highly titaniferous amfiboles. A great share of the dikes belongs to minerals containing volatile components: mica, analcite, chlorite, talc, serpentine, suggesting that the magmatic melt is saturated with volatile components. The same is supported by amygdules being an integral part of the rocks and by the presence of rounded formations – spherules - in the dikes. Compositions of olivine, clinopyroxene, iron-magnesium mica of the dikes differ from kimberlite and lamproite ones by the content and ratios of MgO, TiO₂, Al₂O₃, FeOt. Chromo-spinelide is as a rule poorly chromous. The entry of titanium mainly into silicate phases should be noted as a typical feature of the mineral composition. Considering the petrochemical and mineralogical features of the rocks we can state that the rocks studied bear no relation to the kimberlite formation.

The references:

Minerals: Reference Book. M.: Nauka, - 1981, v. 3, n. 2. 614 p. (in Russian)

Dobretsov N.L., Kochkin Yu.N., Krivenko A.P., Kutolin V.A. Rock-forming pyroxenes. M.: Nauka, 1971. 454 p. (in Russian)

Bogatikov, Ryabchikov, Kononova et al. Lamproites. M., Nauka, 1991. 302 p. (in Russian)

Schulze D. J. A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds// Lithos. 2003. № 71 P. 195– 213