Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Microxenoliths of metosomatized spinel lherzolites in dunites from the Guli Pluton

Panina L.I.

Sobolev Institute of Geology and Mineralogy, Siberian Branch of the RAS, Novosibirsk, Russia

panina@igm.nsc.ru

Fragments of microxenoliths of spinel lherzolites have been found for the first time in phlogopitized dunites of the alkali-ultrabasic carbonatite Guli Pluton. The Guli Pluton (Vasiliev, Zolotukhin, 1975; Egorov, 1991) is localized at the border of the Siberian Platform and Mz-Kz Khatanga trough at the joint of the most active detachments during the period of the Early Triassic cycle. The pluton is surrounded by volcanites of several formations consisting of basalts, trachybasalts, trachyrhyolites, ultrabasic foidites, alkaline picrites, and meimechites. Among intrusive rocks ultrabasic ones make up 70-80% of the outcropping part of the pluton. Dunites are traced as arcuately curved belt extending for more than 40 km with an average width of 9-10 km. They are splitted by late stock-like intrusions of alkaline rocks and carbonatites. Among dunites there also occur bodies and dykes of olivine melanephelinites, picrite porphyrites, and meimechites. Dunites unlike olivines from other massifs of the Maimecha-Kotui province normally contain chromospinelides and clinopyroxene. The amount of the latter in places increases to 20-30% of rock volume, providing a gradual transition to peridotites which form schliere-like segregations in dunites.

Olivine, being the main rock-forming mineral of dunites and peridotites, accounts for 95-70 vol.% of rock and forms solid monomineral aggregates. Its grains are anhedral, poorly elongated or isometric. In peridotites they are corroded and do not have clear borders. The size of grains ranges from hundredth of microns to 3-5 millimeters. Olivine is quite often replaced by serpentine. Clinopyroxene fills the interstices between olivine grains, is anhedral, and in places replaced by phlogopite and/or brownish hornblende. Chromospinelides – Cr-, Ti-bearing magnetites – form rounded segregations and small patches up to 1-3 mm in diameter and are nonuniformly distributed in the rock. Grains of chromite, less often, small plates of ilmenite are in close association with them.

Microfragments of spinel lherzolite xenoliths found by us are slightly melted, vary in the form of segregation, size and mineral composition. They are fine-grained aggregates from fractured grains of spinelide, orthopyroxen, clinopyroxene, serpentine and other minerals. Cracks diverging from these aggregates are common. The simplest microfragments of xenoliths consist of Cr-spinelide core and numerous surrounding sharply angular grains of orthopyroxene with the same optical orientation (fragmented microcrystals?). Aggregations of chromospinelide grains have rounded, partly faceted irregular shapes, with maximum sizes up to 50х80 μm. The size of orthopyroxene grains varies from 0.5-1 to 10-15 μm. The most complex segregations besides Cr-spinelide core and orthopyroxene grains also contain anhedral fractured grains of clinopyroxene, olivine, and laths of phlogopite, added by single grains of albite, serpentine and, less often, barite, calcite and chalcosine (fig.). No definite sequence of segregation of minerals relative to Cr-spinelide grains was observed. Most often orthopyroxene grains are confined to one of the side of Cr-spinelide, whereas clinopyroxene grains and laths of phlogopite - to the other one. Only in one case we observed a consistent replacement of Cr-spinelide grains by ilmenite, and then by grains of orthopyroxene, clinopyroxene, and phlogopite. The grains of phlogopite, serpentine, albite, calcite, barite, and chalcosine tend to the peripheral parts of the mentioned segregations and, likely, belong to the products of metasomatic processing of lherzolite xenoliths.

Рис. Microxenoliths of metosomatized spinel lherzolite in olivine from dunites of the

Guli Pluton (reflected light).

The chemical composition of minerals from lherzolite xenoliths markedly differs from the composition of the same minerals in dunites and peridotites. For example, in olivine the maximum (91-89%) content of forsterite component was found in dunites and regularly decreases to peridotites (Fo=85-83%) and phlogopitized dunites (Fo=83,1%), dropping in the closest vicinity of lherzolite xenoliths to 81-82%, and in xenoliths, to 80 %. In a reverse direction changes the amount of MnO in olivines: from 0.19, 0.2 to 0.33 and 0.36, 0.43 wt.%, respectively. The maximum content of CaO (0.75 wt.%) are typical for olivine of peridotites, and the minimum (0.08-0.02 wt.%) – for phlogopitized dunites and lherzolite xenoliths. The content of NiO in olivines varies from 0.35 wt.% in dunites to 0.18 wt.% in peridotites at intermediate values in phlogopitized dunites and xenoliths. The concentration of Cr in olivine of hyperbasic rocks in general equals hundredths of a percent but in olivines of xenoliths it increases to 0.2 wt.%. Clinopyroxene in peridotites corresponds to diopside composition. It contains (wt%): 1.8 Al₂O₃, 1.5-1.8 TiO₂, 0.1-0.4 Cr₂O₃, to 0.1 MnO and 0.4-0.7 Na₂O at Mg# = 86-87%. Clinopyroxenes from lherzolite xenoliths belong to subcalcic diopsides, have low contents of Al and Ti, low Mg/(Mg+Fe) value (83-84%) and elevated concentrations of Na₂O and Cr₂O₃ (1.2 and 0.5 wt.%, respectively). Orthopyroxene from lherzolite xenoliths belongs to bronzite and has a persistent composition. Its Mg/(Mg+Fe) value is 82-83%. The mineral contains low amounts of TiO₂ and Al₂O₃ (0.11 and 0.17 wt.%, respectively), to 0.5 wt.% CaO, 0.1 wt% Cr₂O₃, and 0.6 wt.% NiO. Chromospinelides in dunites, peridotites and lherzolite xenoliths are enriched in Ti and Cr, and have increased contents of Al and Mg and marked contents of Mn. In xenoliths they are richer in FeO (76.8 vs 65-65.6 wt.%), Cr₂O₃ (10.8 vs 5.3-7.1 wt.%) and poorer in MgO (2 and 4.8-6.6 wt.%, respectively), Al₂O₃ (1 vs 2-3.4 wt.%), TiO₂ (2.2 vs 12.5-10.3 wt.%), and MnO (0.3 vs 0.6-1 wt.%). Ilmenite in lherzolite xenoliths in the content of TiO₂ (52.5 wt.%), FeO (39.2 wt.%), MnO (1.5 wt.%), and MgO (5.5-5.8 wt.%) is similar to ilmenite from porphyric peridotite but contains more of these components than that from phlogopitized dunites but less Cr₂O₃ than that from peridotite (0.14 vs 0.51 wt.%). Phlogopites in lherzolite xenoliths compared to phlogopites from dunites have higher iron contents (Fe/(Fe+Mg) = 26 vs 20%), more TiO₂ (6.2 vs 3.7 wt.%), and are enriched in Cr₂O₃ and NiO (0.52 and 0.17 wt.%, respectively), whereas phlogopites from dunites lack Cr and Ni, but contain up to 0.3 wt.% BaO. Serpentine in xenoliths belongs to the variety with high content of Fe. Unlike that in dunites, this contains more SiO₂ (41.9 vs 39.4 wt.%), FeO (12.5 vs 3.9 wt.%), MnO (0.21 vs 0.05 wt.%) and CaO (0.51 vs 0 wt.%) but considerably less MgO (28.4 against 40.9 wt.%), Al₂O₃ and Na₂O. Barite and albite in xenoliths have a standard composition but calcite in addition to Ca contains 0.9 wt.% FeO and 1.4 wt.% MgO.

Thus, lherzolite xenoliths differ from dunites and peridotites not only in the presence of bronzite but also in the chemical composition of the same minerals. That is: in xenoliths compared to intrusive ultrabasic rocks olivine has higher iron content and has more Mn and less Ca and Ni; clinopyroxene also contains more iron and less Al and Ti but more Na; Cr-spinelide from xenoliths contains more Fe and Cr, but less Ti, Mg, Al, and Mn.

Finding of microfragments of xenoliths of metasomatized spinel lherzolites in olivine of dunites from the Guli Pluton evidences that ultrabasic rocks were formed with participation of mantle peridotites. Primary spinel lherzolites, probably, underwent mantle metasomatism. The agents of the latter, as suggested from the presence phlogopite, albite, calcite, serpentine, and barite in xenoliths, were fluidized alkaline carbonate-silicate melts rich in water and SO₄ and possessing oxidizing properties. Afterwards, metasomatized spinel lherzolites, most likely, underwent cataclastic (dislocational) metamorphism, which caused fracturing of rocks and, apparently, their melting. The high-magnesian composition of parental ultrabasic meimechite magmas was, obviously, the result of interaction of melts from metasomatized lherzolites with the minerals of harzburgites. Most likely, alkaline silicate-carbonate metasomatism, dislocational metamorphism and melting of spinel lherzolites resulted from the ascending of a highly heated plume.

A similar point of view as to the origin of meimechite magmas was earlier suggested by I. A. Ryabchikov and coauthors (2009). The authors think that primary meimechite magmas resulted from partial melting of fertile lherzolite in asthenosphere and further interaction of produced melts with harzburgites that were enriched in incompatible elements as a result of infiltration of diapir melts of low degrees of partial melting.

This work was supported by RFBR (grant 11-05-00283a)

References:

Vasiliev Yu.R., Zolotukhin V.V. Petrology of ultrabasic rocks from northern Siberian Platform and problems of their genesis. Novosibirsk: Nauka. 1975. 271 p.

Egorov L.S.. Ijolite-carbonatite plutonism (on the example of the Maimecha-Kotui complex of Polar Siberia). L.: Nedra, 1991. 260 p.

Ryabchikov I.D., Kogarko L.N., Solovova I.P. Physicochemical conditions of magma formation in the basement of the Siberian plume: data from study of melt microinclusions in meimechites and alkaline picrites from the Maimecha-Kotui province. Petrologiya. 2009. v. 17 (3). P. 311-322.