Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Geochemical particularities of basites from ancient paleorifts of the Siberian platform

Kopylova A.G., Tomshin M.D.

Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russia

kopylova@diamond.ysn.ru

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The eastern periphery of the Siberian platform in the Middle Paleozoic time was characterized by active igneous activity, connected with deposition (заложение) of the series of paleo-rift systems in the Early Devonian times. The biggest ones of them are Viluy and Olenek branches of Pre-Verkhoyansk paleorift. Viluy-Markha dike belt (VMDB) with length of 700 km and a width of 30 km in the south to 150 km in the northern part, of was formed along the north-western edge of Viluy paleorift. At the south-western edge and centriclinale closing of Olenek paleorift, basite dike swarms are combined into Molodo dike belt (MDB) with length of about 300 km, width 180 km. Igneous activity within the limits of rift zones was long-lasting, and its geodynamic environment was heterogenous. Regime of extension was changed many times to regime of compression and back again. Central parts of dike swarms in the predominant regime of extension are dominated by basites filled with plagioclase-pyroxene-olivine mineral association with minor (>3%) content of phenocrysts in the rocks of marginal zones. Petrochemical and geochemical data show closeness of the material composition of VMDB and MDB typical basites (Table). Magnesiality index in them is Mg#40-44, titanium content varies in the range of TiO₂ 2-3%. They are also close in values of microelement contents, REE sum (136,6 and 133,4 respectively), La/Yb_n ratio (4,8 and 4,7) and other indicator ratios. On silica – alkali sum diagrams, they form single set, that probably indicates homogenous composition of mantle sources the melts and uniformity of conditions, forming basite bodies of the both paleorifts. Absence of Ta-Nb minimum at multi-diagrams, relatively high values Nb/Nb* (1,24 and 1,01) exclude significant impact of the crust material on the parent melt composition. Few dike swarms are formed with signs of delays and intense crystallization of the melt in pre-chamber conditions at the outer zone of dike belts at a distance of 20-70 km from their axial parts in the areas dominated by regime of compensating compression. This process is accompanied by significant amounts of protophases. The dikes of leucocratic sub-alkali gabbro-dolerites with up to 30% content of plagioclase protophases, with plagioclasite schlieren are located in peripheral fractures of the Molodo swarm. In such conditions there is an accumulation of the set of the most mobile elements – alkalis, H₂O, all incompatible elements, but mostly mobile large-ion lithophile elements – Rb, Ba, Sr, Pb.

VMDB: 1 – typical dolerites; 2 – plagiodolerites of Chimidikyan dike swarm, 3 – dike dolerites with horizon of anorthosite gabbro-dolerites, differentiates of Nyurba dike: - fine-grained dolerites of marginal zones, 5 – quartz gabbro-dolerites, 6 – monzonite-porphyries;  MDB: - 7 typical dolerites, 8 – plagiodolerites and leucocratic gabbro-dolerites, 9 – dolerites close to the pipe

In VMDB, Chimidikyan swarm of en echelon dikes of plagiodolerites, located at the northern flank of the belt, is an example of intrusions, formed from the melt, crystallized in compression conditions in ramp structures of the rift. Plagiodolerites of the swarm differ from all other VMDB basites by high content of An_60-50 (up to 60-70%) plagioclase, oxide-ore minerals (up to 7-8%), in interstices of quartz-feldspar aggregate, apatite and sphenes. Absence of olivine in magmatites of Chimidikyan swarm allows phenocryst sedimentation, that is indirectly shown by by Mg, Fe, Ni, Co, Cr lower content in them. At the same time, plagiodolerites have significantly higher content rock-forming elements (Ti, K, P) and admixture ones (Rb, Ba, Th, U, LREE, Zr, Nb, Hf). REE sum (254,17) increases mainly by LREE. High La/Yb_n=11,2 ratio, most likely, is due to active LREE accumulation in a large volume of plagioclase amount, unusually large for basites. Low basicity of protoplagioclase in leucocratic gabbro-dolerites and plagiodolerites, absence of early high-temperature and high-baric Fe-Mg minerals indicate that, melt crystallization started in near-surface conditions in the magma channel, or in the intermediate chamber.

There is a group of bodies with isolations of anorthosite gabbro-dolerites among VMDB discordant intrusions. They are formed by the melt with anorthosite tendency of differentiation, being realized in deep-seated intermediate magmatic chamber setting [2]. Bytownite, chrysolite, chrome-spinel, pyrope, moissanite are recognized in them as early minerals. The melt began to evolve at a depth of 35-40 km from the paleosurface in intermediate magmatic chamber, at 1450-1300⁰C as shown by the results of homohenization of melt inclusions in bytownite. Analysis of microelement distribution in the rocks containing protophases of the basic (mafic-) plagioclase showed that, the depth of formation does not change trend of the melt differentiation. Rare earth elements sum (Σ=187,76) and La/Yb_n=6,36 ratio increase for typical basites. LILE and HFSE are accumulated, but no so actively as in plagiodolerites. The strontium content increased by 2-3 times, positive europium anomaly (Eu/Eu*=1,11) are distinctive features of anorthosite gabbro-dolerites, which is connected with formation of high-temperature, high-calcium plagioclases.

Two-phase Nyurba dike, intruding kimberlites of the pipe of the same name, - on the one hand, is an intrusion close to the pipe, whose marginal zone dolerites contain >4% TiO₂, but on the other hand – striking example of manifestation of monzonit trend of basite melt differentiation [3]. Fine-grained rocks of marginal zones of the dike are replaced by quartz gabbro-dolerites, and its central part is filled with monzonite-porphyries. Low Mg# = 36 values, low Ni, Co, Cr contents and high contents of incompatible elements in the rocks of marginal zones indicate that, the melt fractionation began before its rise to the chamber. In intra-chamber conditions further separation of the melt occurred with gradual formation of differentiates more and more rich in SiO₂, alkalis, incompatible elements (Table). Sharp increase of silicicity (more than 5,5%) and almist double increase of potassium, during transition from quartz gabbro-dolerites to monzonite-porpyries can be explained just by processes of intra-chamber differentiation. It is connected with long interaction of basalt melt with substance of fluid, rich in potassium, volatile components in shallow (15-20 km from paleo-surface) intermediate magmatic chamber at P=5-8 kbar. Monzonite  melt as a second phase was intruded into central part of the dike, forming separate body, composed of monzonite-porphyries. Monzonite-porphyries sharply differ from all basites of VMDB in material composition. These are fine-grained porous rocks of porphyritic habit, where phenocrysts are represented by plagioclase (An_50-44), groundmass – aggregate of quartz, orthoclase, with hornblende prisms, apatite crystals and titanomegnetite. Maximum contents of Rb, Th, U, Nb, Ta, Hf, Y and REE (Σ=611,6) are identified in monzonite-porpyries (La/Yb)_n ratio increases up to 9,8. In comparison with other dike differentiates, they have relatively low Sr contents and maximum negative europium anomaly (Eu/Eu*=0,65). In MDB, high-Ti dikes close to the pipe, as well as Nyurba dike, are formed from the melt, which passed the stage of differentiation before delivery to modern chamber. This is indicated by low Mg#=34 value and high contents of incompatible elements. La/Yb)_n=5,95 in MDB basite close to the pipe are much higher then in predominating group of the rocks. Among these dikes, there are two-phase ones. The melt was inflowing through MDB general faults, two-phase occurrence, in this case, is connected with short-term closing of the system with almost no change in the composition.

    Paleorift dike belt record several groups of basites, differing in their material composition. The most common basites were formed in the regime of extension - magma has not experienced significant changes after leaving magma formation zone. These are typical basites of the Middle Paleozoic peleorifts of the eastern part of the Siberian platform. Group of leucocratic gabbro-dolerites and plagiodolerites, which were formed when extension was changed to compression at the peripheral zones of dike belts. Such magmatites reflect manifestation of lateral zonation within the limits of thick dike belts. A separate group consists of the bodies of magmatites, which material composition records long-term fractionation of the melt in pre-chamber deep-seated setting. As a result of long-term closing of the system, anorthosite or monzonite type of magma differentiation are formed in deep-seated magmatic chambers. A separate group consists of dikes of high-Ti basites, grouping close to kimberlite pipes.

Literature:

Oleinikov B.V., Tomshin M.D. Deep-seated differentiation of magma from platform basites // Rep. AN SSSR. 1976. V. 231. № 1. P. 177-180

Oleinikov B.V., Tomshin M.D., Koroleva O.V. et al. Deep-seated evolution of sub-alkali tholeiite-basaltic magma in the regime of paleoriftogenesis (example of Chara-Sin zone). Preprint. –Yakutsk. YF SO AN SSSR publishing, 1984. P. 32.