2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Geochemistry of alkali rocks of Anabar-Udzhin interfluve

Kopylova A.G., Tomshin M.D.

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

kopylova@diamond.ysn.ry

               Ebekhain dike belt records spatially located at the same area dikes and dolerite sills, derivatives of tholeiitic magma, and dikes of trachydolerites, monzoniteporphyries and trachyandesites, formed from melts of alkali olivine-basalt composition. Geochemical composition features indicate that, tholeiitic magma was supplied from mantle source, corresponding to spinel stability, and alkali-basalt magma – from deeper source, equilibrated with garnet.

         Manifestations of alkali-basalt magmatism in Siberian platform have the local character and are represented by single bodies of trachydolerites. One of such provinces is located in Anabar-Udzhin interfluve. Here, series of contiguous dikes of subalkalic composition (trachydolerites, monzoniteporphyries and andesites) is combined into extended Ebekhainsky dike belt (Tomshin et al., 1997). According to geological data, alkali differences of the rocks break through spread Permian-Triassic sills and dikes of dolerites, that shows their younger age. Presence of alkali rock clasts in Carnian conglomerates (T3) shows that, dikes of Ebekhainsky belt were formed at the turn of Lower and Middle Triassic. The goal of this report – geochemical specification of the composition of the region main rocks. Chemical and microelemental composition of dolerites is stable enough and is typical for traps of Siberian platform with moderate content of alkalies (alkalies sum < 2,5-3%), with dominant role of sodium (Oleinikov, Tomshin, 1991). Range of REE distribution is characterized by moderate concentrations (La/Ybn=3,45-3,62), weakly expressed Eu minimum (Eu/Eu*=0,59-0,74) and almost horizontal trend HREE (Gd/Lu=1,37). Spider-diagram clearly shows vegative anomaly Nb-Ta and Ti, characteristic of Siberian traps.

         All studied bodies of trachydolerites in petrographic respect meet the composition of clinopyroxene trachydolerite porphyrites. The belong to the class of rocks undersaturated SiO2 (Q from -7,8 to -10,9). Very significant is unusual high content of titanium (>6% TiO₂), phosphorus (>1,2% P₂O₅), volatiles, increased (at the mean up to 2%) calcium in trachydolerites. Enrichment of trachydolerites with elements of titanium group – Zr, Nb, Y, Yb, Hf, Ta is primarily typical for microelements distribution in trachydolerites, LILE content increases markedly. Total content of rare-earth elements (∑=670) is recorded in trachydolerites, being maximum among all differences of the rocks of Anabar-Udzhin interfluve, herewith slope of spectrum is rather steep La/Ybn=24б 87б La/Smn=2,59, Gb/Ybn=5,42. Portion of siderophile elements – Ni, Co, Cr, Sc in trachydolerites is lees then in dolerites.

           Monzoniteporphyries are the most rich in silica and alkalis. Alkalinity rise takes place mainly due to increased potassium content, which in some cases is > 7% K₂O. Na₂O proportion increases significantly in mozonite-porphyries (sometimes > 5%). Aluminum role increases with rock leucocraticy rise. Among all rocks of Anabar-Udzhin interfluve, monzonites record minimum content of magnesium, ferrum, calcium, and microelemets of ferrum group. In comparison with trachydolerites, titanium proportion is less in them (in average up to 2,12% TiO₂). Contents of large-ionic lithophiles (Rb, Ba, Sr, Th, U), in monzoniteporphyries as well as in trachydolerites are high enough and differ sharply from their values in dolerites. Monzoniteporphyries are characterized by the greatest degree of differentiation of rare-earth elements La/Yb_n=30,5-45,0, while regarding heavy rare-element content – Ho, Er, Tm, Yb, Lu monzoniteporphyries are close to dolerites. Number of elements of titanium group – Zr, Hf, Ta, Nb in mozoniteporphyries is significantly greater than in dolerites, but notably lower their values in trachydolerites. Well-defined negative anomalies on Nb, Ti, Sr are recorded on multi-elemental spectra of monzoniteporphyries. Regarding size of negative anomaly on niobium (Nb/Nb*=0,45) monzonites are comparable with dolerites (Nb/Nb*=0,55).

               Trachyandesite bodies are found only in combination with intrusions of monzoniteporphyries. Chemical composition of trachydolerites takes intermediate position between trachydolerites and monzoniteporphyries. They record quite high content of SiO₂ (49-54%), on average, more titanium than in monzonites (>3% TiO₂), total ferrum (>9% FeO), phosphorus (0,75% P₂O₅). Total content of alkali reaches 7,5% with equal or sometimes prevailing role of potassium. MgO, CaO, Ni, Co, Cr content is slightly higher than their values in monzonites. Like trachydolerites and monzoniteporphyries, trachyandesites are enriched in incompatible elements. REE distribution spectra for all trinity of alkaline rocks are steep, subparallel and unlike dolerites of the region, they don’t record europium minimum. Negative anomalies Sr and Ti are clearly manifested in multi-elemental trends of alkaline rocks, and trachydolerite spectrum doesn’t record niobium minimum. Significant rise of the melt alkalinity and accordingly enrichment of trachydolerites, trachyandesites and monzonites in light lanthanids is possibly due to deeper plunge of magma formation spots and fusion of alkaline olivine-basalt melt, from which firstly dikes of  trachydolerite porphyrites were formed, and then dikes and stock-like bodies of monzonites and trachyandesites were formed (Tomshin, 2010). We use Tb/Yb_n ratio as indicator assessment of melt formation depth. This ratio is higher 1,8 value in magmas, formed from mantle source, regulated by garnet, and is lower 1,8 in less deep melts, equilibrium with spinel (Wang et al., 2002). The lowest values of Tb/Yb_n ratio are recorded in dolerites (in average 1,2), whereas it si much higher 1,8 (3,6 in trachydolerites, 2,6 in monzonites, 3,1 in trachyandesites) in the rocks of alkaline series. It suggest that, tholeiitic magma, forming dolerite bodies, was delivered from mantle source, corresponding to spinel stability, and alkaline-basalt mafma  - from deeper source, equilibrium with garnet. This conclusion is also confirmed by values of ratios Sm/Yb_n = 1,68, Lu/Hf = 0,11, Ti/Y = 290 in dolerites, typical for spinel regulated area. In alkali rocks these ratios are: Sm/Yb_n = 9,38, Lu/Hf = 0,018, Ti/Y = 530 in trachydolerites, Sm/Yb_n =7,05, Lu/Hf = 0,017, Ti/Y = 487 in monzoniteporphyries and Sm/Yb_n = 7,80, Lu/Hf = 0,015, Ti/Y = 470 in trachyandesites, that indicate garnet presence in the source.

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