2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrogeochemistry of Early Mesozoic ultra-basic and basic volcanics in the Southern Tien Shan

Melnichenko A.K. and Varzieva T.B.

Institute of Geology, Earthquake Engineering and Seismology, Dushanbe, Tajikistan; alana-ir@mail.ru

Alkaline and subalkaline volkanics mantle of origin in the Southern Tien Shan may be a source of ore material for mineralization of lithium, cesium, copper, molybdenum, tungsten, tin and mercury.

In the Southern Tien Shan, mainly alkaline and rarely subalkaline volcanics T₃-J₁ (214 ± 8 mln. years) are of the same name complex consist of 2 subphase  of small intrusions (9 km 2), formed in the Early Mesozoic era its  post-consolidate activation. Such rocks are quite usual in Hissar, Zeravshan and Karategin ridges, and compose numerous dikes and various forms of explosive pipes (more than 70; 2x4 - 150-1200 m) with xenolites of mantle and crustal rocks. The first sub-phase consists of volcanic breccia or tuff breccia respectively with magmatic or semi-tuff cement, the second is represented with the difference between the porphyritic rocks of the same composition, composing small stock-shape bodies in the central parts of diatremes and dikes.

Ultra-basic alkali volkanics in the southern Tien Shan are represented with the alkaline picrites, which have massive, brecciated texture and a porphyritic texture. Phenocrysts (7-25%): olivine, analcime, Ti-augite. The bulk mass (%): olivine - 25-30, analcime - 8-18, Ti-augite, diopside - 25-70, plagioclase (labradorite) - 0-2, kersutit - 8-25, Ti-biotite - 3-6. Several diatremes complicated with picrite-bazanite and picrite-basalt. Phenocrysts in the past group are represented with olivine, titan-augite, analcime, plagioclase. The bulk mass (50-80%): Ti-augite, diopside-augite - 30-50, andesine-labrador - 25-40, olivine - 20-35, biotite - 8-10, orthoclase - 3-8. Basic alkali basalts are tephrite with massive amigdaloid, flow texture and porphyry structure. Phenocrysts (5-35%): olivine, analcime, Ti-augite, kersutit, hastingsite, barkevikite, biotite, orthoclase, labradorite, and spinel. The bulk mass (up to, %): Ti-augite - 30, labrador - 35, analcime - 25, olivine - 10, orthoclase - 5, biotite - 5 kersutit - 3, apatite - 2. Subalkaline basalts: leykobasalt, trachyte-basalt-trachidolerite have a massive, amigdaloid texture and serially-porphyry, intersertal, dolerite, pilotaksite, apo-micro-ophyte structure. Phenocrysts (15-30%): Ti-augite, olivine, plagioclase, analcime, biotite in basement of glassy mass. The bulk mass (%): plagioclase (andesine-labrador-bytownite) - 45-65, Ti-augite - 25-40, olivine - 5-30, biotite - 4-25, kersutite - 5-10, orthoclase - 5-20 , analcime - 1-5. Secondary minerals in considered volcanics are following: chlorite, epidote, sericite, carbonate, leucoxene, quartz, albite. From the minority minerals they contain apatite, zircon, fluorite, garnet, galena, chalcopyrite, stibnite, arsenopyrite, cinnabar, pyrite, ilmenite, magnetite, and barite. South-Tienshan alkali volkanics  belong to low-alumina alkaline picrite and moderate-alumina tephrite, respectively to potassium and potassium-sodium series of alkaline row (see table). Basic sub-alkaline basalts in average correspond to moderate-alumina trachy-andesite – trachy-dolerite and potassium-sodium series of sub-alkaline row.

In general, Early Mesozoic ultrabasic and basic volkanics of south-tienshan complex is characterized by magmatism of epi-orogenic rift-genesis within the Hercynian fold system of the Southern Tien Shan, and strontium isotopic composition indicates the formation of magma chambers in the mantle level. Xenoliths of olivine and spinel lherzolite, chromite and gornblendite in explosion pipes indicate that the upper mantle of this region consists of pyroxene-lherzolite rocks.

 Early Mesozoic soth-tienshan volkanics  significantly have enriched with rare lithophile elements and also have high contents of chalcophilic  and  siderophile ones by combining the geochemical characteristics of granitoids and basalts. General rule  of lithophile elements behaviour (except cesium) is a direct function of the content of potassium and fluorine, testifying about the commonality of the source, which can be intra-telluric streams of matter. Their content does not depend on the host sedimentary rocks or granitoids, and on the duration of contact volkanics magma with the crust.

The most general rule for chalcophilic and siderophile elements is the tendency to direct dependence on magnesium and iron, and the inverse of potassium. By quantity of chromium, cobalt, nickel in studied rocks are well separated on the sub-alkaline and alkaline-volkanics, with the first one is richer 2 times  in these elements.

Averages of vanadium, zinc, copper, lead are located along the "basaltic" level. Some elevated mercury levels are consistent with the metallogenic  appearance of region. Small concentrations of gold are similar to its low content in the mantle. South Tien Shan Early Mesozoic volkanics are enriched in molybdenum, tungsten, tin, which are consistent with the fact that mantle-derived magmas in general can be at he source of these elements in the endogenous ore process.

Table. Trace-element compositions of Early Mesozoic ultra-basic and basic basaltoids in the Southern Tien Shan.

Note. A - alkali picrites, B - tephrite, C – sub-alkaline basaltoids (olivine and analcime dolerite – basalt   - trachy-basalts); al1 – alumina;  the tests quantity is shown in parentheses.

Thus, high concentrations of lithium, cesium, copper, molybdenum, tungsten, tin, and mercury in the Early Mesozoic alkaline and sub-alkaline mantle originated  volkanics  indicate that the upper mantle is likely to be the source of ore material in the formation of deposits of these elements in the Southern Tien Shan.