The timing for the onset of plate tectonics, along with the secular changes in the tectonic settings of continental crust formation, continue to be debated. Recent interpretations based on the increase in zircon 176Hf/177Hf ratios at the time of crystallisation (expressed as εHf(t) with respect to chondritic evolution) have been used to ascertain changes in geodynamic settings in the early Earth, specifically in the 3.8–3.6 Ga interval. This increase is widely interpreted as a change in magma generation, from source(s) dominated by ancient crust to source(s) dominated by juvenile inputs from the mantle. At issue, Hf isotope variations remain limited in the early Earth due to the long decay of the LuHf system. This feature, along with the scarcity of rocks and minerals of Eo/Mesoarchaean and Hadean ages, generate large uncertainties over the nature and the timing of the interactions between mantle and crustal reservoirs. The distinction between mantle and crustal sources becomes much clearer in the Palaeoproterozoic, and the study of ancient terranes with several billion years of protracted crustal evolution may hold the key to unlock complex mantle–crust interactions. Here we investigate high-grade metamorphic rocks from the Anabar shield, which contain zircons that crystallised between the Eoarchaean (oldest core at 3814 ± 16 Ma) and the Palaeoproterozoic (youngest core at 2251 ± 15 Ma and youngest rim at 1910 ± 21 Ma). The combination of in situ UPb and Hf isotope analyses in zircon indicates the formation of the continental crust in the Siberian Craton in the Eoarchaean, and a conspicuous metamorphic event at 2.0–1.9 Ga. We demonstrate that 2.0–1.9 Ga zircon ages reflect recrystallisation processes under subsolidus conditions, involving the breakdown of high-Lu/Hf minerals (i.e. garnet and pyroxene). The εHf(t) shift at 2.0–1.9 Ga towards more radiogenic values may not be related to a change in magmatic style and sources, but rather to resetting of the LuHf system in response to heating and metamorphic reactions on a mineral scale. Our findings challenge the widely-evoked mechanism of changes in tectonic style and magma sources to account for vertical arrays in the εHf(t) versus crystallisation age space. This calls for considering alternative options, including those based on petrographic data, when interpreting Hf isotope variations in the Hadean/Archaean detrital zircon record.
New data of geological/ geochemical and petrological lead on alkali granitoids of Elinovskii massive of Middle Devonian. Date of absolute age on U-Pb age on zircon on masspectrometer SHRIMP II let 369 mln. years. Two phases of intrusive described (riebeckite granite and leicocratic sub-alkalic granites) and single dikes leicocratic sub-alkalic granites. Uranium stockwork and skarn fluorite-rare metals-rare earth metals ore mineralization linked with granitoids paragenetic. Zircon (malacon), iitrialite, fergussonite marked in ore mineralization besides of fluorite. Ore mineralization accompany bymetasomatites of feldshpitites type in species albitization of granitoids.
New data on phase relationships, petrology and geochemistry of the Sinyukhinskoye rocks and dikes of the same deposit are presented. Five phases of rock intrusions formed in a homodromic-antidromic sequence are found. The Sinyukhinskoye rocks fall in the oxidized type. In the depth, a source of melting was garnet lherzolites, and, to a lesser extent, spinel lherzolites of an enriched mantle source of the lithospheric slab with a degree of partial melting from 0.2 to 0.4 and low pressure. The formation of intrusive rocks of the deposit took place in the post-collisional environment. The magmatite fluids of the massif and dikes were characterized by high partial pressures of such volatile components as H2O, Cl, F, CO2, which were the main carriers of metals. The rocks exhibit the tetrad effect of the W-type REE fractionation. The dependences of the values of the latter and the concentrations of copper and gold in igneous rocks and fluids were determined by different regimes of acidity and alkalinity of the medium. The deposit belongs to a complex family of ores of the copper-gold-skarn-porphyry type. In the Cheryomukhovaya Sopka and Chir areas, this is also confirmed by the presence of the copper-gold-porphyry type in the ore field, in addition to the gold-copperskarn mineralization.
Rakhanovskiy complex of Big Altai, that it is include territory of Mountain Altai, Rudny Altai, Chinese Altai, formed in 5 phases: 1 – gabbro; 2 – diorites, quartz diorites; 3 – granodiorites; 4 – granites, melanogranites; 5 – leucogranites. Granitoids of Rakhmanovskiy complex сarry to peraluminous type and it formed at melting different sources of lower crust, high crust and mantle and mixing mantle and crust smelting. There is suppose that so crust melts can arise in result termic relaxation in lower crust with melting of quartz eclogites and garnet amphibolites LIL – riched mantle wedge, but mantle-derivative components formed for count of depleted mantle in result adiabatic decompression in high mantle with participation more amount volatile components.
The paper gives new data on the geological structure, ore mineralization, composition, petrography, absolute age, petrology and geochemistry of Timofeev stock of Gorny Altai. The absolute age of monzogabbroes of the stock intrusion lower phase is defined as 397 Ma, that corresponds to the Lower Devonian. Nd-model age of protolith during melting of gabbroids composes 893 Ma, that is characteristic of transit zone from the Middle Paleozoic consolidation to the Altai – Mongolian terrain. According to the chemistry, the Timofeev stock rocks belong to the lime-alkali and shoshonite series. The geodynamic aspect of melt generation is interpreted by mixing alkaline oceanic basalts of the plume setting and normal oceanic basalts of mid-oceanic ridges that took place in the Paleo-Asian Ocean evolution in the Early Paleozoic. Samarium-neodymium isotopic labels of gabbroids indicate the proximity of their values to chondrite meteorites. The ratio of neodymium isotopes with gabbro age indicates that the isotopically varying earth’s crust, close to the depleted mantle, existed under the Korgon trough in the Early Paleozoic. The skarn-iron-ore deposit of the same name is spatially and paragenetically connected with the Timofeev stock.
The paper presents the results of geological, geochemical and isotope-geochronological studies of subvolcanic rhyolites of the Western Gorny Altai and Rudny Altai which have related to two large volcanic systems, the Altai-Minusinsk and Altai-Salair, respectively. The results of U-Pb isotopic dating of zircons revealed two groups of ages ~410-405 and 390-381 million years. Isotope-geochemical characteristics of rhyolites show relatively high values of εNd(T) = +2,7...6,0 with relatively young values of model ages T(DM) = 851-966 Ma in the Rudny Altai and more ancient - up to 1266 Ma in the West Gorny Altai. Geochemical characteristics indicate that its composition is consistent with transitional field between within-plate and island-arc felsic magmas. The results obtained correspond to the two-stage evolution of volcanism and its migration from the continent to the ocean.
