M. A. Mishkin

First results of zircon LA-ICP-MS U-Pb dating of the rocks from the Granulite complex of Khanka massif in the Primorye region

This work is dedicated to the first results of zircon LA–ICP–MS U–Pb dating of the rocks from the granulite complex of the Khanka Massif in the Pri� morye region. Similar to the Khingan–Bureya (Rus� sia) and Jiamusi (China) terranes, the Khanka Massif (terrane) represents one of the principal structural ele�

Early Archean sialic crust of the Siberian craton: Its composition and origin of magmatic protoliths

This study demonstrates that the base of the Archean deep-seated granulite complexes within the Siberian craton consists of a metabasite-enderbite association. The major and trace element distribution patterns revealed that the protoliths of this association are represented by calc-alkaline andesites and dacites, containing several minor sequences of komatiitic-tholeiitic volcanic rocks. The origin of the primary volcanic rocks of the metabasite-enderbite association is inferred on the basis of a model of mantle plume magmatism, which postulates that both andesitic and dacitic melts were derived from the primary basitic crust at the expense of heat generated by ascending mantle plumes. The formation of the protoliths of the Archen metabasite-enderbite association of the Siberian craton began at 3.4 Ga and continued until the late Archean.

The first data on local U-Pb isotope dating of zircons from hypersthene plagiogneisses of the Dzhugdhur block, southeastern Aldan Shield

The Dzhugdzhur block is located in the basins of the Kun Man’e, Ayumkan, and Maya rivers (Fig. 1). The stratifiable nature of granulitic rocks within the Dzhugdzhur block was shown for the first time by V.M. Moshkin [2] who distinguished two strata among them: the lower was that of pyroxene–plagioclase– crystalline schists and the upper was that of biotite– garnet gneisses with marble interbeds. Later, the terri tory of the Dzhugdzhur block was studied by a geolog ical survey and mapped at 1 : 200 000 scale, under the leadership of Yu.N. Gamaleya [3]. During these works, a more detailed stratification of metamorphic units was proposed, with subdivision into four con formingly occurring formations (from bottom to top): Upper Sunnagin, Kyurikan, Sutama, and Khurdukan. However, based on analysis of the published data and our own studies, we decided to turn back to the two strata subdivision from [2]. The lower strata is com posed of hypersthene plagiogneisses (enderbites) interbedding with two pyroxene schists (metabasites). In the upper stratum, there are aluminiferous biotite– garnet gneisses with marble interbeds with another bed of enderbites and pyroxene biotite gneisses (Fig. 1). The lower stratum representing the initial stages of crustal evolution within the Dzhugdzhur block was distinguished in [1] as a metabasite–enderbite associ ation. Hypersthene plagiogneisses (enderbites), which make up most of this association, are composed by plagioclase (50–70%; 40–50% An) with antiperthite interpositions, quartz (5–10%), hypersthene (5–10%), hornblende (8–10%), and monocline pyroxene (up to 5%). Accessory minerals are apatite, zircon, magne tite, and ilmenite. Two pyroxene schists consist of pla gioclase (30–40%; 50–58% An), monocline pyroxene (20–40%), and rhombic pyroxene (5–10%). Some varieties of two pyroxene schists contain garnet or a biotite admixture. Accessory minerals are apatite, zir con, magnetite, and ilmenite. Ultrabasic crystalline schists (two pyroxene, two pyroxene–amphibole, olivine–two pyroxene–amphibole) are composed of orthopyroxene (10–35%), clinoorthopyroxene (10– 45%), olivine (0–19%), and amphibole (0–80%). Accessory minerals are magnetite, ilmenite, spinel, and apatite.

Geochemistry and protoliths of the metabasite-enderbite association of the Dzhugdzhur block, Aldan Shield

The bottom of the stratigraphic sequence of the Dzhugdzhur deep-seated granulite complex was determined to consist of a stratified metabasite-enderbite association. The distributions of major and trace elements indicate that the protoliths of the association were volcanic rocks of the calc-alkaline, komatiite-tholeiite, and picrite series. The model assumed for the genesis of the protolithic volcanics of the metabasite-enderbite association includes two stages. The first of them was responsible for the decompression-induced partial melting of the material of an ascending mantle plume with the derivation of melts of the komatiite-tholeiite series. During the second stage, the volcanics of the calc-alkaline series were produced by the partial melting of the metabasite crust under the effect of the heat of the ascending mantle plume. The protoliths of the metabasite-enderbite association were formed in the Early Proterozoic.

Geochemistry and Metamorphic Parameters of Rocks in the Batomga Granite-Greenstone Terrane, Aldan Shield

The protoliths of the Early Proterozoic metamorphic complex in the Batomga granite-greenstone terrane are proved to comprise two petrochemical series of volcanic rocks: calc-alkaline and komatiite-tholeiite. The metavolcanic rocks of the calc-alkaline series are metamorphosed basalts, andesites, dacites, and rhyolites. The topology of the trace-element patterns of the acid volcanics is similar to that of Archean gray gneisses in platform basements, and this suggests that the petrologic mechanisms that produced the protoliths could be similar. The metavolcanics of the komatiite-tholeiite series are determined to include komatiite and tholeiite basalts. Their chemical composition is consistent with the fractionation model of high-Mg basalts in intermediate chambers under low pressures. The Nb, Y, and Zr concentrations of the metatholeiites testify that their parental melts were derived from a plume source. The metamorphic culmination parameters of the rocks corresponded to the boundary between the amphibolite and granulite facies of elevated pressure.

Geochemistry and genesis of metamorphosed volcanic rocks in the Kholodnikan greenstone belt, southern Aldan Shield

The Kholodnikan Complex consists of two units: lower volcanic and upper volcanic-sedimentary. The distributions of major and trace elements suggest that the protoliths of the lower unit were volcanics of the komatiite-tholeiite series (komatiite-basalt association) and those of the upper unit were volcanics of the calc-alkaline series (andesite-dacite-rhyolite association). The model assumed for the genesis of these associations involves two stages: (1) decompression-induced partial melting of the material of an ascending mantle plume with the derivation of melts of the komatiite-basalt association and (2) derivation of volcanic rocks of the andesite-dacite-rhyolite association via the partial melting of various rocks in the basement of the Aldan Shield under the effect of the heat of the ascending mantle plume. The magmatic protoliths of the Kholodnikan Complex were formed in the Paleoproterozoic at 2.41 Ga.

Magnetite-Ilmenite Equilibria in Archean Enderbites from the Sutam Complex (Aldan Shield)

The composition of disintegrated titanomagnetites and ilmenites from Archean enderbites of the Sutam Complex of the Aldan shield has been reconstructed. The values of oxygen fugacity and temperature of titanomagnetite‐ilmenite mineral equilibrium at 7.5 kbar general pressure are calculated. In the process of granulite metamorphism oxygen volatility varied within = ‐14.823 ± ‐16.868 at temperatures T = 723 ° C‐910 ° C, corresponding to a line parallel to the quartz‐ferrosillite‐magnetite buffer equilibrium, but about 1.5 orders of above it. The probable values of oxygen volatility for protoliths of Sutam enderbites were not lower than calculated ones. Detailed study of iron‐titanium oxide paragenesis occurring in metamorphic rocks has been given little attention so far. In the meanwhile, a purposeful approach with reconstruction of the primary composition of iron‐titanium oxides may supply valuable data on the temperature and oxidation potential at the time of metamorphism. Study of these minerals is especially important when they occur in ancient magmatic rocks. In this case, hypothesizing a minor change in the oxygen potential during metamorphism makes it possible to obtain the value of the oxidation potential of the magmatic rock protolith. In this way we can assess the oxygen regime of the Earth’s crust during the earliest stages of our planet’s evolution. In our paper this task is being solved on the basis of petrological study of magnetite‐ilmenite equilibria occurring in enderbites of the Sutam metamorphic complex. The Sutam Complex forms a tectonic block of the same name to the south of Aldan shield in the Sutam River basin (Fig. 1). It is distinguished by ultrahigh-pressure conditions of regional metamorphism at depth. It was described first by A.A. Marakushev [1]. According f O2 log f O2 log to [2, 3], the Sutam Complex consists of two strata: the low one involving mainly hypersthene plagiogneisses (enderbites) with minor amounts of garnet, garnet‐ biotite plagiogneisses, and garnet‐sillimanite, cordierite, hypersthene‐sillimanite gneisses, and quartzites. Reconstruction of the primary nature of high-grade rocks of the low and upper strata of the complex [2, 3] revealed that volcanic rocks of calc‐alkaline series, andesites, dacites, and rhyodacites (andesite‐dacite association) dominate in the low strata. Volcanic rocks of the komatiite‐tholeiite series presented by peridotite komatiites and komatiitic and tholeiitic basalts (komatiite‐basalt association) make up an insignificant part of the succession (not more than 10%). The initial composition of the upper strata is reconstructed as intercalation of volcanic rocks of the andesite‐dacite association with greywackes, pelites, volcanoclastics, as well as chemogenic‐sedimentary, sil

Origin of the Early Sial Crust and U–Pb Isotope–Geochemical Heterogeneity of the Earth’s Mantle

It is shown that presence of the Early Precambrian sial crust in the Indo–Atlantic segment of the Earth and its absence in the Pacific has been caused by geochemical differences in the mantle underlying these segments. These differences were examined on the basis of Nd–Hf and U–Pb isotopes in modern basalts. The U–Pb isotope system is of particular interest, since uranium is a member of a group of heat-generating radioactive elements providing heat for plumes. It is shown that in the Indo–Atlantic segment, a distribution of areas of the modern HIMU type mantle is typical, while it is almost completely absent in the Pacific segment. In the Archean, in the upper HIMU type paleo-mantle areas, plume generation and formation of the primordial basic crust occurred; this was followed by its remelting resulting in the appearance of an early sial crust forming cratons of the Indo–Atlantic segment.

Geochemistry, origin, and nature of metamorphic rocks of the Batomga granite-greenstone terrane (Aldan Shield)

Two series of volcanic rocks with different petrochemical affinities-calc-alkaline and komatiitetholeiitic series-were identified as protoliths for the Early Proterozoic metamorphic rocks of the Batomga granite-greenstone terrane. The metavolcanic rocks of the calc-alkaline series comprise metabasalts, metaandesites, metadacites, and metarhyolites. The distribution of the trace element abundances in the felsic metavolcanic rocks is similar to that of the Archean grey gneisses from the platform basements, thus suggesting a similar petrological mechanism for the formation of their protoliths.The protoliths for the komatiite-tholeiitic metavolcanic rocks include komatiite and tholeiite basalts. The chemical behavior of the tholeiites tends to support the fractionation of primary high-Mg basaltic magmas in a transient magma chamber at low pressures. The variations in the Nb, Y, and Zr contents of the metatoleiites indicate the derivation of their parental magmas from a plume-related source.

The Hadean Protocrust of the Earth: Formation Model and Probable Composition

1003 Despite the unavailability of Hadean rocks, the problems related to the formation of the Earth’s crust (protocrust) at this stage, i.e., after the cessation of the Earth’s accretion (since 4.44 [1] until 3.9 Ga ago), are at present highly debatable. The indirect evidence for magmatic activity of the Earth in the Hadean is pro� vided by finds of corresponding detrital and xenogenic zircons in younger metasedimentary and metaigneous rocks [2, 3, and others]. During the last decade, several studies were dedicated to detrital Hadean zircons aged 3.9–4.4 Ga from Upper Archean ( ~3 Ga) metaterrig� enous rocks of the Jack Hills and Mount Narrier areas in the Yilgarn Craton of Australia. They include the analysis of their geochemical and geochemical isotope composition and similar data on their mineral inclu� sions. These data are used for interpreting the crust formation at the Hadean stage of the Earth’s evolution [2, 4, and others].