Age and Sources of Dunite from the Konder Massif (Aldan Shield)

The Sm–Nd and Rb–Sr isotope characteristics were studied in clinopyroxenes (Cpx) of ultrabasic rocks (dunite, wehrlite, pyroxenite, and kosvite) from the Konder massif, which is a source of a unique placer platinum deposit. The chemical composition of the clinopyroxenes studied provides evidence for their crystallization from a single melt in the course of magmatic differentiation. The Sm–Nd isotope characteristics of Cpx in dunite from the Konder massif correspond to the regression with an age of 128 ± 40 Ma, which provides evidence for the same age of rocks of the “dunite core,” wehrlite, pyroxenite, kosvite, and alkaline rocks of the subsequent intrusive stage in the Konder massif. Variations in the Sr and Nd isotope characteristics in dunite, wehrlite, pyroxenite, and kosvite result from contamination of the picritic melt with rocks of the continental crust in the course of its cumulative evolution, which allows us to exclude the model of diapiric intrusion of mantle dunite.     

Trace Element and Sr-Pb-Nd-Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites

A phase of Mesozoic extension associated with the terminationof continental collision at the southern margin of the AldanShield produced ultrabasic lamproites in a discontinuous belt500 km long and 150 km wide. The lamproites, locally poorlydiamondiferous, were emplaced as dykes, sills and pipes. AllAldan lamproites have primitive chemical characteristics (e.g.MgO up to 22·7 wt %) and are ultrapotassic (K₂O up to8·3 wt %) and peralkaline with K₂O + Na₂O/Al₂O₃ in therange 0·6–1·16. A distinctive feature ofthese rocks is their low TiO₂ content (0·5–1·4wt %). Aldan lamproites are moderately light rare earth element(LREE) enriched with (La/Yb)_N ranging from 10 to 47. Heavy rareearth element (HREE) abundances are lower than for all otherlamproites by up to a factor of five. Therefore, the combinedmajor and trace element characteristics of the Aldan samplesare not typical of other lamproite occurrences. Large ion lithophileelement concentrations are high (100–800 x Primitive Mantle)but the high field strength elements (HFSE; Nb, Ta, Ti) plusTh and U display unusually low concentrations for rocks of thistype. The style of trace element enrichment recorded by theAldan Shield lamproites is comparable with that of subduction-relatedmagmatism. The Aldan lamproites have among the most extremeinitial isotopic ratios yet recorded from mantle-derived magmas;_Ndi = –10·3 to –22·3, ⁸⁷Sr/⁸⁶Sr_i =0·7055–0·7079, _Hfi = –7·6 to–29·4 and ²⁰⁶Pb/²⁰⁴Pb_i = 16·6–17·4.When interpreted in terms of multi-stage Pb isotope evolution,the Pb isotope data require fractionation from a Bulk Earthreservoir at 3·0 Ga and subsequent evolution with second-stageµ values between 6·4 and 8·0. The inferredArchaean age of the lamproite source is consistent with Nd andHf model ages, which range from 1·5 to 3·0 Ga.Aldan lamproites have _Hf values that range from +3 to –7.Trace element and Sr–Nd–Pb–Hf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative _Hf as a result of metasomatism by slab-derivedhydrous melts that left rutile–garnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotope–trace element correlations and the lowREE contents. Although high-quality trace element data (e.g.HFSE) are not available for most lamproites, it appears thatmany of their source regions contain a component of recycledoceanic crust, possibly including subducted sediment. The sourcesof the Aldan and many other lamproites are distinct from oceanisland basalt mantle sources. This suggests that the long-termstorage of trace element enriched lamproite sources occurredin the sub-continental lithospheric mantle and not at depthwithin the convecting asthenosphere. KEY WORDS: potassic volcanism; isotope geochemistry; fluid enrichment     

Geochronology and Ore Mineralization of the Dzheltula Alkaline Massif (Aldan Shield,South Yakutia)

The Dzheltula alkaline massif is located in the Tyrkanda ore region of the Chara–Aldan metallogenic zone of the Aldan–Stanovy Shield (South Yakutia). The region contains separate placer gold objects, which are being explored at the present time, and ore-bearing Mesozoic alkaline intrusions, which are weakly studied due to their poor accessibility. The Dzheltula massif (DM) is the largest exposed multiple-ring intrusion within the Tyrkanda ore region; therefore, it is considered as a typical object for geological, petrological, geochronological, and metallogenic studies. The DM consists of five magmatic phases of syenite composition. ⁴⁰Ar–³⁹Ar dating has established that the crystallization age of the oldest phase, the leucocratic syenite porphyry (pulaskite), is 121.1 ± 1.3 Ma. The crystallization age of the cross-cutting phases represented by syenite–porphyry dikes (laurvikites and pulaskites) ranges from 120.1 ± 2 to 118.3 ± 2.1 Ma. The youngest phase of the massif, trachyte, crystallized at 115.5 ± 1.6 Ma. According to the mineralogical and geochemical studies, two types of ore mineralization, namely gold and uranium–thorium–rare-earth (U–Th–REE), are established within the DM. The gold mineralization was found in the quartz–chlorite–pyritized metasomatites. It is confined to the NNE- and NNW-trending fault zones and coincides with the strike of the syenite porphyry dike belt. Uranium–thorium–rare-earth mineralization has been established in the quartz–feldspathic metasomatites localized in the outer contact of the massif. The juxtaposition of mineralization of different types in some zones of the Dzheltula syenite massif significantly increases the ore potential of the studied object within the Tyrkanda ore region.     

The Age of Transformation of Rare-Metal Alkaline Granites of the Katugin Massif (Aldan Shield)

The U–Pb (ID TIMS) geochronological studies of single zircon grains and their fragments have made it possible to establish that rare-metal alkaline granites of the Katugin massif (2055 ± 7?2066 ± 6 Ma) underwent high-temperature transformations at age levels of ~1921 ± 5 and 1876 ± 7 Ma.     

Sulfur-Bearing Sodalite,Hackmanite, in Alkaline Pegmatites of the Inagli Massif (Aldan Shield): Crystal Chemistry,Photochromism, and Luminescence

Geology of Ore Deposits - Abstract—Sulfur-containing sodalite, hackmanite, has been found for the first time in alkaline pegmatites of the Inagli alkaline–ultramafic massif, South...     

Crystallization conditions of dunites in the Konder platiniferous alkaline-ultramafic massif of the southeastern Aldan Shield

The investigation of melt inclusions in Cr spinels yielded direct information on the physicochemical parameters of the magmatic processes responsible for the formation of the Konder platiniferous alkaline-ultrabasic massif (southeastern Aldan Shield). The comparative analysis of the composition of the chromites containing the inclusions revealed that the Cr spinels from the Konder dunites differ significantly from their counterparts in the ultrabasic complexes of ophiolites and the modern oceanic crust. In terms of their composition and REE distribution, the clinopyroxene microcrystals from the Konder chromites are significantly different from the pyroxenes in the basic-ultrabasic ophiolite complexes of associations and identical to the minerals from the Kytlym platiniferous massif (the Urals). With respect to the distribution of the major components, the high-magnesian inclusions are identical to alkaline biotite-pyroxene picrites, testifying to the active participation of ultrabasic (picritic) alkaline magmatic systems in the formation of the dunites in the Konder Massif. The results of the ion probe investigation of the inclusions indicate a high water content (up to 0.54 wt %) in the melts. The data on the distribution of the rare and rare-earth elements in the inclusions suggest that the dunites of the Konder Massif crystallized with the involvement of water-saturated magmas at minimal temperatures of about 1230°C. Such temperatures are consistent with the earlier estimates for the melt inclusions in the olivine of the Konder Massif.     

Crystallization conditions of the alkaline-basic dike from the Yllymakh Massif,Central Aldan: Evidence from melt inclusion data in minerals

Alkaline-basic dike from the Yllymakh Massif (Central Aldan) has been studied. Its partially crystallized matrix contains corroded phenocrysts of olivine and hypidiomorphic phenocrysts of clinopyroxene and pseudo-, epileucite. It was found that phenocrysts of clinopyroxene contain abundant primary inclusions, Ti-magnetite and apatite bear only single inclusions, whereas olivine is enriched in secondary inclusions, which are confined to the cleavage of host mineral (along second and third pinacoids) and its cracks. The homogenization temperatures of the primary inclusions in clinopyroxene and secondary inclusions in olivine are approximately equal and lie within 1260–1240°C. The compositions of melt inclusions in olivine and clinopyroxene are also similar and corresponded to the malignite-pseudoleucite phonolite-monzonite pulaskites, which are developed at the Yllymakh Massif. Unheated inclusions in apatite and Ti-magnetite compositionally approach monzonites and nepheline syenites—tinguaites, respectively. It was concluded that the alkaline basaltoid magma was presumably parental magma for the entire rock complex of the Yllymakh Massif. Its crystallization and differentiation presumably provided all observed rock variety from ultrabasics (early derivatives located at depth) and malignites (later derivatives) to leucite phonolites, monzonites, and alkaline pulaskites, which were obtained during subsequent stages of the melt evolution. The parental magma, and especially its derivatives, were enriched in BaO (0.8–0.1 wt %), Cl (0.1–0.3 wt %) and trace elements (primarily, LREE and MREE), which are several times higher than mantle values. At the same time, ion microprobe (SIMS) study showed that derivative melts were dry: contained only 0.01–1.13 wt % H₂O. The trend of melts conserved in the minerals and the massif rocks corresponds to the evolution of alkalinebasaltoid magma with increase in Si, Al, alkalis and decrease in Mg, Ca, and Fe, i.e. the Bowen trend. The considered alkaline-basic dike was presumably formed from the derivative of leucite-phonolite melt, which during emplacement captured olivine xenocrysts from previously fractionated ultrabasic rocks. The parental magma was presumably derived by high-degree melting of garnet-spinel-facies depleted mantle at some influence of crustal material.     

Conditions of crystallization of olivine shonkinites in the Inagli massif (Central Aldan)

The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)_n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.     

Geochemistry of biotite from charnockitoids in the granite-forming series (Nimnyr Block of the Aldan Shield)

The geochemical evolution of biotite (Bt) from garnet-bearing associations in the granite-forming series of the Nimnyr Block (Aldan Shield) is considered. The series includes the primary rock (gneiss or basic rock), autochthonous enderbite- or charnockite-gneiss (Lc1), migmatitic leucosome (Lc2, Lc4), and paraautochthonous charnockite (Lc3-ch1) and (Lc5-ch2 → grn). Bt in each series is characterized by an individual, significant difference in the concentrations of Ti, V, Cr; rarely Al, Fe, Mg, and Rb; and very rarely La, Ce, Nd, and Gd. The uniform trend of evolution of Bt compositions in the series was established for Al, K_Al, (K + Na)/Al, rarely for Mg, Ti, and К. In contrast to the series with ch2, Bt becomes enriched in LREEs from autochthonous Lc1 to allochthonous Lc2 and Lc3. The different compositional variations of Bt in relation some minor elements is explained by the different composition of the material, which underwent melting, temperature variations (from 820 to 750°C), and changes in pH of the mineral-forming medium.     

The first discovery of Hadean zircon in garnet granulites from the Sutam River (Aldan Shield)

For the first time in Russia, a Hadean zircon grain with an age of 3.94 Ga (ID-TIMS) has been discovered in high-aluminous garnet granulites of the Aldan Shield among the U–Pb zircons with an age from 1.92 Ga. In this connection, the problems of its parental source, the petrogenesis of granulites that captured this zircon, and the mechanism of occurrence of these deep rocks in the upper horizons of the crust have been solved. The comparison of the geochemistry of garnet granulites and the middle crust has shown that the granulites are enriched in the entire range of rare-earth elements (except for the Eu minimum), as well as in Al₂O₃, U, and Th and are depleted in the most mobile elements (Na, Ca, Sr). In the upper part of the allitic weathering zone of the middle crust, which formed under conditions of arid climate, this zircon grain was originated from the weathered granites from the middle crust. In the latter case, they were empleced discretely in the upper granite–gneiss crust under high pressure conditions (the rutile age is 1.83–1.82 Ga). The zircon with an age of 3.94 Ga is comparable to the Hadean zircons from orthogneisses of the Acasta region (Canadian Shield, 4.03–3.94 Ga).     

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.     

Paleoproterozoic thermotectogenesis: A rotation-plume model of the formation of the Aldan Shield

The Aldan Shield—the southern segment of the Aldan-Stanovoi sialic nuclear, 1100 km in diameter—is subdivided into an inner granulite-gneiss and an outer amphibolite-gneiss domain. This heterogeneity arose in the Paleoproterozoic as a result of thermotectogenesis, i.e., the sum of magmatism, metamorphism, and deformation superimposed on the older Archean crust. In addition to metamorphic heterogeneity, the main consequences of the Paleoproterozoic thermotectogenesis comprised the emplacement of mafic dikes of various ages and the centrifugal evolution of a radial tectonomagmatic system consisting of complementary granitoids and anorthosites. Thermotectogenesis proceeded in a pulsatory manner with alternation of extension and compression settings in the near-equatorial epi-Archean supercontinent. These consequences and the mechanism of pulsatory evolution are described by a model of plume-related underplating in combination with the change of the Earth’s rotation in the regime of oscillatory evolution of the Earth-Moon system.