Petrology最新文献

The paper summarizes major and trace-element compositions and Sm–Nd isotope data on metabasites (amphibolites) and gabbroids of the Onot granite–greenstone block in the Sharyzhalgai basement uplift, southwestern Siberian craton. The Onot block consists of tectonically combined nappes of the Paleoarchean tonalite–trondhjemite–granodiorite (TTG) complex and the metasedimentary-volcanic complex of the greenstone belt (GB). The Mezoarchean (∼2.88 Ga) metabasalts of the greenstone belt and Paleoproterozoic (∼1.86 Ga) gabbronorites and vein gabbros were formed at rifting and postcollisional extension, respectively. The Archean metabasites of the greenstone belt and enclaves in the TTG complex compositionally correspond to low-Ti tholeiitic basalts and basaltic andesites. The basaltic rocks are characterized by flat REE patterns [(La/Sm)_n = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive ε_Nd(T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative ε_Nd(T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y)_PМ (0.8–1.0), negative ε_Nd(T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm)_n, positive ε_Nd(T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). The trace element-composition of the amphibolites, gabbronorites, and gabbros and the results of geochemical modeling indicate that the parental melts were derived mainly from weakly depleted mantle sources. The Nd isotope composition of the Paleoproterozoic gabbroids resulted from the evolution of the heterogeneous Archean lithospheric mantle. Variations in the isotope and trace-element composition of the amphibolites reflect the initially depleted nature of the Mezoarchean mantle and its metasomatic alteration by fluids/melts, which occurred before its melting at ∼2.88 Ga. The geochemical and Nd isotopic characteristics of gabbronorites and gabbros indicate that the lithospheric mantle had become progressively more heterogeneous by the Paleoproterozoic due to preceding Archean processes. The variable depletion of both the Archean and the Paleoproterozoic mafic rocks in Nb relative to Th and La may be explained by mantle metasomatism and does not reflect the geodynamic settings of the mafic magmatism.

The Eastern Mongolian Volcanic Area (EMVA) is part of the Late Mesozoic–Early Cenozoic volcanic and plutonic belt in Northeastern Asia. The EMVA evolved in three stages, with volcanic rocks of different composition produced during each of the stages and with the parental melts of the rocks derived from different sources and formed by different mechanisms. The rocks of the Early Cretaceous stage (135–100 Ma), which form the volcanic flow complex of the EMVA, are predominantly differentiated alkali basaltoids. Data on isotopic features of these rocks, particularly their Pb isotope composition, allowed us to identify the nature of their sources: peridotites of the Continental Metasomatized Lithospheric Mantle (CMLM) and lower continental crustal eclogitic rocks. The alkali basaltoids of the extrusive complex of the Uldza-gol volcanic field were formed during the next evolution stage of the EMVA at 104–90 Ma. According to their geochemical and isotope features, the melts of these rocks were derived from the same sources as those of the volcanics of the previous Early Cretaceous stage, except only that eclogite material played a more significant role in forming of the Uldza-gol basaltoid melts. During the concluding stage of the EMVA evolution in the Late Cretaceous–Early Cenozoic (87–51 Ma), OIB-like rocks of the basanite–trachybasalt association were formed in the Central Gobi in the southwestern flank of the EMVA. Asthenospheric and recycled pyroxenite components, together with not so much CMLM peridotites, were involved in forming of these rocks. The various sources of the EMVA volcanic rocks reflect two mechanisms of their formation. In the Early to Late Cretaceous, magmatism was triggered by the ascent of the asthenospheric mantle and delamination of the lithospheric mantle, whereas the Early Cenozoic magmatism was induced by the activity of a mantle plume.

The paper presents experimental results on the solubility of fluorides in fluid-saturated melt of alkaline ultrapotassic syenite-porphyry from the Gross gold ore deposit in southern Yakutia at T = 600–800°C, P = 150–260 MPa. The experiments were carried out to confirm the assumption of high solubility of fluorine in ultrapotassic syenite melt, which could contribute to the formation of specific and low-viscosity melts that were emplaced in the form of a syenite porphyry sill. The solid products after the experiments contained aluminosilicate glass, potassium feldspar, fluorite, quartz, and two fluoride phases differing in composition (potassium and aluminum fluoride and potassium and magnesium fluoride). The experimental results led us to determine a high maximum solubility of fluorine in the melt: up to 4.2–4.6 wt %, with the maximum F content found in the lowest temperature melt at 625°C. The solidus and liquidus temperatures of the syenite melt were estimated at 600–625 and 650–800°C, respectively. The aqueous fluoride fluid coexisting in equilibrium with the melt was determined to be alkaline. Potassium feldspar was the first to crystallize from the melt in the experiments, which is consistent with what is observed in samples of the naturally occurring rocks.

The paper presents data on the geological structure of the Kichera zone of the Baikal–Vitim belt (BVB) at the boundary between the marginal part of the Siberian craton and the Barguzin–Vitim superterrane of the Central Asian Orogenic Belt. Early Neoproterozoic (Early Baikalian) and Late Neoproterozoic (Late Baikalian) structures and complexes are identified and characterized in the Kichera zone of the BVB. Data are presented on the geochemistry of the rocks and on their U–Pb isotope age (zircon, SIMS and ID-TIMS) and on the Nd isotope characteristics of rocks from various parts of the Kichera zone, including representative rock association of the Nyurundukan migmatite–tonalite–metabasite complex with MORB-type tholeiites and tholeiites with intraplate geochemical features. It is shown that the sources of the Early Neoproterozoic complexes of the Kichera zone, which were metamorphosed at 0.76–0.74 Ga as a result of accretion events in the marginal part of the craton, were dominated by Early Precambrian recycled crustal material. The Late Neoproterozoic complexes typomorphic of the Kichera zone were formed in the Cryogenian–Ediacaran (720–545 Ma) from prevailing juvenile sources. Our data suggest that the metabasites of the Nyurundukan complex were formed in an environment of segmented troughs of the pull-apart paleorift system of the Kichera zone and can be compared with a reduced complex of continental-margin ophiolites transformed at 630 ± 7 to 615 ± 3 Ma. The destruction of the ancient continental crust of the craton ended with the formation and exhumation of deep rocks in the Late Ediacaran, the emplacement of adakite granites of the postcollisional geochemical type, and the formation of grabens filled with a terrigenous complex. The juvenile and riftogenic crust produced during the Late Neoproterozoic tectonic evolution of the Kichera rift zone does not show any features of mature continental-type crust.

All published experimental data on Rh solubility in silicate melts were combined to derive an equation relating Rh solubility to temperature, oxygen fugacity, and a melt composition. It is demonstrated that Rh is dissolved in a melt dominantly as Rh²⁺ in the entire experimental fO₂ range, from pure oxygen to QFM + 2 (QFM is the quartz–magnetite–fayalite buffer). The temperature dependence of Rh solubility is anomalous. Similar to the solubilities of other noble metals, Rh solubility at a constant fO₂ increases with increasing temperature. The Rh metal/silicate partition coefficient was calculated ((D_{{{text{Me/Sil}}}}^{{{text{Rh}}}}) ≈ 3.5 × 10⁷) for the expected conditions of Earth differentiation into a core and mantle. It is demonstrated that the late chondritic veneer model is able to best explain high Rh contents in upper mantle rocks. The suggested equation makes it possible to discard experimental glasses contaminated with metallic Rh micronuggets and thus to get rid of at least the most gross errors in the determination of Rh partition coefficients between rock-forming minerals and melt.