Petrology最新文献

The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. The mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concentrations of TiO₂ and immobile trace elements (REE, Zr, Nb), and mainly positive ε_Nd(Т) values. The first type of mafic granulites show elevated (La/Sm)_n and enrichment in Th and LREE relative to Nb, which is typical of subduction-related or crustally contaminated basalts. The absence of negative correlation between (La/Sm)_n and ε_Nd(Т) and a clear positive correlation of TiO₂ with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of the first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental lithospheric mantle modified by melts derived from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO₂ content, positive ε_Nd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Mafic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)_n, depletion in Nb relative to Th and LREE, and negative ε_Nd(Т) values. Post-magmatic influence of granitoids leads to the enrichment of mafic granulites in biotite and apatite, an increase in concentrations of K₂O, P₂O₅, a significant enrichment in Zr, Nb, Th, LREE, and negative ε_Nd(Т) values. The difference between mafic granulites of the first and second types is not related to crustal contamination, but is caused by melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted at the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements was likely caused by felsic melts generated from the rocks of subducting plate immediately prior to mafic magmatism.

The processes of crystallization differentiation, retrograde isotopic exchange, and autometamorphism are considered with reference to the Eocene granites of the Raumid massif, which consists of eight intrusive phases and serves as an example of a “natural laboratory”. The work is based on oxygen isotope, petrographic, and geochemical study of representative samples from each intrusive phase of the massif. The isotopic and geochemical studies were carried out for all rock-forming minerals (Qz, Pl, Kfs, Bt) and their altered varieties. Based on geochemical features, the Raumid granites correspond both to A-type granites and to highly fractionated I-type granites. Our results show that the rocks of the Raumid massif are not the geochemical analog of the Eocene granitoids from the Qiangtang terrane of the Central Tibet or the Vanj complex, as it previously assumed (Chapman et al., 2018). We estimated that differentiation of felsic melts of the Raumid pluton occurred at T = 750–800°C, and P = 4.5–7.8 kbar and was mainly controlled by Pl crystallization. The melts were intruded into the hypabyssal zone in at least two stages: early (γ1–γ3) and late (γ4–γ8), although it is possible that the rocks of the γ7 and γ8 phases formed an additional separate stage. The closure temperature of the oxygen isotopic system of quartz (T_Qz) ranges from 420 to 610°C. The effect of the multiple intrusions of the melts on the T_Qz and apparent cooling rates is considered. The study of altered and unaltered minerals showed that autometamorphism partially overlapped with the retrograde oxygen isotope exchange in the cooling rock. The modelled δ¹⁸О values during Pl and Kfs alteration describes well the observed isotope data when the crystallization takes place at limited content of water fluid (W/M = 0.3–0.05) which could release during the Raumid’s magmas crystallization.

Late Mesozoic carbonatites of Central Asia are developed within the Central Asian Orogenic Belt and adjacent territories of the Siberian and North China platforms. In terms of their structural position, age, geochemical characteristics, and other parameters, they differ from other carbonatite occurrences of Central Asia and are distinguished as the Late Mesozoic carbonatite province in Central Asia. The province includes separate areas of carbonatite magmatism, the geological position of which is determined by the relation with Late Mesozoic rift zones of intracontinental Asia. The carbonatites were formed within a relatively narrow time range (between 150 and 118 Ma) at the early evolution stages of these zones. The carbonatite-bearing complexes of the province are represented by subolcanic and volcanic associations of silicate rocks, carbonatites, magmatic non-silicate rocks (phosphates, sulfates, and others), as well as products of hydrothermal activity. The carbonatites are characterized by diverse composition and include calciocarbonatites, magnesiocarbonatites, and ferrocarbonatites. The silicate rocks are dominated by K–Na and K intermediate rocks. All these rocks have similar geochemical features determined by the elevated contents of LREE, Sr, Ba, and Pb, at low Nb and Ta contents. The typomorphic minerals of carbonatites of the province, in addition to carbonates, are fluorite, Ba and Sr sulfates or carbonates, LREE F-carbonates, and apatite. Unaltered carbonatites are enriched in ¹⁸О and ¹³С relative to mantle values, but in general fall within the compositional range of carbonatites around the world. Hydrothermal and supergene processes modified the mineral composition of carbonatites, which was accompanied by a change of the initial Sr, O, and C isotope composition. The Sr and Nd isotope composition of rocks of carbonatite complexes of the province in general depends on the age of the basement of a definite volcanic area. Carbonatites and associated silicate rocks have close isotope characteristics, but carbonatites usually show relative enrichment in (⁸⁷Sr) and depletion in radiogenic neodymium (¹⁴³Nd). The formation of the Late Mesozoic carbonatite province is related to the activity of mantle plumes, which controlled the Late Mesozoic magmatism in Central Asia. The plumes obviously were accompanied by fluid flows enriched in СО₂, F, and S. This caused the enrichment of lithospheric mantle in volatile components, as well as REE, Sr, Ba, and K, which were extracted by a fluid en route to the surface. Subsequent melting of metasomatized mantle produced parental melts of carbonate-bearing rock complexes.

The aegirine end-member (NaFe³⁺Si₂O₆) in clinopyroxenes resulted from incorporation of Fe³⁺ into the mineral structure. Its presence affects the accuracy of reconstruction of the P-T conditions in the high-grade metamorphic rocks and allows the evaluation of the redox conditions of their formation. The content of this end-member in clinopyroxenes is usually estimated using crystal chemical recalculations of microprobe analyses. However, in some publications on eclogites, the comparison of microprobe-based recalculations with Mössbauer data revealed significant difference between the measured and calculated Fe³⁺/ΣFe ratios, which can significantly affect the results of geothermometry. This paper presents the results of the Mössbauer spectroscopy measurements of clinopyroxene fractions separated from three samples of garnet–clinopyroxene granulites from the Udachnaya kimberlite pipe. The ratios Fe³⁺/ΣFe = 0.22–0.26 measured in the clinopyroxenes correspond to 6–10 mol % aegirine. These estimates are in good agreement with the values obtained for the same clinopyroxenes by the recalculation of microprobe analyses using the charge balance method. Following this conclusion, we believe that crystal chemical recalculations of microprobe analyzes of clinopyroxenes from non-eclogitic rocks make it possible to correctly estimate the Fe³⁺ content in them. Similar recalculation of microprobe analyses of clinopyroxenes in crustal xenoliths from other localities, as well as from ferrobasalts of the continental flood basalts provinces, ferrodolerite dikes, and gabbroid xenoliths (similar in bulk chemical composition to many lower–middle crustal xenoliths) revealed significant amounts of previously unaccounted aegirine (up to 13 and 4–9 mol %, respectively), which holds the potential for deciphering redox conditions in many rocks.