Petrology最新文献

The Oulanka group is a compact group of three peridotite–gabbronorite intrusions that is convenient for testing various petrogenetic concepts. The three intrusions are similar in age and occur not far from one another but differ in the composition of their original magmas, are characterized by different sets of cumulus mineral assemblages, and are different in inner structure and rhythmic layering. We applied cluster analysis of the contents of major elements to reproduce the cumulus mineral assemblages of the isochemically altered rocks of the Tsipringa and Lukkulaisvaara massifs. Although the parental magmas of the Kivakka and Tsipringa massifs were of different composition and their crystallization sequences were also different, the vertical sections of these massifs can be clearly subdivided into zones according to their cumulus mineral assemblages, with the limited development of rhythmic interbedding (with individual rhythms ranging from a few to a few dozen meters in thickness). Conversely, the Lukkulaisvaara intrusion does not possess any clearly distinguishable cumulus zones, and large-scale rhythmic layering is traceable throughout the entire thickness of the massif (with rhythms ranging from a few dozen to a few hundred meters in thickness). The different character of the rhythmic layering of the three intrusions may provide an insight into the different scenarios of magma convection in the chambers.

The paper presents data on granites and gneisses recovered by Kulindinskaya-1 hole drilled in the central part of the Siberian Craton. The biotite granites retain a porphyritic texture, correspond to I-type according to their compositional features, are enriched in LREE and moderately depleted in HREE, and have negative Eu, Sr, and Nb and positive Zr anomalies. The U−Pb zircon age of the granites is Neoarchean (2525 ± 10 Ma), with single cores of zircon grains dated at about 2.6 Ga, which likely suggests a crustal source of the granitic magmas. The model age T_Nd(DM) = 2.77 Ga of the granite shows that the crust from which the initial melts were derived had been formed shortly before the melting episode. In terms of age and all characteristics, the granites are close to those of the Yurubchen massif, which was drilled through in the western part of the Tunguska superterrane. The biotite gneiss was apparently derived from sedimentary rocks and was heavily reworked when the granites were emplaced. The enrichment of the gneiss in Cr and Ni is probably inherited from the sedimentary protolith, whereas the REE, HFSE, and LILE concentrations and distribution in the gneiss are similar to those of the granite. The concordant (D < 1%) U−Pb zircon ages (according to LA-ICP-MS data) broadly vary from 3284 to 2620 Ma, with two major peaks at 2717 and 2678 Ma. The model age of the gneiss T_Nd(DM) = 2.91 Ga confirms a contribution of the ancient crustal component to the sedimentary protolith of the rock. The minimum age of the detrital zircon, 2.62 Ga, determines the maximum age limit for sedimentation, and the minimum age limit is set by the age of the granite intrusions at 2.53 Ga. According to our data, the Archean gneisses and granites recovered by the Kulindinskaya-1 drillhole probably compose the eastern part of the Neoarchean Tunguska superterrane. Ereminskaya-101 drillhole, which was drilled 20 km northeast of Kulindinskaya-1, recovered gneisses with model ages T_Nd(DM) from 2.30 to 2.37 Ga, which belong to the adjacent Taimyr−Baikal suture zone with widespread Paleoproterozoic rocks. The contrasting crustal history of the adjacent complexes provides grounds to suggest that they were tectonically combined, which is an additional reason to consider the Taimyr−Baikal suture zone as a Paleoproterozoic collisional orogen.

The paper presents experimental data on tantalite solubility in water-saturated granitoid melts with various alumina and alkaline elements concentrations at T = 650–850°C and P = 100 MPa. The maximum Ta concentration (effective solubility) in melt is shown to be always higher than the Nb concentration. As the melt composition is changed from alkaline to Al₂O₃-enriched, the Ta and Nb concentrations decrease by one to two orders of magnitude, and the Nb/Ta ratio simultaneously decreases (from ~0.8–0.7 to ~0.4–0.1) because the Nb concentration decreases notably more rapidly than that of Ta. This effect is enhanced at decreasing temperature. The effective Ta solubility in melt is demonstrated to be practically independent of the composition of the dissolving mineral of the columbite-tantalite series. The Ta, Nb, Mn, and Fe diffusion coefficients in granitoid melts are estimated. The Ta and Nb diffusion coefficients at T = 750°C and P = 100 MPa are ~10^–10 cm²/s, and those of Fe and Mn are ~10^–8.5 cm²/s. With an increase in temperature from 740 to 980°C, all of the diffusion coefficients increase by approximately 1.5 orders of magnitude. The configurations of the diffusion profiles of Ta concentration in melts change differently depending on change in the composition of the melt, temperature, or pressure.

Rhenium-Os isotope and highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) abundance systematics of Archean komatiites can be used to estimate the stirring rates of the mantle for the HSE and the timing of homogenization of late accreted materials within the mantle. In this study, we report Re-Os isotope and HSE abundance data for ~2.9 Ga komatiites and basalts from the Sumozero-Kenozero greenstone belt in the SE Fennoscandian Shield. The lavas are characterized by excellent preservation of the primary textural, chemical, and Re-Os isotope characteristics. The Re-Os isotopic data for spinifex-textured and cumulate komatiite and massive basalt samples from the lowermost sequences define a precise 10-point isochron (MSWD = 2.6) with an age of 2904 ± 18 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os = 0.10758 ± 18 (γ¹⁸⁷Os(2904) = +0.45 ± 0.17). This is the first direct age determination for the Sumozero-Kenozero lower komatiite-basalt sequences. Our modeling indicates that the mantle source of the komatiites and basalts evolved with a time-integrated ¹⁸⁷Re/¹⁸⁸Os = 0.418 ± 6. This ratio is well within the uncertainty of the bulk chondritic average ¹⁸⁷Re/¹⁸⁸Os = 0.410 ± 51 (2SD), also consistent with the chondritic evolution of the majority of komatiite mantle sources observed globally. The mantle source of the Sumozero-Kenozero komatiites has been calculated to contain the total HSE abundances of 58 ± 7% of those in the estimates for modern Bulk Silicate Earth (BSE). This estimate is in the middle of the range for other late Archean and Proterozoic komatiite systems. Using the estimated HSE abundances in the sources of komatiite systems as a function of their ages and ISOPLOT regression analysis, we calculated the average time in the past by which late accreted materials have been completely homogenized within the mantle to be 2.48 ± 0.23 Ga. These data require that the residence times of the late accreted planetesimals within the mantle, before complete homogenization, were on average 1.92 ± 0.23 Ga. This estimate represents a constraint on the average mixing rates of the mantle in terms of the HSE abundances in the Hadean and the Archean.

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.