Petrology最新文献

Elevated pressure (overpressure) was revealed in the southeastern fragment of the Raahe–Ladoga suture zone in Russia, within the Meyeri tectonic zone. It was caused by structural metamorphic transformations of rocks during collisional interaction of allochthonous and autochthonous blocks. It was supposed that tectonic interaction between the rigid crustal block of the Archean basement of the Karelian craton (autochthon) and the Proterozoic granulite block of the Svecofennian belt (allochthon) provided conditions for the formation of superlithostatic pressure anomalies. Mineral geobarometry and numerical thermomechanical modeling indicated pressures up to 9–11 kbar at a lithostatic pressure of 4–6 kbar. Based on the obtained results, it was argued that the nature of the local superlithostatic pressure (up to 7–9 kbar) established by mineral geobarometry and numerical thermomechanical modeling can be explained by tectonic interaction of blocks with heterogeneous physical and mechanical properties rather than by the errors of the applied mineral geobarometry tools.

Melting of metabasic rocks is a large-scale geologic process contributing to the formation of silicic volcanics and, especially, tonalite–trondhjemite–granodiorite (TTG) complexes, which make up a considerable portion of the ancient continental crust. Based on the phase equilibria modeling using the Perple_X program package, parameterization of melting was conducted for three compositions: anhydrous mid-ocean ridge basalt (MORB), MORB-H₂O (2.78 wt % H₂O), and hydrated basalt (altered oceanic crust, AOC, 2.78 wt % H₂O) at 500–1600°C and 0.0001–3 GPa. The obtained relations show good consistency with limited experimental data and indicate that the volume of melt produced in hydrous systems (MORB-H₂O and AOC) increases rapidly (up to 20 vol %) within 20–30°C above the hydrous solidus, which is followed by a more moderate increase in the degree of melting with increasing temperature. The modeling demonstrated that the near-solidus melts of the hydrous systems are rhyolitic and trachydacitic in composition. An increase in the degree of melting results in a decrease in SiO₂ and alkalis and an increase in CaO, MgO, and FeO contents. Changes in melt volume and composition are considered in connection with peritectic reactions and variations in H₂O content. The application of the parameterization of melting to metabasalts from the downgoing slabs in the Cascadia and Central Aleutian hot subduction zones revealed that these rocks underwent different degrees of melting along respective geotherms, and adakitic magmas are produced by such melting. The proposed parameterization of rock melting is useful for the analysis of the mechanisms of silicic rock formation in different geodynamic environments and can be implemented in the existing petrological and petrological–thermomechanical models.

Unique rocks were found in an outcrop of basement granodiorites in the southern part of the Khangar caldera. The bulk composition of these rocks corresponds to high-Mg andesite (SiO₂ 57–63 wt %, MgO 4–8 wt %, and K₂O 1.4–2 wt %). The rocks contain coexisting quartz, oligoclase, and olivine phenocrysts and a propylitic mineral assemblage (albite, calcite, chlorite, and epidote). The largest phenocrysts are similar in composition to granodiorite minerals (oligoclase An_22–28, quartz, and biotite). The olivine phenocrysts contain melt inclusions of basaltic composition (SiO₂ 45–48 wt %, MgO 7–10 wt %) with a high K₂O content (up to 1.6 wt %). We suggest that these rocks were produced by interaction of basaltic melt with silicic intrusive material and with the xenogenic material of the granitoid intrusion.

Major and trace element data were presented for the Mai Kenetal–Werii metavolcanic rocks within the Arabian–Nubian Shield to examine the petrogenesis and the tectonic setting of the area. Based on field observations and geochemical data, the metavolcanic rocks were classified into basalts, andesites, and dacite-rhyolites. The rocks are dominantly subalkaline (tholeiite to calc-alkaline) in composition. The low contents of Ni (0.1–78 ppm), Co (2–53 ppm), Cr (10–351 ppm), TiO₂ (0.18–1.59 wt %), MgO (0.62–9.16 wt %) and relatively with high contents of Al₂O₃ (10.39–18.51 wt %) indicate that the rocks were formed from more evolved magmas. The chondrite-normalized REE patterns also showed moderate fractionation, with (La/Yb)_N values ranging from 2.28 to 9.22, slightly negative to positive Eu anomalies (Eu/Eu*, 0.50–1.16), and relatively flat heavy REEs ((Gd/Yb)_N = 0.12–1.79). The rocks display slight enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs), representing rocks that were derived from more evolved magmas in a mature island arc setting. Whereas, the low ratios of Nb/La (0.17–1.66) and Nb/Yb (0.61–2.23), combined with low total REE content (ΣREE = 49.58–151.3 ppm), low Nb content (0.6–6.5 ppm), and high Zr/Nb ratios (20.7–96.67) indicate depleted mantle source. In addition, trace element ratios (Y/Nb = 4.11–15.44, Nb/Y = 0.06–0.24, La/Sc = 0.1–2.67, La/Y = 0.32–1.51 and La/Nb = 2.68–6.23) and discrimination diagrams (Zr/4–2Nb–Y, Th–Hf/3–Ta, and Th–Zr/117–Nb/16), indicate that the metavolcanic rocks have a calc-alkaline affinity that was formed in an island arc tectonic setting from subduction-related magmas within Arabian-Nubian Shield.

The Zhaibeishan copper deposit is located at the eastern part of Aqishan-Yamansu metallogenic belt in eastern Tianshan. Zircon U-Pb geochronology, major and trace element, and Sr-Nd isotopic characteristics of Zhaibeishan granite and its relationship with mineralization have been studied. SHRIMP zircon U-Pb dating indicates an Early Carboniferous intrusive time (334.5 ± 2.6 Ma) of the granite. Chemically, Zhaibeishan granites have high silica (71.50–75.06%), aluminum (A/CNK = 1.02–1.23), sodium (Na₂O/K₂O = 0.95–23.83 with 6.89 on average), and total alkalis (Na₂O + K₂O = 6.65–8.43%), and low magnesium (<1%) and titanium (<1%) contents. The Chondrite-normalized REE patterns are characterized by enrichment of LREE relative to HREE (La_N/Yb_N = 4.05–6.85) with moderate negative Eu anomalies (δEu = 0.38–0.73). The Zhaibeishan granites show enrichment of K, Rb, (Large Ion Lithophile Elements), LREE and depletion of Nb, Ta, Ti, and P (High Field Strength Elements), indicating island arc magmatic characteristics. Sr-Nd isotopic data reveal that the I_Sr values range from 0.70473 to 0.70551, while ε_Nd(T) values range from 2.3 to 3.2. We suggest that the Zhaibeishan granites formed in continental arc setting in subduction zone and were probably derived from the product of magma mixing between crust and mantle magmas and experienced subsequent fractional crystallization. Combined with the fluid inclusion and published ore-forming age and isotopic data, we suggest that porphyry mineralization and blind copper orebodies probably exist in the deep part of the Zhaibeishan copper mining area.

The paper presents the first data on the petrography, mineralogy, and geochemistry of jadeitites from the El’denyr massif, Chukotka, Russia, as well as host metalherzolites and amphibolite inclusions in the jadeitites. The jadeitite is composed of an association of jadeite, omphacite, analcime, and pectolite with a Ba−Ti−Si accessory mineral. The host metalherzolite is made of an association of olivine, antigorite, diopside, chlorite, ferrite-chromite, chromium magnetite, and accessory awaruite, heazlewoodite, and pentlandite. The jadeitite contains inclusions with a relict coarse-grained hypidiomorphic-granular texture, which are considered to be relics of the metasomatized protolith of the jadeitite. This protolith was probably high-temperature hydrothermal diopsidite. The inclusions show local recrystallization of primary diopside to aegirine-augite and pseudomorphic development of a fine-grained aggregate of amphiboles (several generations of richterite, actinolite, magnesiokatophorite, K-richterite, and eckermannite), omphacite, pectolite, analcime, phlogopite, accessory maucherite and heazlewoodite after diopside/aegirine-augite and an associated unidentified mineral. The protolith was transformed in several stages before the onset of jadeite crystallization, and these transformations included metasomatic recrystallization and a complete change in its texture. During the last stage, crystallization of the euhedral concentrically zoned jadeite with analcime and pectolite from fluid was accompanied by the recrystallization and dissolution of the last reworked relics of the protolith represented by high-calcium omphacite in microgranular omphacite-jadeite aggregates of jadeitite. The formation of jadeitites and the accompanying metamorphism of the host lherzolites occurred at 500°C and 8.5 kbar, which corresponds to P–T conditions typical of the metamorphism of mantle wedge peridotites in the “warm” subduction regime. The presence of jadeitites in the El’denyr massif and high-pressure metamorphic rocks in the Ust’-Belaya massif, which were studied previously, allows us to consider the Ust’-Belaya terrane as a mélange of a subduction zone active in the Early–Middle Triassic that was deformed and disintegrated during its subsequent exhumation in the Cretaceous.

The paper presents original detailed data obtained by the authors on the Archean Pon’goma-Navolok granulite and charnockite massif in northern Karelia: a geological map of the massif and its surroundings, data on the petrography of the magmatic and metamorphic rocks, and the P–T parameters evaluated for major rock types by the techniques of multimineral thermomabometry and pseudosections. The Pon’goma-Navolok massif is determined to have formed as two intrusive phases at different crustal levels. The first intrusive phase corresponds to the massif of clinopyroxene–orthopyroxene charno-enderbites that crystallized at 8–11.2 kbar and 730–740°C. The second phase comprises dikes of orthopyroxene–biotite charnockites, which formed at 5.6–6.8 kbar and 830–850°C, and biotite granites, which crystallized at 6.8–7.0 kbar and 730–740°C. The dikes most likely correspond to different temperature and water-activity facies. The charnockites and granites were formed by processes of charnockitization and granitization of the charno-enderbites under the effect of saline aqueous solutions. The granulite-facies metamorphism of the metabasite blocks hosted in the charno-enderbite intrusion was of contact nature and was induced by the thermal effect of the charno-enderbites on the roof and wall rocks of the magma chamber. The high metamorphic temperatures of the metabasites (>900°C) and the absence of migmatization aureoles are explained by low water contents in the enderbites.