Petrology最新文献

The Lower Urgen deposit is a newly discovered porphyry Mo deposit in the northern and central Great Xing’an Range. Mineralization predominantly occurs within granite porphyry, yielding a zircon U-Pb age of 142.3 ± 1.5 Ma, thereby endorsing an Early Cretaceous Mo mineralization event. Zircon ε_Hf(T) values (5.5–7.7) and T(DM2-st) (707–844 Ma) suggest that the granite porphyry originated from the partial melting of the Neoproterozoic lower crust. These granite porphyries exhibit coherent geochemical signatures with regional Late Mesozoic Mo-causative granites. Classified as highly fractionated A-type granites, they are enriched in Rb, Th, U, and K, and depleted in Ba, Sr, P, Ti, and Eu. Notably, they possess higher Rb/Sr and Rb/Ba ratios, and lower (La/Yb)_N, Eu/Eu*, LREE/HREE, K/Rb, and Zr/Hf ratios than coeval Cu-causative granites, implying the extent of fractional crystallization plays a pivotal role in determining the mineralization styles (Mo- versus Cu-dominant). Two possible tectonic models are proposed. In one model, Late Jurassic Mo- and Cu-causative granites were formed in an intra-plate extensional setting and compressional setting induced by the flat-slab subduction of the Mongol-Okhotsk Ocean (MOO) plate, respectively, while Early Cretaceous Mo-causative granites were formed in a post-collision extensional setting following the final closure of the MOO. The post-orogenic lithospheric extension model related to the closure of the MOO provides another plausible explanation for the origin of the ore-causative granites. Early Cretaceous highly fractionated A-type granites and Late Jurassic low fractionated adakitic granites represent potential targets for future exploration of Mo- and Cu-dominant porphyry deposits, respectively.

The deformation and metamorphic history of the Precambrian high-grade rocks in the Key Afer area, southwestern Ethiopia within the Mozambique belt is described. It comprises poly-deformed, metamorphosed, and migmatized rocks with intrusion of granitoids and overlain by Quaternary sediments. A combination of field litho-structural mapping, metamorphic mineral assemblages, and microstructural analysis there are three metamorphic events and four phases of ductile deformation, and one Cenozoic brittle fracture (D5) are recognized. The development of the relatively steep NNW-SSE trending S1 relict gneissic banding and the rise of pyroxene and anhydrous minerals indicate that the peak metamorphism (M1) is synchronous with D1. Subsequently, the hydration of M1 assemblages leads to the formation of amphibolite facies (M2). This is followed by the development of amphibolite facies (M2) caused by the hydration of M1 assemblages synchronous with the D2 deformation. It is defined by the major regional fabric (S2) of the area trending NW-SE, tight to isoclinal upright F2 folds, and local L2 lineation. These D2 upright folds are orthogonally superimposed by another upright F3 folds during D3 resulting in a type-I fold interference pattern. The replacement and breakdown of hornblende to epidote, biotite to chlorite, and plagioclase to sericite give a retrogressive event to greenschist facies (M3) syn-D4. It gave rise to NNE-SSW-oriented S4 mylonitic foliations associated with F4 drag folds. Both sinistral and dextral shear sense is recorded but dextral shear sense appears dominant. The fifth phase of deformation (D5) is characterized by brittle fracture and joint structures of the area with varying orientations. The three metamorphic events with deformational episodes of the study show a clockwise P-T path loop from burial to uplift similar to the collision-parallel shearing orogenic setting.

Volcanic sequences of bimodal basalt–trachyte–alkaline rhyolite character with alkaline granites are widespread in central Mongolia. They crop out in small sublatitudinal grabens scattered along the southern and western surroundings of the Khentey part of the Mongol–Okhotsk Belt. According to geochronological data, the bimodal magmatic activity continued from the latest Triassic to earliest Jurassic (at ∼220–195 Ma). Many rocks of the bimodal sequences contain high concentrations of alkalis and rare metals. Fractional crystallization was the leading process for enrichment of rare metals up to their ore-level concentrations in the most differentiated melts. Mafic magmas enriched relative to the OIB in most incompatible trace elements were parental for all rocks of these associations. At the same time, they show elevated Ba and depleted Ta and Nb contents, which indicate that a lithospheric mantle component was involved in their source. The Nd and Sr isotopic ratios of the rocks indicate that the magmas were derived from at least two sources, which are identified as enriched asthenospheric mantle and metasomatized subduction-modified lithospheric mantle. Bimodal magmatism in the Khentey segment of the Mongol–Okhotsk belt appeared ~30 Ma after collision caused by the closure of the Ada-Tsag branch of the Mongol–Okhotsk Ocean at about 250 Ma. Rifting affected the entire surroundings of the Khentey segment of the belt and controlled this magmatism. It was initiated by the collapse of the orogen with delamination of its keel caused the involvement of asthenospheric mantle in the Late Triassic–Early Jurassic magmatism of the region

The P–T paths of the exhumation of Precambrian granulite complexes at craton boundaries usually include two stages: subisothermal decompression and a decompression–cooling stage with a more gently sloped P–T path. Our goal is to understand the possible causes of the change in the slope of the P–T exhumation path of the Central Zone (CZ) of the Limpopo granulite complex, South Africa, located between the Kaapvaal and Zimbabwe cratons. For this purpose, rocks (mainly, metapelites) were studied in various structural settings within the Central Zone, i.e., in dome structures, regional cross folds, and in local and regional shear zones. The metapelites are gneisses of similar bulk composition. The rocks contain various amounts of relics of leucosomes composed of quartz–feldspar aggregates with garnet and biotite, and melanocratic domains that are enriched in cordierite and usually mark shear microzones that envelope and/or break garnet porphyroblasts. Study of polymineralic (crystallized melt and fluid) inclusions in the garnet, its zoning with respect to the major (Mg, Fe, and Ca) and some trace (P, Cr, and Sc) elements, fluid inclusions in quartz, as well as phase equilibria modeling (PERPLE_X) showed that the rocks coexisted with granite melts and saline aqueous carbonic fluids (({a}_{text{H}_{2}text{O}}) = 0.74–0.58) at the peak of metamorphism at 800–850°C and 10–11 kbar. Partial melting of the rocks initiated their subisothermal exhumation to 7.5–8 kbar during diapirism of granitic magmas in the Neoarchean (2.65–2.62 Ga). This is reflected in the specific zoning of the garnet grains in terms of the grossular content. A change in the rheology of the rocks as a result of partial removal and crystallization of melt activated the shear zones during further exhumation to 6–5.5 kbar along a decompression–cooling P–T path at 95–100°/kbar, reflecting the slower uplift of the rocks in the middle crust. This process was resumed due to thermal effects and interaction of the rocks with aqueous fluids (({a}_{text{H}_{2}text{O}}) > 0.85) in the Paleoproterozoic (~2.01 Ga). Such a scenario of metamorphic evolution implies that the Limpopo granulite complex in general and its Central Zone in particular resulted from the evolution of an ultrahot orogen, in which vertical tectonic movements associated with diapirism were coupled to horizontal tectonic processes caused by the convergence of continental blocks.

Raman spectroscopic data of quenching phases in experiments on the dissolution of Pt in reduced carbonic fluid, containing about 30 mol % of CO, both with and without chlorine at P = 200 MPa and T = 950–1000°C are presented. Water content in the fluid was no more than 4.5 mol %. The only soluble form of Pt determined in the acetone solution of the quenching phases and in the experimental products is platinum carbonyl. Low concentrations of carbonyl (no more than a few ppm) become detectable using Raman spectroscopy due to the SERS effect (Surface-Enhanced Raman Scattering), which is possible in the presence of Pt nanoparticles in the objects under study. Platinum nanoparticles, formed at the decomposition of carbonyls, generates specific photoluminescence (PL) peak approximated by Gaussian with parameters FWHM = 1050–1300 cm^–1, k_max = 2050–2100 cm^–1 both in acetone solution and experimental samples. The spectra of CO (main band k ≈ 2050 cm^–1) adsorbed on Pt nanoparticles supported on glassy carbon, formed during the decomposition of excess CO relative to the CCO buffer, corresponded to nanoparticle sizes of about 2 nm. No convincing evidence of a mixed chloride-carbonyl composition of platinum was found in the spectra, which may reflect the lower thermodynamic stability of these mixed complexes at high P-T parameters. Large concentrations of platinum Pt on carbon (up to 2000–3000 ppm) can be explained by the formation of the Pt-C matrix bond and the weakening of the Pt-CO bond in carbonyls, causing their decomposition. Unusual PL peaks were detected in samples from experiments with chlorine-containing fluids, very reminiscent of the PL background of noble metal nanoparticles and attributed to the effect of carbon nanoparticles.

Dolerite dikes were studied in the western part of the Aldan terrane, in the middle reaches of the Tokko River. These dolerite dikes form a swarm of submeridional trend about 1 km wide. The dolerites of the thickest dike preserve their primary textural and structural features and mineral composition: plagioclase + pigeonite + augite + titanomagnetite. Dolerite in the chilled margins and central parts of the dike are homogeneous in composition, corresponds to low-Mg tholeiites, has low contents of Ti and other HFSE, with weak enrichment in light REE and small negative Nb anomalies. Sm–Nd isotope data on magmatic minerals of dolerite from the central part of the dike yield a good linear regression in an isochron diagram that gives to an age of 2510 ± 64 Ma, which probably corresponds to the crystallization age of the basalt. Metadolerites in a thin dike retain plagioclase porphyritic structures, but the pyroxenes are completely replaced by amphibole and chlorite. The metadolerites are contrastingly different in low contents of MgO, Cr, and Ni and in higher contents of TiO₂, Fe₂O₃, P₂O₅, Nb, and all REE. The differences in the composition of the dikes may be explained by the longterm (about 65%) crystallization differentiation of the initial melt and the emplacement of the residual melt from a shallow intermediate magma chamber via opening cracks. Such conditions probably may have existed in tectonically stable intraplate settings. The age of the dolerites of the dike swarm is comparable to that of the anorogenic granites of the Nelyuki complex (~2.4–2.5 Ga), which are widespread in the western part of Aldan granulite–gneiss terrane. Our data bridge some gaps in characteristics of intraplate anorogenic magmatism that occurred in the western Aldan Shield in the Late Archean and marked the final consolidation of a large block of Archean crust in the Chara–Olekma granite–greenstone area.

Buziwannan granodiorite and monzogranite associated with gold–polymetallic mineralization are located in the West Kunlun Orogen Belt in northwest China. Granodiorite was emplaced earlier than monzogranite. To determine the genesis of plagioclase from two intrusions and their relation with mineralization, the major, trace elemental, and Sr isotopic compositions of plagioclase were determined through LA-ICP-MS and LA-MC-ICP-MS respectively. The results indicated that the plagioclase from granodiorite had a high-An (around 40%) core and low-An (around 33%) rim, while the plagioclase from monzogranite was uniform with an An value around 18%. The (⁸⁷Sr/⁸⁶Sr)_i ratios of plagioclase decreased with decreasing An value, which may be caused by small-scale crustal contamination and/or magma mixing. The crystallization process of plagioclase is mainly accompanied by the exsolution of magmatic H₂O, and the pressure changes caused by the loss of magma H₂O. These magmatic fluids are rich in ore-forming elements, such as Au–Ag–Cu–Zn, and form skarn mineralization near the wall rocks. Because of the co-crystallization of plagioclase, hornblende, and biotite, as well as the addition of minor felsic magma with lower Sr isotopic composition, the plagioclase from monzogranite exhibits low and uniform An values. In addition, a large amount of magmatic H₂O carrying ore-forming elements was released during the emplacement of granodiorite, which caused the monzogranite to lose its metallogenic potential.