Petrology最新文献

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.

Crystal size distributions (CSD) of olivine were obtained for 17 samples of plagiodunite and Pl‑bearing dunite from the central part of the Yoko-Dovyren massif, northern Baikal region, Russia. Three types of CSD were identified: loglinear, bimodal, and lognormal. Combining these data with the results of petrological reconstructions, which earlier revealed two main types of the Dovyren magmas (using the method of geochemical thermometry), we proposed a basic scenario of interaction between magmatic suspensions of different temperature to explain the diversity of the CSD. The intratelluric olivine transported by magmas of different temperature, which had not subjected to abrupt cooling or heating in the chamber, retained an original loglinear CSD. For some portions of the hottest magma (∼1290°C), it is assumed that the original olivine evolved into a bimodal CSD due to accelerated crystallization at faster cooling of the high-temperature injections contacting relatively cold crystal mush (∼1190°C). An interpretation of the lognormal CSD suggests that part of the olivine crystals composing the protocumulate systems efficiently interacted with the pore melt infiltrating upward during the compaction of the underlying crystal mush. This led to cycles of partial dissolution and regrowth of the olivine grains resulting in a final lognormal CSD. The infiltrating hot melt, which was undersaturated with immiscible sulfide liquid, could dissolve sulfides preexisting in the low-temperature mush. This produced dunites with lognormal CSD relatively depleted in sulfur and chalcophile elements. The lognormal CSD is considered to be a marker of crystal mush regions through which the focused infiltration of the pore melt proceeded.

The paper discusses possible immiscibility between fluoride salt (“cryolite”) and silicate liquids into which the parental melt of the Katugin massif exsolves, and the petrological implications of this phenomenon. Results of a detailed study of the cryolite and zircon are presented. Liquid immiscibility is demonstrated to have triggered the massive crystallization of zircon and, together with the processes of subsequent evolution of the cryolite melt, contributed to the formation of the large cryolite bodies. Data on mineral-hosted inclusions were used to estimate the crystallization temperatures of fluoride salt and silicate melts and outline the pathways of their evolution during the formation of the massif. It is shown that the granites of the Katugin and West Katugin massifs were most likely derived from distinct sources, that differed mainly in fluorine content. Data on the chemical composition of three zircon generations identified in the granites of the Katugin massif are presented.