1,721,015 research outputs found
Granite geochemistry is not diagnostic of the role of water in the source
No full textThe diverse fluid regimes during melting of the metasedimentary crust have been often discriminated on the basis of the composition of anatectic granitoids, with granites indicating fluid-absent melting conditions and trondhjemitic compositions suggesting the addition of external water in the source region. The lack of abundant metasedimentary-derived trondhjemites in the geological record is supposed to prove the minor role of water-fluxed melting in the crustal maturation. In terms of trace elements, instead, Rb, Sr and Ba contents and their ratios have been commonly used to discriminate dehydration vs. water-fluxed melting scenarios. Here I show that reconciling results of melting experiments, thermodynamic modeling and nanogranitoid study brings out a different picture. Equilibrium thermodynamics cannot properly reproduce melt compositions of the selected benchmark experiments, with the latter having trondhjemitic compositions mainly for the metastable behavior of muscovite during laboratory runs. The formation of sufficient volumes of extractable trondhjemitic melts is related to high pressure melting conditions (≥8 kbar at 700 °C and ≥11 kbar at 750 °C) or K-poor bulk rock compositions, rather than to the only presence of water. At low- to medium-pressure, crustal melts are granites in composition, whatever the fluid regime is. It is inferred that the abundance of anatectic peraluminous granites (compared to metasedimentary-derived trondhjemites) does not imply a dry nature of the orogenic crust. Likewise, the use of LILE (Rb, Sr and Ba) signatures may lead to erroneous conclusions on the fluid regime of the deep continental crust
Nanorocks: a 10-year-old story
Full text linkThe presence of nanogranitoids (crystallized melt inclusions showing a granitic s.l. composition) in regionally metamorphosed migmatitic and granulitic terranes was documented for the first time in 2009. This important finding represented a hallmark event for crustal petrologists, demonstrating that the pristine composition of anatectic melts may be preserved and recovered in small objects hosted in peritectic minerals of high-grade metamorphic rocks. Many further occurrences were documented in rocks worldwide, from diverse geodynamic settings, from the Archean to Miocene, and these inclusions turned out to have diverse composition (from silicatic to carbonatitic). Therefore, we propose “nanorocks” as a comprehensive name for these crystallized melt inclusions. In the last decade, many studies demonstrated their utility in characterizing mechanisms of melting and in tracking crustal magma evolution. Although scholars are now able to easily recognize nanorocks in high-grade metamorphic rocks, only a few research groups remelt them by means of experimental devices because of a time-consuming preparation. However, when cutting-edge techniques are applied in addition to more routine ones, nanorocks do open new perspectives in crustal petrology
Anatectic granites in their source region: A comparison between experiments, thermodynamic modelling and nanogranitoids
No full textAnatectic granites represent the products of crustal maturation by partial melting, therefore, studies on their origin are crucial to understanding how continental crust forms and evolves. This contribution compares composition of melts obtained from three tools allowing the investigation of anatectic granites in their source region: experiments, phase equilibria modelling and nanogranitoids. Benchmark experimental runs are selected along with nanogranitoids from the Ivrea–Verbano Zone (IVZ; NW Italy) and compositions of melt are then calculated using the same bulk rock compositions considered in experimental and natural case studies. The possible effects of melt loss, diverse P–T paths and variable equilibration volumes on melt composition are modeled for IVZ rocks. Results are then compared with those of previous comparative studies to verify the presence of systematic compositional divergences and similarities. Collectively, all data reflect pressure and temperature conditions (≈700–950 °C and 0.3–1.5 GPa) typical of the suprasolidus orogenic crust, with source regions ranging from metapelite, metagreywacke to metagranite. Contents of FeO, MgO, K2O and, to a lesser extent, CaO in the melt show the largest divergences. FeO, MgO and CaO tend to be enriched in experimental melts and nanogranitoids, whereas K2O is mostly overestimated in calculated melts. After a consideration of the common pitfalls for each approach we discuss the potential causes for the main geochemical discrepancies. The most extreme deviations of FeO and MgO, often observed in low-T (≤ 750 °C) melts, seem to be related to the selected melt models, which have been mostly calibrated on the basis of high-T (≥800 °C) experiments. Some experimental melts tend to be strongly depleted in K2O due to the metastable persistence of micas at much higher temperatures in laboratory runs, whereas the compositions of nanogranitoids in some investigated rocks may be partially controlled by diffusion in the melt. Disequilibrium melting of plagioclase may explain the CaO enrichment observed in some experimental and natural melts. When local equilibration volumes are considered in the calculations, a better match is observed for some oxides (e.g. FeO and MgO) between nanogranitoids and their calculated counterparts, even though other significant divergences remain unaffected. The combination of diverse petrologic tools, with proper recognition of their shortcomings, represent the best approach to shed light into the complexities of melting processes in nature which control the composition of anatectic granites at the source
Prolonged metamorphism of garnet-orthoamphibole gneisses from the Fuyun area: New insights into metamorphic evolution of the southern Chinese Altai orogen
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The Euganean Hills (Northern Italy) show their magmatic evolution also through the magmatic enclaves
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Magmatic Processes at Euganean Hills (Veneto Volcanic Province, Italy): Clinopyroxene Investigation to Unravel Magmatic Interactions
No full textThe Euganean Hills (NE Italy) magmatic district represents the final volcanic activity of the Veneto Volcanic Province. Alkaline to subalkaline magmatic suite dominated by intermediate to felsic volcanic rocks characterises the latest volcanic activity of the Euganean Hills. Magmatic (intrusive and volcanic) enclaves are common in Euganean Hills trachytes. We used the ability of clinopyroxene to record variations of P, T, and fO2 to reconstruct the geological history of the volcanic enclaves and trachytic host. Despite similar major and trace elements composition, clinopyroxene from host is higher in Ca and Na (and Fe3+) and lower in Mg than enclaves and is slightly enriched in trace elements but with the same pattern distribution. Minor differences in geochemistry and crystal structure of clinopyroxene from enclaves and trachytic host suggest similar parental magmas that differs by small degrees of fractional crystallisation. Clinopyroxene geobarometry performed combining X-ray diffraction with mineral geochemistry for volcanic enclaves–trachytic host combined with amphibole geobarometry for intrusive enclaves and crystal mushes points to a crystallisation pressure range between 4.8–2.0 kbars. Our data support the model of a complex system of magma chambers at intermediate to shallow crustal level where mafic magma accumulated, evolved by fractionation processes and mixed
Melt inclusions in migmatites and granulites
Full text linkImportant advances have been made during the last 15 years in the study of melt inclusions in minerals from migmatites and granulites. Pioneer work on high temperature metapelitic anatectic enclaves in peraluminous dacites from SE Spain has shown that droplets of granitic melt can be trapped by minerals growing during incongruent melting reactions, and that the composition of such trapped melts can be representative of that of the bulk melt in the system during the anatexis of the rock. Therefore melt inclusions may represent samples of embryionic anatectic granite. In most cases, these melt inclusions define microstructures that are typical of primary entrapment, and show little or no evidence of melt crystallization upon cooling. Rather, the melt solidified to glass due to very fast cooling associated with the submarine extrusion of the dacites. Hence inclusions can readily be analyzed for major and trace elements by conventional methods such as the electron microprobe or by laser ablation-inductively coupled plasmamass spectrometry. Based on the results from these quite unusual anatectic enclaves, one would expect similar melt inclusions to be present also in more conventional, slowly cooled, regionally metamorphosed migmatite and granulite terranes. As a matter of fact, recent investigations confirm this hypothesis. Tiny (<25 μm) inclusions containing a cryptocrystalline aggregate of quartz, feldpars, biotite and muscovite have been found in garnet from the metapelitic granulites of the Keraka Khondalite Belt, as well as in garnet and ilmenite from metapelitic and quartzo-feldspathic migmatites from the Alps, Ronda and the Himalayas. Due to the grain-size, texture and chemical/mineralogical composition, these inclusions are called "nanogranites" and are interpreted to represent a crystallized inclusion of anatectic melt. Exceptionally and spatially associated with the nanogranites, inclusions containing glass have also been observed. In general, the preparation of the samples and analysis of these inclusions in migmatites and granulites require more sophisticated techniques than those applied to inclusions in xenoliths and enclaves, but the information on the composition of crustal anatectic melts can also be obtained. Since its discovery, new occurrences of nanogranite are being reported, or can be inferred from re-assessment of literature data, from migmatites and granulites worldwide. These former melt inclusions open new perspectives both for the microstructural approach to partially melted rocks and for the chemical characterization of natural crustal melts
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