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Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt-reintegration approach
Full text linkReintegrating nanogranitoid inclusion composition to reconstruct the prograde history of melt-depleted rocks
No full textCampotrera ophiolite (Reggio Emilia, Northern Italy): a natural preserve for the geology and mineralogy
No full textCampotrera ophiolite (Reggio Emilia, Northern Italy): an important natural preserve for geology and mineralogy
No full textHigh-temperature metamorphism and crustal melting: Working with melt inclusions
No full textThe application of melt inclusion (MI) studies to migmatitic and granulitic terranes is a recent, small-scale approach for a better understanding of melting in the continental crust. In order to show the role of anatectic MI in providing a wealth of microstructural and compositional information on high-temperature metamorphism and crustal anatexis, we review a series of studies on the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain), which consists of an inverted metamorphic sequence with granulite-facies rocks showing extensive melting on top and amphibolites-facies rocks at the bottom. We studied the microstructures and geochemistry of small (2-10 mu m) primary MI hosted in peritectic garnet of metatexites at the bottom of the migmatitic sequence and of mylonitic diatexites close to the contact with the mantle rocks. The occurrence of MI is a proof that the investigated rocks were partially melted at some time in their history, despite other microstructures indicating the former presence of melt in diatexites were erased by deformation. MI show a variable degree of crystallization ranging from totally glassy to fully crystallized (nanogranites), consisting of Qtz+Pl+Kfs+Bt+Ms aggregates (often modal Kfs > P1 in diatexites). Piston cylinder remelting experiments led to the complete rehomogenization of nanogranites in metatexites at the conditions inferred for anatexis. Compositions of investigated MI are all leucogranitic and peraluminous and differ from those of coexisting leucosomes and from melts calculated by phase equilibria modeling. Systematic compositional variations have been observed between ME in metatexites and diatexites: the former commonly show higher H2O, CaO, Na2O/K2O and lower FeO. The compositions of MI in metatexites and diatexites are interpreted to record the composition of the anatectic melts produced from a peraluminous greywacke i) on, and immediately after crossing, the fluid-saturated solidus of this metasedimentary rock, and ii) during anatexis via biotite dehydration melting at increasing temperature, respectively. While partial melting at the bottom of the migmatitic sequence likely started in the presence of an aqueous fluid phase, MI data support the fluid-absent character of the melting event in diatexites. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes
Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet
No full textStromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojen nappe,
Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+
Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as
armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the
migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in
garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt.
Re-melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria
modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–
H2–O2–C (MnNCaKFMASHOC) system with graphite-saturated fluid shows P–T conditions of
equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental
re-melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated
immediately after entering supersolidus conditions. The P–T estimate for garnet and melt
development does not, however, overlap with the low-temperature tip of the pure melt field in the
phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O–
CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt
compositions formed immediately beyond the solidus with results of phase equilibria modelling points
to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies
are present also when calculated melts are compared with low-T natural and experimental melts from
the literature. Under such conditions, the available melt model does not perform well. Given the
presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that
small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of
continuous melting reactions consuming biotite
Constraining the P-T conditions of melting in stromatic migmatites from Ronda (S Spain).
No full textMelt inclusions in migmatites and granulites.
No full textImportant 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
Age of anatexis in the crustal footwall of the Ronda peridotites, S Spain
Full text linkThis study investigates the age of anatexis of a crustal sequence constituting the footwall of the Ronda peridotite slab, in the hinterland of the Betic Cordillera (S Spain, region of Istán). These rocks represent a polymetamorphic basement involved in the Alpine orogeny and show an increase in the proportion of melt towards the peridotites. Metamorphic conditions in the migmatites vary between T. ≈. 675-750. °C at P. ≈. 0.30-0.35. GPa. The timing of metamorphism and deformation of the migmatites around the Ronda peridotites is controversial and has been previously ascribed to either the Alpine or Variscan orogenies. We present U-Pb SHRIMP dating of zircons from six samples collected across the migmatitic sequence that provide a tighter age constraint on the metamorphism. Zircon ages are related to conditions of metamorphism on the basis of the relationships between zircon microstructures and degree of melting recorded by the host rocks. Anatexis occurred during the late stages of the Variscan orogeny (≈. 280-290. Ma), as indicated by ages of euhedral, oscillatory-zoned domains or new crystals in metatexites and diatexites. Thin, U-rich zircon rims that are affected by radiation damage yield discordant scattered dates between ≈. 260 and 30. Ma, which are interpreted as reflecting a thermal and fluid overprint during the Alpine orogeny that produced recrystallization and Pb loss in Permian zircons. This study identifies a previously unknown Variscan domain within the Betic Cordillera, and indicates, in accordance with previous studies, that Variscan basements recycled during the Alpine orogeny that formed the Betic Cordillera preserve pre-Alpine mineral associations and tectonic fabrics
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