9 results on '"Rolfo, Franco"'
Search Results
2. Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system
- Author
-
Groppo, Chiara, Rolfo, Franco, Castelli, Daniele, and Connolly, James A. D.
- Published
- 2013
- Full Text
- View/download PDF
Catalog
3. The fate of calcareous pelites in collisional orogens.
- Author
-
Groppo, Chiara, Rapa, Giulia, Frezzotti, Maria Luce, and Rolfo, Franco
- Subjects
DEHYDRATION reactions ,MUSCOVITE ,CALCITE ,OROGENIC belts ,CALCAREOUS soils ,PHASE equilibrium ,BIOTITE ,MINERALS - Abstract
Although calcareous pelites are important constituents of sediments involved in orogenic processes, their prograde metamorphism is significantly less studied than that of pure pelites with negligible amounts of modal calcite. This paper presents mineral equilibria modelling in the system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2, with the aim of constraining the prograde evolution of calcareous pelites in collisional orogenic settings. A suite of model bulk‐rock compositions is used to investigate the influence of different proportions of calcite in the protolith on (a) the equilibrium assemblages at different pressure, temperature and fluid composition (P–T–X(CO2)) conditions; (b) the melt fertility and (c) the fluid evolution and the main decarbonation reactions occurring during prograde metamorphism of calcareous pelites. In spite of being purely theoretical, the reliability of the modelling is tested by comparing the predicted assemblages with those observed in a wide set of natural samples from the Himalayan metamorphic core. Comparison between the predicted and the modelled assemblages demonstrates that even a small amount of calcite in the calcareous pelitic protoliths has a strong influence on the final mineral assemblages and compositions, with potential effects on their melt productivity. Specifically, it appears that up to ~800°C, the melt productivity of calcic metapelites remains low, and melt production occurs gradationally because it is mostly controlled by continuous biotite dehydration melting reactions, rather than by muscovite breakdown. Moreover, the study demonstrates that calcareous pelites could be non‐negligible CO2‐source rocks in orogenic settings, and that in such contexts, an internal buffered behaviour is likely for most of them. [ABSTRACT FROM AUTHOR] more...
- Published
- 2021
- Full Text
- View/download PDF
4. Metamorphic CO2 Production in Collisional Orogens: Petrological Constraints from Phase Diagram Modeling of Himalayan, Scapolite-bearing, Calc-silicate Rocks in the NKC(F)MAS(T)-HC system.
- Author
-
Groppo, Chiara, Rolfo, Franco, Castelli, Daniele, and Mosca, Pietro
- Subjects
- *
METAMORPHIC rocks , *CARBON dioxide adsorption , *OROGENIC belts , *COLLISIONAL heating , *PETROLOGY , *PHASE diagrams - Abstract
A reliable quantitative estimate of the metamorphic CO2 flux from collisional orogens is fundamental to our understanding of the deep carbon cycle, but it is still far from being constrained. Among major uncertainties are the poor knowledge of the nature of metamorphic CO2-producing processes and the amount of CO2 potentially released through these reactions. Previous studies of metamorphic decarbonation reactions in metacarbonate rocks mainly used simple model reactions between end-members in simplified model systems. However, fully quantitative modelling of calcsilicate rocks requires an investigation of very complex systems with more than six components. Moreover, scapolite solid solution has been rarely included in previous studies, although this mineral is often a major constituent of calc-silicate rocks. This study focuses on (1) the CO2-producing processes occurring in scapolite-bearing calc-silicate rocks and (2) the discussion of a methodological approach suitable to understand and quantify these processes. In this framework, phase relations and devolatilization reactions in the Na2O-K2O-CaO-(FeO)-MgO-Al2O3-SiO2-(TiO2)-H2O-CO2 [NKC(F)MAS(T)-HC] system are considered, with application to high-grade clinopyroxene+calcite+K-feldspar+scapolite+plagioclase+zoisite calc-silicate rocks from the Himalaya. The NKC(F)MAS(T)-HC equilibria involving scapolite and plagioclase solid solutions are investigated using (1) isobaric T-X(CO2) phase diagram sections and pseudosections and (2) mixed-volatile P-T phase diagram projections. This phase diagram approach allowed us to identify scapolite-bearing, CO2-producing, univariant (i.e. isobaric invariant) equilibria that have never been recognized before, and that could not be detected without considering Na-Ca solid solutions in the calculations. It is demonstrated that the investigated calc-silicate rocks behaved as a nearly closed, internally buffered, system during prograde metamorphism and that most of the observed key microstructures correspond to isobaric univariant or invariant assemblages. In such a nearly closed system, the fluid was mostly produced during prograde heating at the isobaric invariant points, where abrupt changes in mineral modes also occurred. The proposed phase diagram approach further allows quantitative estimation of the amount and composition of the fluid produced at such isobaric invariant points. On average, 2.5 mol of CO2 (110 g) per 1000 cm³ of reacting rock were produced during prograde metamorphism of this calc-silicate rock-type. Because similar scapolite bearing calc-silicate rocks are abundant in the Himalayan orogen, it is suggested that this calc-silicate rock-type might have produced large amounts of CO2-rich fluids during Himalayan metamorphism. A preliminary estimate of these amounts at the scale of the whole orogen suggests a total metamorphic CO2 production of ~(2-7) x 1017 mol, corresponding to (1-3)_1010 Mt of CO2. Integrated over ~20 Myr (i.e. the maximum duration of prograde metamorphism), the calculated metamorphic CO2 flux would be (1.1-3.4) x 1010 mol a-1, corresponding to an annual mass flux of (0.5-1.5) x 10³ Mt a-1. Nevertheless, further studies are still needed to assess more precisely the amount of CO2 released during the Himalayan orogeny. [ABSTRACT FROM AUTHOR] more...
- Published
- 2017
- Full Text
- View/download PDF
5. Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system.
- Author
-
Groppo, Chiara, Rolfo, Franco, Castelli, Daniele, and Connolly, James A. D.
- Subjects
CARBON monoxide ,METAMORPHIC rocks ,SILICATE minerals ,GARNET crystallography ,PHASE equilibrium ,FLUID dynamics ,CARBONATION (Chemistry) - Abstract
The type and kinetics of metamorphic CO
2 -producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2ript> cycle. This paper focuses on CO 2 production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al2 O3 –SiO2 –CO2 –H2 O (CFAS–CO2 –H2 O) system are investigated using P– T phase diagrams at fixed fluid composition, isobaric T– X(CO2 ) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO2 –H2 O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2 -rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle. [ABSTRACT FROM AUTHOR] more...- Published
- 2013
- Full Text
- View/download PDF
6. Partial Melting in the Higher Himalayan Crystallines of Eastern Nepal: the Effect of Decompression and Implications for the ‘Channel Flow’ Model.
- Author
-
Groppo, Chiara, Rolfo, Franco, and Indares, Aphrodite
- Subjects
- *
SNOWMELT , *CHANNEL flow , *CONTINENTAL crust , *EFFECT of temperature on rocks , *METAMORPHIC rocks , *GNEISS , *MATHEMATICAL models - Abstract
Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis. To understand the influence of decreasing pressure on partial melting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet–K-feldspar–kyanite–sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC) of eastern Nepal. The main melt-producing reactions, the amount of melt produced during heating vs decompression, and the effects of melt loss on the mineral assemblages and compositions have been investigated along four ideal P–T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have been interpreted in terms of melt-producing vs melt-consuming reactions (e.g. growth of peritectic garnet with preserved ‘nanogranite’ inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P–T pseudosection modelling predicts that at least 15–20 vol. % of melt was produced at peak P–T conditions through dehydration melting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect of decompression. The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss of most of this melt. The P–T evolution inferred for samples from different, strategically located, structural levels of the Barun Gneiss is consistent with the expectations of a ‘channel flow’ model, including: (1) the clockwise shape of the P–T paths; (2) the estimated P at peak T (new data: 10–8 kbar at 800°C; model: 13–7 kbar at 800°C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel. [ABSTRACT FROM PUBLISHER] more...
- Published
- 2012
- Full Text
- View/download PDF
7. Detection of tectonometamorphic discontinuities within the Himalayan orogen: Structural and petrological constraints from the Rasuwa district, central Nepal Himalaya.
- Author
-
Rapa, Giulia, Groppo, Chiara, Rolfo, Franco, and Mosca, Pietro
- Subjects
- *
DISCONTINUITIES (Geology) , *OROGENY , *PETROLOGY , *STRUCTURAL geology , *PHASE diagrams - Abstract
A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side. Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts. [ABSTRACT FROM AUTHOR] more...
- Published
- 2018
- Full Text
- View/download PDF
8. Early Oligocene partial melting in the Main Central Thrust Zone (Arun valley, eastern Nepal Himalaya)
- Author
-
Groppo, Chiara, Rubatto, Daniela, Rolfo, Franco, and Lombardo, Bruno
- Subjects
- *
OLIGOCENE stratigraphic geology , *CRYSTALLIZATION , *METAMORPHISM (Geology) , *MONAZITE , *ZIRCON - Abstract
Abstract: The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ. While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the P–T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal). Microstructural observations combined with P–T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820°C, 13kbar, the studied sample experienced decompression accompanied by cooling down to 805°C, 10kbar, which caused in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼31Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT. [Copyright &y& Elsevier] more...
- Published
- 2010
- Full Text
- View/download PDF
9. A fluid inclusion study of blueschist-facies lithologies from the Indus suture zone, Ladakh (India): Implications for the exhumation of the subduction related Sapi-Shergol ophiolitic mélange.
- Author
-
Sachan, Himanshu Kumar, Kharya, Aditya, Singh, P. Chandra, Rolfo, Franco, Groppo, Chiara, and Tiwari, Sameer K.
- Subjects
- *
FLUID inclusions , *BLUESCHISTS , *FACIES , *SUTURE zones (Structural geology) , *SUBDUCTION zones - Abstract
The best occurrence of blueschist-facies lithologies in Himalaya is that of the Shergol Ophiolitic Mélange along the Indus suture zone in Ladakh region of north-western India. These lithologies are characterized by well preserved lawsonite-glaucophane-garnet-quartz assemblages. This paper presents for the first time the results of a detailed fluid inclusion study on these lithologies, in order to understand the fluid P-T evolution and its tectonic implications. The blueschist rocks from Shergol Ophiolitic Mélange record metamorphic peak conditions at ∼19 kbar, 470 °C. Several types of fluid inclusions are trapped in quartz and garnet, most of them being two-phase at room temperature. Three types of fluid inclusions have been recognised, basing on microtextures and fluid composition: Type-I are primary two-phase carbonic-aqueous fluid inclusions (V CO2 − L H2O ); Type-II are two-phase (L H2O − V H2O ) aqueous fluid inclusions, either primary (Type-IIa) or secondary (Type-IIb); Type-III are re-equilibrated fluid inclusions. In the Type-I primary carbonic-aqueous inclusions, H 2 O is strongly predominant with respect to CO 2 ; the homogenization temperature of CO 2 range from −7 to −2 °C. The clathrate melting temperature in such inclusions varies in between +7.1 and +8.6 °C. Type-II two-phase aqueous fluid inclusions show a wide range of salinity, from 7.8–14 wt.% NaCl eq (Type-IIa) to 1.65–6.37 wt.% NaCl eq (Type-IIb) with accuracy ±0.4 wt.% NaCl eq . Type-I and Type-IIa primary fluid inclusions are hosted in peak minerals (garnet and quartz included in garnet), therefore they were likely entrapped at, or near to, peak P-T conditions. The dominantly aqueous fluid of both Type-I and Type-IIa inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab. Despite their primary nature, the isochores of Type-I and Type-IIa inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage, suggesting that these inclusions stretched or re-equilibrated during nearly isothermal decompression from 19 kbar to 3 kbar or less, at T = 290 °C. This conclusion is further supported by their large variability in shapes and sizes which range from irregular inclusions (‘C’/arc shaped, hook shape and satellite type). This decompression stage was followed by nearly isobaric cooling, testified by the occurrence of dendritic networks of decrepitated and ‘imploded’ fluid inclusions. [ABSTRACT FROM AUTHOR] more...
- Published
- 2017
- Full Text
- View/download PDF
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.