The metabasites present in Nevado-Filábride Complex (Lugros, Camarate, Caldera and Bacares) were studied by means of Electron Micro Porbe (EMP), Structural Refinement (SREF) and Transmision Electron Microscopy (TEM) in order to characterize the crystal-chemical evolution of amphiboles and clinopyroxenes in these rocks. Two groups of rocks were distiguished in the studied metabasites present in some of the outcrops of Nevado-Filábride Complex (Lugros, Camarate, Caldera and Bacares), according to their fabric, textures, geochemestry and mineral chemistry. In this work they have been named Group-1 and Group-2, respectively, according to the preservation or not of relict igneous textures. The clinopyroxenes from both groups of rocks presented distinctive chemical characteristics: those corresponding to the Group-1, were Ae-poor, and presented a variation between Aug and Jd; those corresponding to the Group-2 rocks presented variable contents in Ae and a much more limited Aug-Jd variation. The Group-1 amphiboles present different compositions as a function of the textural microdomains where they have grown. They show continous zoning from sodic actinolite to edenite up to ferroan-pargasite. Those corresponding to the Group-2 rocks present continous zonation from sodic amphiboles in the core and inclusions, up to ferroan-pargasite and taramite in the rims, going through barroisite and katophorite. The geochemistryof these rocks was also studied. Both groups of rocks may be differentiated by lower contents in Al2O3 and MgO and higher contents in Na2O in the Group-2 rocks. The trace elements contents also show variations. In the Group-1 rocks this changes are coherent with a magmatic diferentiation process while the Group-2 rocks present scatter related with an alteration process. In general they represent tholeiitic-transitional compositions. From the crystal-chemical and mineralogical studies the next observations have been made: Some of the studied pyroxenes were found ordered, belonging to the P2/n space group.The rest were disordered (spatial group C2/c). Since the relationship Jd:Di predicted that all of them would have to be ordered the different causes of the lack of order were explored. One of the motives could be that they might have crystallized at higher temperatures than 825° C, the temperature at which P2/n sodic pyroxenes become disordered.This possibility is not compatible with the regional geology. It was found that the crystals refined as C2/c were richer in Aegirine molecule than those P2/n. The structural parameters were studied and strong correlation among Aegirina content and the c cell parameter and the TAV (Tetrahedral Angle Variancy, Robinson et al., 1971) was found. The samples refined as P2/n had a lower degree of order than that predicted according to their Na/Ca ratio. Antiphase Domains (APD) were studied by TEM. It was observed as the size of the APD was variable. In Ae-poor pyroxenes, the APD size (1200-1400 Å) corresponded to an equilibrium temperature equivalent to that obtained by exchange thermometry. When the Ae contents were something greater (10-15%), the size of the APDs (700 Å) was lower than those predicted by the previous temperature calibrations in similar rocks of others outcrops. Considering a normal crystallization time for a metamorphic process (among 1 and 10 millions of years), they would have crystallized at lower temperature (400-450°C). In zoned crystals of omphacite, with Ae enriched cores and Ae impoverished rims, the rims presented APDs while the cores did not present these microstructures because they were disordered. Three crystals coming from rocks of the Western Alps (Monviso, and Voltri Group) were also studied with the purpose of comparing the behavior of Ae rich pyroxenes in eclogites comming from others metamorphic belts. They presented the highest contents in Ae of all the studied samples and the same crystal-chemical behavior as those from Betic Cordilleras and resulted disordered both in the XRD and TEM studies. The lower degree of order found in the studied samples, as well as the lacking of order in most of the crystals have been interpreted to be related with the Aegirine content in the pyroxene, which produces a progressive order decrease as their Aegirine content increases up to inhibit the order process. That is, considering equal thermal conditions and time, the order process is slowed as the Fe3+ content in the pyroxene increases, up to a point in which APDs don’t nucleate and crystals present spatial group C2/c. In the pyroxenes of Group-1 rocks, which are Ae -poor, variations in the size of the APDs that could not be related with the composition neither with crystalline defects were found. The greatest APDs were equivalent in size to those present in the Group-2 pyroxenes having scarce or nule Ae content, and included lower size APDs. Therefore this characteristic was interpreted as a consequence of a heterogeneous nucleation process produced by a progressive temperature increase. The influence of the aegirina content on the structural and thermodynamical parameters of pyroxenes are affected by the presence of Ae component and this have a direct implication in the geothermometers and geobarometers that consider solid solution models. In amphiboles, the most coincident results between the two analytical methods (EMP and SREF) carried out on the same refined crystals is the normalization of EMP analysis on the basis of 13 cations excluding Ca, Na and K in sodo-calcic amphiboles and 15 cations excluding K in sodic amphiboles. Nevertheless, both schemes produce unsatisfactory results when they are applied to amphiboles with intermediate compositions. The zonation in Group-1 amphiboles is characterized by a continuous increase in the edenite vector and by an increase and subsequent decrease of glaucophane vector. This has been interpreted as an increase and subsequent decrease of pressure while the temperature increased. In the Group-2 amphiboles the zonation is characterized by a decrease of the plagioclase vector and by increase of the edenite vector up to the plagioclase substitution ceases and only the edenite and tschermakite increase is maintained. This is interpretable in terms of a baric decrease which in their later part passes to a thermal increase. From the structural refinement, it was proven that a continuous variation of the structural and chemical parameters in these amphiboles exists and they don’t obey the miscibility gap between sodic and sodo-calcic amphiboles. These amphiboles were studied by High Resolution Transmission Electron Microscopy (HRTEM) and through this technique it could be verified that there was no sort of intergrowth at any scale that could produce intermediate compositions in the EMP analysis. Mineralogical formulae comparable to that obtained by EMP corresponding to the miscibility gap were obtained with the help of microanalysis in lacking defect zones. The fact of these amphiboles present compositions corresponding to the miscibility gap can be explained in two ways: a) the nonexistence of the gap, something that contradicts many previous work; b) the possibility of the amphiboles had crystallized at temperatures higher to the miscibility solvus outlined by Reynard and Ballevre (1988). This last would require temperatures higher than 575°C which are perfectly compatible with the temperature of the high pressure metamorphic climax in these rocks.
Crystalchemical study of metamorphic minerals in basic rocks of the Nevado-Filábride Comples (SE Spain)
CAMARA ARTIGAS, Fernando
1997-01-01
Abstract
The metabasites present in Nevado-Filábride Complex (Lugros, Camarate, Caldera and Bacares) were studied by means of Electron Micro Porbe (EMP), Structural Refinement (SREF) and Transmision Electron Microscopy (TEM) in order to characterize the crystal-chemical evolution of amphiboles and clinopyroxenes in these rocks. Two groups of rocks were distiguished in the studied metabasites present in some of the outcrops of Nevado-Filábride Complex (Lugros, Camarate, Caldera and Bacares), according to their fabric, textures, geochemestry and mineral chemistry. In this work they have been named Group-1 and Group-2, respectively, according to the preservation or not of relict igneous textures. The clinopyroxenes from both groups of rocks presented distinctive chemical characteristics: those corresponding to the Group-1, were Ae-poor, and presented a variation between Aug and Jd; those corresponding to the Group-2 rocks presented variable contents in Ae and a much more limited Aug-Jd variation. The Group-1 amphiboles present different compositions as a function of the textural microdomains where they have grown. They show continous zoning from sodic actinolite to edenite up to ferroan-pargasite. Those corresponding to the Group-2 rocks present continous zonation from sodic amphiboles in the core and inclusions, up to ferroan-pargasite and taramite in the rims, going through barroisite and katophorite. The geochemistryof these rocks was also studied. Both groups of rocks may be differentiated by lower contents in Al2O3 and MgO and higher contents in Na2O in the Group-2 rocks. The trace elements contents also show variations. In the Group-1 rocks this changes are coherent with a magmatic diferentiation process while the Group-2 rocks present scatter related with an alteration process. In general they represent tholeiitic-transitional compositions. From the crystal-chemical and mineralogical studies the next observations have been made: Some of the studied pyroxenes were found ordered, belonging to the P2/n space group.The rest were disordered (spatial group C2/c). Since the relationship Jd:Di predicted that all of them would have to be ordered the different causes of the lack of order were explored. One of the motives could be that they might have crystallized at higher temperatures than 825° C, the temperature at which P2/n sodic pyroxenes become disordered.This possibility is not compatible with the regional geology. It was found that the crystals refined as C2/c were richer in Aegirine molecule than those P2/n. The structural parameters were studied and strong correlation among Aegirina content and the c cell parameter and the TAV (Tetrahedral Angle Variancy, Robinson et al., 1971) was found. The samples refined as P2/n had a lower degree of order than that predicted according to their Na/Ca ratio. Antiphase Domains (APD) were studied by TEM. It was observed as the size of the APD was variable. In Ae-poor pyroxenes, the APD size (1200-1400 Å) corresponded to an equilibrium temperature equivalent to that obtained by exchange thermometry. When the Ae contents were something greater (10-15%), the size of the APDs (700 Å) was lower than those predicted by the previous temperature calibrations in similar rocks of others outcrops. Considering a normal crystallization time for a metamorphic process (among 1 and 10 millions of years), they would have crystallized at lower temperature (400-450°C). In zoned crystals of omphacite, with Ae enriched cores and Ae impoverished rims, the rims presented APDs while the cores did not present these microstructures because they were disordered. Three crystals coming from rocks of the Western Alps (Monviso, and Voltri Group) were also studied with the purpose of comparing the behavior of Ae rich pyroxenes in eclogites comming from others metamorphic belts. They presented the highest contents in Ae of all the studied samples and the same crystal-chemical behavior as those from Betic Cordilleras and resulted disordered both in the XRD and TEM studies. The lower degree of order found in the studied samples, as well as the lacking of order in most of the crystals have been interpreted to be related with the Aegirine content in the pyroxene, which produces a progressive order decrease as their Aegirine content increases up to inhibit the order process. That is, considering equal thermal conditions and time, the order process is slowed as the Fe3+ content in the pyroxene increases, up to a point in which APDs don’t nucleate and crystals present spatial group C2/c. In the pyroxenes of Group-1 rocks, which are Ae -poor, variations in the size of the APDs that could not be related with the composition neither with crystalline defects were found. The greatest APDs were equivalent in size to those present in the Group-2 pyroxenes having scarce or nule Ae content, and included lower size APDs. Therefore this characteristic was interpreted as a consequence of a heterogeneous nucleation process produced by a progressive temperature increase. The influence of the aegirina content on the structural and thermodynamical parameters of pyroxenes are affected by the presence of Ae component and this have a direct implication in the geothermometers and geobarometers that consider solid solution models. In amphiboles, the most coincident results between the two analytical methods (EMP and SREF) carried out on the same refined crystals is the normalization of EMP analysis on the basis of 13 cations excluding Ca, Na and K in sodo-calcic amphiboles and 15 cations excluding K in sodic amphiboles. Nevertheless, both schemes produce unsatisfactory results when they are applied to amphiboles with intermediate compositions. The zonation in Group-1 amphiboles is characterized by a continuous increase in the edenite vector and by an increase and subsequent decrease of glaucophane vector. This has been interpreted as an increase and subsequent decrease of pressure while the temperature increased. In the Group-2 amphiboles the zonation is characterized by a decrease of the plagioclase vector and by increase of the edenite vector up to the plagioclase substitution ceases and only the edenite and tschermakite increase is maintained. This is interpretable in terms of a baric decrease which in their later part passes to a thermal increase. From the structural refinement, it was proven that a continuous variation of the structural and chemical parameters in these amphiboles exists and they don’t obey the miscibility gap between sodic and sodo-calcic amphiboles. These amphiboles were studied by High Resolution Transmission Electron Microscopy (HRTEM) and through this technique it could be verified that there was no sort of intergrowth at any scale that could produce intermediate compositions in the EMP analysis. Mineralogical formulae comparable to that obtained by EMP corresponding to the miscibility gap were obtained with the help of microanalysis in lacking defect zones. The fact of these amphiboles present compositions corresponding to the miscibility gap can be explained in two ways: a) the nonexistence of the gap, something that contradicts many previous work; b) the possibility of the amphiboles had crystallized at temperatures higher to the miscibility solvus outlined by Reynard and Ballevre (1988). This last would require temperatures higher than 575°C which are perfectly compatible with the temperature of the high pressure metamorphic climax in these rocks.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
