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Evolution of REE minerals from diagenetic to amphibolite facies conditions in the Central Alps, with implications to geochronology
A290
Goldschmidt Conference Abstracts 2006
Evolution of REE minerals from diagenetic to amphibolite facies conditions in the Central Alps, with implications to geochronology E. JANOTS, M. ENGI, A. BERGER Institute of Geological Sciences, University of Bern, Switzerland ([email protected]) We report on the REE distribution in metapelites from the northern Lepontine domains (Central Alps), taken along a wellestablished metamorphic field gradient recording conditions from diagenetic to amphibolite facies. Our strategy has been to characterize texturally and chemically the REE-minerals, document their assemblages, and deduce mineral reactions which occurred with increasing metamorphism. In diagenetic to low-grade metamorphic rocks, LREE are contained in roundish inherited and/or minute newly formed monazite (Chemical U–Th–Pb dating), whereas HREE reside in xenotime. With the appearance of chloritoid, monazite vanishes, and LREE are taken up in idiomorphic homogeneous allanite. With the appearance of biotite, allanite acquires a first rim of epidote, and a second one grows contemporaneously with garnet. At the ‘‘chloritoid-out’’ zone boundary, allanite is replaced by monazite associated with plagioclase, biotite and/or staurolite. In our samples, the evolution of the REEphases thus appears to be correlated with the succession of major silicate assemblages. This demonstrates potential for novel geochronology, since all of the samples display REE-minerals, for which ages may be determined and related to the P–T conditions of their formation. In particular, the occurrence of allanite, epidote, and monazite in the same samples offers the unique possibility of dating their prograde evolution using different chronometers. doi:10.1016/j.gca.2006.06.588
Apatite fission track thermochronology: Implications for Brazil–Africa correlations A.R. JELINEK1, F.F. LUFT1,2, F. CHEMALE JR.1 1
University of Rio Grande do Sul, Porto Alegre, Brazil ([email protected]) 2 University of Melbourne, Australia (ff[email protected]) In an attempt to reconstruct the tectono-thermal history in the Southeast continental margin of Brazil and Soth-Southwest margin of Africa, apatite fission track thermochronology was applied in an outcrop samples. In Brazil, the sampling was accomplished along N–S profiles through Floriano´polis Batholith, in the Dom Feliciano Belt, with stratigraphic ages ranging from Neoproterozoic to Early Paleozoic. In the Namibian passive margin the sampling was carried out in Proterozoic granites and gneisses through two coast-perpendicular profiles cross cutting the main structural trend of Kaoko Belt while in the South Africa the sampling was accomplished along an Orange River profile, in Namaqua Belt. The fission track analysis has been carried out at the Isotope Geology Laboratory of the Universidade Federal do Rio Grande do Sul, Brazil. To the Brazilian margin the fission track ages vary from 68 ± 4 to 46 ± 2 Ma and the track lengths ranged from 13.3 ± 0.1 to 10.8 ± 0.3 micras. The Brazilian ages distribution when analyzed with the altitude presents two different groups of samples: (i) one with a positive correlation between ages and altitude, and (ii) the other is constituted by samples collected in the current coastal plain with ages in the same interval of the previous group; however there is no correlation with the altitude. To the African margin the fission track ages vary from 297 ± 26 to 47 ± 4 Ma and from 124 ± 8 to 49 ± 4 Ma to Nabibian and South African margin, respectively and the fission track lengths vary between 13.2 ± 0.2 and 10.0 ± 0.9 micras. In the Namibian margin the ages decrease towards the Purros Mylonitic Zone. Based on thermal history modeling, one abrupt regional event has been recognized in each margin. The African event occurred from Late Cretaceous to Tertiary, was responsible for the development of uplifts in the Great Escarpment regions, associated with the South Atlantic opening. The Late Cretaceous cooling pattern observed in Brazilian margin is interpreted as a geomorphic response to the reactivation of NNE–SSW and WNW– ESE basement structures caused by a change in the spreading geometry of the South Atlantic Ocean. doi:10.1016/j.gca.2006.06.589
Goldschmidt Conference Abstracts 2006
Evolution of REE minerals from diagenetic to amphibolite facies conditions in the Central Alps, with implications to geochronology E. JANOTS, M. ENGI, A. BERGER Institute of Geological Sciences, University of Bern, Switzerland ([email protected]) We report on the REE distribution in metapelites from the northern Lepontine domains (Central Alps), taken along a wellestablished metamorphic field gradient recording conditions from diagenetic to amphibolite facies. Our strategy has been to characterize texturally and chemically the REE-minerals, document their assemblages, and deduce mineral reactions which occurred with increasing metamorphism. In diagenetic to low-grade metamorphic rocks, LREE are contained in roundish inherited and/or minute newly formed monazite (Chemical U–Th–Pb dating), whereas HREE reside in xenotime. With the appearance of chloritoid, monazite vanishes, and LREE are taken up in idiomorphic homogeneous allanite. With the appearance of biotite, allanite acquires a first rim of epidote, and a second one grows contemporaneously with garnet. At the ‘‘chloritoid-out’’ zone boundary, allanite is replaced by monazite associated with plagioclase, biotite and/or staurolite. In our samples, the evolution of the REEphases thus appears to be correlated with the succession of major silicate assemblages. This demonstrates potential for novel geochronology, since all of the samples display REE-minerals, for which ages may be determined and related to the P–T conditions of their formation. In particular, the occurrence of allanite, epidote, and monazite in the same samples offers the unique possibility of dating their prograde evolution using different chronometers. doi:10.1016/j.gca.2006.06.588
Apatite fission track thermochronology: Implications for Brazil–Africa correlations A.R. JELINEK1, F.F. LUFT1,2, F. CHEMALE JR.1 1
University of Rio Grande do Sul, Porto Alegre, Brazil ([email protected]) 2 University of Melbourne, Australia (ff[email protected]) In an attempt to reconstruct the tectono-thermal history in the Southeast continental margin of Brazil and Soth-Southwest margin of Africa, apatite fission track thermochronology was applied in an outcrop samples. In Brazil, the sampling was accomplished along N–S profiles through Floriano´polis Batholith, in the Dom Feliciano Belt, with stratigraphic ages ranging from Neoproterozoic to Early Paleozoic. In the Namibian passive margin the sampling was carried out in Proterozoic granites and gneisses through two coast-perpendicular profiles cross cutting the main structural trend of Kaoko Belt while in the South Africa the sampling was accomplished along an Orange River profile, in Namaqua Belt. The fission track analysis has been carried out at the Isotope Geology Laboratory of the Universidade Federal do Rio Grande do Sul, Brazil. To the Brazilian margin the fission track ages vary from 68 ± 4 to 46 ± 2 Ma and the track lengths ranged from 13.3 ± 0.1 to 10.8 ± 0.3 micras. The Brazilian ages distribution when analyzed with the altitude presents two different groups of samples: (i) one with a positive correlation between ages and altitude, and (ii) the other is constituted by samples collected in the current coastal plain with ages in the same interval of the previous group; however there is no correlation with the altitude. To the African margin the fission track ages vary from 297 ± 26 to 47 ± 4 Ma and from 124 ± 8 to 49 ± 4 Ma to Nabibian and South African margin, respectively and the fission track lengths vary between 13.2 ± 0.2 and 10.0 ± 0.9 micras. In the Namibian margin the ages decrease towards the Purros Mylonitic Zone. Based on thermal history modeling, one abrupt regional event has been recognized in each margin. The African event occurred from Late Cretaceous to Tertiary, was responsible for the development of uplifts in the Great Escarpment regions, associated with the South Atlantic opening. The Late Cretaceous cooling pattern observed in Brazilian margin is interpreted as a geomorphic response to the reactivation of NNE–SSW and WNW– ESE basement structures caused by a change in the spreading geometry of the South Atlantic Ocean. doi:10.1016/j.gca.2006.06.589