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Granites and crustal anatexis
Lithos 93 (2007) v – vi www.elsevier.com/locate/lithos
Editorial
Granites and crustal anatexis
The present special issue of Lithos contains seven papers that have grown out of oral presentations at Symposium 5.5, “Granites and crustal anatexis” at the 2004 Goldschmidt Conference in Copenhagen, convened by T. Andersen, L.D. Ashwal, and O.T. Rämö. Substantial parts of the continental crust and orogenic belts are made up by granitic (sensu lato) intrusions. Granitic magmas are generated by anatectic processes in the deep crust, or by differentiation of mafic, mantlederived magmas — the Goldschmidt symposium was dedicated to anatectic processes and their products, as is the present volume. Understanding anatectic processes in the deep crust is essential for the understanding of continental evolution in general. Since the anatectic region is inaccessible for observation, our knowledge of anatectic processes builds on experimental petrology, and on detailed studies of the geology, geochemistry, mineralogy and isotope geology of the ultimate products of these processes — granitic intrusions exposed at the present surface, which is the focus of this issue of Lithos. The study of granites has evolved in parallel with the development of analytical technology. In recent years, new microanalytical methods such as high-resolution SIMS and laser-ablation ICPMS have been applied to in-situ analysis of stable and radiogenic isotope systems in individual minerals of granites, opening new research frontiers in granite petrology and in geochronology applied to granitic systems. Different approaches to granitic rocks provide information on different aspects of the evolution of granitic magmas, there can therefore be no such thing as a unique approach to the study of granites. The contributions in the present issue illustrate some of the diversity in scope and methods of current research related to anatectic granites. A-type granites comprise rocks of different chemical and mineralogical compositions, reflecting differences in source rock type and physicochemical conditions during anatexis. The oxidized A-type granites make up a par0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.06.001
ticularly problematic group, both in terms of geochemical signature and petrogenesis. The contribution by Dall'Agnol and Oliveira provides new data on the Jamon suite of oxidized A-type granites from Brazil, as well as a general review such granites, comparing their mineralogical and chemical characteristics and their petrogenesis to those of reduced A- and I-type granites worldwide. In their paper on the Gouldsboro granite in Maine, USA, Waight, Wiebe and Krogstad point out the importance of a neglected feature of granitic intrusions: felsic microgranular enclaves. Whereas much research has been devoted to mafic enclaves, their felsic counterparts have largely been neglected. This paper provides new data and an in-depth discussion of felsic enclaves and their importance of the evolution of a complex pluton. Their findings are potentially important for all researchers using isotopic data from complex granitic intrusions to identify source components. Whereas anatexis of subducted oceanic crust is accepted as a mechanism to form I-type granitic magma, less is known about the subduction-induced recycling of continental crust. Since quite extreme collisional settings may be needed to allow such recycling, the process itself is rare. Zhao, Zheng, Wei and Wu present geochronological, geochemical and oxygen isotope data on Cretaceous, post-collisional granites from the Dabie orogen, China. The new data indicate that post-collisional granitic magmas formed by anatexis of subducted continental crust belonging to the Yangtze craton, and suggest that partial melting was triggered by the thermal effects of early Cretaceous mantle upwelling. Andersen, Griffin and Sylvester use granitic rocks as probes of ancient crust–mantle interaction processes in the deep crust, presenting mutually coherent, in situ U– Pb and Lu–Hf isotope analyses of zircons by LAMICPMS for two Mesoproterozoic lithologic units from southern Norway. The targets – ca. 1.2 Ga felsic gneisses and a ca. 0.97 Ga post-tectonic granite that intrudes
vi
Editorial
these gneisses – yield new insights into the interplay of the lithosphere and the sublithospheric mantle in granitoid petrogenesis in the Mesoproterozoic on the southwestern flank of the Fennoscandian (or Baltic) shield. Furthermore, the study provides new constraints for the time-integrated evolution of Hf isotopes, including a piercing point for the depleted mantle curve at ca. 1.2 Ga. Granites contain numerous minor to accessory, U–Th enriched minerals, which are potential U–Pb geochronometers. Zircon has so far received most attention, but insitu zircon geochronology requires the accurate and precise measurement of the isotopic composition of trace amounts of lead by SIMS or LAM-ICPMS. Other minerals, such as monazite, xenotime, thorite and thorianite may be equally important chronometers, and their elevated concentrations of U and/or Th, combined with the virtual absence of common-lead, allow such minerals to be dated by more accessible and less expensive electron microprobe analysis. In their contribution to this volume, Cocherie and Legendre present a method-oriented paper showing how non-isotopic U–Th–Pb chemical dating by electron microprobe analysis can be applied to minerals which are common, but all to commonly overlooked accessory minerals in granites. The major element composition of rock-forming mafic silicate minerals respond to the evolution of magma composition and to changes in volatile fugacities, oxidation state, temperature and pressure. By careful analysis of mainly pyroxenes and amphiboles it is possible to identify trends of magmatic evolution, and the parameters which distinguish and/or control these trends. This is an approach long established in the petrology of mafic and alkaline igneous rocks, but until now much less used in the study of granites. In a two-part study of a suite of spatially
and genetically associated syenites and granites of in southern Brazil, Gualda and Vlach use electron microprobe analyses of mafic silicate minerals as petrological indicators. The analytical data show remarkably large variations in that amphibole and pyroxene chemistry, which are specific for the individual liquid lines of descent in the province. The controlling factors are differences in of oxygen fugacity, and the alkalinity of the magmas. The guest editors want to thank the contributors to this special issue for their effort, and not least for their patience during the much-too-long review and editorial process. We also want to express our sincere thanks to Lew Ashwal, who took a leading role in the preparation and arrangement of the Granites and Crustal Anatexis symposium at the 2004 Goldschmidt Conference; unfortunately he was unable to take part in the subsequent editorial process. Last but not least, those colleagues who have undertaken the important role as reviewers are gratefully acknowledged: Ilmari Haapala, Laura Lauri, Hannu Huhma, Carl Ehlers, Tod Waight, Torkil Røhr, Paul Evins, Muriel Erambert, Lang Farmer, Carol Frost, Paul Wetmore, Roland Maas, Jean-Paul Liegois, Ulf Bertil Andersson and Anonymous. T. Andersen Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway E-mail address: [email protected]. Corresponding author. O.T. Rämö Department of Geology, University of Helsinki, P.O. Box 64, FIN-00014 University of Helsinki, Finland E-mail address: [email protected].
Editorial
Granites and crustal anatexis
The present special issue of Lithos contains seven papers that have grown out of oral presentations at Symposium 5.5, “Granites and crustal anatexis” at the 2004 Goldschmidt Conference in Copenhagen, convened by T. Andersen, L.D. Ashwal, and O.T. Rämö. Substantial parts of the continental crust and orogenic belts are made up by granitic (sensu lato) intrusions. Granitic magmas are generated by anatectic processes in the deep crust, or by differentiation of mafic, mantlederived magmas — the Goldschmidt symposium was dedicated to anatectic processes and their products, as is the present volume. Understanding anatectic processes in the deep crust is essential for the understanding of continental evolution in general. Since the anatectic region is inaccessible for observation, our knowledge of anatectic processes builds on experimental petrology, and on detailed studies of the geology, geochemistry, mineralogy and isotope geology of the ultimate products of these processes — granitic intrusions exposed at the present surface, which is the focus of this issue of Lithos. The study of granites has evolved in parallel with the development of analytical technology. In recent years, new microanalytical methods such as high-resolution SIMS and laser-ablation ICPMS have been applied to in-situ analysis of stable and radiogenic isotope systems in individual minerals of granites, opening new research frontiers in granite petrology and in geochronology applied to granitic systems. Different approaches to granitic rocks provide information on different aspects of the evolution of granitic magmas, there can therefore be no such thing as a unique approach to the study of granites. The contributions in the present issue illustrate some of the diversity in scope and methods of current research related to anatectic granites. A-type granites comprise rocks of different chemical and mineralogical compositions, reflecting differences in source rock type and physicochemical conditions during anatexis. The oxidized A-type granites make up a par0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.06.001
ticularly problematic group, both in terms of geochemical signature and petrogenesis. The contribution by Dall'Agnol and Oliveira provides new data on the Jamon suite of oxidized A-type granites from Brazil, as well as a general review such granites, comparing their mineralogical and chemical characteristics and their petrogenesis to those of reduced A- and I-type granites worldwide. In their paper on the Gouldsboro granite in Maine, USA, Waight, Wiebe and Krogstad point out the importance of a neglected feature of granitic intrusions: felsic microgranular enclaves. Whereas much research has been devoted to mafic enclaves, their felsic counterparts have largely been neglected. This paper provides new data and an in-depth discussion of felsic enclaves and their importance of the evolution of a complex pluton. Their findings are potentially important for all researchers using isotopic data from complex granitic intrusions to identify source components. Whereas anatexis of subducted oceanic crust is accepted as a mechanism to form I-type granitic magma, less is known about the subduction-induced recycling of continental crust. Since quite extreme collisional settings may be needed to allow such recycling, the process itself is rare. Zhao, Zheng, Wei and Wu present geochronological, geochemical and oxygen isotope data on Cretaceous, post-collisional granites from the Dabie orogen, China. The new data indicate that post-collisional granitic magmas formed by anatexis of subducted continental crust belonging to the Yangtze craton, and suggest that partial melting was triggered by the thermal effects of early Cretaceous mantle upwelling. Andersen, Griffin and Sylvester use granitic rocks as probes of ancient crust–mantle interaction processes in the deep crust, presenting mutually coherent, in situ U– Pb and Lu–Hf isotope analyses of zircons by LAMICPMS for two Mesoproterozoic lithologic units from southern Norway. The targets – ca. 1.2 Ga felsic gneisses and a ca. 0.97 Ga post-tectonic granite that intrudes
vi
Editorial
these gneisses – yield new insights into the interplay of the lithosphere and the sublithospheric mantle in granitoid petrogenesis in the Mesoproterozoic on the southwestern flank of the Fennoscandian (or Baltic) shield. Furthermore, the study provides new constraints for the time-integrated evolution of Hf isotopes, including a piercing point for the depleted mantle curve at ca. 1.2 Ga. Granites contain numerous minor to accessory, U–Th enriched minerals, which are potential U–Pb geochronometers. Zircon has so far received most attention, but insitu zircon geochronology requires the accurate and precise measurement of the isotopic composition of trace amounts of lead by SIMS or LAM-ICPMS. Other minerals, such as monazite, xenotime, thorite and thorianite may be equally important chronometers, and their elevated concentrations of U and/or Th, combined with the virtual absence of common-lead, allow such minerals to be dated by more accessible and less expensive electron microprobe analysis. In their contribution to this volume, Cocherie and Legendre present a method-oriented paper showing how non-isotopic U–Th–Pb chemical dating by electron microprobe analysis can be applied to minerals which are common, but all to commonly overlooked accessory minerals in granites. The major element composition of rock-forming mafic silicate minerals respond to the evolution of magma composition and to changes in volatile fugacities, oxidation state, temperature and pressure. By careful analysis of mainly pyroxenes and amphiboles it is possible to identify trends of magmatic evolution, and the parameters which distinguish and/or control these trends. This is an approach long established in the petrology of mafic and alkaline igneous rocks, but until now much less used in the study of granites. In a two-part study of a suite of spatially
and genetically associated syenites and granites of in southern Brazil, Gualda and Vlach use electron microprobe analyses of mafic silicate minerals as petrological indicators. The analytical data show remarkably large variations in that amphibole and pyroxene chemistry, which are specific for the individual liquid lines of descent in the province. The controlling factors are differences in of oxygen fugacity, and the alkalinity of the magmas. The guest editors want to thank the contributors to this special issue for their effort, and not least for their patience during the much-too-long review and editorial process. We also want to express our sincere thanks to Lew Ashwal, who took a leading role in the preparation and arrangement of the Granites and Crustal Anatexis symposium at the 2004 Goldschmidt Conference; unfortunately he was unable to take part in the subsequent editorial process. Last but not least, those colleagues who have undertaken the important role as reviewers are gratefully acknowledged: Ilmari Haapala, Laura Lauri, Hannu Huhma, Carl Ehlers, Tod Waight, Torkil Røhr, Paul Evins, Muriel Erambert, Lang Farmer, Carol Frost, Paul Wetmore, Roland Maas, Jean-Paul Liegois, Ulf Bertil Andersson and Anonymous. T. Andersen Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway E-mail address: [email protected]. Corresponding author. O.T. Rämö Department of Geology, University of Helsinki, P.O. Box 64, FIN-00014 University of Helsinki, Finland E-mail address: [email protected].