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Main Tectonic Events and Metallogeny of the North China CratonPaleoproterozoic Gneissic Granites in the Liaoji Mobile Belt, North China Craton: Implications for Tectonic Setting

Main Tectonic Events and Metallogeny of the North China Craton: Paleoproterozoic Gneissic... [Voluminous Paleoproterozoic gneissic granites occur in the Liaoji belt in the North China Craton (NCC), which were intruded by mafic dykes, and experienced a late-stage amphibolite facies metamorphism. These granites were thought by some researchers to be A-type granites and formed in a continental rifting setting. We propose a different model for the origin of these granites and the tectonic setting, based on the integrated field, petrographic, geochronological, and geochemical studies on a couple of gneissic granite plutons. The gneissic granites were emplaced at 2173–2203 Ma, and the mafic dykes at ca. 2159 Ma, followed by an amphibolite facies metamorphism at ca. 1.9 Ga. The gneissic granites contain mafic microgranular enclaves (MMEs), calcium hornblendes, magnesiohorblendes, accessory titanite, and pyrrhotite. They show calc-alkaline arc magma affinity, with A/CNK = 0.9 − 1.2 (mostly less than 1.1), A/NK = 0.9 − 1.4, SiO2 = 68.3 − 76.9 wt%, low contents of TiO2 (<0.3 wt%), enrichment of LILEs, such as K, Rb, Sr, Cs, depletion of some HFSEs, such as Nb, Ti. These characteristics suggest that these gneissic granites are I-type granites formed probably in a subduction zone. The A-type-like characteristics for some granites (the Dafangshen granite) are attributable to the highly evolved nature of the pluton as shown by the high SiO2 contents (76.7–77.1 wt%), which could have been caused by the high boron contents of the pluton, because addition of boron in magma system tends to prolong magma evolution and lead to significant differentiation. The large variation of whole-rock εNd(t) values (−8.6 to 1.5) and zircon εHf(t) values (−1.3 to 5.6), together with the existence and petrographic features of the MMEs, and oscillatory zoning in plagioclase, suggest that mixing/mingling of lower crust-derived felsic magma with enriched mantle-derived mafic magma might have resulted in formation of these gneissic granites. The existence of Archean inherited zircons in these granites together with the arc affinity suggests a northward subduction for the JLJB in the Paleoproterozoic times.] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

Main Tectonic Events and Metallogeny of the North China CratonPaleoproterozoic Gneissic Granites in the Liaoji Mobile Belt, North China Craton: Implications for Tectonic Setting

Part of the Springer Geology Book Series
Editors: Zhai, Mingguo; Zhao, Yue; Zhao, Taiping
Yang, Mingchun; Chen, Bin; Yan, Cong
Main Tectonic Events and Metallogeny of the North China Craton — Jul 23, 2016
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Publisher
Springer Singapore
Copyright
© Springer Science+Business Media Singapore 2016
ISBN
978-981-10-1063-7
Pages
155 –180
DOI
10.1007/978-981-10-1064-4_7
Publisher site
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Abstract

[Voluminous Paleoproterozoic gneissic granites occur in the Liaoji belt in the North China Craton (NCC), which were intruded by mafic dykes, and experienced a late-stage amphibolite facies metamorphism. These granites were thought by some researchers to be A-type granites and formed in a continental rifting setting. We propose a different model for the origin of these granites and the tectonic setting, based on the integrated field, petrographic, geochronological, and geochemical studies on a couple of gneissic granite plutons. The gneissic granites were emplaced at 2173–2203 Ma, and the mafic dykes at ca. 2159 Ma, followed by an amphibolite facies metamorphism at ca. 1.9 Ga. The gneissic granites contain mafic microgranular enclaves (MMEs), calcium hornblendes, magnesiohorblendes, accessory titanite, and pyrrhotite. They show calc-alkaline arc magma affinity, with A/CNK = 0.9 − 1.2 (mostly less than 1.1), A/NK = 0.9 − 1.4, SiO2 = 68.3 − 76.9 wt%, low contents of TiO2 (<0.3 wt%), enrichment of LILEs, such as K, Rb, Sr, Cs, depletion of some HFSEs, such as Nb, Ti. These characteristics suggest that these gneissic granites are I-type granites formed probably in a subduction zone. The A-type-like characteristics for some granites (the Dafangshen granite) are attributable to the highly evolved nature of the pluton as shown by the high SiO2 contents (76.7–77.1 wt%), which could have been caused by the high boron contents of the pluton, because addition of boron in magma system tends to prolong magma evolution and lead to significant differentiation. The large variation of whole-rock εNd(t) values (−8.6 to 1.5) and zircon εHf(t) values (−1.3 to 5.6), together with the existence and petrographic features of the MMEs, and oscillatory zoning in plagioclase, suggest that mixing/mingling of lower crust-derived felsic magma with enriched mantle-derived mafic magma might have resulted in formation of these gneissic granites. The existence of Archean inherited zircons in these granites together with the arc affinity suggests a northward subduction for the JLJB in the Paleoproterozoic times.]

Published: Jul 23, 2016

Keywords: Gneissic granites; Subduction; Arc magmatism; Paleoproterozoic; North China Craton

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