This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These gran...This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type.展开更多
Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and ...Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma,which caused the mantle upwelling and following mafic and granitic magmatic intrusions.Migmatite in the North Dabie zone,coeval with the formation of low-Mg#adakitic intrusions in the Dabie orogen,was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level.This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning.Based on the geothermal gradient of6.6°C/km for lithospheric mantle and initial partial melting temperature of^1000°C for the lower mafic crust,it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to<45 km when the thickened lower crust was melting.Thus,a two-stage model for mountain-root removal is proposed.First,the lithospheric mantle keel was partially removal by mantle convection at 145 Ma.Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting.Second,partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability,thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse.Therefore,convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.展开更多
基金Supported by the National Natural Science Foundation of China (Grant Nos. 40234052 and 40521001)the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0441)
文摘This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type.
基金supported by the National Basic Research Program of China(2009CB825002)the National Natural Science Foundation of China(40773013 and 91014007)
文摘Systematical studies of post-collisional igneous rocks in the Dabie orogen suggest that the thickened mafic lower crust of the orogen was partially melted to form low-Mg#adakitic rocks at 143–131 Ma.Delamination and foundering of the thickened mafic lower crust occurred at 130 Ma,which caused the mantle upwelling and following mafic and granitic magmatic intrusions.Migmatite in the North Dabie zone,coeval with the formation of low-Mg#adakitic intrusions in the Dabie orogen,was formed by partial melting of exhumed ultrahigh-pressure metamorphic rocks at middle crustal level.This paper argues that the partial melting of thickened lower and middle crust before mountain-root collapse needs lithospheric thinning.Based on the geothermal gradient of6.6°C/km for lithospheric mantle and initial partial melting temperature of^1000°C for the lower mafic crust,it can be estimated that the thickness of lithospheric mantle beneath thickened lower crust has been thinned to<45 km when the thickened lower crust was melting.Thus,a two-stage model for mountain-root removal is proposed.First,the lithospheric mantle keel was partially removal by mantle convection at 145 Ma.Loss of the lower lithosphere would increase heat flow into the base of the crust and would cause middle-lower crustal melting.Second,partial melting of the thickened lower crust has weakened the lower crust and increased its gravity instability,thus triggering delamination and foundering of the thickened mafic lower crust or mountain-root collapse.Therefore,convective removal and delamination of the thickened lower crust as two mechanisms of lithospheric thinning are related to causality.
