Early Yanshanian(Jurassic) granitoids are widespread in the Nanling Range,South China.Whereas large granitic batholiths commonly crop out in the center of the Nanling Range(corresponding geographically to the central ...Early Yanshanian(Jurassic) granitoids are widespread in the Nanling Range,South China.Whereas large granitic batholiths commonly crop out in the center of the Nanling Range(corresponding geographically to the central and northern parts of Guangdong Province),many small stocks occur in the southern part of Jiangxi Province.Most of the small stocks are associated closely with economically significant rare-metal deposits(W,Sn,Nb,Ta).Here we report the results for biotite granites and two-mica granites from three Yanshanian stocks of the Longyuanba complex.LA-ICPMS U-Pb dating of zircon yields an age of 156.1±2.1 Ma for Xiaomubei biotite granite,and U-Pb zircon dating using SIMS yields an age of 156.7±1.2 Ma for Longyuanba-Chengjiang biotite granite and 156.4±1.3 Ma for Jiangtoudong two-mica granite.Biotite granites are silica-rich(SiO 2 =70%-79%),potassic(K 2 O/Na 2 O>1.9),and peraluminous(ASI=1.05-1.33).Associated samples are invariably enriched in Rb,Th,Pb and LREE,yet depleted in Ba,Nb,Sr,P and Ti,and their REE pattern shows a large fractionation between LREE and HREE((La/Yb) N =10.7-13.5) and a pronounced Eu negative anomaly(δEu=0.28-0.41).Two-mica granite samples are also silica-rich(SiO 2 =75%-79%),potassic(K 2 O/Na 2 O>1.2),and peraluminous(ASI=1.09-1.17).However,in contrast to the biotite granites,they are more enriched in Rb,Th,Pb and extremely depleted in Ba,Nb,Sr,P and Ti,and exhibit nearly flat((La/Yb) N =0.75-1.08) chondrite-normalized REE patterns characterized by strong Eu depletion(δEu=0.02-0.04) and clear tetrad effect(TE 1.3 =1.10-1.14).Biotite granites and two-mica granties have comparable Nd isotopic signatures,and their εNd(t) are concentrated in the 13.0 to 9.6 and 11.5 to 7.7 respectively.Their zircon Hf-O isotopes of both also show similarity(biotite granites:εHf(t)= 10.8-7.9,δ 18 O=7.98‰-8.89‰ and εHf(t)= 13.8 to 9.1,δ 18 O=8.31‰-10.08‰;two-mica granites:εHf(t)= 11.3 to 8.0,δ 18 O=7.91‰-9.77‰).The results show that both biotite and two-mica granites were derived mainly from sedimentary source rocks with a minor contribution from mantle-derived materials.In spite of some S-type characteristics,the biotite granites were formed by fractional crystallization of I-type magma and assimilation of peraluminous sedimentary rocks during their ascent to the surface.Therefore,they belong to highly fractionated I-type granites.Two-mica granites exhibit a tetrad effect in their REE patterns,but share the same isotopic features with the biotite granites,suggesting that they are highly fractionated I-type granites as well.Their Lanthanide tetrad effects may be attributed to the hydrothermal alteration by magmatic fluids that have suffered degassing at late stages.Granitic magmas undergoing fractional crystallization and wall-rock assilimation can generate highly evolved granites with no REE tetrad effect in the uni-phase system.However,in the late-stage of magmatic evolution in the multi-phase system(i.e.,magmatic-hydrothermal system),these magmas also can lead to the highly evolved granites exhibiting mew-shaped REE pattern characterized by tetrad effect as the consequence of melt-fluid and fluid-vapor fractionation,and the resultant autometasomatism.We thus suggest that the REE pattern exhibiting tetrad effect feature is an important indicator of rare metal mineralization in the early Yanshanian time in southern China,implying the metamorphism of the ore fluid.展开更多
Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous....Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to^19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.展开更多
基金supported by the Chinese Academy of Sciences(Grant Nos.KZCX1-YW-15-2 and GIGCAS-135Y234151001)the Ministry of Science and Technology(Grant No. 2007CB411403)+1 种基金National Natural Science Foundation of China (Grant Nos.40973025 and 41173039)contribution No.1655 from GIGCAS
文摘Early Yanshanian(Jurassic) granitoids are widespread in the Nanling Range,South China.Whereas large granitic batholiths commonly crop out in the center of the Nanling Range(corresponding geographically to the central and northern parts of Guangdong Province),many small stocks occur in the southern part of Jiangxi Province.Most of the small stocks are associated closely with economically significant rare-metal deposits(W,Sn,Nb,Ta).Here we report the results for biotite granites and two-mica granites from three Yanshanian stocks of the Longyuanba complex.LA-ICPMS U-Pb dating of zircon yields an age of 156.1±2.1 Ma for Xiaomubei biotite granite,and U-Pb zircon dating using SIMS yields an age of 156.7±1.2 Ma for Longyuanba-Chengjiang biotite granite and 156.4±1.3 Ma for Jiangtoudong two-mica granite.Biotite granites are silica-rich(SiO 2 =70%-79%),potassic(K 2 O/Na 2 O>1.9),and peraluminous(ASI=1.05-1.33).Associated samples are invariably enriched in Rb,Th,Pb and LREE,yet depleted in Ba,Nb,Sr,P and Ti,and their REE pattern shows a large fractionation between LREE and HREE((La/Yb) N =10.7-13.5) and a pronounced Eu negative anomaly(δEu=0.28-0.41).Two-mica granite samples are also silica-rich(SiO 2 =75%-79%),potassic(K 2 O/Na 2 O>1.2),and peraluminous(ASI=1.09-1.17).However,in contrast to the biotite granites,they are more enriched in Rb,Th,Pb and extremely depleted in Ba,Nb,Sr,P and Ti,and exhibit nearly flat((La/Yb) N =0.75-1.08) chondrite-normalized REE patterns characterized by strong Eu depletion(δEu=0.02-0.04) and clear tetrad effect(TE 1.3 =1.10-1.14).Biotite granites and two-mica granties have comparable Nd isotopic signatures,and their εNd(t) are concentrated in the 13.0 to 9.6 and 11.5 to 7.7 respectively.Their zircon Hf-O isotopes of both also show similarity(biotite granites:εHf(t)= 10.8-7.9,δ 18 O=7.98‰-8.89‰ and εHf(t)= 13.8 to 9.1,δ 18 O=8.31‰-10.08‰;two-mica granites:εHf(t)= 11.3 to 8.0,δ 18 O=7.91‰-9.77‰).The results show that both biotite and two-mica granites were derived mainly from sedimentary source rocks with a minor contribution from mantle-derived materials.In spite of some S-type characteristics,the biotite granites were formed by fractional crystallization of I-type magma and assimilation of peraluminous sedimentary rocks during their ascent to the surface.Therefore,they belong to highly fractionated I-type granites.Two-mica granites exhibit a tetrad effect in their REE patterns,but share the same isotopic features with the biotite granites,suggesting that they are highly fractionated I-type granites as well.Their Lanthanide tetrad effects may be attributed to the hydrothermal alteration by magmatic fluids that have suffered degassing at late stages.Granitic magmas undergoing fractional crystallization and wall-rock assilimation can generate highly evolved granites with no REE tetrad effect in the uni-phase system.However,in the late-stage of magmatic evolution in the multi-phase system(i.e.,magmatic-hydrothermal system),these magmas also can lead to the highly evolved granites exhibiting mew-shaped REE pattern characterized by tetrad effect as the consequence of melt-fluid and fluid-vapor fractionation,and the resultant autometasomatism.We thus suggest that the REE pattern exhibiting tetrad effect feature is an important indicator of rare metal mineralization in the early Yanshanian time in southern China,implying the metamorphism of the ore fluid.
基金supported by the Chinese Academy of Sciences(Grant Nos.KZCX1-YW-15-2&GIGCAS-135-Y234151001)National Natural Science Foundation of China(Grant Nos.41173039 and 40973025)
文摘Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to^19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.
