Geochronology, geochemistry, and its geological significance of the Permian Mandula mafic rocks in Damaoqi, Inner Mongolia  

作　　者：CHEN Chen ZHANG ZhiCheng GUO ZhaoJie LI JianFeng FENG ZhiShuo TANG WenHao 

机构地区：[1]Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education,School of Earth and Space Sciences,Peking University,Beijing 100871,China

出　　处：《Science China Earth Sciences》2012年第1期39-52,共14页中国科学（地球科学英文版）

基　　金：supported by China Geological Survey (Grant No. 1212010050503);National Natural Science Foundation of China (Grant No. 40672146);National Basic Research Program of China (Grant No. 2007CB411305)

摘　　要：The Hujierte-Chaganhadamiao volcanic-plutonic sequence is located in the Mandula area of northen Damaoqi, Inner Mongo- lia. It contains mainly mafic volcanics, with several ultramafic blocks in its central part and mafic- ntermediate intrusions in the east. The zircon U-Pb ages of the gabbro and basalt samples are 278.5±3.0 Ma （MSWD=）.66） and 273.7±1.0 Ma （MSWD=0.36）, respectively. These ages constrain the magmatism occurred in the Early Permian. The Mandula Permian mafic rocks are characterized by low abundances of REE and slightly LREE-enriched chondrite-normal zed N-MORB-type REE patterns. And these mafic samples have high （143Nd/144Nd）i （0.51262-0.51270）, relatively high pos tive εNd（t） （3.4-8.0）, and high Mg# （49-54）, suggesting derivation from the depleted asthenosphere mantle source. But they al, show the enrichment in large ion lithophile elements （LILE, e.g. Rb, Ba and Sr）, depletion in high field strength elements （I-FSE）; distinctly negative Nb, Ta anomalies and slightly negative P, Ti anomalies; high （875r/86Sr）i （0.70490-0.70537）, low （206pb/204pb）i （17.39-17.93）. All of these resemble the characteristics of the enriched mantle source or involvement of arc materials in the magma genesis. Furthermore, as shown in the correlation plots of eNd（t） VS （87Sr/86Sr）i, （206pb/204Pb）ivs （207pb/204Pb）i and （208pb/204pb）i, La/Ba La/Nb vs La/Nb, and La/Nb ratio, the magma source could have been experienced the contamination and metasomation from the previous subduction process. And the positive correlation between the selected major oxides and trace elements could be explained by the contamination from continental crust and arc materials during the magmatic emplacement. Combined with the distinct geochemistry features of Mandula mafic samples and many previous researches in the study area, the Early Permian magmatism in Mandula occurred under an extensional rift setting, and a juvenile ocean basin probably had already been formed after The Hujierte-Chaganhadamiao volcanic-plutonic sequence is located in the Mandula area of northern Damaoqi, Inner Mongolia. It contains mainly mafic volcanics, with several ultramafic blocks in its central part and mafic-intermediate intrusions in the east. The zircon U-Pb ages of the gabbro and basalt samples are 278.5±3.0 Ma (MSWD=0.66) and 273.7±1.0 Ma (MSWD=0.36), respectively. These ages constrain the magmatism occurred in the Early Permian. The Mandula Permian mafic rocks are characterized by low abundances of REE and slightly LREE-enriched chondrite-normalized N-MORB-type REE patterns. And these mafic samples have high ( 143 Nd/ 144 Nd) i (0.51262-0.51270), relatively high positive ε Nd (t) (3.4-8.0), and high Mg # (49-54), suggesting derivation from the depleted asthenosphere mantle source. But they also show the enrichment in large ion lithophile elements (LILE, e.g. Rb, Ba and Sr), depletion in high field strength elements (HFSE); distinctly negative Nb, Ta anomalies and slightly negative P, Ti anomalies; high ( 87 Sr/ 86 Sr) i (0.70490-0.70537), low ( 206 Pb/ 204 Pb) i (17.39-17.93). All of these resemble the characteristics of the enriched mantle source or involvement of arc materials in the magma genesis. Furthermore, as shown in the correlation plots of ε Nd (t) vs ( 87 Sr/ 86 Sr) i , ( 206 Pb/ 204 Pb) i vs ( 207 Pb/ 204 Pb) i and ( 208 Pb/ 204 Pb) i , La/Ba La/Nb vs La/Nb, and La/Nb ratio, the magma source could have been experienced the contamination and metasomation from the previous subduction process. And the positive correlation between the selected major oxides and trace elements could be explained by the contamination from continental crust and arc materials during the magmatic emplacement. Combined with the distinct geochemistry features of Mandula mafic samples and many previous researches in the study area, the Early Permian magmatism in Mandula occurred under an extensional rift setting, and a juvenile ocean basin probably had already been formed after further development.

关 键 词：Mandula zircon SHRIMP U-Pb age PERMIAN GEOCHEMISTRY extensional setting 

分 类 号：P618.51[天文地球—矿床学] P597[天文地球—地质学]