J. Shellnutt, Tung-Yi Lee, Yoshiyuki Iizuka, Hao-Yang Lee, Chi Thi Pham, Kenshi Suga
{"title":"塞舌尔早古新世北岛闪长岩群的岩石成因","authors":"J. Shellnutt, Tung-Yi Lee, Yoshiyuki Iizuka, Hao-Yang Lee, Chi Thi Pham, Kenshi Suga","doi":"10.2475/001c.94773","DOIUrl":null,"url":null,"abstract":"The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":1.9000,"publicationDate":"2024-03-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Petrogenesis of the Early Paleogene North Island Syenite Complex, Seychelles\",\"authors\":\"J. Shellnutt, Tung-Yi Lee, Yoshiyuki Iizuka, Hao-Yang Lee, Chi Thi Pham, Kenshi Suga\",\"doi\":\"10.2475/001c.94773\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.\",\"PeriodicalId\":7660,\"journal\":{\"name\":\"American Journal of Science\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":1.9000,\"publicationDate\":\"2024-03-21\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"American Journal of Science\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://doi.org/10.2475/001c.94773\",\"RegionNum\":3,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q3\",\"JCRName\":\"GEOSCIENCES, MULTIDISCIPLINARY\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"American Journal of Science","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.2475/001c.94773","RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
Petrogenesis of the Early Paleogene North Island Syenite Complex, Seychelles
The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.
期刊介绍:
The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.