O. M. Turkina, A. E. Izokh, A. V. Lavrenchuk, Ya. Yu. Shelepov
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The basaltic rocks are characterized by flat REE patterns [(La/Sm)<sub>n</sub> = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive ε<sub>Nd</sub>(T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative ε<sub>Nd</sub>(T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y)<sub>PМ</sub> (0.8–1.0), negative ε<sub>Nd</sub>(T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm)<sub>n</sub>, positive ε<sub>Nd</sub>(T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). The trace element-composition of the amphibolites, gabbronorites, and gabbros and the results of geochemical modeling indicate that the parental melts were derived mainly from weakly depleted mantle sources. The Nd isotope composition of the Paleoproterozoic gabbroids resulted from the evolution of the heterogeneous Archean lithospheric mantle. Variations in the isotope and trace-element composition of the amphibolites reflect the initially depleted nature of the Mezoarchean mantle and its metasomatic alteration by fluids/melts, which occurred before its melting at ∼2.88 Ga. The geochemical and Nd isotopic characteristics of gabbronorites and gabbros indicate that the lithospheric mantle had become progressively more heterogeneous by the Paleoproterozoic due to preceding Archean processes. The variable depletion of both the Archean and the Paleoproterozoic mafic rocks in Nb relative to Th and La may be explained by mantle metasomatism and does not reflect the geodynamic settings of the mafic magmatism.</p>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":"30 5","pages":"499 - 522"},"PeriodicalIF":1.0000,"publicationDate":"2022-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"2","resultStr":"{\"title\":\"Composition and Isotope Parameters of Metabasalts and Gabbroids of the Onot Granite–Greenstone Block, Southwestern Siberian Platform, as Indicators of Lithospheric Mantle Evolution from the Archean to Paleoproterozoic\",\"authors\":\"O. M. Turkina, A. E. Izokh, A. V. Lavrenchuk, Ya. Yu. 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The basaltic rocks are characterized by flat REE patterns [(La/Sm)<sub>n</sub> = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive ε<sub>Nd</sub>(T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative ε<sub>Nd</sub>(T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y)<sub>PМ</sub> (0.8–1.0), negative ε<sub>Nd</sub>(T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm)<sub>n</sub>, positive ε<sub>Nd</sub>(T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). 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引用次数: 2
摘要
本文概述了西伯利亚环形山西南部 Sharyzhalgai 基底隆起区 Onot 花岗岩-绿岩地块的主要元素和痕量元素组成以及 Sm-Nd 同位素数据。奥诺特区块由古新世的碳酸盐岩-特朗杰米岩-花岗闪长岩(TTG)复合体和绿岩带(GB)的变质岩-火山岩复合体的构造组合页岩组成。绿岩带的美宙(2.88 Ga ∼2.88Ga)变玄武岩和古近纪的(1.86 Ga ∼1.86Ga)榴辉岩和脉辉长岩分别形成于断裂和碰撞后延伸时期。TTG复合体中绿岩带和飞地的阿新世代谢岩在成分上与低钛托勒密玄武岩和玄武安山岩相对应。玄武岩的特征是REE形态平坦[(La/Sm)n = 0.9-1.9],相对于Th和La,Nb贫化(Nb/Nb* = 0.4-1.1),εNd(T)值范围宽广,大部分为正值(从+5.2到-1.0)。玄武安山岩中不相容元素的富集、Eu的最小值以及εNd(T)的负值都是由古新世TTG片麻岩污染造成的,而古新世TTG片麻岩构成了GB的基底。古新生代榴辉岩具有高Mg#和极低的Ti及不相容元素浓度。岩石的特征是低(Nb/Y)PМ(0.8-1.0)、负εNd(T)值(从0到-1.4),以及相对于Nb的弱Th和LREE富集。脉辉长岩的(La/Sm)n较低,εNd(T)值为+2.8和+0.3,Nb异常为负值(Nb/Nb* = 0.3-0.4)。闪长岩、榴辉岩和辉长岩的微量元素组成以及地球化学建模结果表明,母体熔体主要来自弱贫化地幔源。古近纪辉长岩的钕同位素组成是由异质的阿新世岩石圈地幔演化形成的。闪长岩的同位素和痕量元素组成的变化反映了梅索阿尔奇岩地幔最初的贫化性质及其在2.88 Ga∼熔融之前发生的流体/熔体的元气蚀变。榴辉岩和辉长岩的地球化学特征和钕同位素特征表明,岩石圈地幔在古近纪时由于之前的阿契安过程而逐渐变得更加异质。相对于Th和La而言,阿新世和古近古生代岩浆岩的Nb贫化程度不同,这可能是地幔变质作用造成的,并不反映岩浆岩形成的地球动力学环境。
Composition and Isotope Parameters of Metabasalts and Gabbroids of the Onot Granite–Greenstone Block, Southwestern Siberian Platform, as Indicators of Lithospheric Mantle Evolution from the Archean to Paleoproterozoic
The paper summarizes major and trace-element compositions and Sm–Nd isotope data on metabasites (amphibolites) and gabbroids of the Onot granite–greenstone block in the Sharyzhalgai basement uplift, southwestern Siberian craton. The Onot block consists of tectonically combined nappes of the Paleoarchean tonalite–trondhjemite–granodiorite (TTG) complex and the metasedimentary-volcanic complex of the greenstone belt (GB). The Mezoarchean (∼2.88 Ga) metabasalts of the greenstone belt and Paleoproterozoic (∼1.86 Ga) gabbronorites and vein gabbros were formed at rifting and postcollisional extension, respectively. The Archean metabasites of the greenstone belt and enclaves in the TTG complex compositionally correspond to low-Ti tholeiitic basalts and basaltic andesites. The basaltic rocks are characterized by flat REE patterns [(La/Sm)n = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive εNd(T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative εNd(T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y)PМ (0.8–1.0), negative εNd(T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm)n, positive εNd(T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). The trace element-composition of the amphibolites, gabbronorites, and gabbros and the results of geochemical modeling indicate that the parental melts were derived mainly from weakly depleted mantle sources. The Nd isotope composition of the Paleoproterozoic gabbroids resulted from the evolution of the heterogeneous Archean lithospheric mantle. Variations in the isotope and trace-element composition of the amphibolites reflect the initially depleted nature of the Mezoarchean mantle and its metasomatic alteration by fluids/melts, which occurred before its melting at ∼2.88 Ga. The geochemical and Nd isotopic characteristics of gabbronorites and gabbros indicate that the lithospheric mantle had become progressively more heterogeneous by the Paleoproterozoic due to preceding Archean processes. The variable depletion of both the Archean and the Paleoproterozoic mafic rocks in Nb relative to Th and La may be explained by mantle metasomatism and does not reflect the geodynamic settings of the mafic magmatism.
期刊介绍:
Petrology is a journal of magmatic, metamorphic, and experimental petrology, mineralogy, and geochemistry. The journal offers comprehensive information on all multidisciplinary aspects of theoretical, experimental, and applied petrology. By giving special consideration to studies on the petrography of different regions of the former Soviet Union, Petrology provides readers with a unique opportunity to refine their understanding of the geology of the vast territory of the Eurasian continent. The journal welcomes manuscripts from all countries in the English or Russian language.