{"title":"来自 Alag-Khadny 高压复合体(蒙古西南部)的闪长岩的矿物学、地球化学和钕-锶同位素特征:大陆内部断裂是大陆边缘俯冲的前兆","authors":"S. Yu. Skuzovatov, M. A. Gornova, A. A. Karimov","doi":"10.1134/S0869591122040051","DOIUrl":null,"url":null,"abstract":"<div><p>Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende (<sup>[B]</sup>Na = 0.07–0.16, <sup>IV</sup>Al = 0.79–1.69, <sup>[A]</sup>(Na + K + 2Ca) = 0.14–0.64, <sup>[C]</sup>(Al+ Ti + Fe<sup>3+</sup>) = 0.58–1.29, Fe<sup>2+</sup>/(Fe<sup>2+</sup> + Mg) = 0.18–0.46 at Fe<sup>3+</sup>/(Fe<sup>3+</sup>+Al) = 0.18–0.77), low-to-medium-Ca plagioclase (<i>An</i><sub>24–36</sub>), and epidote–clinozoisite (0.08 < <span>\\({{X}_{{{\\text{F}}{{{\\text{e}}}^{{{\\text{3 + }}}}}}}}\\)</span> < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak <i>P-T</i> conditions up to 570–630°С and 7–9 kbar ascribed to the high-<i>T</i> epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (ε<sub>Nd</sub>(550) from +5.1 to –9.1) and Sr ((<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>550</sub> = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar <i>P-T</i> conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).</p></div>","PeriodicalId":20026,"journal":{"name":"Petrology","volume":null,"pages":null},"PeriodicalIF":1.0000,"publicationDate":"2022-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia): Intracontinental Rifting as a Precursor of Continental-Margin Subduction\",\"authors\":\"S. Yu. Skuzovatov, M. A. Gornova, A. A. Karimov\",\"doi\":\"10.1134/S0869591122040051\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p>Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende (<sup>[B]</sup>Na = 0.07–0.16, <sup>IV</sup>Al = 0.79–1.69, <sup>[A]</sup>(Na + K + 2Ca) = 0.14–0.64, <sup>[C]</sup>(Al+ Ti + Fe<sup>3+</sup>) = 0.58–1.29, Fe<sup>2+</sup>/(Fe<sup>2+</sup> + Mg) = 0.18–0.46 at Fe<sup>3+</sup>/(Fe<sup>3+</sup>+Al) = 0.18–0.77), low-to-medium-Ca plagioclase (<i>An</i><sub>24–36</sub>), and epidote–clinozoisite (0.08 < <span>\\\\({{X}_{{{\\\\text{F}}{{{\\\\text{e}}}^{{{\\\\text{3 + }}}}}}}}\\\\)</span> < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak <i>P-T</i> conditions up to 570–630°С and 7–9 kbar ascribed to the high-<i>T</i> epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (ε<sub>Nd</sub>(550) from +5.1 to –9.1) and Sr ((<sup>87</sup>Sr/<sup>86</sup>Sr)<sub>550</sub> = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar <i>P-T</i> conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).</p></div>\",\"PeriodicalId\":20026,\"journal\":{\"name\":\"Petrology\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":1.0000,\"publicationDate\":\"2022-09-06\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Petrology\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://link.springer.com/article/10.1134/S0869591122040051\",\"RegionNum\":4,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q3\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Petrology","FirstCategoryId":"89","ListUrlMain":"https://link.springer.com/article/10.1134/S0869591122040051","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia): Intracontinental Rifting as a Precursor of Continental-Margin Subduction
Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende ([B]Na = 0.07–0.16, IVAl = 0.79–1.69, [A](Na + K + 2Ca) = 0.14–0.64, [C](Al+ Ti + Fe3+) = 0.58–1.29, Fe2+/(Fe2+ + Mg) = 0.18–0.46 at Fe3+/(Fe3++Al) = 0.18–0.77), low-to-medium-Ca plagioclase (An24–36), and epidote–clinozoisite (0.08 < \({{X}_{{{\text{F}}{{{\text{e}}}^{{{\text{3 + }}}}}}}}\) < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak P-T conditions up to 570–630°С and 7–9 kbar ascribed to the high-T epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (εNd(550) from +5.1 to –9.1) and Sr ((87Sr/86Sr)550 = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar P-T conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).
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.