Jacky Bouloton , Dominique Gasquet , Christian Pin , Aimeryc Schumacher
{"title":"杰比勒特中部(摩洛哥)三叠纪石英闪长岩岩体中的长石异长岩:详细岩石学和地球化学,以及对所谓 \"花岗岩 \"样本的批判性评估","authors":"Jacky Bouloton , Dominique Gasquet , Christian Pin , Aimeryc Schumacher","doi":"10.1016/j.jafrearsci.2024.105349","DOIUrl":null,"url":null,"abstract":"<div><p>Felsic xenoliths occur locally in the Early Triassic quartz-monzodiorite dykes that crosscut the Sarhlef basin in Central Jebilet, Morocco. Irrespective of the secondary features linked to entrapment, three types of xenoliths are distinguished on the basis of textural evidence: 1) isotropic rocks of broad granitic appearance; 2) foliated samples whose planar fabric undoubtedly corresponds to a ghost foliation, 3) a specific xenolith with a blastomylonitic structure.</p><p>Geochemically, samples with Na > K and moderately negative εNd values (-1.5 to - 3.8), reflecting a hybrid origin, depart from samples with K > Na and strongly negative εNd values (-9.1 to - 11.7), pointing to a stronger or even pure crustal derivation. The blastomylonitic sample differs drastically from the rest by its plagiogranitic composition and elevated Nb content, associated with a highly radiogenic Nd isotope signature (εNd<sub>240</sub> = + 6.3). Altogether, these features and available radiometric data suggest a connection of the xenoliths with the Variscan igneous bodies that crop out in the central Jebilet. The potassic xenoliths with unradiogenic Nd are broadly similar to the granodiorite plutons. The xenoliths with Na > K and mildly negative εNd and the blastomylonitic plagiogranite are reminiscent of the bimodal association of microgranites and gabbros of the Sarhlef basin.</p><p>From a structural point of view, samples with a gneissic fabric may reflect the possible occurrence of a metamorphic, migmatitic complex at somewhat deeper crustal level. However, based on detailed mineralogical and geochemical data, the samples previously known as “mid-crustal granulites” turn out to be highly differentiated, aplo-pegmatitic rocks, interpreted as early, incipient partial melts. The presence of high-grade metamorphic rocks at depth is only documented by one small, HT garnet-biotite xenolith, while peraluminous lenses (fibrolite + spinel ± corundum) dispersed in the quartz-monzodiorite dykes suggest that a metapelitic component was involved in the melting process generating these lavas. In summary, this set of felsic xenoliths does not provide much information on the crustal composition under the Jebilet, apart from fairly superficial levels.</p></div>","PeriodicalId":14874,"journal":{"name":"Journal of African Earth Sciences","volume":"219 ","pages":"Article 105349"},"PeriodicalIF":2.2000,"publicationDate":"2024-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Felsic xenoliths in Triassic quartz-monzodiorite dykes from the central Jebilet (Morocco): Detailed petrography and geochemistry, with a critical assessment of the so-called “granulitic” samples\",\"authors\":\"Jacky Bouloton , Dominique Gasquet , Christian Pin , Aimeryc Schumacher\",\"doi\":\"10.1016/j.jafrearsci.2024.105349\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p>Felsic xenoliths occur locally in the Early Triassic quartz-monzodiorite dykes that crosscut the Sarhlef basin in Central Jebilet, Morocco. Irrespective of the secondary features linked to entrapment, three types of xenoliths are distinguished on the basis of textural evidence: 1) isotropic rocks of broad granitic appearance; 2) foliated samples whose planar fabric undoubtedly corresponds to a ghost foliation, 3) a specific xenolith with a blastomylonitic structure.</p><p>Geochemically, samples with Na > K and moderately negative εNd values (-1.5 to - 3.8), reflecting a hybrid origin, depart from samples with K > Na and strongly negative εNd values (-9.1 to - 11.7), pointing to a stronger or even pure crustal derivation. The blastomylonitic sample differs drastically from the rest by its plagiogranitic composition and elevated Nb content, associated with a highly radiogenic Nd isotope signature (εNd<sub>240</sub> = + 6.3). Altogether, these features and available radiometric data suggest a connection of the xenoliths with the Variscan igneous bodies that crop out in the central Jebilet. The potassic xenoliths with unradiogenic Nd are broadly similar to the granodiorite plutons. The xenoliths with Na > K and mildly negative εNd and the blastomylonitic plagiogranite are reminiscent of the bimodal association of microgranites and gabbros of the Sarhlef basin.</p><p>From a structural point of view, samples with a gneissic fabric may reflect the possible occurrence of a metamorphic, migmatitic complex at somewhat deeper crustal level. However, based on detailed mineralogical and geochemical data, the samples previously known as “mid-crustal granulites” turn out to be highly differentiated, aplo-pegmatitic rocks, interpreted as early, incipient partial melts. The presence of high-grade metamorphic rocks at depth is only documented by one small, HT garnet-biotite xenolith, while peraluminous lenses (fibrolite + spinel ± corundum) dispersed in the quartz-monzodiorite dykes suggest that a metapelitic component was involved in the melting process generating these lavas. In summary, this set of felsic xenoliths does not provide much information on the crustal composition under the Jebilet, apart from fairly superficial levels.</p></div>\",\"PeriodicalId\":14874,\"journal\":{\"name\":\"Journal of African Earth Sciences\",\"volume\":\"219 \",\"pages\":\"Article 105349\"},\"PeriodicalIF\":2.2000,\"publicationDate\":\"2024-07-30\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Journal of African Earth Sciences\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S1464343X24001821\",\"RegionNum\":4,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"GEOSCIENCES, MULTIDISCIPLINARY\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of African Earth Sciences","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S1464343X24001821","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOSCIENCES, MULTIDISCIPLINARY","Score":null,"Total":0}
引用次数: 0
摘要
在横切摩洛哥杰比勒特中部萨赫勒夫盆地的早三叠世石英-闪长岩岩体中,局部出现了长石异长岩。无论与夹层有关的次生特征如何,根据纹理证据可将异长岩分为三种类型:1)各向同性的岩石,外观呈宽花岗岩状;2)褶皱样品,其平面结构无疑与鬼褶皱相对应;3)具有爆破绵状结构的特殊异质岩。从地球化学角度看,Na > K 和中度负εNd 值(-1.5 至 -3.8)的样品反映了混合成因,而 K > Na 和强负εNd 值(-9.1 至 -11.7)的样品则表明了更强甚至纯粹的地壳成因。爆斜样品与其他样品的显著不同之处在于它的斜长花岗岩成分和较高的铌含量,以及与之相关的高放射性钕同位素特征(εNd240 = + 6.3)。总之,这些特征和现有的辐射测量数据表明,这些奇石与杰贝雷特中部的瓦里斯坎火成岩体有关。钾质异岩石中的非辐射钕与花岗闪长岩岩体大致相似。具有 Na > K 和轻度负εNd 的斜长岩以及爆破辉长岩让人联想到 Sarhlef 盆地的微花岗岩和辉长岩的双峰组合。从结构的角度来看,具有片麻岩结构的样本可能反映了在更深的地壳层面可能存在变质、偏闪长岩复合体。然而,根据详细的矿物学和地球化学数据,以前被称为 "地壳中层花岗岩 "的样本被证明是高度分化的aplo-pegmatitic岩石,被解释为早期的萌芽部分熔体。深部存在的高级变质岩仅由一块小型 HT 石榴石-生物沸石异长岩证明,而散布在石英-闪长岩岩体中的过铝透镜体(纤维石 + 尖晶石 ± 刚玉)表明,在生成这些熔岩的熔融过程中涉及到偏闪长岩成分。总之,除了相当表层的信息外,这组长英质闪长岩并不能提供太多有关杰比利特地下地壳成分的信息。
Felsic xenoliths in Triassic quartz-monzodiorite dykes from the central Jebilet (Morocco): Detailed petrography and geochemistry, with a critical assessment of the so-called “granulitic” samples
Felsic xenoliths occur locally in the Early Triassic quartz-monzodiorite dykes that crosscut the Sarhlef basin in Central Jebilet, Morocco. Irrespective of the secondary features linked to entrapment, three types of xenoliths are distinguished on the basis of textural evidence: 1) isotropic rocks of broad granitic appearance; 2) foliated samples whose planar fabric undoubtedly corresponds to a ghost foliation, 3) a specific xenolith with a blastomylonitic structure.
Geochemically, samples with Na > K and moderately negative εNd values (-1.5 to - 3.8), reflecting a hybrid origin, depart from samples with K > Na and strongly negative εNd values (-9.1 to - 11.7), pointing to a stronger or even pure crustal derivation. The blastomylonitic sample differs drastically from the rest by its plagiogranitic composition and elevated Nb content, associated with a highly radiogenic Nd isotope signature (εNd240 = + 6.3). Altogether, these features and available radiometric data suggest a connection of the xenoliths with the Variscan igneous bodies that crop out in the central Jebilet. The potassic xenoliths with unradiogenic Nd are broadly similar to the granodiorite plutons. The xenoliths with Na > K and mildly negative εNd and the blastomylonitic plagiogranite are reminiscent of the bimodal association of microgranites and gabbros of the Sarhlef basin.
From a structural point of view, samples with a gneissic fabric may reflect the possible occurrence of a metamorphic, migmatitic complex at somewhat deeper crustal level. However, based on detailed mineralogical and geochemical data, the samples previously known as “mid-crustal granulites” turn out to be highly differentiated, aplo-pegmatitic rocks, interpreted as early, incipient partial melts. The presence of high-grade metamorphic rocks at depth is only documented by one small, HT garnet-biotite xenolith, while peraluminous lenses (fibrolite + spinel ± corundum) dispersed in the quartz-monzodiorite dykes suggest that a metapelitic component was involved in the melting process generating these lavas. In summary, this set of felsic xenoliths does not provide much information on the crustal composition under the Jebilet, apart from fairly superficial levels.
期刊介绍:
The Journal of African Earth Sciences sees itself as the prime geological journal for all aspects of the Earth Sciences about the African plate. Papers dealing with peripheral areas are welcome if they demonstrate a tight link with Africa.
The Journal publishes high quality, peer-reviewed scientific papers. It is devoted primarily to research papers but short communications relating to new developments of broad interest, reviews and book reviews will also be considered. Papers must have international appeal and should present work of more regional than local significance and dealing with well identified and justified scientific questions. Specialised technical papers, analytical or exploration reports must be avoided. Papers on applied geology should preferably be linked to such core disciplines and must be addressed to a more general geoscientific audience.