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
Jacky Bouloton , Dominique Gasquet , Christian Pin , Aimeryc Schumacher
{"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}
引用次数: 0
Abstract
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.