Petrology and geochemistry of metamorphic and metamagmatic rocks in the Kabul block, Kabul, Afghanistan

The study area is located around Kabul Province in the north-east of Afghanistan. In this study, it was aimed to determine the stratigraphic, petrographic, geochemical and structural situation and metamorphism characteristics of the Kabul Block. The study area is located in the tectonically active Alpine-Himalayan orogenic belt, which developed due to the collision between the Indian and Arabian plates and the Eurasian plate in the Late Paleogene in Central Asia. The Kabul Block is a tectonic fragment formed at the intersection between the Indian and Eurasian continents. Basement rocks are exposed in the base of the Kabul Block. Tectonostratigraphically, the basement rocks of the Kabul Block, from bottom to top, consist of the Welayeti formation, Anjerak formation, Kasaba Mountains orthogneiss, Khair Khana Mountains orthogneiss, Sherderwaza formation, and Alghoi metagranitoid. These basement rocks are unconformably covered by the Lataband formation, composed of Miocene-Pliocene aged metasediments. In the study area, the Sherderwaza formation was observed with a tectonic contact on the Welayeti formation. Although there is no contact relationship, it is thought that the Anjerak formation, Kasaba Mountains orthogneiss, and Khair Khana Mountains orthogneiss are also tectonically in contact with the Welayeti formation. The Kasaba Mountains orthogneiss and Khair Khana Mountains orthogneiss cut through the Anjerak formation and settled in the region. The Sherderwaza formation overlies these orthogneisses with a indefinite unconformity. Welayeti formation is of Paleoproterozoic-Neoproterozoic (?) protolith age and from bottom to top; It is made up of "garnet-amphibolite and amphibole schist", "micaschist and garnet micaschists", "quartzite" and "staurolite-kyanite-garnet micaschists". The Eo-Mesoarchean (?) protolith aged Anjerak formation consists of olivine marble, tremolite marble and calcschist. It contains calcsilicate intermediate levels in some places. The Kasaba Mountains orthogneiss is of Neoarchean protolith age and consists of mesocratic orthogneisses with amphibolite interlayers and anatexis observed in some places. The protolith age of Khair Khana Mountains orthogneiss is Paleoproterozoic. This formation commonly consists of leucocratic orthogneisses. Lens-like felsic and mafic granulite blocks and amphibolite-amphibolschist layers. Additionally, anatexis formations are also observed in some places. Paleoproterozoic-Neoproterozoic (?) protolith aged Sherderwaza formation, from bottom to top; It is made up of "migmatite", "paragneiss (biotite gneiss, epidote-biotite gneiss and mica gneiss)", "quartzite", "marble", "amphibolite and amphibole schist" and "micaschist and biotite schist". Neoproterozoic (?) aged Alghoi metagranitoid is consist of metagranites. Garnet micaschists belonging to the Welayeti formation are of shale origin. Migmatites belonging to the Sherderwaza formation were exposed through metamorphism from clayey sandstone, micaschists from sandy claystone, and biotite gneisses from sandstone (arkose-graywacke). These metasedimentary rocks received material from volcanic arc granites of acidic and intermediate composition and were formed on the active continental margin. Garnet amphibolites belonging to the Welayeti formation are of gabbro origin, amphibolites in the Khair Khana Mountains orthogneiss are of dioritic gabbro origin, mesocratic orthogneisses of the Kasaba Mountains orthogneiss are of granodiorite origin, leucocratic orthogneisses of the Khair Khana Mountains orthogneiss are of granite-granodiorite/trondjemite origin, and metagranites of the Alghoi metagranitoid are granite origin. Felsic granulites observed within the Khair Khana Mountains orthogneiss were formed as a result of high-temperature metamorphism of quartzofeldspathic rocks (granite-granodiorite). Mafic granulites observed within the Khair Khana Mountains orthogneiss were formed by high temperature metamorphism from basic magmatites with gabbroic/basaltic composition. All these metamagmatites belong to the subalkaline magma series. Garnet amphibolites belonging to the Welayeti formation belong to the tholeiitic series magma. These rocks are of oceanic crust origin and present mid-ocean ridge basalt (MORB) character. The negative Nb anomaly in these indicates source material associated with subduction. Enrichments in LILE and HFSE indicate that the main magma derives from a subcontinental lithospheric/asthenospheric source. Mesocratic orthogneisses belonging to the Kasaba Mountains orthogneisses have the character of high-K calc-alkaline series magma. These show orogenic features and belong to volcanic arc granites. They show I-type granite characteristics. Mesocratic orthogneisses are close to the composition of the lower continental crust and are similar to the Late Archean-TTG gneisses formed after 3.5 Ga, formed by the partial melting of amphibolite/eclogite at the base of the thickened arc crust. Leucocratic orthogneisses belonging to the Khair Khana Mountains orthogneisses have the character of calc-alkaline series and shoshonite series magma. These show orogenic features and belong to volcanic arc granites. They show I-type granite characteristics. Leucocratic orthogneisses are close to the lower continental crust composition and belong to the Paleoproterozoic (2.3 Ga) TTG gneisses formed by partial melting of amphibolite/eclogite at the base of the thickened arc crust. Amphibolites within the Khair Khana Mountains orthogneiss belong to the "tholeiitic series" magma series. These rocks are of oceanic crust origin and present mid-ocean ridge basalt (MORB) character. Negative Nb anomaly in amphibolites indicates source material associated with subduction. Enrichments in LILE and HFSE indicate that the main magma derives from a lower continental lithospheric/asthenospheric source. Felsic granulites belonging to the Khair Khana Mountains orthogneiss have the character of calc-alkaline series magma. Felsic granulites show orogenic properties. They originate from continental crust and volcanic arcs. These belong to I-type granites. Mafic granulites belonging to the Khair Khana Mountains orthogneiss have the character of calc-alkaline series magma. Basic magmatites that form mafic granulites show mantle source and crust-mantle mixture source characteristics. Basaltic rocks forming mafic granulites belong to the subduction zone. Metagranites belonging to the Alghoi metagranitoid have the character of calc-alkaline series and shoshonite series magmas. Metagranites have I-type granite composition. These show orogenic features and belong to volcanic arc granites. Metagranites show a weak Th depletion. Additionally, they are richer in Nb, La, Ce and Pb values than orthogneisses, and these HFSEs are close to the upper crust composition. Metagranites belonging to the Alghoi metagranitoid, the leucocratic orthogneisses and felsic granulites of the Khair Khana Mountains and the mesocratic orthogneiss of the Kasaba Mountains belong to the CAG-Continental Arc Granitoids tectonic zone. The basement rocks of the Kabul Block, which shows Archean-Proterozoic age data, have been exposed to multi-phase metamorphism and deformations with different index minerals and mineral parageneses. Welayeti formation observed at the base of the Kabul Block was initially formed in the Neoproterozoic (0.83-0.90 Ga) aged has undergone metamorphism "epidote-amphibolite facies" under conditions of 509±20 - 511±20 oC temperature (peak temperature 511±20 oC) - 6.9±0.8 kbar pressure, observed in staurolite-kyanite-garnet micaschists. Later, with the progressive reactions, the formation was formed under the conditions of 616.57±40 – 640.47±30 oC temperature (peak temperature 640.47±30 oC) and 6.7±0.5 kbar pressure, which are observed in garnet amphibolites, and formed the Cambrian-Neoproterozoic (0.5-0.6 Ga) aged amphibolite facies. It has undergone metamorphism in the "kyanite-almandine-muscovite subfacies". In the last phase, the Welayeti formation shows regressive metamorphism effects in the quartz-albite-muscovite-biotite subfacies of the greenschist facies, under 375-430 oC temperature and 2-3.5 kbar pressure conditions. The Anjerak formation, which initially showed metamorphism effects in the granulite facies (>820 oC temperature and >8.2 kbar pressure) and subsequently in the amphibolite facies (520-650 oC temperature and 5.8-7.0 kbar pressure), later It has undergone a regressive metamorphism in the greenschist facies under 375-430 oC temperature and 3.2-4.2 kbar pressure. In the Anjerak formation, the initially observed granulite facies metamorphism must be 1.5-1.6 Ga (Paleoproterozoic) in age. The Kasaba Mountains orthogneiss initially underwent Paleoproterozoic (1.8 Ga) aged granulite facies metamorphism under conditions of 795.27±25 oC temperature and 7.6±1.8 kbar pressure. The orthogneisses were later subjected to Paleoproterozoic (1.77 Ga) regressive granulite facies metamorphism under conditions of 790.09±25 oC temperature and 6.6±1.8 kbar pressure. After initially developing granulite facies metamorphism, the Kasaba Mountains orthogneiss underwent regressive metamorphism in the "sillimanite-almandine-orthoclase subfacies" of the Neoproterozoic (0.82 Ga) aged amphibolite facies under the conditions of 650-690 oC temperature and 5.5-7.5 kbar pressure. The Kasaba Mountains orthogneiss was subjected to greenschist facies in a regressive manner metamorphism in the last phase. This last metamorphism, observed in the quartz-albite-muscovite-biotite subfacies of the greenschist facies, took place under 375-430 oC temperature and 2-3.5 kbar pressure conditions. Khair Khana Mountains orthogneiss underwent Paleoproterozoic (1.8 Ga) aged granulite facies metamorphism, initially under 872±30 oC temperature and 8.23±1.7 kbar pressure conditions, with maximum P-T conditions observed in felsic granulites. This unit was later subjected to Paleoproterozoic (1.77 Ga) aged regressive granulite facies metamorphism under conditions of 864±30 oC temperature and 7.52±1.7 kbar, again with maximum P-T conditions observed in felsic granulites. Khair Khana Mountains orthogneiss, after the initial granulite facies metamorphism, underwent regressive metamorphism in the "sillimanite-almandine-orthoclase subfacies" of the amphibolite facies, under the conditions of 650-690 oC temperature and 5.5-7.5 kbar pressure. The Khair Khana Mountains orthogneiss in a regressive manner was subjected to greenschist facies metamorphism in the latest phase. This last metamorphism, observed in the quartz-albite-muscovite-biotite subfacies of the greenschist facies, took place under 375-430 oC temperature and 2-3.5 kbar pressure conditions. The Sherderwaza formation initially underwent metamorphism in the epidote-amphibolite facies under the conditions of 527.40±50 oC temperature and 7-8 kbar pressure, the effects of which were observed in the epidote-biotite gneisses and biotite gneisses. With increasing temperatures in the environment The Sherderwaza formation was subjected to progressive metamorphism in the "sillimanite-almandine-orthoclase subfacies of the amphibolite facies" under conditions of maximum temperature of 656±50 oC and maximum pressure of 6.2±1.5 kbar in the Neoproterozoic (0.67-0.77 Ga), the effects of which are observed in migmatites and biotite gneisses. After these amphibolite facies metamorphisms, the Sherderwaza formation underwent regressive metamorphism in the "quartz-albite-muscovite-biotite subfacies" of the greenschist facies under the conditions of 375-430 oC temperature and 2-3.5 kbar pressure. The Alghoi metagranitoid was initially exposed to epidote-amphibolite facies metamorphism under 450-550 oC temperature and 4-7 kbar pressure conditions, and later this formation underwent regressive greenschist facies metamorphism under 375-450 oC temperature and 2-3.5 kbar pressure conditions. The Kabul Block is part of the Archean cratons. The Kabul Block is closely related to the orogenies of the supercontinents Columbia in the Paleoproterozoic and Rodinia in the Neoproterozoic. Thus, the Kabul Block is initially associated with the Grenvillian orogeny. The Kabul Block presents close lithological and age similarities with the Tarim Block and the South China Block. The Kabul Block was probably part of Gondwana located between Paleo-Tethys and Neo-Tethys during the Mesozoic. With the closure of Paleotethys in the Mesozoic and the opening of Neotethys in this period, the Kabul Block emerged as a part of Gondwana like a microcraton. The Kabul Block has continued its complex deformation-tectonic development until today, after the collision of the Indian plate with Eurasia and this process are continuing in the region.