Giulio MORTEANI, Cosmas Kongnyuy SHANG, Muharrem SATIR

Archaean zircon U-Pb age paradox in juvenile neoproterozoic granitoids, Central North Sudan, Saharan Metacraton

Özet: Orta Kuzey Sudan’ın, komşu Arkeyan kratonlarının ve Neoproterozoyik Arabian-Nubian Kalkanı’nın aksine, heterojen izotopik bileşime sahip olduğu ve Neoproterozoyik’te tekrar hareketlenmiş olan Saharan Metakratonik kabuğunun bir parçası olduğu uzun zamandır gösterilmekteydi. Biz bu makalede Saharan Metakratonu’nda, çelişkili olarak, Arkeyan kabuğun varlığına işaret etmeyen Arkeyan zirkon U-Pb yaşları sunacağız. İkinci çelişki ise kayaçlar Neoproterozoyik olduğu halde, hem Arkeyan tonalit-tronjemit-granodiyorit (TTG) özellikleri hem de Arkeyan-sonrası granitoyid özellikleri sergileyen jeokimyasal verileri kapsamaktadır. Çalışılan granitoyidler Halfa Terreyni’nde Delgo’nun kuzeyindendir. Kayaçlar, kalk-alkalen ve metaaluminalıdan peraluminalıya geçişli olup, negatif Nb ve Ti sapmalarına sahiptir. Zirkon morfolojisi, katodoluminesans (CL) ve U-Pb yaş verileri dört magmatik zirkon popülasyonunu tanımlamaktadır. En yaşlısı, 3025 My Arkeyan U-Pb yaşıyla karakterize edilmektedir. İkinci zirkon popülasyonunun 728−702 My arası yaşları, araştırma alanının, Doğu ve Batı Gondwana’nın ilk çarpışma kontağı nedeniyle kalınlaşmış astenosferik mantonun delaminasyonunun yol açtığı Neoproterozoyik levha-içi magmatizmanın içerisinde yeraldığını önermektedir. Üçüncü grubun zirkonları, titanit yaşlarıyla özdeş olan ve çalışılan kayaçların bölgesel Pan-Afrikan tektono-metamorfik olaylarıyla birlikte yoğun olarak tekrardan hareketlendiğini gösteren, doruk Pan-Afrikan orojenezi Neoproterozoyik yaşları (630–600 My) vermektedir. Son olarak, dördüncü popülasyonun 554 My yaşlı konkordan zirkonları, alanın daha sonra asla böylesine yüksek sıcaklık ve bölgesel orojenik etkiler geçirmediği önerisini getirmektedir. İlksel Sr (0.702389–0.704011) ve εNd (+5.05±8.66) değerleri önemsiz kıtasal kabuk kirlenmesine uğramış juvenil kaynaklara işaret etmektedir. Nd TDM yaşlarının, Neoproterozoyik (917–653 Ma) olması ve Neoproterozoyik zirkon yaşlarıyla hata payı içinde özdeş olması ve/veya bunlardan kısmen yaşlı olması, bu magmatizmanın primitif özelliğini teyid etmektedir. Bu da, bu zirkon popülasyonunun zenokristik olduğunu ve incelenen terreynin alanına ait yaşları içermediğini ortaya koyarak, bu kayaçlardaki Arkeyan zirkon popülasyonu çelişkisini çözmektedir. Bu sonuçlar, zirkon U-Pb yaşları ile Nd izotop sistematiğinin birleştirilmesinin, kayaçların petrolojisinin ve jeolojik terreynlerin açık bir şekilde tanımlanmasında çok güçlü bir araç olduğunu göstermektedir.

Juvenil neoproterozoyik granitoyidlerindeki arkeyan zirkon U-Pb yaş çelişkisi, Orta Kuzey Sudan, saharan metakratonu

Abstract: It has long been shown that central North Sudan with its heterogeneous isotopic composition, unlike neighbouring Archaean cratons and the Neoproterozoic Arabian-Nubian Shield, is part of the Saharan metacratonic crust that was remobilized in the Neoproterozoic. In this paper, we report an Archaean zircon U-Pb age in the Saharan Metacraton that paradoxically does not indicate the presence of an Archaean crust. The second paradox concerns geochemical data that show Archaean tonalite-trondhjemite-granodiorite (TTG) features and also post-Archaean granitoid features, yet the rocks are Neoproterozoic. The granitoids studied are from north of Delgo in the Halfa terrane. They are calc-alkaline and meta- to peraluminous and have negative Nb and Ti anomalies. Zircon morphology, cathodoluminescence (CL) and U-Pb age data define four magmatic zircon populations. The oldest is characterized by a 3025 Ma Archaean U-Pb age. The 728 to 702 Ma ages of the second zircon population suggest that the studied area was involved in the Neoproterozoic intraplate magmatism that was induced by the delamination of the thickened asthenospheric mantle due to the first collisional contact between East and West Gondwana. Zircons of the third group yield peak Pan-African orogeny Neoproterozoic ages (630 to 600 Ma) that are identical with titanite age data, and show that the studied rocks were intensely remobilized by the Pan-African tectono-metamorphic regional event. Lastly, the 554 Ma concordant zircon of the fourth population suggests that the area thereafter never again experienced such high temperature and pressure regional orogenic effects. Sr initial values (0.702389–0.704011) and εNd values (+5.05±8.66) indicate juvenile sources with insignificant crustal contribution. Nd TDM ages are Neoproterozoic (917–653 Ma) and identical within error and/or slightly older than the Neoproterozoic zircon ages, confirming the primitive nature of this magmatism. This sets a paradox with the Archaean zircon population in these rocks, implying that this zircon population is xenocrystic and has no bearing to the age of the tract of terrane investigated. These results show that combining zircon U-Pb ages and Nd isotope systematics is a very powerful tool in unequivocally defining the petrogenesis of rocks and geological terranes.

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