T61 Life and Death of a Craton: Implications of Archean Crust-Keel Systems for Crustal Growth, Crustal Preservation, and Mantle Evolution

Sunday, 22 September 2019
1:30 PM - 5:30 PM
Room 131ABC, North Building

DOES NORTHERN LIBYA MARK THE NORTHERN BOUNDARY OF REMOBILIZED SAHARAN METACRATON? INSIGHTS FROM ZIRCON U–PB GEOCHRONOLOGY, HF ISOTOPES COMPOSITION AND GEOCHEMISTRY OF BASEMENT UNDERNEATH THE SIRTE BASIN, LIBYA

Paper number: T61-14
Program: Technical Sessions
Time: 4:50 PM – 5:05 PM

Authors

Abstract

Timing and origin of basement underneath northern Libya is not well-understood and inferred as the northern boundary of Saharan Metacraton. In this work, we present the unprecedented LA-ICP-MS zircon U-Pb geochronology and zircon Hf isotope composition from the igno-metamorphic basement underneath the Sirte basin. The analyzed igneous rocks are from the bottom of the deepest drill cores (i.e., as deep as 12000 ft) from oil wells. Geochemical and petrographic data suggest that most of the igneous rocks vary from diorite to granodiorite to granites. In addition to granitic rocks, basement also comprises of gabbro, basalt and meta-igneous rocks. The U-Pb data on twelve samples revealed that the granitic basement has crystallization ages restricted from 588 ± 3 to 562 ± 2 Ma. Few analyses yielded individual dates slightly older than 600 Ma. These ages suggest the presence of some antecrystic zircons. Out of twelve samples from various oil wells, only one sample yielded four individual dates >1000 Ma. One zircon core is dated at 1087 ± 53 Ma. Three zircon spots yielded dates varying from 1921 to 2016 Ma. The scarcity of older ages, suggests that the basement underneath northern Libya is primarily underlain by the Neoproterozoic crust with minimal contribution from older Paleoproterozoic crust. This also suggests that northern Libyan basement formed prior to the culmination of Arabian Nubian shield at ~540 Ma. In contrast to Arabian Nubian shield, northern Libya does not record early subduction-related granite magma generation at 800-700 Ma as recorded in Arabian-Nubian shield and elsewhere in Saharan Metacraton to the south. The weighted mean hafnium isotopic composition is heterogeneous and varies from near-chondritic εHf (t) = +1.6 ± 0.3 to more radiogenic εHf (t) = +8.1 ± 0.6. Out of 113 analyses (600-500 Ma) 104 analyses (> 90 %) yielded positive Hf isotope composition range from +0.4 to +8.7. The larger scatter and predominant positive Hf isotopic composition suggests partial melting of juvenile crust with contribution of mantle derived melts. However, given the scarcity of older ages, absence of xenocrystic inheritance coupled with absence of non-radiogenic hafnium compositions, we suggest the northern Libya is not underlain by the remobilized evolved Paleoproterozoic lithosphere, and thus is not part of the deformed and remobilized Saharan Metacraton; instead it is juvenile Neoproterozoic crust.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-339634

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