Martin J. Whitehouse

Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago.

Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8u2009Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2u2009Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9u2009Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5–3.2u2009Gyr ago from an ancient (3.9–3.5u2009Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2u2009Gyr ago that involved juvenile crust generation by plate tectonic processes.

Zircon U-Pb dating of Mesozoic volcanic and tectonic events in north-west Palmer Land and south-west Graham Land, Antarctica

Abstract New whole rock Rb-Sr and zircon U-Pb geochronological data and Sm-Nd isotopic data are presented from the central magmatic arc domain of the Antarctic Peninsula in the area of north-west Palmer Land and south-west Graham Land, Rb-Sr isochrons indicate an age of 169 ± 6 Ma for basement orthogneisses and 132 ± 9 to 71 ± 9 Ma for plutons. A U-Pb age of 183 ± 2.1 Ma, with no detectable inheritance, on zircons from an orthogneiss from Cape Berteaux provides the first reliable age for the orthogneisses, which are interpreted as metamorphosed silicic volcanic rocks, and Sm-Nd data indicate derivation in a mature volcanic arc. The age indicates they may be correlatives of the Jurassic ‘Chon Aike’ volcanism of the eastern Antarctic Peninsula. A U-Pb zircon age of 107 ± 1.7 Ma on a terrestrial volcanic sequence overlying an uncomformity strongly suggests a mid-Cretaceous age for the extensive volcanic cover of north-west Palmer Land that was previously thought to be Jurassic. The unconformity is interpreted to have been a result of compressional uplift related to the Palmer Land event. This is the first date for the event in the western part of the central magmatic arc terrane of the Antarctic Peninsula.

Neoproterozoic evolution of the eastern Arabian basement based on a refined geochronology of the Marbat region, Sultanate of Oman

Abstract New high spatial resolution secondary ion mass spectrometry (SIMS) U–Pb zircon data from the Sadh gneiss complex and the intruding Marbat granodiorite of the Marbat region, southern Sultanate of Oman, yield Cryogenian magmatic protolith ages for gneisses ranging from c. 850 to 830 Ma. Zircon ages record a c. 815–820 Ma period of deformation and migmatization, followed by intrusion of a hornblende gabbro/diorite and the undeformed Marbat granodiorite at c. 795 Ma. Following break-up and rifting of Rodinia at c. 870 Ma, crustal growth in the Marbat region occurred via arc accretion at c. 850–790 Ma, possibly in the easternmost part of the Mozambique Ocean based on earlier cessation of accretion here compared to the Arabian–Nubian Shield. Similarity of the new zircon geochronology to peaks of detrital zircon ages in the unconformably overlying Ediacaran Marbat sandstone suggests relatively local derivation from uplifted basement for the latter. Supplementary material: Detailed petrographic descriptions and photographs of hand specimens and thin-sections are available at http://www.geolsoc.org.uk/SUP18685.

Zircon U–Pb ages, δ18O and whole-rock Nd isotopic compositions of the Dire Dawa Precambrian basement, eastern Ethiopia: implications for the assembly of Gondwana

New high spatial resolution secondary ion mass spectrometry (SIMS) zircon dating from the Dire Dawa Precambrian basement yields crystallization ages at c. 790u2009Ma and 600u2009–u2009560u2005Ma. Two of the youngest samples are pervasively deformed, indicating that orogenesis continued until c. 560u2005Ma. SIMS δ18Ozrn shows bimodality, with the oldest sample (c. 790u2005Ma) and inherited zircons of that age in the younger samples having values of 7.8u2009–u20099.6‰, whereas the Ediacaran samples have δ18Ozrn values of 4.9u2009–u20097.2‰. These δ18Ozrn ratios are higher than mantle values and indicate a supracrustal input to the source of the Dire Dawa granitoids. All samples have unradiogenic εNd(t) values of −10.3 to −5.8 and Nd model ages of 1.72u2009–u20091.42u2005Ga. These attributes suggest that the Dire Dawa granitoids were mostly derived from reworking of long-lived crustal sources. The occurrence of c. 580u2009–u2009550u2005Ma orogenesis in both the Dire Dawa basement and the juvenile Western Ethiopian Shield and the confinement of c. 630u2005Ma metamorphism to only the latter indicate that these two lithospheric blocks of contrasting isotopic compositions amalgamated at c. 580u2009–u2009550u2005Ma. This suggests that the Mozambique Ocean, which separated these two lithospheric blocks, was completely consumed during the late Ediacaran to early Cambrian.