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Combined U–Pb and Lu–Hf isotope data from zircon populations are widely used to constrain Hadean–Archean crustal evolution. Linear Hf isotope-age arrays are interpreted to reflect the protracted, internal reworking of crust derived from the (depleted) mantle during a short-lived magmatic event, and related 176 Lu/ 177 Hf ratios are used to constrain the composition of the reworked crustal reservoir. Results of this study, however, indicate that Hf isotope-age arrays can also result from complex geodynamic processes and crust–mantle interactions, as shown by U–Pb and Lu–Hf isotope analyses of zircons from well characterized granitoids of the Pietersburg Block (PB), northern Kaapvaal Craton (South Africa). Apart from scarce remnants of Paleoarchean crust, most granitoids of the PB with ages between 2.94 and 2.05 Ga ( n = 32 ) define a straight Hf isotope-age array with low 176 Lu/ 177 Hf of 0.0022, although they show a wide compositional range, were derived from various sources and emplaced successively in different geodynamic settings. The crustal evolution occurred in five stages: (I) predominately mafic crust formation in an intra-oceanic environment (3.4–3.0 Ga); (II) voluminous TTG crust formation in an early accretionary orogen (3.0–2.92 Ga); (III) internal TTG crust reworking and subduction of TTG-derived sediments in an Andean-type setting (2.89–2.75 Ga); (IV) (post-)collisional high-K magmatism from both mantle and crustal sources (2.71–2.67 Ga); and (V) alkaline magmatism in an intra-cratonic environment (2.05–2.03 Ga). The inferred array results from voluminous TTG crust formation during stage II, and involvement of this crust during all subsequent stages by two different processes: (i) internal crust reworking through both partial melting and assimilation at 2.89–2.75 Ga, leading to the formation of biotite granites coeval with minor TTGs, and (ii) subduction of TTG-derived sediments underneath the PB, causing enrichment of the mantle that subsequently became source for high-K granitoids and mafic rocks at 2.68 and 2.05 Ga. Some scatter along the array might have resulted either from significant assimilation of ancient crust, intracrustal Lu/Hf fractionation or melting of heterogeneous mantle sources. Those results show that without any information about the nature and composition of zircon-hosting granitoids, Hf isotope-age data are of limited use to constrain Hadean–Archean magmatogenesis and geodynamics. [ABSTRACT FROM AUTHOR]

Abstract: The magnitude of fault offset, a key parameter of fault geometry and kinematics, provides critical information on the role of the Karakoram fault (KKF) in accommodating deformation of the Tibetan crust. Geologic markers used for estimating the magnitude of offset along the KKF include: (1) geomorphologic features, (2) correlative stratigraphic sequences, (3) ophiolite melange belts, (4) igneous bodies, and (5) regional fault systems. The debated offsets of the KKF range from 40km to as much as 1000km. Conflicting offsets may result from a lack of available offset correlations based on quantitative measurements. The Kunsha granite and the Ayilari granite provide another set of potential markers for estimating the offset along the KKF. U–Pb zircon ages and textural observations provide the basis for correlating these granites. Zircon U–Pb ages show that both the Kunsha granite and the South Ayilari granite crystallized around 50Ma ago, whereas the North Ayilari granite formed mostly around 20Ma ago. Reconstruction of batholiths on two sides of the Namru–Menshi basin, as well as the Yalung–Zangbo suture and the trace of the South Kailas thrust, show the offset along the KKF to be about 52±2km. The chronologic data appear to preclude a 100km offset, based on the correlation between the North Ayilari granite and the Kunsha granite. Our data further disproves the conjecture that the North Ayilari granite is synkinematic granite of the KKF, and thus contradicts a 280–400km ophiolite melange belt offset, since it is based on the an incorrect fault initiation time. We calculate a slip rate of 4.5±0.1mm/yr since 12Ma, which implies that the average long-term slip rate is low. This does not support the lateral extrusion model, which predicts a high slip rate. It does support the concept that the collision between India and Eurasia proceeded by distributed deformation rather than lateral extrusion along faults that bound a rigid Tibet. [Copyright &y& Elsevier]