Jung S, Pfänder JA, Mezger K, Hellebrand E, Brandt S (2022)
Publication Type: Journal article
Publication year: 2022
Book Volume: 418-419
Article Number: 106675
Determining the ages and duration of high-grade metamorphic episodes during orogenesis and concomitant melt extraction events is pivotal in reconstructing and quantifying the chemical and petrological evolution of the Earth's continental crust over time. Major and trace elements combined with multichronometric garnet data from the Pan-African Damara belt of Namibia place absolute time constraints on high-grade regional metamorphism and melting in the lower crust that led to the production of widespread granites emplaced in the upper crust. Some of these granites contain inherited peritectic garnet with inferred igneous rims that record their genesis and evolution from formation by partially melting in the deep crust to the emplacement of their host rock at shallower levels. Particular characteristics of these garnets are textural and chemical disequilibrium features such as resorption, local development of partly euhedral crystal faces and occurrence of quartz-feldspar stringers in the rim of the garnet and trace element abundances with sharp concentration gradients. These features document a previous melting event followed by garnet crystallization in an ascending melt. With the exception of the outermost rim, which records late-stage diffusional features, the garnet crystals have constant Mg and Mn concentrations. In contrast, Ca shows pronounced chemical zoning. Trace elements (HREE, LREE, Zr, Sr, Y) are depleted in the core and enriched in the rim region indicating disequilibrium conditions during garnet growth. A pseudosection calculated with the garnet rim composition and the composition of the host granite yields P-T conditions of 0.62 GPa and 760 °C for the formation of the rim. This result is consistent with a P-T estimate of 0.68 GPa and 760 °C derived from the conventional garnet-biotite thermobarometer. Regional metamorphic conditions were above the cordierite-andalusite stability field with estimated conditions of ~550-600 °C and 0.4–0.5 GPa. This P-T regime indicates that the garnet originally crystallized at deep crustal levels. Diffusion modelling suggests that the observed concentration gradient of Sm in garnet is consistent with a maximum duration of ca. 0.5–1.5 Ma for the inferred melting interval during peak metamorphism in the lower crust. High-precision multi-point Lu-Hf garnet-whole rock data yield dates of 522.0 ± 1.0 and 522.3 ± 0.9 Ma for two samples. Because of presumed short intervals between melting in the lower crust and intrusion of the granite, these dates are interpreted as the age of the melting event during high-grade metamorphism and are temporally indistinguishable from the crystallization interval. Thus, dehydration melting in the deeper crust during peak metamorphism produced peritectic garnet that was incorporated into the granitic melt followed by subsequent new growth of igneous garnet around peritectic cores during intrusion. Previously obtained Sm-Nd garnet whole rock ages are 20–25 Ma younger and likely represent cooling ages. This study shows that granites enriched in mafic components such as garnet and biotite may intrude as mixtures of melt and the peritectic assemblage produced by incongruent melting involving dehydration-melting of biotite in the lower crust. Melting episodes represent short-lived processes relative to the absolute duration of orogenic cycles.
Jung, S., Pfänder, J.A., Mezger, K., Hellebrand, E., & Brandt, S. (2022). Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes. Lithos, 418-419. https://dx.doi.org/10.1016/j.lithos.2022.106675
Jung, S., et al. "Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes." Lithos 418-419 (2022).