Origin of Meso-Proterozoic post-collisional leucogranite suites (Kaokoveld, Namibia): constraints from geochronology and Nd, Sr, Hf, and Pb isotopes |
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Authors: | S Jung K Mezger O Nebel E Kooijman J Berndt F Hauff C Münker |
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Affiliation: | (1) Department Geowissenschaften, Institut f?r Mineralogie und Petrographie, Universit?t Hamburg, Grindelallee 48, 20146 Hamburg, Germany;(2) Institut f?r Geologie, Universit?t Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland;(3) Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia;(4) Universit?t M?nster, Institut f?r Mineralogie, Corrensstra?e 24, 48149 M?nster, Germany;(5) IFM-GEOMAR, Research Division 4, Dynamics of the Ocean Floor, Wischhofstrasse 1-3, 24148 Kiel, Germany;(6) Institut f?r Geologie und Mineralogie, Universit?t zu K?ln, Z?lpicherstr. 49b, 50674 K?ln, Germany |
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Abstract: | Leucocratic granites of the Proterozoic Kaoko Belt, northern Namibia, now preserved as meta-granites, define a rock suite
that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics. Least evolved members
of this ~1.5–1.6-Ga-old leucogranite suite can be distinguished from ordinary calc-alkaline granites that occur elsewhere
in the Kaoko Belt by higher abundances of Zr, Y, and REE, more radiogenic initial εNd values and unradiogenic initial 87Sr/86Sr. The leucogranites have high calculated zircon saturation temperatures (mostly > 920°C for least fractionated samples),
suggesting that they represent high-temperature melts originating from deep crustal levels. Isotope data (i.e., εNdi: +2.3 to –4.2) demonstrate that the granites formed from different sources and differentiated by a variety of processes including
partial melting of mantle-derived meta-igneous rocks followed by crystal fractionation and interaction with older crustal
material. Most fractionation-corrected Nd model ages (TDM) are between 1.7 and 1.8 Ga and only slightly older than the inferred intrusion age of ca. 1.6 Ga, indicating that the precursor
rocks must have been dominated by juvenile material. Epsilon Hf values of zircon separated from two granite samples are positive
(+11 and +13), and Hf model ages (1.5 and 1.6 Ga) are similar to the U–Pb zircon ages, again supporting the dominance of juvenile
material. In contrast, the Hf model ages of the respective whole rock samples are 2.3 and 2.4 Ga, demonstrating the involvement
of older material in the generation of the granites. The last major tectonothermal event in the Kaoko Belt in the Proterozoic
occurred at ca. 2.0 Ga and led to reworking of mostly 2.6-Ga-old rocks. However, the presence of 1.6 Ga “post-collisional”
granites reflects addition of some juvenile mantle-derived material after the last major tectonic event. The results suggest
that similar A-type leucogranites are potentially more abundant in crustal terranes but are masked by AFC processes. In the
case of the Kaoko Belt, it is suggested that this rock suite indicates a yet unidentified period of mantle-derived crustal
growth in the Proterozoic of South Western Africa. |
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