Cosmas Kongnyuy Shang

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma 207Pb/206Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the 207Pb/206Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (207Pb/206Pb zircon evaporation age).

Coeval late-Variscan emplacement of granitic rocks: an example from the Regensburg Forest, NE Bavaria

Detailed field mapping in the Regensburg Forest, West Bohemian Massif, has enabled us to trace the distribution of a group of minor intrusions (granodiorites, granite porphyry dykes, two-mica granites). In order to unravel their genetic relation and to constrain their age, geochemical and geochronological studies were carried out. 207 Pb/ 206 Pb evaporation analyses of zir- cons gave ages of 324 ± 2 Ma for a granodiorite (Stallwang granodiorite), and 323 ± 2 Ma for a granite porphyry dyke. U-Th - Pb dating of monazite from a two-mica granite body gave concordant ages of 323 ± 4 Ma (average 207 Pb/ 235 U-age), 322 ± 5M a (average 206 Pb/ 238 U-age) and 323 ± 5 Ma (average 208 Pb/ 232 Th-age). All these ages define the crystallization times and therefore reflect the simultaneous emplacement of the granitoids. The age determination was largely complicated by the predominance of inherited zircon cores. Combining zircon geochronology with geochemical and Sr - Nd data, it becomes evident that the granitoids were derived from Palaeozoic crustal protoliths which, in turn, contain components from Proterozoic (or older) rocks. A source- rock evaluation based on major element oxides allows discriminating between different protoliths. Lithological compositions similar to presently exposed country rock gneisses (mainly greywacke transformed into more or less homogeneous diatexite) are a potential source for the two-mica granites. The geochemistry of the granite porphyry dykes and the granodiorite is consistent with the experimental results of melting mafic pelitic and amphibolitic protoliths, respectively.