THE CADOMIAN GRANITES OF MANCELLIA, NORTHEAST ARMORICAN-MASSIF-OF-FRANCE - RELATIONSHIP TO THE ST-MALO MIGMATITE BELT, PETROGENESIS AND TECTONIC SETTING

The North Armorican Shear Zone (NASZ) divides the Cadomian belt of the Armorican Massif of France into two major terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The NACT is a pastiche of displaced blocks and terranes (St. Brieuc Terrane, St. Malo Terrane and Mancellian Terrane) which results from the amalgamation of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone. In the NACT, the Neoproterozoic Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. The St. Malo migmatite belt is a syn-tectonic suite of anatectic migmatites derived through partial melting of the Brioverian succession. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that a sample of St. Malo diatexites comes from a population with a different mean than some samples of the Brioverian succession sandstones. Resulting homogeneous diatexites/anatectic granites of the core were emplaced syn-kinematically into Cadomian strike-slip shear zones. Thus the age of anatexis of approximately 540 Ma is also the date of strike-slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the NACT. The Mancellian granites, to the southeast of the St. Malo migmatites, were emplaced at approximately 540 Ma and are overlain uncomformably by Cambrian sediments. Overall geochemistry and a variety of specific geochemical parameters show that the Mancellian granites are similar to the diatexites and anatectic granites of the St. Malo migmatite belt. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that the sample of Mancellian granites comes from a population with a different mean than the sample of St. Malo diatexites. Additionally, both suites of rocks are peraluminous, they have similar distributions of Rb, Sr and Ba, they have K/Rb ratios around 225, and they plot largely within the VAG field using trace element discrimination diagrams for the interpretation of tectonic setting. Samples of the Brioverian succession sandstones, the St. Malo migmatites and the Mancellian granites all exhibit similar primordial mantle-normalized element patterns. Preliminary results of Nd and Sr isotope studies reveal a similar restricted range of values for epsilon-Nd (between -4.0 and -7.3) and epsilon-Sr (between -3 and +42), and depleted model ages in the range 1.7 to 1.5 Ga (Tdmur) for both Mancellian granites and St. Malo migmatites, isotopic features which set these rock types apart from other Cadomian granites within the NACT. The St. Malo migmatite belt and the Mancellian granites are coeval at approximately 540 Ma and both developed by anatexis, at intermediate crustal depths, of the Brioverian succession, within an inverted, but only moderately overthickened, behind-arc basin. In comparison with the St. Malo migmatites, the Mancellian granites represent farther-travelled products of intracrustal melting but their geochemistry still preserves the characteristics signature of the source.