Petrogenesis of magmatic albite granites associated to oogenetic A-typi granites: Na-rich residual melt extraction from a partially crystallized A-type granite mush

摘要

The uncommon association of cogenetic and nearly contemporaneous potassic A-feldspar A-type granites and sodic albite granites is observed within the 347 Ma-old bimodal Saint-Jean-du-Doigt (SJDD) intrusion, Brittany France. A-type granites outcrop as small bodies (<1 km~2) of fine-grained, pinkish to yellowish rock or as meter-thick sills in-between mafic layers. They emplaced early within the thermally "cool" part of the SJDD plu-ton directly beneath the Precambrian host rock forming the pluton roof. Albite granites are fine-grained hololeucocratic yellowish rocks emplaced slightly after the A-type granites in the thermally mature part of the pluton. They form meter-thick sills that mingle with adjacent mafic layers and represent ca. 1 vol.% of the out-cropping part of the pluton. The two granite types are similar in many respects with comparable Sr-Nd-Hf isotope compositions (~(87)Sr/~(86)Sr_(347) = 0.7071 for A-type granites vs. 0.7073 for albite granites; εNd_(347) = +0.2 vs. +0.3; εHf_(347zircon)= +2.47 vs. +2.71, respectively) and SiO2 contents (74.8 vs. 74.4 wt%). On the other hand, they have contrasting con-centrations in K2O (5.30 vs. 1.97 wt%), Na2O (2.95 vs. 4.73 wt%) and CaO (0.48 vs. 2.04, respectively) as well as in some trace elements like Sr (59 vs. 158 ppm in average), Rb (87 vs. 35 ppm), Cr (170 vs. 35 ppm) and Ga (30 vs. 20 ppm). The isotopic composition of the A-type and albite granites is very distinct from that of the associated and volumetrically dominant mafic rocks (i.e.~(87)Sr/~(86)Sr_(347) = 0.7042; εNd_(347) = +5.07; εNd_(347zircon) = + 811), excluding a direct derivation of the felsic rocks through fractional crystallization from the basaltic magma On the other hand, small volumes of hybrid, enclave-bearing granodiorite within the SJDD lopolith suggest mixine processes within a reservoir located at deeper crustal levels. A-type granites may therefore form by magma mixing between the mafic magma and crustal melts. Alternatively, they might derive from the pure melting of an immature biotite-bearing quartz-feldspathic crustal protolith induced by early mafic injections at low crustal levels. Strong field evidences coupled to mineral chemistry and elemental geochemistry strongly support a magmatic origin for the albite granite. Sr, Nd, Hf zircon isotope data, U-Pb zircon ages, as well as data on petrography mineral chemistry and elemental geochemistry attest that A-type and albite granites are closely related. Our preferred petrogenetic model is to consider the albite granite magma as a compositionally extreme melt that was extracted from a partially crystallized A-type granite mush at a late stage of crystallization. Alternatively albite granites could form by melting of plagioclase-rich layers formed during A-type granite differentiation.