Paleozoic olivine-bearing lamprophyre from the Couy (Cher, France) borehole. Mineral composition and alteration phenomena
Wagner, Christiane; Velde, Danielle
published: Mar 4, 1993
manuscript accepted: Jun 30, 1992
manuscript received: Mar 27, 1992
ArtNo. ESP147050501001, Price: 29.00 €
Abstract Stephanian minette dykes cut the metamorphic basement of the Paris Basin and have been sampled in the Sancerre-Couy (Cher, France) borehole. The dyke studied is an extremely fresh minette with relics of unaltered olivine. This minette belongs to the calc-alkaline group of lamprophyres with typical characteristics, i.e. high Mg (mg* = 0.75), high values for compatible and incompatible elements: Ni (314 ppm) and Cr (844 ppm); Rb (309 ppm), Ba (2566 ppm) and Th (28 ppm); significant LREE enrichment; negative Ta and Nb anomalies. Besides relics of fresh olivine, this minette exhibits another peculiarity, i.e. secondary scapolite found in microcracks. Both features may be related to the composition and behaviour of fluids in the late stage of the crystallization sequence: 1) The magnesian (F090-93) and Ni-rich (up to 0.48 wt% NiO) olivine is partly replaced by a three-layer corona made of talc, tremolite then phlogopite. The growth of the corona has been controlled by the diffusion of elements between the olivine and a fluid separated from the cooling magma. This type of alteration is uncommon in lamprophyres where olivine is usually completely replaced by assemblages of talc, chlorite, silica and carbonates. These differences may be due to the restricted circulation of the demixed fluid which allowed for the partial conservation of the olivine. 2) A marialitic Cl-rich (2.3-3.1 wt%) scapolite forms large monocrysts and fills former microcracks together with calcite, K-feldspar, actinolite, diopside and allanite. These minerals crystallized from a second fluid of unknown origin, with a high activity of NaCl, which circulated through the dyke fractures.