Abstract

Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, delta 18 O) data of < 0.2 mu m clay fractions from core samples have been used to enhance our understanding of the evolution of the faulted, carbonate-rich, passive paleomargin in the southeastern French Massif Central. The fault system described at the lower part of the sedimentary sequence was active at 190 + or - 20 Ma during a rifting tectono-thermal activity with important migration of hot fluids. It was active again afterwards, probably after maximum burial occurring at 110-120 Ma, as a result of the compressive Alpine activity in the region. This late shearing induced an overall throw of at least 1160 m and subsequent erosion of as much as 2000 m of sediments. However, it probably did not induce major fluid movements, which means in turn that the mineralizing fluids responsible for economic ore deposits in the margin relate to the 190-Ma-old Liassic hydrothermal event. The Rb-Sr, K-Ar, and oxygen isotopic systematics of the clay fractions suggest that late faulting activity occurred under very low water-to-rock conditions, which did not favor widespread fluid migration. The clay material of the fault yields unreasonably widely scattered Rb-Sr and K-Ar dates, from 70 to 183 Ma, because of variable contents of radiogenic 40 Ar and some unrealistic initial 87 Sr/ 86 Sr ratios, even below 0.700. The 190 Ma illites that crystallized in the fault at 200-210 degrees C also have abnormally low delta 18 O values of about +13.3 per thousand , leading to a delta 18 O value of the paleofluids as high as +8.1 per thousand . The 87 Sr/ 86 Sr ratios and REE distributions of acid-leached clay residues delineate two types of fluids that have probably interacted with the clay minerals. One, with an 87 Sr/ 86 Sr ratio of 0.7250 and a humped REE distribution pattern, is believed to be of hydrothermal origin, and the other, with an 87 Sr/ 86 Sr ratio of 0.7082 and a flat REE pattern, might be of recent continental origin.

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