MINERALOGICAL, GEOCHEMICAL (REE), AND ISOTOPIC (K-AR, RB-SR, DELTA-O-18) EVOLUTION OF THE CLAY-MINERALS FROM FAULTED, CARBONATE-RICH, PASSIVE PALEOMARGIN OF SOUTHEASTERN MASSIF-CENTRAL, FRANCE

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, carbonat e rich, passive paleomargin in the southeastern French Massif Central, The fault system described at the lower part of the sedimentary seque nce was active at 190 +/- 20 Ma during a rifting tectono thermal activ ity with important migration of hot fluids, It was active again afterw ards, probably after maximum burial occurring at 110-120 Ma, as a resu lt of the compressive Alpine activity in the region, This late shearin g induced an overall throw of at least 1160 m and subsequent erosion o f as much as 2000 m of sediments, However, it probably did not induce major fluid movements, which means in turn that the mineralizing fluid s responsible for economic ore deposits in the margin relate to the 19 0-Ma-old Liassic hydrothermal event. The Rb-Sr, K-Ar, and oxygen isoto pic systematics of the clay fractions suggest that late faulting activ ity occurred under very low water-to-rock conditions, which did not fa vor 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 Ar-40 and some unrealistic initial Sr-87/Sr-86 ratios, even below 0.700, The 190 Ma illites that crystallized in the fault at 200-210 degrees C also have abnormally l ow delta(18)O values of about +13.3 parts per thousand, leading to a d elta(18)O value of the paleofluids as high as +8.1 parts per thousand, The Sr-87/Sr-86 ratios and REE distributions of acid-leached clay res idues delineate two types of fluids that have probably interacted with the clay minerals, One, with an Sr-87/Sr-86 ratio of 0.7250 and a hum ped REE distribution pattern, is believed to be of hydrothermal origin , and the other, with an Sr-87/Sr-86 ratio of 0.7082 and a flat REE pa ttern, might be of recent continental origin.