Carbonate-cemented stylolites and fractures in the Upper Jurassic limestones of the Eastern Iberian Range, Spain:: A record of palaeofluids composition and thermal history

Dolomite and calcite cements fill open stylolites, fractures and "pipe-shaped' structures related to faulting in Tithonian-Beniasian limestones of the Maestrat Basin in the Iberian Range (Spain) Due to the grater, susceptibility of the dolomitised limestones to brittle fracturing during the Alpine tectonism, their location and distribution may have important implications for hydrocarbon prospectively within the studied region (Iberian Range). Three generations of structures were recognised (open stylolites, extensional fractures and faults) based on field observations, cross-cutting relationships and cement mineralogy. Petrographic, cathodoluminesence and geochemical analyses (electron microprobe, fluid inclusion, oxygen, carbon and strontium isotopes) of the carbonates helped unravel the origin and evolution of the fluids, from which these carbonates have been precipitated. These cements occur in the following structures: A) The first generation NNE trending fractures formed during the Late Eocene-Miocene compressional stage were filled by calcite cement (delta O-18 VPDB-8.8 parts per thousand and delta C-13 VPDB+0.8). B) The second generation represented by subvertical extensional fractures and the third generation by "pipe-shaped' structures, which are considered to be formed during the Miocene-Pliocene extensional stage, contain four carbonate cement phases : 1) isolated rhombic dolomites; 2) saddle dolomite with fluid inclusions characterised by high salinity (21.5 to 23.5 wt.% eq. NaCl), radiogenic Sr-enriched (0.70796 to 0.70857) in relation to the marine standard, high temperatures (T-h 110-155 degrees C) and low delta O-18 values VPDB (- 11.5 parts per thousand to - 11.3 parts per thousand); 3) calcitized saddle and rhombic dolomite with variable delta O-18 VPDB - 12.2 parts per thousand to - 6.8 parts per thousand and delta C-13 VPDB - 4.4 parts per thousand to + 0.2 parts per thousand, and 4) two phases of calcite cements with moderate to high temperatures (T-h 110-125 degrees C 15 to 19.7 wt.% eq. NaCl and 160-198 degrees C, 5.5 to 9.5 wt.% eq. NaCl), low delta O-18 VPDB values (- 13.4 parts per thousand to - 10 parts per thousand) and positive carbon values (delta C-13 VPDB +0.1 parts per thousand to +1.8 parts per thousand). The diagenetic fluid is interpreted to be initially hot, saline, hydrothermal (temperature higher than the estimate of the ambient temperature) and Sr-enriched (0.70800). The dolomitisation event was followed by an input of meteoric water, which was related to the extensional stage. Finally, late calcite cement precipitated from fluids, which had initially moderate salinity and moderate to high temperature and later evolved to a lower temperature and higher salinity fluid (both Sr enriched). These fluids were probably derived from the progressive mixing of Late Triassic evaporitic brines with descending meteoric water that migrated via fractures during the Miocene-Pliocene Alpine extensional stage. Major vug-filling calcite cement is probably late in timing and related to hydrothermal karst associated with the "pipe-shaped" structure. C) Bed-parallel stylolites and subvertical. extensional fractures, containing idiomotphic quartz crystals for which make it difficult to deduce the timing of its precipitation. Idiomorphic quartz has homogenisation temperatures of 140-180 degrees C and salinities of 13.6 wt.% eq. NaCl. The high homogenisation temperature of this quartz and its association with dickite suggest precipitation from hydrothermal fluid. (c) 2005 Elsevier B.V. All rights reserved.