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–Berriasian 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 ( δ 18O VPDB-8.8‰ and δ 13C 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 °C) and low δ 18O values VPDB (− 11.5‰ to − 11.3‰); 3) calcitized saddle and rhombic dolomite with variable δ 18O VPDB − 12.2‰ to − 6.8‰ and δ 13C VPDB − 4.4‰ to + 0.2‰, and 4) two phases of calcite cements with moderate to high temperatures ( T h 110–125 °C, 15 to 19.7 wt.% eq. NaCl and 160–198 °C, 5.5 to 9.5 wt.% eq. NaCl), low δ 18O VPDB values (− 13.4‰ to − 10‰) and positive carbon values ( δ 13C VPDB + 0.1‰ to + 1.8‰). 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 sali