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

Abstract

Dolomite and calcite cements fill open stylolites, fractures and bpipe-shapedQ 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-utting 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 (δ18O VPDB-8.8x and δ13C VPDB+0.8). B) The second generation represented by subvertical extensional fractures and the third generation by bpipe-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 (Th 110–155 ºC) and low δ18O values VPDB (-11.5‰ to - 11.3‰ ); 3) calcitized saddle and rhombic dolomite with variable d18O VPDB -12.2‰ to -6.8‰ and δ13C VPDB -4.4‰ to +0.2‰ , and 4) two phases of calcite cements with moderate to high temperatures (Th 110–125 ºC, 15 to 19.7 wt.% eq. NaCl and 160–198 ºC, 5.5 to 9.5 wt.% eq. NaCl), low δ18O VPDB values (-13.4‰ to -10‰ ) and positive carbon values (δ13C VPDB+0.1x to +1.8x). 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 bpipe-shapedQ structure. C) Bed-parallel stylolites and subvertical extensional fractures, containing idiomorphic quartz crystals for which make it difficult to deduce the timing of its precipitation. Idiomorphic quartz has homogenisation temperatures of 140–180 8C 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.