RT Journal Article T1 Shallow burial dolomitisation of Middle–Upper Permian paleosols in an extensionaltectonic context (SE Iberian Basin, Spain): Controls on temperature of precipitationand source of fluids A1 Benito Moreno, María Isabel A1 Horra Del Barco, Raúl De La A1 López Gómez, José A1 Fernández Barrenechea, José María A1 Luque Del Villar, Francisco Javier A1 Arche, Alfredo AB This work is focused on carbonate paleosols developed in three stratigraphic sections (Landete, Talayuelas andHenarejos) of theMiddle–Late Permian Alcotas Formation in the SE Iberian Basin. The Alcotas Formation, of alluvialorigin, was deposited in semi-connected half-grabens developed during the early stages of the Permian–Triassicrifting stage that affected the Iberian Basin. The studied sections were located in two of these half-grabens, theHenarejos section being much closer to the basin boundary fault than the other two sections. The mineralogy andtexture of the carbonate precursor of paleosols in the three studied sections are not preserved because originalcarbonate is replaced by coarse crystals of dolomite and/or magnesite. Dolomite crystals are typically euhedral,displaying rhombohedral shapes and reddish luminescence, although in the Henarejos section dolomite displaysnon-planar boundaries and frequently saddle habit. Micas are deformed and adapted to dolomite crystals, which, inturn, are affected by stylolites, suggesting that dolomite precipitated before mechanical and chemical compaction.Carbon and oxygen isotopic compositions of dolomite fromthe three sections showdifferent values (δ13CVPDB meanvalues=−6.7‰,−5.5‰ and −7.5‰; δ18OVPDB mean values=−4.0‰; –5.6‰and−8.2‰, at Landete, Talayuelasand Henarejos sections, respectively). The 87Sr/86Sr ratios are similar in the three sections yielding values between0.71391 and 0.72213. The petrographic and geochemical features of dolomite in the three studied sections suggestprecipitation fromsimilar fluids and during shallow burial diagenesis. Assuming that theminimum temperature fordolomite precipitation in the Henarejos sectionwas 60 °C (as suggested by the presence of non-planar saddle habit),and that the dolomitizing fluid had similar δ18O values at the three localities, then dolomite in the Talayuelas andLandete sections precipitated at temperatures around 16 and 25 °C cooler, respectively. In addition, the δ18OVSMOWvalues of the water from which dolomite precipitated would have ranged between −0.3 and −2.9‰. Dolomite ispartially or totally replaced by non- to dark dull luminescent magnesite in the Landete and Talayuelas sections.Magnesite crystals are affected by stylolites, indicating that it precipitated before chemical compaction. The δ13Cmean values are −6.5 and −6.0‰ and the δ18OVPDB mean values are −6.7 and −7.8‰, in the Landete andTalayuelas sections, respectively. The 87Sr/86Sr ratios of magnesite are similar in both sections yielding valuesbetween 0.71258 and 0.72508. This suggests that they probably precipitated from similar fluids during progressiveburial and at higher temperatures than dolomites at the same sections. Assuming thatmagnesite precipitated fromafluid with similar δ18O values in both sections, then it had to precipitate at a temperature around 8 °C higher inTalayuelas than in the Landete section. Dolomitisation and magnesite precipitation probably occurred via reflux ofsaline to hypersaline brines from the overlying Mid-Late Triassic Muschelkalk and/or Keuper facies. Thetemperatures inferred for dolomite precipitation, however, are too high for shallow burial if a normal geothermalgradient is applied. Thus, it can be inferred that salinefluidswere heated as theyflowed through the syn-sedimentaryextensional faults that controlledMiddle Permian to Middle Triassic sedimentation; consequently fluidswould havebeen at higher temperatures near the Henarejos area, which was closer to the basin boundary fault than at theTalayuelas and Landete areas, whichwere situated further away. This contention is in agreement with recent studieswhich demonstrate that an important thermal event took place during Late Triassic–Early Jurassic times in theIberian Peninsula. PB Elsevier SN 0037-0738 YR 2011 FD 2011 LK https://hdl.handle.net/20.500.14352/42459 UL https://hdl.handle.net/20.500.14352/42459 LA eng DS Docta Complutense RD 10 abr 2025