Timing of post-depositional events in the Burano Formation of the Secchia valley (Upper Triassic, Northern Apennines), clues from gypsum-anhydrite transitions and carbonate metasomatism

The Burano Evaporite Formation from the Secchia River Valley is an up to 22 00 m-thick sequence composed of meter-to decameter-scale interbeds of gypsu m-anhydrite and dolostones with minor halite. The deposit has been affected by a complex array of post-depositional modifications, thermal events and large-scale evaporite dissolution, preventing a satisfactory reconstruction of the environment of deposition. The modifications are intense because th ese rocks were the main decollement horizon during the formation of the Nor thern Apennines chain. The carbonate rocks are massive and-or laminated dolomitic mudstone, wackes tone, oolitic packstones and oolitic, peloidal, bioclastic grainstones, whi ch commonly appear as mega-boudins within a sulfate groundmass. The dolosto nes (delta O-18 = -5.7 to -3.7 parts per thousand; delta C-13 = +1.3 to +3. 0 parts per thousand; PDB) have been affected by Mg-metasomatic replacement by magnesite (delta O-18 = -14.0 to -2.6 parts per thousand; delta C-13 = -2.6 to +1.4 parts per thousand; PDB) induced by hydrothermal circulation. Total homogenization temperatures of fluid inclusions in hydrothermal magne site range from 275 to 310 degreesC. The anhydrite rocks are characterized by flow structures such as centimeter -scale pseudo-lamination composed of aligned prismatic crystals with transp osed isoclinal folds outlined by dolostones fragments. Homogenization tempe ratures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305 degreesC (Emilia) and from 230 to 315 degreesC (Tuscany). The gypsum rocks are composed of xenotopic irregular cloudy crystals and, m ore rarely, by centimeter-scale idiotopic crystals showing the same structu res as the anhydrite rocks. The origin of the gypsum rocks is due to late a lteration of anhydrite by migration of sharp hydration fronts. The hydratio n is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro-relies and authigenic quartz crystals which in clude anhydrite. The role of the Burano Evaporites during the Apennines tectogenesis can be depicted as follows: (a) prevalent deposition of gypsum in the Upper Triass ic; (b) gypsum dehydration at burial conditions to form anhydrite (Cretaceo us?); (c) syn-tectonic flow of anhydrite rocks, brecciation of dolostones; syn-tectonic growth stage of quartz euhedra at deep burial conditions possi bly related to the development of the Oligocene-Miocene greenschist facies Apuane metamorphic complex; (d) hydrothermal deposition of sparry magnesite and partial Mg-metasomatic replacement of dolostones by magnesite; (e) sub -surface dissolution of halite to form thick matrix-supported residual capr ock-like anhydrite mega-breccias; (f) complete gypsification of anhydrite a t sub-surface conditions; and (g) evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of mos t clasts ("Calcare cavernoso"). (C) 2001 Elsevier Science B.V. All rights r eserved.