Fluid source and pressure–temperature conditions of high-salinity fluids in syn-tectonic veins from the Northeastern Apuan Alps (Northern Apennines, Italy) Chiara Montomoli a, * ,1 , Giovanni Ruggieri b,2 , Rodolfo Carosi a,1 , Andrea Dini b,3 , Marianna Genovesi a,1 a Universita ` di Pisa, Dipartimento di Scienze della Terra, Via S. Maria 53, 56126 Pisa, Italy b Istituto di Geoscienze e Georisorse—CNR, Area della Ricerca, Via Moruzzi 1, 56124 Pisa, Italy Received 12 June 2004; received in revised form 3 January 2005; accepted 28 January 2005 Available online 2 August 2005 Abstract Structural studies on syn-tectonic veins cropping out in the northeastern sector of the Apuan Alps metamorphic complex (North- ern Apennines, Italy) revealed two sets of veins: (1) type A fibrous veins within the ‘‘Scisti sericitici Formation’’, related to a late-D1 tectonic phase; (2) B-veins, within the ‘‘Diaspri Formation’’ that developed between late-D1 and D2 tectonic phases. The mineral- ogy of the A (quartz, chlorite, hematite with minor amounts of apatite, allanite-(Ce), thorite and synchysite) and B-veins (quartz only) reflects the mineralogical composition of the host-rocks: quartz, chlorite, white mica and accessory minerals (apatite, zircon, titanite) occur in the ‘‘Scisti sericitici Formation’’, while quartz is the main phase in the ‘‘Diaspri Formation’’. The study of fluid inclusions trapped in quartz crystals of the syn-tectonic veins shows that the inclusions are always oversaturated in NaCl at room temperature and their salinities range from 29.5 to 37 wt.% NaCl equiv. The isochore of the earliest trapped fluid inclusions (type IA; primary inclusions in fiber quartz of A veins), coupled with mineralogical geothermometric data, constrains the pressure–tem- perature conditions to around 325–300 MPa and 370–380 °C during the late-D1 phase. Subsequent trapping of inclusion types IIA, IIB, IIIA, IIIB and VB in the two vein sets probably occurred during a pressure–temperature decrease (down to 220–245 MPa and 260–270 °C) at a lithostatic thermal gradient of 30 °C/km. Type IVB inclusions, on the other hand, were probably trapped at lower pressure (between lithostatic and hydrostatic conditions) during a transient pressure drop resulting from fault-valve action. SEM/ EDS analyses on salts precipitated within opened inclusions confirmed the NaCl-rich compositions of the trapped fluids and also revealed the presence of minor amounts of Ca, K and Mn in the salts. A fluid circulation model, based on mineralogical and fluid inclusion data, was proposed for the high-salinity fluids found in the syn-tectonic veins: the high-salinity and NaCl-rich nature of the trapped fluids suggest that the original metamorphic fluids interacted with evaporite levels present at the base of the Tuscan Nappe, overlying Apuan Alps metamorphic Units. These fluids then infiltrated downward, possibly through shear zones crosscutting the stratigraphic sequence, into the syn-tectonic veins and interacted with the host-rocks to produce a local element mobilization and their precipitation in the vein minerals; as a result the latter reflect the mineralogy of the host-rocks. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Syn-tectonic veins; Fluid inclusions; Pressure–temperature conditions; Apuan Alps 1. Introduction Syn-tectonic veins are widespread in deformed rocks that have undergone various degrees of metamorphism during orogenic events. Their occurrence is indicative 1474-7065/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2005.01.003 * Corresponding author. Fax: +39 50 500932. E-mail addresses: montomoli@dst.unipi.it (C. Montomoli), rug- gieri@igg.cnr.it (G. Ruggieri), carosi@dst.unipi.it (R. Carosi), a.dini@ igg.cnr.it (A. Dini), genovesi.m@tin.it (M. Genovesi). 1 Fax: +39 050 22125800. 2 Fax: +39 050 3152323. 3 Fax: +39 050 3152360. www.elsevier.com/locate/pce Physics and Chemistry of the Earth 30 (2005) 1005–1019