Retrograde P–T evolution and high temperature–low pressure fluid circulation in relation to late Hercynian intrusions: a mineralogical and fluid inclusion study of the Charroux‐Civray plutonic complex (north‐western Massif Central, France)

Abstract

The calc‐alkaline plutonic complex from Charroux‐Civray (north‐western part of the French Massif Central) displays multiphase hydrothermal alteration. Plutonic rocks, as well as early retrograde Ca–Al silicate assemblages, which have crystallized during cooling and uplifting of the plutonic series, are affected by multiphase chlorite–phengite–illite–carbonate alteration linked to intense pervasive fluid circulation through microfractures. The petrographic study of alteration sequences and their associated fluid inclusions in microfissures of the plutonic rocks, as well as in mineral fillings of the veins, yields a reconstruction of the P–T–X evolution of the Hercynian basement after the crystallization of the main calc‐alkaline plutonic bodies. This reconstruction covers the uplift of the basement to its exposure and the subsequent burial by Mesozoic sediments. Cooling of the calc‐alkaline plutonic series started at solidus temperatures (∼650°C), at a pressure of about 4 kbar (1 bar = 105 N m−2), as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data. Cooling continued under slightly decreasing pressure during uplift down to 2–3 kbar at 200–280°C (prehnite–pumpellyite paragenesis). Then, a hot geothermal circulation of CO2‐bearing fluids was induced within the calc‐alkaline rocks leading to the formation of greisen‐like mineralizations. During this stage, temperatures around 400–450°C were still high for the inferred depths (∼2 kbar). They imply abnormal heat flows and thermal gradients of 60–80°C km−1. The hypothesis of the existence of one large or a succession of smaller peraluminous plutons at depth, supported by geophysical data, suggests that localized heat flows were linked to concealed leucogranite intrusions. As uplift continued, greisen mineralization was subsequently affected by the chlorite–phengite–dolomite assemblage, correlated with aqueous and nitrogen‐bearing fluid circulations in the temperature range of 400–450°C. In a later stage, a continuous temperature decrease at constant pressure (∼0.5 kbar) led to the alteration of the dolomite–illite–chlorite type in the 130–250°C temperature range.

Paleo water–rock interaction determined through isotopic tracing (Sr, O, C, U) in fracture-fill minerals: Evidence from the Vienne granitoids (France)

“…Following crystallization, but before complete cooling, the plutonic bodies were percolated by hydrothermal circulations (Freiberger et al, 2001). This is indicated by the pervasive alteration within the granitic matrix, by the fracture fillings and the alteration of the minerals at the fracture walls, and by the fluid inclusions trapped in the newly formed minerals and healed microfractures in the quartz crystals from the enclosing granitoids.…”

“…This is indicated by the pervasive alteration within the granitic matrix, by the fracture fillings and the alteration of the minerals at the fracture walls, and by the fluid inclusions trapped in the newly formed minerals and healed microfractures in the quartz crystals from the enclosing granitoids. The early propylitic alteration was pervasive and does not seem to have produced any major redistribution of the main chemical components in the granitoids (Freiberger et al, 2001). It resulted in the formation of epidote, a chloritization of the femic minerals, an illitization of the feldspars, and carbonation; the sealed fractures also contain hematite.…”

“…Mineral identification in the fracture-fills from the granitoids was performed by optical microscopy, SEM, and X-ray diffraction (Cathelineau et al, 1997;Freiberger et al, 2001). The different mineral assemblages wee classified into three types named after the main mineral identified in the sample, illustrated in the Fig.…”

“…The pressure-temperature evolution of the Vienne granitoids, reconstructed using evidence from fluid inclusions and mineral assemblages (Cathelineau et al, 1997;Freiberger et al, 2001;Boiron et al, 2002), indicates two major fluid events responsible for most of the vein fillings. They are related to the two main geodynamic events that characterize this region: (1) the complex hydrothermal Hercynian cycle s.l.…”

“…Textural and fluid-inclusion data show that most of the fracture-fill minerals crystallized during the geothermal system activities (Bath et al, 2000;Freiberger et al, 2001;Boiron et al, 2002;Fourcade et al, 2002). Small crystals of adularia, barite and pyrite (in places framboidal) are commonly associated with Fe-hydroxides precipitated within finely crystallized clay minerals (Freiberger et al, 2001;Boiron et al, 2002). These small-sizes, featureless, late crystallizations are typical of neoformation and are thought to be related to the youngest groundwater flows.…”