Abstract
Fluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450–550 °C and 0.3–0.6 GPa and below, commonly at conditions of ductile to brittle rock deformation. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th–Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe-Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90–70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36–30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25–12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12–5 Ma).
Highlights
1 Introduction Fluid-assisted fissure-vein and cleft formation occurred in the Alps in metamorphic rocks due to fluid-assisted embrittlement under prograde, peak to retrograde metamorphic conditions at or below 450–550 °C and 0.3–0.6 GPa
Minerals did grow following the initial fissure formation but continued to grow, to crystallize newly or to dissolve during subsequent deformation stages or other causes leading to chemical disequilibrium
6 Discussion The formation of open fissures and fissure monazite crystallization occurs in the upper crust at conditions at or below 450–550 °C and 0.3–0.6 GPa (e.g., Diamond & Tarantola, 2015; Heijboer et al, 2003a, 2006;Mullis, 1974, 1976a, 1976b, 1983, 1991, 1996, 2011; Mullis & Tarantola, 2015; Mullis et al, 1994; Poty, 1969; Poty et al, 2007, 2018; Rauchenstein-Martinek et al, 2016; Sharp et al, 2005), in association with regional scale deformation or localized faulting
Summary
Fluid-assisted fissure-vein and cleft formation occurred in the Alps in metamorphic rocks due to fluid-assisted embrittlement under prograde, peak to retrograde metamorphic conditions at or below 450–550 °C and 0.3–0.6 GPa (e.g., Diamond & Tarantola, 2015; Heijboer et al, 2003a, 2006; Mullis & Tarantola, 2015; Mullis, 1974, 1976a, 1976b, 1983, 1991, 1996, 2011; Mullis et al, 1994; Poty, 1969; Poty et al, 2007, 2018; 14 Page 2 of 25Rauchenstein-Martinek et al, 2016; Sharp et al, 2005). Interaction of fluid-filled clefts with the surrounding rock led to dissolution of minerals in the wall rock and mineral precipitation in the fissures (e.g., Heijboer et al, 2006; Mullis, 1976a, 1995, 2011; Mullis & De Capitani, 2000; Mullis & Wolf, 2013; Mullis et al, 1994; Sharp et al, 2005; Weisenberger & Bucher, 2011). Fissures forming in schists during prograde metamorphism became usually deformed until they formed foliation-parallel (e.g., Miron et al, 2013), more or less boudinaged veins. These veins provided important competence contrast, acting as an incipient point for retrograde formation of open fissures
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