Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

A Iannace,M Gasparrini,T Gabellone,S Mazzoli

doi:10.2516/ogst/2011166

Abstract

The Triassic pelagic carbonates of the Lagonegro Units from the southern Apennines fold and thrust belt host discordant bodies of dolostone. These rocks show textural features typical of zebra or saddle dolomites. In this contribution, the results of field observations, petrography and geochemical analyzes performed on samples from three different outcrops are presented.Field data indicate that the peculiar zebra-type rock and brecciated fabrics were controlled by regular bedding of the micritic, pelagic limestones. The dolomitization resulted in the replacement of the host limestones and subsequent void-filling precipitation of dolomite in a dilatational stress field.Homogenization temperatures (Th) are in the range 80-120°C, with a clear mode of 95 ÷ 10°C. When corrected for maximum pressure, they indicate an upper limit for dolomite formation around 110-115°C. Melting temperatures of ice (Tmi ) point to salinities in the range 2-6 wt% NaCl eq, with a mean of 4.2%. δ 13 C values overlap those of the Upper Triassic seawater, whereas the δ 18 O values are significantly depleted compared to the coeval seawater. 87 Sr/86 Sr values are higher with respect to Upper Triassic seawater, and partially overlap Middle-Upper Miocene values.The performed analyzes indicate a dolomitization process driven by warm fluids (110-115°C) with a salinity close to that of seawater and O-isotope ratios comprised between seawater and formation waters. It is suggested that the dolomitization was accomplished by formation waters squeezed out or from surrounding lithologies or from the underlying melange units. Integration with available thermal data on the collisional history of the belt suggests that the fluid-flow took place after maximum burial, in the early stages of exhumation, in the assumption of a fluid in thermal equilibrium with the host rocks.According to literature data, the dolomitization event was quite widespread. It can be argued that it was part of a major fluid-flux associated with fold and thrust belt development, which possibly affected the Apulian Platform carbonates located in the foreland. The latter, now buried below the nappe stack, hosts some of the major oil fields of continental Europe.

Highlights

Dolomitization is a replacement process requiring a large amount of Mg-rich fluids interacting with a limestone precursor
This study presents petrographic, geochemical and fluid inclusions data that, together with field evidences, strongly suggest that the dolostone bodies were formed during or after the Neogene regional tectonic shortening, due to the actions of overpressured warm fluids
Field evidences indicate that the rock fabric was controlled by regular bedding of the micritic, pelagic limestones; it was the result of replacement and void-filling precipitation in an environment characterized by high pore fluid pressure

Summary

Introduction

Dolomitization is a replacement process requiring a large amount of Mg-rich fluids interacting with a limestone precursor. Among the late-stage models of dolomitization, replacement by warm to hot solutions in the frontal parts of orogenic belts has been increasingly recognized in recent years (Qing and Mountjoy, 1994; Boni et al, 2000a; Swennen et al, 2003; Gasparrini et al, 2006a) This model was introduced to explain the peculiar vuggy dolomites, often displaying zebra textures and saddle-shaped crystals, associated with many MVT-type mineralizations of North America (Jackson and Beales, 1967; Anderson and Macqueen, 1982; Machel et al, 1996). Whatever the origin of the fluid-flow, the dolostone bodies are generally found in the undeformed foreland at the front of the fold and thrust belt and may be associated with deep-seated, sub-vertical normal faults (Taylor and Sibley, 1986; Hurley and Budros, 1990; Berger and Davies, 1999; Wendte et al, 2009; Lavoie and Chi, 2010; Shah et al, 2010), several examples of dolomites formed within the fold and thrust belts itself exist (Qing and Mountjoy, 1994; Swennen et al, 2003; Vandeginste et al, 2005)

Methods

Discussion

Conclusion