Rock‐buffered fluid‐rock interaction in deformed quartz‐rich turbidite sequences, eastern Australia

Abstract

Quartz vein 18O/16O ratios across a 500 km transect through the Lachlan fold belt of southeastern Australia are remarkably uniform (±1–1.5 permil) at both local (centimeter to meter) and regional (over 104 km2) scales. They define isotopic zones that correlate with the tectonic divisions of the Cambrian through Devonian (dominantly Ordovician) quartz‐rich turbidites determined by regional mapping. From west to east these divisions are (1) Stawell, δ18O=14.7±1.1 (47 samples); (2) Bendigo‐Ballarat, δ18O=17.5±1.3 (204 samples); (3) Melbourne, δ18O=19.0±1.6 (80 samples); (4) Tabberabbera, δ18O=16.3±1.9 (12 samples); and (5) Omeo, δ18O=14.4±1.0 (26 samples). Isotope profiles across zone boundaries, and intrazone fault zones particularly within the Bendigo‐Ballarat zone, show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones. δ18O values of veins are insensitive to relative age, type of vein, and immediate host rock lithologies. The δ18O values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid/rock ratios (water/rock ≪ 1). Limited D/H determinations on fluid inclusions fall mainly in the range −70 and −100, with one value as low as −140. These low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium‐depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths. The work suggests that vein formation is possible in regions where the integrated water/rock (w/r) ratio is very low (w/r ≪ 1) as long as a pervasive fluid phase is present. This fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters, and once a quasi steady state has been reached, the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs. Advective cycling of the fluid on this scale is by episodic dilatancy “pumping” in fracture networks associated with localized faulting.

Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major- and trace-element systematics

“…This observation is in agreement with a small-scale (ten centimetres to a few metres) migration of selected elements from wallrock to vein (compare Burkhard and Kerrich, 1988;Gray et al, 1991). This element transfer leading to vein formation could be accounted for by ionic diffusion processes.…”

New evidence for syntectonic fluid migration across the hinterland‐foreland transition of the Canadian Cordillera

“…For example, Marquer and Burkhard [1992] interpreted isotope depletions in shallow level (200°-300°C), carbonate hosted late stage veins and mylonites in the Swiss Alps as evidence that kilometer-scale interconnectivity between shear zones promoted fluid expulsion upward from crystalline basement. In contrast, other case studies infer that regional deformation by pressure solution and faulting need not be accompanied by extreme volume loss or a large time-integrated fluid flux [e.g., Gray et al, 1991;Cartwright et al, 1994;Kirschner and Kennedy, 2001]. On the basis of low isotopic variation between fault rocks and their adjacent protolith, Kirschner and Kennedy [2001] found no evidence for extensive fluid advection outside of very narrow shear zones in the Front Ranges of the Canadian Rockies.…”

“…On the basis of low isotopic variation between fault rocks and their adjacent protolith, Kirschner and Kennedy [2001] found no evidence for extensive fluid advection outside of very narrow shear zones in the Front Ranges of the Canadian Rockies. In a dramatic example from the Lachlan fold belt of southeastern Australia, Gray et al [1991] demonstrated that diffusion and advection over relatively short distances (tens to hundreds of meters) combined to promote the isotopic homogenization of quartz veins in thrust sheets and faults across 80,000 km 2 .…”

Evolution of palaeofluids at the Variscan thrust front in eastern Belgium

“…Oliver 1986Oliver , 1992Moore and Vrolijk 1992). However, several studies of synkinematic veins at the deformation front of mature basins indicate a closed system, where fluids are mainly derived from and buffered by the wall rock (Gray et al 1991;Marquer and Burkhard 1992). During burial, the carbon and oxygen isotopic ratios increase due to intense water-rock interaction Ayalon 1987, 1991).…”