Michael J. Rubenach

The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain

Abstract The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper- and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to c. 12 km. The total amount of extension accommodated by the detachment faults is of the order of 32.6 km corresponding to a β factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.

Metamorphic and metasomatic evolution of the Snake Creek Anticline, Eastern Succession, Mt Isa Inlier

The Snake Creek Anticline is a regional D2 structure, with the overturning of the western limb and the curvature of the axial trace from north to east interpreted as an effect of D3, which produced folds and crenulations with shallowly dipping axial surfaces. Low‐pressure/high‐temperature metamorphism which affected the Mesoproterozoic Soldiers Cap Group reached its peak with the growth of sillimanite during intense foliation development during D2. However, microstructural studies have revealed multiple porphyroblast growth stages, namely: (i) pre‐ to early D1, (ii) late D1 to early D2, (iii) late to post‐D2, (iv) syn‐D3 and (v) syn‐D4. The kyanite/andalusite, sillimanite, and sillimanite/K‐feldspar isograds are D2‐related, whereas the garnet and andalusite/staurolite are composite isograds representing multiple growth events. To explain the occurrence of kyanite partially replaced by andalusite, and the replacement of early growth andalusite by staurolite + muscovite, it is suggested that the pressure‐te...

Isograd pattern and regional low-pressure, high-temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east – west section through the Mt Isa Inlier

The southern Mt Isa Inlier is characterised by high-temperature/low-pressure metamorphism with metamorphic grade varying from greenschist to upper amphibolite facies. Metamorphism across the inlier occurred in a series of events during the Isan Orogeny (1610 – 1500 Ma), although some of these may be local in extent and related to various granitic intrusions. Localised events occurred prior to the Isan Orogeny during the emplacement of the Wonga and Sybella Batholiths, at ca 1740 Ma and 1672 Ma, respectively. The overall pattern is of a series of north – south amphibolite facies belts separated by zones of mainly greenschist facies. Isograd surfaces were initially shallowly dipping before being steepened by subsequent folding. Pelitic rocks change with increasing grade through chlorite, biotite, garnet, staurolite and/or andalusite, sillimanite, and sillimanite/K-feldspar zones. More magnesian-rich schists grew cordierite in place of garnet, with or without staurolite at higher grade. In the absence of pelitic rocks, regional metamorphic patterns in calc-silicate rocks were defined by the clinopyroxene isograd, which approximately corresponds to the sillimanite isograd. Estimated maximum P – T conditions for the highest grade zones are 400 – 500 MPa at >650°C with late (retrograde) kyanite + chlorite in some areas indicating anticlockwise P – T – t paths. Multiple stages of porphyroblast growth have been documented locally, suggesting a number of thermal events throughout the Isan Orogeny. The metamorphic peak occurred in most parts of the inlier between ca 1595 and 1580 Ma. Most workers correlate the metamorphic peak with regional D2. There is a broad-scale relationship between granites and amphibolite-facies metamorphic rocks, but with the exception of abundant migmatites in the sillimanite/K-feldspar zones, the granite bodies intruded before or after the metamorphic peak. A number of models have been proposed for metamorphism in the Isan Orogeny. Our preferred model implicates the intrusion of large volumes of tholeiitic magmas into the lower crust and advective transfer to the mid-crust via crustal melts that now comprise the migmatites of the sillimanite/K-feldspar zones. In addition, heat from older granites with relatively high values of heat-producing elements probably made a local contribution to the thermal budget.

Structural controls on syntectonic metasomatic tremolite and tremolite-plagioclase pods in the Molanite Valley, Mt. Isa, Australia

Abstract In the Molanite Valley west of Mt. Isa, tremolite-bearing metasomatic pods, showing sharp contacts with enclosing rocks, occur over a strike length of 7.4 km in the biotite-zone Bortala Formation. In relatively low-strain rocks of the western part of the valley, plagioclase—tremolite pods have replaced psammite or mica schist surrounding the terminations of buck quartz veins, whereas in intensively sheared rocks to the east, pods of medium to coarse-grained massive tremolite (commonly containing greater than 95% tremolite) have replaced marble, quartzite and psammite. Talc-chlorite schists, formed by dissolution of carbonates during shearing of marble, are associated with some of the massive tremolite pods. The direct relationship between structural domains and metasomatism suggests that migration of externally-derived fluids was controlled by fractures and the shear zone. The fluids, having migrated along fractures, interacted with wall rocks to produce plagioclase-tremolite pods. In the shear zone, buck quartz veins, some tremolite-bearing, show no obvious geometric relationship to massive tremolite pods. It is postulated that the metasomatic fluids fluxed through shear domains and penetrated progressive shortening domains to replace the host rocks and form the massive tremolite pods. Both styles of metasomatic rocks developed from late S 2 to early S 3 during a progressive E-W shortening event, which corresponds to the regional metamorphism. The metasomatic rocks represent only a small part of widespread metasomatism across the Mt. Isa Inlier.

Mafic‐ultramafic complexes of the Greenvale area, North Queensland: Devonian intrusions or precambrian metamorphics?

Abstract In the Greenvale area, mafic‐ultramafic complexes occur along the boundary between the Georgetown and Broken River Provinces, in a zone believed to have been a continental margin in the Precambrian and early Palaeozoic. This margin was the site of repeated metamorphism, deformation, and igneous activity until the Carboniferous; the resulting complication of the geology clearly exemplifies problems in the interpretation of the genesis of ultramafics and ophiolites in highly deformed regions. Following their emplacement, Precambrian layered mafic‐ultramafic complexes were interfolded and metamorphosed with schist and phyllite of the Georgetown Province. The Broken River Province was subsequently the site of lower Palaeozoic igneous activity contemporaneous with the initial deposition of quartz‐rich flysch intercalated with basaltic lavas. In the Gray Creek Complex this phase of igneous activity is represented by gabbro, tonalite, and trondhjemite, and basaltic and calcalkaline dykes which intruded ...

Structural Controls of Metasomatism on a Regional Scale

Examples of structurally controlled regional metasomatism span most metamorphic temperatures, pressures, and tectonic settings. Permeability is crucial, and may be intrinsic or the result of deformation processes such as micro-fracturing (e.g. grain boundary sliding), fracturing, faulting, foliation and shear zone development, and hydro-fracturing. Chromatographic theory has been developed to explain metasomatic zonation, and applies not only where mineral reaction and isotopic fronts formed normal to the fluid flow in a porous medium, but also where flow was parallel to fractures or lithological contacts. Metasomatic fronts may be sharp or broadened by diffusion and dispersion effects during fluid advection. In the upper crust, fluid advection involved in regional metasomatism, such as silicification, albitization, potassic alteration and dolomitization, is controlled mainly by intrinsic permeability, faults, fractures and microfractures in sedimentary basins, volcanic sequences and some granites. For skarn formation, fluid advection is controlled by interface-coupled dissolution-precipitation (“reaction-controlled permeability”) in addition to fractures and microfractures. Cross-layer diffusion in addition to layer-parallel advection is probably important in the formation of skarns along lithological contacts. Although subject to some debate, many workers have provided mineralogical and isotopic evidence for the existence of high time-integrated fluid fluxes and fluid flow directions in many thermal aureoles and regional metamorphic belts. Studies of quartz veins and vein selvages in metamorphic rocks have provided examples both of local derivation of veins and derivation of silica and metasomatic changes in the selvages during advection of external fluids. Also subject to debate is whether metamorphic fluids in the middle crust are solely derived from devolatilization reactions at depth or whether such processes as dilatancy pumping or syn-metamorphic intrusion can lead to lateral or downward fluid advection. Regional metasomatism occurs in many Proterozoic metamorphic belts, but the Mount Isa Inlier is probably the best natural laboratory regarding structural controls. It exhibits enormous strain heterogeneity at all scales that led to fluid channeling, diverse reactive rocks, episodic metamorphic and intrusive events spanning at least 250 Ma, and abundant sources of reactive saline and hypersaline fluids. Examples of widespread metasomatism in the Mount Isa Inlier include Na-Ca alteration associated with extensive breccia development in calcsilicate rocks, albitization of schists and metapsammitic rocks, and the formation in high-strain zones of tremolite pods, quartz-chlorite rocks, and unusual cordierite-rich rocks.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

High radiogenic heat-producing granites and metamorphism - an example from the western Mount Isa inlier, Australia: comment and reply

[Extract] We welcome the paper of McLaren et al. (1999) because it makes an important contribution to our understanding of the source of thermal anomalies in low-pressure metamorphic belts. However, we argue that high radiogenic heat–producing granites such as the Sybella batholith, although locally important in the overall thermal budget, were not the primary cause of low-pressure metamorphism in the Mount Isa inlier, and that synmetamorphic intrusions were a significant additional factor.

The Role of Halogens During Regional and Contact Metamorphism

Halogens are important elements for a range of geological processes during metamorphism from stabilizing mineral phases to being important ligands for mass transfer. Halogens are highly incompatible in most minerals, which makes it difficult to unravel their presence in the past. Minerals useful for understanding halogen behaviour during metamorphism include: scapolite, apatite, titanite, biotite, and amphibole. However, their ability to incorporate halogens depends on parameters such as bulk rock composition, fluid properties, and water-rock ratios. Comprehensive studies of halogens in regional metamorphic rocks and minerals, such as the Clearwater Region, Idaho, USA or the Mary Kathleen Fold Belt, Mt Isa Inlier, Australia, show that halogen contents are highly variable on a bulk rock- and rock layer-scale, reflecting protolith variations. Where low fluid-rock ratios occurred during regional metamorphism, pre-exisiting variations in halogen compositions and ratios across individual layers were not eliminated, resulting in large differences between halogen concentrations on a mineral- and rock-layer scale. Research on F and Cl in apatite in siliceous marbles from five classic aureoles highlights the use of this mineral regarding rock or fluid buffering, and in establishing fluid sources. Chlorine enrichment in biotite and amphibole, associated with regional albitization observed in Cloncurry, Australia or the Bamble Sector Norway, demonstrate advection of saline fluids during albitization and K-feldspar metasomatism that occur in association with regional mineralization. Chlorine-bearing fluids are capable of mobilizing large amounts of metals during large-scale metamorphism on a regional, whole rock, and mineral scale. Consequently, fluid flow could be an essential prerequisite to actively discharge metals from the metamorphic rocks. Recent analytical advancements allow for more routine analyses of halogen contents in minerals and fluid inclusions. For instance, in situ LA-ICP-MS analyses of Cl and Br allow for the reconstruction of the interaction of halogen-bearing fluids with crustal rocks in complex geological settings that have undergone multiple hydrothermal events. In such cases, scapolite can be used as an archive for fluid properties during metamorphism. For example, within the Mount Isa Inlier, it was found that the fluids, which interacted with calc-silicates in the Mary Kathleen Fold Belt, were of bittern brine derivation contrasting with the Cloncurry Region, where the fluids show evidence of dissolved halite. Magmatic fluid interaction with calc-silicate rocks was found to be localized.

The role of transient pressure gradients in mid-crustal advectivemass transfer

Abstract Abrupt pore pressure changes accompanying faulting can compete with, or locally predominate over, thermal gradients, in their capacity to transfer mass in deep hydrothermal systems. Differential mass transfer at 1- to >1000-m scales in the mesothermal Cloncurry Fe oxide-Cu Au district of northern Australia was apparently controlled by pore pressure cycling around dilatant parts of shear and fault zones. Geochemical reaction modelling of pore pressure drops in fluids initially equilibrated with altered wallrocks in the Cloncurry district produces model vein assemblages that mimic those observed in the field, whereas temperature-drop scenarios fail to replicate these veins.