The influence of pre-existing deformation and alteration textures on rock strength, failure modes and shear strength parameters

This study uses the uniaxial compressive strength (UCS), the indirect tensile strength (ITS) and the point load tests (PLT) to determine the strength and deformation behavior of previously deformed and altered tonalite and anorthosite. In general, veined samples show higher strength because the vein material has both cohesive and adhesive properties while fractures have no cohesion, only frictional resistance. This implies that each rock category has to be treated independently and absolute strength predictions are inaccurate. Thus, the conversion factor k is a sample specific parameter and does not have a universal value. The ratio of UCS/ITS appears to be related to the rock strength and can be used to classify rocks based on their strength. The shear strength parameters, the friction angle and the cohesion, cannot be calculated for rocks with pre-existing planes of weakness. Reactivation is favoured only for planes oriented less than 20° to the maximum stress. For planes oriented between 20° and 50° to the maximum stress, failure occurs by a combination of reactivation and newly formed fractures, while for orientations above 50°, new shear fractures are favoured. This suggest that the Byerlee’s law of reactivation operates exclusively for planes oriented ≤10° to the maximum stress

The geological setting of the indium-rich Baal Gammon and Isabel Sn-Cu-Zn deposits in the Herberton Mineral Field, Queensland, Australia

Base metal mineralization at the Baal Gammon and Isabel deposits of the Herberton Mineral Field (HMF) is hosted in metamorphosed greywacke beds in the Hodgkinson Formation, which were intruded by granite, porphyry dykes and overlain by volcanic rocks of the Kennedy Igneous Association during the Carboniferous and Permian. The tin mineralization at the Baal Gammon deposit is hosted by a silicified, chlorite-altered, quartz-feldspar porphyry (UNA Porphyry). The tin mineralization at the Isabel deposit is in polymetallic veins hosting disseminated cassiterite. Polymetallic sulfides (Cu-Zn) and indium (In) mineralization at both deposits overprint the tin mineralization. Chalcopyrite, sphalerite, and stannite host indium in the polymetallic sulfide assemblage at both deposits. Based on overprinting relationships, the timing of tin mineralization is related to the magmatic activity at ca. 320 Ma, whereas the sulfide and indium mineralization are most likely associated with the emplacement of porphyry dykes at ca. 290 Ma. The overall magmatic activity in the HMF spreads between ca. 365 and 280 Ma, with peaks at ca. 337, 322, 305, and 285 Ma. The change from tin mineralization at ca. 320 Ma to sulfide and indium mineralization at ca. 290 Ma indicates a transition from a compressive to an extensional tectonic regime

Metamorphic diamond from the northeastern margin of Gondwana: Paradigm shifting implications for one of Earth’s largest orogens

We describe the first occurrence of diamond-facies ultrahigh pressure metamorphism along the Gondwana-Pacific margin of the Terra Australis Orogen. Metamorphic garnet grains from Ordovician metasediments along the Clarke River Fault in northeastern Queensland contain inclusions of diamond and quartz after coesite, as well as exsolution lamellae of rutile, apatite, amphibole, and silica. These features constrain minimum pressure-temperature conditions to >3.5 gigapascals and ~860°C, although peak pressure conditions may have exceeded 5 gigapascals. On the basis of these data, we interpret the Clarke River Fault to represent a Paleozoic suture zone and at least parts of the Terra Australis Orogen to have formed through classic Wilson cycle processes. The growth of the Terra Australis Orogen during the Paleozoic is largely attributed to accretionary style tectonics. These previously unknown findings indicate that the Terra Australis Orogen was not just a simple accretionary style orogen but rather a complex system with multiple tectonic styles operating in tandem including collisional tectonics

The Dugald River-type, shear zone hosted, Zn-Pb-Ag mineralisation, Mount Isa Inlier, Australia

The Dugald River Zn-Pb-Ag mine is situated in the Mount Isa Inlier, a globally significant base metal province. Zn-Pb deposits in the Mount Isa Inlier are stratabound with four main genetic models, including SEDEX-style, remobilised SEDEX, epigenetic and Broken Hill-type mineralisation applied to interpret their formation. We propose that the Zn-Pb-Ag mineralisation at Dugald River represents a unique, shear zone hosted deposit type that formed through a series of successive deformation events during the Paleoproterozoic Isan Orogeny that concentrated the mineralisation within the Dugald River Shear Zone during two main mineralising phases. The first phase of mineralisation occurred during regional D2 shortening, which is associated with the formation of large-scale F2 folds and a regionally penetrative S2 fabric. During this phase, progressive tightening of upright F2 folds resulted in several sets of secondary space accommodating quartz-carbonate veins that were progressively rotated into parallelism with the pervasive, steep, W-dipping S2 cleavage. The quartz-carbonate veins were coevally replaced by sulphides, which migrated to extensional sites (boudin necks and fold hinges) in tight folds. Thereby creating a sulphide-rich horizon within a developing high strain zone, which during D4 developed into the Dugald River Shear Zone. The second phase of mineralisation occurred during the regional D4 transpressional deformation event and resulted in significant metal enrichment and the current geometry of the ore bodies. The significant enrichment of the mineralisation during D4 resulted from further fold tightening within the high strain zone, which resulted in the attenuation and dismembering of folds and produced a transposed fabric (S4). The sulphide veins were transposed into parallelism with S4 forming sulphide-rich planar ore textures. Strain partitioning at the contact between the ductile deforming sulphide horizon and the brittle deforming slates resulted in the development of an anastomosing shear zone, known as the Dugald River Shear Zone. A right-handed releasing bend in the shear zone produced a dilational jog and a thick, high-grade ore body. The mobilisation of sulphides within the dilational jog involved fragmentation of sulphides and wall rock, brecciation, rotation and rolling of fragments, and the formation of durchbewegung texture

Magmatic Evolution and Rare Metal Mineralization in Mount El-Sibai Peralkaline Granites, Central Eastern Desert, Egypt: Insights from Whole-Rock Geochemistry and Mineral Chemistry Data

The Ediacaran peralkaline granites, which were emplaced during the post-collisional tectonic extensional stage, have a limited occurrence in the northern tip of the Nubian Shield. In this contribution, we present new mineralogical and geochemical data of Mount El-Sibai granites from the Central Eastern Desert of Egypt. The aim is to discuss their crystallization condition, tectonic setting, and petrogenesis as well as the magmatic evolution of their associated mineralization. Mount El-Sibai consists of alkali-feldspar granites (AFGs) as a main rock unit with scattered and small occurrences of alkali-amphibole granites (AAGs) at the periphery. The AAG contain columbite, nioboaeschynite, zircon and thorite as important rare metal-bearing minerals. Geochemically, both of AFG and AAG exhibit a highly evolved nature with a typical peralkaline composition (A/CNK = 0.82–0.97) and formed in within-plate anorogenic setting associated with crustal extension and/or rifting. They are enriched in some LILEs (Rb, K, and Th) and HFSEs (Ta, Pb, Zr, and Y), but strongly depleted in Ba, Sr, P and Ti with pronounced negative Eu anomalies (Eu/Eu* = 0.07–0.34), consistent with an A-type granite geochemical signature. The calculated TZrn (774–878 °C) temperatures indicate that the magma was significantly hot, promoting the saturation of zircon. The texture and chemistry of minerals suggest that they were crystallized directly from a granitic magma and were later subject to late- to post-magmatic fluids. Both granitic types were most likely generated through partial melting of a juvenile crustal source followed by magmatic fractionation. The lithospheric delamination is the main mechanism which causes uplifting of the asthenospheric melts and hence provides enough heat for crustal melting. The produced parent magma was subjected to prolonged fractional crystallization to produce the different types of Mount El-Sibai granites at different shallow crustal levels. During magma fractionation, the post-magmatic fluids (especially fluorine) contribute significantly to the formation of rare metal mineralization within Mount El-Sibai granites

Foliation intersection/inflection axes within porphyroblasts (FIAs): a review of advanced applications and significance

Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) provide a quantitative approach to microstructural and metamorphic analysis that can be used to integrate multiple deformation and metamorphic events and develop lengthy PT path histories during orogenesis. This paper reviews applications of FIAs for determining (1) porphyroblast rotation vs. nonrotation; (2) inclusion trail orientations and orogenic processes involving changes in the direction of bulk horizontal shortening; (3) in situ age determinations for multiple deformation episodes; (4) P-T-t-d paths of poly-metamorphism in an orogenic belt; and (5) constraining relative plate movement paths within ancient orogens. Quantitative structural and metamorphic data correlated directly to relative plate motions, are currently allowing the tectonic reconstruction of multiply deformed and metamorphosed terrains in orogens around the world

Porphyroblast rotation and strain localization: debate settled!: comment

This comment addresses two inferences used by Johnson (2009) to argue for porphyroblast rotation during bulk coaxial shortening. Firstly, he interprets that porphyroblast inclusion trails that are inclined (his figure 1) became inclined because the porphyroblast rotated during growth. He used orientation data from millipede microstructures from one hand sample and concluded on the basis of the total spread of inclusion trail orientations and porphyroblast axial ratios that porphyroblasts had rotated relative to one another during ductile deformation. Secondly, he presents a numerical model that indicates that asymmetrically shaped porphyroblasts rotated during coaxial deformation. However, I will show that the porphyroblasts did not rotate after they grew, in spite of localized shearing along the developing S3 that rotated visible matrix S2 in Johnson's figure 1

1750–1710 Ma deformation along the eastern margin of the North Australia Craton

In this contribution, we present field-based evidence that in the Mary Kathleen Domain from the Eastern Fold Belt of the Mount Isa Inlier the 1790–1750 Ma metasediments of the Leichhardt Superbasin were sheared and folded between 1750 and 1710 Ma. The Mary Kathleen Domain consists of a series of anastomosing high and low strain domains intruded syn- to late-tectonically by a series of 1740–1710 Ma felsic and mafic plutons and dykes. The timing relationships in the high strain domains are unclear due to mylonitisation and transposition but in the lower strain domains are well-preserved. The earliest deformation, D1, consists of a series of low angle truncation surfaces and shear zones that are folded around large scale N-S trending upright folds during D2. The D2 folds were refolded during D3 along NW to EW trending fold axes. An undeformed granitic pluton from the Mt Godkin area dated at ~1710 Ma that crosscuts the D2-D3 folds and the youngest detrital zircon population of ~1750 Ma constrains the timing of D1-D3 deformation between 1750 and 1710 Ma. Later, N-S (D4) and N-E (D5) trending brittle-ductile shears are interpreted to indicate a late Isan Orogeny overprint. The occurrence of contemporaneous folding in the Western Fold Belt suggests that between 1750 and 1710 Ma the eastern margin of the North Australia Craton was experiencing a significant deformation event that can be correlated with similarly aged deformation events from other parts of the North Australia Craton, from South Australia Craton and from East Antarctica. Following the assembly of most of the Nuna Supercontinent by 1.8 Ga, we postulate that the 1750–1710 Ma deformation and plutonism observed in Mount Isa Inlier is part of a series of worldwide orogenic events that contributed to the final configuration of the Nuna Supercontinent

The control of deformation partitioning and strain localization on porphyroblast behaviour in rocks and experiments

Multiple generations of sub-vertical and sub-horizontal foliations preserved as inclusion trails in garnet in mylonitic rocks from the hanging wall of the Main Central Thrust in the Himalayas indicate that these porphyroblasts did not rotate during thrusting. This result is predicated by (i) a consistent succession of 5 changes in FIA trend (foliation inflection/intersection axes in porphyroblasts) for samples where the orientation changes from porphyroblast cores to rims; (ii) sub-vertical and sub-horizontal foliations occur as inclusion trails around each of the 5 FIAs in the succession, which would not be the case if the garnet porphyroblasts rotated during subsequent phases of deformation as tectonism continued; (iii) a change in inclusion trail asymmetry immediately prior to the commencement of mylonitzation indicates top to the south thrusting only if the porphyroblasts had not rotated as they grew; (iv) the latter asymmetry matches truncated crenulation relics preserved within the mylonitic matrix foliation that indicate top to the south thrusting as the latter foliation formed. Partitioning of deformation into shortening and shearing components stops rotation of porphyroblasts during their growth and during following periods of ductile tectonism. This can be replicated via computer modelling by duplicating the crenulation-hinge-like coaxial environment in which porphyroblasts nucleate and grow before the strain intensifies. This was done using Drucker-Prager constitutive models with temperature-dependent strain softening behaviour and resulted in no porphyroblast rotation when followed by non-coaxial deformation no matter how intense. Furthermore, strain localization in the model containing competent objects of variable size, shape and orientation, produced no rotation during deformation involving components of shortening and shearing. These approaches to modelling mechanically resolve the sub-vertical/sub-horizontal foliations defined by inclusion trails and consistent FIA trend successions obtained from the Main Central Thrust rocks as well as in orogens elsewhere

Archaean crustal growth through successive partial melting events in an oceanic plateau-like setting in the Tanzania Craton

The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau-like geochemical signature at ~2800 Ma followed by diorite and tonalite–trondhjemite–granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high-K magmatism between 2700 and 2620 Ma. The εHf values of the detrital zircons range from +2.4 to -1.4 and change with time from radiogenic Hf pre-2700 Ma (98% positive εHf) to unradiogenic Hf post-2700 Ma (41% positive εHf). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and εHf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic-plateau-like environment