Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss

The Twakputs Gneiss is a garnetiferous, K-feldspar megacrystic, biotite granite-granodiorite orthogneiss. It represents a major unit in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Metamorphic Province extending about 250 km between Riemvasmaak in South Africa and Grünau in southern Namibia. The Twakputs Gneiss occurs as foliation-parallel, sheet-like bodies tightly infolded together with granulite-facies paragneisses into which it intrudes along with a variety of other pre-tectonic granite and leucogranite orthogneisses. These rocks were subsequently intruded by late-tectonic garnet-leucogranites, granites and charnockites. The Twakputs Gneiss is a distinctive unit characterised by large ovoid to elongate megacrysts of twinned perthitic K-feldspar, set in a coarse-grained matrix of garnet, biotite, quartz and feldspar. It contains a penetrative foliation defined by the alignment of K-feldspars and streaks of biotite that developed during the main phase D2 of the Namaqua Orogeny (~1.2 to 1.1 Ga). The foliation and an accompanying elongation lineation are more intensely developed along lithological contacts, especially at the margins of the mega-scale F3 domes and basins that refold the regional fabrics. U-Pb zircon dating of the Twakputs Gneiss has yielded concordia ages of between ~1192 and 1208 Ma. Whole-rock geochemistry shows consistent major, trace and REE elemental trends, and thus reflect chemical variability from a single fractionating magma. The Twakputs Gneiss has a granitic to granodiorite composition and is strongly peraluminous. The geochemistry and the ubiquitous presence of garnet and pelitic xenoliths indicate an S-type granite protolith. The Twakputs Gneiss is the most voluminous and widespread member of the Eendoorn Suite which comprises seven textural variants of garnetiferous, K-feldspar-megacrystic granitoid orthogneiss of the same age.

Polyphase gold mineralisation at the Yaou deposit, French Guiana

The Yaou deposit, located in French Guiana within the Guiana Shield, is one of the most promising gold deposits of the regional Palaeoproterozoic greenstone belt. It displays numerous quartz monzodiorite bodies aligned along a sinistral shear zone where a five-deformation phases model is established at the camp scale. The ductile D1/2YA phase is responsible for the main penetrative foliation while the D3YA phase is related to shearing. An intrusive event is identified as being pre to syn-D3YA. The following phase D4YA represents a brittle quartz-carbonate veining set hosted preferentially within intrusive bodies and along the shear zone. A local D5YA brecciation event crosscuts the D4YA veins. Among this deformation history, two auriferous events (D3YA and D4YA) control the overall grade of the Yaou gold deposit. More specifically, most of the Au grade is associated with the main economic D4YA veining event, where the gold is visible and linked to Py4 within an ankerite/hematite rich alteration halo. At the microscopic scale, results of in situ analyses using LA-ICP-MS on pyrite show that metasediment-hosted Py0 is a primary source of submicroscopic gold having a low contribution to the total endowment. Py3 shows some gold content due to possible remobilisation of AuD0YA. Gold in Py4 is found as submicroscopic gold, as micro-inclusions and as infilling fractures in association with elements such as Te, Ag and Bi. Most contribution to the Au grade is from micro-inclusions and, to a lesser extent, from free and submicroscopic gold. The ore shoot locations are lithologically controlled for AuD0YA (metasedimentary unit-hosted), structurally controlled (shear zone-hosted) for AuD3YA and rheologically controlled for the AuD4YA (intrusion-hosted). The deposit is clearly polyphase both at the macroscopic and the microscopic scales, invisible gold is associated with As whereas visible gold is observed as inclusions in pyrite with high contents of Ag, Te and Bi. We define an early low-grade enrichment of AuD0YA to AuD3YA followed by a later high-grade event, AuD4YA supporting polyphase mineralisation processes. This study confirms that orogenic gold deposits can be formed by remobilisation and/or new gold inputs during multiple deformation, veining and hydrothermal events.

Reaction-enhanced ductile deformation during carbonation of serpentinized peridotite

<p>Carbonated serpentinites record carbon fluxes in subduction zones and are a possible natural analogue for carbon capture and storage via mineralization, but the processes by which the reaction of serpentinite to listvenite (magnesite-quartz rocks) goes to completion are not well understood. Large-scale hydration and carbonation of peridotite in the Oman Ophiolite produced massive listvenites, which have been drilled by the ICDP Oman Drilling Project (OmDP, site BT1) [1]. Here we report evidence for localized ductile deformation during serpentinite carbonation in core BT1B, based on observations from optical microscopy, cathodoluminescence microscopy, SEM, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in segments of the core that lack a brittle overprint after listvenite formation [2].</p><p>Microstructural analysis of the serpentinized peridotite protolith shows a range of microstructures common in serpentinite with local ductile deformation manifested by a shape and crystallographic preferred orientation and kinking of lizardite. Listvenites with ductile deformation microstructures contain a penetrative foliation due to a shape preferred alignment of magnesite spheroids and/or dendritic magnesite, bending around Cr-spinel porphyroclasts. Locally the foliation can be due to aligned dendritic overgrowths on euhedral magnesite grains. Magnesite grains have a weak but consistent crystallographic preferred orientation with the c-axis perpendicular to the foliation, and show high internal misorientations. Locally, the microcrystalline quartz matrix also shows a crystallographic preferred orientation with the c-axes preferentially oriented parallel to the foliation. Folding and ductile transposition of early magnesite veins indicates that carbonation initiated before the ductile deformation stage recorded in listvenites with penetrative foliation. On the other hand, dendritic magnesite overgrowths on folded veins and truncated vein tips suggest that folding likely occurred before complete carbonation, when some serpentine was still present. TEM analysis of magnesite revealed that subgrain boundaries oriented at high angle to the foliation can consist of nano-cracks sealed by inclusion-free magnesite precipitates. High dislocation densities are not evident suggesting that dislocation creep was minor or negligible, in agreement with very low predicted strain rates for magnesite dislocation creep at the low temperatures (100 – 200 °C) of serpentinite carbonation. This points to dissolution-precipitation, possibly in addition to grain boundary sliding, as the main mechanism for the formation of the shape preferred orientation of magnesite. The weak magnesite crystallographic preferred orientation may be explained by a combination of initial growth competition in an anisotropic (sheared) serpentine medium with subsequent preferred dissolution of smaller, less favorably oriented grains. We infer that transient lithostatic pore pressures during listvenite formation promoted ductile deformation in the reacting medium through grain boundary sliding accommodated by dilatant granular flow and dissolution-precipitation. Because the reaction product listvenite is stronger than the reacting mass, deformation may be preferentially partitioned in the reacting mass, locally enhancing transient fluid flow and, thus, the carbonation reaction progress.</p><p>[1] Kelemen et al., 2020. Site BT1: fluid and mass exchange on a subduction zone plate boundary. In: Proceedings of the Oman Drilling Project: College Station, TX</p><p>[2] Menzel et al., 2020, JGR Solid Earth 125(10)</p>

Deformation timing and strain in Neoproterozoic strata, Jebel Akhdar, northern Oman

<p>Neoproterozoic rocks exposed in the Jebel Akhdar massif of northern Oman preserve glaciogenic deposits associated with multiple Cryogenian glaciations. Although the depositional history of these rocks is well understood, the significance of post-depositional deformation is poorly constrained. In this study, we examine low-grade metasedimentary rocks exposed in the Ghubrah Bowl, an erosional window in the Jebel Akhdar massif, in order to quantify the 3D finite strain, understand deformation kinematics, and determine the timing of deformation/metamorphism.</p><p>In the Jebel Akhdar massif, the older Ghubrah (Sturtian glaciation) and younger Fiq (Marinoan glaciation) formations comprise a >1 km thick sequence of diamictite interbedded with sandstone, siltstone, conglomerate, volcanic rock, and minor carbonate. Diamictites contain abundant clasts of siltstone and sandstone, with lesser amounts of granite and metavolcanic rock in a fine-grained quartz + sericite ± chlorite matrix. Clasts range from granules to boulders. Harder clasts tend to be subangular and poorly aligned with low aspect ratios, whereas fine-grained rock clasts are well-aligned with large aspect ratios. Bedding generally dips to the NW, but is gently folded in accord with the overall structure of the Jebel Akhdar massif. A penetrative foliation strikes E-W and dips to the S. At some locations, a prominent elongation lineation/pencil structure occurs and plunges gently to moderately to the S.</p><p>R<sub>f</sub>/phi strain analysis in the diamictites reveals a range of 3D strain geometries (apparent flattening to apparent constriction) with strain ratios up to 2.8 in XZ sections. Strain is strongly partitioned, as clasts of igneous rock have low aspect ratios and are only weakly aligned. Penetrative strain in clast-supported sandstones is negligible (XZ ratios of <1.2). Outsized clasts of granite and sandstone are mantled by distinctive symmetric pressure shadows (double-duckbill structures) that include more recrystallized minerals than elsewhere in the diamictite. <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of sericite in pressure shadows yields ages as young as 90 Ma, which are interpreted as mixed ages containing an older detrital component and a younger fraction formed during growth. Deformation is associated with southward emplacement and loading by the Oman ophiolite & Hawasina Group sediments over the autochthonous sequence in the late Cretaceous.</p>

THE NATURE OF DUCTILE DEFORMATION IN THE PHYLLITE-QUARTZITE UNIT (EXTERNAL HELLENIDES)

This work describes the nature of ductile deformation in the Phyllite-Quartzite (PQ) unit in terms of structural evolution and spatial variation of finite strain and vorticity of flow. The PQ unit is affected by at least three ductile deformation (D1, 2, 3) phases. However, the D2 is the dominant phase resulting in the formation of a penetrative foliation (S2) which is by far the most common structural feature in all scales of observation. A stretching lineation (L2), which trends perpendicular to the structural grain of the belt, is well-developed within the S2 plane. Numerous kinematic criteria clearly indicate west (or south)-directed transport of the PQ unit during D2. This phase is also characterized by a systematic non-linear increase of strain ratio (Rxz) with proximity to the Basal thrust. Spatial variation of kinematic vorticity number reveals an increase of pure shear component of D2 deformation towards the middle structural levels of the unit. These results are used to discuss the validity of various geodynamic models related to the exhumation of the PQ unit.

Strain variation associated with the development of a major recumbent curvilinear fold in the Dalradian rocks of NW Achill Island, western Ireland

ABSTRACTThe regionally metamorphosed, Riphean–Cambrian Argyll Group Dalradian rocks of NW Achill Island, western Ireland are disposed in a large-scale, regionally west-facing, tight, recumbent F2 curvilinear fold, with which two ductile shear zones are associated. Clasts in conglomerates within the Dalradian sequence that are deformed by the shear zones preserve evidence for a constrictional overprint of earlier plane strain as the fold became curvilinear, while stretched clasts maintained a constant orientation as the hinge curvilinearity developed. During the constrictional overprint a crenulation fabric, S2b, overprinted a penetrative foliation, S2a, in the shear zones. The S2b has an orientation that varies systematically with that of the fold hinge. It is inferred that, although the S2b surfaces initiated as a dip-slip fabric, there was an increasing degree of strike slip on these surfaces as the fold hinge approached parallelism with the direction of tectonic transport. It is possible that many curvilinear folds have an early history involving plane strain, but that increasing constrictional strain is intrinsic to the later stages of their development.

"Sudbury Breccia" at Whitefish Falls, Ontario: evidence for an impact origin

Sudbury breccias are unusual clast–matrix rock bodies formed in abundance around the Sudbury Igneous Complex, the most obvious manifestation of a major impact event at Sudbury. At Whitefish Falls, ~70 km southwest of Sudbury, similar breccias consisting of clasts of argillite and amphibolite dyke enclosed in a fine-grained matrix of host rock are developed in metamorphosed argillites of the Huronian Supergroup. Pre-brecciation brittle textures in the host argillite and breccia clasts, such as layer-parallel foliation offset by cataclastic fractures, suggest that the host rock was entirely competent prior to brecciation. One composite penetrative foliation and its associated ductile folding were also formed in the argillite host prior to brecciation. Post-brecciation ductile deformation produced a regionally dominant east–west-trending foliation, and two late-stage folding events, and indicate a syn-Penokean age of brecciation. The breccias at Whitefish Falls are enriched in ferromagnesian minerals compared to adjacent, embayed and partially digested, host rock. Flow-foliated breccia matrices surround a highly rounded clast phase. These features are characteristic of impact-related pseudotachylyte, formed during extreme cataclasis and friction melting of the impacted host rock. We propose that these breccias formed by injection of a high-strain, pseudotachylytic melt, triggered by the Sudbury impact event, and focused along a blind superfault, coincident with a post-Penokean high-strain zone.

Deformational history of an outlier of metasedimentary rocks, Coast Plutonic Complex, British Columbia, Canada

The Khutzeymateen assemblage records a portion of the polyphase deformation experienced by rocks within the core of the Coast Plutonic Complex. This series of deformational events probably took place during Late Cretaceous to Early Eocene regional orogenic activity. The Khutzeymateen assemblage is dominated by metamorphosed graywackes and volcaniclastic material. The earliest recognizable deformation involves thrust faulting that juxtaposed rocks of the Khutzeymateen assemblage and Central Gneiss Complex. The next deformational event produced isoclinal folds (F1), a penetrative foliation (S1), and a strong mineral lineation (L1). Both F1 and L1 have a 340°, 15 °orientation. Peak metamorphism (P = 450 ± 50 MPa, T = 650° ± 50 °C) was synchronous with this isoclinal folding event. F1 folding was followed by a brittle chevron folding event (F2) with a 335°, 20° orientation. There is a strong lithologic control on the development of F2 minor folds, which are developed predominantly within regularly layered quartzo-feldspathic lithologies. Open F3 folds (065°, 35°) may have developed by buckling related to differential uplift on the Larch Creek Fault. Post-F3 faults and minor shear zones are developed mostly in the eastern half of the area. The different deformational styles associated with the different deformational events probably reflect variations in the position of this group of rocks with respect to the surface during a single orogenic episode.

Tectonic stacking of metamorphic zones in Cheticamp River area, Cape Breton Highlands, Nova Scotia

Metabasic and quartzo-feldspathic schists and gneisses of the Cheticamp River area can be subdivided into three north–south-trending metamorphic belts. The westernmost belt, the low-grade belt, has been metamorphosed to transitional greenschist–amphibolite facies with development of one penetrative foliation. The central medium-grade belt has been metamorphosed to amphibolite facies with development of two penetrative foliations. The eastern high-grade belt has been metamorphosed to amphibolite facies accompanied by development of strong gneissic segregation and nonanatectic migmatite leucosomes during formation of two penetrative foliations.Deformation has been inhomogeneous in each of these belts. Most rocks in each belt show evidence of post-tectonic porphyroblast growth. However, deformation continued after the metamorphic peak in localized zones. Very high strain occurred in many of these zones during and after the metamorphic peak, so that early formed fold axes were rotated towards the stretching direction. Locally, dynamically recrystallized mylonite has formed. The three belts were juxtaposed under waning metamorphic conditions by relative movement on these high-strain zones. The schist and gneiss complex thus consists of stacked metamorphic zones with the lowest grade rocks lying at the bottom of the stack. Stacking occurred by east to west thrusting of tight macroscopic ductile folds whose lower limbs have been sheared off.There are many lithologic similarities between the three metamorphic belts. The rocks may have been derived from the same protolith and have since been variably deformed and metamorphosed before later juxtaposition. There is no evidence for involvement of an older "basement complex" in the stacking tectonics in the studied area.The schist and gneiss complex has been intruded by post-tectonic plutonic rocks, and locally affected by post-metamorphic brittle deformation.