The Rwenzori Mountains, a Paleoproterzoic crustal shear belt crossing the Albertine rift system

This contribution discusses the development of the Paleoproterozoic Buganda-Toro belt in the Rwenzori mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal scale nappe whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle inverse faulting. The Paleoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori mountains are situated and where the lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori mountain and that this stiff belt may form part of the reason why the Rwenzori mountains are relatively high within the rift. Keywords: East African Rift, Basement, Buganda Toro, Inverse Metamorphic Gradient, Microtectonics, Rwenzori mountain

Geochronology and geochemistry of the northern Scotia Sea: a revised interpretation of the North and West Scotia ridge junction

Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid – Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ±0.3Ma and indicates that magmatism of the Patagonian–Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading centerthat shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30–6Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early – mid Cretaceous in age (137–93Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea

Protracted metallogenic and magmatic evolution of the Kirazlı epithermal Au-Ag and porphyry Cu deposits, Biga Peninsula, NW Turkey: Evidence from zircon U-Pb, muscovite 40Ar/39Ar, and molybdenite Re-Os geochronology

The Kirazlı deposit is located at the center of the Biga Peninsula metallogenic province, in a geological setting characterized by an extensional tectonic environment. A NNW-SSE trending high-sulfidation (HS) orebody with a total reserve of 33.86 Mt @ 0.69 g/t Au and 9.42 g/t Ag lies beneath the Kirazlı Main zone. A porphyry Cu orebody hosted by Eocene intrusive and volcanic rocks has been intersected by drilling within its vicinity. The HS epithermal deposit is hosted by a partly silicified and brecciated Oligocene volcanic and volcaniclastic sequence consisting mainly of basaltic andesite lava flow and lithic/crystal tuff. Lithogeochemistry and zircon U-Pb radiometric ages allow us to distinguish three distinct high-K calc-alkaline magmatic events at ca. 41, 38, and 32 Ma, sourced by metasomatized mantle melts, which have interacted with the crust during their ascent. Porphyry Cu mineralization took place at 36.7 ± 0.4 Ma (muscovite 40 Ar/ 39 Ar age) with subsequent re-opening and base metal deposition. Crosscutting quartz-pyrite-molybdenite veins were emplaced at 33.6 ± 0.2 Ma (molybdenite Re-Os age), and followed by the HS epithermal Au-Ag event at ca. 31 Ma, based on a previous study. Our radiometric data indicate that the Kirazlı deposit has recorded a long-lasting Cenozoic magmatic and metallogenic evolution during about 10 Myr. Our study demonstrates that successive, independent, and overprinting, but genetically unrelated, HS epithermal precious metal, hydrothermal Mo, base metal, and porphyry Cu systems have been active at the same location during protracted extensional tectonics of the Biga Peninsula.</p

A technique to visualise the urethral meatus in difficult male catheterisations.

Goldschmidt 2019, Barcelona, Spain, 18-23 August 2019Constraining how the temperature of rocks changes with time is an important aspect of many geological studies. Geoscientists commonly address this problem by interpreting step-heating Ar-Ar data obtained from feldspars [e.g. 1 and therein] and increasingly more often by interpreting U-Pb data obtained from apatite [e.g. 2 and therein]. Reconstruction of thermal histories using these approaches is underpinned by the assumption that the redistribution of radiogenic Ar in feldspars and Pb in apatite over geological timescales is controlled by volume diffusion. However, is this assumption always valid? Here we revisit the mechanisms of Ar redistribution in famous gem-quality alkali feldpsar from Itrongay pegmatite by combining in situ Ar-Ar dating with cathodoluminescence imaging. Previous in situ Ar-Ar studies of Itrongay feldspar suggested that it has partially lost radiogenic Ar by diffusion [3, 4], supporting the underlying assumption of feldspar ArAr thermochronology. However, our results indicate that this feldspar records a protracted history of interaction with fluids between ~475 Ma (dates in the core) and ~180 Ma (dates at the rim), casting doubt on previous interpretations. Alongside, we have obtained in situ U-Pb dates of three apparently protogenetic apatite inclusions within the studied feldspar crystal. These yield older dates than feldpsar (~490- 535 Ma), and in contrast to feldspar seem to have been partially reset by diffusion, possibly prior to their entrapment. [1] Harrison and Lovera (2013) GSL Spec. Pub., 378, 91- 106; [2] Paul et al. (2018) GCA, 288, 275-300 [3] Flude et al. (2014) Geol. Soc. London Spec. Pub., 378, 265–275; [4] Arnaud and Kelley (1997) GCA, 61, 3227–3255.Science Foundation Irelan

Mesozoic arc magmatism along the southern Peruvian margin during Gondwana breakup and dispersal

A high-resolution U-Pb zircon geochronological study of plutonic units along the south Peruvian margin between 17 degrees and 18 degrees S allows the integration of the geochemical, geodynamic and tectonic evolution of this part of the Andean margin. This study focuses on the composite Jurassic-early Cretaceous Ilo Batholith that was emplaced along the southern Peruvian coast during two episodes of intrusive magmatism; a first period between 173 and 152 Ma (with a peak in magmatic activity between roughly 168 and 162 Ma) and a second period between 110 and 106 Ma. Emplacement of the Jurassic part of the composite Ilo Batholith shortly post-dated the accumulation of the volcanosedimentary succession it intruded (Chocolate formation), which allows to estimate a subsidence rate for this unit of similar to 3.5 km/Ma. The emplacement of the main peak of Jurassic plutonism of the Ilo Batholith was also closely coeval with widespread and repeated slumping (during deposition of the Cachios Formation) in the back-arc region, suggesting a common causal link between these phenomena, which is discussed in the context of an observed 100 km trenchward arc migration at similar to 175 Ma, and the relation with extensional tectonics that prevailed along the Central Andean margin during Pangaea break-up. (C) 2012 Elsevier B.V. All rights reserved

Detrital zircon fingerprint of the Proto-Andes: Evidence for a Neoproterozoic active margin?

Neoproterozoic Palaeogeographic reconstructions of Rodinia conventionally place the western (Proto-Andean) margin of Amazonia against the eastern (Appalachian) margin of Laurentia. Separation and formation of the Iapetus Ocean is generally considered to have occurred later at ~550 Ma. We examine the U-Pb detrital zircon "fingerprint" of autochthonous rocks from the northern and central segments of the Proto-Andean margin, which formed part of the western margin of Amazonia during the Late Neoproterozoic-Phanerozoic. The Proto-Andean margin is clearly the most feasible source region for most of the zircon grains, except for a 550-650 Ma sub-population, broadly age-equivalent to the Brasiliano/Pan-African Orogeny in eastern Amazonia. No obvious source for this detritus is known in the northern and central Andes. Derivation from eastern Amazonia is considered unlikely due to the stark paucity of detritus derived from the core of the Amazonian craton. Instead, we propose that a Late Neoproterozoic magmatic belt is buried beneath the present-day Andean belt or Amazon Basin, and was probably covered during the Eocene-Oligocene. If this inferred Neoproterozoic belt was an active margin, it would record the initiation of Proto-Andean subduction and imply at least partial separation of West Gondwana from its conjugate rift margin of eastern Laurentia prior to ca. 650 Ma. This separation may be linked to the ca. 770-680 Ma A-type magmatism found on eastern Laurentia in the southern Appalachians, and on the Proto-Andean margin in the Sierra Pampeanas and the Eastern Cordillera of Peru. © 2008 Elsevier B.V. All rights reserved

Permo-Triassic anatexis, continental rifting and the disassembly of western Pangaea

Crustal anatectites are frequently observed along ocean–continent active margins, although their origins are disputed with interpretations varying between rift-related and collisional. We report geochemical, isotopic and geochronological data that define an ~ 1500 km long belt of S-type meta-granites along the Andes of Colombia and Ecuador, which formed during 275–223 Ma. These are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon U–Pb dates ranging between 240 and 223 Ma. A model is presented which places these rocks within a compressive Permian arc setting that existed during the amalgamation of westernmost Pangaea. Anatexis and mafic intrusion during 240–223 Ma are interpreted to have occurred during continental rifting, which culminated in the formation of oceanic crust and initiated the break-up of western Pangaea. Compression during 275–240 Ma generated small volumes of crustal melting. Rifting during 240–225 Ma was characterized by basaltic underplating, the intrusion of tholeiitic basalts and a peak in crustal melting. Tholeiitic intrusions during 225–216 Ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust, and we assign this period to the onset of continental drift. Dissected ophiolitic sequences in northern Colombia yield zircon U–Pb dates of 216 Ma. The Permo-Triassic margin of Ecuador and Colombia exhibits close temporal, faunal and geochemical similarities with various crustal blocks that form the basement to parts of Mexico, and thus these may represent the relict conjugate margin to NW Gondwana. The magmatic record of the early disassembly of Pangaea spans ~ 20 Ma (240–216 Ma), and the duration of rifting and rift–drift transition is similar to that documented in Cretaceous–Tertiary rift settings such as the West Iberia–Newfoundland conjugate margins, and the Taupo–Lau–Havre System, where rifting and continental disassembly also occurred over periods lasting ~ 20 Ma

Mode and timing of terrane accretion in the forearc of the Andes in Ecuador

The volcanic basement of the Ecuadorian Western Cordillera (Pallatanga Formation and San Juan unit) is made up of mafic and ultramafic rocks that once formed an oceanic plateau. Radiometric ages from these rocks overlap with a hornblende 40Ar/39Ar plateau age of 88 ± 1.6 Ma obtained for oceanic plateau basement rocks of the Piñon Formation in coastal Ecuador, and with ca. 92–88 Ma ages reported for oceanic plateau sequences in the Caribbean and western Colombia. These results suggest that the oceanic plateau rocks of the Western Cordillera and flat forearc in Ecuador are derived from the Late Cretaceous Caribbean-Colombia oceanic plateau. Intraoceanic island-arc sequences (Rio Cala Group) overlie the plateau in the Western Cordillera and yield crystallization ages that range between ca. 85 and 72 Ma. The geochemistry and radiometric ages of island-arc lavas from the Rio Cala Group, combined with the age range and geochemistry of their turbiditic, volcaniclastic products, indicate that the arc was initiated by westward subduction beneath the Caribbean Plateau. They are coeval with island-arc rocks of coastal Ecuador (Las Orquideas, San Lorenzo, and Cayo Formations) and Colombia (Ricaurte Arc). These island-arc units may be related to the Late Cretaceous Great Arc of the Caribbean. Paleomagnetic analyses of volcanic rocks of the Piñon and San Lorenzo Formations of the southern external forearc show that they erupted at equatorial or low southern latitudes. The initial collision between South America and the Caribbean-Colombia oceanic plateau caused rock uplift and exhumation (&gt;1 km/m.y.) within the continental margin during the Late Cretaceous (ca. 75–65 Ma). Magmatism associated with the Campanian–early Maastrichtian Rio Cala Arc ceased during the Maastrichtian because the collision event blocked the subduction zone below the oceanic plateau. Paleomagnetic data from basement and sedimentary cover rocks in the coastal forearc reveal 20°–50° of clockwise rotation during the Campanian, which was synchronous with the collision of the oceanic plateau and arc sequence with South America. East-dipping subduction beneath the accreted oceanic plateau formed the latest Maastrichtian to early Paleogene (ca. 60 Ma) Silante volcanic arc, which was deposited in a terrestrial environment. Subsequently, Paleocene to Eocene volcanic rocks of the Macuchi unit were deposited, and these probably represent a continuation of the Silante arc. This submarine volcanism was coeval with the deposition of siliciclastic rocks of the Angamarca Group, which were mainly derived from the emerging Eastern Cordillera