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

The temporal evolution of the Mitu group, south-east Peru – first U-Pb age data

The Eastern Cordillera of southern Peru formed along a crustal zone that has been active as part of the western Gondwana margin since the middle Paleozoic. The present study investigates the Mitu Group of south-east Peru in the area of Abancay-Cusco-Sicuani-Titicaca. This unit comprises continental clastic sediments deposited in syn-sedimentary basins during an extensional period in Permo-Triassic times and has not benefitted from a thorough geochemical-geochronological investigation so far. One of the main reasons for this lack of data is a complex structure of the graben system, tectonically complicated by compressional inversion of the extensional basins during Andean orogeny. Due to dominating coarse-grained clastics, the Mitu Group is devoid of fossils and its age is only poorly bracketed to be Permo -Triassic based on its stratigraphic relation to the underlying Copacabana and overlying Pucara groups. The upper levels of the Copacabana have been constrained by palynology to the Artinskian (Doubinger and Marocco, 1981). However, a hiatus may be observed between the Copacabana and the Mitu groups in most places, rendering the age estimate of the basal Mitu imprecise. The Pucara Group, regarded by Rosas et al. (2007) as thermal sag after Mitu extension, is attributed to the late Triassic - early Jurassic on the basis of ammonite fossils and U-Pb zircon ages from ash beds (Schaltegger et al., 2008). The aim of this study is to provide more accurate and precise age constraints for the age and duration of the Mitu Group by using U-Pb geochronology of volcanic zircon in rhyolitic lavas, and of detrital zircon in clastic sediments. For andesitic volcanic lithologies, age approximations will be obtained by Ar-Ar techniques applied to amphibole and groundmass samples. Field data were obtained from a long and apparently complete section through the Mitu, situated 120km SE of Cusco near the city of Sicuani. This section consists of typical Mitu deposits; continental red beds, breccias and andesitic lavas. However, a zircon-bearing rhyolitic lava at the bottom gives us the opportunity to date the start of Mitu sedimentation by U-Pb ID-TIMS; this analysis will provide a precise age for the base of the Mitu group for the first time. In the Sicuani area the Mitu unconformably overlies the Ambo group, suggesting that the entire Copacabana is missing. Laser-ablation ICP-MS U-Pb data of detrial zircons from a sandstone just below the unconformity indicate a maximum age of latest Carboniferous (303Ma) for the underlying Ambo group. This maximum age overlaps with the palynological age of the lower Copacabana (Azcuy et al., 2002), raising the question whether the Ambo and Copacabana are truly diachronous or just coeval units of different sedimentary facies associations. In another section, 100km W of Cusco, near the city of Abancay, we found Mitu sediments overlying the Copacabana Group. Here the Copacabana contains well preserved plant fossils of the lycopsids family also found elsewhere in Peru and Bolivia. Lack of acidic volcanism during Mitu extension in this region prevents from dating of lavas using the U-Pb method. The detrital zircon population in a sandstone in the lowermost part of the Mitu was analysed for U-Pb ages, using LA-ICP-MS techniques. The youngest zircons in the population are around 235 Ma hence providing a maximum age for the onset of Mitu group sedimentation. The Artinskian age for the upper Copacabana from Doubinger and Marocco (1981) has also been obtained from the Abancay region, establishing a hiatus of some 50 Myrs between the two units. The Mitu Group is intruded by a 220 Ma granite body (Lipa and Saraiva, 2008) indicating significant burial of the sediments at this time. 500km SE of Cusco, on the Bolivian shores of lake Titicaca, the Ambo Group features plant fossils of the Lycopsids family like those found in the Copacabana near Abancay. Our detrital zircon LA-ICPMS study on a quartz arenite just below the fossils indicates a maximum U-Pb age of 343Ma. However a zircon-bearing ash bed will allow for more precise calibration of the fossil age by ID-TIMS techniques. The zircon U-Pb data will provide a test whether the Copacabana and the Ambo group are indeed diachronous or just lateral variations of a sedimentary system

Numerical Modelling of Radiogenic Ingrowth and Diffusion of Pb in Apatite Inclusions with Variable Shape and U-Th Zonation

The fundamental premise of apatite U-Th-Pb thermochronology is that radiogenic Pb is redistributed by volume diffusion. In practice, it is often additionally assumed that crystals (1) lose radiogenic Pb to an infinite reservoir, (2) have a simple geometry and (3) are chemically homogeneous. Here we explore the significance of the latter three assumptions by numerical modelling of Pb radiogenic ingrowth and diffusion in apatite inclusions within other minerals. Our results indicate that the host minerals are likely to hamper diffusive Pb loss from the apatite inclusions by limiting the Pb flux across their boundaries, and thus the thermal histories that are reconstructed assuming a fully open boundary may be significantly inaccurate, precluding a meaningful interpretation. We also find that when apatite boundaries are flux-limited, heterogeneities in U and Th concertation within apatite have subordinate effect on bulk-grain U-Th-Pb dates and can cause intra-grain U-Th-Pb dates to increase towards the boundaries. Finally, we show that it is important to correctly account for crystal geometry when modelling intra-grain U-Th-Pb dates. We suggest that the effect of surrounding minerals on diffusive Pb loss from apatite (and loss of other radiogenic isotopes from other minerals) should be examined more closely in future research

Constraining the age of the Mitu Group, South-East Peru: U-Pb ages of detrital and igneous zircons

The present study investigates the Mitu Group of south-east Peru (13-16°S), which consists of continental clastic sedimentary rocks and interbedded basaltic to andesitic lavas. There is a paucity of geochemical and geochronological data from the Mitu Group, and the interpretation of its evolution is complicated by i) rapid changes in fault structure along-strike of the graben system, and ii) inversion during Andean orogenesis. Due to dominating coarsegrained clastics, the Mitu Group is devoid of fossils and its age is poorly bracketed to the Permo-Triassic, based on its stratigraphic relationships with the underlying Copacabana and overlying Pucará groups. The upper strata of the Copacabana Group have been constrained by palynology to the Artinskian, while marine fossils at the base of the Pucará Group indicate a Norian age. The Pucará Group is only present in northern Peru, whereas the Mitu Group has an erosional contact with overlying Cretaceous sandstones in the study area. Preliminary data suggest that the lower Mitu Group is middle Triassic, leaving a significant hiatus between the Copacabana and Mitu groups

Inclusions of Amorphous and Crystalline SiO 2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids

Experimental studies increasingly often report low-temperature (200−800 °C) and low-pressure (0.05−3 kbar) hydrosilicate fluids with >40 wt.% of SiO2 and >10 wt.% of H2O. Compositionally similar fluids were long suggested to potentially exist in natural systems such as pegmatites and hydrothermal veins. However, they are rarely invoked in recent petrogenetic models, perhaps because of the scarcity of direct evidence for their natural occurrence. Here we review such evidence from previous works and add to this by documenting inclusions of hydrosilicate fluids in quartz and feldspar from Itrongay. The latter comprise opal-A, opal-CT, moganite and quartz inclusions that frequently contain H2O and have negative crystal shapes. They coexist with inclusions of CO2- and H2O-rich fluids and complex polycrystalline inclusions containing chlorides, sulphates, carbonates, arsenates, oxides, hydroxides and silicates, which we interpret as remnants of saline liquids. Collectively, previous studies and our new results indicate that hydrosilicate fluids may be common in the Earth’s crust, although their tendency to transform into quartz upon cooling and exhumation renders them difficult to recognise. These data warrant more comprehensive research into the nature of such hydrosilicate fluids and their distribution across a wide range of pressure and temperature conditions and geological systems

Testing the applicability of morphometric characterisation in discordant catchments to ancient landscapes: A case study from southern Africa

The ancient landscapes south of the Great Escarpment in southern Africa preserve large-scale geomorphological features despite their antiquity. This study applies and evaluates morphometric indices (such as hypsometry, long profile analysis, stream gradient index, and linear/areal catchment characteristics) to the Gouritz catchment, a large discordant catchment in the Western Cape. Spatial variation of morphometric indices were assessed across catchment (trunk rivers) and subcatchment scales. The hypsometric curve of the catchment is sinusoidal, and a range of curve profiles are evident at subcatchment scale. Hypsometric integrals do not correlate to catchment properties such as area, circularity, relief, and dissection; and stream length gradients do not follow expected patterns, with the highest values seen in the mid-catchment areas. Rock type variation is interpreted to be the key control on morphometric indices within the Gouritz catchment, especially hypsometry and stream length gradient. External controls, such as tectonics and climate, were likely diminished because of the long duration of catchment development in this location. While morphometric indices can be a useful procedure in the evaluation of landscape evolution, this study shows that care must be taken in the application of morphometric indices to constrain tectonic or climatic variation in ancient landscapes because of inherited tectonic structures and signal shredding. More widely, we consider that ancient landscapes offer a valuable insight into long-term environmental change, but refinements to geomorphometric approaches are needed

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

Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the ⁴⁰Ar/³⁹Ar Method: A Review

The 40Ar/39Ar method applied to K-feldspars and muscovite has been often used to construct continuous thermal history paths between ~150–600 °C, which are usually applied to structural and tectonic questions in many varied geological settings. However, other authors contest the use of 40Ar/39Ar thermochronology because they argue that the assumptions are rarely valid. Here we review and evaluate the key assumptions, which are that (i) 40Ar is dominantly redistributed in K-feldspars and muscovite by thermally-driven volume diffusion, and (ii) laboratory experiments (high temperatures and short time scales) can accurately recover intrinsic diffusion parameters that apply to geological settings (lower temperatures over longer time scales). Studies do not entirely negate the application of diffusion theory to recover thermal histories, although they reveal the paramount importance of first accounting for fluid interaction and secondary reaction products via a detailed textural study of single crystals. Furthermore, an expanding database of experimental evidence shows that laboratory step-heating can induce structural and textural changes, and thus extreme caution must be made when extrapolating laboratory derived rate loss constants to the geological past. We conclude with a set of recommendations that include minimum sample characterisation prior to degassing, an assessment of mineralogical transformations during degassing and the use of in situ dating

Apatite U-Pb Thermochronology: A Review

The temperature sensitivity of the U-Pb apatite system (350–570 °C) makes it a powerful tool to study thermal histories in the deeper crust. Recent studies have exploited diffusive Pb loss from apatite crystals to generate t-T paths between ~350–570 °C, by comparing apatite U-Pb ID-TIMS (isotope dilution-thermal ionisation mass spectrometry) dates with grain size or by LA-MC-ICP-MS (laser ablation-multicollector-inductively coupled plasma-mass spectrometry) age depth profiling/traverses of apatite crystals, and assuming the effective diffusion domain is the entire crystal. The key assumptions of apatite U-Pb thermochronology are discussed including (i) that Pb has been lost by Fickian diffusion, (ii) can experimental apatite Pb diffusion parameters be extrapolated down temperature to geological settings and (iii) are apatite grain boundaries open (i.e., is Pb lost to an infinite reservoir). Particular emphasis is placed on detecting fluid-mediated remobilisation of Pb, which invalidates assumption (i). The highly diverse and rock-type specific nature of apatite trace-element chemistry is very useful in this regard—metasomatic and low-grade metamorphic apatite can be easily distinguished from sub-categories of igneous rocks and high-grade metamorphic apatite. This enables reprecipitated domains to be identified geochemically and linked with petrographic observations. Other challenges in apatite U-Pb thermochronology are also discussed. An appropriate choice of initial Pb composition is critical, while U zoning remains an issue for inverse modelling of single crystal ID-TIMS dates, and LA-ICP-MS age traverses need to be integrated with U zoning information. A recommended apatite U-Pb thermochronology protocol for LA-MC-ICP-MS age depth profiling/traverses of apatite crystals and linked to petrographic and trace element information is presented

Low-temperature (<110°C) thermal history of the Mt Isa and Murphy Inliers, northeast Australia: evidence from apatite fission track thermochronology

The paucity of Phanerozoic rock sequences in the Proterozoic Mt Isa and Murphy Inliers in northern Australia renders it difficult to determine their Phanerozoic tectonic histories. However, thermochronological methods provide a means to assess this problem. Apatite fission track ages vary from 390 to 218 Ma, and mean track lengths range between 11.1 and 13.6 μm. These results record a non-linear cooling history below about 110 ± 10°C. Forward modelling of the fission track data suggests that the first episode of relatively rapid cooling since the Early Carboniferous occurred between ca 350 and 250 Ma at rates of ∼0.9°C/million years and was synchronous with intracontinental deformation associated with the Alice Springs Orogeny and tectonics at the contemporaneous eastern margin of Australia. This event resulted in ≥ 2 km of exhumation across both inliers. Cooling rates also increased after 100 Ma, although estimates of the amount of cooling and exhumation across trhe inliers obtained by forward modelling of the fission track data exceed those determined by constraints provided by the 40Ar/39Ar ages of weathering profiles. This observation adds to the growing amount of evidence that the fission track annealing model employed overestimates cooling rates at low temperatures (<60 – 70°C). The spatial variation of apatite fission track data within the Mt Isa Inlier indicates the three major structural belts, the Western Fold Belt, Kalkadoon – Leichhardt Belt and the Eastern Fold Belt, which are separated by major north – south faults, experienced similar thermal histories on a regional scale. This suggests that the main faults bounding the belts have not experienced major reactivation in a vertical sense, as a single large-scale fault plane, since ca 350 Ma. However, adjacent smaller scale, fault-bounded blocks within the structural belts demonstrate variable cooling histories, suggesting that reactivation of favourably oriented minor faults in the inlier, including segments of the major faults, has occurred over this time interval. Variations in apatite fission track data show that up to 1.5 km of vertical displacement occurred along parts of the Mt Isa Fault Zone and the Pilgrim Fault between 350 and 250 Ma