Mesozoic to recent evolution of the Andean forearc of northern Chile (22-24 s)

The Andean forearc of northern Chile comprises four morphotectonic units, which include from east to west: 1) The Cordillera de la Costa: composed of Jurassic granites and andesites, thought to represent a volcanic arc, the Mejillones terrane, an accreted allochthonous terrane, and the Lower Cretaceous Coloso basin, which formed through forearc extension along the suture between the Mejillones terrane and the Jurassic arc. Palaeomagnetic studies of the above units have identified approximately 29+/-11 degrees of clockwise rotation. Rotation is due to extension (caused by subduction roll back and slab pull), at an angle to the direction of absolute motion of the South American Plate. 2) The Central Depression: a large arid basin containing isolated fault-bounded blocks of pre-Mesozoic metamorphosed igneous rocks, Triassic sediments and volcanics, and Jurassic carbonates, deposited in a. back-arc basin setting. The isolated blocks formed through extension along previous thrust faults, these originated through compression of the back-arc basin due to accretion of the Jurassic volcanic arc. 3) The Precordillera.: composed of Permian-Triassic rift-related sediments and volcanics, Jurassic continental sediments synchronous with back-arc basin sedimentation, and Cretaceous and Oligo-Miocene continental sediments deposited in foreland basins. Palaeomagnetism has identified clockwise rotation in rocks ranging in age from Jurassic-Miocene. Rotation in the Precordillera. affected larger structural blocks than in the Cordillera de la Costa. 4) The Salar Depression: a. series of arid continental basins developed on continental crust. These basins nay have originated in the Triassic, when rifting of the South American craton is thought to have taken place. In conclusion, palaeomagnetic and geological evidence is consistent with the view that the north Chilean forearc was largely under an extensional stress regime. However, the presence of extensive compressional structures in Palaeocene and older rocks in the forearc together with the currently active foreland thrust belt of Argentina. indicate that throughout the evolution of the Andean Orogen, a delicate balance between compressional and extensional tectonic regimes has existed.

Volcanogenic mineralisation in the limestone lake area, Saskatchewan, Canada

This thesis describes the geology, geochemistry and mineralogy of a Lower Proterozoic, metamorphosed volcanogenic Cu-Zn deposit, situated at the western end of the Flin Flon greenstone belt. Stratabound copper mineralisation occurs in silicified and chloritoid-bearing alteration assemblages within felsic tuffs and is mantled by thin (< 3m) high-grade sphalerite layers. Mineralisation is underlain by garnet-hornblende bearing Lower Iron Formation (LIF), and overlain by garnet-grunerite bearing Upper Iron Formation (UIF). Distinctive trace element trends, involving Ti and Zr, in mineralised and footwall felsic tuffs are interpreted to have formed by fractionation associated with a high-level magma chamber in a caldera-type environment. Discrimination diagrams for basaltic rocks are interpreted to indicate their formation in an environment similar to that of recent, primitive, tholeiitic island arcs. Microprobe studies of key mineral phases demonstrate large and small scale chemical variations in silicate phases related to primary lithological, rather than metamorphic, controls. LIF is characterised by alumino-ferro-tschermakite and relatively Mn-poor, Ca-rich garnets, whereas UIF contains manganoan grunerite and Mn-rich garnets. Metamorphic mineral reactions are considered and possible precursor assemblages identified for garnet-, and chloritoid-bearing rocks. Chloritoid-bearing rocks are interpreted as the metamorphosed equivalents of iron-rich feeder zones formed near the surface. The iron-formations are thought to represent iron-rich sediments formed on the sea floor formed from the venting of the ore fluids. Consideration of various mineral assemblages leads to an estimate for peak metamorphic conditions of 450-500oC and > 4Kb total pressure. Comparisons with other volcanogenic deposits indicate affinities with deposits of `Mattabi-type' from the Archean of Ontario. An extrapolation of the main conclusions of the thesis to adjacent areas points to the presence of a number of geologically similar localities with potential for mineralisation.

The stratigraphy of the Fishguard volcanic complex and associated sediments, SW Dyfed, Wales

This thesis describes the geology of a Lower Palaeozoic terrain, situated west of the town of Fishguard, SW Dyfed, Wales. The area is dominated by the Fishguard Volcanic Complex (Upper Llanvirn), and sediments that range in age from the Middle Cambrian to the Lower Llandeilo. The successions represent an insight into sedimentation and volcanism for c. 100 Ma. along the south-western margin of the Lower Palaeozoic Welsh Basin. The stratigraphy of the sedimentary sequence has been completely revised and the existing volcanostratigraphy modified. The observed complexity of the stratigraphy is primarily the consequence of Caldedonide deformation which resulted in large scale repetition. Fold-thrust tectonics dominates the structural style of the area. Caledonide trending (NE-SW) cross-faults complicate preexisting structures. Middle Cambrian (?) sedimentation is documented by shallow marine clastics and red shales deposited within tidal - subtidal environments. Upper Cambrian sedimentation was dominated by shallow marine `storm' and `fair weather' sedimentation within a muddy shelf environment. Shallow marine conglomerates and heterolithic intertidal siliciclastics mark the onset of Ordovician sedimentation during the lower Arenig transgression. Mid-Arenig sediments reflect deposits influenced by storm, fair-weather and wave related processes in various shallow marine environments, including; shoreface, inner shelf, shoaling bar, and deltaic. Graptolitic marine shales were deposited from the upper mid-Arenig through to the lower Llandeilo; during which time sediments accumulated by pelagic processes and fine grained turbidites. The varied nature of sedimentation reflects both localised change within the depositional system and the influence of larger regional eustatic events. Ordovician subaqueous volcanic activity produced thick accumulations of lavas, pyroclastics, hydroclastics, and hyaloclastics. The majority of volcanism was effusive in nature, erupted below the Pressure Compensation Level. Basaltic volcanism was characterised by pillowed lavas and tube networks, whilst sheet-flow lavas, pillow breccias and minor hyaloclastites developed locally. Silicic volcanism was dominated by rhyolitic clastics of various affinities, although coherent silicic obsidian lavas, sheet-flow lavas and pyroclastics developed. Hypabyssal intrusives of variable composition and habit occur throughout the volcanic successions. Low-grade regional metamorphism has variably affected the area, conditions of the prehnite-pumpellyite and greenschist facies having been attained. Numerous secondary phases developed in response to the conditions imposed, which collectively indicate that P-T conditions were of low-pressure facies series in the range P= 1.2-2.0 kbars and T= 230-350oC, under an elevated geothermal gradient of 40-45oC km-1. Polymineralic cataclastites associated with Caledonide deformation indicate that tectonism and metamorphism were in part contemporaneous.