Search results for "Precambrian"

Palaeomagnetic results from the late Precambrian Chela Group of southwest Angola

1992

Abstract Palaeomagnetic results from fourteen sites (56 samples) in the late Precambrian Chela Group of southwest Angola are reported. After alternating field demagnetization eight of the sites have mean directions closely grouped around D = 123°, I = −44 1 2 ° ; the dispersion of these directions increases substantially after tectonic corrections suggesting magnetic overprinting. Intensive thermal demagnetization proved more effective than alternating field demagnetization in resolving magnetic components. After removal of viscous components, samples from five sites (Group I) show a single component, similar to that observed after alternating field demagnetization. At four more sites (Grou…

Reassessment of continental growth during the accretionary history of the Central Asian Orogenic Belt

2014

We argue that the production of mantle-derived or juvenile continental crust during the accretionary history of the Central Asian Orogenic Belt (CAOB) has been grossly overestimated. This is because previous assessments only considered the Palaeozoic evolution of the belt, whereas its accretionary history already began in the latest Mesoproterozoic. Furthermore, much of the juvenile growth in Central Asia occurred in late Permian and Mesozoic times, after completion of CAOB evolution, and perhaps related to major plume activity. We demonstrate from zircon ages and Nd–Hf isotopic systematics from selected terranes within the CAOB that many Neoproterozoic to Palaeozoic granitoids in the accre…

Structure, Composition and Evolution of the South Indian and Sri Lankan Granulite Terrains from Deep Seismic Profiling and Other Geophysical and Geol…

2005

We here argue the need for a multidisciplinary, integrated geophysical/geological survey across southern India and Sri Lanka, part of the largest exposed Neoproterozoic high-grade metamorphic terrain in the world. The purpose is to stimulate deep seismic profiling and other geophysical and geological investigations in order to better understand the structure, composition and evolution of a keystone of Precambrian continental crust and the role it played in the amalgamation and subsequent break-up of the Gondwana supercontinent. Of particular importance is that this lithospheric fragment records a 2 billion year long history of magmatism, sedimentation, deformation and metamorphism and now e…

Devonian arc-related magmatism in the Tseel terrane of SW Mongolia: chronological and geochemical evidence

2009

Abstract: The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Palaeozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 ± 3 Ma (2σ) and 397 ± 3 Ma (2σ) for two calc-alkaline metarhyolite samples indicate arc volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 ± 2 Ma (2σ). Geochemical and Nd isotopic data for low-grade mafic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc–…

Book Review: The trace-fossil record of major evolutionary events.

2020

Book Review: M. Gabriela Mangano & Luis A. Buatois (eds) (two volumes). 2016. The trace-fossil record of major evolutionary events. Volume 1: Precambrian and Paleozoic. Springer. Topics in Geobiology, 39, 358 pp. (ISBN 978-94-017-9599-9). M. Gabriela Mangano & Luis A. Buatois (eds) (two volumes). 2016. The trace-fossil record of major evolutionary eventsThe trace-fossil record of major evolutionary events. Volume 2: Mesozoic and Cenozoic. Springer. Topics in Geobiology, 40, 485 pp. (ISBN 978-94-017-9596-8).

Tectonic models for accretion of the Central Asian Orogenic Belt

2007

The Central Asian Orogenic Belt ( c . 1000–250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic–Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian–Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with…

Sedimentary dynamics and structural geology of pre-rift deposits of the interior basin of Gabon

2002

Abstract Recent studies combining field data (facies and paleocurrent analyses, structural surveys), sub-surface data (core analyses, well logs, seismic sections) and palynological data provide insight into the sedimentary dynamics of pre-rift deposits of the interior basin of Gabon and their structural and paleoclimatic setting. These sedimentary deposits belong to the Noya, Agoula and M’vone series of Late Precambrian to Jurassic age. Although these deposits are potential sources of petroleum, they are partly known because of poor outcrop in a wet equatorial climate, low density and uneven distribution of exploration wells and poor seismic survey coverage. The pre-rift deposits reflect fl…

The role of geochronology in understanding continental evolution

2010

Geochronology has become one of the most essential tools in reconstructing processes of continental growth and evolution, and in situ dating of minerals has become common practice through the development of high-resolution ion microprobes and laser ablation inductively coupled plasma mass spectrometry techniques. Zircon has established itself as the most robust and reliable mineral to record magmatic and metamorphic processes. The combination of mineral ages with Sm–Nd, Lu–Hf and O isotopic systematics constrains magma sources and their evolution, and a picture is emerging that supports the beginning of modern-style plate tectonics in the early Archaean. Major fields for future research in …

Preliminary petrographic and geochemical considerations on the precambrian mafic dykes of the Ilhéus-Olivença area, Bahia

1989

Tectonic evolution in the Archaean and Proterozoic

1991

Kre high heat flow probably favoured global hotspot activity with crustal growth of Iceland type through vertical magmatic accretion. The tectonic environment for the generation of about 3.9-2.5 Ga granite-gneiss-greenstone terrains with volurnlnous production of tonalite-trondhjemite-granodlorite magmas is still uncertain, and both piate ma@ and intrapiate scenarios arc possible. Deformanon atylea in greenstones and adjacent high-grade orthognersses with intercalated shallow-water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking. This probably resulted from collisional and/or rotational motion and produced significant intracrustal melting as early …