The evolution of amphibolites from Site 1067, ODP Leg 173 (Iberia Abyssal Plain): Jurassic rifting to the Pyrenean compression
V. Gardien, G. Poupeau, B. Muceku, R. Hébert, G. Beaudoin, E. Labrin, 2001. "The evolution of amphibolites from Site 1067, ODP Leg 173 (Iberia Abyssal Plain): Jurassic rifting to the Pyrenean compression", Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea, R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, N. Froitzheim
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During ODP Leg 173 (April-May 1997) five new sites (1065, 1067-1070) were drilled at the ocean-continent transition (OCT) zone off the West Iberia margin in the Iberia Abyssal Plain. At Site 1067 the 92 m of cored amphibolites were subdivided into three units based on textural criteria. Unit 1 consists of highly foliated and folded amphibolites and acidic gneiss concordant to and folded along with the foliation of the amphibolite. Unit 2, in the middle part of the section, consists of brecciated amphibolites. Unit 3, at the bottom of the hole, is a weakly deformed zone where magmatic textures are observed in the amphibolites and in associated anorthosites. The amphibolites contain tschermakitic to magnesio-hornblende amphibole, plagioclase, zircon, apatite ± titanite ± Fe-oxide ± quartz. The acidic gneiss consists of garnet, plagioclase, alkali-feldspar, quartz, biotite and zircon. Chlorite, sericite and ilmenite occur as secondary phases. The metamorphic evolution of the amphibolite and acidic gneiss started under amphibolite- facies conditions (Stage I: 670 ± 40 °C and 7 ± l kbar). Further exhumation took place through low-grade amphibolite-facies (Stage II: 550 ± 50°C and 5.5 ± l kbar) to greenschist-facies (Stage III: 500 °C and 3 kbar) conditions contemporaneous with the development of ductile structures. The late metamorphic evolution of the amphibolite ended under ocean-floor conditions. Oxygen isotope ratios and studies of fluid inclusion indicate that a magmatic waterrich fluid in equilibrium with the igneous protolith predominated during Stage I. During Stages II and IIII low-temperature waterrich fluids of metamorphic origin predominated, with a probable contribution of sea water. Apatite fission-track dating indicates that the amphibolites record two thermal excursions below 120°C. The first took place at c. 113 – 100Ma and could be related to Cretaceous rifting. The second was brief, and so did not anneal older tracks; it occurred between 75 and 55 Ma and could be related to the Pyrenean orogeny.
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Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea
Non-volcanic continental margins may form up to 30% all present-day passive margins, and remnants of them are preserved in mountain belts. The papers in this volume demonstrate the benefits of integrating offshore and onshore studies, and illustrate the range of information obtained at different scales when comparing evidence from land and sea. Data sets collected across a range of spatial scales are evaluated: thin sections, cores, outcrops, seismic reflection profiles, and other geophysical data. The outcrop scale is crucial because it enables the spatial gulf to be bridged between DSDP and ODP cores and marine seismic data. There is also the problem that basins on land and beneath the sea inevitably have had different post-rift histories resulting in their contrasting present-day elevation. In mountain belts, portions of continental margins and oceanic crust are superbly exposed, but dismembered by subsequent compressional tectonics. Off present-day passive margins, extensional features have only been slightly deformed, if at all, by compressional movements, but are buried beneath significant thicknesses of post-rift sediments and so can only be sampled by ocean drilling at a small number of points.
The first paper reviews the synergies that have occurred between investigations of the eastern North Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the Alps, and some later papers return to this theme. However, papers describing margins from other parts of the world show that it may be premature to use models based on the Atlantic and the Alps as the paradigm for all non-volcanic margins. The following 25 papers in the book are grouped under the following headings: (1) Margin overviews; (2) Exhumed crust and mantle; (3) Tectonics and stratigraphy; (4)Numerical models of extension and magmatism.