ALBERT

Beschreibung: 〈p〉High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role for understanding the tectonic evolution of orogenic belts (〈cross-ref type="bib" refid="SP474-18-149C19"〉Johnson, M.R.W. & Harley, S.L. 2012〈/cross-ref〉. 〈i〉Orogenesis: The Making of Mountains〈/i〉. Cambridge University Press, Cambridge). They have typically experienced complex changes during subduction and exhumation processes, arising from recrystallization, deformation, element redistribution, fluid–rock interactions and even partial melting, and may therefore carry a valuable record of evolving geodynamic systems in an orogenic belt. Until now, more than 20 UHP metamorphic belts, i.e. belts which contain rocks that experienced pressure–temperature (〈i〉P–T〈/i〉) conditions exceeding the lower limit of the coesite stability field, have been identified all over the world (〈cross-ref type="bib" refid="SP474-18-149C1"〉Carswell, D.A. & Compagnoni, R. 2003〈/cross-ref〉. 〈i〉Ultrahigh Pressure Metamorphism〈/i〉. Eotvos University Press, European Mineralogical Union, Notes in Mineralogy, Budapest; 〈cross-ref type="bib" refid="SP474-18-149C4"〉Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T. & Jahn, B.M. 2009〈/cross-ref〉. Ultrahigh-pressure minerals and metamorphic terranes – the view from China. 〈i〉Journal of Asian Earth Sciences〈/i〉, 〈b〉35〈/b〉, 199–231; 〈cross-ref type="bib" refid="SP474-18-149C17"〉Zheng, Y.F., Zhang, L.F., McClelland, W.C. & Cuthbert, S. 2012〈/cross-ref〉. Processes in continental collision zones: preface. 〈i〉Lithos〈/i〉, 〈b〉136〈/b〉, 1–9). New and innovative studies from different geoscience disciplines have been invaluable in developing a better understanding of the geodynamic evolution of orogenic belts. This special issue contains 15 papers, most of which were presented as part of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’, held at the 35th International Geological Congress in Cape Town, South Africa during 27 August to 4 September, 2016.〈/p〉

Beschreibung: 〈p〉Many archaeological sites with jadeitite artefacts are known in the Caribbean region, but defining the source of the raw material is a major problem because of great mineralogical heterogeneity both in potential sources and in artefacts. The archaeological settlement site of Playa Grande on the northern coast of the Dominican Republic is particularly significant because it yielded evidence of on-site axe manufacture, and lies only 20–30 km NE of a recently discovered potential source area of serpentinite mélanges in the nearby Río San Juan Complex (RSJC). A suite of nine artefacts was chosen from a collection of over 100 excavated woodworking tools rich in jadeite, as well as two blueschist artefacts. Permission to perform destructive analysis allowed data on petrography, mineral chemistry and bulk-rock chemistry to be obtained. Seven of the nine artefacts are jadeitite 〈i〉sensu stricto〈/i〉 (〉90 vol% jadeite), which are identical to material known from the RSJC. Two artefacts are jadeite–lawsonite rocks. These and the two blueschists show only minor differences from corresponding rocks of the RSJC source. With this direct linking of source and site material, it is now possible to better define source discriminators for the Caribbean and to assess sampling bias.〈/p〉

Beschreibung: The age of peak-metamorphic conditions of the ultrahigh-pressure ( UHP ) metamorphic rock sequence constituting the Dora-Maira Massif is well established at 35 Ma. In order to understand the behaviour of excess argon during sequential pressure–temperature stages of exhumation from 〉3.5 to less than 0.5 GPa, distinct generations of chemically and petrographically well-characterized white mica were studied with the multigrain, stepwise-heating 40 Ar/ 39 Ar dating technique. Mica separates were obtained from boudins of coesite-bearing UHP pyrope quartzite and phengite schist enclosed within them, from retrograde assemblages evolving from the pyrope quartzite, and from biotite–phengite orthogneiss surrounding the boudins. Assuming the 35-Ma age as a benchmark, it is possible to reconstruct directly the quantity of excess argon during these various stages of the exhumation history. The amount of excess Ar roughly correlates with the Si content of phengite, which is an indicator for the pressure at which phengite incorporated excess Ar. All mica samples crystallized at pressures 〉1.0 GPa exhibit significant excess argon and aberrant ages significantly higher than 35 Ma. Between 1.0 and 0.5 GPa excess Ar diminishes and realistic ages are approached. Phengite (Si 3.55 ) separates from two localities of fresh coesite-bearing pyrope quartzite equilibrated at ca . 3.8 GPa show similar U-shaped spectra with high apparent ages in the first or second degassing steps, dropping to a plateau-like value of 71.0 ± 1.1 Ma in one case and yielding an apparently trustworthy plateau age of 103.4 ± 1.0 Ma in the second. Retrograde phengite generations from pyrope quartzite crystallized between 3.2 and 1.5 GPa also display a U-shaped spectrum with a broad depression corresponding to a minimum age of 96.9 ± 1.6 Ma. Retrograde phlogopite, representing part of the breakdown assemblage of pyrope and formed between 1.9 and 1.2 GPa, documents several sharp steps in age values from 548 Ma towards 850 Ma, whereas further degassing steps are characterized by a nearly linear increase of age values from 852 to 973 Ma. A plateau age of 56.3 ± 0.9 Ma was calculated for phengite from a phengite-schist inclusion within pyrope quartzite. The spectra of phengite from the country-rock biotite–phengite gneiss surrounding the pyrope quartzite boudins are hump-shaped. The total degassing ages decrease with decreasing grain size (and decreasing metamorphic pressure from 1.6 to 0.3 GPa, as signalled by a concomitant decrease of Si per formula unit) from 43.25 ± 0.60 Ma (1–2 mm) to relatively realistic values of 33.50 ± 0.50 Ma (90–180 μm) and 31.55 ± 0.50 Ma (〈100 μm). The latter age correlates with very low calculated excess argon values. The data demonstrate that even retrograde micas are affected by excess argon, and that "ages" derived from inverse isochron diagrams ( 36 Ar/ 40 Ar vs . 39 Ar/ 40 Ar) as well as "perfect" plateau ages need to be treated with caution and constrained by other geochronological systems.

Beschreibung: Many archaeological sites with jadeitite artefacts are known in the Caribbean region, but defining the source of the raw material is a major problem because of great mineralogical heterogeneity both in potential sources and in artefacts. The archaeological settlement site of Playa Grande on the northern coast of the Dominican Republic is particularly significant because it yielded evidence of on-site axe manufacture, and lies only 20–30 km NE of a recently discovered potential source area of serpentinite mélanges in the nearby Río San Juan Complex (RSJC). A suite of nine artefacts was chosen from a collection of over 100 excavated woodworking tools rich in jadeite, as well as two blueschist artefacts. Permission to perform destructive analysis allowed data on petrography, mineral chemistry and bulk-rock chemistry to be obtained. Seven of the nine artefacts are jadeitite sensu stricto (〉90 vol% jadeite), which are identical to material known from the RSJC. Two artefacts are jadeite–lawsonite rocks. These and the two blueschists show only minor differences from corresponding rocks of the RSJC source. With this direct linking of source and site material, it is now possible to better define source discriminators for the Caribbean and to assess sampling bias.