Ulf Linnemann on the Assembly of Central Europe in the Paleozoic
Ulf Linnemann is Leader of the Geochronology Lab at the Senckenberg Collections of Natural History in Dresden. He has developed the systematic dating of detrital zircon populations into a powerful tool for reconstructing tectonic plate movements through geological time, and the paleogeography that led to the transport of zircons from one plate to another. He explains how he identified distinct detrital zircon provinces in Europe and used these to work out how the plates of modern-day central Europe came together.
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Podcast Illustrations
All illustrations courtesy of Ulf Linnemann unless otherwise noted
Zircons crystallize out of a cooling magma. The illustration shows one such setting — in a magma chamber under a volcano.
The various steps required to extract zircon crystals from rocks before they can be dated. These steps are very labor-intensive. At the end of the podcast, Ulf Linnemann says how much it would enhance his research if these preparatory steps could be made faster and more efficient. The zircon crystals suitable for detrital zircon analysis are between 65 and 125 microns across.
Zircon crystals can be imaged by using a method called cathodoluminescence. A beam of electrons in a scanning electron microscope is directed onto a crystal, causing the crystal to luminesce. The luminescence is sensitive to small variations in the internal structure of a zircon, and so it can reveal the various zones within a zircon. These may reveal that a zircon formed in more than one phase of crystallization. The images are used as a guide for the ion probe or laser instruments that can then be targeted to the different regions to obtain the ages of the various zircon zones. The average grain used in detrital zircon studies is about 100 microns across.
The zircon provinces of Europe when it was a part of the Gondwana supercontinent, which in turn was part of Pangea. Each of these provinces is characterized by a distinctive age distribution of detrital zircon ages.
A quartzite sampled by Ulf Linnemann on the island of Bornholm in the Baltic Sea. The detrital zircons in this sedimentary rock of Neoproterozoic-Cambrian age are representative of the southern margin of the Baltica craton.
The detrital zircon population that characterizes the South Baltica zircon province. This population is found all along the so-called Tornquist Zone, which marks the southern boundary of Baltica and extends 2000 km from the Baltic Sea to the Black Sea.
Ulf Linnemann sampled these sandstones in the Brabant Massif in Germany as representative of Middle Cambrian rocks of Avalonia.
The detrital age distribution that characterizes the Avalonian zircon province. As explained in the podcast, parts of this age distribution bear the signatures of other tectonic plates — mainly that of Baltica.
Ulf Linnemann sampled sandstones from the Cliff of Atar on the Reguibat Shield of Mauretania as representative of the Northwest African craton.
Detrital zircon age distribution characteristic of the Northwest African zircon province.
These Neoproterozoic rocks were sampled in the Saxo-Thuringian Zone of the Lausitz Block as representative of Cadomia.
Detrital zircon population characteristic of the Cadomian zircon province. The population of zircons from 2 billion years and older is derived from the Northwest African craton.
The detrital zircon age distributions (grey histograms) of all four blocks (Baltica, Avalonia, Cadomia, and West Africa) are shown side by side in a single diagram to make it easier to compare them to each other. The figure shows that the detrital zircon age distributions of the four zircon provinces are very different from each other. As Ulf Linnemann explains in the podcast, the ability to characterize a block by its detrital zircon population age distribution makes this a valuable tool for reconstructing the relationships between the plates and hence inferring their relative positions during the Paleozoic. In addition to the zircon populations, the diagram shows how a different set of measurements — that of hafnium isotopes in the rocks — can also serve to characterize the different tectonic blocks. The colored bands, lines, and arrows relate to the hafnium analysis. (MORB = mid-ocean ridge basalt; εHf = a measure of the difference between the hafnium isotope ratios in the rock and that of the Earth as a whole; CHUR = chrondritic uniform reservoir, which is assumed to be representative of the bulk Earth.)
The various geological zones that make up present-day Germany include basement rocks from three tectonic blocks — the Baltica craton, and the Avalonian and Cadomian terranes.
The present-day distribution of Avalonian and Cadomian basement rocks. Younger, Mesozoic rocks cover the white areas in between the exposed outcrops. The Avalonian terrane extends as far as the Appalachians on the other side of the Atlantic.
Classification of the detrital zircon record of Avalonia in the early Paleozoic as preserved in sandstones of the Brabant Massif in Germany.
This series of plate location reconstructions is based in part on the detrital zircon analysis described in the podcast. In a given time slice defined by the age of a sedimentary rock, the relative proportions of its detrital zircons characteristic of each of the zircon provinces indicate how close to the sedimentary rock the blocks corresponding to those zircon provinces were. Thus, 330 million years ago, the presence of detrital zircons from Baltica in each of the Avalonian, Cadomian, and even Northwest African blocks indicates that the plates had docked with each other by that time.
