Harzburger Gabbro

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The Harzburger Gabbro is an intrusion of mafic and ultramafic , plutonic rocks on the north-western edge of the Harz . Her seat took place towards the end of the Variscan Orogeny at the turn of the outgoing upper carbon / lower perm .

Etymology and initial mapping

The Harzburger Gabbro, also Harzburger Gabbromassiv or Harzburger Basit-Ultrabasit-Complex , was named after Bad Harzburg , which lies on its northern edge . It was geologically mapped for the first time in 1888 by Karl August Lossen . In 1896, near Wernigerode, the Natural Science Association of the Harz dedicated a monument made of classic Harz rock to Lossen , on the east side of which you can also find a Harzburg gabbro from the Radau valley (stone no. 22).

Type locality and outcrops

In the Radautal in the middle of the intrusion is the type locality for the peridotite rock Harzburgite , which was first described petrographically by Karl Heinrich Rosenbusch in 1887 . The best outcrops in the Harzburg gabbro can be found today in the Radautal, in the Bärenstein quarry, which is still being dismantled.

geography

Gabbro quarry near Bad Harzburg

The intrusion consists of two oval massifs that are separated from each other by a sunken fracture zone . Its maximum length in north-northeast-south-southwest direction is 6 kilometers, at its widest point in the northern part of the massif, almost 4 kilometers are reached. The Radau flows through both partial massifs, its tributary the Baste only the southern part. The Ecker runs through the eastern section in the north, the Riefenbach the outermost northwest edge.

geology

Geological map of the Harz with the Harzburg gabbro (green / black dotted)

The name gabbro is somewhat misleading, as the bulk of the intrusion consists of a gabbronorite or norit gabbro. Localized and occur Norit ( Glimmerhaltig ) Olivinnorit , Gabbrodiorit , diorite and quartz diorite to, more rarely troctolite , Bronzitit ( Orthopyroxenit ) ferrogabbro , Ferroolivingabbro and Ferrodiorit . The Harzburgites form small, island-like occurrences within the Gabbronorite, which are elongated in the north-northeast to northeast direction. Their occurrence is limited to the south of the north massif and the south massif. To date, dunites have only been encountered in holes approximately 350 meters deep. The predominant Gabbronorites can be divided into an Augit and an Olivin facies .

The intrusion took place in the western section in folded , weakly to medium-metamorphic sedimentary rocks of the older Paleozoic Era ( pelite and pebbly slate of the Upper Devonian and Lower Carbons - north-northeast continuation of the Sösemulde ). The sediments were changed in contact metamorphosis and are now available as Hornfelse . In the eastern section, the Eckergneiss and the Brocken granite were touched.

According to Roland Vinx (1982), the Harzburger gabbro a layered arrangement and thus forms a layered intrusion ( English layered intrusion ).

Due to the tectonic overprinting of the massif, however, the stratification is rarely visible in accumulated layers in the centimeter and decimeter range. In addition, it is severely disturbed by faults that generally run southeast-northwest .

Intrusion age

A. Baumann et al. a. (1991) dated the intrusion age of the Harzburg gabbro with 297 to 293 million years BP . This corresponds to the outgoing Upper Carboniferous, or more precisely the Kasimovian to Gzhelian ; according to Felix Gradstein u. a. (2004), however, this period already belongs to the lowest Permian ( Cisuralium - Asselium to Sakmarium ). At about the same time, the Brocken granite and the oker granite also appeared.

Internal structure

The Harzburg Gabbromassiv has a wide range of different types of rock, ranging from ultramafites and mafites to rocks of intermediate composition. The most strongly differentiated rocks diorite and quartz diorite occur on the north-western edge of the massif and show traces of contamination with the host sediments. They contain, for example, assimilated foreign rocks ( sediment xenolites such as lime silicate marbles and lime silicate corn rocks). But even the gabbronorites are contaminated, recognizable by the blue corundum in sediment xenolites and by fine-grained metasediment inclusions consisting of a Felsic bitotite-quartz-plagioclase mineralogy.

Internally, a clear sequence can be seen in the vertical, which begins in the lying part of the massif with the ultramafitites dunite and Harzburgite and mafit-rich gabbros (lying series with conspicuous, rhythmic layering) and via the gabbronorite finally leads to ferrogabbros and ferro-olivingabbros in the hanging wall (with monotonous, cryptic layering). At the same time, the iron number steadily increases from the bottom (Fe # = 0.2) upwards (Fe # = 0.95) and accordingly the magnesium number Mg # steadily decreases.

The north massif shows roughly a fractional , concentric structure with a central norite surrounded by Gabbronorites. On its southern edge there are more primitive cumulative rocks in the form of oliving abbros.

mineralogy

The gabbronorite, the most common type of rock, has a holocrystalline structure and is composed of the following minerals , with clino- and orthopyroxes having roughly equal proportions:

The more or less strongly serpentinized dunites , however, have a cumulative structure (adkumulus type) with over 90 percent by volume of olivine. Pargasite (hornblende), phlogopite and plagioclase or clinopyroxene occur as intercumulus phases . Chromium spinels are enclosed as idiomorphic crystals in these intercumulus minerals, but never in olivine.

The Harzburgites are also serpentinized. Their structure is of the orthocumulus type with olivine as the cumulus phase and clinopyroxene, hornblende, phlogopite, plagioclase and orthopyroxene as intercumulus crystals.

The Norite also show a cumulative structure. In this case, the cumulus phases are orthopyroxes and plagioclase.

The diorites and quartz diorites have a heterogeneous, hypidiomorphic granular texture. They consist of large plagioclase and orthopyroxe grains with smaller, interstitial hornblende, biotite and quartz grains.

Crystallization sequence

Using the microscopic findings , the following crystallization sequence can be determined for the intrusion:

  • Olivine → Pargasite, Phlogopite, Spinel → Orthopyroxen, Plagioclase → Clinopyroxen, Plagioclase

or simplified:

  • Olivine → spinel → plagioclase and orthopyroxene → clinopyroxene

Chemical composition

Due to the fractionation and assimilation processes, the chemical composition of the Harzburg intrusion varies greatly. However, two main trends can be identified:

  • the accumulation series Dunit-Harzburgit-Norit-Olivingabbro
  • the non-cumulative series gabbronorite ( fayalite- leading) ferrogabbro- ferrodiorite

The compositions of both series (but especially the non-cumulative series) are additionally overlaid by hybrid rocks (metasediments, quartz-feldspar rocks as well as diorite and quartz diorite), which results in a significant scatter.

Cumulative series

The magnesium numbers in the cumulative series range between 0.76 and 0.89. The SiO 2 content fluctuates between around 35 and 55 percent by weight and is negatively correlated with the magnesium numbers. Also Al 2 O 3 (20 to 2 percent by weight), CaO (15 to 0 percent by weight) and Na 2 0 (1.2 to 0 weight percent) are clearly negatively correlated. As expected, only MgO (10 to 37 percent by weight) shows a clear positive correlation.

Non-cumulative series

The magnesium numbers of the non-accumulates are all below 0.76. Their SiO 2 content is mainly between 45 and 55 percent by weight and shows only a relatively weak, positive correlation with the magnesium numbers.

Apart from this trend, the diorites and quartz diorites are at 60 percent by weight, the metasediment and quartz-feldspar hybrids can even reach up to 75 percent by weight SiO 2 . The magnesium numbers of the assimilated rocks are all below 0.50.

Al 2 O 3 (13 to 17 percent by weight) and CaO (5 to 10 percent by weight) also have weakly positive correlations . MgO is clearly positively correlated (0 to 10 percent by weight). Negative correlations can be seen with TiO 2 (2.5 to 0.5 percent by weight), MnO (0.4 to 0.15 percent by weight), FeO (total iron - 20 to 7 percent by weight) and Na 2 O (3 to 1 percent by weight) .

Analyzes

To illustrate the chemical compositions, the following rock analyzes from boreholes and the calculated composition of the initial magma:

Chemical composition of rocks of the Harzburg Gabbromassiv in wt.%
oxide Dunite (7 samples) Harzburgite Gabbronorite Primitive
tholeiite magma
SiO 2 36.74 38.45 39.05 51.50
Ti0 2 0.06 0.22 0.11 0.45
Al 2 O 3 2.20 3.14 10.28 16.02
Fe 2 O 3 4.57 4.82 4.93
FeO 5.00 6.08 3.62 7.09
MnO 0.15 0.18 0.12 0.15
MgO 40.67 35.61 25.89 10.15
CaO 1.08 2.21 5.87 10.16
Na 2 O <0.10 0.13 0.44 1.25
K 2 O 0.04 0.12 0.07 0.80
P 2 O 5 0.02 0.04 0.01 0.07
H 2 O 7.66 7.53 8.47 2.1
Mg # 0.88 0.86 0.85 0.71

Tholeiitic starting magma

The chemical compositions of the non-accumulates define, in particular outside the contamination area, i.e. H. above Mg # = 0.50, a clear trend that justifies the assumption of a fractionated starting magma. Since the separated cumulus minerals are in chemical equilibrium with a primitive initial melt, Sano u. a. (2002) calculated their magnesium number as 0.71, assuming the most magnesium-rich olivine cumulate with Fo 89.5 and including a participation coefficient between olivine crystals and melt of K D = 0.3. The corresponding chemical composition with 51.5 percent by weight SiO 2 is of a sub-alkaline , basaltic nature and corresponds, in particular due to the low TiO 2 and the increased MgO values, to an island arc - tholeiite (and not, as actually to be expected, a continental flood basalt ) .

Economic use

The predominantly medium-grain Harzburger gabbro is mainly used in road construction as gravel and gravel because of its good physical properties ( frost resistance , outstanding compressive strength , high abrasion resistance ) .

literature

  • Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke Verlag, Stuttgart 1985.
  • Roland Vinx: The Harzburg Gabbromassiv, an orogenetically shaped layered intrusion . In: New Yearbook for Mineralogy - Treatises . tape 144 , issue 1, 1982, pp. 1-28 .
  • S. Sano et al. a .: Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany . In: Journal of Petrology . tape 43 , no. 8 , 2002, p. 1529-1549 .

Individual evidence

  1. ^ KA Lossen: Communications from Mr. KA Lossen about the geological map in the Harzburg district (in 1888) . In: Yearbook of the Royal Prussian State Geological Institute . tape 1888 , no. VIII , 1889, p. XXXV-XLIII .
  2. ↑ Loss memorial . ( Memento of the original from August 6, 2011 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF) Geopark Harz-Braunschweiger Land; accessed on January 14, 2014  @1@ 2Template: Webachiv / IABot / www.harzregion.de
  3. ↑ Friedhart Knolle , Béatrice Oesterreich, Rainer Schulz, Volker Wrede: The Harz - Geological excursions. Perthes, Gotha 1997, ISBN 3-623-00659-9 , p. 145.
  4. ^ W. Sohn: The Harzburger Gabbro . In: Geological Yearbook . tape 72 , 1956, pp. 117-172 .
  5. ↑ R. Vinx: The Harzburg Gabbromassiv, an orogenetically shaped layered intrusion . In: N. Jb. Miner. Depending on the band 144 , 1982, pp. 1-28 .
  6. ↑ A. Baumann et al. a .: Isotopic age determinations of crystalline rocks of the Upper Harz Mountains, Germany . In: Geologische Rundschau . tape 80 , 1991, pp. 669-690 .
  7. ↑ F. Gradstein et al. a .: A geological time scale 2004 . Cambridge University Press, 2004, ISBN 0-521-78142-6 .
  8. ↑ G. Müller: The igneous rocks of the resin . In: Clausthaler Geologische Abhandlungen . tape 31 , 1978, p. 92 .
  9. ↑ L. Harries: Sapphire blue corundum from the resin . In: Lapis . tape 11 , 1999, p. 30 .
  10. ↑ a b c S. Sano u. a .: Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany . In: Journal of Petrology . tape 43 , no. 8 , 2002, p. 1529-1549 .
  11. ↑ Joachim Marten, Uwe Steinkamm: Gabbro - 150 years of stone industry in the Radautal . Ed .: ISV Ilseder Mischwerke. Dr. Schmidt group. Bode Verlag, Haltern 1988, p. 48 .