Harzburgite

Harzburgite is the name of an ultramafic, plutonic peridotite - rock.

Etymology

Harzburgite in 1887 described petrographically for the first time by Karl Heinrich Rose bush and named by him after the type locality in Radautal in Bad Harzburg in the resin. The rock forms part of the Harzburg gabbro and is found as a cumulate in the footwall of this massif.

Definition

Harzburgite consists mainly of olivine, which can be between 40 and a maximum of 90 percent by volume. Orthopyroxene is present with 5 to 60 percent by volume. Clinopyroxene is subordinate and reach a maximum of 5 percent by volume.

Mineral constituents

The in fresh, unweathered state deep dark green to black-green rock, in addition to defining minerals olivine, orthopyroxene and clinopyroxene relatively small amounts of plagioclase, spinel, phlogopite, amphibole ( hornblende - Pargasite ) and accessory apatite and ore minerals contain.

As an example of the modal mineral composition of Harrzburgit the type locality was given, the 43 volume percent olivine, orthopyroxene 56.5 volume percent, 0.5 percent by volume of plagioclase and traces of ore and apatite contains. Clinopyroxene is absent.

Structure

In addition to typical mantle rock structures ( described in detail in lherzolite ) shows the harzburgite of the type locality for example, a Kumulatgefüge, with olivine as inclusions in plagioclase and pyroxene poikilitische present ( Poikilitisches structure).

Chemical composition

The chemical composition of Harzburgiten be illustrated by the following examples: a global average based on 206 analyzes, followed by the harzburgite of the type locality (2 analyzes) and the extremely depleted, Greenland Wiedemmann - harzburgite. CIPW the standard of the average value is also shown.

Harzburgites are predominantly quartz - undersaturated, olivine - and hypersthene - normative rocks.

Occurrence

In addition to their occurrence along oceanic spreading centers located harzburgites found in layered intrusions in association with dunite, Norit, gabbro, Gabbronorit, diorite and quartz diorite. As Volcanic bombs and harzburgite xenoliths appear among the ejecta of volcanoes, but they can be dragged into dykes ( lamprophyres, etc.) and in kimberlites.

Formation

Harzburgite occur predominantly at oceanic Riftsystemen due to the formation tholeiitischer basalt melts.

The starting material for this is lherzolite - rock from the upper mantle, which is subject to the ascent to the spreading or rift zones of strong pressure reduction. This leads to a partial melting out of up to 20 % of the original Lherzoliths, with priority given to certain mineral interests ( such as clinopyroxene, plagioclase ) go from the starting material in the tholeiitic melt. While the melt further rises and eventually solidifies into new oceanic crust, the remaining Gemengteile ( Restit ) in the hanging wall of the Lherzoliths remain as residual or Residualschicht.

The remaining material is as depleted or depleted harzburgite rock (English depleted ) rated because it has very clearly lost in incompatible elements such as aluminum and calcium. These elements do not fit due to its ionic radius and the electric charge of their number in the crystal lattice of the rock shell, and are therefore preferably used in the melt. The residual material remain particularly magnesium and iron, and small amounts of chromium, nickel and cobalt, but which may reach locally minable concentrations.

Localities

In layered intrusions:

  • Germany - Harzburg gabbro (type locality)

At oceanic spreading centers:

At transform faults and associated fault zones ( engl. fracture zones ):

  • United States Washington - Ingalls Ophiolithkomplex - Middle to Upper Jurassic

In Subduktionskontext ( supra- subduction zones - mantle wedge ):

  • China: Tibet - Shigatse - Tethyan ophiolites

Generally in ophiolites:

As bombs and xenoliths:

  • Greenland - Wiedemann Fjord
  • Jordan - Shamah - basalt plateau
  • South Africa - Kaapvaal craton
Fractional crystallization (geology) Seafloor spreading Norite Tholeiitic magma series Mantle (geology) Crust (geology) Aluminium Washington (state) Digital Object Identifier
377566
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