Harzburgite

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The ultramafic rock, harzburgite, is a variety of peridotite consisting of the minerals, olivine and low-Ca pyroxene (enstatite), and named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral. Garnet-bearing harzburgite is much less common, found only as xenoliths in kimberlite.

Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called lherzolite. The molten magma extracted from harzburgite may then erupt on the surface as basalt. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-free peridotite called dunite. Harzburgite may also form by the accumulation of olivine and low-Ca pyroxene in large magma chambers of basalt deep in continental crust (ultramafic to mafic layered intrusions).

[edit] Occurrence

Harzburgite is the most common variety of peridotite found in ophiolites, which are thought to represent oceanic crust exposed during collision with continental crust. Examples of ophiolites with extensive harzburgite include the Troodos ophiolite in Cyprus, the Semail ophiolite in Oman, the Coast Range ophiolite of California, and the Bay of Islands ophiolite in Newfoundland. Interestingly, the so-called "abyssal peridotites" dredged from the seafloor near fracture zones (where oceanic lithosphere may be exposed at the surface) are typically lherzolites, not harzburgites. Lherzolites may also form by the extraction of magma from more pyroxene-rich peridotite, but are less depleted than harzburgites. This is consistent with the observation ophiolites probably form in the fore-arc region of island arcs, not at mid-oceanic ridges.

Harzburgite may also be found in some Alpine peridotite massifs that consist mostly of lherzolite. Alpine or orogenic lherzolites represent subcontinental mantle lithosphere (the upper mantle below continental crust) exposed during the plate tectonic collision of continental plates. Examples of orogenic lherzolite massifs with harzburgite include the Lherz massif in France (Pyrenees Mountains), the Lanzo massif in northern Italy, and the Horoman massif in Japan.

Garnet-bearing harzburgite is found only as a xenolith in some kimberlite pipes, which are found almost exclusively in ancient continental cratons of Archean or Paleoproterozoic age. The mantle lithosphere under these cratons is particularily thick (up to 300 km) and cool. This causes the Ca-rich pyroxene (diopside) to recrystallize as the mineral garnet, lowering the abundance of high-Ca pyroxene. Garnet harzburgites are less depleted in the basalt component than most ophiolite harzburgites. Garnet harzburgite xenoliths are found mostly in kimberlites in South Africa.

Cumulate harzburgite is found in some large layered mafic intrusions. At the Earth's surface, basaltic magmas typically crystallize the minerals: olivine, plagioclase, and augite (a high-Ca pyroxene); low-Ca pyroxenes can only co-exist with olivine at low pressure in lavas that are high in both MgO and SiO2 (boninites). At pressures greater than 5 kilobars (0.5 GPa, or 5000x atmospheric pressure), olivine and low-Ca pyroxene (enstatite or bronzite) may crystallize together from normal basalt magmas to form harzburgite. These conditions were common in some layered mafic intrusions, most of which are Proterozoic in age, which formed from enormous sill-like intrusions of basalt into lower continental crust. The classic example of a Proterozoic layered intrusion with cumulate harzburgite is the Stillwater complex of Montana, USA.

[edit] References

• Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, ISBN 0-7167-2438-3