- Harzburgite
The
ultramafic rock, harzburgite, is a variety ofperidotite consisting mostly of the twomineral s,olivine and low-calcium (Ca)pyroxene (enstatite ); it is named for occurrences in theHarz Mountains ofGermany . It commonly contains a few percentchromium -richspinel as an accessorymineral .Garnet -bearing harzburgite is much less common, found most commonly asxenolith s inkimberlite .Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called
lherzolite . The moltenmagma extracted from harzburgite may then erupt on the surface asbasalt . If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite calleddunite . Harzburgite may also form by the accumulation of olivine and low-Ca pyroxene in largemagma chamber s of basalt deep incontinental crust (ultramafic to mafic layered intrusions ).Occurrence
Harzburgite is the most common variety of peridotite found in
ophiolites , which are thought to representoceanic crust and the underlying oceanic mantle exposed during collision withcontinental crust . Examples of ophiolites with extensive harzburgite include the Troodos ophiolite inCyprus , the Semail ophiolite inOman , the Coast Range ophiolite ofCalifornia , and the Bay of Islands ophiolite inNewfoundland . Interestingly, the so-called "abyssal peridotites" dredged from theseafloor near fracture zones (where oceaniclithosphere 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 that many ophiolites probably form in the fore-arc region ofisland arc s, not atmid-oceanic ridge s.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 lherzolitemassif s with harzburgite include the Lherz massif inFrance (Pyrenees Mountains ), the Lanzo massif in northernItaly , and the Horoman massif inJapan .Garnet -bearing harzburgite is found as axenolith in somekimberlite pipes, which are found almost exclusively in ancient continentalcraton s ofArchean orPaleoproterozoic age. The mantle lithosphere under these cratons is particularly thick (up to 200km or more) and cool. Garnet harzburgites are less depleted in thebasalt component than most ophiolite harzburgites. Garnet harzburgite xenoliths from kimberlites inSouth Africa have been particularly well-characterized.Cumulate harzburgite is found in some large layered mafic intrusions. At the
Earth 's surface, basaltic magmas typically crystallize the minerals: olivine,plagioclase , andaugite (a high-Ca pyroxene); low-Ca pyroxenes can only co-exist with olivine at low pressure in magma that is high in both MgO and SiO2 (boninite s). At pressures greater than 5kilobar s (0.5 GPa, or 5000xatmospheric pressure ), olivine and low-Ca pyroxene (enstatite orbronzite ) may crystallize together from normal basalt magmas to form harzburgite. These conditions were common in some layered mafic intrusions, most of which areProterozoic 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 theStillwater igneous complex ofMontana .References
* Blatt, Harvey and Robert J. Tracy, 1996, "Petrology: Igneous, Sedimentary and Metamorphic", 2nd ed., Freeman, ISBN 0-7167-2438-3
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