Peridotites occurrences

During the last two decades, many studies have reported major- and trace-element data on orogenic, ophiolitic, and oceanic peridotites. Whole-rock compositions representative of the main peridotite occurrences are illustrated on element versus AI2O3 diagrams for major elements (Figure 5), minor transition elements (Figure 7),... 

Occurrence paragneisses, methamorphosed limestones, amphibole peridotites Name basaltic hornblende... 

Rocks consisting essentially of olivine alone are known as dunites, the name coming from the occurrence of this rock in the Dun mountains of New Zealand. In the United States, this mineral is found in North Carolina, South Carolina, and Georgia, where corundum is associated wtith the dunite in commercial quantities. The olivine of peridotites alters readily to the mineral serpentine, often to such an extent that the rock itself is called a serpentine. As mentioned above, the pendotites may contain chromite or other valuable minerals, often to such an extent that they may be commercially exploited, for nickel, platinum, and precious garneL... 

Perhaps one of the most important consequences of a peridotite composition for the upper mantle is that the phase transitions in olivine that are manifested as seismic discontinuities should exhibit thermally controlled variations in their depth of occurrence that are consistent with the measured Clapeyron slopes (Bina and Helffrich, 1994) of the transitions. In particular, the olivine-wadsleyite transition at 410 km should be deflected upwards in the cold environment of subduction zones while the disproportionation of ringwoodite to silicate perovskite and magnesiowiistite at 660 km should be deflected downwards, thereby locally thickening the transition zone. In anomalously warm regions (such as the environs of mantle plumes as described below), the opposite deflections at 410 and 660 should locally thin the transition zone. The seismically observed topography of 20-60 km on each of the 410 and 660 is consistent with lateral thermal anomalies of 700 K or less (Helffrich, 2000 Helffrich and Wood, 2001). 

We shall hereafter follow the current terminology. It is convenient to distinguish three main types of mantle occurrences and to examine them in the following sequence (i) orogenic peridotite massifs, (ii) ophiolitic mantle rocks, and (iii) oceanic peridotites. 

Brueckner and Medaris (2000) reviewed the main occurrences of orogenic garnet peridotites and pointed to their diversity, even within individual HP/UHP terranes. They attributed these differences to different initial environments and different evolutions during the tectonic cycles that ultimately exposed them at the surface, and proposed a classification based on these criteria. The garnet peridotites are first... 

Western Alps. The LP orogenic peridotites are particularly well exposed in the Alps (Figure 2), where they form several large occurrences of ultramafic rocks (—100 km or more) along the Alpine and North Apennine mountain arc (Figure 2). In general, the massifs underline the sumre zone between the southern Alpine and European plates. The most significant bodies include ... 

Lensch G. (1971) The occurrence of sapphirine in the peridotite complex of Finero, zone of Ivrea, western Italian Alps. Contrib. Mineral Petrol 31, 145-153. 

Pearson D. G., Davies G. R., Nixon P. H., and Milledge H. J. (1989) Graphitized diamonds from a peridotite massif in Morocco and implications for anomalous diamond occurrences. Nature (London) 338, 60—62. 

Morten L. (1987) A review of xenoUthic occurrences and their comparison with Alpine peridotites. In Mantle Xenoliths (ed. P. H. Nixon). Wiley, London. 

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