Peralkaline rocks

The Island of Pantelleria shows many characteristics that are commonly found in volcanoes from continental rift settings, such as the abundance of acid peralkaline rocks, the compositional gap between mafic and silicic compositions and the smooth inter-element trends. Therefore, its petro-genesis shares the same problems as magmatism from continental rifts (see Peccerillo et al. 2003 for discussion). The similar isotopic compositions of... 

Morra V, Secchi FA, Assorgia A (1994) Petrogenetic significance of peralkaline rocks from Cenozoic calc-alkaline volcanism from SW Sardinia, Italy. Chem Geol 118 109-142... 

Intrusive Deposits. Deposits included in the intmsive deposit type are those associated with intmsive or anatectic rocks of different chemical composition, eg, alaskite, granite, monzonite, peralkaline syenite, carbonatite, and pegmatite. Examples include the uranium occurrences in the porphyry copper deposits such as Bingham Canyon and Twin Butte in the United States, the Rossing Deposit in Namibia, and Ilimaussaq deposit in Greenland, Palabora in South Africa, and the deposits in the Bancroft area, Canada (15). 

The Boucher Brook Formation is the upper-most unit in the California Lake Group and consists of grey shale, siltstone, chert, alkalic basalt, and minor peralkaline felsic volcanic rocks including the alkaline basalt of the Camel Back Member (Fig. 1). 

Case-3 Rare Earth Mineral (REE) Deposits. The Thor Lake rare metals deposits are hosted by the peralkaline Blachford Lake intrusion, an Aphebian ring complex emplaced in Archean-supracmstal rocks of the Yellowknife... 

Sicily (7 Ma to Present) Etna (0.5-Present), Iblei (7-1.5), U-stica (0.75-0.13), Pantelleria (0.3-0.005), Linosa (1-0.5), Sicily Channel seamounts - Stratovolcanoes, diatremes, small plateaux, etc. formed of tholeiitic and Na-alkaline rocks (basanite, hawaiite, trachyte, peralkaline trachyte and rhyolite). 

Fig. 8.12. Classification of Pantelleria peralkaline acid rocks (SiC>2 > 63 wt % after Le Maitre 1989).

The Sicily Province consists of tholeiitic to Na-alkaline magmas. Most rocks are mafic in composition, ranging from basalt to hawaiite and nephelinite, except at Pantelleria where peralkaline acid volcanics (pantellerites) dominate. A few trachytic rocks occur at Etna, Ustica and as lithic ejecta at Linosa. 

A strongly silica undersaturated Na-alkaline series ranging in composition from basanite-tephrite to phonolite, with some of the most evolved rocks reaching a peralkaline composition. This is best represented by volcanic rocks from Montiferro. 

White (2003) has demonstrated that this relationship is inadequate for metaluminous to peralkaline systems and proposes an equation for the latter based on the agpaitic index and the Si02 content of the rock ... 

London (1994). This amount of P2O5 is equivalent to the dissolution of 0.3 to over 3.0 modal per cent apatite. If the melt does not leave the rock, the phosphates will precipitate as the melt crystallizes. Apatite solubility is also enhanced in low ASI melts (peralkaline), because of the presence of alkali phosphate complexes (Ellison and Hess 1988). 

Ellison AJG, Hess PC (1988) Peraluminous and peralkaline effects upon "monazite" solubihty in high-silica liquids. EOS Trans Am Geophys Union 69 498 Ferry JM (2000) Patterns of rrrineral occurrence in metamorphic rocks. Am Mineral 85 1573-1588 Finger F, Broska 1, Roberts MP, Schermaier A (1998) Replacement of primary monazite by apatite-allarrite-epidote coronas in an amphibolite facies grarrite gneiss from the eastern Alps. Am Mineral 83 248-258... 

The Strange Lake complex (a.k.a. the Lac Brisson complex), on the border of Quebec and Labrador, Canada, is a peralkaline granite which intruded metamorphic rocks and quartz monzonite. It occurs 250 km northeast of Schefferville, Quebec, and 150 km west of Nain, Labrador. The emplacement age of the earliest intrusion is approximately 1240 2 Ma (Miller et al. 1997). The location is given in Fig. 2.14. 

In many localities anomalously high concentrations of uranium of probable hydrothermal origin have been noted in, or associated with, volcanic rocks. Volcanic rocks serve as hosts for important uranium deposits in several countries. Uranium is most commonly associated with highly silicic types of volcanic rocks, but peralkaline and peraluminous variants also can be uraniferous. Some deposits in Australia bear some relation to basaltic eruptives. It is estimated that about 15.5% of the... 

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