Magmatic/metasomatic amphibole in the mantle: evidence from the Balmuccia lherzolite massif, Ivrea zone, western Italian Alps

Conference · Tue Jan 01 00:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6322645

Shervais, J W

Amphibole and mica are the most important alkali-rich phases stable under mantle P-T conditions, and constitute the primary source of the alkalis needed to form basaltic magmas. Yet, the origin and distribution of these phases in the upper mantle is poorly understood. The Balmuccia lherzolite massif is a small slice of subcontinental mantle in the western Alps that intruded granulite facies lower crust 250 m.y. ago, and was exposed during the Alpine orogeny. Amphibole (HB) ranging in composition from Ti-pargasite to kaersutite in a minor but widespread accessory phase in the lherzolite, in websterite and bronzite dikes of the Cr-Di suite, and in pyroxenite and gabbro dikes of the Al-Aug suite. Textural and compositional relations show that HB in the Cr-Di and Al-Aug suite dikes formed by crystallization of residual magma, or by reaction between this magma and earlier formed phases (primarily spinel). Hb also occurs in thin (1-2 cm) veins of hornblendite and pyroxene hornblendite, and in the HB-enriched wallrock adjacent to these veins. Similar Hb veins in the Lherz massif have been shown to form from an alkali and water-enriched residual magma derived by filter pressing of Al-Aug pyroxenites. This origin is not possible at Balmuccia because the Hb veins have higher Ti, Cr, K/Na and Mgnumbers than any accessory Hb in Al-Aug suite dikes. Fractionation trends in the veins are controlled by HB. This requires that the HB veins formed from a separate and distinct parent magma that metasomatically enriched the lherzolite on a local scale. However, remelting of the widespread pyroxenite dikes may constitute a more important source of alkalis than the rare HB-rich veins.

• Research Organization: Univ. of South Carolina, Columbia (USA)
• OSTI ID: 6322645
• Report Number(s): CONF-8510489-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
BASALT
MINERALOGY
PETROGENESIS
ITALY
MAGMA
CHEMICAL COMPOSITION
ALPS
AMPHIBOLE
CONTINENTAL CRUST
EARTH MANTLE
GEOLOGY
MAGMATISM
METAMORPHISM
MICA
OROGENESIS
PETROLOGY
EARTH CRUST
EUROPE
IGNEOUS ROCKS
MINERALS
MOUNTAINS
ROCKS
VOLCANIC ROCKS
WESTERN EUROPE
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)
580400 - Geochemistry- (-1989)