Low-pressure differentiation of melanephelinitic magma and the origin of ijolitic pegmatites at La Madera, Córdoba, Argentina.

GALLISKI, M.A., LIRA, R. Y DORAIS, M.,

CANADIAN MINERALOGIST

Mineralogical Association of Canada

Lugar: Ottawa; Año: 2004 vol. 42 p. 1799 - 1823

0008-4476

The ijolitic pegmatites of La Madera are exposed in a quarry in Córdoba, Argentina. They occur as dykelets and dykes 0.2 to 0.5 m wide and 5 to 20 m in length, emplaced in a volcanic olivine melanephelinite of probable Late Cretaceous age. The dykes are composed of pyroxene, nepheline, analcime, devitrified glass, magnetite, phillipsite-Na, magnesiokatophorite, eckermannite,biotite, calcite and chlorite. The pyroxene is strongly zoned, with compositions grouped about four different Mg# values (80, 70, 55 and 20), showing a segmented trend of increasing alkalinity from diopside to aegirine-rich aegirine-augite. Nepheline (Ne66Ks31Qtz3) is locally replaced by zeolites in the more leucocratic dykes. Idiomorphic analcime is a locally important hydrousphase, that is invariably replaced by phillipsite-Na. Apatite and perovskite are REE- and especially Sr-enriched minerals. The devitrified glass of the mesostasis contains amygdules that are, in places, squeezed between crystals and mesostasis. The amygdules are composed dominantly of phillipsite-Na, with small quantities of calcite, and scarce chlorite, apatite and Fe-oxides,with textural relationships suggesting formation by liquid immiscibility. Textural relationships and whole-rock compositions show that the ijolitic pegmatites were formed by H2O-undersaturated, P2O5-, CO2- and incompatible-element-bearing melts that were derived by fractional crystallization of a parent olivine melanephelinite. The pegmatite-forming melt was collected andtransported upward by diapiric transfer of low-density melt fractions; transport occurred in discrete units now represented by scattered segregation vesicles. These melts were emplaced and crystallized in the irregular fractures of the consolidated upper levels of the lava flow. During this process, the residual pegmatite-forming melt evolved until two immiscible fractions began toseparate. One melt that led to the formation of phillipsite-Na amygdules, resembles nepheline syenite in its major-element composition.The other is represented by a more basic dark glass, K-, Fe-, Mg-, and P-enriched, and Na-, Al- and Ca-depleted compared to the leucocratic fraction.Keywords: ijolitic pegmatites, melanephelinite, liquid immiscibility, analytical data, isotopic data, La Madera, Argentina.