Las pegmatitas ijolíticas de La Madera, Provincia de Córdoba: Quimismo de sus minerales y modelo genético

GALLISKI M. A. ; DORAIS M.; LIRA R.

Buenos Aires

Congreso; XIII Congreso Geológico Argentino; 1996

Asociación Geologica Argentina

<!-- /* Font Definitions */ @font-face {font-family:"Cambria Math"; panose-1:2 4 5 3 5 4 6 3 2 4; mso-font-charset:0; mso-generic-font-family:roman; mso-font-pitch:variable; mso-font-signature:-1610611985 1107304683 0 0 159 0;} @font-face {font-family:Calibri; panose-1:2 15 5 2 2 2 4 3 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:-1610611985 1073750139 0 0 159 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin-top:0cm; margin-right:0cm; margin-bottom:10.0pt; margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoPapDefault {mso-style-type:export-only; margin-bottom:10.0pt; line-height:115%;} /* Page Definitions */ @page {mso-footnote-numbering-restart:each-section; mso-endnote-numbering-style:arabic;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Abstract: The ijolitic pegmatites of La Madera are exposed in a quarry in Córdoba, Argentina. They occur as segregated veins 0.2 to 0.5 m wide and 5 to 10 m in lenght which are emplaced in an olivine melanephelinite of probable Late Cretaceous age. The veins are composed of pyroxene, nepheline, analcite, devitrified glass, titanomagnetite, phillipsite, magnesio-katophorite, eckermannite, biotite, calcite and chlorite. The pyroxene is strongly zoned with compositions grouped about four different #Mg at 80, 70, 55 and 20 and showing a segmented trend of increasing alkalinity from diopside through aegirine-augite to aegirine. K-bearing nepheline (Ne65Ks31Qz3) is the second most abundant mineral but is locally replaced by zeolites in the more leucocratic veins. Idiomorphic analcite is localized, hydrated phase, that is always replaced by Na-bearing phillipsite. Apatite and perovskite are REE and especially Sr-enriched minerals. The devitrified glass of the mesostasis contains amygdules, which are squeezed between crystals and mesostasis in places; they are composed of phillipsite, calcite and scarce chlorite, apatite and Fe-oxides, with textural relationships suggesting that were formed by a liquid immiscibility process.. Whole rock analyses of major and trace elements, and their textural relationships, show that the ijolitic pegmatites were formed by H2O undersaturated, P2O5, CO2 and incompatible element-bearing melts, which were derived by fractional crystallization of a parent olivine melanephelinite. The pegmatitic melt was collected and transported upward from magma lower levels by diapiric transfer of low-density melt fractions; transport occurred in discrete units now represented by scattered 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 pegmatitic melt increased in H2O saturation until the two immiscible phases were separated. One melt has the composition of phillipsite amygdules which resemble nepheline syenite in its major element chemistry. The other is represented by a more basic dark glass, K, Fe, Mg enriched, and Na, P, Al and Ca depleted compared to the leucocratic phase.