Abstract
Two porphyritic granitoids (Huaco and Sanagasta) in the Velasco district of the Pampeana Pegmatite Province in Northwestern Argentina are recognized. They are considered the fertile granites of the beryl-bearing pegmatites and can be described as post-orogenic and peraluminous A-type granites formed in an intraplate tectonic setting during an extensive regime, whose magma source is predominantly of cortical origin and to a lesser extent, mantle-derived. The pegmatites are classified as Rare Elements of the beryl type and beryl-columbite-phosphate subtype, while the entire district shows characteristics related to the NYF (Nb-Y-F) petrogenetic family. From rocks and cogenetic minerals of an individual ‘Ismiango’ pegmatite of the Velasco district, two Rb/Sr isochrons have been constructed. They define an age of 330.3 ± 8.3 and 331.7 ± 2.3 Ma and fall in the Lower Carboniferous period, consistent with the age of the parental and host-rock, the Huaco granite. As the Ismiango pegmatite has a similar composition and structure to the other beryl mineralized pegmatites of the Velasco district, the obtained age is attributed extensively for the entire district. According to the initial 87Sr/86Sr value obtained of 0.713, the pegmatite-magmatism of the Velasco District might be mainly derived from the crust with some minor participation of mantle materials.
Highlights
Granitic pegmatites are igneous rocks characterized by large crystals with mineralogical and textural variations between different sectors of these bodies, as well as a chemical heterogeneity that can be manifested even within the same grain (London 1992)
The Rb-Sr geochronology study presented in this paper has been performed using cogenetic ‘material’ sampled from the Ismiango pegmatite: aplite from the border section of the pegmatite and minerals from the intermediate zone, such as microcline, plagioclase, muscovite, beryl and triplite (Fig. 3A)
The dominant beryl color in the Ismiango pegmatite is yellow, which would have formed later than the green color that occurs in the pegmatites of the Velasco district (Sardi and Heimann 2014)
Summary
Granitic pegmatites are igneous rocks characterized by large crystals with mineralogical and textural variations between different sectors of these bodies, as well as a chemical heterogeneity that can be manifested even within the same grain (London 1992). In many cases, these rocks are of great economic importance since they are sources of various rare metals and minerals for industrial use, jewelers, and collectibles. All of these reasons have, for a long time, increased the interest of numerous researchers in the study of pegmatites, who despite having a rich and abundant knowledge on their mineralogy and geochemistry, still have to deal with the scarcity of studies on isotopic composition in pegmatites. The rock-forming minerals of pegmatites commonly contain variable amounts of potassium, such as K-feldspars and micas Geochemical behaviors, such as ionic size and charge, allows the tracing of radiogenic element Rb in order to substitute the major element potassium in these minerals. The isotopic Rb-Sr method can be properly used for geochronology studies on pegmatites (e.g., Lima et al 2009, Barros et al 2017)
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