The Miocene-Pliocene Macusani Volcanics,SE Peru |
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| Authors: | Michel Pichavant Daniel J Kontak Louis Briqueu Jacinto Valencia Herrera Alan H Clark |
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| Institution: | (1) Centre de Recherches Pétrographiques et Géochimiques, BP 20, F-54501 Vandoeuvre les Nancy, France;(2) Department of Geological Sciences, Queen's University Kingston, K7L 3N6 Ontario, Canada;(3) Instituto Peruano de Energia Nuclear Av. Canada, 1470 Lima 13, Peru;(4) Present address: Nova Scotia Department of Mines and Energy, PO Box 1087, A1B 2X1 Halifax, Nova Scotia, Canada |
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| Abstract: | The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An10–20) and group II (An30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar,
was intermediate between WM and QFM and
was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar.
(calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H2O-saturation. f
HF is around 1 bar but the
ratios are significantly different between samples.
ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H2O-undersaturated conditions.CRPG Contribution n 769 |
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