Journal of Conference Abstracts

Volume 2 Number 1

vsg - Minsoc '97

The Frontal Cordillera of South America (27° ­ 36°S): Along Strike Variability in Pre-Andean Granites

H. M. Orme1 (, N. Petford1, M. P. Atherton2, D. A. Gregori3, M. A. Ruvinos3 & S. G. Pugliese1

1 School of Geological Sciences, Kingston University, Penrhyn Road, Kingston-upon-Thames, Surrey KT1 2EE, U.K.

2 Dept of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3BX, U.K.

3 Departamento de Geologia, Universidad Nacional del Sur, San Juan 670, 8000 Bahia Blanca, Argentina.

The Frontal Cordillera (FC) of South America is a chain of mountains 800 km in length, situated at 27-36°S between the Pre Cordillera to the east and the Principal and Coastal Cordilleras to the south west and north west respectively. It is made up of mid Palaeozoic metamorphic and sedimentary rocks which are discordantly overlain and intruded by volcanics and granitic plutons of upper Palaeozoic to Triassic age.

It has long been recognised that the tectonics of the Andes mountains have been controlled for a long period of time by interactions between the oceanic plate margin and the western sea board of the South American continent. Tectonics, sedimentation and magmatism predominantly occur in mountain chains, or 'Cordillera' parallel with a N/S strike to the present day Peru-Chile trench. The FC is one such range.

In order to observe the spatial and temporal changes in cordilleran magmatism along strike of the Andes between 27° and 36°, granitic plutons from the northerly Chilean FC have been compared geochemically to those of the southerly Argentinian FC. Preliminary results indicate that the rock types to the south are considerably more monzonitic, and therefore may represent an along strike transition from typical cordilleran calc-alkaline magmatism to extension related anorogenic magmatism with time.

Strain-Driven Reversal of the Orthoclase ­ð Microcline Phase Transformation During Phase Separation in Braid Microperthites

Ian Parsons1 (, Martin R. Lee1 (, William L. Brown2 ( & Kim A. Waldron3 (

1 Dept. Geology and Geophysics, Univ. Edinburgh, Edinburgh EH9 3JW, Scotland.

2 CNRS-CRPG, BP 20, 54501, Vandœuvre-lès-Nancy Cedex, France.

3 Dept. Geology, Colgate Univ. Hamilton, NY 13346, USA.

Alkali feldspar crystals in syenites usually have bulk compositions near Ab60Or40 and are braid micro- or crypto-perthites consisting of coherent, intersecting lamellae of low microcline, parallel to {6­6­1}, enclosing lozenges of low albite, on a scale from 40­500 nm. Perthite morphology is controlled by the minimization of coherency strain and varies with bulk composition and cooling rate. At the relatively low cooling rates which lead to the braid texture, feldspars with more Ab- or Or-rich bulk compositions have zig-zag or straight intergrowths, the former with intermediate microcline, the latter with low sanidine or tweed orthoclase. Microcline forms when coherency strains are minimised by migration of lamellar interfaces from (6­01) to {6­6­1} but the potassium feldspar is stranded as tweed orthoclase when this migration cannot occur.

Braid intergrowths frequently coarsen to up to 20 µm in width at crystal margins and around inclusions, giving rise to a texture called a 'pleated rim'. We have studied the development of pleats, using SEM and TEM, in the Klokken and Coldwell syenites. Coherent pleats form from the braid microtexture by interaction of the spontaneous coherency strains with the surface of the crystal. They are regions in which the bulk composition of the cryptoperthite is alternately Ab and Or rich; coarsening has occurred by phase separation. Near the inner parts of pleats there is a transitional zone in which intergrowths straighten (in response to the change in bulk composition). Coherency strains induce a phase transformation involving Si,Al disordering, of low microcline into low sanidine, now tweed orthoclase, even though the crystal is in the hydrostatic T-stability field of microcline. Spaced edge dislocations develop as intergrowths straighten, allowing ingress of water into the crystals, producing, in the outer parts of pleats, coarsely exsolved, non-coherent, microporous patch and vein perthites in which microcline may re-form.

vsg - Minsoc '97
6-9 January 1997
University of Cambridge, Cambridge, UK

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