RT Journal Article
T1 The Layos Granite, Hercynian Complex of Toledo(Spain): an example of parautochthonous restite-richgranite in a granulitic area
A1 Barbero González, Luis C.
A1 Villaseca González, Carlos
AB The Layos Granite forms elongated massifs within the Toledo Complex ofcentral Spain. It is late-tectonic with respect to the F2 regional phase and simultaneous with themetamorphic peak of the region, which reached a maximum temperature of 800-850°C andpressures of 400-600 MPa. Field studies indicate that this intrusion belongs to the "regionalmigmatite terrane granite" type. This granite is typically interJayered with sill-like veins andelongated bodies of cordierite/ garnet-bearing leucogranites. Enclaves are widespread andcomprise restitic types (quartz lumps, biotite, cordierite and sillimanite-rich enclaves) andrefractory metamorphic country-rocks including orthogneisses, amphibolites, quartzites, conglomeratesand caJc-silicate rocks.These granites vary from quartz-rich tonalites to melamonzogranites and define a S-type trendon a QAP plot. Cordierite and biotite are the mafic phases of the rocks. The particularly highpercentage of cordierite (10%-30%) varies inversely with the silica content. Sillimanite is acommon accessory mineral, always included in cordierite, suggesting a restitic origin. Themineral chemistry of the Layos Granite is similar to that of the leucogranites and country-rockperaluminous granulites (kinzigites), indicating a close approach to equilibrium. The uniformcomposition of plagioclase (An2S)' the high albitic content of the K-feldspar, the continuousvariation in the Fe/Mg ratios of the mafic minerals, and the high Ti content of the biotites(2·5-6·5%) suggest a genetic relationship.Geochemically, the Layos Granite is strongly peraluminous. Normative corundum liesbetween 4% and 10% and varies inversely with increase in Si02. The CaO content is typicallylow «1·25%) and shows little variation; similarJy the LILE show a limited range. On manyvariation diagrams, linear trends from peraluminous granulites to the Layos Granite andassociated leucogranite can be observed. The chemical characteristics argue against an igneousfractionation or fusion mechanism for the diversification of the Layos Granite. A restiteun mixing model between a granulitic pole (represented by the granulites of the ToledoComplex) and a minimum melt (Jeucogranites) could explain the main chemical variation of theLayos Granite. Melting of a peiitic protolith under anhydrous conditions (biotite dehydrationmelting) could lead to minimum-temperature melt compositions and a strongly peraluminousresiduum.For the most mafic granites (61-63% Si02), it is estimated that the trapped restite componentmust have been around 65%. This high proportion of restite is close to the estimated rheologicalcritical melt fraction, but field evidence suggests that this critical value has been exceeded. Thishigh restite component implies high viscosity of the melt which, together with the anhydrousassemblage of the Layos Granite and the associated leucogranites, indicates H20-undersaturatedmelting conditions. Under such conditions, the high viscosity magma (crystal-liquid mush) had arestricted movement capacity, leading to the development of parautochthonous plutonic bodies.
PB Royal Society of Edinburgh, [1980]-2007.
SN 0263-5933
YR 1992
FD 1992
LK https://hdl.handle.net/20.500.14352/58652
UL https://hdl.handle.net/20.500.14352/58652
LA eng
DS Docta Complutense
RD 3 may 2024