The Layos Granite, Hercynian Complex of Toledoud(Spain): an example of parautochthonous restite-richudgranite in a granulitic area

摘要

The Layos Granite forms elongated massifs within the Toledo Complex ofudcentral Spain. It is late-tectonic with respect to the F2 regional phase and simultaneous with theudmetamorphic peak of the region, which reached a maximum temperature of 800-850°C andudpressures of 400-600 MPa. Field studies indicate that this intrusion belongs to the "regionaludmigmatite terrane granite" type. This granite is typically interJayered with sill-like veins andudelongated bodies of cordierite/ garnet-bearing leucogranites. Enclaves are widespread andudcomprise restitic types (quartz lumps, biotite, cordierite and sillimanite-rich enclaves) andudrefractory metamorphic country-rocks including orthogneisses, amphibolites, quartzites, conglomeratesudand caJc-silicate rocks.udThese granites vary from quartz-rich tonalites to melamonzogranites and define a S-type trendudon a QAP plot. Cordierite and biotite are the mafic phases of the rocks. The particularly highudpercentage of cordierite (10%-30%) varies inversely with the silica content. Sillimanite is audcommon accessory mineral, always included in cordierite, suggesting a restitic origin. Theudmineral chemistry of the Layos Granite is similar to that of the leucogranites and country-rockudperaluminous granulites (kinzigites), indicating a close approach to equilibrium. The uniformudcomposition of plagioclase (An2S)' the high albitic content of the K-feldspar, the continuousudvariation in the Fe/Mg ratios of the mafic minerals, and the high Ti content of the biotitesud(2·5-6·5%) suggest a genetic relationship.udGeochemically, the Layos Granite is strongly peraluminous. Normative corundum liesudbetween 4% and 10% and varies inversely with increase in Si02. The CaO content is typicallyudlow «1·25%) and shows little variation; similarJy the LILE show a limited range. On manyudvariation diagrams, linear trends from peraluminous granulites to the Layos Granite andudassociated leucogranite can be observed. The chemical characteristics argue against an igneousudfractionation or fusion mechanism for the diversification of the Layos Granite. A restiteudun mixing model between a granulitic pole (represented by the granulites of the ToledoudComplex) and a minimum melt (Jeucogranites) could explain the main chemical variation of theudLayos Granite. Melting of a peiitic protolith under anhydrous conditions (biotite dehydrationudmelting) could lead to minimum-temperature melt compositions and a strongly peraluminousudresiduum.udFor the most mafic granites (61-63% Si02), it is estimated that the trapped restite componentudmust have been around 65%. This high proportion of restite is close to the estimated rheologicaludcritical melt fraction, but field evidence suggests that this critical value has been exceeded. Thisudhigh restite component implies high viscosity of the melt which, together with the anhydrousudassemblage of the Layos Granite and the associated leucogranites, indicates H20-undersaturatedudmelting conditions. Under such conditions, the high viscosity magma (crystal-liquid mush) had audrestricted movement capacity, leading to the development of parautochthonous plutonic bodies.