In-situ granite melt formation in granulites

The proposed project will attempt to provide new insights into the origin of the so-called I-type and A-type granites. At the present time, different theories exist as to how these granite types originate. The model of an origin from the mantle region, through fractionation of basaltic parental melts, contrasts with a model based on the possibility of high-temperature re-melting of magmatic, or restitic crustal material. The latter crustal remelting model shall be investigated and constrained during this project by using rare examples of meta-igneous, high- temperature granulites presently exposed at the earths surface. Such rocks can be found in the classic granulite occurrences in south Bohemia, whose petrogenetic evolution included the partial dehydration melting of crustal granitoid material during the Variscan orogeny at pressures of ca. 16 kbar, and temperatures of ca. 1000C. Apart from traditional petrographic methods of investigation, such as fieldwork and thin-section microscopy, comprehensive whole rock geochemical analysis will be carried out for major, trace and rare-earth elements, focussing on leucosome and restitic portions within the different compositional granulite types. In addition, mineral analyses (including trace elements), by means of electron microprobe and laser ablation ICP-MS, should enable conclusions to be made concerning the compositions of co-existing interstitial partial melts, via Kd relations. A special importance during this investigation is attributed to accessory minerals such as zircon, rutile, monazite, apatite and sphene, as these are considered in part to be newly formed magmatic phases in the granulites, and their behaviour during partial melting greatly influences the trace element systematics of resultant melts. Detailed micro- textural studies of polished thin-sections and grain mounts using CL and BSE imaging techniques should help to exploit the genetic information stored in these geochronologically important minerals.

In the frame of project P 15133 a special rock type of the Bohemian Massif was investigated, the so-called South Bohemian granulites. These light, garnet- and kyanite-bearing rocks formed during the Variscan orogeny (i.e. ca. 340 Million years ago) in great depths (~ 50 km below the earths surface) and at high temperatures (~ 1000 C). They were later exhumed by tectonic processes and provide an opportunity to study geological processes which occur deep below mountain chains. These processes include the partial melting of lower crustal rocks in response to high temperatures. This partial melting gives birth to another important rock type, the granites. Granites form when melt migrates out of granulitic rocks, rises and accumulates in plutonic bodies. The research work carried out in the frame of this project was mainly dedicated to the understanding of the initial processes of granitic melt formation in granulitic environment. In the centre of interest was the geochemical composition of this initial melt and the behavior of the so-called accessory minerals (zircon, monazite, rutile, apatite), which are the carriers of many important trace elements.