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Contacts Between High-P Eclogites and Gneisses in the L¹dek-Œnie¿nik Metamorphic Unit, the West Sudetes
| Content Provider | Semantic Scholar |
|---|---|
| Author | Stawikowski, Wojciech |
| Copyright Year | 2002 |
| Abstract | In the L1dek-Œnie?nik Metamorphic Unit (LSMU), amidst mostly orthogneises and metasediments, there are Carboniferous (U)HP rocks forming small dispersed eclogite bodies and a vast granulite massif. A mechanism for their exhumation from a depth of c. 100 km is poorly understood. It is unknown if and how much of the surrounding rocks have also undergone similar P-T paths. Having identified a Grt(Gros30-50)-Zo-Ti(Rt)-Qtz assemblage in quartz-plagioclase-mica rock immediately adjacent to eclogites, Brocker and Klemd (1996) suggested that at least part of the LSMU gneisses also underwent the (U)HP metamorphism but the critical mineral assemblages have been obliterated during exhumation. Similar suggestions were made by Smulikowski (1979), Borkowska et al. (1990), Dumicz (1991, 1993) and Steltenpohl et al. (1993). Tectonic emplacement of eclogites inside the adjacent gneisses is also assumed but the interpretations of their exhumation differ in important details (Don, 1989; elaŸniewicz and Bakun-Czubarow, 2002). In hope for the elucidating the above problems, the present author has started detailed studies at the contact zones between eclogites and gneisses in the LSMU. Three occurrences have been examined: one in the Miedzygorze gneiss unit, and two in the Giera3tow gneiss unit (Strachocin and Sowia Kopa near Stronie Œl1skie – the last one was mapped earlier as amphibolites of the Stronie fm.). In both the gneissic units, (U)HP metabasites occur as long, narrow belts inside gneisses, parallel to their foliation (Fr1ckiewicz and Teisseyre, 1973; Cymerman and Cwojdzinski, 1986). The eclogites are usually mantled by amphibolites, often strongly deformed and showing signs of migmatisation. According to the present author, the surrounding gneisses are mylonitised and often migmatised metagranites. In Stronie Œl1skie, post-eclogite amphibolitic rocks also occur within mica schists. Eclogites of both units strongly differ in their provenance, peak P-T conditions, textures and mineral composition (Smulikowski, 1967; Bakun-Czubarow, 1998). The eclogites from Miedzygorze have MORB protolith and underwent the UHP metamorphism (P>29 kbar, T = 660–780 °C) followed by retrogression under the amphibolite facies conditions, whereas those from the Giera3tow unit (granulite massif) have calc-alkaline basalt provenance and underwent higher temperature metamorphism (P>28 kbar, T = 700 to 800 °C), but along different P-T paths involving granulite facies episode. In the case of eclogites from Strachocin and Sowia Kopa no evidence for UMPM has been found yet (Bakun-Czubarow, 1998). In the Miedzygorze unit, the inner parts of metabasitic lenses are built of fresh or weakly retrograded UHP eclogites, sometimes having laminated texture, with locally changing proportions within the assemblage of Grt-Omp-Rt±Qtz±Phe±Ky±Hbl±Dol±Zo (Bakun-Czubarow, 2001). The outer parts are built of amphibolites touched by migmatisation. They are composed of Amp-Pl-Bt-Qtz-Ti-Ep-Ap-Zr. Amphiboles occur as pargasite-magnesiohornblende and younger actinolite; plagioclases contain An15–35. P-T conditions for hornblende-plagioclase pairs are estimated at T=635±30 °C (Holland and Blundy, 1994) and domain with frozen Devonian fabrics was decoupled from stabilised southward dipping fabrics of the mantle lithosphere along a flat rheologically weak lower crustal horizon. Thus, the observed discrepancy between orientation of the upper crustal boundaries and the deep crustal orogenic fabrics on one side, and the large-scale fabric of the mantle lithosphere root on the other side, may be explained by the proposed kinematic model. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://geolines.gli.cas.cz/fileadmin/volumes/volume14/G14-084.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
