HIGH-POTASSIUM ROCKS IN THE IL'CHIR OPHIOLITE BELT ( East Sayan )
A.G. Mironov and S.M. Zhmodik*
Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
* Institute of Geology and Mineralogy, Siberian Branch of the RAS,
3 prosp. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Ophiolites, high-potassium rocks, carbonization, metasomatism, shearing, mylonitization
Pages: 1128-1137
Abstract
High-potassium rocks have been found among ultrabasites of the Il'chir ophiolite nappe. Stock- and sill-like bodies are composed of greenish-gray to dark-gray holocrystalline (sometimes, porphyritic) rocks. Three rock varieties are recognized: albitized monzonitoids (wt.%): SiO2 - 53.2-53.8; Al2O3 - 14.6-14.9; MgO - 3.3-8.3; Na2O - 5.0-5.9; and K2O - 0.05-0.1; syenites (orthoclasites) (wt.%): SiO2 - 63.0-66.1; Al2O3 - 17.2-18.6; Na2O - 0.2-3.1; and K2O - 8.2-16.7; granosyenites (wt.%): SiO2 - 71.5-77.3; Al2O3 - 11.5-13.5; Na2O - 0.1-1.72, and K2O - 9.2-10.7. The K-richest varieties are composed of allotriomorphic grains of perthites, K-antiperthites, and, sometimes, plagioclase. Biotite, amphibole, and chlorite occur in small amounts (1-5 vol.%). The rocks are strongly altered to blastocataclasites with a feldspathic groundmass. Along with intense shearing and pseudoporphyritic texture (porphyroclasts), a distinctive feature of K-feldspar and plagioclase preserved as porphyroclasts in relics is their carbonization. Neogenic K-feldspar, albite, and dark-colored minerals lack cloud-like carbon pigmentation. Potassium-rich rocks have high contents of Ba and Rb and show a distinct Eu minimum on their REE patterns. According to Rb-Sr dating, these rocks have an age of 419 ± 30 Ma (87Sr/86Sr = 0.7061 ± 0.0021), which is, most likely, close to the age of the last metamorphism. Indirect data evidence that primary island-arc rocks might have formed in island-arc setting, possibly, in the foreland of island arcs.
|
