| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 87 === Qilian mountain belt is the suture zone of Sinokorean plate and Qaidam plate. Central Qilian mountain is primary made up by the metamorphic basement of amphibolite faces. We study the composition distribution and chemical zoning of garnets in metapelites of Huang -Yuan Group, and use these data to drive P-T paths. The study includes two sets of areas, one is Quala River, Chiaman River and Sanchiabeichigo, are across the uplift belt of Central Qilian; the other one is Laleekwan, Shibantsun, Huangshui and Huangmoutsun.
The main work in this study is to use optical microscope to identify the minerals. We found sillimanite in the samples of sillimanite zone, and kyanite and staurolite in kyanite-staurolite zone samples. Several samples collected beside the intruded igneous body are found andalusite and cordierite, so we can guess that contact metamorphism may occur in some areas of the metamorphic basement. We also used electro-microbe to analyze minerals composition, and used geothermobarometer and Gibbs'' method to calibrate the metamorphism P-T paths. The garnets in kyanite-saturolite zone and garnet zone have growth zoning, and some samples that have shooter diameter show flat compositional profiles. The garnet samples from the sillimate zone displayed flat patterns because of high temperature.
The result of biotite-garnet geothermometer and garnet-muscovite-plagioclase-biotite geobarmeter yield increase in metamorphic pressure and temperature from south to north of the first set of study areas. Some samples have higher pressure may because of shear zone passing. The temperature and pressure from core to rims of garnet primary increase, some samples reveal increase in temperature and depress in pressure.
The result of dating tells us that the metamorphic age of study area should indicate Caledonian orogeny (422Ma) , so Central Qilian Monument may doesn''t have Luliangian orogeny, or the age of the Caledonian orogeny overprinted on the metamorphic basement of Central Qilian , and reseted the age.
Concluding the calculated result of geothermobarometry and Gibbs'' method, we can find that one deep burial event may occurred at the early Caledonian continent-continent collision in Central Qilian, and began to uplift and erosion, and this process may had few times igneous intrusive, but just the sample M011 had recorded this event. Later happened the deep burial event , this event may be the major metamorphic event of Central Qilian. The age of this event is 422Ma, and all samples recorded this event. Crust uplifts and erosions until now.
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