| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 91 === The North Qilian Orogen was a Caledonian suture zone located between the Alashan terrane and the Central-Southern Qilian terrane. The studied area consists of three units of rock series: the Precambrian Basement, the Caledonian volcanic-sedimentary associations, mafic-ultramafic associations, and intermediate-felsic plutonic rocks, and Late Paleozoic to Cenozoic cover. This research is focused on petrographic, geochemical, and geochronological studies of the Precambrian basement rocks exposed at Tuolai pasture, Salahe, Qilian county, and Shifengyaigou.
All basement rocks have been subjected to high greenschist-amphibolite facies metamorphism. In the neighborhood of the Qilian county town, the metamorphic rocks also have been mylonited. The REE patterns of the metasedimentary rocks show LREE enrichment (100× Chondrite), HREE depletion (10× Chondrite), and negative Eu anomalies (Eu/Eu*＝0.5-0.7), like the compositions of the post-Archean shales. The almost constant Th/Sc ratio (Th/Sc＝1) and lower εNd vaule (εNd(0)≦-10) of metasediment suggest that a major sedimentary provenance of an old upper continental crust. At the Longkonggou profile, the different geochemical features suggest a minor sedimentary provenance of a young undifferentiated arc. The Neodymium isotopes also suggest that the metasediment can be divided into Northern Alashan type and Southern Central-Southern Qilian type (Qaidam type).
The Metabasites of the studied area, such as amphibolites, indicate some geochemical features similar to that of the continental flood basalts. The magma was derived from an enriched mantle or plume-like source, and was subjected to crustal contamination. The basalt from Xiaobabao river is identified as a normal-type mid-oceanic ridge basalt, and is probably the remnants of the paleo-Qilian oceanic crust.
The 40Ar-39Ar dating indicates that the biotites extracted from the gneiss and schist reveal the plateau ages of 400.5 ± 2.3 and 434.1 ± 2.5 Ma respectively. They show that the Argon isotope system of the Precambrian basement rocks were resetted by the Caledonian orogeny. An exceptional hornblende plateau age of 535.7 ± 2.9 Ma from amphibolite in Longkonggou profile show the Precambrain basement terran now located at Tuolai pasture was located at the far south when the paleo-Qilian ocean was closed in the Silurian-Devonian period.
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