The Qilian Fold Belt is a suture belt between the China-Korea craton and Qaidam craton. There are manly Paleozoic volcanic rocks of marine origin occurred in the Qilian suture belt. According to the previous studies, these volcanic rocks are mainly spilite and keratophyre. In the present study, we have used methods of petrography and mineral chemistry to study alteration and low-grade metamorphism of Ordovician volcanic rocks in Mayahsuehshan, Minleh and Changma areas. These three areas are located in eastern, middle and western of Northern Qilian Fold Belt, respectively. On the basis of the parageneses of secondary minerals, it is indicated that the metamorphic grade of the Lower Ordovician volcanic rocks is equivalent to greenschist facies, whereas that of the upper part of the Middle Ordovician volcanic rocks is equivalent to prehnite-pumpellyite facies. The lower part of the Middle Ordovician volcanic rocks is a transition zone between prehnite- pumpellyite and greenschist facies. The metamorphic grades are thus lower for the younger volcanic rocks. The metamorphic grades of Minleh and Changma areas are prehnite-pumpellyite and prehnite-actinolite facies, respectively. The mineral parageneses of the volcanic rocks are generally consistent with that of spilite or keratophyre. It was considered earlier that the spilite and keratophyre were formed by direct crystallization from a mixture of magma and seawater. However, metamorphic grades of the volcanic rocks are shown to be equivalent to that of the intercalated metaclastic rocks in the present study. Therefor, it is implied that the spilitic volcanic and metaclastic rocks in the Mayahsuehshan area have formed as a reasult of regional metamorphism. The spilitic and keratophyric volcanic rocks from Minleh and Changma areas, respectively, are also inferred to have formed as a result of regional metamorphism. It is therefore more adequate to name them metabasalts or metandesites. Mineral assembleges and mineral chemistry are different in different parts of the rocks, because of varied effective compositions during metamorphism. According to the chlorite compositions, the chlorite/smectite mixed layering is not prevailing in the volcanic rocks of the three areas. However, interstratification of chlorite and illite appears in groundmass of some volcanic rocks in Minleh area. There are few chromium-spinel in the volcanic rocks of Mayahsuehshan area. The plots of Mg/(Mg+Fe) versus Cr/(Cr+Al) of the chromium-spinel imply that the volcanic rocks in Mayahsuehshan area may have occurred in island arc environments.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0729100-212353 |
| Date | 29 July 2000 |
| Creators | Hsu, Ta-Wei |
| Contributors | Houng-Yi Yang, Ching-Lang Tsai, Yen-Hong Shau, Wei-Teh Jiang |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0729100-212353 |
| Rights | unrestricted, Copyright information available at source archive |
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