中文摘要

徐達偉

祁連褶皺帶是中朝古陸和柴達木古陸之間的一個縫合帶,在縫合帶中分佈有許多早古生代海相火山岩,以往的研究認為本地區的火山岩類型以細碧岩和角斑岩為主。本研究針對甘肅省馬雅雪山、民樂以及昌馬地區之火山岩,從岩象和礦物化學的角度來探討火山岩所受的蝕變作用或變質作用。這些地區分別位於北祁連褶皺帶東段、中段和西段,年代屬於奧陶紀。馬雅雪山地區早奧陶至中奧陶的火山岩地層,根據共生的次生礦物群所顯示,其變質度從早奧陶地層的綠色片岩相到中奧陶上部地層的葡萄石-綠纖石相,而中奧陶下部則屬於兩變質相間的過渡帶,亦即本區地層由老到新,變質度有逐漸降低的趨勢。民樂與昌馬地區變質火山岩,變質度分別為葡萄石-綠纖石相和葡萄石-陽起石相。這些火山岩所含的次生礦物群大致符合細碧岩或角斑岩之定義。過去認為北祁連褶皺帶的細碧-角斑質火山岩,在成因上是由於岩漿與海水混染後的結晶產物。本研究顯示馬雅雪山地區火山岩的變質度和相間之碎屑岩的變質度一致,因此火山岩和碎屑岩應受同一區域變質作用而轉變成變質火山岩和變質碎屑岩。馬雅雪山、民樂和昌馬地區的細碧-角斑質火山岩應為區域變質作用下的產物,因此將其稱為變質玄武岩或變質安山岩較為恰當。由於受到變質作用時岩石中不同區域的有效成分之影響,岩石中的共生礦物群或次生礦物成分亦有所差異。此三地區火山岩中綠泥石成份顯示,綠泥石與膨潤石混層情形並不普遍,而民樂地區則有部分基質中的綠泥石夾有伊萊石。馬雅雪山地區的火山岩,含有少量原生的鉻尖晶石,從鉻尖晶石的Mg/(Mg+Fe)對Cr/(Cr+Al)投圖結果,顯示本區火山岩應該是島弧環境下噴發所生成。

Abstract

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.