緬甸晚新生代火山岩之地球化學及岩石成因

Late Cenozoic Volcanic Rocks from Burma: Geochemical Characteristics and Petrogenesis

楊曉明(Hsiao-Ming Yang)

指導教授 ： 鍾孫霖

國立臺灣大學/理學院/地質科學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02870

摘要

緬甸位於印度與歐亞兩大陸塊碰撞帶的東緣，其境內最顯著的地質現象是南北走向的右移斷層Sagaing Fault。Sagaing fault往北和東喜瑪拉雅造山結(eastern Himalayan Syntaxis) 相接，向南與安達曼海(Andaman Sea)以及印尼的蘇門答臘斷層(Sumatra fault)相連，形成一系列碰撞擠壓－陸塊脫逸－海盆張裂的特殊環境。位於這個從擠壓到張裂環境的過渡帶上，緬甸在整個東南亞的新生代地體構造中扮演要角，是研究印度－歐亞板塊碰撞演化的關鍵地區。
本研究針對緬甸中部晚新生代火山岩的氬-氬定年結果顯示，Monywa地區與Popa火山的岩漿活動主要發生在兩個時期:中新世中期與更新世。其中，Popa火山的定年結果為13.25 ± 0.07 Ma和13.27 ± 0.10 Ma，比前人依地層層序推估的上新世年代早很多。Monywa地區的定年結果則為15.46 ± 0.08 Ma，同屬於中新世中期岩漿活動的產物。
緬甸晚新生代火山岩屬於高鉀鈣鹼系列，中新世的火山岩以安山岩(SiO2 = 53.2 ~ 61.2 wt%)為主，更新世組成則以玄武岩到玄武岩質安山岩(SiO2 = 44.1 ~ 56.4 wt%)為主。其中Monywa地區部份更新世火山岩屬於鉀玄岩系的白榴基玄武岩(absarokite)，具有極為富集的輕稀土元素，其高鎂(Mg# = 70.6 ~ 73.6)及低矽含量(SiO2 ≈ 45 wt%)的特徵顯示它們是原始岩漿，可由富集的含角閃石地函橄欖岩經小程度部份熔融(~ 5 %)而形成。Popa火山的更新世火山岩以玄武岩為主(SiO2 = 47.6 ~ 56.4 wt%)，具有富集的輕稀土元素，它們的鎂值雖低(Mg# = 51.5 ~ 60.7)，但都沒有銪元素負異常的出現。相對地，Monywa地區與Popa火山的中新世火山岩較酸性(SiO2 = 53.2 ~ 61.2 wt%)，低鎂值(Mg# = 26.0 ~ 59.2)，但具有明顯的銪元素負異常，代表經歷了強烈的斜長石結晶分化作用。在不相容元素的分布上，所有緬甸晚新生代火山岩都具有大離子半徑元素的富集以及高場力鍵結元素虧損的現象，為隱沒帶或活動大陸邊緣島弧火成岩的地球化學特徵。在鍶-釹同位素組成上，它們則具有高的釹同位素(εNd = +1 ~ +4)以及低的鍶同位素值(87Sr/86Sr ≈ 0.7045)，沒有明顯的地殼物質或隱沒沉積物混染的情形。緬甸晚新生代火山岩的鍶-釹同位素組成分布範圍相當均一，顯示岩漿源區具有非常相似的同位素組成。
根據Sagaing fault的右移速率(20 ~ 50 mm/yr)，推估在13 Ma時，緬甸火成岩區位於目前的南邊約260~650公里處，因為當時印度洋板塊的隱沒作用，造成以安山岩為主的島弧岩漿。隨後，Sagaing fault的活動而帶動的安達曼海張裂，改變了這個區域的構造環境，可能使得火山活動停息。直到~ 1 Ma，緬甸陸塊幾乎已達現今的位置，而在Sagaing fault活動所主導的地體背景下，形成局部拉張的環境，使得富集的岩石圈地函或殘存的地函楔產生小程度部分熔融，重新開始了這個地區新的岩漿活動。

關鍵字

緬甸 ； 晚新生代火山岩 ； 高鉀鈣鹼系列 ； Sagaing斷層 ； 小程度部分熔融 ； 氬氬定年

並列摘要

Burma is located in the eastern margin of the India-Asia collision zone. The most significant geologic feature in the region is arguably the Sagaing Fault, representing a dextral strike-slip fault system that links the eastern Himalayan Syntaxis in the north and the Andaman Sea in the south. This region is situated in a unique and “transitional” position between contractional (Himalayan) and extensional (Andaman Sea) tectonic settings, and furthermore characterized by the eruption of a series of Middle Miocene to Quaternary volcanoes along the Sagaing Fault. Here we report geochemical and Sr-Nd isotopic analyses of these young volcanic rocks from Mt. Popa and Monywa areas, Central Burma.
This study carried out Ar-Ar dating of some early stage of volcanic eruptions in the region. New Ar-Ar age results indicate two distinct stages of the volcanism, in mid-Miocene and Pleistocene, respectively. Two andesites from Mt. Popa yielded Ar-Ar ages of 13.25 ± 0.07 Ma and 13.27 ± 0.10 Ma, significantly older than previous conception of Pliocene ages. A drilled basalt from Monywa area yielded Ar-Ar age of 15.46 ± 0.08 Ma, which is broadly coeval with the mid-Miocene eruption in Mt. Popa volcano.
All the Burmese volcanic rocks show a potash-rich nature, with most of Mt. Popa samples (21 out of 25 analyses) plotting in the high-K calc-alkaline suite and Monywa samples (5 out of 8 analyses) in the shoshonitic suite. Some of the latter, specifically, resemble absarokite in terms of mineral constituents and trace element characteristics. All these rocks display significant depletions in the high field strength elements (HFSE; e.g., Nb, Ta and Ti), enrichments in the large ion lithophile elements (LILE; e.g., Cs, Rb, Ba) and light rare earth elements (LREE; e.g., La and Ce). Their overall incompatible trace element distribution patterns are similar to those of arc magmas formed in the subduction zone. Moreover, the Burmese rocks show a rather uniform isotopic composition, marked with high Nd (εNd = +4 to +1) and low Sr (87Sr/86Sr ≈ 0.7045) isotopic ratios. Consequently, we propose that the magma source of the high-K calc-alkaline rocks from Mt. Popa is the remnant and juvenile mantle wedge in the region despite the Indian Ocean slab beneath this part of the South Asia continent has already ceased its subduction. Under this framework, the Monywa shoshonites/absarokites represent products of small-degree melting (~5%) of a amphibole-bearing peridotite source in the mantle wedge.
Our results bear some implications for regional tectonic evolution. Given the dextral slip rate of ~20-50 mm/yr observed in the Sagaing fault, Mt. Popa volcano that erupted andesites at ~ 13 Ma was located ~250-600 km south of it present location as the result of the Indian Ocean slab’s subduction. Then, the volcanism may have become dormant owing to the active Sagaing fault and the Andaman Sea opening that eventually transformed the controlling tectonic force of the region from subduction to strike-slip. At ~ 1 Ma, when Mt. Popa nearly arrived to the present location, volcanism was renewed via small-degree melting of the remnant, enriched mantle wedge due probably to a trans-tension setting affiliated with regional tectonic transition.

並列關鍵字

Myanmar ； Burma ； High-K calc-alkaline suite ； Sagaing fault ； small-degree partial melting ； Ar-Ar dating

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