Magma genesis of Popa volcano
Popa volcano
Geologic and tectonic settings of Myanmar are the same with Japan; this country lies at convergent margin, and therefore, a lot of earthquakes and some volcanoes are present. Several geological research of Myanmar have been conducted last decade and new discoveries have reported. For example, it was reported that three Quaternary volcanoes are present in the Central Lowlands; Popa, Monywa, and Singu (Fig 1). Popa volcano, the largest of these volcanoes, should be an active volcano because last eruption was reported to be less than 10,000 years before present (Fig 2).
Fig 1. Simplified geologic map of Myanmar. Locations of Quaternary volcanoes in central Myanmar with main tectonic features and depth contour lines of the upper surface of the subducting slab (blue lines) are shown.
Fig 2. Popa volcano.
Establishment of growth history
Indian plate, which is subducted from Bengal Nay, is present beneath central Myanmar, and plate-top depth is greater than 120 km (Fig 1). Similar to Northeast Japan, subduction-related magmatism (i.e., arc magmatism) served a lot of eruptions in central Myanmar and constructed stratovolcanoes.
Information about the construction history for these Myanmar volcanoes is limited, and we report the first comprehensive geological dataset for Popa volcano. New results from field observations, radiometric age determinations and geochemical analyses outline the first detailed growth history for this arc stratovolcano.
The products of Popa have been divided into 'older volcanics (10~20 million years ago)' and 'younger volcanics (younger than 1 million years ago)''. Between these two stages, Pliocene sediments of the Irrawaddy Formation are present (3 million years ago). The main body of Popa is composed of the younger volcanics, which is subdivided into lower plateau (960~650 kilo years ago) and upper stratovolcano stages (younger than 330 kilo years ago).
Fig 3. Geological map and stratigraphy of Popa volcano. Sample locations and radiometric ages are also shown.
Magma genesis
We found that the Popa stratovolcano is predominantly composed of volcanic rocks with adakitic affinity (I call adakite here after) based on petrological, geochemical, mineralogical and Sr-Nd isotopic data. Adakite is characterized by its high Sr/Y than typical arc volcanic rocks (Fig 4a). The adakites were erupted in western Aleutian arc and St Helens in America. The technical term 'adakite' is named after Adak Island, the place of first research for adakite was conducted, in western Aleutian arc.
Fig 4. Chemical characteristics of Popa volcanic rocks. (A) shows Popa adaite is plotted on an area of adakite compositions. (B) shows isotopic compositions of Popa adaite are different from those of normal adakites.
Adakite is proposed to be formed by partial melting of subducted oceanic plate (Fig 5a). However, we have founded that Popa adakites have lower 143Nd/144Nd than normal arc volcanic rocks (Fig 4b), and therefore, we have concluded that magma genesis of Popa adakites are different from normal adakites. We have suggested that Popa adakites were produced by partial melting of mantle wedge with wet condition because water from the subducted oceanic crust has low 143Nd/144Nd. The partial melts were probably evolved in magma chambers to produce the adakite magma (Fig 5b).
Fig 5. Magma genesis of (A) normal adakite magma and (B) Myamnar (Popa) adakite magma.