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Relationship better the Igneous Activity and the Tectonic Setting

Plate tectonic interpretations relating to the igneous activities of Myanmar were proposed by many workers. According to the previous workers, tectonic setting of the study area might be reconstructed as below.

At early Mesozoic, the Indian plate subducted under the Eurasian plate at the eastern part of Indo-Burman Ranges. The flysch-type Triassic sediments were laid down in the trench during Mid-to-Late Triassic. At the end of Triassic, calc-alkaline magmatic volcanic arc complex was (Khin Zaw, 1986). developed at the site of the present day Central Volcanic Line

This flysch-type Triassic sediments were reported in the southern adjoining area.

(Thet Tun, 1994), and its un, 1994), and its northern continuation in Bonchaung-Sadwin area (Tin Nyunt 0o, 1991). These sediments and associated rocks were described under the name of Shwedaung Formation by DGSE (1976). This unit is not exposed in the present area.

But,it is certain that the subduction and trench sedimentation had occurred in this area at Mid-to-Late Triassic time.

Maung Thein (1983) postulated that this east-dipping subduction zone was shifted to the west throughout Mesozoic and Cenozoic, but this westward migration and plate convergence does not necessarily rule out the change of Paleo-Benioff zone’s dipping from low to high (Mitchell and Garson 1972, Mitchell, 1973).(see in Khin Zaw, 1986).

At the end of Jurassic, the magmatic-volcanic arc collided with the continental foreland at the east. During this collision of continental foreland with the older magmatic volcanic arc, the continental crust had thickened and it is followed by the underthrusting of continental margin under the overriding older arc and a zone of remelting along the upper part of continental, sialic basement occurred. Therefore, the granitoid plutons of the central granitoid belt of Myanmar would have initiated during Late Jurassic to Early Cretaceous interval (Khin Zaw, 1986).

Myint Thein, Hla Moe, and Ko Ko Gyi (1996) recently reported that the arc Volcanism, represented by Mawgyi Andesite, erupted during the Jurassic to Early Cretaceous.

Based on the above interpretations, an andesitic volcanism seem to be taken place Based on above at the far west of the remelted zone during the collision of continental foreland with the older magmatic-volcanic arc. The partial melting of subducted oceanic crust and overlying sediments would have provided the eruption of andesitic rocks (30-called Mawgyi Andesite) in thinner part of continental margin and it was followed by the eruption of rhyolitic magma which was the differentiation product of the andesitic magma.

During Cretaceous to Eocene interval, the subduction at the eastern part of the

present day Indo-Burman Ranges was shifted to the west. Due to this westward migration, the magmatic front at the east at the margin of Shan Plateau was also shifted tothe west with formation of a new magmatic-volcanic arc at the site of present Central Volcanic Line extending from Mt. Popa through Monywa. Salingyi. Wuntho to . This renewed magmatic-volcanic arc is characterized by older, Late Cretaceous, epizonal I-type granitoid plutons of western granitoid belt at north (eg.Wuntho) and younger Mio-Pliocene to sub-recent, largely andesitic, porphyry Cu-related (e.g., Monywa and Mt. Popa) at south (Khin Zaw, 1986).

These events occurning during Late Cretaceous to Eocene interval was supported yLate Cretaceous intrusion of Kanzachaung Batholith. The diorite and leucogranite of area were possibly later phase, differentiated portion of the major granodiorite pluton occurring in surrounding area. In the study area the eruption of dacite (so-called Kawdaw Dacite) would be the latest activity of Cretaceous-Eocene igneous activities, related to the subduction of Indian plate beneath the Eurasian plate.

The post-Eocene tectonic history is profoundly related to the Sagaing (transform). Fault which was responsible for the development of the present configuration of most granitoid plutons in Myanmar.

The brief account of tectonic evolution of the study area may be constructed as. At the end of Jurassic, the older magmatic arc collided with the continental

follows.

foreland to the east. This collision caused the intrusion of granodioritic pluton thíckened part of the continental crust during the Jurassic to Early Cretaceous. At thesame time, the subduction of oceanic plate beneath the continental plate gave rise to are volcanism, defined as Mawgyi arc by Myint Thein, Hla Moe, and Ko Ko Gyi (1996), in the thinner part of the continental margin.

Particularly, at Late Cretaceous, the major granodioritic pluton with its later phaseintrusions invaded into the volcanic are (Mawgyi arc). Finally, the local eruption took as a latest activity of Cretaceous Focene activities in the study area.

The schematic tectonic model illustrating the igneous activities of the study area modified from Khin Zaw (1986).

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