Late Cenozoic Volcanic Rocks from Burma: Geochemical Characteristics and Petrogenesis
Date Issued
2008
Date
2008
Author(s)
Yang, Hsiao-Ming
Abstract
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.
Subjects
Myanmar
Burma
High-K calc-alkaline suite
Sagaing fault
small-degree partial melting
Ar-Ar dating
Type
thesis
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