Petrology and Geochemistry of Basalts from Tonnge Area, Tigyaing Township, Sagaing Region, Myanmar

Tonnge basalt is a relatively small basaltic field situated in the Tigyaing Township, Sagaing Region, Myanmar. This basaltic field is considered as one of the occurrences of younger extrusive rocks along the major strike-slip fault of central Myanmar. The basaltic lava flows erupted and rested upon the Upper Miocene-Pliocene Irrawaddy Sandstone. Petrological and geochemical studies of the Tonnge basalts have not been carried out yet. This paper re-ports the results of petrological and geochemical investigation on the basalts from Tonnge area. These data have provided insight into the origin and petrogenetic processes during evolution. The petrographic characteristics of the basalts have been studied using polarizing microscope. The major- and trace-element compositions of basalt samples were analyzed using X-Ray Fluorescence. The Tonnge basalts are porphyritic, holocrystalline and consist of phenocrysts of olivine, plagioclase ± clinopyroxene which are set in the fine-grained intergranular groundmass of olivine, pyroxene, and opaque minerals. The basalts show typical characters of alkali olivine basalts. Trace element assemblage of the studied basalts plots on various discrimination diagrams as within-plate basalt erupted in the continental setting. A relatively small volume, monogenetic nature and unique geochemical characteristics of the basaltic lavas suggest that they are derived from a single, short-lived, discrete parental magma as a result of partial melting.


Introduction
Myanmar is located in the eastern marginal zone of India-Asia collision and marked by a prominent still-active dextral Sagaing Fault which connects the eastern Himalayan Syntaxis to the north and the Andaman Sea to the south [1] [2] [3]. This major strike-slip fault medially divides Myanmar into the eastern part (Shan-Thai Block) and the western part (West Burma Block) [4]. The former consists of the Shan Plateau, the Mogok Metamorphic Belt and the Shan Scarps, whereas the later comprises the Indo-Burma Range, the Wuntho-Popa magmatic arc and overlying Cretaceous-Paleogene sediments [3] [5] (Figure   1(a)).
Cenozoic magmatism is one of the prominent features in the geologic history of Myanmar. Magma generation processes are related to the India-Asia collision that was commenced during Early Eocene and associated regional plate reorganization, a transition from oblique subduction to dextral movement in Miocene, and subsequent "rollback" of the subducted Indian oceanic lithosphere in the Quaternary [3]. Based on geochronological data from the previous studies [3] [6] [7], the volcanic centers in the central Myanmar are mostly recognized as Quaternary in age.
Tonnge basalt is considered to be one of the occurrences of younger extrusive rocks along the dextral Sagaing Fault of central Myanmar. It is an intraplate

Geology of the Tonnge Basalt
The basaltic field in the Tonnge area displays a general circular outline ( Figure   1  The loss on ignition (LOI) was measured for all samples by weight difference after ignition at 1000˚C.

Petrography
The    Sample ID  Tn-01  Tn-02  Tn-03  Tn-04  Tn-05  Tn-06  Tn-07  Tn-08  Tn-09  Tn-am Tn-am1 TN- [12] showing the basalt samples from Tonnge area in the field of trachy basalt. The line separating the sub-alkaline and alkaline subdivisions was after Irvine and Baragar [14]; (b) K 2 O vs. SiO 2 variation diagram [13] showing the alkaline composition of Tonnge basalts. The classification for K 2 O-SiO 2 contents and shoshonitic series was after Gill [15], and Peccerillo and Taylor [13], respectively.   Figure 6. Tectonic discrimination diagrams for basalts from Tonnge area (a) Nb-Zr-Y diagram [16]; (b) Ti-Zr-Y diagram [17]; (c) Nb/Y-Ti/Y diagram [18] and (d) Zr vs. Zr/Y diagram [19]. with SiO 2 contents. It indicates that possible fractionation of early formed mafic minerals such as olivine occurs during evolution of magma and no apparent fractional crystallization involved during the ascent of the basaltic magma. Incompatible trace element abundances and HFSE ratios are considered as suitable parameters in order to discriminate the tectonic environment for the eruption of basaltic magmas. The basalt samples are plotted on the various tectonic discrimination diagrams such as Nb-Zr-Y, Ti-Zr-Y, Nb/Y-Ti/Y and Zr vs. Zr/Y (Figures 6(a)-(d)). In the plot of Nb-Zr-Y diagram (Figure 6(a)) [16], all basalt samples are plotted in the field of the within-plate alkali basalt (WPA). The plots of these samples also cluster in the field of within plate basalts on the Ti-Zr-Y (Figure 6(b)) [17] and Nb/Y-Ti/Y diagrams (Figure 6(c)) [18]. On Zr-Zr/Y diagram, it is possible to discriminate the basalts of Tonnge area as continental basalt (Figure 6(d)) [19].

Conclusions
Petrographical studies of basalts from the Tonnge area indicated that they were composed primarily of olivine, plagioclase and pyroxene. They were classified as olivine basalt as olivine was considered to be the most common essential mineral phase, followed by other minerals such as plagioclase, pyroxene, and Fe-Ti oxides. Absence of quartz and presence of fairly abundant olivine (as both phenocrysts and groundmass) in the studied samples indicated that they were typical alkali olivine basalts [20].
Geochemical and tectonic discrimination diagrams indicated that the basaltic lavas were formed by the eruption from central vent in the continental intraplate setting. The unique geochemical characteristics of basalts indicated that they were derived from the discrete magma batch as a result of partial melting of continental lithospheric mantle source and evolved independently by variable degrees of fractional crystallization. The abundance of olivine phenocrysts in the basalts suggested that the melt has ascended from its source and subsequently detained in the magma chamber and phenocrysts were settled down during subsequent ascent. A positive correlation between decreasing Ni and Cr with respect to decreasing MgO suggested that fractionation of olivine and clinopyroxene occurred during the evolution of the basaltic magma. The higher ratios of K 2 O/P 2 O 5 in the basalts pointed to the fact that assimilation of magma with the silicic crystal component or with wall rocks occurred during their emplacement and/or ascent as within-plate basalt in the continental setting.