Petrogenesis of the Payangazu Complex in Southern Mandalay, Central Myanmar and Its Tectonic Implications

Abstract

The Payangazu complex in the central Myanmar is composed mainly of quartz diorite, granodiorite, and some synplutonic mafic dikes. The quartz diorite and granodiorite have zircon U-Pb ages of 130.5±4.0 (MSWD=3.5) and 118.4±2.5 Ma (MSWD=2.4), respectively. Rock samples of the quartz diorite and granodiorite are metaluminous, enriched in large-ion lithophile elements like LREE, Rb, Th, and U, and depleted in high field-strength elements such as HREE, Nb, Ta, P, and Ti, indicative of arc-type magmatic affinities. Whole rock samples of the quartz diorite have eHf(t) value of +0.6, initial 87Sr/86Sr ratios of 0.708 6 to 0.710 0, and eNd(t) values of -4.8 to -4.9; whereas rocks of the granodiorite are relatively isotopically enriched, with eHf(t) values of -5.1 to -7.2, initial 87Sr/86Sr ratios of 0.711 7 to 0.711 8, and eNd(t) values of -8.7 to -8.8. The isotopic data together with the high Mg# (both the quartz diorite and granodiorite have Mg# values of >40) suggest a strong involvement of mantle materials in the genesis of the parent magmas. The possible petrogenetic process may be that the ascending of melts from partial melting of metasomatized mantle wedge triggered by dehydration of subducted slab resulted in partial melting of the lower crust and mixed with the latter. These Early Cretaceous intrusions from the complex are older than those found in the eastern Wuntho-Popa arc in western Myanmar, eastern Himalaya, and western Yunnan which are interpreted to be related to the Neo-Tethyan subduction, and have eNd(t), eHf(t) values lower than the latter. On the contrary, the ages and geochemical characteristics of the Payangazu complex are consistent with some of the intrusions in the northern magmatic belt in Tibet, eastern Himalaya, and western Yunnan which are believed to be associated with the subduction of the Bangong-Nujiang Ocean crust. Thus, we propose that the Early Cretaceous intrusions in the central Myanmar are most likely related to the southward subduction of an ocean slab that was possibly an extension of the Bangong-Nujiang Ocean.