Petrogenesis and tectonic implications of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar

Abstract

Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An37–91), whereas plagioclase in the plagiogranites is less calcic (An1–40). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97–52.88wt% SiO2, which show negative correlations with Al2O3, CaO and MgO, but positive correlations with Na2O, P2O5 and TiO2. Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87Sr/86Sr ratios of 0.703938–0.706609 and εNd(t) values of +2.4–+7.2, and relatively variable Hf isotopes, with εHf(t) values of +13.4–+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93–77.93wt% SiO2, 1.27–10.79wt% Na2O and 0.05–0.71wt% K2O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO2 contents (0.03–0.2wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB but were generated by hydrous melting of gabbros. This process is also supported by the nearly constant contents of both La and Yb with increasing SiO2 of the plagiogranites. The Sr-Nd-Hf isotopes of the plagiogranites indicate the involvement of crustal component in their genesis. We proposed that the Myitkyina plagiogranites were generated by partial melting of the mafic protoliths in the subducted slab.