Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): Implications for tectonic settings

摘要

Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinet lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12-45.19 wt.% MgO, 1.1-2.74 wt.% Al2O3 and 0.67-2.67 wt.% CaO. Their spinels have Cr# values of 0.12-0.4, yielding fractional melting degrees of 3-15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08-48.73 wt.% MgO, 0.09-0.99 wt.% Al2O3 and 0.07-0.8 wt.% CaO. Their spinels have Cr# values of 0.3-0.73, giving 12-21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88-48.16 wt% MgO, 0.13-1.65 wt.% Al2O3 and 0.1-1.68 wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO2 contents (i.e., <0.2 wt.%) have variable Cr# values of 0.28-0.72, yielding 11-21% degrees of fractional melting. Clinopyroxenes in all Kalaymyo peridotites display flat patterns in MREE and HREE, but variably LREE-depleted patterns. They also show remarkably negative Sr and Zr anomalies. Plagioclases in the Kalaymyo plagioclase peridotites display a significant Eu positive anomaly and have similar contents of LREE to the co-existing clinopyroxenes. In contrast, they have much lower contents of HREE and MREE than the clinopyroxene. Clinopyroxenes in the Myitkyina harzburgites display consistent patterns from HREE to MREE but different patterns in LREE. Clinopyroxenes with LREE-depleted patterns display remarkable negative Sr and Zr anomalies, whereas clinopyroxenes with LREE-flat patterns do not show Sr or Zr anomaly. However, the LREE contents of clinopyroxenes in both Kalaymyo and Myitkyina peridotites are too enriched to be produced by degrees of melting corresponding to spinel Cr# values. This indicates that they were variably re-enriched after melt depletion, which is also supported by mineral microtextures, e.g., embayment of olivine within orthopyroxene. Plagioclase in the Kalaymyo peridotites was produced during melt-peridotite reaction rather than through breakdown of spinel. Chemical compositions support the co-existence of both refractory and fertile peridotites in both Kalaymyo and Myitkyina ophiolites. The refractory peridotites have compositions similar to the forearc peridotites, whereas the fertile peridotites are compositionally similar to the abyssal peridotites. Therefore, both ophiolites have experienced complicated evolution in different tectonic settings. (C) 2016 Elsevier B.V. All rights reserved.