Mineralogical Study Of Peridot From Pyang-Gaung,Myanmar

Pyang-Gaung is located at the northern edge of the famous Mogok Stone Tract,it is the main source of large grain peridot in the world gem market.The tectonic setting of Pyang-Gaung is in the Mogok Metamorphic belt,the peridotite in Pyang-Gaung belongs to orogenic peridotite.The gemology study of Pyang-Gaung peidot shows that the refrative index and special gravity are within the theoretic value of peridot.The characteristic inclusion in peridot is the "lily pad" and smoke-like veiling,which suggest an uplift process.The peridotite samples are carefaully observed,they are dunite,huzburgite and lherzolite,all of them are partially altered,the X-ray powder diffraction data shows that the altered mineral is mainly serpentine.Dunite has the highest degree of alteration.The inclusions in the olivine are antigorite,magnetite and brucite,which is a common phenomenon in the orogenic peridotite.Lherzorite contains a relatively high proportion of euhedral magenite but no chromite is found,so we suppose there was a serpentinazation before the peridotite formed,which altered the chromite to magnetite.The minerals in the peridotite often have exsolution which can suggest an uplifting process,for example the olivine has directional arranged chrome spinel lamellae and orthpyroxene has clinopyroxene exsolution.The elctronic probe data shows that Pyang-Gaung peridot has a very high Fo arond 95,close to the pure fosterite,Mg#?96,indicating the peridotite isn't directly from mantle,but formed trough a metamorphic process.The CaO congtent is 0.001-0.037 wt.%,suggesting that Pyang-Gaung peridot isn't form through magmatic crystallization.Using the LA-ICP-MS to anylise the trace element in the peridotite minerals.Clinopyroxene has a W REE pattern,showing a LREE enrichment,which suggest there was fluid metasomatism.But the olivine extremely deplete in REE,it can't be the outcome of fluid crystallization.According to the evidences above,we infer a probable genetic model of Pyang-Gaung peridot:firstly,the mantle peridotite intruded into crust and was altered to serpentinite through some matasomatism process.Later on,the high pressure metamorphsim in the Mogok Metamorphic Belt push the serpentinite to experience a dehydrate process and recrystallized to peridotite again,which contains the large grain gem peridot.During the orogenic belt uplifting process,the gem bearing peridotite expose to the earth's surface and experience serpentinization again.