Browsing by Author "Nyunt, Thet Tin"

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    Petrological and geochemical contribution to the origin of jadeitite and associated rocks of the Tawmaw Area, Kachin State, Myanmar
    (2009) Nyunt, Thet Tin; Massonne, Hans-Joachim (Prof. Dr.)
    Jade is the trade term for the jewellers but mineralogically two types of jade exist: jadeite (pyroxene end-member NaAlSi2O6); and Ca-amphibole (end-member Ca2Mg5Si8O10 (OH)2), nephrite. Jadeite jade (petrological term: jadeitite) is rarer and also more valuable than nephrite jade. Jadeitite is found only in a few countries including Myanmar, Japan and Guatemala where corresponding deposits are associated with major strike slip faults and high-pressure tectonic zones and always associated with serpentinites and high-pressure rocks. The recent geotectonic evolution of Myanmar is related to the India-Asia collision. In the central part of Myanmar the N-S trending Central Burma Basin is located which is filled with Eocene to Plio-Quaternary sediments. The sedimentary association is related to a fore-arc basin which resulted from northeastward-directed subduction of the Bengal oceanic crust beneath Myanmar. The study area, the Tawmaw area, is located in the northern part of the Central Burma Basin at the western part of the Sagaing strike-slip fault zone around 15 km northwest of the city of Pharkant. This area is part of the so-called “jade mines area” or “jade belt” which extends over ca. 2600 km2. Glaucophane schist is the most common rock which is exposed together with epidote schist in the eastern part of the Tawmaw area where serpentinized peridotite-dunite and jadeite-bearing rocks also occur. Garnet-mica schists also occur in tectonic contact with the serpentinized peridotite-dunite in the western part of the Tawmaw area. A larger elongated ultramafic body of late Cretaceous to early Eocene (dismembered) ophiolite unit occur in the central part. Jadeitite is found as vein-like or lens-shape bodies in the serpentinized peridotite-dunite body. In this jadeitite-bearing unit it is possible to distinguish between two main zones. The Outer Zone, occurring at the border of ultramafic rocks, is made up of thin layers chlorite, actinolite schists and amphibole felses (so-called wall rocks), which can, however, also lack. Jadeite and kosmochlor are also included in the amphibole felses together with chromite. The Central Zone is composed of the vein-like or massive lenses of jadeitite but can also contain albitite. The petrological study revealed that the supply of chromium in jadeite is from chromite of the former (and still adjacent) peridotite. This can be clearly seen in maw-sit-sit and amphibole felses where kosmochlor formed from destabilized chromite keeping the size of the original chromite grain. Thus, chromium is immobile in contrast to strongly mobile Na. Moreover, it means that sperpentinite and not the country rocks surrounding serpentinite was replaced by the wall-rocks. In the (nearly) pure jadeitite, vesuvianite, identified by RAMAN-spectroscopy, was found for the first time in one sample only. Vesuvianite normally occurs in low-pressure environments such as contact aureoles, rodingite/metarodingite, and skarns. This study shows that this phase can also appear in a high-pressure environment. The mineral assemblage of vesuvianite + end-member jadeite does not show any indication for non-equilibrium. The composition of this vesuvianite is characterized by up to 1.5 wt.% Na2O that is higher than for vesuvianite described from elsewhere. A major mechanism to incorporate Na into vesuvianite is the charge-balanced substitution Ca + Al = Na + Ti. A detailed petrological investigation of mica schists and blueschists in the immediate vicinity of the serpentinite was undertaken to understand the formation of jadeitite and jadeite-albite rocks in the Tawmaw area. P-T conditions of garnet- and glaucophane-bearing mica schists were calculated using the computer program PTGIBBS (Brandelik and Massonne, 2004) for the thermodynamic calculation of mineral equilibria. In addition, P-T pseudosections, considering the corresponding chemical bulk rock compositions, were calculated with the computer program package PERPLE_X (Connolly, 2005). The obtained results indicate that the P-T condition of metamorphic stage I (e.g. related to the core of garnet in mica schists) were 470-540° C and 16-19 kbar. During stage III (rim composition of garnet, phengite rim) conditions of 5.5-8.5 kbar and 570-630°C were reached. The P-T condition of stage II are intermediate between those of stages I and III. The P-T conditions estimated for glaucophane schists are similar to those of the garnet mica schist. For the core of the garnet, 470-490°C, 18-20 kbar are derived, and the P-T condition of rim compositions of garnet from the glaucophane schist result in 560-620°C, 7-12 kbar. The here derived P-T evolution of the glaucophane schist and garnet mica schist is different to previous interpretions of other authors (Shi et al. 2003, 2001, Goffé et al. 2000). Monazite in garnet mica schists were analysed with the electron microprobe also for their U, Th, and Pb contents. 43 analyses yielded an age range between <15 and 120 Ma.