Petrology of Metamorphic Rocks Exposed in Mohauk Area, Momeik Township, the Boundary of Shan and Mandalay Region
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1 Yadanabon University Research Journal, 2019, Vol-10, No.1 Petrology of metamorphic rocks exposed in Mohauk area, Momeik Township, the boundary of Shan and Mandalay Region Tin Zaw Oo, Nwe Ni Win Abstract The research area is situated about 7 miles from Momeik and 8 miles north west of Mogok in the boundary of Shan and Mandalay Region. Generally, the study area falls within the northern part of the Mogok Metamorphic Belt (MMB). It is bounded by King chaung in western part and the well-known left-lateral strike slip fault, Momeik Fault, in northern boundary of the study area. And thus, the main lithologies exposed are variety of Mogok Series, they are paragneiss and orthogneiss, many types of marbles and calc-silicates that host gem-quality rubies, sapphire and some quartzite. These rocks are intruded by peridotite, granites and various suite of syenites. According to mineralogical and textural evidences study area is affected regional metamorphism and, locally, contact metamorphism (metasomatism). On the basis of mineral assemblages, upper amphibolite to granulite facies has been recognized. The time of metamorphism of the study area can be assigned to Early Cretaceous to Early Eocene and peak- condition occurred in early Cretaceous to Oligocene and younger event would take later, probably in Early Miocene. Key words: Mogok Metamorphic Belt, mineral assemblages, granulite facies Introduction The study area is situated about 8 miles north west of Mogok in Mandalay Region. The area is bounded by the latitude 23° 00' to 23° 07' N and the longitude 96° 22' 30" to 96° 30' 00" E and vertical grids 310 to 440 and horizontal grids 455 to 580 in UTM map No 2396 08. It covers about 175.5 square kilometers, with 13.5 kilometer in length and 13 kilometer in width.The location map and geological map of the study area is shown in figure (1) and (2). Map Symbols Town & village Stream River Highway Road Study area Figure (1) Location map of the study area. Lecturer, Dr, Department of Geology, Yadanarbon University Lecturer, Dr, Department of Geology, Yadanarbon University 2 Yadanabon University Research Journal, 2019, Vol-10, No.1 GEOLOGICAL MAP OF THE HTINSHUTAUNG AREA, MOGOK TOWNSHIPS Htinshutaung Late Miocene Map Index Middle 93 A/8 Oligocene Miocene 93 A/4 93 A/12 to Middle Miocene 93 B/1 93 B/5 93 B/9 Jurassic ? 93 B/2 93 B/6 93 B/10 Figure (2) GeologicalEarly map of the Htinshutaung area, Mogok Township Paleozoic Late Proterozoic 3 Yadanabon University Research Journal, 2019, Vol-10, No.1 In the research area, the metamorphic rocks cover almost of all of the area with minor intrusion of different types of magmatic rocks. Generally, based on the observations of mineral assemblages, the metamorphic rocks of the study area fall in the amphibolite to granulite facies of medium to high temperature orogenic metamorphism during the Cretaceous to early Eocene time and metamorphic grade increases from north-west to south-east in the area. Mineral Assemblages and Metamorphic Facies Assemblages of coexisting minerals in the metamorphic rock units provide the principal evidence for the metamorphic condition under which they were formed. These mineral assemblages listed below are from the important and distinct rock units established. Certain mineral assemblages may vary considerably in amount because of diverse lithologies and different bulk compositions. Accessory minerals are not listed in the mineral assemblages of some rocks. The distinctive mineral assemblages of the petrographical images are shown in figure (3). For convenience and to emphasize bulk compositional controls on the assemblages of various rock units, they are divided into (7) typical mineral assemblages (table 1). Grt Qtz Bt Qtz Sil Pl Kfs a 1mm b 1mm c 1mm d 1mm Figure (3). Photomicrophs of representative mineral assemblages and textural relationships in Grt,Bt,Sil,Crd gneiss of MMB. (a)Kfs+Bt+Qtz+Pl assemblages in Bt gneiss; (b) Grt+Sil+Qtz association in Grt gneiss; (c) Grt+Crd+Bt+Qtz with pressure shadow (d) Grt+Opx+Qtz assemblages in granulites.(XN. X 30) 4 Yadanabon University Research Journal, 2019, Vol-10, No.1 Table (1). The prominent mineral assemblages and metamorphic facies in calcareous and pelitic rocks of the study area. Index mineral, Pelitic rocks Calcareous rocks Facies (pelitic) Chlorite K-feldspar + quartz + Greenschist Biotite plagioclase + biotite chlorite + muscovite + calcite+diopside+ graphite Lower Garnet quartz + garnet + biotite +phlogopite±amphibole Amphibolite Calcite+spinel+diopside+ garnet+sillimanite+biotite Upper Sillimanite forsterite+phlogopite± graphite± + corderite Amphibolite corundum Garnet, garnet+sillimanite+quartz Calcite+forsterite+diopside+spinel Granulite Pyroxene orthopyroxene +K-feldspar ±corundum Study Area Figure (4). Temperature-pressure diagram showing two major types of metamorphic facies series proposed by Miyashiro (1973, 1994) (in Winter, 2010). The shade rectangle represents the condition of present area. Types of Metamorphism The dominant metamorphism of the study area is regional metamorphism which prevailed in metasedimentary rocks of prominent calcareous and peletic compositions. The regional metamorphism in the area is characterized by foliations, lineations, recrystallization, neomineralization, crenulation, minor folding and contorted foliations. The metamorphic rocks are relatively folded and brecciated throughout the study area, and they show a range in grade of metamorphism from amphibolite facies to granulite facies. The patterns of mineral zones have been disrupted to some degree by intrusions, faulting or other deformation and polymetamorphism. 5 Yadanabon University Research Journal, 2019, Vol-10, No.1 Mineral Zone The mineral paragenesis recognized in study area may be delineated to be indicative of the amphibolite facies to granulite facies in pelitic rocks. On the basis of mineral paragenesis, metapelitic rocks of the study are divided into biotite, garnet, and sillimanite zone, in order of increasing metamorphic grade. These zones are defined by the index minerals and appearance and disappearance of these minerals in pelitic rocks. Simplified zonal pattern of metapelitic rocks is shown in figure (5). The overall progression of mineralogical changes in the area is biotite-garnet- sillimanite. Characteristic mineral assemblages of the pelitic zones are as follows; Biotite zone - K-feldspar + quartz + biotite + plagioclase Garnet zone - garnet + biotite + quartz + chlorite Sillimanite zone / upper sillimanite zone - garnet + sillimanite + biotite + orthopyroxene Cordierite-garnet-K-feldspar zone - garnet + sillimanite + biotite + cordierite + K-feldspar The distinct zonal pattern, biotite zone, garnet zone and sillimanite zone, and mineral paragenesis in the present area is somewhat similar to that of Abukuma-type Facies series of Japan and Buchan-type series, and believed to be rather higher grade of Barrovian series of Scotland. The occurrence of migmatite gneiss with the leucosome intercalation (stromatic migmatite, Yardley, 1989) implies the higher temperature metamorphism. However these migmatites are often of much more restricted extent, closely associated with the intrusion of deeper-derived hot magmatic rocks. Garnet zone is defined by the first appearance of pyralspite garnet and by the absence of biotite, and chlorite change in composition in rock which is garnet-in isograd (Winkler, 1979). It is easily to trace in the field because garnet appears in a wide range of rock composition as well as in pelites. Frequently it forms conspicuous porphyroblasts and aids to distinguish almandine rich garnets typical of garnet zone from spessartine garnets which may develop at lower grades in Mn-rich sediments, but are usually fine-grained (Yardley, 1998). A mineral assemblage in garnet zone of the present area is generally almandine-rich garnet- chlorite-biotite-muscovite-quartz. Htinshutaung Figure.(5). Schematic metamorphic zone map of the present area (Nouthern part of Mogok), showing distribution of zonal pattern and location of magmatic rocks. The location of granulite sample is marked by star 6 Yadanabon University Research Journal, 2019, Vol-10, No.1 In sillimanite and upper sillimanite zone, the sillimanite occurs in the form of very fine needles which is matted together with biotite and quartz. There is no andalusite and staurolite. According to (Yardley, 1998) the upper sillimanite zone is characterized by the coexistence of sillimanite and K-feldspar, rather than by only one mineral and there is no longer muscovite in sillimanite zone. Muscovite breakdown may involve a melt phase and is close to the first developed second sillimanite isograd (Yardley, 1998). The mineral assemblage in the sillimanite/ upper sillimanite zone is garnet-sillimanite-biotite-quartz. At still higher grades, pelitic rocks develop assemblages with cordierite, garnet, K- feldspar and sillimanite in the cordierite-garnet-K-feldspar zone. Cordierite and garnet develop depending on the pressure condition. Cordierite is favoured by low pressure and garnet by higher pressure. In the cordierite-garnet-K-feldspar zone, garnet- sillimanite-biotite- cordierite and K-feldspar are recognized. The garnet-corderite-K-feldspar assemblage is typical of high grade pelitic migmatite, and is often taken to mark the beginning of the granulite facies, which represents the highest grade zone of the present area. Winkler (1979) also noted that the transformation of low grade to medium grade is marked by the "almandine-in" in pelitic rocks