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Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes (GREAT 2008), 4th IGCP 516,and 5th APSEG;

November 24-26, 2008, Bangkok, Thailand

Extended AbstractThe geology o Arunachal Pradesh is very signifcant as it shows juxtaposition o three dierent units, namely the

Himalayan block, the Mishmi terrain and the Central Burmese terrain and this area is sandwiched between two nearly

orthogonal suture zones; the Himalayan collision belt to the north and the Indo-Burman collision belt to the east. The

eastern part o Arunachal Pradesh orms the Eastern syntaxis region where a bend in regional strikes o the majortectonic units rom ENEWSW to NWSE is developed (Thakur and Jain, 1975).

The Lesser Himalayan unit in this region comprises o low grade metamorphic rocks. It rests on the Mishmi thrust

and thrusted over the recent sediments o Lohit River (Gururajan & Choudhury, 2003). The rocks o this unit are chlo-

ritebiotite schist, phyllite, quartzite and some metabasics and the overlying Mishmi crystalline rocks contain gar-

netstaurolitekyanite bearing mica schists. The pelitic schist o biotite zone contains chlorite, biotite, muscovite and

quartz. Very small tiny crystals o Mn-rich garnets are also ound in this grade. Detailed petrographic study shows a

compositional banding demarcated by Q and M domains is characteristic o this grade. Muscovite, chlorite, biotite

grains show strong alignment parallel to the band which gives rise to the regional oliation plane (S2) and this regional

oliation plane was olded and crenulated to give rise to locally crenulation cleavage. In garnet and staurolite grade

rocks, the porphyroblastic minerals are staurolite and/or garnet along with quartz, plagioclase eldspar, biotite, musco-

vite, graphite chlorite. In garnet zone the grain size o the rocks is much coarser than the previous zone. The garnet

porphyroblasts show multiple episodes o growth. Large garnet grains contain inclusions o quartz mainly aligned as

an internal oliation (Si) which is at places parallels with the external oliation (Se) and sometimes oblique with Se. In

some cases garnet grains contain numerous inclusions o quartz but the inclusions do not show any preerred orien-

tation. Garnet shows mainly three types o age relation with respect to the dominant schistosity, (1) pre-tectonic with

random inclusions at the core and swerving o S2 around the porphyroblasts, (2) early to syn tectonic where the Si is

oblique with Se and Se warps the garnet grains and (3) syn to late tectonic garnet where the Si is continuous with Se.

The amount o chlorite decreases in this zone

In Staurolite zone rocks, porphyroblastic minerals are highly ractured and contain inclusions o quartz. Some stauro-

lite grains also contain inclusion o muscovite. Chlorite is totally absent in this rock. The schistosity S2 wraps around

the garnet grains. Thereore these garnets are pre-tectonic with respect to S2. The inclusions o quartz within garnet

grains are random.

Petrographic study reveals ollowing key reactions:

1. Chl + Ms Grt + Bt + Qtz + H2O .. (1)

And/or

Chl + Qtz Grt + H2O . (2)

2. Chl + Ms + Qtz

St + Bt + H2O . (3)

Both isopleth geothermobarometry and geothermobarometric study suggest a prograde evaluation o P 10 Kbar and

T 600C - 650C at garnet grade rock. Isopleth thermobarometry has been done by constructing pseudosections in

Petrology of Lesser Himalayan Schists of Eastern Arunachal Pradesh, India:

Implication of Inverted Metamorphic Sequence

Paramita Paul

Department o Geological Sciences, Jadavpur University, Kolkata 700032, India.

E-mail: paramita.geo@gmail.com

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Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes (GREAT 2008), 4th IGCP 516,and 5th APSEG;

November 24-26, 2008, Bangkok, Thailand

CaO-MnO-Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O system using the sotware Perple_x 07. The pseudosections are

contoured to show variation o mode and compositional isopleths o key minerals. These contoured pseudosections areused to determine the PT condition o a particular rock. In geothermobarometric study the peak PT conditions were

estimated rom garnetbiotite (GrtBt) FeMg exchange thermometer and garnetaluminosilicateplagioclasequartz

(GASP) barometer simultaneously in GB GASP program. An additional calculation has been done according to the

average P-T method o Holland & Powell (1994), using the Thermocalc program. The mineral compositions were de-

termined by spot analysis with an electron microprobe.

According to isopleth thermobarometry as well as geothermobarometric study the rocks o this region show

clock wise P-T path that means ollowing an initial increase in P reaching Pmax the rocks suer near isobaric heating,

urther heating accompanying decompression reaching Tmax and then isothermal decompression & fnally isobaric

cooling.

Keywords: Lesser Himalayan schists, Metamorphism, Arunachal Pradesh, India

References

Gururajan N. S. and Choudhury B.K., 2003. Geology and tectonic history o the Lohit Valley, Eastern Arunachal

Pradesh. Journal of Asian Earth Sciences., 21, 731-741.

Thakur, V.C. and jain, A.K., 1975. Some observation on deormation, metamorphism and tectonic signifcance o

the rocks o some parts o the Mishmi Hills (Lohit District) (NEFA).Arunachal Pradesh. Himalayan Geology,

5, 339-364.

Powell R. and Holland TJB. 1994. Optimal geothermometry and geobarometry. American Mineralogist, 79, 120-

133.