petrography of metapelitic granulite ppt (113)

Ipsita Mitra

Regd. No:12221621013 of 2012-2013

Roll no:12221613

Theoretical modelling of peak mineral assemblages of metapelitic granulites in Chilka area.

by

contents

• 1. petrographic analysis of metapelitic granulite by microscopic study

• 2.petrography of metapelitic granulite by perple_x software

• The present study area is located at the north-eastern part of Chilka area,India. It extends along a linear belt from Khurda to Sunakhala covering approximately 700 sq.km area.

• The Chilka Lake domain comprises of metapelitic granulites with small amounts of magmatic leptynite enclosing charnockite and intrusion of anorthosites.

• Metapelitic and charnockite ground granulites exposed around chilka preserves multiphase P-T record

Objective of microscopic study

• To study the mineral content &textural relationship within the rock.

• To develop the sequence of development of the minerals.

Texture

• Orthopyroxene, cordierite, garnet, quartz,k-feldspar, plagioclase,spinel,biotite are the major constituents.

• The overall texture of the rock is massive. Quartz, feldspars and smaller cordierite make up the matrix whereas garnet, larger cordierite and orthopyroxene grains constitute the porphyroblastic phases. The distribution of quartz feldspar and cordierite grains is random.

Texture

garnet

orthopyroxene

biotite

cordierite

quartz

Sequence of development

• Bt1 + Qtz + (Sil)/(Pl) → Grt +Opx1+Crd1+ melt ………(1)• The peak mineral assemblage is represented by Grt + Opx +

Crd + Qtz (+ Pl ± Kfs in leucosome). • Grt + (Sil/Qtz) → Opx2 + Crd2 + Pl2 (+Spl) ………….(2)• Opx2 + Crd2 +H2O→Bt+ Qtz.

Microscopic study

• Symplectites of garnets:

• Cuspate grain boundaries

• Evidence of melting:

• Embayment of garnet: Garnet has been partially engulfed by Opx and Cordierite

Phase diagram modelling of the metapelitic granulite

• Petrographic analysis by perple_x software.• Perple_x is the thermodynamic calculation that helps in creating pseudosection

.It is a collection of programmes for calculating phase diagrams.• Here the temperature range is 873-1273k and pressure range is-5000-10000b.• The purpose of the work is to accurately pinpoint the temperature and pressure

condition during which peak metamorphism was achieved for the studied metapelitic granulite.

• Pseudosection is a phase diagram for a particular chemical system or bulk composition that illustrates several important fields of stability of mineral assemblages for that system.

• Pseudosection only include those reactions that affect a particular composition.The Pseudosection shows fields of different phase assemblages for single bulk composition of rock

PSEUDOSECTION Wt % of compounds : K2O ,Na2O, CaO , FeO , MgO ,Al2O3 ,SiO2, TiO2, H20

1.57 , 1.23, 0.94, 11.69, 11.17, 13.05, 59.37, 0.77, 0.50

Pressure : 5-10 Kbar

Temperature : 873K-1273K

PSEUDOSECTION SHOWING PLAGIOCLASE(GREEN) AND GARNET(PINK) ISOPLEUTHS

INTERPRETATION

• From the petrographic analysis(mineral, composition chemistry and texture study) the T-P window of the peak assemblages are found Grt + Opx + Crd + Qtz (+ Pl ± Kfs in leucosome.

• The point of intersection of Gt (HP) and Pl (h), within this T-P window gives the temperature and pressure of the peak mineral assemblages. . During decompression event, porphyroblastic garnet breaks to produce orthpoyroxene + spinel + cordierite in domain .The retrograde path calculated using the bulk composition with the help of pseudosection modelling exhibits the development of symplectic orthopyroxene + cordierite+- spinel from decomposition of garnet

• Gt(HP)-0.57 and Pl(h)-0.43 isopleuths lies within the T-P window and from the point of intersection of this two isopleuths the temperature and pressure of the peak assemblage is estimated as-

• Temp:860-880°C• Pressure:7.2-7.5 Kb

References

Dobmeier,C.J.Raith, M.M:2003.Crustal architecture and evolution of the

Eastern Ghats Belt and adjacent regions of India.

Geochronology of the 983‐Ma Chilka Lake Anorthosite, Eastern Ghats Belt, India: Implications for Pre‐Gondwana Tectonics Author(s): Nilanjan Chatterjee, James L. Crowley, Amalbikash Mukherjee, and Subhasish Das Source: The Journal of Geology, Vol. 116, No. 2 (March 2008), pp. 105-118.

Dobmeier,C.,Simmat,R.,2002.Post –Grenvillean transpression in the chilka lake area,Eatern Ghats eBelt-implications for the geological evolution of peninsular india.Precambrian Res.113,243-268.