RECONNAISSANCE Rb-Sr DATING OF THE PRECAMBRIAN ROCKS

OF SOUTHERN PENINSULAR INDIA

PRESENTED BYDEEPCHAND VGOL 160507DEPARTMENT OF GEOLOGYUNIVERSITY OF KERALA

ByA. R. CRAWFORD

Department of Geophysics and Geochemistry, Australian National University, Canberra

Rb-Sr Dating

Isotopes of Rb85Rb (72.2%) 87Rb (27.8%)

87Rb has a half-life of 4.92×1010 years

Isotopes of Sr84Sr (0.56%) 86Sr (9.86%)87Sr (7.0%) 88Sr (82.58%)

87Rb 87Srβ-decay

λ= 1.42 ×10-11 yr-1

Isochron

CONTENTS

Introduction

Study Area

Previous Works

Method of Analysis and Treatment of Results

Mysore-Hyderabad Region

Tamizhagam-Kerala Region

Results and Conclusion

INTRODUCTION

• Many Rb-Sr dating analyses and their isochrones are reported for the crystalline complex of southern Peninsular India. But data are still insufficient for a reliable geochronology.

• Apparent ages of rocks ranging from over 3000 m.y. to 720 m.y. in this region

• The oldest rocks with an age of 2700 m.y. have been found in Kerala (Nilgiri Hills)

• Rocks aged below 2700 m.y. also dated from the locations like Mysore-Hyderabad craton, Chitradurga, Closepet Granite of Kerala, Chamundi Hill Granite and Ramanathapuram granite

STUDY AREA • A major division of peninsular India was made into three regions:

Northern peninsular India, north of the Narmada-Son lineament

Southern peninsular India (west of the Godavari)

An eastern region (with Shillong Plateaus )

• This paper deals with the second region, which extends up to Karimnagar District (AP) to the foot of the western Ghats.

• This study area is subdivided in to two based on Geology and Geochronology.

1.Mysore-Hyderabad (with the part of Maharashtra).

2.Tamizhagam-Kerala

Figure 1. Map of India, Pakistan and Ceylon showing Precambrian regions

Figure 2. Simplified geological map of southern peninsular India, after G. S. I. maps.P. Phanerozoic rocks

K. Kurnool Series and equivalents;

Horizontal hatching, CuddapahSystem and equivalents

Dotted, charnockites

Diagonal hatching, DharwarSystem Peninsular Gneiss and undifferentiated rocks.

Sample localities are shown by GA number

PREVIOUS STUDY • Smeeth and Watson (1918) were perhaps the first in the world to adopt this method

• Attempts were made very early 19th centaury to use radiometric data to solve geological problems in India.

• Great pioneer of the application of radioactivity to geology, Arthur Holmes, did work on India from 1918 (incorporated data on samarskite from the Nellore mica belt)

• Isotopic dating in the modern sense dates from the introduction of techniques using mass-spectrometers.

PREVIOUS STUDY

• Venkatasubramanian (1953), who pioneered the Rb-Sr method in India, by analyzing a phlogopite from Kerala. He obtained an age of 1630 ± 200 m.y.

• Venkatasubramanian and Krishnan (1960) dated the Yediyoor monazite.

METHOD OF ANALYSIS AND TREATMENT OF RESULTS• Preliminary examination by X-ray fluorescence and chemical techniques

• Mass-spectrometric analysis by the isotope dilution method

• The results were calculated using a computer program (Dr. P. A. Arriens.)

• The decay constant used is 1.39x10-11 yr-1

• Main aim of the samples were analyzed, to obtain isochrons

• Statistical analysis has been applied. Using the method of least-squares fitting(York, 1966) and two-error regression method (McIntyre,1966)

METHOD OF ANALYSIS AND TREATMENT OF RESULTS• The computer program with this recommends one of four answers.

• Model I Isochron assumes all errors to be experimental.

• Model II Geological variation in Sr87/Sr86 relating it proportionately

• Model III Assumes any variation in Sr87/Sr86 to be independent of Rb87/Sr86

• Model IV combines the two

• The program requires prior estimates for the experimental precision.

• For this MS2-SG and Nuclide Analysis Associates machines used in the analyses.

• MSWD

• mean square weighted deviation (MSWD)

MYSORE-HYDERABAD REGION

• Topography

MYSORE-HYDERABAD REGION

• Stratigraphy

Classic crystalline complex (Archaean Complex)

Major unconformity between the more metamorphosed Precambrian and the overlying Cuddapah rocks.

Dharwar System Rocks Dharwar Schists Metamorphism increases

southwards Igneous content of Dharwar

rocks increase towards the south-east.

Champion Gneiss Type area -Kolar Goldfields Fine-grained very micaceous

rock of greasy appearance and is highly Sheared.

Intrudes the schists and is associated with a conglomerate

Peninsular Gneissbanded gneisses (the oldest)granitic gneissesgneissic granites Granites (the youngest).

Charnockites charnockite problem Fermer line charnockite region to be the oldest part

of the complex

Clospet Granites Bellary Gneiss coarse-grained and porphyritic granite Cuts across the strike of foliation of the

Peninsular Gneiss

Dykes Dharwars- acid and basic

dykes are associated with the meta-volcanics dark hornblende dykes occurring as a swarm in the Peninsular

Gneiss of the south Norite dykes- Bangalore as a swarm Olivine-norite dykes Mysore Felsite and feldspar porphyry dykes Sriringapatnam Granite porphyry dykes occur near

Arsikere,

MYSORE-HYDERABAD REGION

• Geochronology

One hundred and two samples were collected in this region. Sixty-one totalrock and twelve mineral analyses were made.

Twenty-one samples were collectedalmost entirely of basic lavasnot provide verysatisfactory material for dating

Isochron age 2345 ± 60 m.y. (Model I) (selected group).Initial ratio:0.7045 ±0.0009MSWD 23.31

Kolar Gold Minepoorly enriched

Four samples of Chitradurga Granitevery well-enriched in radiogenic Sr and can stand

as individual age determinationsAge of the Granite estimated as between 2400 and

2450 m.y.

Five samples of the Granite were collectedrich in radiogenic strontium

maximum indicated age of 2620 m.y

• Thirteen samples of Peninsular Gneiss and associated rockswere collected

(a)Peninsular Gneiss sensu stricto of BangaloreAge 2580 ± 50 m.y.Initial ratio 0.7022 ± 0.0013MSWD 1.00

(b) Peninsular Gneiss sensu lato of BangalorcAge 2590 ± 35 m.y.Initial ratio 0.7016 ± 0.0009MSWD 3.20

(c) Peninsular Gneiss of South MysoreAge 2585 ± 40 m.y.Initial ratio 0.7016 ± 0.0011MSWD 3.83

(d) Peninsular Gneiss of all MysoreAge 2585 ± 30 m.y.Initial ratio 0.7017 ± 0.0008MSWD 3.43

Seven total rock and two K-feldspar analyses were made and one of a biotite.Isochron age (with GA 1820) 2380 ± 35 m.y. (Model III)Initial ratio 0.7050 ± 0.0016MSWD 27.28

Four samples of the granite forming the isolated mass of Chamundi HillIsochron age 790 ± 60 m.y. (Modell)Initial ratio 0.7050 ± 0.0013MSWD 0.14

Two feldspar porphyry dykes were sampled near SrirangapatnamThe maximum indicated ages are 970 and 1140 m.y. respectively

The area of gneiss at Honnali on the Tungabhadra River

Hyderabad City Granites: Three of the four samples are of granitesHyderabad city granitesAge 2495 ± 265 m.y. (Model I)Initial ratio 0.7037 ± 0.0091MSWD 0.37Hyderabad city granites with Karimnagar granitesAge 2490 ± 115 m.y. (Model II)Initial ratio 0.7045 ± 0.0044MSWD 2.22

MYSORE-HYDERABAD REGION

• The age of crystallization of the Peninsular Gneiss is approximately 2550 m. y. The age applies to a vast area extending from South Mysore north to Karimnagar in Andhra

• The age of the volcanism in the Dharwar rocks younger than the Peninsular Gneiss (2350 m.y.)

• lavas were extruded during folding and imply that they were formed at an early stage in the evolution of the geosyncline.

• The Champion Gneiss of Mysore remains of uncertain age

• Feldspar porphyry dykes in south Mysore are not precisely dated but are younger than 1000 m.y

TAMIZHAGAM-KERALA REGION

• Topography

TAMIZHAGAM-KERALA REGION• Stratigraphy

region is crossed by the boundary Fermor (1936),drew between the charnockites and the non-charnockites

Charnockites main rock type granular green hypersthene

rock first name Nilgiri Gneiss. Job Charnock source of the stone was St.

Thomas's Mount, Madras. Charnockites Series (Hollond) Granulite facies granulitic or gneissic

Peninsular Gneiss coarse-grained with

granitic or pegmatitic texture

intrusive in habit Regional and thermal

metamorphism

Other rocks granitic gneisses, migmatites

Coimbatore Sivamalai, soda-syenites Salem schist belt Subramaniam Diabase dykes,Pink

granites garnetiferous and sillimanite gneisses Tiruchirapalli funnel-shaped

anorthosite-gabbro Closepet Granite of Mysore Ramanathapuram biotite and

hornblende gneisses

TAMIZHAGAM-KERALA REGION

• Geochronology

• Not provide much information about the age of the rocks.• MAX age 3810 m.y. (garnetiferous feldspar gneiss)

Isochron Age (GA 1847, 1852, 1853, 1856)Age 2615 ± 80 m.y. (Model I)Initial ratio 0.7023 ± 0.0012MSWD 5.04

• Sampled from Pallavaram and St. Thomas's Mount.• give a good range for plotting an isochron.

Isochron age 2580 ± 90 m.y. (Model III)Initial ratio 0.7056 ± 0.0040

• samples are very poorly enriched• Less information gathered about their age• The lack of an adequate number of samples• Biotite of GA 1865 gives a maximum indicated age of 520 m.y.

Isochron age 1020 ± 670 m.y. (Model I)Initial ratio 0.7008 ± 0.0046MSWD 0.20

Analytical data from Tamizhagam and Kerala Region.

TAMIZHAGAM-KERALA REGION• Comments on Geochronology

Rocks of an age of about 3000 m.y. occur in South Kerala and in the Nilgiri Hills

Most of the charnockites rocks are rather younger but may still be slightly older than the Peninsular Gneiss of Mysore Hyderabad (2600 m.y. )

Migmatites of an age of about 2100 m.y(both regions)

next youngest rocks are the soda- syenites of Sivamalai

upper Precambrian granite of Ramanathapuram in the south east (700 m.y.)

Biotite of 700my age east of Coimbatore suggests that either intrusive of the same age may exist inland in the region, or that some event affected it very widely at this time.

CONCLUSION

• There are still too-few data to establish a reliable geochronology.

• There are two problems.

• obtaining reliable apparent ages for formations clearly, defined.

• these ages mean in terms of geological events

• Some results have been achieved in the area where the geology is well-established.

eg.Peninsular Gneiss apparent age of about 2585 m.y.

• Isochron of Chitradurga is much less satisfactory than that for the Peninsular Gneiss.

• The discovery in three separate areas of rocks giving apparent ages of over 3000 m.y. shows that the Peninsular Gneiss is not the oldest formation in the region.

CONCLUSION

• The Chitradurga Granite has an age of between 2450 and 2400 m.y.

• The Closepet Granite isochron is not wholly satisfactory

• The Champion Gneiss is. difficult to date by Rb-Sr.

• Charnockites too have often proved difficult to date, but there is doubt that their ages vary from over 3000 m.y

• Westernghat belts of rocks of principal ages of 2400 and 2100 m.y.

• They locally contain older relic sand have been affected by several later events. This is a provisional view.

• future geochronological work on the whole of this vast and highly interesting area.