Ion Microprobe U‐Pb Age Determinations on Zircon from the Late Archean Granulite Facies Transition Zone of Southern India

The southern Dharwar Craton of Karnataka and Tamil Nadu states in south India exposes a depth profile of Late Archean crust. Amphibolite facies felsic to intermediate Peninsular Gneiss of protolith ages 3.4–2.9 Ga with subordinate infolded supracrustal relics and the large Closepet Granite body give...

Full description

Saved in:
Bibliographic Details
Published in:The Journal of geology 2003-07, Vol.111 (4), p.407-425
Main Authors: Mojzsis, S. J., Devaraju, T. C., Newton, R. C.
Format: Article
Language:English
Subjects:
Amphibolite facies
Archean eon
Biotite
Cratons
Geological time
Geology
Gneiss
Granite
Granulite facies
Metamorphism
Quarries
Rocks
Stone
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:The southern Dharwar Craton of Karnataka and Tamil Nadu states in south India exposes a depth profile of Late Archean crust. Amphibolite facies felsic to intermediate Peninsular Gneiss of protolith ages 3.4–2.9 Ga with subordinate infolded supracrustal relics and the large Closepet Granite body give way southward to granulite facies (orthopyroxene‐bearing) gneisses over a transition zone about 40 km wide, with mineralogic pressure indicators increasing from about 4 kbar (corresponding to about 15 km paleodepth) to about 7 kbar (corresponding to near 25 km paleodepth). Previous whole rock and zircon geochronology have established that amphibolite facies metamorphism, migmatization, and granite intrusion took place in the latest Archean at 2.53–2.51 Ga. However, some workers have considered the foliated granulites as products of an earlier metamorphism, distinct from the terminal Archean events. This work presents whole rock geochemical studies coupled with ion microprobe207Pb/206Pb ages and Th/U ratios of zircons from banded granulites and orthopyroxene‐bearing pegmatoids from three quarries at Chillapura, near Halaguru, Karnataka, at the southern end of the Closepet Granite and well into the granulite transition zone. The majority of the zircons, including all those from the pegmatoidal charnockites, show nearly concordant ages at 2.53 Ga and thus provide a firm temporal link with amphibolite facies metamorphism and granite emplacement at higher crustal levels. Moreover, we propose that the very low Th/U ratios of many zircons (≤0.001) could not reflect precipitation from siliceous magmas of the compositions of their host rocks but more probably record equilibration with a high U/Th metamorphic fluid. Two zircons from one banded granulite showed nearly concordant207Pb/206Pb ages at 2.96 Ga and Th/U ratios at 0.6–1.0, compatible with primary crystallization from a mantle‐derived magma with canonical Th/ \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\mathrm{U}\,\approx 4$ \end{do
ISSN:0022-1376
1537-5269
DOI:10.1086/375285