Transcript PowerPoint - 18.1 Mo

The Closepet granite, Dharwar
craton: petrogenesis and
emplacement history
Jean-François Moyen
Université Claude-Bernard, Lyon
Hervé Martin, Université Clermont-Ferrand
Mudlappa Jayananda, Bangalore University
Anne Nédélec, Université Toulouse
The Dharwar craton
Deccan trapps
Goa
Late-Archaean
granites
Granulites
Peninsular
Greenstone
gneisses
belts
Closepet granite
Cuddapah basin
W.D.C.
E.D.C.
Bangalore
Nilgiri hills
Madras
The Closepet granite
Northern intrusions:
Elliptic, well-delimited
intrusions
The gap:
No granitic outcrop
Transfert zone:
Large mass of porphyritic
granite
Root zone:
Sheets and plugs injected
into the gneisses
1- Petrogenetic processes in
the « root zone »
2- Contrasted emplacement
modes at different structural
levels
1- Petrogenetic processes in
the « root zone »
The « Root zone »
3 main facies :
•Granites and migmatites
• Porphyritic monzogranite
• Cpx-bearing monzonite
Field evidences for magma mixing
•Mechanical interactions (mingling)
•Chemical interactions (mixing s.s.)
~ 1 mm
Low strain
High strain
Isotopic evidences for magma mixing
Harker plots: end-members of the
mixing
Trace elements modeling of the
mixing
The felsic end-member
 Anatectic granites (partial melting of Peninsular Gneisses)
Modeling the melting of
Peninsular Gneisses
Determination of the mafic endmember
Source of the mafic end-member
• 51 % SiO2 : Mantle-derived magma
2.5 Ga depleted
mantle
 Source: enriched mantle
Petrogenetic
model
Modeling the melting of an
enriched mantle
How can we form a Jadeite-bearing lherzolite ?
Sanukitoids in the East Dharwar
Elements of the Bukkapatnam granite ?
Madanapalle dark granodiorites
The « Dod gneisses »
The Bisanattam pluton
Closepet
granite
Krishnagiri Hb-bearing
granodiorites
Madras
Bangalore
Injections in
Krishnagiri tonalites
Kolar schist belt
The Dod gneisses: petrography
Diorites, monzodiorites and
granodiorites
Lots of microgranular
mafic enclaves
Qz + Pg + KF + Bt + Hb ± Cpx
Ap + Ilm + Sph + Zn
Sanukitoids: geochemistry
Making sanukitoids
Closepet parental liquid
Petrogenetic relationships
2- Contrasted emplacement
modes at different structural
levels
•Strain pattern and fabric development
•Granite emplacement at different levels
•Relationships with basement
•Liquid/solid partitionning
Conditions of the deformation
Orthogneissification
Shear zonesShear
invaded
zones
by
Magmatic-state
invaded
late
aplites/pegmatites
by C/S
latefoliation
fabric
melts
« Proto-shear
zone »
Conditions of the deformation (2)
Magmatic or sub-magmatic deformation Sub-solidus structures
(K-spar orientation)
Development of fabric during cooling
 fabric developped all along the cooling from magmatic- to
solid-state, under the same strain pattern
Outcrop-scale structures
AMS structures
 syntectonic emplacement of the Closepet granite in a strikeslip setting
Northern intrusions:
Elliptic, well-delimited
intrusions
The gap:
No granitic outcrop
Transfert zone:
Large mass of porphyritic
granite
Root zone:
Sheets and plugs injected
into the gneisses
Schematic section in the root zone
SPOT images
 Network of shear zones invaded with granites
Northern intrusions:
Elliptic, well-delimited
intrusions
The gap:
No granitic outcrop
Transfert zone:
Large mass of porphyritic
granite
Root zone:
Sheets and plugs injected
into the gneisses
Cross section in the transfert zone
SPOT images
interpretation
 Partition of the deformation between low strain and high
strain areas
 Magma ascent focussed in high strain zones
Northern intrusions:
Elliptic, well-delimited
intrusions
The gap:
No granitic outcrop
Transfert zone:
Large mass of porphyritic
granite
Root zone:
Sheets and plugs injected
into the gneisses
Northern intrusions
The northern intrusions
 Homogeneous and apparently undeformed granites
Similar to granites found at deeper levels
The Hampi intrusion
AMS foliation
and lineation
« The gap »
Meaning of
the gap
 Melt/solid partitionning
at a rheological interface
(filtering of the « mush »)
How deformation is accomodated
Intrusion zone:
Filling of small pockets
 The
In the
gap:same structural context, major control by the
Filtering of solid
basement rheology
 Melt/solid partitionning (petrological and geochemical
Transfert zone:
consequences ?) Mass transfert of magma
Melt/solid partitionning
 Major role of a structural limit (the gap)
Deep crust (root zone):
Melt injection in shear zones
percolation