Synplutonic mafic dykes from late Archaean granitoids in the Eastern Dharwar Craton,southern India |
| |
| Authors: | M Jayananda T Miyazaki R V Gireesh N Mahesha T Kano |
| |
| Institution: | (1) Department of Geology, Bangalore University, Bangalore, 560 056, India;(2) Institute for Earth Evolution, JAMSTEC, Yokosuka, Japan;(3) Department of Earth Sciences, Yamaguchi University, Yamaguchi, Japan |
| |
| Abstract: | We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56–2.52 Ga calcalkaline to potassic
plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole
rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central
part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The
enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic
enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes
together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic
link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width
and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more
leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting
their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and
mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial
hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected
during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization
with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit
only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host
magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic
mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes
with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization
of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity
rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity
of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and
resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic
magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected
the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic
event and spatially associated with reworking and cratonization of Archaean crust in the EDC. |
| |
| Keywords: | Synplutonic mafic dykes Granite plutons Mafic Magmatic Enclaves Magma chamber Eastern Dharwar craton |
| 本文献已被 SpringerLink 等数据库收录！ |
|
