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M. Ramakrishnan 《Journal of the Geological Society of India》2009,73(1):101-116
Mafic rocks of Western Dharwar Craton (WDC) belong to two greenstone cycles of Sargur Group (3.1–3.3 Ga) and Dharwar Supergroup
(2.6–2.8 Ga), belonging to different depositional environments. Proterozoic mafic dyke swarms (2.4, 2.0–2.2 and 1.6 Ga) constitute
the third important cycle. Mafic rocks of Sargur Group mainly constitute a komatiitic-tholeiite suite, closely associated
with layered basic-ultrabasic complexes. They form linear ultramaficmafic belts, and scattered enclaves associated with orthoquartzite-carbonate-pelite-BIF
suite. Since the country rocks of Peninsular Gneiss intrude these rocks and dismember them, stratigraphy of Sargur Group is
largely conceptual and its tectonic environment speculative. It is believed that the Sargur tholeiites are not fractionated
from komatiites, but might have been generated and evolved from a similar mantle source at shallower depths. The layered basic-ultrabasic
complexes are believed to be products of fractionation from tholeiitic parent magma. The Dharwar mafic rocks are essentially
a bimodal basalt-rhyolite association that is dominated by Fe-rich and normal tholeiites. Calc-alkaline basalts and andesites
are nearly absent, but reference to their presence in literature pertains mainly to carbonated, spilitized and altered tholeiitic
suites. Geochemical discrimination diagrams of Dharwar lavas favour island arc settings that include fore-, intra- and back-arcs.
The Dharwar mafic rocks are possibly derived by partial melting of a lherzolite mantle source and involved in fractionation
of olivine and pyroxene followed by plagioclase. Distinctive differences in the petrography and geochemistry of mafic rocks
across regional unconformities between Sargur Group and Dharwar Supergroup provide clinching evidences in favour of distinguishing
two greenstone cycles in the craton. This has also negated the earlier preliminary attempts to lump together all mafic volcanics
into a single contemporaneous suite, leading to erroneous interpretations. After giving allowances for differences in depositional
and tectonic settings, the chemical distinction between Sargur and Dharwar mafic suites throws light on secular variations
and crustal evolution. Proterozoic mafic dyke swarms of three major periods (2.4, 2.0–2.2 and 1.6 Ga) occur around Tiptur
and Hunsur. The dykes also conform to the regional metamorphic gradient, with greenschist facies in the north and granulite
facies in the south, resulting from the tilt of the craton towards north, exposing progressively deeper crustal levels towards
the south. The low-grade terrain in the north does not have recognizable swarms, but the Tiptur swarm consists essentially
of amphibolites and Hunsur swarm mainly of basic granulites, all of them preserving cross-cutting relations with host rocks,
chilled margins and relict igneous textures. There are also younger dolerite dykes scattered throughout the craton that are
unaffected by this metamorphic zonation. Large-scale geochemical, geochronological and palaeomagnetic data acquisition through
state-of-the-art instrumentation is urgently needed in the Dharwar craton to catch up with contemporary advancements in the
classical greenstone terrains of the world. 相似文献
2.
K. Naha A. Rai Choudhuri V. Ranjan R. Srinivasan 《Journal of Earth System Science》1995,104(3):327-347
The supracrustal enclave within the Peninsular Gneiss in the Honakere arm of the Chitradurga-Karighatta belt comprises tremolite-chlorite
schists within which occur two bands of quartzite coalescing east of Jakkanahalli(12°39′N; 76°41′E), with an amphibolite band
in the core. Very tight to isoclinal mesoscopic folds on compositional bands cut across in the hinge zones by an axial planar
schistosity, and the nearly orthogonal relation between compositional bands and this schistosity at the termination of the
tremolite-chlorite schist band near Javanahalli, points to the presence of a hinge of a large-scale, isoclinal early fold
(F1). That the map pattern, with an NNE-plunging upright antiform and a complementary synform of macroscopic scale, traces folds
'er generation (F
2),is proved by the varying attitude of both compositional bands (S0) and axial pranar schistosity (S
1), which are effectively parallel in a major part of the area. A crenulation cleavage (S
2) has developed parallel to the axial planes of theF
2 folds at places. TheF
2 folds range usually from open to rarely isoclinal style, with theF
1 andF
2 axes nearly parallel. Evidence of type 3 fold interference is also provided by the map pattern of a quartzite band in the
Borikoppalu area to the north, coupled with younging directions from current bedding andS
0
-S
1 inter-relation.
Although statistically theF
1 andF
2 linear structures have the same orientation, detailed studies of outcrops and hand specimens indicate that the two may make
as high an angle as 90°. Usually, in these instances, theF
1 lineations are unreliable around theF
2 axes, implying that theF
2 folding was by flexural slip. In zones with very tight to almost isoclinalF
2 folding, however, buckling attendant with flattening has caused a spread of theF
1 lineations almost in a plane. Initial divergence in orientation of theF
1 lineations due to extreme flattening duringF
1 folding has also resulted in a variation in the angle between theF
1 andF
2lineations in some instances. Upright later folding (F3) with nearly E-W strike of axial planes has led to warps on schistosity, plunge reversals of theF
1 andF
2 axes, and increase in the angle between theF
1 andF
2 lineations at some places. Large-scale mapping in the Borikoppalu sector, where the supposed Sargur rocks with ENE ‘trend’
abut against the N-‘trending’ rocks of the Dharwar Supergroup, shows a continuity of rock formations and structures across
the hinge of a large-scaleF
2 fold. This observation renders the notion, that there is an angular unconformity here between the rocks of the Sargur Group
and the Dharwar Supergroup, untenable. 相似文献
3.
The V-Ti magnetite layers (lodestone) occur within the layered gabbro-anorthosites-ultramafic rocks emplaced into the migmatitic
gneisses close to the high grade Archeaen Sargur supracrustal rocks in the Kurihundi area. The ore petrographic studies of
the lodestone reveal the presence of primary Ti-magnetite, ilmenite, ulvospinel, pleonaste, hematite and pyrite, chalcopyrite,
pyrrhotite and secondary Ti-maghemite, martite and goethite as well as secondary covellite. These layers contain Ti-magnetite
(60%) and ilmenite (30%) with silicates (<5%) exhibiting granular mosaic texture with well-defined triple junctions and are
classified as adcumulus rocks. The grain-boundary relationships in the ores indicate considerable postcumulus growth and readjustment
due to combined effects of sintering and adcumulus growth. Intergrowth textures (ulvospinel, ilmenite and pleonaste in Ti-magnetite
and hematite in ilmenite) reflects exsolution features crystallized from solid-solutions compositions under different conditions
of oxygen fugacities. Larger bodies of pleonaste and ilmenite in Ti-magnetite become lensoid or rounded in outline and these
morphological modifications took place during the regional upper amphibolite to lower granulite facies metamorphism at 2.6
Ga ago. The lodestone contains high TiO2 (20 to 22.59 wt%), with V2O5 (0.85 to 1.15%) and Fe2O3
t (72.03 to 74.25%). Ti-magnetite shows alteration to Ti-maghemite, martite and goethite due to low temperature oxidation and
hydration during weathering. 相似文献
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