Paleomagnetism and U–Pb geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37Ga

Halls, H. C.; Kumar, Anil; Srinivasan, R.; Hamilton, M. A.

doi:10.1016/j.precamres.2007.01.007

Cited by 163 publications

(92 citation statements)

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“…In broad terms, the N-S set appears to be younger than the E-W set. The dykes show three groups of ages, viz., 2.4 Ga, 2.0-2.2 Ga and 1.6 Ga (Radhakrishna et al 2004;Halls et al 2007), of which 2.0-2.2 Ga and 1.6 Ga groups are most prominent. Further, the occurrence of dyke clusters in WDC is in conformity with regional metamorphism of greenschist facies in the north to granulite facies south, corresponding to the northward tilt of the craton, exposing deeper crustal levels in the south (Pichamuthu,1959;Ramiengar et al 1978).…”

Section: Proterozoic Mafic Dyke Swarmsmentioning

confidence: 99%

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Ramakrishnan¹

2009

J Geol Soc India

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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 s...

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Section: Proterozoic Mafic Dyke Swarmsmentioning

confidence: 99%

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Ramakrishnan¹

2009

J Geol Soc India

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“…Thus they should be of SD to pseudo-single-domain (PSD) behavior (Butler and Banerjee, 1975). Rocks containing these magnetites have been demonstrated to record stable magnetization (Hargraves and Young, 1969;Morgan and Smith, 1981;Xu et al, 1997;Selkin et al, 2000;Halls and Zhang, 2003;Halls et al, 2007). Questions of critical importance to paleomagnetism are the timing and temperature at which the exsolution occurs and whether the NRM of the exsolved magnetite is a TRM or a thermochemical remanent magnetization (TCRM).…”

Section: Introductionmentioning

confidence: 99%

“…TCRM is a less efficient magnetization process than TRM, and paleointensity experiments using TCRM would underestimate the actual intensity of ancient field (Stacey and Banerjee, 1974). Unfortunately, there is no mineralogical evidence for NRM of exsolved magnetite in plagioclase to be TRM (Armbrustmacher and Banks, 1974;Halls and Zhang, 2003;Feinberg et al, 2005). A paleomagnetic study proposed that alteration by CO 2 fluid may modify and remagnetize exsolved magnetite in plagioclase (Halls et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

“…Unfortunately, there is no mineralogical evidence for NRM of exsolved magnetite in plagioclase to be TRM (Armbrustmacher and Banks, 1974;Halls and Zhang, 2003;Feinberg et al, 2005). A paleomagnetic study proposed that alteration by CO 2 fluid may modify and remagnetize exsolved magnetite in plagioclase (Halls et al, 2007). On the other hand, Halls and Zhang (2003) explained their paleomagnetic results from exsolved magnetite as either TRM or high temperature (450∼550…”

Section: Introductionmentioning

confidence: 99%

“…A paleomagnetic study proposed that alteration by CO 2 fluid may modify and remagnetize exsolved magnetite in plagioclase (Halls et al, 2007). On the other hand, Halls and Zhang (2003) explained their paleomagnetic results from exsolved magnetite as either TRM or high temperature (450∼550…”

Section: Introductionmentioning

confidence: 99%

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Nonlinear thermoremanence corrections for Thellier paleointensity experiments on single plagioclase crystals with exsolved magnetites: a case study for the Cretaceous Normal Superchron

Usui

Nakamura

2009

Earth Planet Sp

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Single plagioclase paleointensity experiment has been an excellent tool to reliably estimate ancient geomagnetic field intensity. Although transparent plagioclases with magmatic nanophase magnetites have shown their potential for paleointensity estimation, in most cloudy plagioclases with exsolved magnetites, the problems of strong anisotropy as rod-shape magnetites, the hyperbolic tangential saturation of thermoremanence (TRM), and slow cooling of host plutons need to be addressed. In this paper, we propose experimental schemes to address these problems with considerations of error estimation and uncertainty for paleointensity experiments on exsolved magnetite. First, in order to experimentally check the effect of the hyperbolic tangential saturation of TRM, we performed Thellier simulation experiments using laboratory total TRM as simulated natural remanence (NRM). Single cloudy plagioclases were sampled from early Cretaceous granite in the Kitakami massif, Northeast Japan. We designed the experiment where the total TRM and the partial TRMs in the Thellier experiments were acquired under different field intensities. For these experiments, correction for hyperbolic tangential TRM acquisition yielded precise field intensity estimations. Next, Thellier experiments on NRM of the crystals from the same granite were performed as a demonstration of correction schemes for both magnetic anisotropy and hyperbolic tangential TRM acquisition. Precise determination of anisotropy tensor seems to be the major challenge for single plagioclase paleointensity estimation with exsolved magnetite.

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The intensity of the geomagnetic field from 2.4 Ga old Indian dykes

Valet

Besse

Kumar

et al. 2014

Geochem Geophys Geosyst

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Precambrian paleomagnetic records from dyke swarms provide a unique source of information regarding the Archean geomagnetic field and more specifically the average field strength produced by the early dynamo. We sampled 16 paleomagnetic sites from the Dharwar giant dyke swarm in southern India which was emplaced between 2.365 and 2.368 Ga. Despite taking great care in selecting locations exempt of any geological disturbance, only two of these sites provided primary directions with very steep inclinations and therefore were emplaced in close to a magnetic pole. Paleointensity experiments were conducted on a subset of samples from the dyke margins. The characteristic magnetization is carried by single domain magnetite grains with a very narrow range of unblocking temperatures inferred from the sharp decrease by at least 75% of their remanence above 520 C. The paleointensity results indicate an average low field of 9.2 6 7 mT, consistent with reported values from Canadian dyke swarms for the same period. These results combined with the Thellier-Thellier determinations obtained so far for the Precambrian suggest that a low field period prevailed from circa 2.3 to 1.8 Ga, while the preceding and following time intervals are characterized by significantly stronger paleointensities. Although this suite of episodes is not fully incompatible with previous models for the long-term evolution of the geodynamo, it is tempting to make the link with the recent suggestion of an early dynamo sustained within a conductive magma layer at the base of the mantle from 3.5 to 2.5 Ga which progressively declined until convection became sufficiently efficient to reactivate a strong dynamo process within the Earth's liquid core.

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Paleomagnetism and U–Pb geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37Ga

Cited by 163 publications

References 30 publications

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Precambrian mafic magmatism in the Western Dharwar Craton, southern India

Nonlinear thermoremanence corrections for Thellier paleointensity experiments on single plagioclase crystals with exsolved magnetites: a case study for the Cretaceous Normal Superchron

The intensity of the geomagnetic field from 2.4 Ga old Indian dykes

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