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Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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Basu, Abhijit

Provenance and Al/Si Order-Disorder of Detrital Alkali Feldspars

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Authors

Abhijit Basu ¹

Affiliations
1 Center for Astrophysics, Harvard College Observatory and Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, Massachusetts 02138, US

Source

Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 18, No 9 (1977), Pagination: 477-492

Abstract

The principle that detailed characterization of minerals enhances petrogenetic interpretation is extended to sedimentary petrology by estimating the Al/Si order-disorder of detrital alkali feldspars from Holocene fluvial sands derived from granite plutons. A weathering-independent characterization of the influence of source rocks on daughter sands is based on structural state determination of 42 detrital alkali feldspars. Structural states were estimated from refined unit cell parameters derived from slow oscillatory difractograms. .

None of the studied feldspars show anomalous cell dimensions as inferred from b-c plots. In addition, estimates of Or content from refined cell volumes correspond very well with microprobe measurements of the Or contents of the same grains. In crystalline rocks, however, many alkali feldspars have anomalous cell dimensions; their estimated and measured Or contents are also often widely different. This suggests that detrital alkali feldspars may be amenable to rapid x-ray methods of structural state determination.

Detrital alkali feldspars in sands derived from pre-Mesozoic plutons are triclinic with a relatively low structural state (average Δ bc=0.975; t₁(o)=0.479).

The age of the parent pluton may be a factor in controlling the relative structural states of detrital feldspars derived from the pluton. Alternatively, the difference in ordering between the two groups of feldspars might reflect the differences in the thermal history of the different zones of the parent batholith supplying feldspars. It is possible that the rapidly cooled outer shells of plutons are quenched at higher temperatures than their relatively slowly cooled cores. Thus the outer shells would tend to produce alkali feldspars with relatively higher structural states than those coming from the core. If so, a vertical sequence of sediments derived from systematic unroofing of a batholith should reflect this change in feldspar structural state.

Full Text

Petrology of the Nepheline Syenites from Santuri Purulia District, West Bengal

The Proterozoic Puzzle of Peninsular India

Ediacaran Fossils in Meso- and Paleoproterozoic Rocks in Peninsular India Extend Darwin

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Authors

Abhijit Basu ¹

Affiliations
1 Department of Geological Sciences, Indiana University, 1001 East 10^th Street, Bloomington, IN 47405, US

Source

Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 73, No 4 (2009), Pagination: 528-536

Abstract

Typically or arguably Ediacaran fossils (635 Ma to 543 Ma) are reported by several research groups from one unit of the Chhattisgarh and two units of the Vindhyan Supergroups in peninsular India. Depositional ages of the host sediments, however, are inferred to be ∼1000 Ma and ∼1630 Ma as determined by U-Pb dating of magmatic and detrital zircons in rhyolitic tuff (∼ porcellanite) and sandstones, provenance considerations and paleopole positions. The contradiction of absolute ages results from inferring the Ediacaran age strictly on the basis of fossils. I argue that the fossils reported from the Chhattisgarh and Vindhyan Supergroups should be considered mostly Mesoproterozoic and late Proterozoic in age. I also argue that although the Ediacaran Period records explosive diversity of preserved fossils, many forms very likely appeared much earlier with variable degrees of preservation or none at all at times, and, that their age-ranges extend to the Paleoproterozoic. I hypothesize that the rate of increase of biological diversity was lower than the rate of preservation in certain geological intervals, especially immediately after extinction events.

Keywords

Ediacaran Fossils, Paleoproterozoic, Darwin.

Full Text

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