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INTRODUCTION
INTRODUCTION
The unique feature of the earth is its continental crust, which covers around 40% of the
earth's surface area. In contrast to the earth's oceanic crust, which is predominantly
thin, basaltic and young (<200 Ma), the continental crust is thick (20-60 km) with an
average thickness of 39 km (Christensen and Mooney, 1995). The only accessible part
of the earth to the man, for its resources, is continental crust. Geologists could study
only this part of the earth with direct evidences. So, one of the fundamental problems in
geology concerns how the continental crust evolved. The studies involving ore genesis
and soil formation are of crucial importance to the mankind and directly related to the
evolution of the earth's crust. This is only the evolved nature of the earth crust unlike
other planets, which could support the life since its prokaryotic nature to the most
evolved Homo sapiens.
The chemical fractionation of the earth during its evolution by magmatic
processes and subsequently by surface sedimentary processes have been going on
continuously since its birth at 4.6 Ga. Although, the records preserved in sedimentary
rocks indicate that the formation of continental crust has been an ongoing process
throughout the earth's long history, the creation of crust has not always had the same
character. lqor example the boundary between Archean and Proterozoic eons, around 2.5
Ga ago, a distinct change in the rock record occurred. The composition of the earth's
upper crust before this break contained less evolved continents, composed of mixture of
basalt and sodium rich granites, these rocks together make up the so called tonalite-
Introduction
trondjemite-granodiorite suite; but the present upper crust is dominated by K-rich
granites (Taylor and McLennan, 1995). Many studies on sediment chemistry, especially
that of mudrocks, have concluded that global scale changes in average composition
reflect changes in the average composition of crystalline source rocks (Taylor and
McLennan, 1985; 1991; Gibbs et al., 1986; Condie and Wronkiewicz, 1990).
The mineralogical and bulk chemical compositions of sedimentary rocks are
used to determine provenance, palaeoclimates and tectonic activities, and to study the
evolution of the crust (Condie, 1967; Sibley and Vogel, 1967; Wieldman and Condie,
1973; Pettijhon, 1975; Dickinson and Valloni, 1980; McLennan and Taylor, 1982;
Nesbitt and Young, 1982; Kronberg et al., 1986; Cullers et al., 1988; Ojakangas, 1988;
Condie and Wronkiewicz, 1990; Visser and Young, 1990; McLennan and Taylor, 1991;
McLennan et al., 1993; Potter, 1994). Several studies report mineralogical and bulk
chemical compositions of sedimentary rocks (Shaw, 1956; Pearson, 1979; Dickinson
and Suczek, 1979; Lajoie and Ludden, 1984; McLennan et al. 1983; 1993; Condie,
1993; Johnson and Basu, 1993; Van de Camp and Leake, 1994). Many studies on
weathering have documented the chemical changes within the weathering profile
(Duddy, 1980; Nesbitt, 1979; Nesbitt et al., 1980; Fritz, 1988; Middleberg et al., 1988;
Nesbitt and Young, 1989; Prudencio et al., 1993; Mongelli, 1993; Condie et al. 1995),
but the effects of chemical weathering on the composition of sediments derived remain
poorly documented (Johnson, 1993). There are very few detailed studies (e.g., Nesbitt et
al., 1996; Tripathi and Rajamani, 1997, communicated) which discuss the relative
importance of weathering profiles and unweathered bed rock in controlling the
composition of recent sand and mud.
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Introduction
Chemical weathering and mechanical erosion are the most important surfacial
processes in the destruction of bedrock and production of sediments. Chemical
weathering proceeds through reaction of organic and inorganic acids of soil waters with
bedrock minerals (Garrels, 1967; Nesbitt and Young, 1984; Drever, 1988) such as
feldspars the most abundant minerals of the upper crust (Wedepohl, 1969, Nesbitt and
Young, 1984). Large amounts of clay minerals are produced during weathering, which
contribute to the development of mineralogical and geochemical differentiation in
weathering profiles. The fragile portions of weathering profiles are readily removed by
the erosion and provide detritus for the sedimentary rocks. The rates of weathering and
erosion may vary over time as a result of changes in climate and tectonic setting, leading
to production of sediments of variable composition. Therefore the main objective of the
present study is concerned with
the presently ongoing sedimentary geochemical
processes in the region to understand past surface geochemical processes leading to the
formation of sedimentary rocks. It will also provide an insight into the geochemical
behaviour of the elements in the present goo-environmental condition and its relevance
to the environmental problems.
OBJECTIVE AND SCOPE OF THE STUDY
The process of sedimentation, including weathering, erosion, sedimentary sorting and
diagenesis, essentially involve water/rock interaction and result in many fundamental
chemical changes. The weathering and alteration processes constitute one of the major
part of this study, which would provide an insight to the chemical changes and
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Introduction
behaviour of the elements during water-rock interaction under surface temperatures and
underground elevated temperatures.
The study of Delhi quartzites using their geochemistry has been intended to
understand their sources and processes by which they have been formed. The
provenance is in turn important to understand the geological history, tectonic setting and
therefore crustal evolution during the deposition of these sediments (Taylor and
McLennan, 1985; Hemming et al., 1995; Fedo et al., 1996). An understanding of
weathering of quartzites would also provide key constraints to understand the
weathering of quartzite's protoliths and clastics supply.
Recently it has been realised that the aeolian component is omnipresent on the
earth surface and its contribution to the soils is significantly important (Reheis et al.,
1995; Pye, 1987; McLennan, 1995). Sediments present in the topographic lows on the
Delhi ridge has been studied to understand their nature, aeolian, alluvial or residual. The
textural mineralogical and chemical signatures have been used to understand the
processes involved in their transportation, and deposition and to evaluate the nature of
their sources or provenance.
The study on the origin of kaolinite and sand deposits after pegmatites and
surrounding quartzites has been aimed to understand about the conditions under which
they have formed. This study provides an insight into the behaviour of REE and other
elements in such an intensive leaching process under elevated temperatures. Using the
REE as proxy for the actinides is important to understand the behaviour of the nuclear
wastes under the similar geo-environmental conditions (Rard, 1988; Wood, 1990).
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