C. Manikyamba et al., GEOCHEMICAL HETEROGENEITIES OF METAGRAYWACKES FROM THE SANDUR SCHIST BELT - IMPLICATIONS FOR-ACTIVE PLATE MARGIN PROCESSES, Precambrian research, 84(3-4), 1997, pp. 117-138
Metagraywackes found at four stratigraphic levels in different blocks
of the Sandur schist belt represent unmixed sediments, exhibiting grea
t diversity in petrography, mineralogy, major, trace and rare earth el
ement (REE) geochemistry. Most of these rocks have a low (Q+F)/C ratio
and compared to AUCC (Average Upper Continental Crust) are depleted i
n incompatible elements like Rb, Ba, Th, U, Nb, Ta, K, Na, Ca, Ce and
Sr and enriched in compatible elements such as Mg, Cr, Ni and Sc. They
are enriched in Fe2O3 (upto 25%) and their Mg/K ratio varies from <1
to 1000. La/Sc, Th/Sc, Th/Yb and Co/Th ratios, along with several diag
nostic geochemical twins such as U-Th, Hf-Zr and Nb-Ta, indicate that
upper continental crust has contributed marginally to the composition
of most of these graywackes. In each block, except one, two types of R
EE patterns are found. Type I patterns have moderate Sigma REE, La-N/Y
b-N less than or equal to 3 and no significant Eu anomalies. On the ot
her hand, type II patterns show LREE enrichment resulting in significa
nt increase in Sigma REE and La-N/Yb-N between 5 and 15. Type II patte
rns are common in the Deogiri block. Type I patterns are similar to th
ose found in modern deep sea forearc basins. Significant negative Eu a
nomalies are found in a few cases of type II graywackes where recyclin
g of sediments such as micaceous quartzite is evident. La:Th:Se and Co
/Th, Th/Sc and La/Sc ratios show that most of these graywackes were su
pplied from a source that was mainly basic volcanic rocks, banded iron
formations and cherts. However, significant compositional variations
are found from block to block and within the same block. These are att
ributed to the proportional differences in their local sources. The ov
erall composition indicates that the graywackes of different blocks ha
ve been deposited in various environments of oceanic subduction comple
x and have not sampled much of the upper continental crust. The compos
itional, structural and metamorphic discontinuities observed in the be
lt are best explained by a small plate tectonic model, involving accre
tion of diverse blocks to a pericontinental passive margin succession.
It is argued that even in the Archaean, volcanosedimentary processes
at young undifferentiated arcs involving ocean (floor) ridge basalts (
AORB) have generated turbidites which in many geochemical features res
emble those found in modern forearc basins. (C) 1997 Elsevier Science
B.V.