Geochemical heterogeneities of metagraywackes from the Sandur schist belt: implications for active plate margin processes

Metagraywackes found at four stratigraphic levels in different blocks of the Sandur schist belt represent unmixed sediments, exhibiting great diversity in petrography, mineralogy, major, trace and rare earth element (REE) geochemistry. Most of these rocks have a low (Q+F)/C ratio and compared to AUCC (Average Upper Continental Crust) are depleted in 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 diagnostic 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 REE patterns are found. Type I patterns have moderate Sigma REE, La-N/Yb-N less than or equal to 3 and no significant Eu anomalies. On the other hand, type II patterns show LREE enrichment resulting in significant increase in Sigma REE and La-N/Yb-N between 5 and 15. Type II patterns are common in the Deogiri block. Type I patterns are similar to those found in modern deep sea forearc basins. Significant negative Eu anomalies are found in a few cases of type II graywackes where recycling 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 supplied 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 attributed to the proportional differences in their local sources. The overall composition indicates that the graywackes of different blocks have been deposited in various environments of oceanic subduction complex and have not sampled much of the upper continental crust. The compositional, structural and metamorphic discontinuities observed in the belt are best explained by a small plate tectonic model, involving accretion 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 resemble those found in modern forearc basins. (C) 1997 Elsevier Science B.V.