Textural, geochemical, and isotopic data from silicified rocks and associated chemical sedimentary rocks in the ~ 2.7 Ga Abitibi greenstone belt, Canada: Insight into the role of silicification

Abstract

Silica-rich Precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust. Because these rocks are important geochemical and paleontological archives, we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving Archean silica cycle. Here, we investigate a series of sub- to greenschist facies Si-rich Archean rocks from the ~ 2.7 Ga Abitibi greenstone belt, Canada, that represent chemical sedimentary rocks and rocks formed via silica-addition through the process of silicification. We report data for major and trace element geochemistry, multi-crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry, and texture-specific silicon isotope values measured using secondary ion mass spectrometry on Neoarchean chemical sedimentary rocks, their silicified equivalents, and associated silicified volcanic rocks. We find that in such a well-preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin, distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification. Chemical sedimentary rocks display a wide range of 30Si-depleted silicon isotopes values that vary with stratigraphy similar to other Archean iron formation. Silicified volcanic rocks possess 30Si-enriched values, similar to Archean silicified basalts. We conclude that because silicon isotope values of iron formation shift toward 30Si-enriched values up stratigraphy, basinal changes in the composition of the silicon isotope reservoir may be preserved. Silicon isotope values of silicified volcanic rocks by contrast, likely represent precipitation from an isotopically heavy silicon reservoir, influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock (basalt or andesite). Overall, we confirm that Neoarchean silicified rocks are 30Si-enriched like their Paleoarchean counterparts.