Coupled Microfracturing and Chemical Weathering of Precambrian Quartzite in the Extremely Humid and Tectonically Active Shillong Plateau, NE India: Implications for In Situ Quartz Weathering and Quartz Silt Production

Abstract

The origin of quartz silt, the most abundant detrital particle, is controversial. Quartz silt is generally considered to be broken quartz sand, attributed to glacial, eolian, and/or long fluvial transport. In this article, we highlight the origin of in situ quartz silt in deep weathering profiles (≤7 m) over Precambrian quartzite (>92% quartz with or without 6%–8% mica, feldspar-free), a rock known to be resistant to weathering, in the Shillong Plateau (NE India). This region is one of the most tectonically active and the rainiest place on Earth. We discuss, for the first time, detailed field petrographic and microstructural evidence including diverse microfracturing and dissolution morphologies in quartz, from bedrock through saprolite (2–3 m) and nodular zone (NZ; ∼0.5 m) to soil (≤2–5 m), for quartz silt production through coupled mechanical-chemical processes over 3–4 My in the plateau. Progressive size reduction of quartz from medium sand (1.13–1.36 Φ) in bedrock quartzite through fine sand in saprolite (2.55–2.67 Φ) and the NZ (2.12–4.4 Φ) to coarse and medium silt in soil (4–5 Φ), with concomitant increase in clay mineral (≤80%), characterizes the profiles. Quartzite microfracturing, linked to exhumation and/or seismicity, provided pathways for acidic water to attack and promote chemical weathering in quartz, which in turn effected microfracture widening, porosity enhancement, and weakening of quartz in a positive feedback loop. Mica-microbe interactions facilitated fluid acidity, causing Al and Fe mobilization and more clay mineral production, and enhanced corrosive potential for in situ quartz dissolution in feldspar-free quartzite. Coupled brittle disintegration and chemical weathering of even extremely resistant quartz-rich rocks thus contribute toward long-term regolith development and global quartz silt production.