Oxygen isotope study of Precambrian granites from the Illinois Deep Hole Project

Abstract

Seventeen whole rock samples and 27 mineral separates from the Precambrian granites in drill cores UPH‐1, ‐2, and ‐3 have been analyzed. The δ180 values for quartz remain exceedingly uniform (δ =7.6–8.6‰) throughout the entire length of the three cores and are indicative of their primary igneous values. The δ180 for the feldspars are also uniform for samples located 70 m or deeper below the unconformity (δ =6.0–6.9‰) However, in the upper 70 m of the granite body, a gradual upward increase of δ180 in the feldspars is observed (to 8.8‰). Similarly Δ18 quartz‐feldspar show typical plutonic values (1.4–2.1) in the main part of the Precambrian sections except in the upper 70 m, where ΔQ‐F become progressively smaller upward until a reversed fractionation (negative ΔQ‐F) is observed in samples several meters below the unconformity. The increase of δ180 in the feldspars is interpreted as a result of hydrothermal alteration of the granites at low temperatures (probably between 110°–260°C). The source of the hydrothermal fluid responsible for the alteration most likely derived from the formation waters in the upper Cambrian‐Ordovician sedimentary rocks overlying the unconformity. The alteration occurred at least 135–200 m.y, ago when there was also a lead mobilization event. The relative low primary 18O/16O ratios in the granites suggest that the granites may have derived from partial melting of low −180 metamorphic rocks in the lower crust.