Prograde and retrograde fluid-rock interaction in calc-silicates northwest of the Idaho batholith: stable isotopic evidence |
| |
Authors: | Claudia I Mora John W Valley |
| |
Institution: | (1) Department of Geological Sciences, University of Tennessee, 37996 Knoxville, TN, USA;(2) Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., 53706 Madison, WI, USA |
| |
Abstract: | Carbon and oxygen isotopic analyses of silicate and carbonate minerals indicate that isotopic compositions in metasediments
of the Wallace Formation (Belt Supergroup) exposed northwest of the Idaho batholith have been affected by both prograde and
retrograde fluid-rock interaction. Silicates retain isotopic fractionations that reflect equilibration at peak metamorphic
temperatures. In contrast, calcite oxygen isotopic compositions range from δ18O(Calcite)=+2.3 to +18.6‰ SMOW (standard mean oceanic water) and indicate that some calcites have exchanged with low-δ18O meteorichydrothermal fluids. Values of Δ18O (Quartz-Calcite) as large as +15.5 clearly indicate that the isotopic depletion of these calcites postdates the peak of
regional metamorphism. Carbon isotopic compositions of 18O-depleted calcites are not significantly shifted relative to δ13C values in undepleted calcites, suggesting that the retrograde fluid was carbon-poor. Petrographically, retrograde fluid-rock
interaction is associated with the occurrence of fine-grained, highly-luminescent calcite overgrowths on less-luminescent,
metamorphic calcites, slight to moderate argillic alteration, and pseudomorphing of scapolite porphyroblasts by fine-grained
albite. Retrograde isotopic depletions may be related to shallow meteoric-hydrothermal fluid flow developed around the Idaho
batholith after intrusion and rapid uplift of the terrane. Peak metamorphic isotopic compositions in the Wallace Formation
reflect mineralogically heterogeneous protolith compositions and isotopic fractionation due to devolatilization and/or infiltration.
Variability in oxygen isotopic compositions on the order of 4–6‰ within the same rock type can be attributed to the combined
effects of inherited isotopic compositions and isotopic shifts resulting from prograde devolatilization. Isotopic and compositional
heterogeneity on the scale of mm to m precludes generalization of isotopic gradients on a regional scale. The isotopic data
presented here, and metamorphic fluid compositions determined in previous studies, are best reconciled with heterogeneous
bulk compositions, dominantly channelized prograde and retrograde fluid flow, and locally low fluid-rock ratios. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|