Oxygen and sulfur isotope studies of the Wolf River Batholith in Wisconsin and related Precambrian anorogenic granitic rocks in the midcontinent of North America
Abstract
Oxygen and sulfur isotope compositions have been determined for the granitic and related rocks from the Wolf River Batholith, Wisconsin; the Illinois Deep Holes; the Wet Mountains plutons, Colorado; and subsurface sample from Illinois, Iowa, S. Dakota, Nebraska, Missouri, Kansas and Oklahoma. Hydrothermal alteration resulting in a decrease of $\Delta\sb{\rm Q-F}$ values was observed locally throughout the Wolf River Batholith. Feldspars of different colors (pink, gray and red) were separated whenever feasible and analyzed. Most red feldspars (An$\sb{10-30}$) show the highest and constant $\delta\sp{18}$O values (9.3-10.0 permil) suggesting near complete isotope exchange with hydrothermal fluid. Based on $\delta\sp{18}$O values and alteration temperatures estimated from fluid inclusions (260-350$\sp\circ$C), $\delta\sp{18}$O of the fluid is calculated to be 5.0 $\pm$ 1.4. Sedimentary formation water in Wisconsin is most likely the source of the fluid. Sulfur content and isotope composition of granitic rocks of the Wolf River Batholith range from 30 to 117 ppm and from 1.1 to 6.5 permil respectively and are considered to be magmatic. The positive correlation observed between oxygen and sulfur isotope data of granitic rocks may be due to assimilation of the Penokean plutonic rocks by a primary magma of deep-crustal origin, or to mixing at depth of a primary magma with another magma having higher $\delta\sp{18}$O and $\delta\sp{34}$S. Oxygen isotope fractionations among quartz, feldspar and biotite indicate two different isotope exchange processes at two different stages: (1) the continuous retrograde isotopic exchange shortly after emplacement and (2) the hydrothermal alteration at a much later time. When data from the present study are combined with data from previous studies on similar Precambrian anorogenic granitic rocks in the midcontinent of North America, a regular increase of $\delta\sp{18}$O$\sb{\rm Q}$ from NE to SW is observed: Wolf River Batholith, WI (mean $\delta\sp{18}$O$\sb{\rm Q}$, 7.9); Illinois Deep Holes (8.2); St. Francois Mts., MO (9.5); and Wet Mts., CO (9.8). The $\delta\sp{18}$O$\sb{\rm Q}$ values of subsurface samples range from 7.3-10.5. Thus, all the granites may have formed by partial melting of a low-$\sp{18}$O source material in the lower crust, with an increasing contribution of high-$\sp{18}$O material such as metasedimentary and/or metavolcanic rocks toward the south.
Degree
Ph.D.
Advisors
Shieh, Purdue University.
Subject Area
Geochemistry
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