Masters Theses

Date of Award

3-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

K. C. Misra

Committee Members

H. Y. McSween, L. A. Taylor, T. C. Labotka

Abstract

The Late Proterozoic (?) Ashe Formation is a complex assemblage of pelitic schists and gneisses, amphibolites and ultramafic rocks. Although studies of metamorphism of each of these rock types have been conducted, little work has been done to determine the nature of the protoliths for the Formation and the tectonic environment in which the original rocks were deposited. To explore this aspect of the Ashe, a detailed geochemical study of amphibolites within the Ashe and the overlying Alligator Back Formation has been carried out.

Petrographic study of 78 samples, 62 from the Ashe and 16 from the Alligator Back, shows that the assemblages present in amphibolites from the Ashe are (+qtz + plag + Fe-oxide): hnb + ep, hnb + bio + ep, hnb + gar, hnb + bio, hnb + bio + gn, hnb + gar + ep and hnb + gar + cumm. The last assemblage, hnb + cumm + gar, has not been reported for these rocks. Assemblages in the Alligator Back Formation are (+qtz + plag + Fe-oxide): hnb + ep, hnb + bio + ep and hnb + bio + gar.

Whole rock analyses of 74 samples from these formations reveal three distinct rock types. Group 1 contains rocks characterized by relatively high average values of TiO2 (3.0%), Zr (200 ppm), Y (45 ppm), Nb (22 ppm), P2O5 (0.4%) and FeO* (15.8%). Group 3 is characterized by relatively low averave values of TiO2 (0.5%), Zr (40 ppm), Y (15 ppm), Nb (<4ppm) and P2O5 (0.05%). This group also contains high values of CaO (11-15%) and Ni (110 ppm). Group 2 contains the most samples and is characterized by intermediate values of Ti02 (1.4%), Zr (110 ppm), Y (30 ppm), Nb (10 ppm) and P2O5 (0.2%). Spatially group 1 and 2 rocks occur randomly throughout both formations and often are closely associated, whereas group 3 samples are situated only within the Ashe. On the basis of several discrimination diagrams (Ti-Zr-Y, Ti/V, Y/Nb-TiO2), it is concluded that group 1 rocks were originally continental flood basalts, whereas group 2 rocks are predominantly metamorphosed ocean floor basalts. Rare earth element patterns support these conclusions. Group 3 rocks are problematic in that their trace element abundances suggest they are metmorphosed intrusive rocks, while their Mg/(Mg + Fe) ratios suggest that they are basalts. This group probably represents a protolith of basaltic composition, yet with a high phenocryst/liquid ratio.

This study shows that amphibolites from within the Ashe Formation are much more variable than was previously considered, however, it is possible to constrain the depositionaly setting of the unit to a rifted passive margin containing crust transitional from continental to oceanic. These data and conclusions are also consistent with chemical data from other ancient and present-day rifts, and also with regional stratigraphy.

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