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Precambrian geology of the central Mazatzal Mountains, Arizona. Lead isotope heterogeneity in Precambrian igneous feldspars


Ludwig, Kenneth Raymond (1974) Precambrian geology of the central Mazatzal Mountains, Arizona. Lead isotope heterogeneity in Precambrian igneous feldspars. Dissertation (Ph.D.), California Institute of Technology.


Part I: The Mazatzal Mountains are located in Gila, Yavapai, and Maricopa counties in central Arizona. The rocks in the central part of this range are dominantly Precambrian, and the only younger stratified rocks are of Tertiary or younger age. The Precambrian rocks have undergone a Precambrian regional metamorphism to lower greenschist grade. Two main mappable units of stratified rocks are present: the Alder series and the Red Rock rhyolite. Previous work (Wilson, 1939) had indicated that the Alder series is a clastic sedimentary pile, and that it lies in fault contact with an older Red Rock rhyolite. This work shows that the Alder series consists of about one-third directly accumulated volcanic material, another third of slightly reworked volcanic material, and the remaining third of argillites, quartzose sandstones, and limestones. The total thickness of the section is unknown, but is at least 14000' (4200 meters). The overall character of the Alder series is eugeosynclinal, in view of its great thickness, the abundance of volcanics, the thick sections of volcanic sandstones and wackes, and an overall trend towards more shallow-water deposited rocks towards the top of the section. The volcanics of the Alder series, however, are predominantly felsic (dacites to rhyolites), and basalts and pillow lavas, though present, are not abundant. The Red Rock rhyolite is a thick (at least 1000 meters) pile of extrusive rhyolitic volcanics including abundant ash-flow tuffs. There is no evidence of a fault between the Red Rock rhyolite and the Alder series. The Red Rock rhyolite lies with depositional contact and without apparent unconformity on the uppermost Alder series beds. In the lower Gold Creek area, a feeder dike to the Red Rock rhyolite penetrates the uppermost Alder series, and broke the surface at the time of accumulation of the Red Rock rhyolite. The lithologies of the Alder series show a fairly continuous evolution from sediments and volcanics accumulated in relatively deep water to sediments and volcanics accumulated in progressively more shallow water, and finally to sediments and volcanics accumulated under intermittently subaerial conditions. The basin of accumulation of the Alder series became progressively more shallow, at least partly because of the large volumes of volcanic material which accumulated there, until the eruption of the tremendous amounts of volcanics of the Red Rock rhyolite obliterated the remaining vestiges. The Red Rock rhyolite is petrographically and chemically indistinguishable from some of the volcanics in the uppermost Alder series. It evidently accumulated from sources which were active in latest Alder series time and which contributed to the uppermost Alder series section. Thus at least locally, the Alder series - Red Rock rhyolite contact intertongues. The thickness of the Red Rock rhyolite and the abundance of associated intrusive increases to the northeast, suggesting a probable direction of the source area, perhaps the Precambrian alkali rhyolite complex of Tonto Basin described by Conway (1973). Several generations of shallow intrusive rocks penetrate the Alder series and the Red Rock rhyolite. Two types of felsic porphyries were emplaced before folding, while one type of felsic porphyry was emplaced after folding. Mafic volcanic sheets and dikes which intruded in and near the Red Rock mass of the Red Rock rhyolite may have been emplaced during folding. Chemical analyses were obtained for nine samples of the Pre-cambrian rocks of the central Mazatzal Mountains. These analyses suggest that the volcanic rocks of the lower Alder series are dominantly dacitic, while the upper Alder series volcanics are dominantly rhyolitic. The chemical composition of the upper Alder series volcanics and the Red Rock rhyolite are generally quite similar to modern, unaltered alkalic rhyolites. The structural framework of the area is dominated by tight, northeastward-trending folds. The Red Rock rhyolite is exposed at the cores of synclines, while a faulted anticlinal structure cuts off the oldest Alder series beds to the southeast of Mt. Peeley. These large-scale structures are of Precambrian age. Isotopic age determinations using the U-Pb method on cogenetic zircon fractions were performed on upper Alder series volcanics. The apparent age indicated by the analyses is 1730 ± 20 m.y. The upper Alder series volcanics are not distinguishable in apparent age from the Red Rock rhyolite as determined by Silver (1964). This apparent age is distinctly younger than the ages obtained for previously-correlated rocks in the Prescott-Jerome area, and confirms the suspicions of Anderson et al (1971) that the Yavapai schist of the Prescott-Jerome area is not correlable with the type Alder series. Part II: The lead of high-purity, acid-washed K-feldspar concentrates from five Precambrian granites was removed in a stepwise fashion by two different techniques. Both the technique of stepwise volatilization under vacuum and the technique of stepwise partial attack by hydrofluoric acid yielded leads from the same K-feldspar concentrate which varied significantly in their isotopic composition. The patterns of lead isotopic variation from the feldspar concentrates show that the lead isotopic heterogeneity is due to a variable component of radiogenic lead which was generated since the crystallization of the rock. Two classes of lead isotopic variation were observed: (I) Parallel and colinear variation in the 206/204, 207/204, and 208/204 ratios, suggesting that the feldspar incorporated lead derived from long-term uranium and thorium decay from a source with constant Th/U, and (II) Variation in the 206/204 ratio independent of any 207/204 and 208/204 variation. This suggests that the feldspar contains lead derived from the continuing separation and concentration of a U^(238) intermediate-daughter, probably Rn^ (222). The amount and type of radiogenic lead incorporated by the five feldspar concentrates correlates with the concentrations of uranium and thorium in the rocks and with the geologic age and history of the rocks. Thus the rock with the highest uranium content (the Lawler Peak granite) has the K-feldspar with the greatest amount of uranium-derived radiogenic lead, and the rock with the highest thorium content (the Marble Mts. granite) has the K-feldspar with the greatest amount of thorium-derived radiogenic lead. The calculated Th/U values for the sources of the radiogenic leads ranges from two to eleven. Th/U values in this range are reasonable for several common granitic accessory minerals of appreciable U and Th content, but are higher than the Th/U values of the zircons in the rock. For two of the rocks, the Payson granite of Arizona and the Giants' Range granite of Minnesota, the composition of the radiogenic lead of the K-feldspar concentrates suggests that the rocks were disturbed by nearby Precambrian intrusives. The apparent time of incorporation of the radiogenic lead of the K-feldspar of the Giants' Range granite is 1010 ± 150 m.y., which correlates well with the time of intrusion of the Duluth gabbro complex at 1120-1140 m.y. (Silver and Green, 1972). Although the least radiogenic lead fractions derived from the K-feldspar concentrates were significantly closer to the composition of the original feldspar lead, in no case was the original feldspar lead isolated. Moreover, because the least volatile lead fractions of some of the feldspar concentrates were more radiogenic in character than fractions of lesser volatility, it appears that the technique of stepwise volatilization is not promising for the routine isolation of original feldspar leads. The technique of stepwise hydrofluoric acid attack, however, yielded a greater isolation of original feldspar leads in fewer and simpler steps. This technique may be useful for the routine removal of the greatest part of the radiogenic lead component of Precambrian feldspars, and with further development may lead to the complete isolation of the original leads of such feldspars. The isotopic compositions of the least radiogenic fractions of the three 1450 ± 20 m.y. southwestern granites have almost no variation in 207/204 (15.39 ± .02) and less than 1% variation in 206/204 (16.32± .08). Calculations using the patterns of lead isotopic heterogeneity of these feldspars suggest that the original 206/204 values of these rocks were close to 16.16. The observed least radiogenic 208/204 value of the high-thorium (142 ppm) Marble Mts. granite is distinctly higher than those of the other two 1450 ± 20 m.y. granites. These patterns suggest that these rocks were derived from a common, long-lived source, which source underwent a differentiation so as to cause a change in Th/U and Th/Pb a few hundreds of millions of years before the emplacement of the granites.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Silver, Leon T.
Thesis Committee:
  • Unknown, Unknown
Defense Date:31 July 1973
Funding AgencyGrant Number
National Science FoundationGA-15989
Atomic Energy CommissionAT(04-3)-767
Record Number:CaltechTHESIS:09092010-080806480
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6025
Deposited By: Benjamin Perez
Deposited On:10 Sep 2010 15:33
Last Modified:26 Dec 2012 04:30

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