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11 0 11 III1 0 11 1111 111 VIII 11 il 3 1818 00069108 7 Middle Tertiary plutonism in the Santa Catalina
and Tortolita Mountains, Arizona
By .rt S. C. Creasey, Norman G. Banks, R. P. Ashley, and Ted G. Theodore U.S. Geological Survey, Menlo Park, California 94025
U.S. Geological Survey Open-file Report 76-262 1976
This report is preliminary and has not been edited or reviewed for conformity with Geological Survey standards and nomenclature.
2634 7!3
CONTENTS
Abstract
Introduction
General geology
Radiometric ages
Epilogue
References cited ILLUSTRATIONS
Page
Figure 1. Geologic sketch map of the Santa Catalina Mountains -- 3
2. Locations of samples listed in tables 1, 2, and 3. Stipple pattern indicates the outcrop areas of the quartz monzonite of Samaniego Ridge 7
3. Bar graphs of new fission-track and K-Ar ages of the quartz monzonite of Samaniego Ridge and the quartz monzonite of the Tortolita Mountains, Santa Catalina and Tortolita Mountains 11
4. Bar graphs of published K-Ar ages of quartz monzonite of Samaniego Ridge and gneissic rocks, Santa Catalina and Tortolita Mountains 13
TABLES
Table 1. New fission-track and K-Ar ages of the quartz monzonite of Samaniego Ridge and the quartz monzonite of the Tortolita Mountains, from the Santa Catalina and Tortolita Mountains 6
2. Previously published K-Ar ages of the quartz monzonite of Samaniego Ridge, Oracle Granite, and gneissic rocks, Santa Catalina and Tortolita Mountains 8
3. Previously published Rb-Sr ages of the quartz monzonite of Samaniego Ridge, Oracle Granite, and gneissic rocks, Santa Catalina Mountains 14
4. Chemical, spectrographic, normative, and modal analyses of rocks from the Santa Catalina and Tortolita Mountains 18
Analytical data for fission-track ages for Santa Catalina and Tortolita Mountains 19
Analytical data for K-Ar ages for the Santa Catalina and Tortolita Mountains 20
ii 1
ABSTRACT
Recent reconnaissance geologic mapping in the Santa Catalina and Tortolita Mountains of southeastern Arizona, supplemented by new and published potassium-argon and fission-track ages, suggests that a large middle Tertiary (about 25 m.y.) composite batholith crops out extensively in both mountains. More than two-thirds of the batholith and the contiguous wallrocks are gneissic, the gneissosity comprising strong cataclasis and mylonitization, penetrative planar and linear structures, and crystallization of muscovite and biotite in the foliation planes. New radiometric ages indicate the deformation followed the crystalliza- tion of the batholith so closely that the K-Ar dating method cannot distinguish a difference, whereas previously published ages from the gneisses indicate a short time interval between the two events.
INTRODUCTION
The authors recently mapped the geology of the Santa Catalina and a small part of the Tortolita Mountains in reconnaissance (Banks, 1976; Creasey and Theodore, 1975), and made 26 new radiometric age determinations to help interpret the plutonic and structural history of the region. A synopsis of the geology of the Santa Catalina Mountains, which includes some heretofore unpublished geology of the northern flank of the mountains, is shown on figure 1.
The Santa Catalina Mountains were mapped twice by the U.S. Geological Survey previous to our work, but neither of the earlier maps were published. The first map was made by C. F. Tolman, Jr., at a scale of 1:125,000 in 1911-12. He also prepared a report in 1914, but neither map nor report was published. In 1930, B. N. Moore of the U.S. Geological Survey was assigned to update the report for publication. In 1938, he submitted a report, but before the report was approved for publication, he left the Geological Survey, and again the report was never published. However, a summary of Moore's report, prepared by B. S. Butler and R. M. Hernon, and a black-and-white version of the map were released in open file (Moore and others, 1949). In addition, many maps from theses by students from the University of Arizona cover separate parts of the Santa Catalina. Except for a mid-Tertiary rather than a Late Cretaceous or early Tertiary age for emplacement of the batholith and development of the gneiss, our interpretations do not differ substantially from those of B. N. Moore.
New K-Ar and fission-track ages are listed in table 1, and to permit comparison, previously published K-Ar and Rb-Sr ages of rocks in the Santa Catalina Mountains are listed on tables 2 and 3. These data are summarized by the bar graphs of figures 3 and 4. Chemical and spectrographic analyses, modes, and norms of our dated rocks and two additional samples are listed at the end of the report (table 4). The 2 locations of all samples, including ours and those available for dated samples reported in the literature are indicated on figure 2. The analy- tical data supporting the radiometric ages are tabulated at the end of the report.
GENERAL GEOLOGY
The Santa Catalina Mountains are one of the structurally and lithologically complex mountains of the Basin and Range province in southeastern Arizona (fig. 1). At the northern end of the Santa Catalina, the Mogul fault is the dominant structure. North of the Mogul fault, only Oracle Granite, locally cut by dikes and quartz veins, crops out, whereas south of the fault and along the northeastern flank, the range consists of Precambrian, Paleozoic, and Mesozoic sedimentary rocks, all intruded by quartz diorite, quartz monzonite, and granodiorite porphyry.
The central core of the Santa Catalina Mountains consists of a large composite batholith of middle Tertiary age. Apparently the batholith extends to the northwest into the Tortolita Mountains, where it has been partly mapped in reconnaissance by the authors, and to the southeast into the Rincon Mountains. The batholith comprises at least two intrusive phases hereafter referred to as the quartz monzonite of Samaniego Ridge and the quartz monzonite of the Tortolita Mountains. Except for two dikes of the quartz monzonite of the Tortolita Mountains, just east of the Pirate fault, only the quartz monzonite of Samaniego Ridge crops out in the Santa Catalina Mountains. The main mass of the quartz monzonite of the Tortolita Mountains lies to the west of figure 1 in the Tortolita Mountains.
The southwestern flank of the Santa Catalina consists of a mixture of quartz monzonite of Samaniego Ridge and roof pendants, principally of Oracle Granite, but also of granodiorite and Precambrian rocks. The quartz monzonite forms a sill-like body more than 2 km thick which intruded the contact between the Apache Group composed of sedimentary rocks lying to the north and the Oracle Granite on the south.
Roof pendants and xenoliths of Apache Group, Pinal Schist, Paleozoic and Mesozoic sedimentary rocks, and quartz diorite (Leatherwood) occur here and there in the quartz monzonite. They are common near Mount Lemmon, which is the highest peak in the Santa Catalina.
Emplacement of the batholith was essentially coeval with the development of extensive penetrative planar and linear structures that characterize the gneisses forming the southeastern forerange of the Santa Catalina Mountains and extending both northwestward into the Tortolita Mountains and southeastward into the Rincon Mountains, which join the Santa Catalina Mountains along a low divide known locally as the Redington Pass. The contact between gneissic and nongneissic parts 3 45'
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