GEOLOGICAL ANALYSIS OF APLITE DIKES, TEXAS CANYON STOCK, ARIZONA
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Authors HARDIN, JUSTIN O’LEE
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GEOLOGICAL ANALYSIS OF APLITE DIKES, TEXAS CANYON STOCK, ARIZONA
By
JUSTIN O’LEE HARDIN
______
A Thesis Submitted to The Honors College
In Partial Fulfillment of the Bachelors degree With Honors in Geosciences
THE UNIVERSITY OF ARIZONA
A U G U S T 2 0 1 6
Approved by:
______
Dr. George Davis Department of Geosciences 1
ABSTRACT
An area of the Texas Canyon stock near Dragoon, Arizona, containing exposures of aplite dikes (thin, resistant, fine crystalline intrusions) (Wells and Bishop, 1954), was scouted and geologically mapped in order to find the extent and character of a dike swarm located in that area. A sample of the aplite material was collected to be studied with geochronology techniques to determine its age, to serve as a guide to interpret the timing of the development of the fracture pattern in the granite, with the orientations of the aplite dikes studied in mind. The dikes in the swarm were found to dip steeply (70⁰ to 90⁰); the overwhelming majority had strikes roughly east-northeast, which is the same direction the swarm itself is aligned. The swarm was found to be approximately 580 meters in north-south length and 76 meters in east-west width, with an average dike thickness of 5-15 centimeters.
The results of geochronology on the aplite revealed that the date of its intrusion (as well as the fracture pattern in the granite filled by aplite) is approximately 56.3 million years ago. This knowledge will be useful in the understanding of other plutons in the region that show the same patterns.
INTRODUCTION
GOALS & OBJECTIVES
The purpose of this study is to document the distribution and properties of aplite dikes within an area of the Texas Canyon Stock granite located near Dragoon, Arizona. Aplites are magmatically late, very fine-grained, quick-cooling intrusive rocks (Wells and Bishop, 1954). Figure 1 displays their general appearance and relationship to the surrounding granite. The study area is located entirely on property belonging to the Amerind Foundation, an organization and museum that performs archeological research and preservation on artifacts of native cultures.
2
Figure 1: The granite is the darker rock with larger crystals on the outside, while the lighter rock with finer crystals is an aplite dike. At the
top of the large boulders, the higher resistance to weathering possessed by aplite is visible, as it is the only remaining rock at the top. The aplites here show closely spaced horizontal joints. The large phenocrysts visible in the surrounding granite are orthoclase
feldspar in composition.
The interest in the focus of this study, aplites in the Texas Canyon stock, came about during a structural geology field trip to investigate fracture patterns in the granite. During this trip, I became interested in the character and origin of the aplite dikes that pervade the granite in some areas. This experience raised questions in my mind about the aplites, including their history and structural relationship to jointing in the granite. Furthermore, although the area was geologically mapped in the 1950s, the extent of prior investigation of the aplite was minimal, consisting only of a note of their presence and general petrological classification in Cooper and Silver (1964). The overall question became one of what circumstances produced the jointing, how it affects the aplite dikes, and when.
3
LOCATION OF STUDY AREA – AMERIND FOUNDATION
“The Foundation is a nonprofit research institution devoted to the study, preservation, and interpretation of prehistoric and historic Native American cultures” (Amerind 2012). Without the access to the study area graciously provided by the Foundation, this study would not have been possible. Figure 2 displays the location of the field area.
Figure 2: Location of field area shown in red. Area of bottom map is represented on top map by black rectangle. 4
GEOLOGICAL SETTING
TEXAS CANYON STOCK
The Texas Canyon Stock is the singular rock unit in the area of study. Much of the previous work carried out on this unit was performed by John R. Cooper and Leon T. Silver in their paper Geology and Ore Deposits of the Dragoon Quadrangle, Cochise County, Arizona, published in 1964. As such, it is extremely valuable both in describing the granite and as a context for understanding aplite composition and geochronology.
Although referred to as granite in a general sense, the Texas Canyon stock is specifically composed of quartz monzonite. Quartz monzonite is petrologically classified as “30 to 35 percent quartz, 30 to 40 percent plagioclase, 25 to 30 percent potassium feldspar, and about 5 percent mica.” The stock intrudes Paleozoic strata, as well as Precambrian basement including the Pinal schist, a 1.65 billion year old metamorphic complex (Meijer, 2014). Cooper and Silver (1964) reached the conclusion that the aplite was intruded along the joints in the quartz monzonite stock, but they did not provide supporting documentation. They also concluded, as an inference based on geological evidence, that the age of the Texas Canyon quartz monzonite falls somewhere between early and late Tertiary (Cooper and Silver, 1964). An important objective of this study is to properly analyze these conclusions.
The elevation across the area covered by the stock varies, sometimes sharply, ranging from no less than 1414 meters above sea level to a peak of1460 meters. 4791 Some exposures of granite are relatively flat and expansive, emerging only a meter or two out of the ground. Other localities, including many in and around the study area, consist of tors of granite often as high as 6 meters or more abruptly rising from the ground as well as large boulders.
APLITES
Upon “scouting out” the area to determine the locations of aplite exposures, it became clear that some portions of the area were marked by abundant dikes, while others lacked them almost completely. Geological mapping resulted in the recognition of a dike swarm. “Dike swarm” is a formal term referring to an extremely concentrated or dense collection of dikes found at a location within an intrusive body. They are not exclusive to granites or aplites. 5
Mapping this dike swarm became a primary objective. The swarm is oriented ENE, and consists solely of dikes with a near-vertical (70⁰ to 90⁰) inclination. It measures about 580 meters north- south and 76 meters east-west.
An aplite dike is generally defined as a thin intrusion into granite, containing extremely small crystals (aphanitic texture) of almost solely quartz and feldspar. This composition, along with purely sodium plagioclase and muscovite, marks the crystallization of aplite at magmatically low temperatures just above 700 ⁰C (Bowen, 1922). Previous work puts the composition of aplite dikes in the Texas Canyon stock as “40 to 45 percent quartz, 25 to 35 percent microcline, 20 to 30 percent albite, and about 5 percent muscovite” (Cooper and Silver, 1964).
METHODS
GEOLOGICAL MAPPING
Data collection for the aplites was performed by walking about the study area and stopping at excellent aplite exposures (marked as stations; see the stations map). The very first step was to take one day performing “reconnaissance,” simply walking about the potential field area and noting locations where aplites were abundant, sparse, or absent. For each aplite in an area (station), the thickness was recorded, along with the trace lengths of individual aplite dikes as expressed in horizontal and vertical exposures, when available (for example, horizontal traces were not available for aplites that were apparent in vertical walls). Dip directions and dips were recorded for a selection of aplites within each group; aplites with clearly contrary orientations to the average were always noted. Photos were taken at any location that would provide a good demonstration of some quality of the aplite dikes or swarm. It became apparent when making observations and collecting data that the aforementioned dike swarm existed, so part of the process was determining its borders and path through the study area via geologic mapping. Google Earth applications helped greatly in capturing an image of the fracture pattern in the area as well as an image of the position and orientation that the basemap would be constructed from.
Unfortunately, the identifiable boundaries of the dike swarm are unclear at the north and south because of alluvial cover; this makes the edges of the swarm vague or gradational in those directions. Bodies of Texas Canyon stock exist directly to the west and to the northeast 6
(separated by a patch of alluvium). This makes the western border of the dike swarm very clear, as it was easy to see where the abundance of dikes began to wane. The large exposure of quartz monzonite in the northeast contains few to no aplite dikes, so it provides a good constraint on the extent of the dike swarm. A “boundary zone” classification was created on the geologic map for areas where the probable border of the dike swarm is covered by alluvium, or areas where the frequency of dikes is less than in the center of the swarm, but not extremely scarce. What is visible of the dike swarm is about 580 meters (1,900 feet) in north-south length and about 76 meters (250 feet) in east-west width. Figure 4 displays the completed geologic map of the study area.
It should be noted that there is a bias in this study against aplites with a thickness less than 2.5 centimeters (due to reasons of visibility as well as comprehensiveness) as well as against dikes that occur at higher elevations. Some portions of the field area are very difficult to access, due to the geomorphology of the area as described in the section on “Geological Setting.”
Figure 3: West-directed view demonstrating geomorphology in area.
7
focused geological map of study area. of study map focusedgeological -
Aplite :
4 Figure 8
GEOCHRONOLOGY
University of Arizona LaserChron provided the opportunity to apply standard methods to determine the age of the aplite sample. The sample was collected (using a rock hammer and chisel) from a relatively thick aplite dike at the location marked “Sample” on Figure 7. Next, a rock saw was used to remove weathered material from the outside of the sample; a small piece was also set aside to be sent off for thin section preparation. The clean sample was then crushed via pounding by hand and sifting it through a grain film, after which the very fine product was sent to the mineral separation lab. Zircons were then picked from the resultant grain sample using a microscope and tweezers. The zircon grains were mounted with standards to be viewed under the scanning electron microscope and then placed into the mass spectrometer for spot measurements of U-Pb. Finally, the resultant data were reduced with the Excel programs commonly used by Arizona LaserChron.
In analysis of zircons using the U-Pb system, a series of reference (known age, used for error correction) zircon grains as well as the sample grains are ablated in order to find the abundances of lead-206, lead-207, uranium-235, and uranium-238 within (Ibanez-Meija et al., 2015). Uranium-238 is the parent isotope in a decay chain with daughter isotope lead-206, and uranium-235 is the parent isotope in a decay chain with daughter isotope lead-207. Thanks to the constancy of nuclear decay, if the abundances of both elements are known, an age can be calculated, with the assumption that additional amounts of such material have not been added over time; the analysis is strengthened by the simultaneous presence of two decay chains.
9
PETROGRAPHY
Petrographic compositional studies were performed based on thin-section examination using a petrographic microscope. Estimates of percentages of quartz, plagioclase, potassium feldspar, and other mineral percentages were made using a 75-grain sample selection and then compared to the original study by Cooper and Silver (1964).
FINDINGS
PETROGRAPHY OF APLITE DIKES
Cooper and Silver (1964) categorize the aplites of the Texas Canyon Stock as “40 to 45 percent quartz, 25 to 35 percent microcline, 20 to 30 percent albite, and about 5 percent muscovite.” Based on petrographic examination of a thin section from the aplite sample collected as part of this study, independent comparison to the observations of Cooper and Silver (1964) could be made using a sample of 75 grains. The percentages of microcline and albite within the sample fell within the ranges supplied by Cooper and Silver, and with the sample size given, the percentages of other minerals (almost all muscovite) and quartz were within the margin of error. The particular sample may have had a slightly higher abundance of quartz.
Grain type Tally Percentage of total Quartz 35 46.7 Plagioclase 21 28.0 K-feldspar 16 21.3 Other 3 4.0 Total 75 100.0
Table 1: Grain counts for minerals in aplite sample, based on petrographic analysis.
10
Figure 5: Photomicrograph in cross-polarized light identifying appearances of different mineral
grains.
ORIENTATIONS OF APLITE DIKES
Figure 6 displays stereonets of the orientations (strike and dip) of the aplite dikes in three distinct regions: south, central, and north. These regions were treated as structural domains within which orientations of aplite dikes could be averaged, and across which orientations of aplite dikes could be compared and contrasted. Each structural domain contains 7 to 8 stations, the locations of which can be seen in Figure 7. A stereonet displays both the direction of the dike as well as the steepness, or dip, of its plane; the closer to the center of the stereonet, the steeper the dip. One may imagine that if, in the stereonet, the aplite dike is very close to the center and occupies a line from the southwest to the northeast direction, then standing at the location of that dike you would find it running in said directions and being nearly vertical in orientation. Stereonets are useful because they provide an excellent tool for viewing statistically the 11 orientation occupied by the average aplite, how spread out that average is, and what outliers exist. The aplite dikes overwhelmingly have an average strike of ENE, but they spread as much as 45⁰ away from that direction, at least for dikes that do not appear to be outliers. The average dip is very clearly about 80⁰, as there are no outliers in terms of dip. Eighty-six total dikes were catalogued in the study, which were reduced to 36 averages for the stereonets. Averages were calculated from similar orientations at each station, so some stations had two distinct average orientations.
THICKNESS OF APLITE DIKES
Figure 8 is a histogram of the thicknesses of the measured aplite dike. It shows the number of aplite dikes between 1 and 5 centimeters thick (in the “1.0” bin), between 5 and 10 centimeters thick (in the “5.0” bin), and so on and so forth. This is useful for establishing an average thickness of the dikes in the study area, as well as being a tool for easily showing just how large the dikes can become, and how common those sizes are. The dikes range from 2.0 to 91.4 centimeters in thickness, and the majority are between 5.0 and 15.0 centimeters thick. Eighty-five dikes catalogued were measured for thickness. All of the original data recorded in the field for thicknesses, orientations, etc. are in the Appendix.
12
s.
Note Note that there is no significance to the . .
Figure 7
Geologic Geologic map with overlay of stereonets displaying orientations of aplite dikes (strike and dip). White lines separate south, :
north, north, and central regions. The geographic distribution of stations can be seen in Figure 6 the line by constrained areas the of sizes the of limitations the to isdue that themselves; stereonets the sizes of relative 13
on top of geological map. geological topof on
and sampling location sampling and
Map of data collection station locations station collection data Mapof
:
Figure 7 Figure
14
thick thick were not considered for the study.
2.5 2.5 centimeters
inherent bias against small dikes exists, due to visibility from a distance. a from tovisibility due exists, dikes small against bias inherent
Histogram Histogram of aplite dike thicknesses. Note that aplite dikes less than :
Figure 8 an histogram, the of theappearance Despite 15
VARIATIONS IN ORIENTATIONS AND THICKNESSES
As best viewed in Figure 6, aplite dikes in the swarm vary across the study area in their degree of discordance from the dominant ENE direction. Figures 9 and 10 demonstrate this variation.
Figure 9: Northeast-directed view demonstrating two sets of orientations in dike swarm at its approximate center.
Figure 10: North-directed view south of Figure 9 location, an area near the center of the dike swarm with more uniform orientation. 16
Figure 11 shows the higher resistance of aplite as compared to the surrounding quartz monzonite, as already shown also in Figure 1. Aplites in the study area also sometimes vary in orientation due to the nature of the intrusion, as shown by Figure 13. Figure 12 shows a good close-up of two different aplite dikes meeting, demonstrating no sharp boundary when looking closely.
Figure 11: Exposure of aplite where all of the surrounding granite has weathered away, showing its greater resistance to weathering compared to the quartz monzonite.
17
Figure 12: Melding of two aplite dikes as exhibited on a loose boulder.
Looking closely, no sharp boundary is seen between the dikes. 18
Figure 13: An aplite dike with highly variable orientation. 19
At the north and south ends of the aplite dike swarm, the thickest dikes are seen (see stations N01, S09, and S10 in Tables 3 and 4 for exact thicknesses; locations on Figure 7). Figures 14 and 15 show examples of some of these very thick dikes. The thickest dike in the entire area, at 91.4 centimeters thick, is visible in these photos. Figure 16 is a demonstration of how the orientations of some aplites are measureable, but not the thicknesses.
Figure 14: East-directed view at N01 of very thick aplite dikes; the thickest is 91.4 centimeters, while the dike to right of the pencil is 5.1 centimeters thick.
Figure 15: North-directed view (alternate angle) of dikes at N01 as described in Figure 14 caption, better showing the 91.4 centimeter thick dike.
20
Figure 16: Northwest-directed view of a situation in which it is possible to get orientations of some
aplite dikes, but not thicknesses (due to safety). The dikes at the top of the photo cannot be measured for thickness, but since there is a direct line of sight to them, their orientations can be approximated.
FRACTURE PATTERN
Three sets of joints run through the quartz monzonite country rock in the field area; Figure 17 shows these fractures from a satellite view. Heidrick and Rehrig (1972) identify these joint sets as striking ENE, NNW, and WNW, and they are common amongst stocks of Laramide age in the region.
21
colors are granite, while dark colors are alluvium and are colors while alluvium dark are colors granite,
pattern. s toLight fracture show
photo satellite
edited
-
edited and editedand contrast
- Color
:
in white. area outlined Study ge.
Figure Figure 17 folia 22
APLITE GEOCHRONOLOGY
After using software to place 20 micron targets on individual zircon sample and standard grains for the mass spectrometer, the mass spectrometer cycled through each spot analysis automatically. Live preliminary results were displayed on a screen throughout the process, after which the data were reduced and organized into a usable format. The most important part of Table 2 below is the “Best age” column in yellow, which shows the age returned for each
individual sample spot. The error range is in the column to its right.
8.8
5.2
3.3
1.8
2.3
1.7
1.3
2.1
2.4
1.2
1.3
1.3
2.5
±
12.2
15.2
17.7
16.2 16.7
(Ma)
96.3
63.0
60.1
59.2
58.0
57.1
57.0
56.9
56.2
55.8
55.5
55.1
184.3
1629.3
1622.7
1066.9
1059.4
1046.9 (Ma)
age Best
±
12.2
15.2
17.7
16.2
16.7
32.6
92.9
58.4
46.3
50.8
46.0
41.6
58.4
27.6
43.4
25.0
(Ma)
105.9
114.0 7.7
2.5
25.8
10.6
56.4
25.6
38.4
805.7
161.3
721.6
263.2
101.1
1629.3
1622.7
1066.9
1059.4
1046.9
1340.5
207Pb* 206Pb*
8.6
3.5
2.1
2.7
1.9
1.5
3.2
2.5
1.7
1.4
1.4
2.5
±
22.1
21.6
20.4
23.9
24.2
13.3
(Ma)
62.0
58.9
61.8
56.8
54.6
56.3
42.3
54.9
55.4
48.9
56.1
300.4
130.3
1615.6
1593.7
1042.1 1024.1
1044.9
235U
207Pb*
Apparent ages (Ma) ages Apparent
8.8
5.2
3.3
1.8
2.3
1.7
1.3
2.1
2.4
1.2
1.3
1.3
2.5
±
37.7
35.8
28.7
33.8 34.8
(Ma)
96.3
63.0
60.1
59.2
58.0
57.1
57.0
56.9
56.2
55.8
55.5
55.1
184.3
1605.1
1571.9
1030.3
1007.6
1043.9
One billion and 1.6 billion year ages are ages year billion 1.6 and billion One
238U*
206Pb*
0.97
0.95
0.96
0.98
0.97
0.94
0.77
0.91
0.85
0.87
0.83
0.81
0.65
0.72
0.67
0.90
0.82
0.97
corr.
error
2.7
2.6
3.0
3.6
3.6
4.8
5.4
5.3
3.1
3.9
2.9
2.3
3.7
4.3
2.2
2.3
2.4
4.5 ±
(%)
0.2827
0.2761
0.1733
0.1692
0.1758
0.0290
0.0151
0.0098
0.0094
0.0092
0.0090
0.0089
0.0089
0.0089
0.0088
0.0087
0.0086
0.0086 238U
206Pb*
2.7
2.7
3.1
3.7
3.7
5.1
7.0
5.9
3.6
4.4
3.4
2.9
5.8
5.9
3.2
2.6
3.0
4.6
± (%)
Isotope ratios Isotope
3.9092
3.8047
1.7907
1.7417
1.7985
0.3443
0.1370
0.0630
0.0597
0.0627
0.0575
0.0552
0.0570
0.0426
0.0555
0.0561
0.0493
0.0568
235U* 207Pb*
0.7
0.8
0.9
0.8
0.8
1.7
4.4
2.4
1.9
2.2
1.9
1.7
4.4
4.1
2.4
1.2
1.7
1.1
± (%)
9.9718
10.0071
13.3438
13.3937
13.4772
11.6136
15.1566
21.4856
21.6222
20.2907
21.6487
22.2295
21.4875
28.7050
21.7402
21.3728
24.1578
20.8172
207Pb*
206Pb*
1.8
1.9
4.2
3.9
4.5
3.5
3.8
2.6
3.7
5.7
7.7
0.8
1.6
4.7
6.2
4.9
7.2 14.2
U/Th
5970
9171
9503
6258
2013
1540
7718
2671
73479
35353
22479
28662
47238
34740
19543
36485
496940
212361 204Pb
206Pb
95
66
61
75
149
180
141
941
160
169
202
217
544
324
790
295
U
1342
1502 (ppm)
Full reduced abundances and age results for each sample spot. sample each for results age and abundances reduced Full
Analysis
TXC-001-SAMPLE 1 Spot 4 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 1 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 3 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 5 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 2 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 7 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 8 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 6 Spot 1 TXC-001-SAMPLE
Table 2: 2: Table 2014). (Meijer, respectively schist, and Pinal strata Paleozoic from in brought grains zircon to attributed
TXC-001-SAMPLE 1 Spot 14 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 11 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 12 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 17 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 21 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 15 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 18 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 19 Spot 1 TXC-001-SAMPLE
TXC-001-SAMPLE 1 Spot 16 Spot 1 TXC-001-SAMPLE TXC-001-SAMPLE 1 Spot 20 Spot 1 TXC-001-SAMPLE 23
Figure 18, below, is the graph determining the best age for the aplite itself. Each red bar represents the top 11 rows of the best age column in Table 2, which represent probable aplite ages, extended out to their respective error ranges. The plot shows a final aplite age of 57.0 Ma (represented by the green bar), with an error range of 0.98 Ma in either direction.
Figure 18: Final age determination for the aplite.
DISCUSSION
The finding by Heidrick and Rehrig (1976) that stocks of similar ages in the Arizona region had the same joint patterns was significant. It meant that the stress directions being felt by all of these rocks, at the same time, were the same. Rather than local conditions, the stocks were being influenced by motions on a much larger scale, showing that the tectonic actions of plates as far away as the California boundary of the North American and Pacific plates were creating stress on rocks cooling in southeastern Arizona. These are called far-field stresses. Although the 24 aplite dikes overwhelmingly follow a similar course as the ENE joints, perpendicular to the greatest principal stress direction, a few outliers were seen (as shown in Figure 6) that roughly follow the NNW orientation.
Despite these rough matches, within the “dike swarm” (especially toward the areas of highest aplite dike abundance in the center of the swarm) the dikes often follow winding and fast-changing strikes, sometimes melding with each other showing no clear cross-cutting or exhibiting structural balance by combining or splitting into dikes with about the same combined thickness. It is clear that the aplite swarm is aligned along a “weak lane” in the granite, the orientation of major jointing, but it does not appear that all of the aplite dikes have simply filled openings where joints previously existed. Otherwise, the pattern would be expected to be much more uniform. The distribution of orientations of the aplites also raises questions about the timing of aplite injection relative to joint formation. Again, the great majority of dikes are along the ENE set of major joints. From this, it could be surmised that most or all of this ENE set of joints had developed before the injection of aplite. In contrast, the NNW striking set of minor joints appears nearly as abundant as the ENE set, and yet very few dikes appear along the NNW orientation. Even where they do, they are generally more discordant than the other aplites relative to the major joint set. It may be that aplite injection had largely completed before the NNW set developed, and definitely before the WNW set, which is devoid of aplites. This scenario would have led to only a few dikes along the NNW orientation. Another possibility is that the ENE set of joints is purely extensional, while the NNW and WNW sets are the result of shear components and thus not as favorable for aplite injection. The results of geochronology helps make the picture more clear and give ages to these events.
Personal communication from Jay Chapman (July 22, 2016) reveals that a previous geochronological study, using similar methods, of the Texas Canyon quartz monzonite proper (not aplite) delivered an age of 55.6 Ma, a much more precise age than that given in previous literature by Cooper and Silver (1964). The reduced data of probable aplite dates returned by the geochronology study put the aplite dike sampled at 57.0 million years old. I had expected that the results of radiometric age dating conducted in this study would reveal that the aplite had been injected several million years after the emplacement of the Texas Canyon stock and, subsequently, the formation of its major joint pattern. With the age limits of the granite given by 25
Cooper and Silver (1964) falling between early and late Tertiary, the final age achieved for the aplite would have supported this hypothesis.
The aplite age of 57.0 Ma, coupled with a granite age of 55.6 Ma (Jay Chapman, personal communication, July 22, 2016), sheds new light on the magmatic emplacement history of the granite and the aplite dikes intruded into it. This new information reveals that the injection of the aplite dikes and the cooling of the pluton were, geologically speaking, contemporaneous. Again, given that aplites are magmatically late, this is a surprise, and transforms understanding of the history of the aplite dikes. Contrary to the starting hypothesis, the far-field stresses at the time of aplite injection must have been the same as those that produced the ENE-striking set of joints in the granite pluton, which were also virtually contemporaneous with the cooling of the pluton itself. This also tightly constrains the age in which the far field-stresses that created the ENE- striking joint pattern in the Texas Canyon stock were operating, because the cooling, jointing, and aplite injection all occurred in very quick succession. With errors in mind, this age is approximately 56.3 million years ago (as the previously gleaned ages show aplite older than relatively nearby granite).
How are the few wayward aplites explained? As only one sample from an ENE-striking aplite was age-dated, it may be possible that there were multiple stages of aplite injection, and a much weaker one occurred later which formed a few aplite dikes striking NNW. Another possibility is that this later stage of aplites occurred very early in formation of the associated joints.
CONCLUSION
Age dating of the aplite, coupled with helpful personal communication from Jay Chapman (July 22, 2016) regarding a more exact age of the surrounding quartz monzonite reveals that aplite injection, plutonic cooling, and formation of the major joint pattern (ENE) were all nearly geologically contemporaneous. All of these events took place around 56.3 Ma. Other minor joint patterns in the granite are not precisely datable with the information gleaned. This study may provide a framework for helping to put dates to past stress directions of other igneous bodies by dating intrusions that fill joint patterns.
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ACKNOWLEDGEMENTS
Greatest thanks to Dr. George Davis, my advisor, for providing revision and reference suggestions, inspiration, and instruction, and to Sally Meader for contributing to the George H. Davis Undergraduate Research fund and helping to make this study possible. I also thank Mark Pecha for assisting significantly in the age dating process and scheduling, including getting my sample through mineral separation, providing information, and reserving time for me to use the Element 2 mass spectrometer. I thank Jay Chapman for providing unpublished information he had which helped the completeness of the interpretation, as well as Chelsi White, Mekha Pereira, and Dan Alberts, who all in some way assisted with preparation of the sample mount, identification and verification of zircon grains, the photomicrograph process, operation of the Element 2 mass spectrometer, and preparation of BSE images. My field partners Ryan Ureña, Scott Mooney, Theodore Perkins, and Vincent LeBlanc also receive recognition as well as my gratitude for taking time out to accompany me during data collection and assure safety.
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REFERENCES CITED
Amerind Foundation, 2012, Amerind Foundation [Brochure], Dragoon, AZ.
Bowen, N. L.,1922, The reaction principle in petrogenesis: The Journal of Geology, v. 30, no. 3, p. 177-198.
Cooper, J. R. and Silver, L. T., 1964, Geology and ore deposits of the Dragoon quadrangle, Cochise County, Arizona: Geological Survey Professional Paper 416: Washington, D.C.: U.S. Government Printing Office, p. 78-82.
Ibanez-Mejia, M., Pullen, A., Arenstein, J., Gehrels, G., Valley, J., Ducea, M., Mora, A., Pecha, M., and Ruiz, J., 2015, Unraveling crustal growth and reworking processes in complex zircons from orogenic lower-crust: The Proterozoic Putumayo orogen of Amazonia: Precambrian Research, v. 267, p. 285-310.
Meijer, A., 2014, The Pinal Schist of southern Arizona: A Paleoproterozoic forearc complex with evidence of spreading ridge–trench interaction at ca. 1.65 Ga and a Proterozoic arc obduction event: Geological Society of America Bulletin, v. 126, no. 9-10, p. 1145-1163.
Rehrig, W. A. and Heidrick, T. L., 1972, Regional fracturing in Laramide stocks of Arizona and its relationship to porphyry: Economic Geology, v. 67, p. 198-221.
Rehrig, W. A. and Heidrick, T. L., 1976, Regional tectonic stress during the Laramide and late Tertiary intrusive periods, Basin and Range province, Arizona: Arizona Geological Society Digest, v. 10, p. 205-228.
Wells, K. and Bishop, A. C., 1954, The origin of aplites: Proceedings of the Geologists' Association, v. 65, no. 2, p. 95-114.
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APPENDIX
Station # Thickness (in) Dip direction (az) Strike (az) Dip (⁰) Horizontal trace (m) Vertical trace (m) Avg. str. Avg. dip N01-A 9.5 140 50 76 4.5 1.5 46.5 79.5 N01-B 2.0 141 51 78 4.0 N01-C 15.5 130 40 80 1.5 0.5 N01-D 35.0 135 45 84 3.0
N02 9.5 357 267 87 11.0 0.5 267.0 87.0
N03-A 2.3 327 237 83 1.5 1.0 222.4 82.0 N03-B-α 3.6 317 227 83 6.0 1.0 46.0 85.0 N03-B-β 4.4 306 216 83 6.0 1.0 N03-C 2.2 335 245 78 5.5 1.0 N03-D 1.2 277 187 83 1.5 N03-E 3.0 136 46 85 4.0 4.0
N04-A 1.1 281 191 81 5.0 1.0 206.0 84.3 N04-B 0.8 315 225 86 5.0 1.5 53.0 70.0 N04-C 4.1 143 53 70 5.5 2.0 N04-D 1.2 292 202 86 1.0
N05-A-α 1.6 210.3 73.3 N05-A-β 2.5 14.0 67.0 N05-A-γ 6.0 1.0 N05-B 5.0 1.0 N05-C 2.3 307 217 83 7.0 3.0 N05-D 1.1 4.0 0.5 N05-E 10.5 104 14 67 7.0 8.0 N05-F 1.0 304 214 89 4.0 1.0 N05-G 2.0 5.0 1.0 N05-H 1.1 290 200 48 1.5 3.0
N06 1.1 113 23 71 6.5 3.5 23.0 71.0
N07-A 9.0 100 10 80 3.0 15.5 81.5 N07-B 3.9 111 21 83 3.0
N08-A 5.7 295 205 82 4.5 7.0 205.0 82.0 N08-B 130 40 76 4.5 40.0 76.0
N09-A 2.8 100 10 80 2.5 4.0 10.0 80.0 N09-B 3.0 305 215 80 2.5 4.0 217.5 82.5 N09-C 1.1 310 220 85 2.5 4.0
N10-A 2.0 125 35 76 3.5 2.5 30.0 76.0 N10-B 1.9 4.0 N10-C 3.1 115 25 76 4.0
N11-A 3.0 340 250 83 3.5 3.5 262.0 86.3 N11-B 2.4 4.0 3.0 N11-C 1.0 4.0 3.0 N11-D 1.0 4.0 3.0 N11-E 1.8 4.0 3.0 N11-F 8.6 359 269 88 4.0 3.0 N11-G 1.5 357 267 88 4.0 3.0
N12-A 1.0 64 334 82 6.5 334.0 82.0 N12-B 3.6 140 50 88 4.4 50.0 88.0
N13-A 1.4 96 6 70 2.0 32.0 78.0 N13-B 2.1 145 55 83 2.0 N13-C 2.4 125 35 81 1.5 N13-D 2.0 N14-A 3.4 134 44 77 2.5 28.0 76.7 N14-B 2.0 2.5 N14-C 6.0 2.0 N14-D 3.0 111 21 83 1.0 1.5 N14-E 1.9 109 19 70 1.0 1.5
N15-A 2.6 11.0 3.0 52.0 78.0 N15-B 3.5 142 52 78 11.0 3.0
Table 3: Full raw aplite data table, migrated from field forms to digital format.
An absent horizontal trace indicates a dike exposed on a vertical face, while an absent vertical trace indicates a dike exposed on a horizontal surface. Absent thickness indicates an inability to directly measure it due to safety. Part one. 29
Station # Thickness (in) Dip direction (az) Strike (az) Dip (⁰) Horizontal trace (m) Vertical trace (m) Avg. str. Avg. dip S01 0.9 333 243 85 2.0 243.0 85.0 S02-A 3.5 120 30 77 4.0 3.5 30.0 77.0 S02-B 2.9 7.0 4.0 S03 0.8 214 124 83 3.0 124.0 83.0 S04-A 2.0 175 85 68 1.0 90.0 72.0 S04-B 0.5 185 95 76 1.0
S05-A 1.3 123 33 81 1.0 0.5 36.5 84.0 S05-B 3.6 130 40 87 3.0 0.5 S05-C 2.0 3.0
S06-A 8.5 281 191 87 7.0 4.0 199.5 84.0 S06-B 2.0 6.0 2.0 S06-C 2.8 298 208 81 3.5 2.5
S07-A 0.9 295 205 83 1.5 205.0 83.0 S07-B 1.5 90 0 81 1.0 0.0 81.0
S08-A 8.8 134 44 82 4.0 2.0 44.0 82.0 S08-B 13.5 280 190 87 4.0 2.0 190.0 87.0 S08-C 7.5 4.0 2.0
S09-A 9.0 285 195 81 3.0 156.0 82.0 S09-B 2.0 70 340 86 3.0 4.0 348.5 87.0 S09-C 36.0 87 357 88 2.5 4.0 S09-D 12.0 207 117 83 1.0 4.0
S10-A-α 23.0 117 27 77 12.5 24.5 81.3 S10-A-β 27.0 131 41 81 12.5 356.0 82.0 S10-B 5.0 86 356 82 12.5 S10-C-α 15.0 114 24 85 12.5 S10-C-β 21.0 96 6 82 10.0
S11-A 12.0 118 28 87 4.0 11.0 31.3 82.8 S11-B 7.5 119 29 83 8.5 16.0 S11-C 9.5 137 47 84 8.5 16.0 S11-D 10.0 111 21 77 8.5 16.0
Table 4: Full raw aplite data table, migrated from field forms to digital format. An absent horizontal trace indicates a dike exposed on a vertical face, while an absent vertical trace indicates a dike exposed on a horizontal surface. Absent thickness indicates an inability to directly measure it due to safety. Part two.