UNLV Retrospective Theses & Dissertations
1-1-1989
Petrogenesis of Late Cenozoic alkalic basalt near the eastern boundary of the Basin-and-Range: Upper Grand Wash Trough, Arizona and Gold Butte, Nevada
Erin D Cole University of Nevada, Las Vegas
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Repository Citation Cole, Erin D, "Petrogenesis of Late Cenozoic alkalic basalt near the eastern boundary of the Basin-and- Range: Upper Grand Wash Trough, Arizona and Gold Butte, Nevada" (1989). UNLV Retrospective Theses & Dissertations. 45. http://dx.doi.org/10.25669/uilu-4lw7
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Petrogenesis of Late Cenozoic alkalic basalt near the eastern boundary of the Basin-and-Range: Upper Grand Wash Trough, Arizona and Gold Butte, Nevada
Cole, Erin D., M.S.
University of Nevada, Las Vegas, 1989
UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106
PETROGENESIS OF LATE CENOZOIC ALKALIC BASALT NEAR THE EASTERN BOUNDARY OF THE BASIN-AND-RANGE: UPPER GRAND WASH TROUGH, ARIZONA AND GOLD BUTTE, NEVADA
By
Erin D. Cole
A thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science
in
Geology
Department of Geoscience University of Nevada, Las Vegas August, 1989 The thesis of Erin D. Cole for the degree of Master of Science in Geoscience approved.
Chairperson, (Dr. E. I. Sihith
*... u / , Examining Committee Member, Dh E. M. Duebendorfer
M . I Examlning'Coymnittee Member, Dr. S. M. Rowland
Examining Committee Member, Dr. D. PypPyper-Smith Abstract
Geochemical, isotopic, paleomagnetic, and geochronological studies of Late
Miocene to Pliocene alkalic basalt in the upper Grand Wash Trough, Arizona and
at Gold Butte, Nevada may place constraints on the petrogenetic history of the
basalt and on the geometry of the crustal structures that may have served as
conduits. The upper Grand Wash Trough lies near the east-central boundary of
the Basin-and-Range Province in northwestern Arizona. Gold Butte lies 20 km
west of the upper Grand Wash Trough.
Basalt of the upper Grand Wash Trough is divided into three magma types.
Type A basalt (9.15-9.47 Ma, Ce/Yb = 10-25, epsilon Nd = -1.14, and epsilon Sr
= 7.6) occurs near Gold Butte, Nevada. Type B basalt (<4.7 Ma; Ce/Yb = 10-
20, epsilon Nd = - 6.95 and epsilon Sr = 1.46) occurs primarily in the central- upper Grand Wash Trough at Pakoon Springs and East St. Thomas Gap. Type C basalt flows and dikes (<4 Ma, Ce/Yb = 20-70, epsilon Nd = 3.49 and 6.75 and epsilon Sr = -4.57 and -17) occur primarily in the northern upper Grand Wash
Trough at Mud Mountain.
Geochemical modelling suggests that type B basalt can be derived from independent batch melts (5 to 10%) of spinel peridotite with limited fractionation
(5 to 9% olivine plus Fe-Ti oxide). Similarly, geochemical modelling suggests that Gold Butte lavas can be derived from 2 to 3.5% batch melting of spinel peridotite with small amounts of fractionation (3-5% olivine and Fe-Ti oxide). . The Sr, Nd and Pb isotopic data confirm the geochemical models and
indicate that all three magma types are mantle derived and underwent little
interaction with crustal material. Isotopic compositions of upper Grand Wash
Trough and Gold Butte basalt are more similar to coeval alkalic basalt of the
western Colorado Plateau and are unlike coeval basalt of the central Basin-and-
Range Province.
The restricted range in geochemical diversity, the lack of extensive
fractionation suggested by geochemical modelling, combined with a similarity of
paleomagnetic poles and a lack of paleosols or uncomformities in the flow stacks
of type B basalt flows, suggests that the eruptive episode for type B lavas was
short lived. The rapid ascent of magma through the crust requires high-angle
zones of weakness to serve as conduits for magma transport.
Gold Butte flows (type A) overlie the trace of the Gold Butte fault, a
major N-NE striking strike-slip fault. Because paleomagnetic poles of the lava
flows display little dispersion, and the flow stack is conformable, it is suggested
that magma ascended through the crust rapidly. The rapid ascent of magma suggests that the Gold Butte fault may be a crustal penetrating structure that
allowed magma to escape to the surface, contrary to some existing models on the
role of strike-slip faulting in extensional terrains. Table of Contents
A bstract...... iii
List of Figures ...... vii
List of Tables ...... viii
Acknowledgments ...... ix
Introduction ...... 1 P u rp o se ...... 1 Location and Previous Work ...... 1 Regional Late Cenozoic Basaltic Volcanism ...... 4 Structural Setting ...... 8 Description of Upper Grand Wash Trough and Gold Butte B asalt ...... 10
Geochemical Constraints on Crustal Structure ...... 15
Methods ...... 18
Petrography ...... 20 Lava Flows ...... 20 Dikes and Plugs ...... 21
Geochemistry...... 23 Major and Trace Element Variation ...... 27 Isotopic D ata ...... 33 Magma Types of Gold Butte and upper Grand Wash Trough Basalt ...... 38 Geochemical Models ...... 41 Magma Type A ...... 41 Magma Type B and C ...... 44
Paleomagnetometry ...... 46
Structural and Regional Implications of Upper Grand Wash Trough and Gold Butte Basalt ...... 52 Implications of Upper Grand Wash Trough Basalt ...... 52 Structural Implications of Gold Butte Basalt ...... 53
v Regional Implications of Gold Butte Basalt ...... 54
Sum m ary ...... 55
References C ited ...... 57
Appendix 1 ...... 61
Appendix 2 ...... 68
vi List of Figures
Figure Page
1 Location M ap ...... 2
2 Geologic Map of Gold Butte and Upper Grand Wash Trough areas...... 5
3 Photograph of the Upper Grand Wash Trough ...... 11
4 Air-photo of Gold Butte basalt ...... 13
5 Geologic Map of the Mosby Peak Cinder Cone ...... 14
6 Map of the Western Colorado Plateau Region ...... 17
7 Microphotograph of an Olivine Phenocryst ...... 22
8a Major Element Variation Diagrams ...... 28
8b Trace Element Variation Diagrams ...... 31
8c Ce/Yb vs. Cr Diagram ...... 32
9a Epsilon Nd vs. ^Sr/^Sr Diagram ...... 35
9b 208?b /204?b and 207Vb/204Vb vs 206Pb/204Pb D iagram ...... 36
9c Trace Element Ratio Diagrams ...... 37
10 Spider Diagram ...... 43
11 Ce vs. Cr and Ce vs. Co Evolution Diagrams ...... 45
12 Stereoplot of Paleomagnetic D ata ...... 48
13 Polarity Time Scale ...... 50
14 Photograph of Gold Butte lava flows ...... 51
vii List of Tables
Table Page
1 K-Ar dates for upper Grand Wash Trough and Gold Butte basalt ...... 3
2 Errors for Trace Element Analyses ...... 19
3 Major and Trace Element Analyses ...... 24
4 Isotope Geochemistry ...... 34
5 Criteria for Magma Type Classification ...... 40
6 Elemental Abundance for Mantle Source and Trace Element Distribution Coefficients ...... 42
7 Summary of Paleomagnetic D ata ...... 47
viii Acknowledgments
I would like to acknowledge the Nevada Nuclear Waste Project Office for
funding. I would also like to thank my advisor, Dr. E. I. Smith for his patience,
encouragement, and beneficial discussions throughout the project. Thanks also go
to the Center for Volcanic and Tectonics Studies.
I am grateful to Dr. Doug Walker at the University of Kansas for
performing isotopic analyses, and to Dr. John Geissman at the University of New
Mexico for his help and the use of the paleomagnetic laboratory.
I would also like to acknowledge the other members of my committee, Dr.
S.M. Rowland, Dr. E.M. Duebendorfer, and Dr. D. Pyper-Smith for reading the
manuscript and providing helpful recommendations. Thanks also to Dr. Clay
Crow, Visiting Assistant Professor, for his review on the manuscript.
I would also like to express my gratitude to my friends and colleagues in the Geoscience Department, especially fellow Graduate Students, Lance Larson and Cheryl Canaan and Research Associates Dan Feuerbach and Terry Naumann.
Special thanks go to my husband, George P. Biberstein, for the encouragement and patience he has given me throughout my education. INTRODUCTION
Purpose
Geochemical studies of volcanism in extensional terrains may place
constraints on crustal structure. The purpose of this study is to apply
geochemical, paleomagnetic, isotopic, and field data to determine the petrogenetic
history of the upper Grand Wash Trough and Gold Butte basalts and to place
constraints on the geometry of the structures that may control their emplacement.
Location and Previous Work
Late Cenozoic (3.99 to 6.9 Ma) alkalic basalt occurs in the upper part of the Grand Wash Trough near the eastern boundary of the Basin and Range in the northwestern corner of Arizona (Figure 1). The Grand Wash Trough is a north- trending structural depression that lies between the Grand Wash Cliffs on the west and the Virgin Mountains, White Hills and Cerbat Mountains on the east.
The upper Grand Wash Trough is confined to that portion of the trough north of
Lake Mead. Alkali basalt also crops out near Gold Butte, southeastern Nevada, but the Gold Butte basalt (9.4-9.1 Ma) is more than 2 Ma older than the oldest flows of the upper Grand Wash Trough (6.9 Ma; Table 1).
Moore (1968) noted the location of a number of dikes and described a * / Arizona Basin and 5 _ / Range /S > • 3 /Colorado z
Callville Mesa Lss Vegas ♦
Figure 1. Location map of the Lake Mead area. Inset shows upper Grand Wash Trough (boxed area), Gold Butte (+ ) and Death Valley- Pancake Range (stippled box).
2 Table 1: K-Ar age of Upper Grand Wash Trough and Gold Butte Basalt Sample # Location %K Radiogenic K-Ar age (Lat. and Long.) Ar (pm/g) +/- error G8-2-2-LN Dike in Quail Springs Wash 1.713 17.00 5.72 Ma 36 16’18" N +/- 0.33 114 16'47" W
G8-3-40-LN Lowest Gold Butte Flow 1.176 18.7 9.15 Ma 36 15’56" N +/- 0.13 114 15'25* W
G8-3-43-LN Upper Gold Butte Flow 1.277 20.98 9.46 Ma 36 15'56" N + /-0.12 114 15’25" W
G9-7-78-LN Lowest Mud. Mt. Flow 0.873 6.1 4.02 Ma 36 40’ N +/- 0.08 114 4 7 'W
G9-5-79-LN Upper Pakoon Springs Flow 1.25 10.194 4.7 Ma 36 24’ N +/- 0.07 113 56’ W
G9-8-80-LN Upper Grand Wash Bay Flow 0.812 5.626 3.99 Ma 36 16’ N +/- 0.06 113 59’ W
PED-35-66* * Upper Cottonwood Wash Flow 1 9 4.73 Ma 36 37.2’ N +/- 0.18 113 53.38'W
PED-34-66* Lower Cottonwood Wash Flow 0.6 57 6.87 Ma 36 37.62’ N +/- 0.20 113 53.36’W * Best et al. (1980) - Whole Rock * * Damon unpublished data - Reported by Reynolds et al. (1986) all others based on plagioclase concentrate techniques
3 4
volcanic center near Mosby Peak (Figure 2). Best and Brimhall (1974) included a
few samples from the northernmost basalt flows in the upper Grand Wash Trough
in a regional study of the petrogenesis of Late Cenozoic basalt flows in the
western Colorado Plateau (Figure 1). They classified the samples as hawaiites
and suggested that the lavas were derived from partial melts of mantle peridotite
at depths of less than 65 km that were modified to varying degrees by polybaric
fractionation during ascent. Leeman (1974) suggested an oceanic-type mantle
origin for basalt of the northern part of the upper Grand Wash Trough on the
basis of Sr isotopic compositions (87St/ 86St=0.7028 to 0.7041). Reynolds et al.
(1986) reported two K-Ar dates (6.9 and 4.73 Ma) based on whole rock analyses
of basalt from the upper Grand Wash Trough (Table 1).
Morgan (1968) demonstrated that two high-angle faults cut exposures of
Gold Butte basalt, and suggested that basalt is not offset by the Gold Butte fault,
a major northeast-striking strike-slip fault.
Regional Late Cenozoic Basaltic Volcanism
Late Cenozoic basalt occurs in a broad region in southwestern Utah,
northwestern Arizona, and southern Nevada. The closest Pliocene to Holocene
basalt fields to the upper Grand Wash Trough lie to the east in the western
Colorado Plateau, to the southwest near Fortification Hill, and to the west in the
Death Valley-Pancake Range belt (Figure 1).
Pliocene to Holocene lava fields of the western Colorado Plateau contain abundant basalt cinder cones and consist of a variety of rock types including alkali Explanation to accompany Figure 2
OT Quaternary and Late Tertiary Sediments
Tertiary Basalt r _0 Early Tertiary (Miocene and earlier) Sediments T Mesozoic Strata t f Z
Paleozoic Strata s Precambrian Rocks / N Basalt Dikes
High-angle Normal Fault (bail and bar on X down-thrown side)
J Low-angle Normal Fault (teeth on down- thrown side)
Strike-slip Fault
J Thrust Fault (teeth on upper plate) KILOMETERS
South Virgin Mount N
Figure 2. Generalized geologic map of the upper Grand Wash Trough and Gold Butte areas. Locations: GB - Gold Butte; GWB - Grand Wash Bay; ESTG - East St. Thomas Gap; PS - Pakoon Springs; MM - Mud Mountain; MPC - Mosby Peak Cone; PC - Pocum Center; CW - Cottonwood Wash. Modified from Moore (1972), Morgan (1968), Hamblin (1970), and Wernicke and Axen (1988).
6 7 olivine basalt, hawaiite, and basanite (Best and Brimhall, 1974). Chemical variations may reflect varying degrees of partial melting of the mantle over a range of pressures combined with fractional crystallization during ascent to the surface (Best and Brimhall, 1974). Alkali basalt of the western Colorado Plateau exhibits low ^ S r/^ S r (0.70324-0.70448), unradiogenic Pb isotope compositions
(206Pb/204Pb = 17.85-18.77; 207Pb/204Pb = 15.49-15.6; 208Pb/204Pb = 37.33-
38.62), and moderate 143Nd/144Nd (0.5125-0.51275) (Alibert et al., 1986). On the basis of isotopic compositions of Quaternary to Oligocene (30 Ma) volcanic rocks from the western Colorado Plateau, Alibert et al. (1986) suggested that the mantle source for alkalic basalt of this region must include both asthenospheric mantle, characterized by high 143N d /144Nd (0.51290), high 206Pb/204Pb (19.5), and low ^ S r/^ S r (0.70344) and a component with low Sm/Nd, tentatively identified as "recycled abyssal/hydrothermal sediments."
Southwest of the upper Grand Wash Trough, in the Lake Mead area, alkalic basalt cinder cones and lavas (4.7 to 5.9 Ma) comprise the Fortification
Hill field (Feuerbach et al, in prep.). Feuerbach et al. (in prep.) suggested that basalt of the Fortification Hill field was generated by independent batch melts of mantle peridotite followed by olivine and Fe-Ti oxide fractional crystallization.
Initial ^ S r/^ S r for a basalt (4.7 Ma) of the Fortification Hill field is 0.7048. An alkali basalt dike south of Hoover Dam on U.S. 93 in Arizona is characterized by low initial 87S t /86S t (0.7035) and moderate ^ N d / ^ N d (0.5127) (Alibert et al.,
1986). 8 Late Miocene to Quaternary alkali basalt (0.04 to 10.2 Ma) comprises the
Death Valley-Pancake Range volcanic field near the south-central part of the
Basin-and-Range Province in southern Nevada and California (Figure 1). The southern Death Valley-Pancake Range basalt is dominantly hawaiite and is characterized by low 143m / 144m (0.511303-0.511772), high S7Sr/56Sr (0.70671-
0.70747) and variable Pb isotopic compositions (206Pb/2 207Pb/204Pb = 15.556-15.598; 208Pb/204Pb = 38.205-38.995) (Farmer et al., 1989). The high87Sr/86Sr ratios are not attributed to crustal contamination because the ratios are remarkably homogeneous. Instead, Farmer et al. (1989) attributed isotopic compositions of southern Nevada basalt to derivation from lithospheric mantle. Basalt from the northern Death Valley-Pancake Range is characterized by higher 143Nd /144m (0.512351-0.512060), lower ^ S r/^ S r (070294-0.7047), and higher206Pb/204Pb (> 18.9) than those of the southern Death Valley-Pancake Range and are interpreted to have been derived from upwelling asthenospheric mantle (Farmer et al., 1989). Structural Setting Lucchitta (1979) suggested that the Grand Wash Trough is an asymmetrical half-graben that formed during mid- to late-Miocene time by down-to-the-west movement associated with the Grand Wash fault. This fault lies on the eastern edge of the Grand Wash Trough and parallels the lower Grand Wash Cliffs escarpment for approximately 170 km. It forms the west-central boundary of the Colorado Plateau (Anderson and Mehnert, 1979). The upper Grand Wash Trough is bound on the west by the Virgin Mountains, a broad northeast-trending structural dome (Moore, 1972). Wernicke (1985) proposed that the breakaway zone of a mid- to late-Miocene detachment fault system lies on the western margin of the Virgin Mountains (Figure 2). This detachment zone may represent the easternmost boundary of extensional allochthons in the central Basin-and-Range Province. Wernicke (1985) and Wernicke and Axen (1988) suggested that the Virgin Mountains were uplifted, folded and faulted in response to tectonic denudation related to regional extension and detachment faulting to the southwest of the Virgin Mountains. A number of north-striking high-angle normal faults occur in the Virgin Mountains and in the upper Grand Wash Trough (Figure 2). Moore (1968) suggested that the displacement related to some of these faults may be as great as 4 km. Structures in the South Virgin Mountains and near Gold Butte include east-northeast striking strike-slip faults, low-angle normal faults and abundant north-striking high-angle normal faults (Morgan, 1968). The east-northeast- striking Gold Butte fault juxtaposes Precambrian metamorphic rocks and steeply dipping Paleozoic and Mesozoic strata north of the fault against Precambrian metamorphic and granitic rocks south of the fault (Longwell et al., 1965). The Gold Butte fault, active from 12-10 Ma (Morgan, 1968; Bohannon, 1984), appears to have offset Paleozoic strata nearly 7 km in a left-lateral sense (Morgan, 1968; Anderson, 1973). However, westward rotation of the strike of the Paleozoic strata north of the Gold Butte fault, near the Gold Butte basalt is more consistent with right-lateral movement. Other workers have suggested that the Gold Butte fault 10 may be a right-lateral strike-slip fault (J. Fryxell, personal comm. 1989; Eschner, 1989). Description of upper Grand Wash Trough and Gold Butte Basalt Individual basalt flows in the upper Grand Wash Trough and at Gold Butte range from 1 to 3 m thick and form flat, nearly featureless mesas (Figure 3). Locally, the flows reach up to 5 m thick where basalt occupies paleochannels cut in the underlying sandstones and conglomerates informally named the "red sandstone unit" by Bohannon (1984). Where slumping has occurred on the edge of the mesas, the slumped flows are tilted up to 30 degrees towards the mesa. Flows at Mud Mountain (Figure 2) are divided into upper and lower units by 5 to 10 m of intervening sandstones and conglomerate. The lowest flow is dated at 4.02 Ma (Table 1). Near Grand Wash Bay (Figure 2), flows are similarly divided. About 5 m of intervening red sandstone and conglomerate separate the lower from the upper flow. The upper flow was dated at 3.99 Ma (Table 1). The lava flows at Pakoon Springs and East St. Thomas Gap (Figure 2) lack intervening sediments, erosional unconformities or paleosols. The uppermost flow at Pakoon Springs was dated at 4.7 Ma (Table 1). Basalt dikes are located at Mud Mountain, Mosby Peak, Pocum Center, and Cottonwood Wash. Dikes at Mud Mountain strike N5W to N55W and range in length from 0.6 to 2.5 km (Figure 2). Dikes at Mosby Peak radiate from the center of a cinder cone. Dikes at Pocum Center strike N-S and E-W. The dike 11 f U \ tori',* ’f 11 * . 'j *■/ > o « *1 12 in Cottonwood Wash, 4 km east of the Mosby Peak volcano, is sinuous but generally strikes N35W and is 0.8 km in length. A breccia plug nearly 25 m high and 150 m across is located 1 km southeast of Pakoon Springs (Figure 2). Gold Butte flows crop out 20 km west of the upper Grand Wash Trough on the west side of the South Virgin Mountains (Figure 2). The thickest section of Gold Butte basalt is composed of seven flows with a total thickness of 25 m. The uppermost flow was dated at 9.46 Ma and the lowest flow at 9.15 Ma (Table 1). A basalt dike was dated at 5.72 Ma. The Gold Butte flows cover the trace of the Gold Butte fault. However, the northeasternmost extent of Gold Butte basalt appears to be cut by a northeast-trending structure that is parallel in map view with the Gold Butte fault (Figure 4). If the Gold Butte basalt is cut by a strand of the Gold Butte fault, then the Gold Butte fault may have been active as late as about 9.15 Ma. The upper Grand Wash Trough and Gold Butte volcanic field contains few cinder cones in contrast to the Late Cenozoic basalt volcanic fields on the Colorado Plateau (Best and Brimhall, 1974) and near Lake Mead (Feuerbach et al., in prep.). Only two vent areas were identified, and both of these occur in the north-central part of the upper Grand Wash Trough near its margins. The Mosby Peak cinder cone is composed of agglutinated and poorly sorted scoria and ash, bombs, flows and radiating dikes (Figure 5). The cinder cone was probably localized by a high-angle, west-dipping, north-striking normal fault (Moore, 1972). The Pocum center (Figure 2) is composed of cinder, scoria, and bombs associated with a number of north-south and east-west striking basalt dikes. 13 -3 JJ "3 ^ 0 V3 <+* V' Q *-• «y m « iz X> U *■* 4> JZ— *-*3 m 60 CQ 1 2 12 >» g-° E ^ 3 « 5 SS 31) <^N 0) 603 E Quaternary 0 and Late Tertiary Sediments Tb Coarse Ash Agglutinated scoria Tertiary QT basalt dikes Tb Tertiary basalt flows Mosby Peak Tb 350 QT m eters Figure 5. Geologic map of the Mosby Peak cinder cone and associated flows. 14 15 No cinder cones were identified in the southern half of the upper Grand Wash Trough or at Gold Butte, but field evidence suggests that at least some vents must be present. For example, the lower flow near Grand Wash Bay contains distinctive plagioclase phenocrysts and was traced north from Grand Wash Bay for a distance of only 4 km. The nearest cinder cones are nearly 20 km to the north of the northernmost exposure of this unit, thus suggesting a nearby center for this flow. GEOCHEMICAL CONSTRAINTS ON CRUSTAL STRUCTURE Below, I present two examples of studies that use geochemistry and related techniques to place constraints on crustal structure or the residence time of magma in the crust. In the western Lake Mead area, Pliocene alkali basalt of the Fortification Hill volcanic field erupted following major extension; intermediate volcanism was coeval with active and waning extension (Smith et al., 1989; Feuerbach et al., in prep). Feuerbach et al. (in prep.) suggest that regional extension and active faulting may result in ponding of basaltic magma in crustal chambers beneath major low-angle faults and shear zones where fractionation and crustal contamination may occur. Following extension, high-angle faults may penetrate through a cooler, more brittle crust (Damon, 1971) and perhaps a more dense crust (Glazner and Ussier, 1989), enabling alkali basalt magma to pass through the crust without significant crustal contamination or differentiation. 16 East of the upper Grand Wash Trough, in the western Colorado Plateau region of northwestern Arizona and southwestern Utah, Pliocene to Holocene basalt centers are associated with major north-south striking high-angle normal faults (Hamblin, 1970 Figure 6). Best and Brimhall (1974) suggested that the chemical variation within western Colorado Plateau basalt may have resulted from varying degrees of partial melting of mantle peridotite with subsequent polybaric fractionation. Based on the petrogenetic evidence (Best and Brimhall, 1974) and on the abundance of eclogite and peridotite xenoliths, Best (1970) suggested that the basanite and alkali basalt magma travelled through the crust quickly. Leeman (1974) concurred with Best and Brimhall (1974) and concluded that the Sr isotopic composition of basalt of the western Colorado Plateau (87Sr/86Sr=0.7028-0.7041) was indicative of an oceanic-type mantle source. Anderson (1988) proposed that volcanic centers in a northeast-trending belt extending from St. George to Panguitch in southwestern Utah are aligned along structures that may extend through the crust. The structures may have served as conduits for magma escape. This study uses geochemical, palemagnetic, isotopic and field data to determine the petrogenetic history of the upper Grand Wash Trough and Gold Butte basalt, and in a manner similar to the examples above, places constraints on the geometry of structures that may have controlled their emplacement. Nevada Arizona ► * > ► Gold Butte ** Normal Faults, dashed where inferred N Cinder Cones ''O' ^ Strike-slip Faults, dashed where inferred 0 30 km Figure 6. Generalized map of the western Colorado Plateau region showing major normal faults and cinder cones (modified from Hamblin, 1970). Also shown are the upper Grand Wash Trough and Gold Butte areas. 17 18 METHODS Four stratigraphic sections of upper Grand Wash Trough basalt, one section of Gold Butte flows, and dikes and plugs were sampled for chemical, petrographic, and paleomagnetic analyses. Whole-rock major-element analyses were done by Inductively Coupled Plasma techniques (ICP) at Chemex Labs, Inc., Sparks, Nevada. Trace element concentrations were determined by Instrumental Neutron Activation Analysis (INAA) at the Phoenix Memorial Laboratory, University of Michigan. Errors for trace elements are listed in Table 2. Six samples were analyzed for isotopes of Sr, Nd, and Pb by thermal ionization mass spectrometry at the University of Kansas. Six samples were collected for K-Ar dating. Dates were performed by Paul Damon and M. Shafiquillah using plagioclase concentrate techniques at the Laboratory of Isotope Geochemistry at the University of Arizona, Tucson. Twenty-five orientated hand samples were collected from basalt flows in the field and cored for paleomagnetic analyses. The remanent magnetic direction held by the samples during alternating field demagnetization (AF up to 140 mT) was determined by using a superconducting rock magnetometer at the paleomagnetic laboratory at the University of New Mexico, Albuquerque. The process of AF demagnetization consists of placing the cored sample into a coil of wire carrying a current that creates an alternating magnetic field. The alternating field is raised to a peak value and lowered to zero. The remaining magnetism Table 2: Errors for Trace Element ______Analyses ______% % % ______Avg. Min Max Cr 1.63 1.02 2.64 Co 1.25 0.90 3.79 Sc 0.37 0.30 0.50 V 3.04 2.24 3.76 Hf 8.43 4.40 13.60 Th 9.86 4.39 14.30 Ba 13.63 9.29 16.40 Ni 14.62 12.80 18.70 Ta 9.16 5.59 15.00 Zn 7.82 5.18 14.57 La 1.31 0.80 2.44 Ce 4.26 1.31 9.55 Sm 0.67 0.42 1.38 Eu 5.12 0.47 7.60 Yb 7.51 2.14 11.80 Lu 9.05 6.61 13.80 19 20 held by the sample is then measured. This process was repeated 15 to 20 times from 2 to 140 mT in a laboratory isolated from the Earth’s ambient magnetic field. Structural corrections were applied to samples collected from slumped flows. PETROGRAPHY A summary of petrographic data is presented below. Detailed petrographic descriptions and modal percentages determined by 500 point counts per thin section are given in Appendix 1. Lava Flows The Gold Butte basalt flows contain 9 to 15% olivine phenocrysts containing Fe-Ti oxide inclusions in an intergranular to pilotaxitic matrix of plagioclase, olivine, clinopyroxene and Fe-Ti oxide. Olivine phenocrysts (1-4 mm) are embayed and coated with rinds of iddingsite. Two samples contain trace amounts of blocky, anhedral plagioclase (1-3 mm). Basalt flows at East St. Thomas Gap, Pakoon Springs and the lower flow near Grand Wash Bay contain olivine phenocrysts (4-10%) with Fe-Ti oxide inclusions, and have a subophitic to intergranular matrix composed of plagioclase, clinopyroxene, olivine and Fe-Ti oxide. A trace amount of blocky, anhedral plagioclase was observed in several samples from East St. Thomas Gap. The upper flow near Grand Wash Bay contains plagioclase (30%) and olivine (10%) 21 phenocrysts in a fine-grained matrix of plagioclase, olivine, clinopyroxene, and Fe- Ti barbs. Olivine phenocrysts are almost entirely altered to iddingsite. Lower flows at Mud Mountain are mesocrystalline and contain plagioclase laths 1 to 7 mm long (60%-70%) in a matrix of olivine (10%), clinopyroxene (15%), and Fe-Ti oxide (3-5%). Upper flows contain 2-3% olivine phenocrysts with rare inclusions of Fe-Ti oxide in a matrix composed of blocky plagioclase, clinopyroxene, iddingsitized olivine and barbs of Fe-Ti oxide. Olivine phenocrysts have a thin jacket ( < 0.1 mm) of fresh olivine around an inner rim of iddingsite (Figure 7). This texture suggests that the iddingsite formed as a magmatic phase. A sample from the lower flows at Mud Mountain contains a rounded quartz crystal (5 mm) with a reaction rim of clinopyroxene and glass, and a grain of poikilitic clinopyroxene (1 mm). Flows at the Mosby Peak volcano are microporphyritic and contain phenocrysts of olivine (20%) with inclusions of Fe-Ti oxide, and zoned clinopyroxene (5%). Olivine phenocrysts are generally less than 1 mm in size and show only minor alteration to iddingsite. The matrix is intergranular to felty with microlites of plagioclase, clinopyroxene, olivine and Fe-Ti oxide. Poikilitic clinopyroxene (0.5 mm in size) was observed in one flow. Dikes and Plugs The dike at Gold Butte contains phenocrysts of plagioclase (9.4%), clinopyroxene (9.2%), and olivine (5%) in a pilotaxitic matrix of plagioclase, clinopyroxene and Fe-Ti oxide. The breccia plug near Pakoon Springs contains Figure 7. Microphotograph of an olivine phenocryst (2 mm in length) in sample 7-34, upper flow at Mud Mountain. Note rind of iddingsite (A, reddish-brown) and surrounding fresh jacket of olivine (B). 22 23 olivine (8%) and plagioclase phenocrysts (8%) in an aphanitic matrix. Dikes atMosby Peak contain phenocrysts of olivine (15%) with Fe-Ti oxide inclusions, and clinopyroxene (5%). Olivine phenocrysts are generally less than 1 mm in size and commonly display thin (<0.1 mm) discontinuous zones of iddingsite, that are surrounded by thin (<0.1 mm) jackets of olivine. The pilotaxitic groundmass is composed of microlites of plagioclase (60%), olivine (6%), clinopyroxene (2%) and abundant Fe-Ti oxide (15%). The west-northwest-trending olivine basalt dike in Cottonwood Wash contains phenocrysts of subhedral olivine (13%) with inclusions of Fe-Ti oxide, and subhedral clinopyroxene (3%). The pilotaxitic matrix is composed of plagioclase microlites, olivine, rare clinopyroxene and Fe-Ti oxide. Two dikes near Mud Mountain (Samples 39 and 76) contain 5 to 7% olivine phenocrysts that are altered to iddingsite. Iddingsite zones are commonly coated by a jacket of olivine. The pilotaxitic matrix is composed of both lath shaped and blocky plagioclase with clinopyroxene, olivine and Fe-Ti oxides. A third dike near Mud Mountain (Sample 77) is highly vesicular (20%) and contains 6% olivine phenocrysts (0.5 to 2 mm) with rare Fe-Ti oxide inclusions in a closely spaced to felty matrix of plagioclase microlites with olivine and Fe-Ti oxides. 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Ce/Yb varies from 10 to 25 over a range of Cr contents from 224-280 ppm (Figure 8c). Upper Grand Wash Trough basalt from Pakoon Springs and East St. Thomas Gap range in Si02 content from 44 to 49 wt %. Major elements and transition elements display relatively flat slopes when plotted against Mg# (Figure 8a and b). Ce/Yb ratios range from 10 to 22 as Cr content ranges from 204 to 337 ppm (Figure 8c). Basalt flows and dikes from Mud Mountain are generally higher in Ti02, Na20 and total rare earth element (REE) content than those from Pakoon Springs and East St. Thomas Gap. Ti02 and Na20 decrease with increasing Mg# (Figure 8a). A12 Oj , FeO and MgO and transition elements are generally less abundant in Mud Mountain and Mosby Peak basalt than those from Pakoon Springs and East St. Thomas Gap (Figure 8a and 8b). Ce/Yb ranges from 20 to 68 for the flows and dikes at Mud Mountain as Cr varies from 130 to 459 ppm (Figure 8c). 50 g 0 44- 40 0.45 0.50 0.55 0.60 0.65 0.70 1 2 - O 0 8" 4 - 0.45 0.550.50 0.60 0.65 0.70 20 1 5 - CNO 5 10 " 5 - .0.50.50 0.55 0.60 0.65 0.700.45 Mg# Figure 8a. Major elements plotted against degree of evolution as measured by Mg#. Symbols: circle - Gold Butte; diamond - Pakoon Springs and East St. Thomas Gap; triangle - Mud Mountain; square > Grand Wash Bay; inverse triangle - Mosby Peak and Cottonwood Wash dikes. 28 o □ o o 8 s g oo u 6-. 4- 2 ■ 0.45 0.550.50 0.60 0.65 0.70 3 ■ 2 - 0.45 0.50 0.55 0.60 0.65 0.70 5 - 4-- 3 - 2 - 0.45 0.50 0.55 0.60 0.65 0.70 Mg# Figure 8a. Continued. 29 iue8. Continued. 8a. Figure MnO P205 Ti02 0.00 0 0.40- - 0 .8 0 0.3 0.0 0.1 0 . 0 2 00 . 0 .5 .0 .5 .0 .5 0.70 0.65 0.60 0.55 0.50 0.45 2- - 3 2 ■■\ 0.45 4 .50.50 0.45 -• -• - .00.55 0.50 * □ A * A A .50.60 0.55 ^ Mg# 30 a ••S v 8 0.60 «8> v A 0.65 .50.70 0.65 0.70 500 ------1 o oo 3 0 Figure Figure 8c. Ce/Yb plotted against Cr (ppm). Symbols as in Figure 8a. q V ® o 33 Isotope Geochemistry Upper Grand Wash Trough and Gold Butte basalt is characterized by variable and relatively unradiogenic Sr and Pb compositions and moderate to high ^ N d /^ N d (Table 4). Most of the basalt from the upper Grand Wash Trough and Gold Butte plot within the mantle array on the epsilon Nd vs ^Sr/^Sr diagram (Figure 9a). Pb isotopic compositions of upper Grand Wash Trough and Gold Butte basalt are unradiogenic (Table 4) and plot in a narrow range above the "northern hemisphere reference line" (Figure 9b). The northern hemisphere reference line is a best fit Pb isochron established for basalt from mid-ocean ridges and oceanic islands including Hawaii, Iceland, Azores, Canaries and Cape Verde (Hart, 1984). Sr, Nd and Pb isotopic compositions suggest that the Grand Wash and Gold Butte basalt were generated within the mantle and underwent little crustal contamination. Minor and trace element ratios are commonly used to identify the source rock and to properly interpret the isotopic history of basalt (Hawkesworth et al., 1987). All the samples from Gold Butte and the upper Grand Wash Trough have trace element characteristics of oceanic and uncontaminated continental basalt and do not plot within trends established for subduction related magmatism or fractional crystallization within the crust (Figure 9c). Sr, Nd.and Pb isotopic compositions of upper Grand Wash Trough and Gold Butte basalt are quite different from isotopic compositions reported by Table 4: Isotope Geochemistry Sample Number G8-3-6-LN G8-4-13-LN G8-5-14-LN G8-6-39-LN G8-7-33-LN G9-8-62-LN Error O ° CO O 2 CD g Lli CO . 0 s T3 £ Q2 2 1 s o T3 t 5 I | ■o * S' o e re a w mre 5 i § E •§ fi o> w e 8 (0 3 Q O 3 3 « . 5 Q. o OC CO W CO CO iS* O -C CM CO t- COK* O co ° d § ; " ni CMCM CM S CO S S CM CM © w§o ^ w § o ? * T* LQ O T*O) COLO CO T* CO S # # » S n r m l f n ” g S g S II f I 1E § P n P m CM ^ S O 3CO 3 O (Oco T* CO CM CO O a - • o o ^ o o • • CO o m - o in to O) N CO N O) d CO CO O © CMC CO OCO C OC M CNI C ° pC o r- o O g o CO O O CO CO CMCM h* t cm O) - co CO i ) OS t CM CO CM to U) S O O CM 0> CO ^ ° 55 ^ ?! id CM d O) CM O)U) U) CO 0 O LU ^ ^ CO Q. T* T* » C Z s 1; 3 n „ £ * - 5 ^ 5 * - £ CO N it N £2 * & 00r*10 00 CO « ^ 3 £ in fs. co $ £ T* S S z co _ z S S M- CO O) CO e§ 5 I Q. g ^ E E ^ g Q. 3 o o o ® o c in o in LO ° n r r in 1 CM - « CO 0 $2 o o> CO O) 0 CO h- N. T* 1 N. - s « « « CO dKin K cd « z T3 « >I*LO Is* o> T* m c O 1 O) O O S O CM CO CM o> > o^ m co ^ o> 'tO RN ”SS§ T" m a> N. CO r CO O r C in m Oo ▼-uj▼- C O 3 ^ co in 2 9 s Tm LO O) co CO CMS CM 0. CM 0. O 0. 5 O O CMs W £ ^ 3 £ 3 £ 3 - CO r CO r t- + m in m 0 LO 0> CM CO N, r* in IO 1 1 c i co in co- co O § *+ CO CO CO CO >CO a> m h- S CO cd 0 co 1 t 3 2 8 m 5 !? s II 3 0. s f CM o> CO 3 E o E . § . . + Q.Q. CM - (D n 0 h- m K 3 . : •a >> •a re i ^ ■i if o re oo <3"8 ^ ® o S 5 H 1w- 10 ©£© g Ec O- o 25 re J- E O £ 3 -O o p* O O c i2 'g 2 5 C © O c v> (If > 34 35 CT> O VO Q. 41 JS r» OV 3 ■* !3 s u 2 r®°OSr S(/i 'c - r i 2J « '23 c e JS ~ . s 3 —k* vftl —• O e-j—! cah" s 0 f l £ o O 2 S h.2 • s *.s O I s > w S' CM W >^i3 al M C .S 03 o OO 3 ov 2 I PN uojjsda IZ Figure 9b. J0sPb/2wPb and and J0sPb/2wPb 9b. Figure 208Pb/204Pb 207Pb/204Pb 36.5 38 15.3 39.5 15.4 15 15.7 . . 5 6 17 -• 17 - 207?b/204Pb - 18) Smos s nFgr 9a. Figure in as Symbols (1984). central and southern for Fields basalt. Butte Gold and Trough Colorado Plateau basalts enclosed in parallel dashed lines in the the in lines dashed parallel in (western enclosed Plateau basalts Colorado Plateau western Colorado the from and basalts Nevada plot) from Farmer et al., 1989. NHRL from Hart Hart from 1989.NHRL al., et Farmer from plot) —L- n a d a v e N 207Vb/204Pb 206Pb/204Pb Southern 18 18 Plateau Western Colorado Western 36 Western Colorado Western Plateau vs. 2®Pb/2MPb for upper Grand Wash Wash Grand upper for 2®Pb/2MPb vs. 19 19 Nevada Central 20 20 Figure 9c. Trace element criteria used to evaluate source characteristics for for characteristics source evaluate to used criteria element Trace 9c. Figure Ti/Yb Sr/Yb 1E— 1 1000-- 1000 100 100 1E4-- 1E5 10 ------1 basalts (MORB), withinplate basalts and picrites. After After picrites. and basalts withinplate (MORB), basalts aksot e a. 18) Smos s n iue 9a. Figure in as Symbols (1987). al. ridge et Hawkesworth Mid-ocean for trend main the enclose lines Parallel basalts. Nd/Yb Nd/Yb 37 10 10 100 100 38 Fanner et al. (1989) for basalt from southern Nevada. Basalt from the southern Death Valley-Pancake Range is characterized by lower epsilon Nd, higher epsilon Sr, and higher 207'Pb/204Vb and 20&Pb/204Pb (Figure 9a, b and c) than basalt from the upper Grand Wash Trough and Gold Butte. Basalt from upper Grand Wash Trough and Gold Butte is more similar in isotopic compositions of Sr, Nd, and Pb to basalt from the western margin of the Colorado Plateau. Alibert et al. (1986) suggested that the mantle source of alkali basalt of the western Colorado Plateau was composed of an asthenospheric mantle and an endmember with unradiogenic Sr and Pb and low Sm/Nd that they identified as abyssal clays and hydrothermal sediments that accumulate on the sea floor and are later subducted to sink into the mantle. Upper Grand Wash Trough and Gold Butte basalt show more scatter on the epsilon Nd vs ^Sr/^Sr diagram, an thus the mantle source for alkali basalt must be more variable than that for western Colorado Plateau basalt. However, two samples plot within the field for western Colorado Plateau basalt, and one sample is relatively depleted in epsilon Nd and lies below the mantle array. Therefore, the presence of a component with unradiogenic Sr and low Sm/Nd similar to that identified by Alibert et al. (1986) in the mantle source for upper Grand Wash Trough and Gold Butte basalt can not be ruled out. Magma types of Gold Butte and Grand Wash Basalt On the basis of geochemistry, isotopic composition and age at least three magma types can be distinguished (Table 5): 39 Magma type A. Late Miocene Gold Butte basalt (9.15-9.47 Ma) characterized by low Ce/Yb ratios (10-25), high Mg# (0.0.59-0.62), a relatively narrow range Cr (224-280 ppm) contents, negative epsilon Nd (- 1.14), high positive epsilon Sr (7.6) and high 208Pb/204Pb (38.068). Magma type B. Early-Pliocene (<4.7 Ma) basalt at Pakoon Springs and East St. Thomas Gap with variation in Cr contents from 204 to 337 ppm, low Ce/Yb (10-20), Mg# that vary from 0.56 to 0.60, negative to low positive epsilon Nd (-6.95 and 1.27), low epsilon Sr (-13.77 and 1.46) and moderate 208Pb/204Pb (38.042). Magma type C. Early- to mid-Pliocene basalt flows from Mud Mountain (lowest flow dated at 4.02 Ma) and the upper flow near Grand Wash Bay (3.99 Ma) with a high and relatively wide range in Ce/Yb (20-70), a relatively low and wide range in Mg#s (0.48-0.62), a wide range in Cr contents (130-459 ppm), higher epsilon Nd (3.49-6.75), lower epsilon Sr (-4.573 and -17) and lower 208Pb/204Pb (37.704) compared to magma type B. Because of chemical similarities with Mud Mountain flows, upper Grand Wash Trough and Gold Butte basalt dikes are tentatively included in this magma type. 40 (0 co oj CO 05 o °J Si * CO O 2 S Sjeg o CM «- ,i t rr ® id N o ■ » CO ^ ° 8 ■O’ Q) O T- T » CO o> 2 T3 C CO 00 s. CO CO CO o to CM 2 O CO CM CO 05 iT m fx. I I I (0 oo CM tO O I I I ^ o I CO TT O N - 7 00 to fx. CM l - 1 CO CO CM o ci CM >< to c CD CL CL a.•Q < 1_ T3 a> CD CD CO z CMs O) c c s (0 CO 2 2 u- £ *-4CD a53 '53 a o oo o i o o CD 0) CM z 41 Geochemical models Geochemical modelling was done to place constraints on the evolution of the upper Grand Wash Trough and Gold Butte basalt. Models were performed by using batch melting equations described by Allegre and Minster (1978). Mineral-liquid-distribution coefficients from Arth (1976) and Honjo and Leeman (1987) were used for trace element models (Table 6). A model spinel peridotite was chosen to represent the mineralogy of the mantle source because it is a common ultramafic inclusion in alkali basalt from the Colorado Plateau (Alibert et al., 1986; Best, 1980) and the Basin-and-Range (Ormerod, 1988; Wilshire et al., 1988). The source was assumed to have two times chondritic abundances of the light REE and chondritic abundances for the heavy REE. These values are within the range reported by Wilshire et al. (1988) for spinel peridotites. Mantle concentrations of the transition (Cr, Co, Sc, and V) and high-field-strength elements (Mn, Th, and Hf) were obtained from Taylor and McLennan (1985) and Ringwood (1975), and are listed in Table 6. Gold Butte (Magma Type A) A 2% batch melt of a spinel peridotite containing 45% olivine, 24% orthopyroxene, 20% clinopyroxene and 12% spinel is a good match to the most primitive basalt at Gold Butte (M g#=0.62) (Figure 10). This mineralogy of the model source is within the range of natural spinel peridotites (Jagoutz et al., Table 6: Elemental Abundance Assumed for Mantle Source and Trace Element Distribution Coefficients Mantle Mineral-Liquid distribution coefficients Abundance Olivine Cpx Opx Spinel Magnetite (ppm) Ce 2.87 0.0069 0.15 0.0032 0.03 0.03 Sm 0.694 0.0066 0.5 0.01 0.05 0.268 Eu 0.262 0.0068 0.51 0.02 0.05 0.05 Yb 0.372 0.014 0.62 0.072 0.1 0.08 Lu 0.057 0.016 0.56 0.084 0.1 0.08 Cr 2040 2.1 8.4 1.8 40 5 Co 100 3.8 1.2 0.6 2.5 10 V 128 0.03 1.3 1.4 0.1 45 Sc 15 0.25 2.5 0.56 0.6 0.05 Hf 0.27 0.04 0.12 0.01 0 — Th 0.2 0.0055 0.007 0.13 0 0.28 Ta 0.04 0.03 0.15 0.06 0.6 __ Abbreviations: Cpx-clinopyroxene; Opx-orthopyroxene Note: distribution coefficients from Arth (1976) and Honjo and Leeman (1987) mantle abundance from Taylor and McLennan (1985) and Ringwood (1975) 42 1 0 0 8^|jpuoqo/>iooy < o y « " (/) " ” o ” " o " " o Figure 10. Spider diagram (data normalized to chondrite) displaying similarity between modelled rock and the most primitive (Mg# =0.62) Gold Butte flow. Symbols: circle - Gold Butte flow; triangle - modelled rock. 44 1979). Varying degrees of partial melting of a spinel peridotite source does not account for the more highly evolved alkali basalt at Gold Butte (M g#=0.62-0.59). On the other hand, 3 to 5 percent fractional crystallization of olivine and Fe-Ti oxide from independent batch melts (2 to 3.5%) of the mantle source produces the observed chemical variations (Figure 11). Because the Gold Butte dike is markedly different from the flows in chemistry, petrography and age, it is not included in the geochemical model. Upper Grand Wash Trough Basalt (Magma T^pes B and C) Alkali basalts from Pakoon Springs and East St. Thomas Gap were treated as a single magma type (type B) for modelling because they are similar in chemistry and petrography and their outcrops are close together. The mantle source for the type B basalt may be similar to that for Gold Butte basalt except that it may contain less spinel. The mineralogy of the model source for central upper Grand Wash Trough basalt is 42% olivine, 26% orthopyroxene, 22% clinopyroxene and 10% spinel. Five to 10.5% batch melting of this source and removal of 5 to 8 % olivine plus 0.1 to 0.8% Fe-Ti oxide produces evolution vectors that account for the observed variation in chemistry of magma type B basalt (Figure 11). Samples classified as magma type C (upper flows at Mud Mountain and near Grand Wash Bay and dikes at Mosby peak, Cottonwood Wash and Gold Butte) appear to have undergone a more complicated petrogenetic history than [PM 8 0 -- FXL 6 0 -- (3 ;0 .0 2) Q) o ( 2 ; 0 . 4 ) q (4 : 4 0 -- 6 % (8:0.8)o o* 7% 2 0 -- •10% + 200 250 300 350 400 Cr PM 8 0 -- FXL 6 0 - d) O (2:0.4)o 3% 4 0 - 5% 7% 2 0 - 1 0 % 35 40 45 50 55 60 Co Figure 11. Ce vs. Cr and Ce vs. Co diagrams with partial melting vectors (PM - labelled in percent batch melt) for Gold Butte flows (circles) and Pakoon Springs and East St. Thomas Gap flows (diamonds), and ffactonation vectors (FXL). FXL vectors are labelled to correspond to amount of olivine and Fe-Ti oxide removal. For example, from a 2% batch melt of the Gold Butte source, 3% olivine and 0.02% Fe-Ti oxide is fractionated (3; 0.02). 45 46 those of magma type B. The presence of zoned olivine, poikilitic clinopyroxene, and rounded quartz with a reaction rim of clinopyroxene and glass, indicates that the formation of type C basalt may have involved high pressure fractionation and/or crustal contamination. Geochemical model(s) were not derived for these rocks because there was not enough data to construct a feasible model. The depth of melt generation can be estimated by determining the phase assemblages in the model mantle source. Because geochemical models suggest that the alkalic basalt was derived from a source containing spinel, partial melting probably occurred at pressures greater than 8 kb; corresponding to a depth of about 25 km (Takahashi and Kushiro, 1983). PALEOMAGNETOMETRY Paleomagnetic poles for twenty-five samples from the upper Grand Wash Trough and Gold Butte lava flows are listed in Table 7. Paleomagnetic poles for the seven flows at Gold Butte show normal magnetization, while those at Pakoon Springs are reversed; but poles are tightly clustered (Figure 12). The two sample population means and angular standard deviations and the magnitude of the resultant vectors (R) for each population are presented in Figure 12. Permissible values of R range from 0 to 1; if R = 1, the observations are assumed to be tightly clustered with little dispersion (Davis, 1973). Paleomagnetic poles for Gold Butte lava flows have little dispersion (R=0.955) and can be related to the normal polarity event (chron) 5 (interval between 9.17 and 9.47 Ma) between reversals 5-1 and 5-2 of the Cretaceous- Table 7: Summary of Paleomagnetic Data Sample Number Dec. Inc. Angular Demag Stratigraphic error steps (mT) Postion Gold Butte G8-3-18-LN 309.9 43.2 0.9 5 -1 4 0 1 G8-3-41-LN 336.3 44.5 0.6 5 - 1 4 0 2 G8-3-42-LN 349.3 47.5 1.1 5 - 1 4 0 3 G8-3-44-LN 3.5 49.3 1.6 1 5 -1 4 0 4 G8-3-45-LN 342.2 55.8 0.9 5-140 5 G8-3-46-LN 353.5 48.2 0.6 1 0 -1 4 0 6 G8-3-24-LN 2.6 40.8 2 2 5 -1 4 0 7 East St. Thomas Gap Flat - lying G8-4-25-LN 217.6 -63.3 0.5 40 - 140 1 G8-4-26-LN 169.5 -48 0.4 40 - 140 2 G8-4-47-LN 184 -50 0.5 40 - 140 3 Tilted G8-4-50-LN 184.1 -40.1 1.2 1 4 -1 2 5 1 G8-4-49-LN 218.5 -65.1 0.9 15 - 140 2 G8-4-48-LN 226.1 -26.8 1.4 55 - 140 3 G8-4-28-LN 4.5 -1.3 2.3 70-125 4 Pakoon Springs G8-5-57-LN 173.7 -48.3 2 4 5 -1 3 0 1 G8-5-56-LN 168.1 -57.2 1 25 - 140 2 G8-5-55-LN 144.8 -58.3 1 7 0 -1 4 0 3 G8-5-63-LN 177.2 -50.4 0.9 40 - 140 4 Grand Wash Bay G8-8-53-LN 146 -59.8 3.6 14-125 1 G8-8-51 -LN 352.5 75 2 25 - 140 2 Mud Mountain G8-7-37-LN 204.7 32.5 3 40 - 140 1 G8-7-36-LN 169.8 40.1 0.8 5-140 2 G8-7-58-LN 236.8 -44.0 1.2 25 -1 4 0 3 G8-7-34-LN 156.3 -43.6 1.2 25 - 140 4 Abbreviations: Dec. - Declination; Inc. - Inclination; mT - milli Tesla Flows numbered from bottom to top 47 • • • A ° 0 Summary of Paleomagnetic Statistics ______Gold Butte Flows n = 7 mean = 345.7 R = 0.9554 Se = 17.12 degrees Pakoon Springs Rows n = 4 mean = 166.1 R = 0.9762 ______Se = 12.5 degrees Abbreviations: n - number of samples; R - magnitude of resultant vector; Se - angular standard deviation Figure 12. Equal area lower hemisphere projection of paleomagnetic data. Table summarizes statistics to demonstrate cluster of Gold Butte and Pakoon Springs samples. Open symbols for reversed poles, closed for normal. Symbols: circles - Gold Butte; squares - East St. Thomas Gap; diamonds - Pakoon Springs; triangles - Mud Mountain; inverse triangles - Grand Wash Bay. 48 Tertiary-Quaternary Mixed Polarity Superchron (KTQ-M) (Cox and Hart, 1986) by K-Ar dates of 9.15 and 9.46 Ma (Figure 13). Therefore, both paleomagnetic and K-Ar age data suggest that volcanism at Gold Butte occurred within a time interval of less than 300,000 years. However, the seven flows at Gold Butte are separated only by their vesiculated tops (Figure 14); there are no paleosols or unconformities observed in the flow stack. Therefore, on the basis of the field evidence, it is suggested that the Gold Butte flows were extruded in a time interval substantially shorter than 300,000 yrs. In fact, the flows may represent pulses of a single eruption. Paleomagnetic poles of Pakoon Springs basalt flows also cluster tightly (R=0.976) and can be related to chron 3r of the KTQ-M Superchron (Figure 13) by the K-Ar date of 4.7 Ma. Chron 3r comprises the interval between 4.79 and 5.41 Ma, therefore, volcanism at Pakoon Springs is constrained to have occurred within a 620,000 year period. Basalt flows at East St. Thomas Gap show similar poles to those at Pakoon Springs (Figure 12; Table 7), but the lowest of the flat flows and its two slumped equivalents (structurally corrected for tilt due to slumping) has a significantly different pole (displaced 40 to 50 degrees to the west). At the position in the stratigraphic section where the shift is recorded, the flows are conformable and no paleosols were observed. At Mud Mountain and near Grand Wash Bay, intervening sediments separate basalt flows with markedly different paleomagnetic poles. Lower flows at Mud Mountain (4.02 Ma) have anomalous poles; they are south-seeking but normally magnetized (Figure 12). These flows may have been extruded during an i r ± - - - Z Gold Butte flows (9.15 to 9.47 Ma) (■ 9 : J►. *s “ “ Pakoon Springs flows (4.7 Ma) ---c-Lower flow at Mud Mountain (4.02 Ma) 3 N 'Upper flow near Grand Wash Bay (3.99 Ma) Figure 13. Polarity time scale (from Cox and Hart, 1986) showing intervals assigned to upper Grand Wash Trough and Gold Butte lava flows, as constrained by K-Ar age data. 50 Figure 14. Photograph of Gold Butte lava flows in Quail Springs Wash, looking south. Total thickness of flow stack in view is 20 m. 51 52 interval in which the earth’s geomagnetic field was undergoing a transition. Such a reversal occurred at about 4 Ma (Figure 13). Near Grand Wash Bay, at least one complete reversal of the geomagnetic field occurred after extrusion of the lower flow (south-seeking; reversed) and before extrusion of the upper flow (3.99 Ma) (north-seeking; normal). Paleomagnetic and field evidence from the central upper Grand Wash Trough and Gold Butte suggests that there was little time between eruptions of lava flows in these areas. STRUCTURAL AND REGIONAL IMPLICATIONS OF GOLD BUTTE AND UPPER GRAND WASH TROUGH BASALT Structural Implications for Upper Grand Wash Trough Basalt Geochemical data suggest that basaltic magma originated in the mantle and rose through the crust rapidly and was not significantly modified by fractional crystallization. Limited isotopic data indicate that magma underwent little interaction with crustal material. Field, K-Ar and paleomagnetic data suggest that eruption episodes were short-lived. All of the available evidence suggests that basalt rose through the crust relatively rapidly. Rapid ascent requires crustal- penetrating high-angle zones of weakness to serve as conduits for magma transport to the surface. Crustal penetrating zones of weakness may have been created by high-angle normal faulting in the upper Grand Wash Trough. Wernicke and Axen (1988) proposed a model for the formation of high-angle 53 faults in the upper Grand Wash Trough in which they suggested that the rebounding footwall of the Virgin-Beaverdam breakaway zone underwent folding and steep faulting to accommodate uplift due to isostatic rebound. According to this model, high-angle normal faults may have been created in the Virgin Mountains and in the upper Grand Wash Trough during isostatic rebound following tectonic denudation southwest of the Virgin Mountains. These high- angle faults may have created zones of weakness that were used as conduits for mantle derived magma, in a relatively narrow region ( 10-20 km) east of the detachment system. Alternatively, because western Colorado Plateau alkali basalt is contiguous with upper Grand Wash Trough basalt, an emplacement mechanism similar to that invoked for western Colorado Plateau basalt may account for upper Grand Wash Trough basalt. Volcanic centers on the Colorado Plateau are associated with north-striking high-angle fault zones (Hamblin, 1970; Anderson, 1988; Moyer and Nealy, 1988). Huntoon and Sears (1974) suggested that two of these faults, the Bright Angel and Eminence faults, underwent a history of Phanerozoic movement, beginning in Precambrian time. These structures are likely to be zones of crustal weakness that served to localize volcanic fields at Mt. Floyd, Mt. Hope, Kaiser Spring, and in the Castaneda Hills (Moyer and Nealy, 1988). Structural Implications of Gold Butte Basalt Basalt at Gold Butte overlies the trace of the Gold Butte fault, a northeast- striking strike-slip fault (Figure 5). It is probable, but not proven, that the Gold 54 Butte fault controlled the emplacement of the Gold Butte basalt. Because vents or dikes related to the Gold Butte basalt are not observed in the well-exposed Paleozoic section north of the fault, or in the Precambrian rocks south of the fault, it is unlikely that basalt erupted from either of those areas and merely flowed down Quail Springs Wash. Vents or dikes may be buried beneath the alluvium overlying the trace of the Gold Butte fault. Because the Gold Butte basalt apparently is derived from a mantle source and underwent only small amounts of fractional crystallization, crustal penetrating zones of weakness are required to transport the magma. The Gold Butte fault is the most likely structure to serve as a conduit. Therefore it may be a crustal penetrating structure. If this is true, then models that suggest that strike-slip faults in the Lake Mead area are transfer structures in the upper plates of shallow detachment fault systems (Weber and Smith, 1987) may not apply to all strike-slip faults in the Lake Mead area. Regional Implications of Gold Butte Basalt Gold Butte basalt (9.46 to 9.15 Ma) is the oldest alkali basalt dated in the Lake Mead region. While alkali basalt erupted at Gold Butte, basaltic-andesite compound cinder cones formed between 8.7 and 10.3 Ma in the Callville Mesa- West End Wash area (Figure 1) (Feuerbach et al., in prep.). Alkali basalt volcanism occurred between 6.02 and 4.7 Ma in other parts of the Lake Mead region. Feuerbach et al. (in prep.) suggested that Callville Mesa-West End Wash basaltic-andesite was generated by mixing of magma with the composition of 55 alkali basalt with an upper crustal component. The coeval eruption of alkali basalt and basaltic-andesite may be circumstantial, or it may imply differences between the two areas in either the geometry of structures that may control the emplacement of lavas, the crustal composition, the thermal state of the crust (Glasner and Ussier, 1989), and/or the mantle source of the magmas. SUMMARY Field, geochemical, isotopic and paleomagnetic data suggest that the basalt lava flows in the upper Grand Wash Trough and at Gold Butte erupted quickly. Flows represent partial melts of mantle spinel peridotite and record only small amounts of fractionation. Conclusions drawn from each data set are in agreement. At Gold Butte, the restricted range in chemical variation (Mg# 0.59 to 0.62; Cr 224-280) the lack of petrographic diversity, similar K-Ar dates (9.15- 9.47 Ma), similarity of paleomagnetic poles and lack of paleosols or uncomformities in the flow stack all indicate that the eruptive episode was short lived. Geochemical modelling supports this conclusion and suggests that 2 to 3.5% batch melting and 3 to 5% fractional crystallization is required to produce the observed chemical variation. Basalt from Pakoon Springs and East St. Thomas Gap is also restricted in chemical diversity (Mg# 0.56-0.6; Cr 245-337) and thus extensive fractional crystallization did not play a role in their development. In addition, flows are not separated by sedimentaiy units or paleosols and have similar paleomagnetic poles. 56 At Mud Mountain, lower flows are petrographically distinct from upper flows. Lower flows are distinguished from upper flows by containing abundant phenocrysts of plagioclase and/or clinopyroxene in addition to olivine. Paleomagnetic poles are also different for lower and upper flows, but the difference in time can not be constrained because only one flow was dated at Mud Mountain. Available data gathered from basalt flows in the central upper Grand Wash Trough and Gold Butte suggest that magma rose through the crust quickly and required high-angle zones of weakness that penetrate deeply into the crust to serve as conduits for magma escape. 57 REFERENCES CITED Alibert, C., Michard, A., and Albarede, F., 1986, Isotope and trace element geochemistry of Colorado Plateau volcanics: Geochemica et Cosmochim Acta, v. 50, p. 2735-2750. Allegre, C.J., and Minster, J.F., 1978, Quantitative models of trace element behavior in magmatic processes: Earth and Planetary Science Letters, v. 38, p. 1-25. Anderson, R.E., 1973, Large magnitude Late Tertiary strike slip faulting north of Lake Mead, Nevada: U.S. Geological Survey Professional Paper 794, 18 p. Anderson, R.E., and Mehnert, H. H., 1979, Reinterpretation of the history of the Hurricane fault in Utah: in Basin and Range Symposium, Newman, G. W., and Goode, H. 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Smith, E.I., Feuerbach, D.L., Naumann, Terry and Mills, J.G., 1989, Geochemistry and evolution of mid-Miocene igneous rocks in the Lake Mead area of Nevada and Arizona: Geological Society of America, Memoir 174, "The Nature and Origin of Cordilleran Magmatism.", Anderson, J. L. editor (in press). Takahashi, E., and Kushiro, I., 1983, Melting of dry peridotite at high pressures and basalt magma genesis: American Mineralogist, v. 68 , p. 859-879. Taylor, S.R., and McLennan, S.M., 1985, The Continental Crust: its composition and evolution, Blackwell Scientific Publications Inc., Palo Alto, 312 p. Weber, M.E., and Smith, E.I., 1987, Structural and geochemical constraints on the reassembly of mid-Tertiary volcanoes in the Lake Mead area of southern Nevada: Geology, v. 15, p. 553-556. Wernicke, B., 1985, Uniform-sense normal simple shear of the continental lithosphere: Canadian Journal of Earth Sciences, v. 22, p. 108-125. Wernicke, B., Axen, G.J., 1988, On the role of isostasy in the evolution of normal fault systems: Geology, v. 16, p. 848-851. Wilshire, H.G., Meyer, C.E., Nakata, J.K., Calk, L.C., Shervais, J.W., Nielson, J.E., and Schwarzman, E.C., 1988, Mafic and ultramafic xenoliths from volcanic rocks of the western United States: U.S. Geological Survey Professional Paper 1443, 179 p. 61 APPENDIX 1: Sample location and Petrography Gold Butte Flows and Dike Sample Number: G8-3-3 through G8-3-9; G9-3-40 and G9-3-43 Location: 36 15’56"; 114 15’25". Flows near the head of Quail Springs Wash Rock Name: Alkali olivine basalt Description: Porphyritic with few vesicles (< 1%). Olivine phenocrysts (1-4 mm) comprise 9 to 15% of the samples. Sub to anhedral olivines are embayed and coated with thick rinds of red brown iddingsite. Olivine interiors contain many irregular fractures that are coated with iddingsite. The matrix is pilotaxitic to intergranular with subhedral plagioclase laths (46 to 54%; < 0.5mm), olivine (15-21%; <0.5 mm), clinopyroxene (4-12%; < 0.5 mm) and Fe-Ti oxides (4-12%; <0.1 mm). A few of the samples show areas of mild propylitic alteration. Blocky, zoned plagioclase with undulatory extinction and irregular margins (< 3 mm) was observed in two samples. Sample Number: G8-2-2-LN Location: 36 16T8"; 114 16’47": Dike 2 km west of thick section of flows in Quail Springs Wash Rock Name: Alkali olivine basalt Description: Porphyritic. Subhedral plagioclase phenocrysts (10%, <7 mm) are twinned (albite, pericline and carlsbad) and zoned. Clinopyroxene phenocrysts (7%, <2 mm) are subhedral. Two grains of clinopyroxene are poikilitic, with inclusions of plagioclase. Subhedral olivine phenocrysts (2%, < 2 mm) with magnetite inclusions have little alteration to iddingsite. The pilotaxitic matrix is composed of plagioclase, olivine and clinopyroxene less than 0.5 mm. Fe-Ti oxide is typically less that 0.1 mm. East St. Thomas Gap Flows Sample Number: G8-4-10 to 4-13, 4-32, 4-68 to 4-72 Location: 36 28’24"; 114 1’42": Lava beds east of St. Thomas Gap, on west 62 side of Grand Gulch Road. Rock Name: Alkali olivine basalt Description: Porphyritic and vesicular (up to 11%). Olivine phenocrysts (3- 10%; 3-7 mm) have thick rinds of red brown iddingsite and magnetite inclusions. A few grains have a thin, discontinuous rim of fresh olivine around the iddingsitized rim. A subophitic to intergranular matrix is composed of plagioclase laths (49- 61%; 1-2 mm), clinopyroxene (7-14%; 0.5-1.5 mm), olivine (10- 21%; 0.5-lmm) and Fe-Ti oxides ( 1-7%; <0.5 mm). Pakoon Springs Flows and Breccia Plug Sample Number: G9-5-64 through G9-5-67 Location: 36 24’11"; 113 56’30", 1 km southwest of Pakoon Spring. Rock Name: Alkali olivine basalt Description: Porphyritic and vesicular (up to 7%). Subhedral olivine phenocrysts (9-13%; 2-3 mm) with magnetite inclusions are embayed and iddingsitized. A thin discontinuous rim of fresh olivine around the iddingsitized rind was observed in a few grains in one flow. The subophitic to intergranular matrix consists of plagioclase laths (54-58%; <0.5 mm, olivine (12-15%; < 0.5mm), clinopyroxene (8-15%, < 0.5mm) and opaque Fe-Ti oxides (2-7%; < 0.2mm) Sample Number: G8-5-14-LN Location: 36 24’30"; 113 56T0": Breccia plug < 1 km southeast of Pakoon Springs Ranch Rock Name: Alkali olivine basalt Description: Porphyritic with a trace of vesicles. Olivine phenocrysts (7%; 1- 4 mm) with abundant magnetite inclusions show little alteration to iddingsite. Plagioclase phenocrysts (7%; <2mm) are twinned (albite, carlsbad and pericline) and subhedral. A 7 mm wide sausseritized plagioclase xenocryst was observed. The matrix is glassy. Grand Wash Bay Flows Sample Number: G8-8-52-LN Location: 36 16’7"; 113 59’42": Lower flow near Grand Wash Bay Rock Name: Alkali olivine basalt Description: Porphyritic and vesicular (10%). Olivine phenocrysts ( 8 %; 2-4 63 mm) have a thin rind of iddingsite and irregular interior fractures. The intergranular matrix is composed of plagioclase laths (61%; < 0.5mm) with pericline, albite and carlsbad twins, clinopyroxene (11%; <1.5 mm), olivine ( 6 %; <0.5mm) partially altered to iddingsite, and Fe-Ti oxide barbs and blocks up to 0.5 mm (8 %). Sample Number: G9-8-62-LN Location: 36 16’7"; 113 59’42": Upper flow near Grand Wash Bay Rock Name: Alkali olivine basalt Description: Phyric ("turkey-track") and vesicular (3%). Subhedral plagioclase phenocrysts (39%; 3-8 mm) are twinned (albite, pericline and carlsbad) and lath shaped. A few plagioclase phenocrysts are blocky, zoned and have undulatory extinction. Subhedral olivine phenocrysts (11%; 1 to 5 mm) are almost wholly altered to iddingsite and some are skeletal. The groundmass is intergranular and composed of plagioclase laths, iddingsitized olivines (< 1 mm), Fe-Ti oxide barbs (< 0.5mm) and clinopyroxene (<0.3 mm). Mud Mountain Flows Sample Number: G8-7-33 and G8-7-34-LN Location: 36 40’4"; 113 47’24": Upper flows at Mud Mountain above Cane Springs. Rock Name: Hawaiite Description: Porphyritic with a trace of vesicles. Anhedral Olivine phenocrysts (2-3%) are partially altered to iddingsite, contain few magnetite inclusions and have a thin jacket of fresh olivine around an inner rind of iddingsite. The matrix is intergranular and composed of blocky, zoned plagioclase with irregular outlines and undulatory extinction and plagioclase laths (60- 63%; <0.5 mm) with clinopyroxene (9-19%) olivine (4-15%) and Fe-Ti oxides (8-11%). 7-34 contained a rounded xenocryst of quartz (5 mm) with a reaction rim of clinopyroxene and glass and a subhedral poikilitic clinopyroxene (1 mm). Sample Number: G8-7-60 and G8-7-61-LN Location: 36 40’4"; 113 47’24": Lower flows at Mud Mountain above Cane Springs. Rock Name: Alkali olivine basalt Description: Holocrystalline and vesicular (3-5%). Subhedral plagioclase phenocrysts (63-69%; 3-7 mm) are lath shaped and twinned (albite, pericline and carlsbad). A few are blocky, zoned and 64 have undulatory extinction. The matrix is composed of olivine (10-13%; 1-3 mm) with magnetite inclusions and little alteration to iddingsite. Clinopyroxenes (10-18%; 2-4 mm) are zoned and exhibit diallage structure. Fe-Ti oxides (3-5%) are less than 0.5 mm. Mud Mountain Dikes Sample Number: G8-6-39-LN Location: 36 39’54"; 113 49’48": dike on west side of Mud Mountain. Rock Name: Alkali olivine basalt Description: Porphyritic with few vesicles (3%). Sub to anhedral olivine phenocrysts ( 2-7 mm) have inclusions of magnetite and iddingsitized outer rims and interior fractures. A fresh jacket of olivine around the iddingsite rim is common. The intergranular to pilotaxitic matrix is composed of plagioclase laths (62%, <0.5 mm), iddingsitized olivine (12%, <0.5 mm) and Fe-Ti oxide barbs (4%, < 0.5mm). A vein 8 mm wide occurs within the sample and is composed of blocky plagioclase (< 0.5mm) with undulatory extinction with rare olivine (<0.3 mm) and abundant Fe-Ti oxides (<0.1 mm) Sample Number: G9-6-77-LN Location: 36 40’8"; 113 49’24": dike on west side of Mud Mountain. Rock Name: Alkali olivine basalt Description: Porphyritic and vesicular (21%). Olivine phenocrysts (5.6%, 1-6 mm) with rare magnetite inclusions are eu- to subhedral with little alteration to iddingsite around the rim and in irregular interior fractures. The pilotaxitic matrix is composed of tightly bound plagioclase microlites (60%, <0.5 mm), iddingsitized olivine (7.8%, < 0.3 mm), clinopyroxene (0.6%, <0.3 mm), and Fe-Ti oxides (4.2%, <0.7 mm). A trace (0.2%) of blocky plagioclase with undulatory extinction was observed. Sample Number: G9-6-76-LN Location: 36 39’40"; 113 50’: dike on west side of Mud Mountain. Rock Name: Alkali olivine basalt Description: Porphyritic and vesicular (5%). Anhedral olivine phenocrysts (10%, 1-4 mm) with magnetite inclusions have thin rinds of iddingsite and thin fresh jackets of olivine around the iddingsite rim. A trace of blocky plagioclase with undulatory extinction (1 mm) was observed. The pilotaxitic matrix is composed of plagioclaths laths (57%, < 0.3mm) clinopyroxene ( 8 %, < 0.5mm), iddingsitized olivine (10% < 0.3mm), and large (0.5-0.75 mm) 65 and small (< 0.2 mm) Fe-Ti oxide grains. Mosby Peak Sample Number: G9-10-74-LN Location: 36 34’12"; 113 58’30"; Mosby Peak central and longest dike. Rock Name: Alkali olivine basalt Description: Porphyritic and nonvesicular. Sub to anhedral olivine phenocrysts (14.2%, 1-3 mm) with inclusions of magnetite and thin rinds of iddingsite with fresh jackets of olivine around the iddingsite rim. A trace of poikilitic clinopyroxene (1.5 mm) with olivine and magnetite and a trace of blocky resorbed plagioclase with undulatory extinction was observed. The pilotaxitic matrix is composed of tightly bound microlites of plagioclase (61.4%, <0.5 mm), iddingsitized olivine (6.4%, <0.3 mm), clinopyroxene (2%, <0.3 mm) and abundant Fe-Ti oxides (15.2%, <0.2 mm). Cottonwood Wash Sample Number: G9-10-75-LN Location: 36 33’30"; 113 55T2": Dike in Cottonwood Wash. Rock Name: Alkali olivine basalt Description: Porphyritic with few vesicles (0.4%). Subhedral olivine (13.2%, 0.5-2 mm) phenocrysts with magnetite inclusions are partially altered to iddingsite. Some olivine phenocrysts are skeletal. Subhedral plagioclase laths (9.8%, 1-3.5 mm) are twinned and zoned. Clinopyroxenes (3%, 1-3 mm) are glomeroporphyritic and exhibit diallage structure. The pilotaxitic matrix is composed of plagioclase microlites (53%, <0.5 mm), olivine (9.4%, <0.5 mm), clinopyroxene (2%, < 0.3mm), and Fe-Ti oxides ( 8 %, <0.3 mm). Appendix 1: (cont.) Table of modal mineralogy Sample # Phenocrysts I Groundmass & a. Q. & 8 o . u S T ~~ .1 5 0 X x > x a> o> g CO • c £ C SS "8 CD CO i _ Z n i 0 0 T" y > u o CM OCM CO Od CO h= £ H r- M* CO mcm cm >© o a> a © 0 CO CO CO i - Z T— CO M; CO CM CO o c CM M* M00N. N 0 0 CM o < T” CM (O CD Z GO j - 5 0 N CO MC M* d c CM CM lO o t o t Ocd c CO CM T— T~ CO (0 o 0 GO CO — Z d c o d c d c M- CO r* H H o t CO o ® © O( (0 (0 CO j 0 CO OCO CO CO CO —I z CM 5 0 ) O CO o t a> 5 0 d c d c O o o o 0 CO z M CM CM CM n i d i £ m d i fs. 0 0 0 5 0 o l _ CO M- v. rv z CM 1 o MCO CM MCO CM o CM o CD -^ 5 0 5 0 . N z Z CM J. o t lO N CM y-' CM ■M- o c o t 0 CM t r J - CO o CM CM CO CM T— M* CM CL i 8 c % © 0 J - z 5 0 n i s a> o cvi CM CM d c d c Mco c CM H . N £ MT’~ CM o M; © CM O CD 0 S J - Z 5 0 n i d c OCO CO lO d c o c / " r f T a 0 CO CO l _ Z 5 0 n i Oin i lO CO CM y* d i CM cvi CM CO CO CM CM d i CO o CM T— y 66 67 X i ‘S © © © £ E Q O CO CM CO CM in id © 2 r-' o o o ~© IS C s i— o) 2 O) © in CO 00 CO h* CM CO CM a> o cvi d cvi cvi rj- cvi cd o c w© CM 0 3 o h- COs c © in CM CM CO U) CM (D CM CM T3 si o CM cd id Q.© £ CO c CM co CO GO CM LO to ©_ o3 *“ o od o> cvi o c o & © * - o 2£ o E CMCM o o CM CO I s cvi CO CO cd in £ in a> CO in in 5 0 >% 0. 1 -o 3 03 s. 1 E ° . | •oc CMCM00 CO CO CM Mg# Mg# = [Mg/(Mg+Fe) atomic ratio] Mg = MgO/40.305 Fe = FeOt * 2 * 0.9 / 159.676 Epsilon Sr and Epsilon Nd Epsilon Sr = 87 Sr/8gSr (measured) - 1 * 10E4 57Sr/ 87St/ 86St (UR) = 0.7045 UR - model whole earth reservoir Epsilon Nd = 143N d /144Nd (measured) -1 * 10E4 143Nd/ Nd (CHUR) 143N d /144Nd (CHUR) = 0.512638 CHUR - model chondritic reservoir