Geology and Petrology of the Basalts of Crater Flat: Applications to Volcanic Risk Assessment for the Nevada Nuclear Waste Storage Investigations

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LOS ALAMOS SCIENTIFIC LABORATORY Post Office B0x 1663 Los Alamos. New Mexico 87545

I,; I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I I An Affrmative Action/Equal Opportunity Employer

This work was supported by the US Depart. ment of Energy, Nevada Nuclear Waste Storage Investigations.

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UNITED STATES DEPARTMENT OF ENERGY ) CONTRACT W7405-FENG.

I LA-8845-MS UC 70 Issued: June 1981

Geology and Petrology of the Basalts of Crater Flat: Applications to Volcanic Risk Assessment for the Nevada Nuclear Waste Storage Investigations

D. Vaniman B. Crowe

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GEOLOGY AND PETROLOGY OF THE BASALTS OF CRATER FLAT: APPLICATIONS TO VOLCANIC RISK ASSESSMENT FOR THE NEVADA NUCLEAR WASTE STORAGE INVESTIGATIONS

D. Vaniman and B. Crowe

ABSTRACT

Volcanic hazard studies of the south-central Great Basin, Nevada, are being conducted for the Nevada Nuclear Waste Storage Investigations. This report presents the results of field and petrologic studies of the basalts of Crater Flat, a sequence of Pliocene to Quaternary-age volcanic centers located near the southwestern part of the Nevada Test Site. Crater Flat is one of several basaltic fields constituting a north-northeast-trending volcanic belt of Late Cenozoic age extending from southern Death Valley, California, through the Nevada Test Site region to central Nevada. The basalts of Crater Flat are divided into three distinct volcanic cycles (3.7, 1.1, and 0.3 Myr) based upon geologic mapping, potassium-argon (K-Ar) dating, and magnetic polarity determinations. The cycles are characterized by eruption of basalt-magma of hawaiite composition that formed cinder cone clusters and associated lava flows. Total volume of erupted magma for respective cycles is about 0.5 to 4.0 x 10 1 km'; volumes of indi- vidual cinder cone and lava flow centers are about 0.3 to 1.5 km3. The basalts of Crater Flat are sparsely to moderately porphyritic; the major phenocryst phase is olivine, with lesser amounts of plagioclase, clinopyroxene, and rare amphibole. Basalts of the 3.7-Myr cycle contain glomeroporphyritic clots of bytownite and augite typical of hawaiite basalts in the southwestern United States. Major and trace- element differences between cycles, as well as the variations within cycles (in particular the 11-Myr cycle), cannot be explained simply by crystal- liquid fractionation. However, the consistent recurrence of evolved hawailte magmas in all three cycles points to- crystal fractionation from more primitive magmas at depth. A possible major i

transition in mantle source regions through time may be indicated by a transition from normal to Rb- depleted, Sr-enriched hawaiites in the younger basaltic cycles. The recurrence of small volumes of hawaiite magma at Crater Flat supports assumptions required for probability modeling of future volcanic activity and provides a basis for esti- mating the effects of volcanic disruption of a repository site in the southwestern Nevada Test Site region. Preliminary data suggest that succes- sive basalt cycles at Crater Flat may be of de- creasing volume but recurring more frequently.

I. INTRO DUCTION The Nevada Nuclear Waste Storage Investigations (NNWSI) are evaluating the suitability of the Nevada Test Site (NTS) for location of a high-level radioactive waste repository. Current geologic exploration studies within the NTS are focused on Yucca Mountain (Dixon et al. 1980), a large fault block composed of multiple sequences of ash-flow tuff erupted from the Timber Mountain-Oasis Valley cauldron complex (Byers et al. 1976). Yucca Mountain is located within the south-central Great Basin, a physio- ) graphic subprovince of the larger Basin and Range Province, which includes much of the western United States. The Great Basin is a tectonically active region. Its geologic history is characterized by extensional block faulting that produced linear mountain ranges separated by broad alluvial basins (Nolan 1943; Stewart 1978; Christiansen and McKee 1978). This faulting was closely associated in time and space with silicic volcanic activity at major cauldron complexes. Silicic volcanic rocks as old as 40 to 45 Myr are present in the central Great Basin. Younger volcanic rocks occur within broadly arcuate belts that are successively younger to the south and toward the margins of the Great Basin (Armstrong et al. 1969; Scott et al. 1971; Noble 1972; Stewart and Carlson 1978). Silicic rocks of late Miocene age are most abundant within an east-west-trending belt of the south-central Great Basin. This belt extends from southeastern Nevada through the NTS region and may bend to the northwest along the Walker Lane structural trend (Stewart and Carlson 1978). Since about 14 Myr ago, two major changes in the patterns of tectonic and volcanic activity have occurred. First, there has been a progressive concentration of tectonic activity toward the margins of the Great Basin (Scholtz et al. 1971; 2 Christiansen et al. 1978). Second, silicic volcanic activity has been re- placed by basaltic volcanism including minor amounts of bimodal basalt- rhyolite volcanism (Christiansen and Lipman 1972; Christiansen and McKee 1978). This basaltic activity occurs within distinct belts along the eastern and western margins of the Great Basin (Stewart and Carlson 1978; Best and Hamblin 1978) and within a less prominent northeast-trending belt in the south- central Great Basin that extends through the NTS region (Fig. 1; Crowe and Carr 1980). Volcanic hazard studies, being conducted as a part of the NNWSI, are attempting to assess the risk of disruption of a waste repository within the NTS by future volcanic activity. Crowe and Sargent (1979) compared the geology and geochemistry of the Silent Canyon and Black Mountain peralkaline volcanic centers, the latter representing the youngest major silicic volcanism within the NTS region. They concluded that the Black Mountain cycle represents a renewed phase of silicic volcanism following the Timber Mountain- Silent Canyon volcanic cycle. This suggests that there is a small but finite possibility of recurrence of silicic volcanism within the NTS area. Crowe and Carr (1980) provided a preliminary assessment of the risk of basaltic volcanism within the southern Great Basin. They briefly described the Late Cenozoic volcanic geology of the southwestern NTS region, calculated the probability of disruption, and examined the disruption effects due to intrusion of a repository by basaltic magma. In this report, a continuation of previous work, we describe the detailed geology, geochronology, and petrology of the basalts of Crater Flat. This basaltic field is located within and adjacent to Crater Flat, an alluvial basin west and southwest of Yucca Mountain (Fig. 1). The basalts of Crater Flat record three small volume magma pulses that are spa- tially and temporally distinct (3.7, 1.1, and 0.3 Myr). Each pulse erupted basalt that may be classified as hawaiite following the definition by Best and Brimhall (1974).

II. GEOLOGY AND PETROGRAPHY OF THE BASALTS OF CRATER FLAT Crater Flat contains over 15 small basaltic volcanic centers that consist of cinder cones and associated lava flows. The distribution and tectonic setting of the volcanic rocks has been described by Crowe and CUrr (1980). The rocks are divided into three distinct cycles or magma pulses based on geologic

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Fig. 1. Distribution of Late Cenozoic basaltic volcanism in the south-central Great Basin. Modified from Stewart and Carlson 1978, and unpublished work by W. J. Carr. Gravity symmetry axis is the line of bilateral symmetry of the observed Bouguer gravity field of the Great Basin (after Eaton et al. 1978). LC, Lunar Crater volcanic field; RR, Basaltic rocks of the Reveille Range; QC, Basaltic rocks of the Quinn Canyon Range; BR, Basalt of Basalt Ridge; SC, Basaltic rocks of the Silent Canyon cauldron; SB, Basaltic rocks of the Sleeping Butte cauldron; BB, Basalt of Buckboard Mesa; PR, Basaltic rocks of Paiute Ridge; NC, Basaltic rocks of Nye Canyon; CF, Basaltic rocks of Crater Flat; 114, Basalt of Dome Mountain; SM, Basalt of Skull Mountain and Kiwi Mesa; GM, Basaltic rocks of the Greenwater Mountains; DV, Basaltic rocks of southern Death Valley; WL, Walker Lane; LV, Las Vegas shear zone; DV-FC, Death Valley- Furnace Creek fault.

N - field relations, potassium-argon ages (Table I), and magnetic polarity determinations (Fig. 2).

* 3.7-Myr cycle (Tb, Tbp) Rocks of the older cycle-consist of deeply dissected cones and flows with locally exposed feeder dikes. ' They crop out in the central and southeastern part of Crater Flat (Fig. 2).

* 1.1-Myr cycle (Qb1, Qbp1): Basaltic rocks of this cycle consist of cinder cones and lava flows located along a northeast-trending structural arc near the center of

TABLE I POTASSIUM-ARGON WHOLE-ROCK AGES OF BASALTS FROM CRATER FLATa

.K2 0 Age Group and Center Sample No. (Myr)

0.3-Myr basalt cycle T.SY -1 1.818, 1.824 0.29+0.2 Lathrop Wells Cone 1.820, 1.810 1.1-Myr basalt cycle TSV-128 1.723,,1.712 1.14±0.3 1.723, 1.727

Black Cone TSV-2 1.806, 1.797 1.09±0.3 ". 1.797, 1.799

TSV-2A 1.632, 1.634 1.07±0.4 1.637, 1.635 Little Cones TSV-3 1.680, 1.683 1.11±0.3 1.678, 1.680 3.7-Myr basalt cycle CF-72-24-8 -1.57 3.84+0.2 1.60

. CF-79-26-1 1.77 3.64±0.1 1.76

aSamples from Lathrop Wells and western centers determined by R. J. Fleck, U.S. Geological Survey, Menlo Park, California; southeastern Crater Flat by R. F. Marvin, U.S. Geological Survef, Denver, Colorado.

Uncertainties represent the larger of either the value calculated using the expression of Cox and Dalrymple (1967) or the calculated standard deviation of the reolicate analyses. D

I . Crater Flat. From northeast to southwest, the major cones in this cycle include a northernmost cone (un- ) named), Black, Red, and Little Cones.

* 0.3-Myr cycle (Qb2, Qbp2, Qbs): The youngest cycle is marked by cones and flows of the Lathrop Wells center. This center is located in the southeast corner of Fig. 2, outside of Crater Flat.

III. 3.7-MYR BASALT CYCLE The older basalts of Crater Flat crop out as deeply dissected cinder cones with minor lava flows (central Crater Flat) and as moderately extensive lava flows with no associated cone deposits (southeastern Crater Flat). The cinder cone and flow deposits are significantly eroded. The cones retain no evidence of original forms or slopes; cone scoria is preserved only where onlapped by lava flows or where dikes have ncreased the erosional resistance of the deposits. Dikes exposed within the dissected cones trend north-south to north-northeast parallel to basin-range faults within Yucca Mountain (Fig. 2). This suggests the older basalts were erupted along preexisting basin-range faults (Crowe and Carr 1980). The dikes are of variable but generally small ) width (0.3 to 2 m) and are laterally discontinuous (Figs. 3 and 4). They pinch and swell and branch and coalesce, with dips ranging from vertical to less than 20° (Fig. 4). Locally the dikes are arcuate with inward dips (Fig. 4). The dikes are feeder intrusives that fed former surface eruptions and underlie and intrude cinder cones. Lava flows of the older basalts (central Crater Flat) thicken adjacent to the cone scoria indicating the cones were the source of the flows. The flows are highly modified by erosion. They lack flow fronts and primary flow topography. Lava surfaces are pediments with areas of desert pavement and local thin soils. Two separate and more extensive lava flow sequences crop out in southeastern Crater Flat (Fig. 2) and are equally modified by erosion. They are locally offset by north-northeast-trending normal faults with displacements down to the northwest. There are no vent or cone scoria deposits associated with either lava sequence. Aeromagnetic data suggest the flows are not continuous in the subsurface. The flows were probably erupted from separate vents that are now buried by alluvium. The basalts of the older cycle are moderately porphyritic; total phenocryst content ranges from 12 to greater than 20 modal percent and averages 13 )

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I , EXPL ANA 71ON ) CORRELATION OF MAP UNITS

I OUArERNARY BASE QTo SURGE

I PLIOCENE BASALT: LAVA FLOWS PYROCLASTIC ALLUVIUM DEPOSITS

7; TERTIARY SLIDE BLOCK OF PALEOZOIC ROCKS [ MIOCENE

rUFF, LAVA, AND SEDIMENTARY ROCKS

I PALEOZOIC SEDIMENTARY ROCKS )

- ~ CONTACT

.* . FAULT, dotted where concealed; bar and ball on downthrown Side

AhL&A&&. Bse of slide block, dotted where concealed Direction of loro flow

* Buried volcanic Center, located from aeromagnetic anomaly

Fig. 2. (Cont.) Generalized geologic map of the southern Crater Flat area (from Crowe et al. 1980). )

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r- Pd-,, Geologic contact ~~~~'I Inferred cinder cone boundary Pb 1 Flow foliation

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Fig. 3. Geologic map of the northern outcrop area of the 3.7-Myr basalt of Crater Flat. Map compiled by direct transfer from uncorrected aerial photographs.

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EM. j tN rv l - ' Geologic contact "°/ -as '-off ho ,,,% Inferred cinder cone t-Z>^s' '° Aft boundary

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j70 (ten t # 20 Strike and dip of cinder c %0c8] ) { cone bedding

3 fewo Pb J Lava flow foliation f' Magnetic Polarity station asi1[ \> 2 \n R-reversed ~~,I~~~f20i ~~N-normal)

Pb Pliocene olivine basalt flow 7Pc Pliocene cone facies

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Fig. 4. Geologic map of the north-central part of the outcrop area of the 3.7-Myr basalt of Crater Flat. Compiled by direct transfer from uncorrected aerial ) Dhotographs. to 14% (Table II). The older basalts can be distinguished from the basalts of younger cycles by their greater total phenocryst abundance and by the presence of clinopyroxene, plagioclase, and olivine as coexisting phenocryst phases in some samples. Olivine (Fo80 75) is the major phenocryst phase in all samples of older basalt. Crystals are subhedral to euhedral, commonly embayed due to resorption and partly to completely altered to iddingsite. Phenocrystic plagioclase (An82 - 68 ), euhedral with moderate normal zoning, is present in the majority of samples. Groundmass feldspars may be zoned from labradorite to alkali feldspar. Rare isolated plagioclase phenocrysts are strongly resorbed and cloudy due to the presence of abundant glass inclusions--these crystals are probably xenocrystic. Clinopyroxene phenocrysts are present in about one- half of the basalt samples studied. Glomeroporphyritic clots are abundant in many samples and have a varied assemblage including olivine, olivine + plagioclase, olivine + plagioclase + clinopyroxene, and plagioclase + clinopyroxene, the latter being most abundant. These clots do not occur in the younger basalts of Crater Flat. As discussed elsewhere, glomeroporphyritic clots ap- parently formed under hi-gher temperature or higher pH20 than the latter stage phenocryst and groundmass assemblages. Except for rare samples showing evidence of resorption, the phenocryst zonation trends and euhedral shapes argue against a xenocrystic origin. Groundmass phases within the older basalts include olivine (largely altered to iddingsite), pyroxene, feldspar, iron-titanium oxides, and rare apatite. Phlogopite, which is pleochroic from clear to pale red, occurs in the groundmass of coarse-grained basalts and as a vein fill in dike rocks. These occurrences suggest that phlogopite formed as a late-stage deuteric phase. Textures of the older basalts are generally intergranular with interstitial spaces between plagioclase laths filled by pyroxene, olivine; iron-titanium oxides, and rare glass. Textures of basalt samples collected from the inter- ior of thick lava flows (southeastern Crater Flat) are diabasic. Vesicle and vein-fill phases in the older basalt are primarily calcite with minor amounts of a pale brown, fibrous to amorphous zeolite(?). K-Ar determinations of two samples of the older basalts yield an average age of about 3.7 Myr (Table I). These dates are consistent with the reversed magnetic polarity of the basalts and indicate a correspondence to the Gilbert Reversed Magnetic Epoch (Mankinen and Dalrymple 1979).

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I *k ) TABLE II

MODAL AND PETROGRAPHIC DATA, CRATER FLAT BASALTSa

1st cycle: 3.7 Myr

Sample F878-14 CF12-6-12 CF12-6-10 CF12-7-6 CFl2-7-1 F378-15 av

Phenocrysts o lvelne 7.4 7.7 S.8 3.3 6.1 8.2 6.4 cilnopyroxene - 3.2 4.4 7.5 - 1.5 2.8 feldspar 6.7 9.3 2.7 5.1 1.5 4.2 amphibole

Groundmass ol IV ne 9.1 2.8 6.5 5.7 9.0 5.5 pyroxene 9.5 12. 0 16.7 10.6 16.5 13.4 13.1 feldspar 63.9 62. 3 60.4 62.9 68.0 60. 1 62.9 amphibole -b biotite 0.4 1.0 tr. 0.2 ilmenite 0.1 0.1 magnetite 3.0 2.6 2.9 5.7 3.3 2.5 3.3 hematite 0.4 0.4 °-lb apatite 0.2 tr. Glass or Mesostasis 0.4 3.9 3.5 1.3

CYesicles)a (7.6) (1.4) (4.0) (1.4) (1.8) (5.2) (3.6) ) Phenocryst Sizes () 0.2-1.0 0.2-1.0 0.2-1.2 0.2-1.5 0.2-1.0 0.2-2.0

Grounduass plagloclase dimensions 0.lx0.01 0.lx0.03 0.2x0.05 0.lxO.01 0.lSXO.015 0.i5XO.015 (average in mm)

Sampl e dike dike flow dike flow flow

aNote: Modal data for the first 14 entries in each coluin are normalized to vesicle-free abundance. Percentages listed as vesicles- represent the volume of vesicles in each sample.

bSymbol tr.- ndicates that mineral is present in trace amounts (less than O.1S); a dash indicates that mineral or phase is not present.

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I KL TABLE 11 (Cont.)

MODAL AND PETROGRAPIIIC DATA, CRATER FLAT BASALTSa

2ndCycle: 1.1 yr

Northern Bl ack Cf______Red Little .4.01 Little N.E. - C~r2-i t B 8- 10o f8?8-1 f078-4 FD78-5 CffI2-4- CI24-6 C12-141 FI2-4-12A FB78-9- CF12-4-13A CF12-4-13 av Phenocrysts 0.8 1.1 2.0 1.1 2.6 1.8 1.5 2.6 1.6 1.4 2.4 3.4 1.9 cl inopyroxene feldspar amphibole _ 0.5 0.9 0.1 Groundmass oilv Tne 14.7 8.7 10.1 3.7 6.3 6.0 10.1 9.9 4.3 3.8 9.3 5.6 7.7 pyroxene 5.7 4.8 4.5 11.4 8.1 7.8 11.3 7.7 7.7 13.4 7.8 9.5 8.3 feldspar 67.7 66.2 62.8 77.8 70.7 75.4 68.1 62.2 66.8 62.3 68.0 b 74.7 61.6 6t; amph1bole .3 tr. b1otite -rb Ilmenite 0.1 - 0.6 tr. tr.b magnetite 3.4 3.0 2.0 2.6 2.6 3.8 3.1 3.5 6.9 6.0 6.9 8.6 4.4 hematite 1.3 0.1 apatite Glass or Hesostasis 6.5 16.3 18.6 2.8 9.8 5.3 5.9 14.1 12.8 0.7 11.7 9.8 9.5 (Yesicles a (4.6) (5.4) (10.8) (29.8) (14.0) (20.2) (4.6) (9.0) (24.8) (16.2) (33.2) (44.1) (18.1) Phenocryst Si zes (om) 0.2-1.0 0.2-1.2 0.2-0.5 0.2-1.0 0.2-0.75 0.2-2.0 0.2-1.0 0.2-1.0 0.2-0.5 0.2-0.5 0.2-0.5 0.2-0.5 Groundnass plagloclase dimensions 0.2x0.025 0.2x0.02 0.2x0.02 0.18x0.02 0.02 .02 0.2x0.03 0.2xO.025 .2xO.02 0.08X0.00 0,O.005 0XO.01 0O.01 (average in n)

Sample flow flow flow dike flow flow flow bomb bomb flow bomb bomb

aNote: Modal data for the first 14 entries in each column are normalized to vesicle-free abundance. Percentages listed as 'vesicles represent the volume of vesicles in each sample.

bSymbol tr.' indicates that mineral is present in trace amounts (less than 0.11; a dash indicates that mineral or phase is not present. 7

TABLE II (Cont.) ) MODAL AND PETROGRAPHIC DATA, CRATER FLAT BASALTS

3rd Cycle: 0.3

Lathrop Wells

CF11-7-1 CF11-7-2 F378-7 av Phenocrsts 2.6 3.0 2.9 .2.8 cli nopyroxene feldspar amphi bole Groundmass ol1vine 6.9 6.2 9.3 7.5 pyroxene 4.1 4.6 2.6 3.8 fel dsoar 68.8 55.7 63.2 62.6 amphibole biotite ilmenite magneti te 2.2 2.4 1.2 1.9 hematite apati te Glass or Mesostasi s 15.5 28.1 20.8 21.5 ) (Vesicles)a C ) i: ) C ) Phenocryst Sizes (mm) 0.2-1.5 0.2-0.5 0.2-1.25 Groundnass pl agioclase dimensions 00.15x0.01 0.12x0.02 0.lxO;01

Samp e fl ow bomb flow

aNote: Modal data for the first 14 entries in each column are nomal i zed to vesicle-free abundance. Percentages listed as vesicles' represent the volume of vesicles in each sample.

)

1 1

I .. Magma volume calculations were determined for the older basalts based on outcrop area, area of inferred subsurface outcrop from aeromagnetic data, and measured thicknesses. Lava volumes were converted to magma volumes assuming a magma density of 2.7 g/cm 3; pyroclastic volumes were calculated using a cone porosity of 25% and assuming that 400 of the tephra was deposited over one cone diameter from the vent. Bomb density values were taken from McGetchin et al. (1974). The total volume of magma erupted during the older basalt cycle is about 4.0 x 1O-1 km3.

IV. 1.1-MYR BASALT CYCLE Cinder cones and associated lava flows dated at 1.1 Myr define a northeast-trending arc within the central part of Crater Flat (Figs. 2 and 5). This basalt cycle may follow one of a system of faults of northeast trend within the Walker Lane fault system in the southern Great Basin (Carr 1974; Crowe and Carr 1980). Little Cones, the southwesternmost center of the arc, consists of two separate cinder cones. The southwestern cone (base diam 0.3 km) is deeply rilled with approximately 200 of the cone removed by erosion. The cone is breached on the south side by the vent of a small lava flow. This flow is largely concealed by colluvium with local outcrops; original flow margins can be inferred from slope changes in the colluvial surface. Aeromagnetic data suggest the presence of an older flow, now buried. This flow appears to have extended in the same direction as the younger flow, but about 1 km farther. The second cone of the Little Cones (Fig. 2) has an inferred base diameter of 0.2 km and is equally modified by erosion. Based upon outcrops and aeromagnetic data, this cone appears to lack associated lava flows. Red Cone and Black Cone, the middle cones in the northeast-trending arc, are very similar in field occurrence, mineralogy, and petrology; Red Cone will be described in some detail. The oldest deposits of Red Cone consist of small coalesced cinder cones (base diameter <0.2 km) that occur southeast and south of the main cone (Fig. 6). These cones are deeply eroded and onlapped by aa flows from Red Cone. Red Cone itself, the largest cone of the center, is a typical Strombolian cinder cone. It has a base diameter of 0.5 km and origin- ally rose about 80 m above the alluvial surface. Approximately 20% of the cone has been removed by erosion. Two small dikes, which are probable off- shoots of the main conduit, are exposed in the western cone wall. The summit )

I A I

Fig. 5. 1.1-Myr basalt cycle viewed from the southwest. Volcanic cones include from right to left, Little Cone, Red cone, and Black Cone. The northernmost cone is not visible in the photograph.

crater of Red Cone was infilled by inward-dipping spatter (Fig. 6) with bombs exceeding 2 m in length. This spatter draped the vent of Red Cone during the waning stages of activity when the ejection velocity of bombs was insufficient to crest the walls of the summit crater. During growth of Red Cone, aa flows extruded from southeastern and possibly southwestern flank vents. These stub- by flows partly surrounded and onlapped the older cinder cones and extended slightly more than 1 km from the vent. Steep lava flow fronts are preserved, althouch primary surface flow toooaraphy is completely modified by erosion.

16 a--- a- O -

- Geologic Contact _ Inferred cinder cone boundary - Basalt dike

20 Strike and dip of cinder cone bedding is Aid 05ei I p Lava flow foliation - I Magnetic polarity station R-Reversed C. \~~~~~~~~~~N-Normal 20 )O Lava flow source and \4 "I. flow direction 1 / Oc Quaternary scoria colluvit

- __. Ocf Quaternary cone facies ats Quaternary vent spatter .I I~~~~~~ I -# t Quaternary basalt I .

Scale 1:12000 I. I -' Kilometefs I6 - 0.5 1.0

Fig. 6. Geologic map of the Red Cone volcanic center. Compiled by direct transfer from uncorrected aerial photographs. P

- 7- - -

Fig. 7. Black Cone volcanic center. Note the dissection of the cinder cone, capping lava fill sequence, preservation of original lava flow fronts, as well as modification of the original lava flow surface.

The Black Cone center includes several coalesced cinder cones located directly south of Black Cone (Fig. 7). Aa flows vented from the north- northeast and southeast sides of Black Cone (Fig. 2). Black Cone is capped by inward-dipping flows that ponded within the summit vent of the cinder cone. The northernmost center of the 1.1-Myr basalt cycle is more deeply in- cised than other centers of the arc. Margins and flow tops of lava outcrops are completely modified by erosion. Local scoria deposits in the northern part of the center are probably remnants of the original cinder cone. All primary cone features have been destroyed and the deposits are lower i0

I topographicaliy than~the lavas. The greater dissection o the northernmost canter probably is due to both higher elevation and a steeper drainage gradi- ent within this part of the Crater Flat basin. The basalts of the intermediate age cycle are aphyric to sparsely porphyritic (less than 34 total phenocrysts, Table II). Olivine is the only major phenocryst phase and occurs as subhedral to euhedral crystals that are fresh or exhibit minor alteration to iddingsite at grain margins and along frac- tures. Olivine compositions range from Fo77 to F 62 and are more iron rich than olivines from either the older or younger cycles. Basaltic hornblende (high-Ti amphibole) occurs as phenocrysts in one cone from the Little Cone center and in the groundmass of basalt samples from Red Cone. It is markedly pleochroic (shades of red-brown) and is fringed by reaction rims composed of granular intergrowths of plagioclase, pyroxene, and iron-titanium oxides.

Plagioclase (from An71 to more alkaline compositions) is the major groundmass phase, present as microlites and as larger crystals that approach micropheno- dryst size. Additional groundmass phases are olivine, pyroxene, ron-titanium oxides, and variable amounts of deep brown glass. A two-pyroxene groundmass assemblage (high-Ca and low-Ca pyroxene) occurs in Black Cone and the northernmost cone. Textures of the lavas are mostly intergranular, though some have pilotaxitic textures. Samples containing appreciable amounts of groundmass glass show intersertal or hyalopilitic textures. The basalts are largely un- altered; clays are present as vesicle filling along with small amounts of calcite. K-Ar ages for Little Cones, Black Cone, and northernmost cone centers are all about 1.1 Myr (Table I). Magnetic polarity determinations for all the centers are reversed, in agreement with the K-Ar ages; the basalts of the 1.1- Myr cycle thus belong to the Matuyama Reversed Magnetic Epoch. Calculated magma volume for the basalt cycle is 3.0 x 10 1 km3.

V. 0.3-MYR BASALT CYCLE The 0.3-Myr basalt cycle of Crater Flat includes the Lathrop Wells center located about 5 km southeast of the southeastern edge of Crater Flat (Fig. 2). Here, a large cinder cone with two small satellitic cones overlie and are flanked to the east by aa flows (Fig. 8). The satellitic cones are overlapped by deposits of the main cone. The large cone, referred to as the Lathrop Wells cone, has a height/width ratio of 0.23. The summit crater and the cone

I I ) I- o 0

3 'U a, 4.'

4 u 0 I u .25 li a I- . A ' ..A . i E I 0 L

j 1% r I 4- 4.) 'I L.

.U '4- -,- N'"% 8

f Lj .5-u toI. f ) U, - I a, 4 C'U u u

i *'':

-C

e % a c ,.-I- I, , f../0 , - - '_ , , .' _-, v A _4.' CJs E-l - C )

I ;I ------tWw- - ~~f * es:

Fig. 9. Well-bedded base-surge deposits exposed in the -northwestern part of the Lathrop Wells Cone. are elongate to the northwest, probably due to prevailing winds from the southeast. The cone -appears unmodified by erosion except for minor slumping of steep cone slopes. The probable oldest deposits of the Lathrop Wells cone are well-bedded base-surge deposits (Fig. 9) that are exposed only on the northwest side of the cone where they overlap a topographic ridge upheld by tuff. They probably underlie the scoria deposits of the cone and thus record an epi- sode of phreatomagmatic activity during the early eruptive stages of the center. Two aa flows vented at several sites along the east flank of the Lathrop Wells cone (Fig. 8). Flow vents are marked by arcuate spatter ridges

I .i extending east and southeast of the cone. The lavas have unmodified flow margins and rubbly flow surfaces consistent with their young age. They are local- ) ly covered by aeolian sands. The basalts of the Lathrop Wells center are sparsely porphyritic with olivine as the major phenocryst phase (3 modal percent). In thin section the basalts can be distinguished from the intermediate cycle basalts by slightly greater olivine contents and a greater abundance of deep brown interstitial glass. Moreover the olivine phenocrysts have slightly more magnesium-rich cores (Fo8 0_77) than olivines of the 1.1-Myr cycle (Fo7 7 -76). Groundmass phases in the basalts include plagioclase (zoned from An68 to more alkaline compositions) and minor amounts of olivine, pyroxene, and iron-titanium oxides plus interstitial glass. Basalt textures are hyalopilitic to pilotaxitic and reflect the high content of groundmass glass. Lavas of the Lathrop Wells center have been dated at about 0.3 Myr, consistent with the lack of erosional modification of both cones and flows. The basalts are normally magnetized and thus assigned to the Brunhes Normal Mag- netic Epoch. Calculated magma volume is about 0.5 x 10 km3

VI. MINERAL CHEMISTRY ) Mineral compositions of basalts from Crater Flat were determined by electron microprobe. An automated Cameca electron microprobe was used, with ac- celerating potential fixed at 15 kv and sample current at 0.015 A on thorium. Counts were collected for a maximum of either 20 s or 30 000 counts for each element. Complete tables of representative mineral compositions are included in the Appendix (Tables A-I to A-V). Brief descriptions of the mineral data and applications to petrology follow. A. Olivine Data on olivine for the basalts of Crater Flat are summarized in Fig. 10. This figure indicates continuous zonation of most olivine phenocrysts; the zoning is normal, without notable reversals. Phenocryst olivine is more Mg- rich than groundmass olivine; the apparent overlap of Red Cone phenocryst and groundmass compositions in Fig. 10 is not real because two samples with different zonation ranges have been superimposed in one diagram. Olivine phenocrysts in fact may have formed with rims as Fe-rich as the groundmass olivines; if so, the Fe-rich rims have been totally altered.to iddingsite. )

I , N

The distribution of re and M between olivine and basalt c melt may provide evidence for or against equilibrium crystallization; i equilibrium crystallization has occurred and the composition of early crystallized olivines can be determined, the olivine-melt system can be used to estimate the highest (or first) temperature of olivine appearance. The Mg:Fe composition of the basalt may be assumed to approximate the initial liquid composition if no other Fe-, Mg-rich phases preceded olivine in the crystallization sequence. Experimental studies (Knutson and Green 1975) indicate that hawaiites similar to the basalts of Crater Flat are cosaturated with olivine and clinopyroxene ± plagioclase ± amphibole. Because olivine forms early, the natural olivine/rock compositions can be compared using experimentally calibrated olivine/liquid compositions (Shibata et al. 1979). Experimental studies (Roedder and Emslie 1970; Longhi et al. 1978) docu- ment an exchange distribution coefficient (KD) that is constant at 0.30 to 0.33 for basalts similar in composition to the basalts of Crater Flat. The appropriate KD curves are drawn in Fig. 11, with plotted points representing

3.7-Myr Basalts

* . P . . - Fa I I , I, . _ I -m I I I Fig. 10. Ca-Mg-Fe compositions of pyroxene and amphibole, and Mg-Fe compsition of livine and biotite in basalts of Crater Flat.

7.,

I *' I

)

Mg -

Rfld0miL . . F0. --U-- I I ru

)

FI i| & n x X i l i i i Fa

Fig. lO.(Cont.) Ca-Mg-Fe compositions of pyroxene and amphibole, and Mg-Fe composition of oli- ) vine and biotite in asalts of Crater Flat .

2 CdMg I\ A AX A\ /E CFS 1.1-Myr Basalts go Lttl Cone S.W.

* Groundmass pyroxent 0 Phenocryst gnvine

Ma V V V V V V V V ^Fe

Fal I fI l; iII I I I's

CaM

Mg V V

, ~_. , . . . *At rol .- [ . - I I _ IIII rar.

Fig. 1O.(Cont.) Ca-Mg-Fe compositions of pyroxene and amphibole, and Mg-Fe composition of olivine and biotite in basalts of Crater Flat.

£5

I 'I 1.1-Nlyr 3asaltu / \ ~~~~~~~~~~RedCne ! /* Groundmass Pyroxen. * &oundmassi mphlbole

Pnocryst * Groundmoss

mg. / y v V V V V V V V _ Ft

Rim ffi I & 11ri9 a ., i i i i - iFo

)

Fo, 4dli4 - I I I IF

Fig. 10.(Cont.) Ca-Ilg-Fe compositions of pyroxene and amphibole, and Mg-Fe composition of olivine and biotite in basalts of Crater Flat. )

I I -0.8

C_ 5 0.4-

0 No K c- 0.2 N u..

0~~~~~~~~~

0.2 0.4 0.6 0.8 1.0 (\x Fe \~)liquid FXe+ M8

Fig. 11. Comparisons of cation fraction Fel (Fe 2 + Mg) in olivines and liquids (inferred from rock compositions) of the basalts at Crater Flat. Symbols represent (1) the 3.7-Myr basalts, (2) the 1.1-Myr basalts, both nepheline and hypersthene normative, and (3) the 0.3-Myr basalts. Arcuate lines enclose the compositional range where olivine and liquid may be in equilibrium; olivine resorption or accumulation will result in points plotting above the arcuate lines, whereas olivine loss will result in points plotting below the arcuate lines. -The composition of Fe' in the original liquid is assumed to be 0.9 x total Fe, following the arguments of Shibata et al. (1979) for maintaining a liquid composition near fayalite-magnetite-quartz stability.

I 01 I olivine/rock pairs from Crater Flat samples. Considerable effort was under- taken to find the most Mg-rich olivine phenocryst cores in the basalt samples in order to obtain an analysis approaching the composition of the first olivine that crystallized from the basalt. If the first-formed olivine reacts with the melt, it will become more Fe rich and project upward above the KD = 0.30 to 0.33 envelope in Fig. 11. A displacement to the left of the envelope will occur if olivine has accumulated in the sample, resulting in an increase of the apparent Mg content of the host liquid. From Fig. 11, it is apparent that olivine phenocryst cores in several of the 3.7-Myr basalts are more Fe rich than permitted by an equilibrium model; plotted points are displaced upward, suggesting re-equilibration of the initial Mg-olivine cores to a more Fe-rich olivine. This interpretation is supported by the coarse grain size and the scarcity of glass or mesostasis in the 3.7-Myr basalts, features in accord with slow cooling that would permit re- equilibration of olivine with subliquidus Fe-enriched liquids. It is possible to estimate the liquidus temperature (first olivine preci- pitation) using relations developed by Roedder and Emslie (1970) and expanded by Longhi et al. (1978) and Leeman (1978) using samples that maintain the equilibrium olivine/melt relation (Fig. 11). These temperature estimates are less accurate in samples with high Na and K contents, and although the hawaiites of Crater Flat are not alkali rich, they contain enough Na and K to yield anomalously low temperature estimates. Leeman (1978) has provided some guidelines for evaluating temperature estimates in alkaline basalts; using his Fig. 5 and K, relationships, we estimate the temperature of olivine appearance at 1200 0C in the basalts of Crater Flat. This estimate is crude because of the moderate alkali content of Crater Flat basalts and carries a large uncertainty of ±750C. B. Feldspar With the exception of feldspar phenocrysts in the 3.7-Myr basalts, the most Ca-rich plagioclase cores are high-Ca labradorite (An70) in the basalts of Crater Flat. Plagioclase zonation is limited in some samples; in other samples disequilibrium zonation may extend in shallowly bowed paths across the feldspar solvus to a Ca-free alkali feldspar composition (Fig. 12). Note that the cores of plagioclase phenocrysts in the 3.7-Myr basalts of Crater Flat are significantly Ca enriched (An 80; Fig. 12 and Table A-II). The Ca-enriched phenocrysts of the 3.7-Myr basalts have important implications for the petrogenesis of basalts of this cycle. )

28 Analogous to the formation of the most Mg-rich olivine in the initial stages of a crystallizing melt, Ca-rich plagioclase cores represent the first plagioclase to form. Drake (1976) described several empirical relations that relate plagioclase/melt compositions to crystallization temperature. These relations are reliable for a broad range of basalt types, provided the vapor

pressure of water (pH20) is not high during crystallization. Experimental data (Knutson et al. 1975) indicate that plagioclase is a near-liquidus phase

for ow-H 20 hawaiite compositions, and the coupled Al-Ca substitution in plagioclase resists re-equilibration at subliquidus temperatures even in slowly cooled basalt samples. Using plagioclase/rock relations fitted to the plagioclase/melt relations of Drake (1976), temperature estimates for the basalts at Crater Flat are listed in Table III. The average plagioclase/rock temperature estimate for the basalts of Crater Flat is 1208 ± 160C. Within the error for this method (80'C), this

3.7-Myr Bash

* Phamoyg . & _oumdm

Fig. 12. Or-Ab-An compositions of feldspar in basalts of Crater Flat. 29

I I 1.14Myr Bsaflts Orclrwonas Fewsa;u of ) NOrU Con*

Rod Con* )

a ae OVM, \~~~~~a ~A L Ab

L"U Ccn. N. L

Lb Fig. 12.(Cont.) Or-Ab-An compositions of feldspar in basalts of Crater Flat. )

I 1 03-Myr Basafts Groundmass Foldspen. Lathrop won$ Cn*

Fig. 12. (Cont.) Or-Ab-An compositions of feldspar in basalts of Crater Flat.

TABLE III

TEMPERATURE ESTIMATESa FRO4 PLAGCLASE/ROCK CPOSITIONS IN THE BASALTS CF CRATER FLAT

Relic or Groundmass Phenocryst 3.7-Myr Basalts Plagioclase Plagloclase

CF12-6-1 1218"C 1374 C 1.1-Myr Basalts

FB78-10 (Northern Cone) 1218-C FB78-5 (Black Cone) 1187 C CF12-4-11 (Red Cone) 1201 C 1285 C CF12-4-12A (Little Cone S.W.) 11859C CF12-4-13A (Little Cone N.E.) 1208%C

0.3-Myr asalts

CF11-7-1 (Lathrop Wells) 1225C

1Uncertainties in lagloclase/rock temperature estimates are +80C.

I TI estimate is in close accord with the olivine/rock temperature estimate of 1200"C. The similarity in calculated temperatures is expected from experimental and petrographic evidence for multiple phase cosaturation in hawaiites. The 3.7-Myr basalts of Crater Flat yield phenocryst plagioclase/rock temperature estimates that are significantly higher than the plagioclase/rock temperature estimates obtained from groundmass plagioclase. These high temperature estimates for the older basalts are based on phenocryst plagioclase grains with very Ca-rich cores (An7 5 -83, Fig. 12). Drake (1976) noted that basalts that crystallized under appreciable pH20 generated anomalously Ca-rich plagioclase, although this effect was reversed at pH20 > 10Kb. Although the pH20 effect has not been quantified for anomalously Ca-rich plagioclase, the data compiled by Drake (1976) indicate that Ca-rich plagioclase phenocrysts, as found in the 3.7-Myr basalts of Crater Flat, may occur by crystallization at pressure (that is, at moderate depth) before eruption. The same effect may also lead to the anomalously high temperature estimates from very rare relic plagioclase in the basalts of Red Cone (Table III). C. Pyroxene Pyroxene end-member compositions in the basalts of Crater Flat are summarized in Fig. 10 for the components CaSiO3 - MgSiO3 - FeSiO3. Representa- tive pyroxene analyses are listed in Table A-III. The basaltic pyroxenes are diopsidic augites with a narrow range of compositions. Pyroxene minor-element contents are very low; Al+Ti+Na+Mn+Cr contents are less than 0.35 and commonly below 0.2 on a six-oxygen mineral formula basis. The groundmass pyroxenes of all basaltic cycles are broadly similar and less Mg rich than the clinopyroxene phenocrysts that occur in basalts of the 3.7-Myr cycle (Fig. 10). The Mg-rich pyroxene phenocrysts in the older basalts represent early crystallization, coinciding with the development of olivine and feldspar phenocrysts at depth before eruption. Coprecipitation of clinopyroxene, olivine, and plagioclase phenocrysts in the 3.7-Myr basalts of Crater Flat is indicated by the occurrence of all three phases in some glomeroporphyritic clots. Other compositional features also point to the earlier growth of Mg-rich clinopyroxene phenocrysts: the phenocrysts have a distinctly higher Al/Ti ratio (6.5 vs

3.5, Table A-III), and the phenocrysts commonly contain Cr203 in amounts above microprobe detection limits (0-0.1 wt.%), whereas groundmass pyroxenes seldom contain detectable Cr.

I . The groundmass- minerals of dike and lava flow samples from the two northernmost cones of the 1.1-Myr cycle (Fig. 2) include both low-Ca and high- Ca pyroxenes (Fig. 10). These cones consist of hypersthene-normative basalts in which low-Ca pyroxene and olivine both formed during late crystallization. If temperatures of groundmass crystallization are inferred using the two- pyroxene geothemometer of Wells (1977), the results are anomalously high (2100C). These unreasonably high temperature estimates indicate that the two-pyroxene association at the northernmost cone and at Black Cone is not an equilibrium pyroxene intergrowth. D. Oxide Mineralogy The common primary oxide mineral in Crater Flat basalts is magnetite, with much smaller amounts of primary Ilmenite (Table A-IV). Scarcity of primary ilmenite can be related to the alkaline nature of Crater Flat basalts, in 2 which Ti is incorporated into aluminous pyroxene (R +TiAl206) rather than ilmenite (FeTiO3) to liberate the silicon required by formation of alkaline phases. Magnetite and ilmenite both occur in the older and intermediate age basalts, although the oxidation and exsolution of magnetite to ilmenite, hematite, and pseudobrookite obscure the primary magnetite compositions and pre- vents useful application of magnetite-ilmenite geothermometry. Microprobe studies show that the groundmass Fe-Ti oxides in the basalts of Crater Flat are low in minor elements such as Cr,. Al, and low in Mg/(Mg Fe). The spinels enclosed in and therefore coprecipitating with olivine phenocrysts are considerably more Mg-, Al-, and Cr-rich than spinels found in the basalt groundmasses (Table A-IV). Early coprecipitatfon of olivine and Mg-, Cr-, Al-rich spinel is consistent with the evidence from clinopyroxene- olivine-feldspar glomeroporphyritic clots suggesting that saturation in most major phases began early in the crystallization histories of the basalts at Crater Flat. E. Amphiboles and Biotites Biotite grains occur in the groundmass of some samples of the older basalt cycle. These grains are Mg-rich phlogopites (Fig. 10), which formed late in the crystallization history of the host basalts. The occurrence of biotite in the 3.7-Myr basalts can be attributed to protracted late-stage crystallization under relatively hydrous conditions. Amphibole grains occur both as phenocrysts and as groundmass minerals in basalts of the 1.1-Myr cycle (Fig. 10). There is little difference in

I .I i .

composition between phenocryst or groundmass amphibole; both are high-Ti basaltic hornblendes. The occurrence of amphibole is restricted to basalts of Red Cone and Little Cone that are located at the southwest end of the 1.1-Myr basaltic arc. The occurrence of amphibole as phenocrysts indicates that the host magmas were not dry and may have been relatively water rich at the time of eruption. The inference of high volatile (water) content in these samples is supported by the very high vesicle content (33 to 44%; Table II) of basalts with amphibole phenocrysts. Representative amphibole analyses are included in Table A-V. The importance of amphibole in the basalt fractionation histories is discussed in the following section.

VII. MAJOR-ELEMENT CHEMISTRY OF THE BASALTS AT CRATER FLAT Whole-rock major-element analyses (Si, T, Al, Fe, Mg, Ca, Na, K, and P) were obtained by electron microprobe analysis of glass beads fused from whole- rock powders on an Ir-strip resistance furnace. Details of this technique and analytical uncertainty are given by Baldridge (1979). The basalts of Crater Flat all fall within the classification of hawaiite as used by Best and Brimhall (1974) for this abundant basalt type in the western Colorado Plateau. The principal features of this basalt type are (1) normative plagioclase An content between 40% and 52% and (2) transitional alkaline affinities, with compositions generally near the normative nepheline/ hypersthene divide. The major-element chemistry and calculated cation norms for basalts from Crater Flat are listed in Table IV. The textural and mineral- ogical descriptions of Colorado Plateau hawaiites by Best and Brimhall (1974) are very similar to the descriptions of the basalts of Crater Flat outlined above, including the common occurrence of diopsidic augite + bytownite glomeroporphyritic clots as in the 3.7-Myr basalt cycle at Crater Flat. An important characteristic of hawaiite basalts is their transitional alkaline composition. MacDonald and Katsura (1964) defined a generally ac- cepted division between tholeiitic and alkaline basalts based on a plot of total alkalis versus silica content (Fig. 13); hawaiites such as the basalts of Crater Flat project slightly above the MacDonald-Katsura line. A recent revision of alkaline and tholeiitic lineages by Chayes (1979) shifted the dividing line upwards, leaving an undefined zone within which most hawaiites plot. This transitional alkaline characteristic is an important feature of Miyashiro's (1978) classification for Straddle-A type alkaline basalts. The

I It TABLE V

BASALTS OF CRATER FLAT: MAJOR-ELEMENT CHEMISTRY AND CALCULATED CATION NORMS

3.7-Hbr Cycle 1.1-Ntr Cycle 0.3-Myr Cycle Northern 81 ack Red Little Little Little Cone Cone Cone Cone S.W. Cone N.E. Cone N.E. Lathrop el1s Calc. CF CF CF Fe Fa FB FB CF CF CF CF CF CF Parent 12-6-10 12-6-12 12-7-8 78-15 78-17 78-10 78-5 12-4-11 12-4-12A 12-4-13A 12-4-138 11-7-1 11-7-2

''1°2 48.4 49.4 48.5 49.3 48.1 48.8 49.9 51.0 50.9 47.8 48.7 47.4 48.5 48.4 1102 1.55 1.60 1.62 1.64 1.77 1.70 1.41 1.45 1.19 2.05 2.31 2.24 1.82 1.76 A) 03 13.3 15.5 15.2 15.7 :15.2 15.6 17.0 17.2 17.2 16.0 16.3 15.8 16.6 16.4 FtO 11.0 10.8 11.5 11.3 11.8 11.2 9.9 9.9 9.8 10.8 11.5 11.5 10.8 10.7 Milo 0.21 0.23 0.21 0.23 0.21 0.24 0.21 0.19 0.23 0.21 0.14 0.17 0.20 0.19 ill° 10.4 7.1 8.6 7.0 7.8 7.0 5.0 5.2 5.2 4.8 5.1 5.1 5.9 5.8 (:dO 10.9 9.8 9.2 9.1 10.2 9.2 9.0 8.7 8.8 10.7 8.6 9.6 8.8 9.1 11a 0 2.42 3.00 2.93 3.05 2.71 3.08 3.39 3.42 3.36 3.82 3.79 3.71 3.50 3.70 K20 1.23 1.58 1.37 1.49 1.50 1.53 1.57 1.60 1.69 1.91 2.07 2.03 1.76 1.77

P205 0.55 0.62 0.68 0.71 0.71 0.74 1.13 1.09 1.20 1.47 1.21 1.24 1.14 1.40 100.0 99.6 99.8 99.5 100.0 99.1 98.5 99.8 99.6 99.6 99.7 98.8 99.0 99.2

Qza - - Or 7.2 9.4 8.1 8.9 8.9 9.1 9.4 9.5 ' 10.0 11.4 12.3 12.2 10.5 10.5: P1 42.8 51.3 50.4 52.4 48.6 52.5 58.1 57.8 57.4 44.5 51.2 45.0 55.2 53.1 (An) (50.9) (47.2) (47.8) (47.4) (51.3) (46.8) (46.6) (46.7) (1^17.0) (47.5) (41.8) (46.2) (43.5) (43.5) tie 0.0 - 0.4 - 6.7 2.7 '5.8 0.7 2.1 23.1 16.4 13.6 12.8 16.8 13.2 8.5 7.4 7.2 18.1 10.8 15.7 9.5 10.4 0.0- 1.1 1.3 4.9 2.1 9.3 13.0 112.9 - 1 (I 22.2 17.0 21.7 16.0 20.0 17.8 9.1 6.9 7.2 12.0 15.8 14.2 17.8 17.3 'it 1.3 1.3 1.3 1.3 1.4 1.3 1.2 1.2 1.2 1.3 1.4 1.4 1.3 1.2 11. 2.2 2.2 2.2 2.3 2.5 2.4 2.0 2.0 1.7 2.9 3.2 3.2 2.6 2.5 Ap 1.2 1.3 1.4 1.5 1.5 1.6 2.4 2.3 2.5 3.1 2.6 2.6 2.4 3.0 aNote: cation norms are calculated assuming atomic ratio Fe2 /Fe 3 + * 9/1. Row (An) represents the cation percent anorthtte in plagioclase (1't.). All analyses obtained by electron microprobe analysis of fused rock powder. The derivation of the calculated parent composition (Mg - 0.65) s dIscussed on the following page. primary requirements of this classification are that the less evolved members of a volcanic sequence straddle the dividing line between normative nepheline and hypersthene fields, whereas the more evolved members of the sequence include two distinct fractionation trends, one into the nepheline field and another into the hypersthene field. The atomic ratio of Mg/(Mg+Fe), or Mg', provides a reliable measure of how evolved a sample is. Figure 14 is a plot of Mg' vs nepheline (Ne) or hypersthene (Hy) normative composition for the basalts of Crater Flat. The diverging trends in Fig. 14 are characteristic of Miyashiro's Straddle-A classification. A low Mg' ratio near 0.5 (Fig. 14) is also characteristic of hawaiite basalts (Knutson and Green 1975; Green et al. 1974). This feature is perhaps the most important of all hawaiite charac- teristics, for the low Mg' ratio requires that all hawaiites be derived from more primitive basalts. Where hawaiites are erupted along with their probable parental precursors, the parental magmas are distinctly alkaline (Green et al. 1974). Where, as at Crater Flat, the parental magmas do not occur at the surface, an alkaline parentage can only be inferred. A parental composition, with Mg' = 0.65, has been calculated from the least evolved basalts at Crater Flat: those of the 3.7-Myr cycle that plot near the straddling nepheline-hypersthene divide in Fig. 14, with Mg' compositions of 0.57. Inspection of bivariate oxide plots has shown that olivine, amphibole, and clinopyroxene are phases that might be removed from an Mg' = 0.65 basalt to arrive at the Mg' = 0.57 Crater Flat composition. By trial- and-error modeling of olivine, clinopyroxene, and amphibole fractionation, we have found that clinopyroxene and/or olivine addition to the Mg' = 0.57 composition lead t calculated parental compositions that also plot near the straddling position. These calculated parental magmas are represented by the question mark at Mg' = 0.65 in Fig. 14. Amphibole addition results in parental compositions that plot well within the nepheline field, which is not expected for the least-fractionated members of a Straddle-A association. It would be highly unlikely for the least evolved magmas to arise far from the straddling position, fractionate towards that position, and then diverge from the straddling position in two opposed directions. These calculations suggest that olivine or clinopyroxene may be involved in the evolution of the 3.7-Myr Crater Flat basalts from parental magma. The parental composition that is listed in Table IV requires fractionation of 5 olivine plus 12% clinopyroxene to lead to the Mg' = 0.57 basalt of the 3.7-Myr cycle. With this 4 C a.

S10, Fig. 13. Variation of Na2O + K20 vs SiO2 (wt%) in the basalts of Crater Flat. Symbols represent (1) the 3.7-Myr basalts, (2) the 1.1-Myr basalts,,and (3) the 0.3-Myr basalts.

H4E I Ny12 -

&U U4

Noi4

Fig. 14. A plot of cation normative hyersthene or nepheline content vs atomic ratio of Mg to Mg + Fe2+ in the basalts of Crater Flat. Fe2+, symbolized by "Fe*, is standardized as 90" of total atomic Fe analyzed as FeO. Weight ratios of La/Sm and wt% TiO 2 are shown for various normative fields. Divergence into both hypersthene- and nepheline-normative fields from less-evolved "straddling" compositions Is characteristic of Miyashiro's (1978) straddle-A-type alkaline basalt clan. The arrow labelled "kaersutite removal" is discussed in the text and reproduced in Fig. 15.

I :, fractionation scheme, the calculated parental magma has an A1203/CaO ratio of 1.2, a ratio that we have chosen as realistic for the parental magma (for support of this argument, see Frey et al. 1978). No attempt has been made to calculate parental compositions for the younger basalts of Crater Flat, those of the 1.1- and 0.3-Myr cycles, because those basalts were certainly derived by more complex fractionation histories. To a certain extent these complexities can be seen in Fig. 14, particularly in the origin of the hypersthene-normative 1.1-Myr basalts. The solid arrow in Fig. 14 shows a trend of amphibole fractionation; the evidence for amphibole fractionation includes the steady increase of La/Sm along this trend (a point discussed below) and a steady decrease of TiO 2 along this trend. No other early-crystallizing phases except amphibole could account for these TiO 2 and La/Sm trends in a hawaiite composition. The evidence of Ti variation is particularly telling, for no other phase but amphibole is likely to remove appreciable amounts of Ti from the fractionating melt. The evolved Ne-normative basalts are almost certainly products of complex olivine, clinopyroxene, and amphibole fractionation. The irregular variation of La/Sm and Ti with Mg' (Fig. 14) suggests that amphibole fractionation was prominent in some samples (for example, La/Sm >10) but less important in others, particularly within the nepheline field. Parental compositions have not been calculated for the nepheline-normative basalts, because the many possible fractionation histories involved introduce a large degree of ambiguity. A projection of normative mineral compositions employs chemical distinc- tions based on most of the major elements. Six of the major elements or major element groups (SiO2. A1203 , CaO, FeO-MgO, Na2 -K 20, TiO2) play a significant role in plotting the position of a basalt on the projection of Fig. 15. In this multielement projection, the three Crater Flat basaltic cycles can be distinguished. As in Fig. 14, there is a lobe of the 3.7-Myr basalts extending towards the hypersthene-normative 1.1-Myr basalts. The arrow in Fig. 15 is consistent with a single model of amphibole fractionation, a model more fully developed in Fig. 14. However, because of the approximate 2.6-Myr age difference between these two basalt cycles, they cannot be the products of a single fractionation event. This point is important, for it proves that a specific pattern of amphibole fractionation has been repeated at least twice among the basalts of Crater Flat.

I * OLH Hy

Fig. 15. Cation-basis normative diopside-olivine-nepheline-hypersthene plot for the basalts of Crater Flat. Symbols are defined in Fig. 13; the question mark signifies a proposed parental composition (Table IV), and the arrow reproduces the kaersutite removal trend of Fig. 14.

A comparison of Figs. 13, 14, and 15 leads to the following conclusions concerning the origins of the basalts at Crater Flat. First, the three cycles of hawaiite volcanism at Crater Flat are compositionally distinctive. Second, where pronounced compositional variation does occur, as within the 1.1-Myr cycle, the variation cannot be modeled by fractionation of one erupted variant from another; all variants of the 1.1-Myr hawaiites arose either from one parent magma by varied modes of fractionation or from two or more cosanguineous parent magmas. Third, the various volcanic cycles are distinctive but each cycle reproduces hawaiite-clan volcanism. This third conclusion is very important, for it strongly suggests that the mechanics of mantle melting and parental magma evolution have remained fundamentally unchanged beneath Crater Flat for the past 3.7 Myr. This last conclusion is strengthened by the repeated occurrence of a specific amphibole fractionation trend among the 3.7- and 1.1-Myr basalts of Crater Flat.

I 'I VIII. GENERAL TRACE-ELEMENT CHEMISTRY PD Rb-Sr SYSTEMATICS OF THE BASALTS AT CRATER FLAT Trace-element abundances were obtained by instrumental neutron activation analysis (INAA). Both thermal and epithermal neutron irradiations were used. Several 250-mg aliquots of each whole-rock powder were encapsulated in poly- ethylene vials and irradiated in the following neutron fluxes at the Los Alamos Omega West Reactor: (+Thermal = 1 x 1013 and OEpithermal = 5 x 10' 0 n/cm2/s). Different irradiation lengths, decay intervals, and counting times were employed to determine Sc, V, Cr, Mn, Co, Ga, As, Rb, Sr, Cs, Ba, La, Ce, Sm, Eu, Tb, Yb, Hf, Ta, and Th by direct counting of gamma radiation on large Ge(Li) crystals coupled to 4096-channel pulse-height analyzers. All gamma-ray spectra were stored either directly on Digital Equipment Corporation (DEC) RL02 disks or on magnetic tape for subsequent transfer to disk. Data reduction was done off-line on a DEC PP 11/34 minicomputer under the RSX-11M operating system. Uranium was determined by delayed neutron assay (DNA). All procedures are described in Gladney et al. (1980a,b,c). As with major elements, trace-element concentrations discriminate between the three basal tic cycles at Crater Flat. The incompatible trace elements are distinctively enriched or depleted in the various basaltic cycles of Crater ) Flat (Table V). The 3.7-Myr cycle is relatively low in most incompatible elements including the light rare-earth elements (La and Sm, Fig. 16), the high- valency actinide elements (U and Th, Fig. 17), and large cations such as Sr (Fig. 18). Despite a broad range in composition, the 1.1-Myr basalts are re- markably enriched in all incompatible trace elements except Rb (Fig. 18); the northeastern cinder cone of the Little Cone center is an exception to the 1.1-Myr enrichment in most incompatible elements, although it is comparable to other 1.1-Myr basalts in Sr enrichment. The final basalt cycle at Crater Flat (0.3 Myr) is generally intermediate in trace-element composition, between the two preceding basaltic cycles. The enrichment of incompatible trace elements In the two younger cycles of Crater Flat basalts is much greater than in other comparable hawaiite basalts (Price and Taylor 1980; Frey et al. 1978; Fitton and Hughes 1977). The implications of this enrichment are discussed below. The origins of the basalts at Crater Flat are partially obscured by their evolved nature. Basalts are known to have high Mg' values (>0.65) at their source regions in the upper mantle, and basalts with lower Mg' values have evolved from their parental compositions. The basalts at Crater Flat, with )

I I TABLE V

TRACE ELEMENTa COMPOSITIONS OF CRATER FLAT BASALTS (ppm)

3.7-Myr Cycle

FB CF CF CF CF FB CF FB 78-14 12-6-12 12-6- 10 12-7-6 12-7-1 78-15 12-7-8 78-17 Cs 0.92. 0.51 0.72 0.57 0.41 0.75 0.83 0.68 Rb 66 36 65 22 18 28 30 39 Ba 1040 1260 780 950 1020 890 710 930 Sr 920 800 840 770 800 770 750 780 La 104 66 63 72 58 - 73 Ce 188 126 128 136 119 - 140 Sm 11.5 8.5 8.1 9.0 9.3 - 9.1 Eu 3.3 2.5 2.5 2.6 2.9 - . 2.8 Yb 2.4 2..6 2.9 2.8 3.2 - 2.7 Th 7.5 6.2 6.2 6.0 4.6 5.4 5.6 6.4 U 1.5 -1.1 1.2 1.2 1.0 0.8 1.2 1.1 Hf 7.5 6.6 6.2 6.5 6.6 5.9 7.9 6.4 Ta 1.54 1.27 1.24 1.38 1.50 1.10 1.40 1.26 V 173 220 187 217 259 248 243 189 Sc 22 27 29 27 30 27 29 25 Ga 18 18 21 16 15 19 18 18 As 1.5 0.6 1.4 0.8 2.4 1.2 1.0 0.5

aAll analyses reported in ppm. Data obtained by nstrumental neutron activation. Pelative errors are 10: for Sr, La, Eu, Tb. Th. U, Hf. Ta, V, and Sc; 15: for Cs, Ba, Ce, Ga, and As; 202 for b, n, and Yb.

I I TABLE V (Cont.) I . TRACE ELEMENTa COMPOSITIONS OF CRATER FLAT BASALTS (ppm)

1.1-Myr Cycle 0.3-Myr Cycle

Northern Cone Black Cone Red Cone Little Cone S.W. Little Cone .E. Lathro e!lls -

Cf FB FU FB FB CF CF CF CF FO CF CF CF Cf f B 12-6-3 7-10 711-L 78-4 8-5 12-4-4 12-4-6 12-4-11 _j?.A.1jA 78-9 12-4-13A 12-4-138 11-7-1 11-7-2 Cs 1.1 2.0 2.5 2.6 2.0 2.6 2.0 2.6 0.87 0.72 1.4 1.1 1.5 1.1 2. 3 14, 19 45 20 36 20 36 33 31 14 24 14 32 19 18 Oa 13110 1410 1420 1010 1140 1340 1350 1500 1430 1390 1280 1170 1330 1310 1 J!j0I Sr liV10 1170 1040 1100 1200 1600 1340 1230 1750 1900 1320 1180 1380 1450 1290 La - 122 116 - 121 111 93 94 88 Ce - 217 206 - 202 207 186 - 181 184 s - 12.3 11.6 - 11.4 13.1 12.9 - 12.0 11.9

Eu - 3.7 ' 3.4 - - ~~3.2 3.7 3.7 - 3.6 3.5 Yb - 3.0 2.6 - 2.5 2.6 2.7 - 2.5 2.7 Th 10 9.6 11 9.9 10 15 14 12 7.6 10 5.0 4.9 6.7 6.4 7.5.0 U 3.0 3.0 3.4 3.2 3.4 4.4 3.6 3.6 3.3 1.9 0.9 1.5 2.2 2.0 2.4 lit llf 8.2 8.7 9.1 8.8 8.5 8.1 8.8 8.9 9.4 8.6 8.2 8.0 8.0 8.2 1 1.b3 Ta 1.64 1.77 1.69 1.63 1.76 1.65 1.78 1.59 1.88 2.1 1.83 1.56 1.56 1.62 V 75 190 145 160 151 166 181 148 179 200 224 209 207 152 178 Sc 18 19 22 20 22 19 19 20 19 19 19 18 19 19 2. Ga 18 16 17 19 21 20 17 17 19 22 20 16 18 19 18 As 1.6 1.3 1.9 1.6 1.2 2.3 2.1 1.8 0.8 0.8 1.3 5.0 1.3 1.2 1.5

aAll analyses reported In ppm. Data obtained by instrumental neutron activation. Relative errors are 10 for Sr, La. Eu, T. Th, U, lf, Id V, and Sc; 15S for Cs, Ba, Ce, Ga. and As; 201 for R, S, and b. 15

10.

0 so 100 150 200 La (ppml

Fig. 16. Plot of Sm vs La for the basalts of Crater Flat; symbols represent (1) the 3.7- Myr basalts, (2) the 1.1-Myr basalts, and (3) the 0.3-Myr basalts. The stip- pled field represents the common range of compositions for tholeiitic to alkaline basalts, including most hawaiites. Olivine (0L), clinopyroxene (CPX), and amphibole (AMPH) compositions are shown as calculated for minerals in equilibrium with the range of basalt compositions at Crater Flat. The younger basalts at Crater Flat have very high La/Sm ratios (discussion in text).

1*0 __ U l b (p p m) lb Ispm

Fig. 17. Plot of Th vs U for the basalts of Crater Flat; symbols as in Fig. 16. Note the constant ratio but increased content of U and Th in the youngest basalts !2 *'' 100- oft

cc~~~~'

2 233

Sf tppml

Fig. 18. Plot of Rb vs Sr for the basalts of Crater Flat; symbols as in Fig. 16. The youngest basalts (2, 3) have high Sr contents but very low Rb/Sr ratios; high radiogenic Sr content in the 0.3-Myr basalts (3), Sr'7/Sr8" = 0.7075, strongly suggests Rb depletion. Arrows indicate the paths of fractionation from the field of common tholeiitic to alkaline basalt compositions; this path is in- variably one of Rb-enrichment. The line RbiSr = 0.032 represents a whole- earth model ratio (Carter et al. 1978).

Mg' values of 0.58 to 0.46 (Fig. 14), are so evolved. The Mg' value of a parental magma is reduced by removal of Mg-rich silicate phases, of which olivine, clinopyroxene, and kaersutite (or another Ti-rich amphibole) are possible candidates that may lead to the hawaiites of Crater Flat. Plagioclase removal does not affect the Mg' value of the evolving magma and can be ruled out for the Crater Flat basalts on the basis of smooth chondrite-normalized patterns for all of the rare earth elements, including Eu, in even the most lanthanide-enriched basalts of the 1.1-Myr cycle. Without samples of the parental magma, the development of detailed crystal fractionation schemes for Crater Flat basalts is highly speculative. However, the incompatible trace-element compositions of the 1.1- and 0.3-Myr basaltic cycles provide some indication of which minerals were removed from parental magma to generate Crater Flat basalts. Minerals that tend to reject all lanthanide elements, but disfavor light lanthanide elements (for example, La) more than other lanthanide elements (for example, Sm), will increase the concentrations of La and Sm and raise the La/Sm ratio in the evolved magma.

44 .Fractionation of kaersutite or clinopyroxene leads. to this result. Figure 16 shows that the La/Sm ratios in the 1.1- and 0.3-Myr Crater Flat basalts are very high (10 to 14), much higher than in other hawaiites evolved to similar Mg' values (La/Sm <6, Price 1980; Frey et al. 1978). High La/Sm ratios may become common as more trace-element data are obtained for basalts in the NTS area, but the immediate inference from this data is that large amounts of clinopyroxene or kaersutite,- or both, were fractionated from the magmas that were parental to the two younger basaltic cycles at Crater Flat. The importance of amphibole fractionation has been discussed above in conjunction with Figs. 14 and 15. Clinopyroxene and kaersutite fractionation probably included the crystallization and removal of olivine, to account for the combined decrease in Mg' and pronounced increase in incompatible elements. The less evolved basalts of the 3.7-Myr Crater Flat cycle are not greatly enriched in La/Sm and could be derived from parental magma(s) through crystal fractionation dominated by olivine, though the calculation of parental magma types suggests that clinopyroxene was also involved. Whatever their parentage, the hawaiites of Crater Flat, particularly the younger basalts, did not rise abruptly from their mantle sources, but were derived from parental magmas that were held at depth and partially crystallized before eruption. Processes that enrich a magma in one incompatible element will generally result in enrichment in other incompatible elements, though the final ratios between incompatible elements may vary. The 1.1-Myr basalts of Crater Flat are enriched in almost all incompatible trace elements, with the notable exception of Rb, relative to the 3.7-Myr basalts. Figure 18 shows the low Rb content and high Sr content in Crater Flat basalts relative to other common basalt types. The high Sr content of the younger basalts of Crater Flat may .be attributed to the extensive crystal fractionation that is required to ex- plain the other incompatible-element enrichments. However, all realistic fractionation models should increase Rb as much as Sr or more. This low-Rb anomaly in the younger Crater Flat basalts strongly suggests an b depletion of the mantle source region, before the melting event that generated the 1.1- and 0.3-Myr basalts of Crater Flat. The scenario of Rb depletion is complicated by isotopic data. Analyses published by Leeman (1970) show that the 0.3-Myr Lathrop Wells basalt is enriched in Sr87, the radiogenic daughter of Rb, with an Sr87186 ratio of 0.7075. The low b content of this and other Crater Flat samples rules out

I T, .he ossibiliy of crustl contani naticn. Thus the mantle source -or the 0.3-Myr basalt (and probably the other basalts of Crater Flat as well, though isotopic data have not been collected for these samples) was enriched in Rb through some event in the distant past. The requirements for past Rb enrichment are illustrated by the line Rb/Sr 0.032 in Fig. 18. This is a model whole-earth ratio (Carter et al. 1978), which would lead to a present-day Sr 87/86 of 0.705, if left undisturbed throughout the earth's history. Some dis- turbance involving Rb enrichment must have occurred in the ancient mantle that was to become the Late Cenozoic source region for the basalts of Crater Flat. A single-stage Rb enrichment model based on the highest Rb/Sr content of the 3.7-Myr basalts would place this enrichment event at about 900-Myr ago. The Rb-enriched mantle remained static until the Late Cenozoic and generated a large amount of Sr87 from the high b concentration. Finally, this mantle source region lost much of its Rb. The loss of R could not have been coupled to a loss of other trace elements, or the great enrichment of lanthanides and other incompatible elements would not be seen in the younger basalts of Crater Flat (Figs. 16 and 17). A selective depletion of Rb is possible through flux- ing by aqueous volatile-rich fluids; data of Shaw (1978) show that in such fluids the solubility of b is greater than the solubility of other incompatible elements. Alternatively, Rb might be selectively lost by destabilization of an Rb-rich mantle phase (for example, phlogopite) in an upwelling mantle environment. The isotopic data suggest that Rb depletion must have occurred in the near past. Could this event have been the magma genesis of the 3.7-Myr Crater Flat basalts? .Or was it a much larger event, associated with one or more of the silicic caldera-forming eruptions of the Timber Mountain-Oasis Valley cauldron complex? A proof of this second possibility would provide documentation of an important mechanism for generating mantle nhomogeneities. Another possibility for selective Rb loss would be the general crustal thinning and mantle upwelling associated with late Cenozoic Basin-Range tectonism. Further petrographic, chemical, and isotopic studies will address these questions.

IX. VOLCANIC RISK ASSESSMENT Recurrence of basaltic volcanism within the Crater Flat area is of con- cern to siting a waste repository at Yucca Mountain: the Quaternary-age Lathrop Wells center is located less than 20 km from the southern edge of the

-36 Yucca Mountain-exploration block. Crowe and. Carr (1980) defined maximum probability limits (10-8 to 109 /year) for the likelihood of volcanic disruption of a repository at Yucca Mountain. They briefly considered the direct disruption effects of volcanism and examined the regional volcanic setting of basaltic volcanism within the south-central Great Basin. Several conclusions from this work add data with respect to the earlier volcanic risk assessment. First, field, geochronologic, and geochemistry studies all support the recognition of cycles of basaltic activity within the Crater Flat area. Each cycle is distinct in space and time and can be discriminated through major- or trace-element abundances. Absolute volumes of erupted lava for each cycle are relatively small (<1 km 3), and the actual number of eruptive vents for each cycle is variable but generally small (less than 10 vents per cycle). Thus, if this pattern of past basaltic activity can be assumed to continue into the future, it is likely that future volcanism in the Crater Flat area will be of relatively small volume with a limited number of volcanic vents. Second, there is no clear evidence of an increase in rates of volcanic activity or volumes of erupted magma within the last 3.7 Myr. This is illustrated by Fig. 19, a plot of calculated magma volume vs time. Two interpretations are suggested by this figure although the interpretations are sharply limited by the small number of data points. There is a near-linear decline in volume of magma for successively younger volcanic cycles. This suggests a possible waning in basaltic activity within the last 3.7 Myr. This trend con- trasts with a decrease in the intervals between eruptions with time that could indicate an acceleration of basaltic activity. Both of these interpretations need to be tested through examination of the history of basaltic volcanism (postsilicic volcanism) for the entire NTS region. Such studies are in progress. Third, compositional studies indicate that each of the three volcanic cycles at Crater Flat produced similar hawaiite magmas. There is a strong sug- gestion of source region variation with time from the fact that the oldest basaltic cycle includes samples that are not Rb-depleted, unlike the younger basaltic cycles. On the other hand, compositionally similar basalt types were erupted repeatedly within the Crater Flat area, reflecting relatively constant conditions of magma generation through time.

I , 1.0_.

0.5.

I >.2 0.1

2g22 0.05f

0.01 I' - 2.6 4 32I0 TIME (million years)

Fig. 19. Plot of volume vs age for the three basaltic cycles at Crater Flat.

The above data argue that, within the Crater Flat area for the last 3.7 !4yr, basalt types have remained relatively similar and volumes have been small. The general assumptions of continuity in magmatic processes for the Crater Flat area (last 3.7 Myr) required for probability calculations thus far are broadly supported by the continuing field, dating, and petrologic work. Two additional areas of investigation are required. (1) The history of basaltic volcanism for a larger area of the NTS region needs to be studied. Young basalts are present at two additional localities. Two cinder cone and lava flow centers dated at about 0.3 Myr are present north of Crater Flat (SB of Fig. 1); the basalts of Buckboard Mesa (BM of Fig. ) have been dated at about 2.8 Myr (W. J. Carr, personal communication 1980). Scattered basalts younger than 11 yr are also present within the NTS area. These basalts will be compared petrologically with the basalts of Crater Flat. The concept of discrete cycles or pulses of basaltic activity will be tested through regional studies and the volume/time plot completed for Crater Flat basalts will be expanded to include the entire NTS region. (2) The Lunar Crater volcanic field of central Nevada (probably Pliocene and Quaternary age) is the northernmost basalt field of the volcanic belt. Volumes of basalt in this field

I ., exceed several tens of cubic kilometers. Cone density of uaternary-age cinder cones within the Lunar Crater field is about 0.1/km2, in contrast to the Quaternary cone density for the NTS region of about 10-3 to 10-4/km2. It is important to determine why the contemporary rates and volumes of basaltic activity for the Lunar Crater volcanic field are so much greater than for the Crater Flat field. Studies under way indicate the compositional range of basalt types is much greater in the Lunar Crater field than the Crater Flat field. These fields need to be contrasted petrologically and geochemically in order to further understand basaltic volcanism in the southern Great Basin.

ACKNOWLEDGMENTS W. S. Carr, U.S. Geological Survey, participated in many aspects of the geologic studies of the basalts of Crater Flat. We benefited from his know- ledge of the tectonic and volcanic history of the Great Basin. We are grate- ful to E. S. Gladney of the Los Alamos Health Sciences Division for his excel- lent work in INAA analysis of our samples. We also gratefully acknowledge the assistance of R. J. Fleck and R. F. Marvin of the U.S. Geological Survey who determined the K-Ar whole rock ages for the basalts. The manuscript was reviewed by F. M. Byers, W. J. Carr, and A. C. Waters. Editorial review was contributed by M. G. Wilson.

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McKay, E. J. and K. A. Sargent, "Geologic Map of the Lathrop Wells Quadrangle, Nye County, Nevada, U.S. Geol. Survey Geol. Quad. Map GQ-883, scale 1:24,000 (1970).

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APPENDIX

TABLES OF MINERAL ANALYSES

Table A-I: Olivine analyses normalized to 4 oxygens. Table A-II: Feldspar analyses normalized to 8 oxygens. Table A-III: Pyroxene analyses normalized to 6 oxygens. Table A-IV: Oxide analyses normalized to 2 cations (rhombic) or 3 cations (isometric). Table A-V: Amphibole analyses normalized to 23 oxygens.

[Note: (n.a.) not analyzed; (-) = below microprobe detection limits.]

Z s

I *. TABLE A-I t)- I- OLIVINE ANALYSES, CRATER FLAT ASALTS

3.744yr Cycle

fU78-14 CF 12-7-6A CF12-7-1

Phenoc rys ts Groundindss Phenocrysts Phenocrysts Groundmass

Si02 38.2 37.9 36.6 36.7 38.4 38.9 39.7 39.5 39.0 35.5 35.7 Al 03 n.a. n.a. - 0.06 n.a. n.a. 0.06 - - 0.07 0.20 7°2 0.12 0.13 - - - - - 0.15 0.16 FeO 21.9 23.1 34.5 34.0 18.8 19.3 19.3 19.4 21.9 39.9 38.9 MnO 0.17 0.24 0.91 0.83 0.21 0.22 0.25 0.24 0.23 0.78 0.79 Mg0 40.9 39.4 29.8 29.4 42.4 41.2 42.5 41.9 39.7 25.4 24.0 CaO 0.14 0.20 0.15 0.15 0.14 0.12 0.20 0.21 0.21 0.43 0.69 ------Cr 2 03 0.09 0.10 z 101.3 100.0 102.2 101.4 100.0 99.7 102.0 101.2 101.0 102.2 101.2

Si 0.980 0.983 0.990 0.999 0. 984 0.9U7 0.995 0.999 0.998 0.981 0.996 Al n.a. - 0.001 0.1 - - 0.001 0.006 I1 - 0.002. 0. 2 - - - - 0. 002 0.002 Fe 0.469 0.500 0.779 0.773 0.402 0.420 0.403 0.410 0.468 0.927 0.905 Hn 0.003 0.005 0.021 0.018 0.005 0.005 0.005 0.004 0.004 0.018 0.018 Hg 1.564 1.524 1.201 1.194 1.620 1.598 1.587 1.577 1.518 1.054 1.030 Ca 0.003 0.005 0.004 0.004 O.003 0.003 0.005 0.005 0.007 0.012 0.020 Cr - 0.001 0.001 _ _ _ lcatlons 3.019 3.017 2.998 2.992 3.014 3.013 2.996 2.995 2.995 3.001 2.977

Fo 0.77 0.75 0.61 0.61 0.80 0.79 0.U0 0. 79 0.76 0.53 0.53 I'd 0.23 0.25 0.39 0.39 0.20 0.21 0.20 0.21 0.24 0.47 0.47 vf-

TABLE A-I (Cont.)

OLIVINE ANALYSES, CRATER FLAT ASALTS

1.1-Myr Cycle Northern Cone 81ack Cone

CF12-6-3 F878-1 F878-4

Phenocrysts gohs Phenocrysts Grounmhass Phenocrysts Groundeass

SiO2 38.6 38.1 36.2 38.5 38.5 37.3 36.8 39.1 37.1 36.3 35.8 Al203 ------TiO 2 - - 0.09 _ - 0.10 0.11.. - - 0.13 0.11 FeO 21.5 27.5 33.5 22.5 26.3 30.8 33.3 22.4 31.3 35.8 37.5 MnO 0.24 0.41 0.74 0.38 0.38 0.63 0.64 0.32 0.59 0.76 0.75 MgO 40.5 35.6 30.4 40.0. 36.7 32.6 30.3 40.7 .32.0 28.2 26.5 CaO 0.14 0.17 0.32 0.16 0.17 0.33 0.38 0.16 0.27 0.30 0.52 Cr 2 03 ------z 101.0 101.8 101.2 101.5 102.0 101.8 101.5 102.7 101.3 101.5 101.2

Si 0.991 0.997 0.985 0.985 0.997 0.993 0.996 0.987 0.996 0.994 0.995 - Al - - - .. ------TI - - 0.001 - - 0.001 0.001 - - 0.002 0.001 Fe 0.460 0.601 0.760 0.482 0.571 0.687 0.753 0.473 0.701 0.820 0.873 Mn 0.004 0.010 0.017 0.007 0.007 0.014 0.014 0.007 0.013 0.017 0.017 Mg 1.547 1.388 1.233 1.527 1.419 1.295 1.222 1.533 1.278 1.151 1.097 Ca 0.003 0.004 0.008 0.003 0.005 0.009 0.010 0.003 0.007 0.008 0.015 Cr ------rcations 3.006 3.000 3.004 3.004 2.999 2.999 2.996 3.003 2.99S 2.992 2.998

Fo 0.77 0.70 0.62 0.76 0.71 0.65 0.62 0.76 0.65 0.58 0.56 Fa 0.23 0.30 0.38 0.24 0.29 0.35 0.38 0.24 0.35 0.42 0.44

(3m MS TABLE A-I (Cont.)

.,. OLIVINE ANALYSES, CRATER FLAT BASALTS

1.1-Hyr Cycle Red Cone Little Cone. S.U.

.... . CEi2A-.h____. __ _ _ 12=__2 Ph!enocr-sts Groundimass Phenocrysts Groundmass Phenocrysts

S102 39.0 38.2 36.8 36.1 39.1 37.2 36.2 35.6 38.4 38.5 38.5

A1203 - - 0.06 - - 0.05 0.08 0.07 - -

1102 - - - 0.11 - - 0.11 0.12 0.09 - _ FeO 21.7 26.6 32.1 34.0 23.0 33.4 36.7 40.8 22.3 24.3 25.1 MnO 0.32 0.50 0.74 0.90 0.27 0.68 0.83 0.88 0.45 0.63 0.66 HgO 39.6 36.5 31.2 28.8 39.4 30.6 27.4 24.4 38.6 37.9 37.3 CadO 0.16 0.20 0.20 0.46 0.13 0.25 0.33 0.29 0.23 0.28 0.32

Cr203 ------z 100.8 102.0 101.0 100.4 101.9 102.1 101.6 102.2 100.1 101.6 101.9

Si 1.001 0.993 0.993 0.994 0.997 0.999 0.997 0.994 0.999 0.995 0. 96 Al - - - 0.001 - - 0.001 0.002 0.001 - - TI - - - O.Ol - 0.001 0.002 0.001 - - Fe 0.465 0.577 0.727 0.704 0.491 0.750 0.845 0.951 0.484 0.524 0.S43 Mn 0.006 0.010 0.017 0.021 0.005 0.015 0.019 0.020 0.009 0.013 0.014 Hg 1.516 1.414 1.255 1.114 1.502 1.225 1.125 1.016 1.496 1.459 1.437 Ca 0.003 0.005 O.OU5 0.013 0.003 0.007 0.009 0.008 0.006 0.007 0.008

Cr - - - _ _ _ Icatlons 2.991 2.999 2.997 2.998 2.998 2.996 2.997 2.Y93 2.996 2.998 2.998

Fo 0.77 0.71 0.63 0.60 0.75 0.62 0.57 0.52 0.76 0.74 0.73 Fa 0.23 0.29 0.37 0.40 0.25 0.38 0.43 0.48 0.24 0.26 0.27 TABLE A-I (Cont.)

OLIVINE ANALYSES, CRATER FLAT BASALTS

1.1-yr Cycle Cont'd.) 0.3-Myr Cycle

Little Cone, .E. La'throp Wells Cone CF12-4-13H FB78-7

Phenocrysts Groundmass Phenocrysts Groundmass

Si02 38.1 37.5 36.1 36.1 39.0 38.8 37.3 36.2 0.05 0.04 0.11 0.29 A12 03 n.a. n.a. TI 02 - 0.08 0.17 0.14 - 0.14 FeO 22.8 24.9 31.5 31.7 19.9 21.8 27.6 31.5 0.26 0.36 1.02 0.98 0.26 0.27 2 0.48 0.54 HgO 39.1 37.8 30.5 30.6 41.2 40.3 34.8 29.9 "go CaO 0.15 0.25 0.43 0.45 0.13 0.16 0 0.48 0.95 Cr203 - - 0.09 0.01 t 100.4 100.9 99.9 100.1 100.5 101.3 101.8 100.8 99.5

Si 0.989 0.981 0.989 0.988 0. 7 0.992 0.989 0.988 0.995 Al n.a. n.a. 0.001 0.001 - 0.002 0.009 TI 0.001. 0.003 0.002 0.002 Fe 0.495 0.541 0.722 0.725 0.426 0.466- O. 569 0.612 0.722 Mn 0.005 0.008 0.023 0.023 0.005 0.O05 0.009 0.010 0.012 Mg 1.515 1.471 1.247 1.249 1.571 1.535 1.431 1.375 1.223 Ca 0.004 0.007 0.012 0.013 0.003 0.003 0.005 0.013 0.027 Cr 0.001 ('.001 1cations 3.008 3.009 2.998 3.002 3.002 3.001 3.003 3.000 2.990

Fo 0.76 0.73 0.63 0.63 0.79 0.77 0.72 0.69 0.63 Fa 0.24 0.27 0.37 0.37 0.21 0.23 0.28 0.31 0.37

)I (II TABLE A-l

FELDSPAR ANALYSES, CRATER FLAT ASLTS

3.l-Hyr Cycle

IS /81-4 CF12-6-12.

Phenocrysts Groundeass Pheocrysts

49.3 0.5 60.2 S?.! 55.2 6Z.4 65.8 41.6 60.6 51.1 lSoJ IJ.2 3l.S 30.1 29.2 26.8 22.3 31.s 32.4 30.9 33.4 ft0 o. o 0.81 0.99 3.36 0.98 0.60 0.41 0.61 0.71 0.49 "-p 0. )9 0.09 - 0.01 0.09 - 0.61 ".4. A.A. NI. tdo . 14.1 14.4 £2.9 30.8 3.9, 0.48 16.1 14.2 13.9 A.S. A.A. A.A. n.A. ... '".. N.A. 3.4. A.S. N.G. sto ado N.A. N.A. . n.h... A.A. R.A. h.A. B.S. A.A. 4.4. NA20 2.64 2.61 3.36 3.9i S.11 S.0O 3.23 2.21 3.11 3.3 KIo 0.16 0.20 0.09 0.22 0.Jo 6.09 11.4 0.24 0.34 0.3s 9 100.S 100.7 99.1 300.0 99.3 100.0 99.4 99.4 59.9 100.6

Si 2.24S 2.290 2.316 2.392 2.634 2.191 L.aos 2.208 2.316 2.314 Al 1.129 1.682 1.636 3.661 1.440 1.181 0.942 1.164 1.664 3.616 lSt.Al J.914 3.912 3.962 . sss 3.964 3.984 3.947 3.912 3. 980 3.99 Fc 0.025 0.030 0.038 0.042 O.0J6 0.422 0.016 0.023 0.026 0.0£1 H9 0.00s 0.00s - 0.004 0.006 - 0.031 A.A. A.S. 3.8. Es 0.171 0.116 0.712 0.628 0.624 0.390 0.022 0.198 0.696 0.675 sr N.A. N..A.4. A.A. ".S. S.d. A.A. U... A.S. I .e" a n.s. N..G.A. n.s. B.S. N.d. n.a. B.S. A.S. B.S. Ua 0.223 0.2S3 0.303 0.34S 0.43 0.431 0.286 0.39 0.21 0.296 K 0.008 0.010 0.004 0.012 0.017 0.g4 0.66s 0.013 0.039 0.020 ix-.cti033 3.038 1.012 1.oss 1.033 1.033 0.998 3.025 1.033 1.016 3.001 ctlos 4.992 4.984 6.001 4.990 4.987 4.982 4.912 S.0S 4.996 4.991

Or 0.01 0.03 0.m3 0.03 0.02 0.36 0.68 0.0£ 0.02 0.02 Ab 0.22 0.26 0.30 0.3s 0.45 0.46 0.29 0.20 0.28 0.30 0.11 0.13 0.70 0.64 U.sJ 0.19 0.03 0.19 0.70 0.68 .to I

TABLE A-II (Cont.)

FELDSPAR ANALYSES, CRATER FLAT BASALTS

J.J-Hyr Cycle

CFl,--10 CE2-t1.6 phe1ocr ilts __e__or_ _ts '

S102 50.0 521 S4.1 Sl.S *. 64.4 41.1 48.4 41.S 49.1 10.4 10.1 54.1 59.J Jo.7 30. S 28.8 21.2 20.1 19.7 32.6 11.6 331 32.1 31.6 31.3 28.0 24.6 tao O.SI 0.96 0.42 0. 4 0.18 0.19 0.12 0.62 O.S9 O.S6 0.6 0.44 1490 0.12 0.11 0.04 0.81 0.11 0.01 A.S. O.&. R.*. .. 4. N.S. A. L.a. L.S. Lao 14.8 13.4 11.1 0.84 12.9 14.1 1.2 IS.$ I4.S 13.8 10.9 S.8, 101.1 1.1 '-0 O.J0 0.18 0.74 0.11 0.20 A.. .4. *.* ".a. A..S. L.S. N.A. - 840 0.31 0.09 0.25 L.4. M.S. L.a. P.A. L.a. L.A. L.A. n.a. J.OS 3.58 4.91 4.21 1.S6 2.19 2.09 2.66 2.89 3.41 .S9 k.11 0.29 2° 0.?I 0.2J 0. s 0.W8 10.0 0.09 O.2O 0.09 0.21 0.20 0.24 0.40 1.31 99.9 JOD.5 300.6 103.0 100.1 99.2 98.S 100.1 200.3 100.2 9S.4 ".2 98.6

1S 2.298 2.340 2.161 2.1113 2.*50 2.18 2.212 2.19 .241 t. 4 2.299 2.414 '7.689 Al 3.880 1.512 2.530 2.4?SI0.7s3 O.WS1.102 3.Ujs 2.11 132 1.814 I.t3O 3. G 1.912 I.SOJ. 1.311 | .Is$ I St-Al 3.95 3.912 3.961 J.969 3.464 3.993 3.911 3.991 3.991 j.981 4.000 F. 0.01S 0.0)6 0.014 0.0.486 02 0.0219.314 U.01 I 0.07 o0.0 0.012Z0 0.022 0.021 0.020 0.0?4 0.0w8 iI 0.00 0.O6 0.001 0.00 0.010 N.A. A.S. B.S. A.S. L.i. A.S. L.*. f.a. I0.O.O03 JI C 0. 19 0.so 0.1US 0.1JWI O.0D89 0.838 0.132 0.19" 0.12 O.yob 0.611 0.31 0.281 St 0.006 0.004 0.001 O.01)4 O. 005 0.. L.a.S NL.a..a. L.a. ... 8.4. as - O.WOt N.A. L.a. L.S. L.A. L.a. l.a.M... I.e. 0.0J14 Ns 0.211 0.314 0.429 0.114 0.218 O.2S1 0.186 0.238 0.255 0.30 0.406 0.621 R 0.01? 0.013 0.0281 0.028 0.142 0. 5,6 O.0U4 0.033 o.004 0.OU8 Q.010 0.013 0.051 0.01 a.-ctlons I.OSI 1.02J .LON I3.0Ot4 1.01 9 1.0JS 1.024 1.011 I.028 0.n2? 1.01 .04 2.000 I catlons 5.W0 4.99S S.WI *.4?S9. S.a 5.005 S.Ou s.aw. 5.004 S.0S 4. "S S.004 4.945 S.OO

or 0.01 0.01 0.07 0.Oj 0.5s o.S 0.02 0.03 0.00 0.02 0.0l 0.03 0.us 0.0 Ab 0.?1 O.12 0.44 0.58 0.16 U.3J 0.11 0.7 0.1 0.232 0.28 0A 8.42 0.u An 0.12 0.61 O.SS 0.41 0.09 u.u4 1.87 0.14 0.81 0.18 0.13 0.68 0.S4 0.29

(I AL TABLE A-II (Cont.) (X. FELDSPAR AALYSES, CRATER FLAT BASALTS

3.144yr Cycle

Northern Cone 813ck Cone Red Con. Red Cne Cont) (f12-6-3 f`1-1 1118-4 Cf 12-4-4 CF12.4-6 firawam Groundss Crouadmass Grounmaess Groundeass 13.8 Sb?2 49.9 Sl.0 50.9 51.4 51.2 14.9 1.8 63.3 13.8 88.1 13.4 51.3 29.8 30.3 30.3 29.9 29.4 21.0 28.1 26.8 28.0 18.3 30.9 30.2 30.4 30.6 29.3 A1203 Fe0 0.98 0.82 I.01 0.81 1.03 0.85 0.88 0.98 0.98 0.63 0.91 3.OS 0.89 1.04 I .6d 0.12 14go i.e. i.A. N.A. N.A. i.e. N.A. 0.12 0.11 0.10 N.A. N.A. 0.09 0.11 C&O 14. 14.0 13.8 13.9 13.8 Il.S 13.8 12.4 11.8 0.29 13.1 13.3 33.7 13.8 Sro P.e. M.S. N.A. M.S. N.A. i.A. i.e. i.e. i.e. N.A. N.e. M.e. i.e. i.e. 80 N.e. i.e. N.A. i.C. i.e. i.A. i.e. i.e. N.e. i.e. i.A. i.e. N.e. i.e. ~2° 3.39 .8 3.41 3.80 3.61 4.80 3.41 4.23 4.68 3.15 3. " 3.90 3.i3 3.36 4.34 z20 0.24 0.22 0.26 0.24 0.22 0.41 0.24 0.38 0.45 10.8 0.22 0.21 0.26 0.28 0.41 £z go.8 99.1 99.1 99.8 99.3 59. 98.2 98.2 .8 99.9 101.0 100.8 1300.3 300.S 101.3

S 2.320 2.37 2.333 2.349 2.31 2.502 2.396 2.481 2.441 3.001 2.332 2.318 2.341 2.329 2.413 Al 1. 24 1.636 I.841 1.811 1.S94 1.450 I.S 1.462 1.499 0.911 1.641 I.613 I.16 1.56o St-Al 3.944 3.913 3.974 3.980 3.9SI 3.9S52 3.933 3.929 3.948 3.918 3.9Y9 3.969 3.912 3.961 3.963 Fe 0.036 o.031 0.038 0.031 0.039 0.032 0.034 0.233 0.016 0.022 0.033 0.039 0.033 0.019 0.062 Mg R.. i.e. ".e. i.&. i.e. M.S. 0.006 0.00 0.0006 - i.e. i.e. 0.005 0.006 0.006 Ca 0.129 0.885 0. 69 .680 0.83 0.562 0.685 0.S17 0.1SS 0.013 0.62 0.4 0. 68 0.812 0.562 Sr i.&. i.e. i.e. i.e. i.e. N.e. N.A. i.e. N.A. i.e. N.e. i.S. n.e. ad i.e. i.4. M.e. N.e. N.e. i.e. N.e. N.e. i.e. i.e. N.e. i.e. i.e. Ns 0.281 0.282 0.303 0.319 0.321 0.424 0.306 0.319 0.412 0.329 0.322 0.342 0.292 0.294 0.111 K 0.013 0.012 0.01 0.013 0.012 0.022 0.011 0.021 0.025 0.821 0.012 0.012 0.026 0.022 la-catlons l.09 1.010 1.025 1.043 .o55 1.040 1.044 I.00 1.054 0.949 1.029 1.040 1.013 1.021 1.029 X catbons 5.009 4.983 4. 999 S. WI S.008 4.99z 4.911 4.989 S.O00 4.961 5.008 S.009 4. S8 4.992 4.992

Or 0.01 0.01 0.03 0.03 0.02 0.02 0.01 0.02 0.02 0.85 0.01 0.01 0.02 0.02 0.02 Ab 0.28 U. 29 0.31 0.32 0.32 0.42 0.31 0.31 0.41 0.34 3.32 0.34 0.30 0. 3 0.40 An 0.11 0.10 0.88 0.87 0.61 0.56 0.8 0.81 0.51 0.01 0.81 O.8s 0.68 0.8 0.58 (l I

TABLE A-TI (Cont.)

FELDSPAR ANALYSES, CRATER FLAT BASALTS

1.1 4yr Cycle .3.eyr Cycle

tlittle Cone 5.11. LiI~l Co. .1. Latbrop Wells Cap* r11O-1 lE*tt V a010.1 "waf8es.

12.1 St. 5).3 1.1 51.6 SI.1I 1.6 49. S2.3 S).I SS.I 20.3 25.5 20.1 Zl.7 29.6 20.9 25.0 11.1 30.0 26. 26.? F tO 0.U0 1.01 1.08 0.15 1.20 0.89 1.18 0. 1. 03 O.9 1.01 N0 0.09 0.13 0.09 .e. R.a#. 0.10 o.S 0.09 0.09 (*O 12.9 12.1 11.0 4.96 1Z. 12. 32.0 13.2 12.) II.z 10.0

a...a^a. A.S. nRe. MAs.. A.. n. 0.43 0.44 0.31 0.38 io0 A... A.S. a... A.S. P.&. 4.e. , .4. - 0.24 0.11 O.Z4 I.9S 4.1 4.44 S.10 3.ro J. 3.22 3. 1 4.45 4.09 K20 0. 26 0. 2 0. 3 S. 0. 4 0.39 0.38 0.21 0.39 o.S0 0.94 9.6 9.9 99.I 99.2 'S5 99.1 9. 99.0 100.6 100.2 100.0

51 2.403 2.318 ?.435 2.199 2.J66 2.407 2.360 2. 1 2.311 t.436 2.529 Il 1.S11 1.549 1.514 1.159 I.b09 1. 54 1.89 1.700 1. m 1.542 1.4?9 J. 0 J."? I. 2 3.956 3.911 3.9 41 3. 96 3.991 ).1 3.930 3.VS8 Fe 0.036 0.040 0.040 0.079 0.045 0.03) 0.44 0.021 0.039 0.031 0.040 C. 0.DOS 0.005 0.001 - n.j. n.e. .. 0.06 0.009 0.00 O.0S 0. G)s 0.611 0. Sa0 0. 20 0.629 0.602 0. 02 O.6S O.59" 0.545 0.44 5. n.e. as... n"e. .&. R.&. II.&. A.S. 0.011 0.010 0. 0 0. I i it a.*. .. *a.. .. n ".e. * as.^ M... A.A. 0.004 0.001 O.04 "a 0.352 0.3?1 O.)9 0.09 0.33* U.JS? 0.348 0. 5t1 0. 25 0.391 0.431 K O.Ols 0.011 0.019 0.90 0.019 0.0?2 O.021 0.01 0.022 0.028 O.OS4 la-catlees 1.043 1.051 1.038 1.030 1.029 1.014 I.075 e.m 3.004 .ots l.0W9

I cations 4.9d3 s.000 4.990 4. "a S. M 4.99 ou.0 4.996 4.985 4.993 4.961

Or 0.01 0.02 0.02 - 0.9 0.02 0.02 0.0? 0.02 0.02 0.03 0. Pb 0.35 0.31 0. 40 U.St . 4 0.31 0.30 0. 30 0.3 0.41 0.44 An 0.64 0.61 0.s0 0.20 0.64 0.03 0.62 ." - 0.63 .S4 0.50

o TABLE A-111

PYROXENE ANALYSES

I1l.Nvr CYcIe -- zS:: Mart mrnlom Uk 9..nR ed Cone Cf_12-6_3. fU/81 i 47- 6 s102 51.4 1.3 2.4 5.31.1 50.6 S 0.6 63.8 52 1.4 Croumnd sza A1203 2.22 2.09 1.68 0.50 2.14 2.96 3.22 3.29 0.65 2.28 3.25 2.62 2.S0 1.92 1102 1.30 0.98 0.90 0.39 1.06 1.30 I.1S 1.42 0.46 0.68 1.23 1.09 1.06 0.19 30.7 10.4 10.2 23.3 11.4 10.4 12.3 33.5 22.5 1.2 9.5 30.5 30.6 11.4 N"o 0.30 0.32 0.30 0.65 0.30 8.25 0.39 0.42 0.7s 0.21 0.21 0.25 0.25 0.34 14.8 14.9 lb.3 20.S 16.1 14.3 s.5 13.1 19.1 16.4 14.1 14.0 14.1 IS.81 1'° Ca" I1.1 19.2 18.8 1.6 18.3 19.9 17.8 18.1 2.80 I8.8 19.6 19.8 19.9 11.81 0.49 0.43 0.47 0.06 0.36 U.41 0.39 0.59 0.20 0.27 U.36 0.43 0.33 04r203 U.32 Cr2')3 £ 98.9 99.6 100.0 98.3 99.8 100.1 9.8 101.4 99.3 9.1 91.0 100.9 100.8 300.4

SI 1.931 I.S2S 1.949 1.9/0 3.931 1.092 3. o 1.91.1 I.9S3 1.914 1.ow0 1.916 1.912 1 1.932 VAI 0.065 0.01S 0.01 0.021 0.08 0.108 0.10 0.09 0.007 0.086 0.120 0.082 0.06 U.U8b stet 2. 0 2. OU 2. O ".91 2.W 2. iO0 2.000 2.0 2.00 2. 00 2.000O 2.000 2.00 2.O 0.033 0.017 0.021 - U.010 0.021 0.042 0.04 0.021 0.013 0.026 0.033 0.020 0.01 t TI 0.036 0.021 0.026 0.009 0.02S 0.035 0.044 0.039 0.012 0.038 0.033 0.029 0.029 0.021 Fe 0.331 U.325 0.315 0.161 0.38 0.324 0.389 0.354 0.715 0.288 0.300 0.326 0.324 0.354 NA 0.009 0.009 0.008 0.020 0. 9 0.0WY 0.012 0.013 0.024 0.037. 0.001 O.OD) 0.001 0.010 mg 0.829 0.832 0.846 1.172 0.043 0.197 0./59 0.715S .093 0.914 0.825 0.761 0.311 0.8?3 Ca 0.133 0.167 0.747 0.063 0.734 0.19 0.16 0.131 0.113 0.71 0. 790 0.192 0.791 0.101 he 0.03S 0.032 0.034 0.003 0.025 0.021 0.028 0.041 0.035 0.018 0.025 0.030 0.022 0.022 Cr zoct and 3.92 2.OL%9 1.997 2.014 2.008 2.006 1.990 1. 953 1.953 2.009 2.006 3.99 20 04 2. O4 Ications 3.SY2 4.009 3.59/ 4.00S 4.008 4.006 3.990 3.993 3.913 4.009 4.006 3.911 4.04 4.004

Io 0.38 0.40 0. 39 0.03 0.38 0.41 0.38 U.40 0.06 0.38 0.41 0.42 0.41 0.31 En 0.44 0.43 0.44 0.59 0.44 0.42 0.41 0.41 0.7 1 0.41 0.43 0.41 0.42 0.4S Fs 0.t3 0.17 0.11 0.38 0.18 U.A1 U.21 0.19 0.31 O.1S 0.16 0.21 U.2 0.28 AMIl 2.1 3.4 2.8 2.] 3.2 3.6 .I 3.1 2.3 5.5 4.4 4.0 3.I 4.0 TABLE A-111 (Cont.) PYROXENE ANALYSES

3.l.Hyr Cycle Cf12-4- 1678-35 M-14_ CF12-14A6

-- tmulQfts- 49.1 50.3 S3.6 50.6 48.6 49.9 s.4 50.6 Soo 50.4 50. 5 S0. 6 s0. 3.0 S3.6 2 2.94 2.95 1.73 4.06 2.7 . 3.71 3.6s 6.1 4.38 Al 0 3.6. 3.11 3.44 3.S0 3.69 2 1 1.29 1.04 0.f6 1.40 1.34 3.30 L.06 1.62 1.41 1.01 0." 0.9 1.31 1.14 9.3 10.7 6.5 7.2 1.0 6.0 6.1 Feo 10.4 9.9 1.6 7.S 1.2 9.6 10.3 0.34 o.32 flO 0.22 0.23 0.20 0.32 0.39 0.22 0.19 "tno 0.34 0.2s 0.39 0.33 0.3 13.8 IS.3 13.? 14.6 14.1 3s.0 14.3 14.6 3s.0 14.7 14. 3.s 13.4 33.6 "go 20.8 20.5 21.6 21.1 22.9 23.0 22.1 21.6 c&O 20.2 20.3 21.3 12.0 22.6 20.9 0.47 0.27 0.30 0.24 0.36 o.29 0.32 0.34 0.2s 0.20 0.2s O.3S 0.35 0.3 No20 0.36 0.04 - - . 0.22 0.3? O.09 0.20 Cr 203 99.9 99.? 99.6 300.6 100.6 100.9 100.3 300.6 C 303.0 100.4 99.4 99.7 100.6 99.s

1.926 1.669 1.79? 3.65 3.697 1.8 I.Mo I.686 3.8 3.683 3."4 3.654 3.62 so 1.669 0.333 0.344 0.108 O.07s 0.133 0.203 0.350 0.303 IVAl 0.333 0.120 0.112 0.334 0.119 0.056 2.000 2.0O 2.000 2.00o 2.000 2.000 2.000 2.000 2.000 2.000 2.0uo 2.000 2.00 2.000 tel 0.018 0.034 0.03 - O.OS 0.060 0.040 0.024 IAl 0.027 o.ols 0.o 0.038 0.040 0.020 0.036 0.028 0.026 0.037 0.030 0.030 0.025 0.044 0.0.J9 0.027 0.021 0.026 0.033 0.048 1 0.266 0.322 0.296 0.332 0.197 0.21 0.213 0.246 F. 0.322 0.309 0.242 0.233 0.223 0.306 0.006 0.006 0.eos 0.030 0.0O 0.006 0.e05 0.006 " 0.030 0.0My 0. 003 0.003 0.003 0.006 . 0.71 0. 2s 6.7Sl 0.602 0.60s 0.30 0.163 0.833 0.036 0.6s 0.609 0.1s4 0.749 mg 0.630 0.626 6.661 0.640 0.900 0.901 0.6O" 0.856 c 0. ml 0.608 0.64 0.816 0. 94 0.839 0.026 0.034 0.017 0.020 0.037 0.024 0.020 0.021 0.024 0.017 0.014 0.037 0.025 0.024 Na 0.004 - - - 0.oos 6.030 0.00 0.005 Cr . . . . 0.002 0.003 tct find 2.001 2.009 2.009 2.033 2.012 2.0o9 2.006 2.000 Vill 2.006 2.01s 2.000 2.006 2.012 1.96 4.001 4.009 4.009 4.011 4.309 4.009 4.0ON 4.000 Ccatloes 4.008 4.01S 4.008 4.006 4.012 3.586

0.41 0.46 0.47 .4S 0.43 0.42 0.44 0.46 0.46 0.44 0.44 0.45 0.43 we 0.42 0.43 0.40 0.40 6.43 0.40 0.42 0.42 to 0.41 0.42 0.43 0.42 0.42 0.40 0.39 0.10 0.12 0.33 0.3 0.14 0.16 0.3 0.12 0.32 0.16 0.37 O.AS 0.1 Fs 0.37 4.4 3.4 2.7 6.4 7.0 6.2 4.2 Alm71 3.6 3.1 S.6 S.6 6.2 2.3 3.0

01 .,. 0) TABLE A-Ill (Cont.)

PYROXENE ANALYSES

I 1441LS-cdtcIC 8C±... _ __°_- Q.l3Itr Cjcle U11 Cone .14...... -WH..11l (f.w fl. ______F27H-S Cf12.4-lie 157.F1- Siot IUO.7 4 d. _ 4crF b I Cr4 8 4d8s.4- W0.b 49. s0.0 St.l

A1203 4.85 6.2 4.92 3.89 6.9 5.4 2.54 3.63 2.98 2.23 1302 3. 0 1.92 1. S 1.93 2.95 2.23 1.32 1.10 I.59 1.13 Fell 6.6 9.U 6.6 10.0 30.S 9.5 11.1 10.2 9. 9.2 hooU 0.20 0.21 0.23 0.36 0.32 0.St 0.2? 0.20 0.21 0.23 hgO 13.6 31.5 14.1 14.1 11.' 13.1 14.6 13.4 13.8 IS.0 Cau 20.9 2.9 20.9 19.9 19.0 19.8 18.8 20.0 20.4 20.9 Cr2o 0.56 0.38 0.43 0.34 0.56 0.60 0.42 0.68 0.48 0.34 CrO 3 100.8 300.6 100.1 100.2 99.8 100.2 100.0 99.6 99.4 100.1

So 1.89 3.1002 1.634 1. hS6 1. 19S 3.605 1.S02 1.611 I. 5 1. S I W^ - 0.141 0.398 0.166 U. 144 0.205 0. IS 0.098 0.123 0.IIS 0.095 VIAl Itt 2. o0 2.000 2.000 2. W0 2.000 2.0 00 2. 00 2.000 2.000 2.000 111 0.069 0.011 0.049 0.026 0. U0 0.041 0.013 0.0J6 0.035 0.046 0.053 0.041 0.0SJ 0.0O4 0.061 0.036 0.04) 0.044 0.029 Fe 0.265 0.218 0.26) 0. J13 0.3130 0.29) 0.341 0.321 0.310 0.265 No O.0S 0.00S 0.006 0.011 0.00 0.036 0.001 0.005 U.O06 0.006 0.147 0.741 0.71 0. 716 0.659. 0.162 0.82S 0.750 0.115 0.32 Call 0.826 0.828 0.613 0.7ff 0.166 0. 190 0. 15 0.600 0.82S 0.631 wie 0.041 0.021 0.030 0.024 0.041 0.042 0.029 0.00 0.034 0.024 Cr tact and [a C 2.0W0 2.009 2.008 2.0W I .990 2. Ws 2. N 2.009 2.0W9 2.001 ticat leas 4.WI3 4.009 4.008 .00 3.994 4.009 4.0Ws 4.009 4.009 4.001 Fs lctcn ho U.4SS0.45 U.44 0.42 0.44 0.43 0.39 0.43 0.43 0.43 0.41 0.40 0.42 0.42 . d 0.41 0.41 0.40 0.41 0.42 0.34 U.IS U. 14 U.36 0.3 8 0.16 0.18 0.11 0.Is AIi U.6 4.6 S.l 4.5 j.2 J.6 3.9 3.1 j.4 3.0 TABLE A-IV

OXIDE MINERAL ANALYSES, BASALTS OF CRATER FLAT

3.7-Myr Cycle 1.1-lyr CycleSlac Cone Northern Cone 8lackt Cone CF12-7-6A Bo 8_- CF12-6-3 Magneti tc Magneti te Magneti te in In In lmenite Magnet i te IlmenI te Magnet I te 01 ivine Magneti te Magneti te 01 tvIne Magnet ite 01 IvIne 0to? 47.0 8.6 46.8 13.73 3.41 15.9 20.2 13.0 15.0 7.9 Al203 0.14 5.00 0.11 3.61 13.4 2.79 1.33 4.81 1.46 8.2 FeO 37.6 37.7 37.0 40.0 31.0 40.3 44.6 36.7 40.0 31.1 11.1 46.8 11.9 38.6 29.6 34.5 28.7 36.6 37.9 40.H Fe203 M90 1.03 0.36 0.60 *0.52 0.44 0.58 0.66 0.38 0.58 0.43 MnO 1.03 0.36 0.60 *0.52 10.44 0.58 0.66 0.38 0.58 0.43

Cr203 0.12 1.37 18.9 1.54 0.16 j.23 0.11 5.3

V2 03 .A. n.a. n.a. o.a. n.a. 0.62 0.71 0.59 0.53 ().52 98. ' 99.8 98.9 100.5 100.3 99.8 99.4 99.8 98.3 99.6

TI 0.892 0.239 0.885 0.375 0.088 0.435 0. 9 0.350 0.424 0.20 Al 0.004 0.217 0.003 0.155 0.542 0.120 0.058 0.203 0.064 0.341 Fe2+ 0.794 1.163 0.778 1.216 0.893 1.225 1.372 1.100 1.252 0.916 3 Fe 0.211 1.301 O.Z26 1.056 0.768 0.947 0. 98 0.989 1.068 1.081 Mg 0.077 0.065 0.094 0.144 0.182 0.193 0.166 0.238 0.154 0.219 Mn 0.022 0.011 .0.013 0.016 0.013 0.018 0.021 0.012 0.018 0.013 Cr 0.004 0.039 O.514 0.044 0.005 0.092 0.003 0.147 V n.h. n.a. n.a. n.a. .a. 0.018 0.021 0.017 0.016 0.015 I 2.000 3.000 2.000 3. 000 3. 0 3.UUo 3.000 3.000 3.000 3.000

magnetite 0.74 0.s5 0.89 0.45 0.31 0.53 0.49 0.71 Ulvospinel 0.26 0.45 0.11 0.55 0.69 0.47 0.51 0.29 1imenite 0.89 0.89 a;401 hematite 0.11 0.11 O. TABLE A-IV (Cont.) OXIDE IIHERAL AIJALYSES, BASALTS OF CRATER FLAT

1.1-Hyr Cycle 0.3-Hyr Cycle

* Red Cone Littlee Cone M.E. Lathrop Wells CF12-4-4 CFl22-4-130- FU78-7 Magnetite Magnetite In In Magnetite 01Ivine Flagnetite .Hnetite Wagnetlte 1oivne

T10 19.7 7.5 17.8 14.7 14.3 9.5 2.79 Al203 1.40 7.1 4. 70 3.46 7.3 FeO 43.4 31.0 41.8 39.5 37.5 31.2

Fe2 O3 30.5 45.8 32.8 37.2 37.1 41.8 HgO 3.58 5.0 3.5? 3.70 4.62 6.3 HnO 0.79 0.38 0.63 0.48 0.50 0.36

Cr2 03 0.13 2.43 0.31 2.43 3.04 9203 0.71 0.34 n.a. n.A. 0.56 0.54 100.2 99.6 99.4 I00.6 100.5 100.0

TI 0.540 0.I99 0.489 0.395 0. 85 0.249 Al 0.060 0.297 0.120 0.19U 0.146 0.302 2 Fe ' 1.321 0.9Z2 1.276 1.13 1.123 0.911 3 Fe 0.836 1.226 0.901 1.002 0.999 1.100 Hg 0.194 0.266 0.194 0.197 0.246 0.327 Hn 0.024 0.011 0.019 0.015 0.015 0.011 Cr 0. 004 0.068 0.009 0.069 0.084 W 0.021 0.010 n.j. n.a. 0.016 0.016 3.000 3.000 3.000 3.000 3.000 3.000

Magnetite 0.31 0.72 0.38 0.50 0.48 0.62 Ulvospinel 0.69 0.28 0.62 0.50 0.52 0.38 Ilmenite hematite TABLE A-V

N4PHIBOLE ANALYSES, BASALTS Of RED CONE AND LITTLE CONE N.E.

1.1-Myr Cycle Little Cone N.E. Red Cone CF1 2-4-l 3B CF12-4-4 Phenocrys ts Groundmass

Sio2 39.4 40.0 40.8 40.9

Al 203 14.3 12.9 13.8 13.4 TiO2 3.88 3.94 2.99 3.17 FeO 11.4 11.2 11.0 10.6 MnO 0.05 0.09 0.11 M9O 13.6 13.9 14.4 14.2 CaO 11.2 11.8 11.5 11.6

Na2 0 2.53 2.45 2.55 2.59

K20 1.20 1.00 0.80 0.80 Cr2 03 n .a. n .a. 0.08 0.14

(H 20) 2.5 2.8 2.0 2.5 100.0 100.0 100.0 100.0 I Si 5.851 5.950 5.992 6.026 IVAl 2.149 2.050 2.008 1.974 YIAl 0.345 0.205 0.384 0.349 Ti 0.431 0.441 * 0.328 0.351 Fe 1.410 1.390 1.351 1.305 Mn 0.004 0.006 0.010 Mg 3.000 3.082 3.151 3.117 Ca 1.785 1.879 1.806 1.829 Na 0.727 0.704 0.719 0.730 K 0.222 0.200 0.144 0.144 Cr n.a. n.a. 0.006 0.011 zcations 15.920 15. 905 15.895 15.846

Mg/(Fe+Mg) 0.68 0.69 0.70 0 .70

67 *U.S. GOVERNMENT PRsNTiNG OFFICE: 19.O.-t777-022167

aS % I IN

TABLE A-V

A4PHIBOLE ANALYSES, BASALTS OF RED CONE AND LITTLE CONE N.E. 1.1-Myr Cycle

Little Cone N.E. Red Cone CFl2-4-13B CF12-4-4 Phenocrysts Groundmass

SiO2 39.4 40.0 40.8 40.9 Al203 14.3 12.9 13.8 13.4 TiO2 3.88 3.94 2.99 3.17 FeO 11.4 11.2 11.0 10.6 MnO 0.05 0.09 0.11 MGO 13.6 13.9 14.4 14.2 CaO 11.2 11.8 11.5 11.6 Na20 2.53 2.45 2.55 2.59

K2 0 1.20 1. 00 0.80 0.80 Cr2 03 n.a. n.a. 0.08 0.14 (H20) 2.5 2.8 2.0 2.5 100.0 100.0 100.0 100.0

Si 5.851 5.950 5.992 6.026 I AI 2.149 2.050 2.008 1.974 Vi Al 0.345 0.205 0.384 0.349 YIA 0.431 0.441 0.328 0.351 Fe 1.410 1.390 1.351 1.305 Mn 0.004 * 0.006 0.010 Mg 3.000 3.082 3.151 3.117 Ca 1.785 1.879 1.806 1.829 Na 0.727 0. 704 0.719 0.730 K 0.222 0.200 0.144 0.144 Cr n.a. n.a. 0.006 0.011 zcations 15.920 15.905 15.895 15.846

Mg/(Fe+Mg) 0.68 0.69 0.70 0.70

67 *U.S. GOVERNMENT PRINTING OFFICE; 1941-0-777 022187 * bv

hinted In tbe United States of Amen¢c Available from National Technicel Inormation Service US ep nt ef Commerce 525 Pon Royal Road Spnfield. VA 22161 Microfiche 3.0 (AOl) Domestic NTIS Domestic 4t7S Domestic NTIS Domestic Nris Pae Range prie Prie Code PageRange Pice Price Code PageRange hice ice Code pe lange price Price Code

001025 S 5.00 A02 1175 311.00 AO 301-32 31700 AI4 451475 S23.00 A20 026450 4.00 A03 174-200 12.00 A09 326-350 IS.00 AUS 474-500 24.00 A21 05 475 7.00 £04 201.225 13.00 AIO 35 1-37 19.00 A16 501525 25.00 A22 076-100 3.00 AS 226-250 14.00 All 376400 20.00 A17 S26-SSO 2600 £23 101-125 9.00 £06 251.275 15.00 A12 401425 21.00 Al 551-575 27.00 £24 126-150 50.00 o 6?N1O 16.00 A13 426450 22.00 A19 576400 26JO0 A25 601up A99 tAdd S O for each additmonaJ25-page maemet o portion tber f rtom 601 pages op.

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