Ultramafic inclusions in late Miocene alkaline basalts

American Mineralogist, Volunw 70, pages 668477, 1985

Ultramafic inclusions in late Miocene alkaline basalts fromFry and Ruby Mountains, San Bernardino County, California

S. L. Nnvnt-r. Pnrnn ScnrrrM.,lxl. eNp Pnrnn SloI,rn

Department of Earth SciencesUniuersity of California, Riuersiile, Califurnia 92521

Abotract

Flows, dikes, and remnant constructional features of ultramafic inclusion-bearing alkalinebasalts, well known from the central Mojave, are also present near its southern boundarywith the San Andreas fault at Fry and Ruby Mountains. Ultramafic inclusions from theselocalities are predominantly spinel lherzolites and harzburgites. A single, 20Ggram garnetwebsterite inclusion found at Fry Mountain is only the second one reported in alkaline basaltfrom the western United States. All ultramafic inclusions from these localities exhibit tectonictextures and are believed to be accidental fragments of upper mantle material incorporatedinto basanite melts at temperatures between 950 and 1200'C and pressures between 9 and23kbar. Inclusion-bearing basaltic volcanism in the Fry and Ruby Mountain areas occurredbetween 6 and l0 million years ago, whereas comparable volcanism in the central Mojavebeyond the transpressional tectonism ofthe San Andreas fault has continued into the Quater-nary.

Introduction

Studies of ultramafic inclusions provide crucial input formodels concerning the thermal structure of the uppermantle and the origin of alkaline basaltic melts. Compo-sitional data on the coexisting minerals in these inclusionscan be used to estimate the pressure and temperature con-ditions of their source regions. Ultramafic inclusions fromthe central Mojave Desert of California have been exten-sively studied (e.g., at Dish Hill by Wise, 1966 and 1969;Wilshire et al., l97l and 1980; Wilshire and Trask, 1971;and at the Cima volcanic field by Katz and Boettcher,1980). This paper presents petrologic data and thermo-barometric calculations for a suite of ultramafic inclusionsfrom two new localities near the southern margin of theMojave Desert. These may represent the oldest occurren@sof ultramafic inclusion-bearing alkaline basalts in theMojave Desert. The Fry Mountain locality is of particularsignificance because a garnet websterite inclusion foundthere is only the second such described from alkaline ba-salts in the western United States.

Geologic s€tting

Fry Mountain (34'30'N, 116'43'W) and Ruby Mountain(34'16'N, 116'40'W) are the two most prolific ultramaficinclusion localities within an approximately 1000 squarekilometer area of predominantly inclusion-free alkaline ba-saltic volcanism in the south central Mojave Desert andadjacent San Bernardino Mountains (Fig. 1). At theselocalities, ultramafic inclusions, ranging in size from I to 12

r Present address: Department of Geology, University of Cali-fornia, Davis, California 95616.

0003-{04x/85/0708-0668$02.00

cm, occur within both flows and the exposed feeder dikeswhich have intruded Mesozoic plutonic rocks. Cindercones, common in central Mojave ultramafic inclusionlocalities, are rare and where present (e.9., at Fry Moun-tain) are deeply dissected (see Neville, 1982 and Neville andChambers, 1982 for detailed descriptions of field relations.)

Alkaline basalts in the Fry and Ruby Mountain areasare older than those of the central Mojave. Radiometricage determinations on alkaline basalts from the area shownin Figure I range from 6 to 9 m.y. (Peterson, 1976; Ober-lander, 1972). One inclusion-bearing flow at Fry Mountainhas been dated by K/Ar at 8.9+0.2 m.y. The oldest flowsin the central Mojave are very late Miocene (7.6 m.y. in theCima Field according to Dohrenwend et al., 1984), al-though inclusion-bearing volcanism has been active until atleast the late Pliocene: Katz and Boettcher (1980) report acarbon 14 date of 33G480 years on charcoal from a Cimaflow.

Basalt and megacryst petrography and chemistry

The alkaline basalts from Fry and Ruby Mountains aremarkedly uniform in their petrographic characteristics.These rocks are fine-grained and sparsely phyric; micro-phenocrysts generally comprise less than 15% of the modeand include olivine (0.75 mm) and lesser titaniferous augite(0.50 mm). Pilotaxitic groundmass minerals include otvine,clinopyroxene, plagioclase, Fe-Ti oxides, apatite, and insome samples phlogopite. Deuteric zeolites (natrolite andanalcite?) occur interstitially in the groundmass, in micro-fractures, and in vesicles. Megacrysts found in flows whichcontain ultramafic inclusions include: (1) strained anhedralolivine (up to 3.5 crn) at Fry Mountain; (2) translucent,white anorthoclase (AbtsOrrTAnor) at Ruby Mountain;

668

43 X- "ti: a" 'l^o l>"",'s%\ 53"',"u offKi-

frwY MouNrA<,

w F .o"!-

^ rfrO

sAN BeRNIRU'

r r rLE sAN BERNARDINO

669

sitionally identical, cxcluding inclusions, whether they areinclusion-bearing or inclusion-free. However, basalts whichcontain relatively high modal abundances (approximately5%) of kaersutite and clinopyroxene megacrysts areTiOr-poorer than basalts with lesser megacryst contents.

Inclusion petrography

Thirteen ultramafic inclusions from Fry Mountain and 26 fromRuby Mountain were analyzed modally, with ,100-500 pointscounted on each sample. Modal compositions of these samples,shown in Figure 3, indicate that lherzolite and subordinate harz-burgite are the most common inclusion types. Additional identi-fied ultramafic lithologies comprise singular occriirences of garnetwebsterite (Fry Mountain), olivine websterite (Ruby Mountain),and amphibole-olivine websterite (Fry Mountain). All of the spinellherzolite and harzburgite inclusions have petrographic character-istics of the Cr-diopside series of Wilshire and Shervais (1975).

Spinel lherzolite inclusions (Fig. 4) are composed of a mosaic ofanhedral forsteritic olivine (1.0 to 3.0 mm), anhedral to subhedralenstatite (1.0 to 3.5 mm, colorless with common clinopyroxenecxsolution lamellac approximately 2 microns thick), anhedral di-opside (0.5 to 1.0 mm and colorless, with rare orthopyroxene ex-solution lamellae), and anhedral brown to green spinel (0.5 to 1.0mm} which comprises up to 3% of the total mode. Texturally,spinel lherzolites are porphyroclastic (Pike and Schwarzman,1977), and characterized by kinked or strained porphyroclasts s€t

Table l. Composition of alkaline basalts from Fry and Ruby

"""r,"tr, a"*-t"

"Fry Mouota ln Ruby Ut.

FMA-2 FltA-6 FUG-32 FMCS-3I FUCS-32 FI1C-34 Rr-3 ru-20

Fig. 1. Ultramafic inclusion localities in the south-centralMojave Desert and adjacent San Bernardino Mountains. Theinset map in the upper right shows the location of other majorMojave Desert alkaline basalt localities including: Amboy Crater(A), Cima Domc (C), Dish Hill (D), Malapi Hill (M), and PisgahCrater (P). Generalized geologic units shown include: pre-Tertiarybasement (stippled), late Tertiary ultramafic inclusion-absent alka-line basalts (diagonal rule), late Tertiary ultramafic inclusion-bearing alkaline basalts (solid black), and alluvium (white).

and black, vitreous, conchoidally fractured (3) clinopyrox-ene and (4) kaersutite (both up to 5 cm) at both Fry andRuby Mountains. Kaersutites typically exhibit some trac€sofcleavage whereas clinopyroxene megacrysts do not.

Whole rock, major element analyses of basalts (minusinclusions if present) were determined by atomic absorp-tion and X-ray fluorescence spectroscopy (see Neville, 1982for analytical procedures). Representative analyses are pre-sented in Table 1. According to the classification of Wise(1969), all of the basalts analyzed are basanites (i.e., withSiO2 < 46wt.o/o,normativenepheline > 5o/o,andKrO >1.5 wt.%.) The alkali variation diagram of Figure 2a dem-onstrates the limited SiO, compositional variation of thebasalts from Ruby and Fry Mountains (closed circles) aswell as other petrographically similar inclusion-bearingand inclusion-free basalts from localities depicted in Figure1. As can be seen in Figure 2a,, alkali basalts from younger,central Mojave Desert localities (open symbols) are gener-ally more SiO2-rich. A better discriminant for dis-tinguishing these two suites of alkaline basalts is theirTiOr-MgO relationships (Fig. 2b). The basalts of the Fryand Ruby Mountain suite are generally enriched in TiO,and MgO with respect to the basalts of the Central Mojavesuite.

Basalts of the Fry and Ruby Mountain suite are compo-

NEVILLE ET AL.: ULTRAMAFIC INCLUSIONS IN ALKALINE BASALTS

Tlo2

A7203

Feo 11.94 IO.47 10.97 I t .34

I ' t n o 0 . 2 0 0 , 1 9 0 . 1 8 0 . 1 9

i l g o 7 . 2 a 8 . 3 2 1 I . 5 8 9 . 9 7

c a o 4 . 6 2 7 . 9 4 8 . 9 3 9 . 5 7

N a 2 O 3 . 9 9 4 . f 4 3 . 3 f 4 . 1 6

K z O 1 . 7 0 2 . 2 0 l . 8 r 0 . 7 6

L o r 1 . 9 9 r . 5 7 1 . 4 2 0 . 5 r

T o r a l 9 8 . 7 3 9 E . 9 3 9 8 . 9 5 1 0 0 . 4 9

s lo2 44 , t9 44 .89 43 . r4 43 .80

1.66 1 .43 3 .22 3 ,54

I 5 . l 6 I 5 . 7 8 r 4 . 3 9 1 6 . 6 5

4 2 . 8 2 4 4 . 1 0

3 . 6 9 3 . 1 9

t 4 . 2 3 1 4 , 3 E

t2 .66 t 1 .09

0 . 2 r 0 . 1 8

7.9a 11 .40

9 . 0 2 9 . 0 3

3.33 7 .26

2 . O 7 1 . 8 8

2.00 0 . E7

98,01 99 .38

CIPW Norn

t2 .78 I l .3 t7 . 5 5 6 . 7 9

18.49 19 .3 I1 1 . 8 3 1 r . 5 0

11.49 10 .547 , 5 A 8 . 0 r3 . 9 1 2 . 5 3

9.73 r5 .226 , 4 4 5 . 2 r

2 . 4 46 . 1 5

r00.00

q -o r 1 0 . 4 2 1 3 . 3 9 1 0 . 9 9a b 7 3 . 8 3 1 2 . 2 2 5 . 2 0a n 1 9 . 0 9 1 8 . 3 3 1 9 . 5 Ene 17.40 12.95 lz.7l

44.65 45.26

2 . 5 9 2 . 6 r

1 5 . 9 2 1 5 . 9 9

12.64 12.54

o . 2 2 0 . 2 1

7 . 9 2 7 , 9 6

8 . 4 6 8 . 4 0

4 . 3 6 4 . 4 2

1 . 6 3 r . 6 4

2 . 3 6 r . 6 3

100.75 r00 .66

9 . 4 2 9 . a 21 1 . 7 8 I 2 . 7 879.22 t9 .17r 4 . 0 3 I 3 . 5 3

4 .5010.9724.41r 3 . l 9

rc IO.29d i e n 6 . 6 E

f 6 3 . 6 0

enh y f 6

8 . 9 2 1 0 . 4 3 9 , 2 96 . 4 2 7 . 9 3 6 . 1 82 . 5 0 2 . 5 0 2 . 5 2

1 0 . 3 9 1 0 . 3 27 . 4 4 7 . 7 9

2 . 7 8 2 . 7 55.00 5 .0 r

100.00 100.00

9 . 5 7 9 . 4 25 . 9 3 5 . 9 33 . 6 4 3 . 4 8

f o 8 . E 9 1 . 0 . 8 9 1 5 . 8 0 1 1 . 4 9o I f a 5 . 9 3 5 . 3 6 6 . 1 5 7 . 6 9

ntt I

totel

2 . 6 4 2 . 3 4 2 . 4 4 2 . 4 6 2 , 8 77 . I 9 6 . 6 9 6 . 2 7 6 . 7 2 7 . 3 0

100.00 100,01 100.00 100.02 lo0 .0 l

. FRY ANO RU€Y @NTAiN AREA^ CIMA MME (KATZ ohd @ETTCHER, I9&)6 MALAMI HILL (STULL ONd MCMILLAN, 1973}a DISH H|LL (WISE, l$6; WTLSHIRE dd OTHERS,l9Tl)o Aw CRAT€R (BRKER, r$gi W|SE, 1969)o PrsGAt CRATER (WrSE, 1969)

s i o 2 w T %

670 NEVILLE ET AL.: ULTRAMAFIC INCLUSIONS IN ALKALINE BASALTS

Fig. 2. Major element compositional variations of alkaline ba-salts from the Fry and Ruby Mountain area, in comparison toyounger, Central Mojave Desert alkaline basalts. (A) NarO+ KrO versus SiOr. The dashed line separates fields of alkaline

and subalkaline basalts (Irvine and Barager, l97l). Dashed lineencloses field for basanites from the Fry and Ruby Mountainareas. (B) TiO, versus MgO relationships. Symbols are thc sameas in (A).

in a finer-grained (<2 mm) "matrix" of recrystallized unstrain€dand nonrecrystallized strained grains. Olivines exhibit strain fea-tures such as undulatory extinction and kink banding; ortho-pyroxenes commonly exhibit undulatory extinction and bent (210)cleavage planes. The strain bands in olivine and orthopyroxenegcnerally divide singlc grains into two or three zones having differ-ent optical orientations (Fig. d). Clinopyroxene exhibits minorstrain features and spinel exhibits little.

Harzburgites possess the same mineralogic and textural featuresas the spinel lherzolites but contain less than 5% modal clinopy-roxene. In the samples studied, brown spinel is a common access-ory (0.1-2 modal %).

A single 2fi) gram garnet websterite inclusion (sample FMCN-2) was discovered in the Fry Mountain field. Modally this noduleconsists ol 360/o garnet, 33% orthopyroxne, 2EVo clinopyroxene,3% spinel. Orthopyroxene and clinopyroxene are anhedral to sub-hedral with average grain sizes of 1.0 to 1.5 mm. The ortho-pyroxenes have a faint brownish tint and well developed (210)cleavage. Clinopyroxenes are colorless and have poorly developedcleavage. Exsolution is rare in both pyroxenes. Garnct and spineloccur as irregular, anhedral blebs interstitial to the larger pyrox-ene crystals (Fig, aa). Garnet grains have brownish rims, which incrossed polarized light appear to be composed of a radially fi-brous aggregate; microprobe analysis indicates these rims arecompositionally identical to the garnet. Green spinel is intimatelyassociated with the garne! since cvcry spinel grain is completelyor partly encloscd by an aggregate of garnet grains. A single, large(10 mm), subhedral garnet crystal was observed in one thin section

(Fig. aa). Hand sample examination suggests such largc crystalsare rare. Strain features in this sample are limited to undulatoryextinction and lesser bent cleavage planes within som€ pyroxenecrystals.

A single porphyroclastic olivine websterite inclusion was foundat Ruby Mountain. It contains approximately 18% forsteritic oli-vine, 60% enstatite, 21% diopside, and lo/o brown spinel. Thcaverage grain size is 1.5 mm. Exsolution lamellae are not presentin either of the two pyroxenes. Undulatory extinction is prevalentwithin olivine and orthopyroxene, less obvious in clinopyroxene,and not seen in spinel.

A single 400 gram amphibole-bearing inclusion (sampleFMCN-I) discovered at Fry Mountain has a mode of 22o/o oli-vine,50% clinopyroxene, 14% orthopyroxene, and 14% amphibo-le. Olivine (l-2 mm) is anhedral, generally ovoid, and exhibitsundulatory extinction. Anhedral light green clinopyroxene (1-2mm) is twinned and exhibits irregular, wormy exsolution of ortho-pyroxene. Orthopyroxene (0.712.5 mm) is faint pleochroic brownto colorless and has well developed (210) cleavage, anhedral tosubhedral crystal form, and lamellae of exsolved clinopyroxene.Amphibole is pleochroic (dark yellow-brown to faint yellow), sub-hedral, and poikilitically encloses orthopyroxeng clinpyroxene,and olivine. Microprobe analyses (Table 4) indicate that this am-phibole is kaersutite. Strain textures are uncommon in thissample, and although olivine and orthopyroxene display someundulatory extinction, the sample does not have a pervasive tec-tonite fabric. Rather, it is texturally dominated by the large poiki-litic amphibole and smaller poikilitic orthopyroxene and clinopy-roxene. Optically continuous amphibole poikilocrysts of dimen-sions 25 mm x 30 mm enclose clinopyroxene, orthopyroxene, andolivine (Fig. 4b). Recrystallized (?) orthopyroxene (non-strained,no exsolution lamellae) encloses ovoid olivine crystals as doessome clinopyroxene. Several grains of partly resorbed, strained,orthopyroxene occur throughout the sample and are apparentlyrelict crystals.

In addition to the ultramafic inclusions described above, grani-toid inclusions are also commonly observed in basalts from Fryand Ruby Mountains. They were presumably derived from localMesozoic plutons. No other types of inclusions were identified.

OL

^ d

/ \

. RUBY MONTAIN INCLUS(

O FRY MOUNTAIN SPINEL I

. FRY rcUNTAIN HORNBLEOLIVINE WEBSTER

; oLHERzoLrrE

I . 9 "

rNS

HEF

NOET E

O L I V I N E W E B S T E R I T E .

W E B S T E R I T E T

Fig. 3. Mineral modes of ultramafic inclusions from Fry andRuby Mountains. Classifcation after the LU.G.S. Subcommissionon the Systematics of Igneous rocks (1973).

=

+

;!

.


Fig. 4. Ultramafic inclusions in basanites from Fry Mountain. (A) Garnet websterite (FMCN-2) with large garnet in upper left ofphotograph, although most garnets form as small rims on green spinels which are interstitial to larger pyroxenes; (B) Amphibole(kaersutite) olivine websterite (FMCN-l) with large kaersutite poikilocryst enclosing olivine and pyroxenes; (C) Spinel lherzolite(FMCS-7) with typical porphyroclastic textures described in tcxt; (D) Spinel lherzolite (FMCS-3) with strain fcatures in olivines thathave been accentuated by oxidation. Abbreviations: olivine: ol; garnet : ga; spinel : sp; clinopyroxene : cpx; orthopyroxene : opx.All photographs taken in plane-polarized light.

671

Inclusion mineral compmitions

Minerals from six selected ultramafic inclusions wereanalyzadi four spinel lherzolites (i.e., samples RMA, RMD,FMCS-3, FMCS-7), the garnet websterite (FMCN-2), andthe amphibole olivine websterite (FMCS-I). Analyses wereperformed on a MAC5 model SA3 electron microprobeoperated at 15 kV and l-50 nA sample current. Data were

reduced using real time Bence-Albee matrix correctionsemploying the alpha factors of Albee and Ray (1970). Rep-resentative analyses of pyroxenes, olivines, spinels, garnets,and amphiboles from these samples are presented in Tables14.

Clinopyroxenes are diopsidic and exhibit little compo-sitional variability in Mg/(Mg + Fe) (Table 2 and Fig. 5).Compositions in spinel lherzolite are Enso-s3wo4r-43, in


Table 2. Representative compositions of pyroxenes from ultramafic inclusions and xenocrysts of Fry and Ruby Mountains

Ruby lbunteln

RI,IA R}IA R}ID RI|D Rn24B2t PMcs-l FUCS-L FMCN-2 FMCN-2

Fry lbuntaln

FUCS-3 FIICS-3 FMcs-7 FMcs-7 FuB**

sto2Tlo2

ezo3Cr2O3

Feo*

I€O

CaO

Na2O

MnO

To tal

s1T1A1CrFeMSCaNalftr

51.75 53 .63 52 .42

0 . 5 6 0 . 1 5 0 . 4 5

7 . 1 3 5 . 1 1 5 . 1 5

0 . 7 r 0 . 3 8 0 . 7 8

3 . 1 6 6 . 4 6 2 . 7 9

r4 .93 3 t .44 15 .11

18.96 0 .94 20 .34

r . 9 5 0 . 1 4 1 . 9 0

0 . l 4 0 . 1 3 0 . 0 8

99.29 98 .38 99 .02

J , . 8 1 7 1 . 8 8 5 1 . 9 0 80 . 0 1 5 0 . 0 0 4 0 . 0 1 20.305 0 .212 0 .22r0 . 0 2 1 0 . 0 I 1 0 . 0 2 30 . 0 9 6 0 . 1 9 0 0 . 0 6 50.607 L .649 0 .8200 . 7 3 7 0 . 0 3 5 0 . 7 9 4o. r37 0 .009 0 .1350.004 0 .004 0 .002

47.52 5r .25

1 . 8 1 t . 0 4

9 . 2 0 5 . 5 3

0 . 0 0 0 . 5 9

8 . 7 2 5 . 7 8

12.64 15 .31

L7.64 L9 .57

L . 7 4 1 . 2 8

0 . l l

99 .27 100.46

s3.77 50.43 52.98

0 .46 0 .42 0 .07

4 . 3 4 9 . r 2 7 . 7 5

0 . r0 0 .32 0 . 19

LL.42 3.60 7.93

29.92 14.45 3r .25

L . 2 5 2 L . t 6 0 . 7 3

0 .09 0 .53 0 .02

o .25 0 .07 0 .12

r01.60 r00, t0 10r,04

52.33 55 .43

o . 3 4 0 . 0 6

5 . 9 0 4 . 2 7

0 . 9 5 0 . 4 4

2 . 8 7 6 . 2 6

15.97 33 .52

2 0 . t 4 0 . 8 1

0 . 9 8 0 . 0 5

0 . 0 7 0 . t 3

9 9 . 5 5 r 0 0 . 9 9

54 .36

o , t 2

3 . 8 2

0 ,3E

9 .49

30 .75

o . 7 9

0 . 1 8

I00.11

Fomule Prcportlona Based 0n 4 Cetlong

52.23 54.83 47.80

0 . 4 9 0 . 1 0 1 . 7 0

7 . 2 2 4 . 7 2 8 . 0 r

0 .70 0 .26 0 .0 r

2 .85 6 .2 t 10 .32

15 .01 32 .73 r1 .45

19 .45 0 .74 19 .09

1 .36 0 .07 L .70

0 .06 0 .12

99.37 99.78 100.07

L .892 1 .900 1 .896 1 .7580.002 0.013 0.004 0.048o .172 0 .310 0 .193 0 .35 r0.0r2 0.020 0.0070 .179 0 .087 0 .180 0 .32 rr .706 0.814 L.687 0.6210 .030 0 .758 0 .o27 0 .7710.004 0.096 0.005 0.L230.004 0.002 0.004

r . 899 1 .755 r . 8590.003 0.0s0 0.0280 .158 0 .40 r o .2360 .011 0 .017o .277 0 .269 0 .176L ,602 0 .696 0 .8280.030 0.698 0.7610 .015 0 . r 25 0 .0900.005 0.003

r . 866 1 .833 1 .8 r9 r . 9020.0r2 0.012 0.002 0.0090 .178 0 .391 0 .3 r4 0 .2530.003 0.009 0.005 0.0270 .332 0 . r 10 0 .22a 0 .0671 .548 0 .783 1 .500 0 .8650.047 0.824 0.027 0.7850.007 0.037 0.00I 0.0700.008 0.002 0.004 0,002

AII Fe Glculated aa FeO.Ruby ilouDtaln cllrcpyroxere [egecryet.Fry lbuntaln clircpyrcrene @gacryat .

the amphibole olivine websterite (FMCS-I) are close toEnnrWoas, and in the garnet websterite averageEnroWoo2. AlrO, contents for diopsides range from 5.2 to7.4 wt.% in lherzolites, average 5.5 wt.% in amphiboleolivine websterite, and are 9.1 wt.Vo in garnet websterite.

Orthopyroxenes are Mg-rich with compositions En's_eo(Table 2 and Fig. 5). Mg-Fe partitioning between enstatiteand coexisting olivine is essentially l:1 for both peridotiteinclusions and amphibole olivine websterite and is 1: I be-tween orthopyroxene and clinopyroxene except for spinellherzolite inclusion RMD. Alumina contents of enstatitesin periododite inclusions range from 3.1 to 5.8 wt.% and ingarnet websterite alumina averages 7.8 wt.%. Althoughclinopyroxene ex$olution was observed in several samples,

it could not be analyzed due to the small width of lamellae.AlrO, contents of coexisting spinel and either enstatite ordiopside exhibit a positive linear relationship as noted byCarswell (1980) and Brown et at. (1980). Clinopyroxenemegacrysts are either titaniferous augites or titaniferoussalites (Enrr--rr'Wo36--42, Fig. 5). All clinopyroxene mega-crysts are higher in Al, Ti, and total Fe and lower in MgCr, and Ca than clinopyroxenes from spinel lherzolite.Charge balance considerations require that virtually alliron in pyroxenes from both inclusions and megacrysts bedivalent.

Olivines Clable 3) from 3 of the 4 lherzolite nodules havea restricted Md(Mg * Fe) compositional range betweenFose.3 and Foco.c. Inclusions RMD and FMCS-I (am-phibole olivine websteritQ contain olivines whose compo-sitions are close to Fosr. Both RMD and FMCS-I exhibittextures (e.g., RMD pyroxenes exhibit spongy border zonesand FMCS-I contains poikilitic amphibole) which implypartial re-equilibration at P-T conditions different thanthe conditions of initial formation.

Spinels exhibit significant compositional variability in Crand Al (Table 3). In lherzolites, CrrO, ranges from 8.4 to19.2 wt.o/o and AlrOr from 45.5 to 59.9 wt.%. Spinels ingarnet websterite average 3.5 wt.% Cr2O3 and UJ wt.%AlrOr. Mg and Fe content varies also, but not as much asCr and Al. Trace amounts of Mn occur in the tetrahedralsites of most spinels. The compositions of spinels in termsof Cr x 100/(Cr + Al) and Mg x 100/(Mg + Fe2+) areillustrated in Figure 6. Irvine (1967) has defined a field forspinels from ultramafic nodules in alkaline volcanics and

A GARNET WEBSTERITE. SPINEL LHERZOLITEr HORNBLENDE q IVINE

WEESTERI TEO MEGACRYSTS

FeS i03M g S i O 3

BF€S io3

Fig 5. Quadrilateral compositions of pyroxenes from in-clusions and megacrysts from Fry (A) and Ruby (B) Mountains.Tie lines connect compositions of coexisting orthopyroxene andclinopyroxene used in geothermometric calculations,

M q

NEVILLE ET AL.: ULTRAMAFIC INCLUSIONS lN ALKAUNE BASALTS

Table 3. Representative compositions of olivines and spinels from Fry and Ruby Mountain inclusions

OIlvlne8 Splnelg

sto2

Tto2

A1203

C12o3

Feol

Mgo

Cao

MnO

Nto

Total

40 .99

0.00

0.00

0.00

9 . 8 9

5 0 . 3 6

0 . 0 7

0 . l 3

o . 4 2

101.86

40.46 39 .50

0 . 0 r 0 . 0 1

0.o0 0 .00

0.00 0 .00

1 0 . 1 5 1 6 . 8 5

48.28 43 .43

0.07 0 .07

0 . l 8 0 . 2 9

0 . 4 5 0 . 3 7

9 9 . 6 0 1 0 0 . 5 2

39 , r3 40 .93

0 .03 0 .000.00 0.00

0 .03 0 .04

15.92 9.39

43 . t 2 50 .56

0.09 0.08

o . 2 3 0 . 1 5

o .23 0 .43

98 .78 101 .58

0 . r 3 0 . 1 7 0 . r 3

0 . 1 4 0 . 0 9 0 . 0 3

58.02 46.20 63.94

9 . 1 8 1 9 . 6 5 3 . 5 2

1 0 . 6 3 t 7 . 8 4 1 1 . 6 9

19.89 t5 .29 20 .09

0.00 0 .00 0 .00

0 . 1 1 0 . 2 8 0 . 0 5

98.10 99 .52 99 .45

0 . 1 4 0 . l 8

0 . 1 2 0 . l r

53 . l5 59 .92

1 3 . 3 5 8 . 4 5

t 4 . 2 5 1 1 . 2 4

20.45 21.26

0.00 0 .00

0 . 0 2 0 . 1 1

r 0 1 . 4 8 1 0 1 . 2 7

Fomula PEoport lons Baaed On 3 Cat lons

stTtA1CrFe!4cCallnNtFo

0.9988 0 .99610.0001 0 .0002

0 . 2 0 9 5 0 . 3 5 5 4I . 7 7 7 L t . 6 3 2 A0.0017 0 .00190.0038 0 .00610.0088 0 .00748 9 . 3 8 1 . 8

1 .0011 0 .9843 0 .9846

0.0006 0 .00070.3407 0 .1890 0 .19881 . 6 4 4 8 1 . 8 1 2 6 1 . 8 0 3 70.0024 0 .0020 0 .00200.0050 0 .0030 0 .00260.0048 0 .0082 0 .008282.6 90 .4 89 .9

0 .0033 0 .0047 0 .0032o.0027 0 .0018 0 .00051 . 7 9 1 5 1 . 5 0 8 4 1 . 9 1 3 60.1901 0 .4304 0 .07070.2329 0.4132 0.24830.7768 0 .5314 0 .7606

o.oo24 0 .0064 0 .00r0

0.0036 0 .00440.0023 0 .00211 . 6 2 1 3 1 . 7 8 3 10 . 2 7 3 1 0 . 1 6 8 60 . 3 0 8 5 0 . 2 3 7 30.7887 0 .8004

0.0005 0 .0022

I : Total Fe calculated as Feo.

kimberlites, and most spinels plot within this field. Theonly exceptions are in one lherzolite nodule (FMCS-7) andthe garnet webseterite (FMCN-2). The spinels are compo-sitionally similar to those in peridotite at Cima Dome andDish Hill and to spinel in the Dish Hill garnet clinopyroxe-nite (Fig. 6).

Garnets in the garnet websterite (FMCN-2) arePy"rAlrrGrr2 (Table 4). The garnets exhibit negligiblezoning or differences in composition whether they occur asdiscrete gtains or as rims on spinel. The garnets from thisinclusion are compositionally similar to those in the DishHill garnet clinopyroxenite (i.e., Py".Alr3Gr1n, Shervaisand others, 1973) except that the Fry Mountaio garnetscontain 1wt.% less AlrO. and about I wt.% more MgO.

Kaersutite poikilocrysts comprise approximately 14% ofthe mineral mode of inclusion FMCS-I from Fry Moun-tain. Cognate megacrysts of kaersutite from Ruby Moun-tain (Table 4) have lower MgO and NarO, slightly lowerCr2O3, and higher K2O and total iron. The Dish Hill veinamphibole in peridotite (Wilshire and others, 1971) isalmost identical in major element composition to the am-phibole in sample FMCS-I.

Thermobrrometry

The pressure-temperature crystallization conditions ofultramafic inclusions can be estimated or at least bracketedby assuming equilibrium partitioning of cations amongcoexisting minerals. Numerous reliable geothermometersare available, although geobarometers (particularly forspinel lherzolites) are limited. Since compositional zoningwithin minerals from individual inclusions from Rubv and

Fry Mountains is minimal, the compositions presented inTables 2-4 were used in all calculations.

The most commonly used geothermometers are based onFe/Mg partitioning between coexisting clinopyroxene and

J

Uz.o-o

+O

/ l

/ l/ 1

II

Io /

/ .l oI\ r o'f,u---

a A

,/ - ,*, oro ar*, - MOUNTAINS PERIDOTITE

I

i . "#L$oJl,"t"/ - cru l pERroorrrE

- (KATZ, t98r)

^ D I S H H I L L P E R I D O T I T E" ( w t s E , t 9 6 6 )

^ 33i#ii5fcll+?'13?..luo,t

AND orHERs,

too 80 60 40 ?o

Mg x IOO/Mg*Fe++ SPTNEL

Fig. 6. Crl(Cr + Al) versus Mg/(Mg + Fe++; compositional re-lationships of spinels in ultramafic inclusions from Fry and RubyMountains and other Mojave Desert localities. Dashed line en-closes the approximate field of spinels in peridotite inclusions andkimberlites as defined by Irvine (1967).


Tablc 4. Representative compositions of garnet and amphibolefrom Fry Mountain websterite inclusions and amphibole

megacrysts from Ruby Mountain

able results when temperatures are calculated using themethod of Wells (1977). Olivines in ultramafic inclusionsfrom Fry and Ruby Mountain range from 0.06 to 0.13wt.yo CaO, thus lelding a pressure range of 1F-28 kbar byassuming a mean CaO content of 0.08 and a possible tem-perature range of 93O-l(n0'C.

Maximum pressures of stability for spinel lherzolite inthe CMAS system have been experimentally delineated bya number of workers (e.g. MacGregor, 1965; O'Hara et al.,1971; Newton,1978; and O'Neill, 1981). Recently, ONeill(1981) has experimentally investigated the effects of Cr2O3on the spinel-garnet transition. At a given isotherm, in-creasing Cr2o3-content of spinel will dramatically increasethe equilibrium pressure for the reaction: spinel + ortho-pyroxene : garnet + olivine. Assuming 1000'C for theequilibrium temperature of the Fry and Ruby Mountaininclusions, the range in Cr2O3 content of spinels (3.5 tot9.7 wt.% Cr2O3, Table 3) yields equilibrium pressures of18-23 kbar. If FeO contents of inclusion olivines (whichaverage Fono) are accounted for, these pressures are low-ered to approximately 15-20 kbar (O'Neill, 1981, Fig. 5).

The Fry Mountain garnet websterite must also haveequilibrated at pressures below 20-23 kbar if the observedassemblage of orthopyroxene, clinopyroxene, spinel, andgarnet is stable. The spinel-garnet transition in peridotitesis defined by the reaction: spinel + orthopyroxene :garnet + olivine (O'Neill, 1981). Thus in the model system,MgO-SiOr-Al2O., the three phase sub-assemblage of or-thopyroxene + garnet * spinel is stable on the lowerpressure (and/or higher temperature) side of this reaction,for bulk compositions (such as websterites) that are MgO-depleted with respect to lherzolites or harzburgites. Re-connaissance calculations based on a combination of esti-mated modes and mineral compositions indicate that thegarnet websterite (FMCN-2) inclusion has significantlylower (MgO/(MgO + Al2O3 + SiOrX:9.26; than does arepresentative spinel lherzolite inclusion (e.g., FMCS-3with mode of 46V' olivine, 38% orthopyroxene, t3% clin-opyroxene, and 3%o spinel, and MgO/(MgO + Al2O3 +SiO, : 0.43) from the same locality. The temperatureestimates (Table 5) do not indicate that the garnet web-sterite inclusion equilibrated at significantly different con-ditions than the spinel lherzolites from Fry Mountain.Therefore, we suggest that the garnet websterite inclusion

Table 5. Equilibration temperatures for ultramafic inclusionsfrom Fry and Ruby Mountains, based on various two pyroxenc

geothermometers

M o r i ( 1 9 7 7 ) HeIIB (1977) wood & Benno (1973)

Cernet

FMCN-2

Anph lbo le

FMcs-l rul2lglslo2

Tto2

41203

Cr2O3

Feo2

llgO

Cao

ho

K20

Na2O

4 2 . 5 6

0 . r 3

23.98

o . 4 7

1 9 . 3 5

0 . 3 1

40.69

4 . 9 6

0 , r 7

8 . 3 0

1 5 . 0 5

I0 .80

. 1 0

t . 3 2

3 . O 2

39.88

5.04

r3.44

o . 2 3

r 3 . 8 6

10.70

10.43

. 1 4

l . b )

8 Ca!1on6

s i 2 .9898Tr .0066Al 1.9E68Cr .0267Fe .5423Hg 2 .0275Ca .4025Mn .o lE4

l5 Cat lons (uinus Na,K)

6 . 0 0 4 5. 5 5 I 1

2 . 3 6 7 8.0194

r . 0 2 5 03 .3 r r8L . 7 0 7 4

. 0 1 2 9

6 . 0 6 9 5

2 . 4 t 2 6.o279

1.7 6492 , 4 2 9 0r . 7 0 0 5

, 0 1 7 7

. 8 1 9 l

K -- .24a2Na .8701

Degacrya t

Total Fe es FeO

orthopyroxene. Table 5 presents the result of calculationsbased on the methods of Mori (1977), Wells (1977), andWood and Banno (1973) for six Fry and Ruby Mountainultramafic inclusions. Calculated temperatures for individ-ual inclusions generally have a 200"C scatter; the Morigeothermometer yields the highest estimates and the Wellsmethod the lowest. The temperatures range from a mini-mum 934'C (Wells geothermometer on RMD) to a maxi-mum of 1255C (Mori geothermometer on FMCS-l). Forany one particular method, however, temperatures calcu-lat€d for the entire suite do not vary by more than 75"C.All of the calculated temperatures are similar to or lowerthan the 1200-1300"C liquidus temperatures estimated forbasanitic melts at pressures between 25 and 30 kbar, withHrO contents between 2 and 7 wt.% (Green, 1973). Thusall the inclusions probably equilibrated at comparable tem-peratures, not far removed from those of the source regionsof their basanitic melt hosts.

Geobarometric estimates for ultramafic inclusions arepredominantly based on Al2O3 solubility in ortho-pyroxenes (e.g., MacGregor, 1974'1. Unfortunately, thealumina isopleths within the spinel field are not particu-larly pressure sensitive (see Hertzberg L978, Fig. 6). Mini-mum pressures for Ruby and Fry Mountain inclusions,calculated using Hertzberg's (1978) equation 31, are ap-proximately 36 kbar, which is unreasonably high for spinellherzolites.

Recently, a geobarometer based on calcium content ofolivine coexisting with orthopyroxene and clinopyroxenewas suggested by Finnerty and Rigden (1981). They dem-onstrated a method that produces petrologically reason-

RilD

RHA

FUCS-7

F!{CS-3

FUCS-1

FMCN-2

1188 I 35

1178 r 35

1220 * 35

1233 r 35

1255 r 35

1197 r35

934 t 70

952 r 70

984 r 70

1003 r 70

992 t 70

955 r 70

1009 r 60

1053 r 60

10E5 I 60

Ir02 r 60

1037 i 50

1041 r 60

NEVILLE ET AL.: ULTRAMAFIC INCLUSIONS IN ALKALINE BASALTS 675

most likely derived by recrystallization of a mor€ alumi-nous, less MgO-rich protolith than did the spinel lherzo-lites, but apparently under similar conditions, temperaturesbetween 950 and 1200"C and pressures below 23 kbar. Fur-ther evidence for a relatively low pressure origin of thegarnet websterite inclusion is indicated by garnet-orthopyroxene barometry (equation 5 of Harley andGreen, 1982), which lelds equilibrium pressures of ap-proximately 10 kbar.

All of the ultramafic inclusions found at Fry and RubyMountains are interpreted to be accidental upper mantlefragments as indicated by their tectonite textures, mineralassemblages, and thermobarometry. The kaersutite-bearingolivine websterite inclusion is also interpreted to be anaccidental upper mantle fragment that has been subjectedto mantle metasomatism (e.g., Wilshire et al., 1980; Boett-cher and O'Neil, 1980). The depth to the mantle in theMojave and adjacent San Bernardino Mountains is esti-mated to be between 30 and 35 km (Hadley and Kanamori,1977). Thus, minimum pressures for mantle-derived in-clusions are between 8.5 and 10.0 kbar, consistent with thebarometric calculations and the plagioclase-spinel transi-tion (Green and Hiberson, 1970).

Garnet websterite

Garnet-bearing peridotite has been classified as a distinctnodule group, the garnetiferous ultramafic group, by Wil-shire and Shervais (1975); according to them this inclusiontype does not belong to the Cr-spinel series. Garnet peri-dotite is extremely rare in basaltic rocks and when presentis thought to represent accidental upper mantle fragments(Shervais et al., 1973\. It is believed that the garnet web-sterite collected at Fry Mountain is only the second garnet-bearing inclusion in basalt to be found in the westernUnited States, the first being a garnet clinopyroxenite dis-covered at Dish Hill (Shervais et al., 1973). Other garnetperidotite discoveries in the western United States havebeen confined to kimberlite hosts in the Colorado Plateauregion (e.g., Helmstaedt and Doig, 1975).

The garnet websterite of Fry Mountain and the garnetclinopyroxenite of Dish Hill have dissimilar modal compo-sitions (Fig. 3). Orthopyroxene, garnet, and spinel are en-riched in the Fry Mountain inclusion relative to the DishHill inclusion. Mineral compositions, however, do notdiffer significantly.

Texturally, these two inclusions share some common fea-tures: (l) both exhibit tectonic deformation textures; (2)garnet occurs mainly as reaction rims on spinel and assubordinate distinct grains; and (3) recrystallization tex-tures are present. Important textural dissimilarities are: (1)grain sizes differ greatly between the two and (2) exsolutionfeatures are prevalent throughout the Dish Hill sample andare rare in the Fry Mountain inclusion.

To explain its deformation and recrystallization texturesShervais et al. (1973) interpreted the Dish Hill garnet clin-opyroxenite as an accidental inclusion derived from theupper mantle. They believe that garnet formed in the Dish

Hill sample by subsolidus reaction between clinopyroxeneand spinel and by exsolution from clinopyroxene. Thegarnet websterite from Fry Mountain is also an accidentalupper mantle fragment, but lacks textural indications ofrecrystallization by the reaction mechanisms postulated forDish Hill.

Alkaline basaltic volcanism of the Moiave Desertregron

The outcrop characteristics, bulk chemical composition,and mineralogy of the basalt province that includes the Fryand Ruby Mountain localities resemble those of the otherlate Cenozoic, alkaline basalts farther north in the MojaveDesert. The entire set of Mojave occurrences is in turncorrelative (Glazner, l98l;Katz and Boettcher, 1980), withthe late Cenozoic alkaline volcanism of the Basin andRange province (Leeman and Rogers, 1970; Best and Brim-hall, l974la tectonic setting characterized by high heatflow and a thinning crust. In both the Basin Range andMojave Desert provinces, magmas appear to have beengenerated at depths ranging from 40 to 60 km by no morethan 2oo/o partial melting of a peridotite mantle (Leemanand Rogers, 1970).

The MgO-SiO2-TiO2 contents, the tectonic setting andthe detailed age distributions do, however, reveal signifi-cant differences between the basalts that bear inclusions atthe Fry and Ruby Mountain localities and their counter-parts to the north in the Central Mojave. The Fry andRuby Mountain basalts are relatively primitive (i.e., some-what richer in MgO) and this may reflect changes in thepressures, temperatures, or compositions of source regionsfor the alkaline basaltic magmas during the last l0 m.y.Available age determinations indicate that the alkaline ba-saltic volcanism in the Fry and Ruby Mountain area beganno more than 10 m.y. ago and ceased approximately 6 m.y.ago. The latest age assessments for alkaline basalts in theCentral Mojave (Dohrenwend et al., 1984) indicate that thevolcanism began approximately 6 m.y. ago and has contin-ued into historic time.

The Fry and Ruby Mountain localities are part of abasalt province that is preserved in the eastern San Ber-nardino Mountains and immediately adjacent basins. Thispart of the Transverse Ranges belonged to the MojaveBlock until the onset of a late Cenozoic, transpressionaluplift (Sadler, 1982) that was probably a response to thelocal orientation of the San Andreas fault. Although themost spectacular uplift, associated with thrust faults, ap-pears to have occurred 2-3 m.y. ago, Sadler and Reeder(1983) argue that the M m.y. inception of basin formationat the sites of later thrust faulting is the first record oftranspressional tectonics. One experimental model of trans-pressional uplift (Bartlett et al., 1981) indicates amushroomJike cross section with a deep, tapered "root"zone. The local termination of alkaline basalt volcanism inand near the San Bernardino Mountains and its northwardshift coincident with the onset of transpressional effects


may be evidence of "root" zone influenc€s as deep as 4(Fd)km-the likely depth of magma genesis.

Acknowledgments

We thank Lewis Cohen and Harry McSween for thehelpful comments and review of this manuscript and Anth-ony Finnerty for sharing his thoughts on the thermobar-ometry of ultramafic inclusions. Our appreciation also toLinda Bobbitt (drafting), Robert Hicks (photography), andAnita Gentry (typing). An intramural research grant toPeter Schiffman from the University of California at River-side helped to defray the cost ofelectron microprobe analy-ses.

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677

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Ultramafic inclusions in late Miocene alkaline basalts