Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States

20 AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC XENOLITHS IN BASALTIC ROCKS FROM WESTERN UNITED ST ATES* By H. G.

WILSHIRE

and 1.

W. SHERVAIS

u.s. GeologIcal Survey, Menlo Park, California 94025, USA ABSTRACT Ultramafic xenolIths in basalts from the western United States are divided into AI-augite and Cr-diopside groups. The AI-augite group is characterized by AI, Ti-rich augites, comparatIvely Fe-nch olivine and orthopyroxene, and AI-rich spmel, the Cr-dlOpside group by Cr-rich clinopyroxene and spinel and by Mg-rich olivme and pyroxenes. Both groups have a wide range of subtypes, but the AI-augite group is dominated by augite-rich varieties, and the Cr-dlOpside group by olivine-nch Iherzolites.

Al AUGITE GROUP Pyroxene-rich members are interpreted as dikes, and olivine-rich members as their metamorphIc wall rock. This interpretation is based on occurrence of: (I) anastomosing thin pyroxenite bands withm olivme-nch xenoliths; (2) fragments of olivine-rich rock within the pyroxenite; (3) pyroxenite bands that crosscut metamorphic structures in olIvine-rich rock; (4) igneous textures m the pyroxemtes and metamorphic textures in the olivinench rocks; and (5) compositional trends such that peridotite in contact with pyroxenite changes from the contact outward toward compositions of the Cr-dlOpslde group. These structures, textures, and compositional relatIOnshIps resemble those of dikes and wall rocks in some alpine peridotites. Due to the preservation of igneous structures and textures in pyroxenites of the AI-augIte group and similarity of Sr Isotopic content of AI-augite rocks and basalts that brought them to the surface, we interpret the common pyroxenites as fragments of feeder dikes m the source zone of the host basalts. The composition of magma from which pyroxenites crystallized was modified by reaction with peridotIte wall rocks and by vein-forming processes of fractionation. The wall rock peridotite is interpreted as Cr dlOpside peridotite which was involved both in partIal fusion that yIelded basaltic magma and in reaction with magma denved from a broader zone of melting.

Cr- DIOPSIDE GROUP ThIS group reveals a similar, earlier hIstory. Although uncommon, intersectmg Cr-diopside pyroxenites like the AI-augite dIkes occur; they are found espeCIally where abundant relIc grains mdlcate less-than-usual deformatIOn and recrystallIzation. Dunite mcluslOns are found in lIttle-deformed Cr-diopslde pyroxemtes as they are in the AI-augite series. Chemical data suggest local preservation of trends between pyroxenite -pendotIte phases like those of the AI-augIte series. Complex clinopyroxenite vein systems occur in some alpine peridotites but are uncommon. The simllanties between the AI-augite and Cr-diopside groups support our hypothesis that the lIthologIC variations in both groups reflect partIal melting events that resulted in formation of igneous pyroxenite dikes or layers, WIth or WIthout marginal reaction zones in the peridotite wall rock, and larger zones of residual pendotlte from whIch the melts were derived. The Cr-diopside group formed the country rock of the AI-augite senes, but Itself reflects earlier lIke events.

* PublIcatIOn authonzed by the DIrector, U.S. GeologIcal Survey. 257

258

H. G. WILSHIRE AND J. W. SHERVAIS

INTRODUCTION The work of POWERS (1955), FRECHEN (1948, 1963), and more recently of FORBES and KUNO (1965), WHITE (1966), GREEN and RINGWOOD (1967), KUNO (1969), TRASK (1969), CARTER (1970), KUTOLIN and FROLOVA (1970), JACKSON and WRIGHT (1970), DAWSON and SMITH (1973), and many others has emphasized the variety of ultramafic xenoliths found in basaltic rocks rather than the uniformity that was emphasized in most earlier works. In particular, many recent studies have concentrated on a variety of xenolith characterized by black, aluminous augite and relatively iron-rich olivine and spinel that contrast with the more common peridotites that are characterized by Cr-diopside, magnesian olivine, and Cr-rich spinel. WHITE (1966) designated the former group the "dunite-wehrlite-gabbro" suite to distinguish it from his "lherzolite" group which includes the magnesian types. A group of rocks that is more-or-Iess the same as White's dunite-wehrlite-gabbro group has subsequently been called the wehrlite group, dunite-wehrlite group, black clinopyroxene type, black type, pyroxenite suite, and other less descriptive names. Considerable confusion has resulted because each major group of ultramafic rocks contains the subtypes -lherzolite, wehrlite, pyroxenite, dunite, etc.-used to name the main groups. Our proposal to designate these rocks the AI-augite and Cr-diopside ultramafic groups, respectively, does not completely escape this pitfall, but may alleviate the confusion. In contrast to the lack of agreement on names, the view that members of the AI-augite ultramafic group are cognate cumulates is virtually unchallenged (FRECHEN, 1963; FORBES, 1963,1967; WHITE, 1966; GREEN and RINGWOOD, 1967; AOKI, 1968; KUNO, 1969a, b; TRASK, 1969; BEST, 1970; CARTER, 1970; DAWSON et al., 1970; FRISCH and SCHMINCKE, 1970; ISHIBASHI, 1970; K UTOLIN and FROLOVA, 1970; McIVER and GEVERS, 1970; TAZAKI, 1971; HARRIS et al., 1972; UPTON and WADSWORTH, 1972; DAWSON and SMITH, 1973; IRVING, in press). A few authors (VILMINOT, 1965; AOKI and KUSHIRO, 1968) suggest a cognate relationship without specifying a cumulus origin, and FUSTER et al. (1970) suggest a noncognate cumulus origin. Members of the Cr-diopside ultramafic group are generally considered to represent mantle material from which variable amounts of basaltic liquid have been removed (FORBES and KUNO, 1965; WHITE, 1966; GREEN and RINGWOOD, 1967; KUNO, 1969a, b; TRASK, 1969; and many others). In this paper we describe the structural and textural relationships of members of the AI-augite and Cr-diopside ultramafic groups as observed in xenoliths from the western United States. These relationships suggest that the postulated cumulus origin of the AI-augite ultramafic group is a highly oversimplified view. The structural, textural, and mineralogical relationships of members of this group resemble in many respects the relationships of the less deformed members of the Cr-diopside ultramafic group and both may have originated by the same processes.

CLASSIFICATION There are seven main classes of xenoliths in basaltic rocks of the western United States: (1) xenoliths of crustal rocks that are clearly unrelated to the basalt. These include silicic igneous and metamorphic rocks
259

AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC XENOLITHS

ultramafic group; (6) feldspathic ultramafic group; and (7) garnetiferous ultramafic group. Groups (4), (5), and (6) are all spinel-bearing rocks, and some in group (7) contain spinel in addition to garnet. The mineralogy (except for presence of plagioclase), textures, and relative proportions of subtypes of the feldspathic ultramafic group are like those of group (4). Xenoliths representing the garnetiferous ultramafic group are quite rare in basaltic rocks of the western United States (SHERVAIS et ai., 1973). These seven classes of xenoliths, except (1), are not sharply defined groups and understanding their origin is as much dependent on recognition of rock associations and intergradations as on determination of textural, structural, and compositional relationships. In this paper our attention is devoted to xenoliths representing the Cr-diopside ultramafic group (4) and AI-augite ultramafic group (5). Because of the relative abundance in western United States of olivine- and pyroxene-rich members of the AI-augite ultramafic group, we focus attention on those rocks. However, it should be emphasized that similar rocks are subordinate members of dominantly gabbroic suites at some localities (CUMMINGS, 1972; STOESSER, 1973) where their origin may be entirely different from that which we propose here. The distinctive characteristics of the AI-augite and Cr-diopside ultramafic groups under discussion here are shown in Table 1. While the relative proportions of these two groups vary from locality to locality (WILSHIRE et ai., 1971), Cr-diopside ultramafic rocks are clearly the dominant type, as they appear also to be world-wide (KUNO, 1969b). Figure 1 illustrates

.,

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FIG I. TrIangular plots shOWing. distributIOn of lithologIes In AI-augite and Cr-dlOpSlde ultramafic group, Boundaries and names follow lUGS (1973) recommendatIOns the relative abundance of rock types representing each group. Boundaries between rock types are drawn on 5 percent lines according to the recommendation of JACKSON (1968; oral communication, 1971); the modal distribution clearly illustrates the suitability of these class limits. Names and additional class limits in Fig. 1 are those recommended by the International Union of Geophysical Sciences (1973). Although both major groups include representatives of most subtypes, rock types near the clinopyroxene-olivine line (Fig. I) are clearly dominant in the AI-augite group and olivine-rich lherzolite is clearly dominant in the Cr-diopside group.

260

H. G. WILSHIRE AND

J.

W. SHERVAIS

STRUCTURAL RELATIONSHIPS

AI-AUGITE ULTRAMAFIC GROUP Contacts between different members of the AI-augite group are especially well displayed in xenoliths from San Carlos, Arizona (FREY and PRINZ, 1971), Kilbourne Hole, New Mexico (CARTER, 1970), and Wikiup, Arizona (WILSHIRE et al., 1971). Xenoliths having a single contact separating olivine-rich from augite-rich parts are the most common. The grain size of the augite-rich part generally increases away from the contact (reaching dimensions of 2 cm or more), but even at contacts the pyroxene is coarser than in the olivine-rich part. Olivine decreases both in size and abundance in the augite-rich rock away from the contact, but shows no systematic size variation within olivine-rich rock. Contacts tend to be sharp and planar in small xenoliths, but many larger xenoliths show irregular and locally gradational contacts. Gradational contacts (PI. I, Fig. 2) are less commonly recognized within single xenoliths, but modal compositions intermediate between dunite and clinopyroxenite in many samples may represent portions of larger scale gradations from olivine-rich to clinopyroxene-rich rocks. Xenoliths which show two or more contacts are less common, but a substantial number have been recovered. Of these, the majority consist of thin pyroxenite bands between thicker olivine-rich bands, but some show the reverse. The proportions of rock types in Fig. 1 indicate that olivine-rich members of this group are somewhat less abundant or available for incorporation in the basalt than are the augite-rich members. The scale of the banding, based on direct measurement and on the dimensions of xenoliths recognized as members of the AI-augite group, varies from 1 mm to 10 cm or more (average of 140 samples is 3.6 cm) for augite-rich bands, and from 5 mm to 14 cm or more (average of sixty samples is 3.4 cm) for olivine-rich bands. However, in many xenoliths the banding is highly irregular (PI. I, Fig. 3). Even in small xenoliths the augite-rich rocks form irregular, anastomosing bands (PI. I, Fig. 3) that isolate pods of the olivine-rich members of the AI-augite group. These relations are the rule in large xenoliths (PI. I, Fig. 4). Crosscutting pyroxenite bands that differ in proportions of spinel and grain size are not uncommon (PI. I, Fig. 4), but generally the anastomosing bands appear to have formed concurrently. Directional structures are uncommon in the olivine-rich members of this group, but four samples (PI. I, Fig. 5) from Kilbourne Hole have prominent planar or linear alignments of olivine that are crosscut by the augite-rich bands.

Cr-DIOPSIDE ULTRAMAFIC GROUP Contacts between different members of the Cr-diopside ultramafic group are found in xenoliths from most localities, but are best observed in xenoliths from San Carlos, Arizona. Xenoliths with single sharp contacts between pyroxene-rich and olivine-rich rocks are particularly common (Table 1). As a rule, pyroxene is coarser grained in the diopside-rich rocks, and olivine is coarser grained in the olivine-rich rocks. Relic pyroxene grains with exsolution lamellae in diopside pyroxenite bands are as large as 6 cm across, in contrast to an average grain size of a few millimeters for all lithologies. Gradational contacts between olivine-rich and diopside-rich rocks are uncommon, and are characterized by uniformity

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FIG. 2. PhotomIcrograph of contact between spinel (black)-rich. AI-augite c1inopyroxenIte (C) veins and olivine (whlte)-rich wehrhte (W). Thin offshoots of pyroxenite extend diagonally from upper left and nearly Isolate a fragment of dUnIte (D). Contacts are gradatIOnal. FIG. 3. AnastomosIng AI-augite c1inopyroxenIte veins (dark gray) In ohvIne-rich host rock (light gray). FIG. 4. AI-augIte vein network in dunite (D)-wehrhte (W) host rock. Note dunite inclusion in pyroxenIte left center PyroxenIte veIn at right is coarser-grained than pyroxenite at bottom and crosscuts it. DUnIte In corner between these two veins is saturated with chnopyroxene formIng wehrlite (W), all contacts of whIch are gradational. FIG. 5. Photomicrograph of thIn AI-augIte ohvIne c1inopyroxenIte vein (horizontal, borders dotted in ink) that crosscuts foliation (trace dips diagonally to left) of lherzolite host rock. FIG. 6. BranchIng Cr-dlOpslde websterite bands (dark) In lherzolite. FIG. 7. DUnIte (D) "depletion zone" separating Cr-diopside lherzolite on right from websterite on left Note olIVIne-rich inclusions in websterite. 261

262

H. G. WILSHIRE AND 1. W. SHERVAIS TABLE I. PRINCIPAL FEATURES OF AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC GROUPSt Cr-dlOpslde group

AI-augIte group

LHERZOLITE; dumte. olIvine webstente; websterite, wehrlIte; olivine ciinopyroxenite harzburglte, ciinopyroxemte; [olIvIne orthopyroxemte], [orthopyroxemte]

OLIVINE CLINOPYROXENITE; WEHRLITE, CLINOPYROXENITE; dunlte; iher7olIte; [olIvIne webstente]; [webstente]

Structures A. Layenng and cross cuttIng

COMMONLYCONCORDANT SOLID FLOW LAYERS; branchIng but not cross cuttIng

B. Contact relatIOns

SHARP; PLANAR; GRADATIONAL

C InclusIOns

Olivine-nch InclUSIOns In pyroxene-nch rocks Common

COMMONLY BRANCHING AND CROSS CUTTING IGNEOUS LAYERS: pyroxene-rich dikes cro~s folIatIOn of olIvIne-rich wall-rock SHARP, IRREGULAR: GRADATIONAL OLIVINE-RICH INCLUSIONS IN PYROXENE-RICH ROCKS Uncommon

D. GraIn onentatIon

AJ Inera/ogy A. Relative abundance

'j'ECTONITE TEXTURES; RECRYSTALLIZA TlON TEXTURES; UnmIXIng textures

Pyroxene-nch rocks. IGNEOUS VEIN TEXTURES, unmlXlng textures; recrystallIzatIon textures; reactIon textures: cumulus textures; cataclastic tcxtures O/nlne-rich rocks RECRYSTALLIZATION TEXTURES, tectomte textures

OLIVINE; ORTHOPYROXENE; clInopyroxene; spInel; pargasIte; phlogoplte

CLINOPYROXENE, OLIVINE; spIneL orthopyroxene; kaer~utite; TI-phlogopIte

Cpx Opx 01 Ca 45 I Mg 49.3 896 868 Fe 5.6 10.4 122 Al 5.5 4.7 .84 Cr .56 TI

Sp Fe 8.1 AI 61.3 Cr 30.6

Cpx Opx 01 Ca 46.0 Mg 46.4 86.6 81.8 Fe 7.8 114 18.5 AT 7.7 4.7 Cr .23 Ti 128

Sp Fe 7.4 Al 886 Cr 4.0

t All upper case IndIcates important or domInant feature or rock type; lower case IndIcates feature or rock type present but subordinate; lower case In brackets Indicates rare feature or rock type. t CompOSItions above lIne are normalIzed At.°o and represent averages of fifteen or more analyses In each of twelve samples. CompOSitIOns below lIne are wt.°o oxides. of grain sizes. Xenoliths with two or more contacts most commonly show relatively thin diopside-rich bands between thicker olivine-rich bands. A greater volume of olivine-rich members available for incorporation in the basalt is shown by the proportions of xenoliths from 3 mm to 10 cm or more (average of ninety samples is 3.0 cm) for diopside-rich bands, and from 5 mm to 15 cm or more (average of 484 samples is 4.1 cm) for olivine-rich bands. The banding in both large and small xenoliths is typically planar and regular. In San Carlos xenoliths. however, branching (PI. I, Fig. 6) and intersecting pyroxene-rich layers are found. In places the bands appear to have been folded or faulted, but no crosscutting relationships between pyroxene-rich bands and directional elements in the fabric of olivine-rich members

AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC XENOLITHS

263

of the group have been observed. Inclusions of olivine-rich rocks are found locally in Cr-diopside websterites (PI. I, Fig. 7), especially in little-deformed pyroxenites. All of the principal subtypes of the Cr-diopside group have been observed in contact with one another in single xenoliths. Common physical arrangements of rock types are: websterite, olivine websterite, harzburgite, wehrlite, and olivine clinopyroxenite bands separated by lherzolite; webst~rite or wehrlite and lherzolite separated by harzburgite; and pyroxene-rich rocks and lherzolite separated by dunite (PI. I, Fig. 7). In some xenoliths the distribution of rock types is very complex, probably as a result of deformation.

TEXTURAL RELATIONSHIPS AI-AUGITE ULTRAMAFIC GROUP Textures of dunite, and of olivine-rich lherzolite and wehrlite, are typically granoblastic (PI. I, Fig. 2). At most localities, rocks with visible linear or planar fabric elements are scarce, but locally, as at Lunar Crater, Nevada (TRASK, 1969), they are common. Deformation and development of granoblastic-polygonal textures range from minimal to extensive. Olivine commonly has kink bands subparallel to (100), orthopyroxene has kink bands subparallel to (001), and clinopyroxene is typically undeformed except locally by cataclasis or weak polygonization. Spinel is not visibly deformed, and occurs as anhedral inclusions in olivine or as irregular grains molded between other mineral grains. Pyroxene-rich members of the AI-augite ultramafic group typically have xenomorphicgranular or polygonal textures (PI. II, Fig. 8a) which are neither clearly igneous or metamorphic (see VERNON, 1970). Green to gray spinels mostly form irregular grains molded between silicate minerals, but also occur as irregular inclusions in pyroxene. Most of the pyroxenites and olivine clinopyroxenites are uncteformed. However, a few xenoliths from Dish Hill, California, show extensive recrystallization textures. Relic augites in these rocks are deformed and have well-developed orthopyroxene exsolution lamellae. Several pyroxenites and pyroxene-rich wehrlites from Potrillo maar, New Mexico (CARTER, 1970), have well-developed cataclastic textures; annealed mylonitic bands, separate larger porphyroclastic (MERCIER, 1972) bands that are partly annealed but not apparently sheared. Exsolution lamellae of orthopyroxene and in places of opaque minerals is present in some augite pyroxenites, but the majority do not have optically visible exsolution features. Reaction textures involving clinopyroxene + spinel ---> plagioclase + olivine reactions are locally well developed, and some pyroxenites have patches of interstitial glass from which small euhedra of plagioclase, olivine, and opaque minerals have crystallized. Contacts between olivine- and augite-rich rocks are texturally gradational even where sharp modal breaks occur. Where the contacts are modally gradational, there is no visible change in texture across the contacts. However, in the transitional zones, the distribution of olivine, augite, and spinel is typically highly irregular (PI. I, Figs. 2,4). Cr-DIOPSIDE ULTRAMAFIC GROUP Textures of all members of the Cr-diopside group are normally metamorphic. Whether the dominant mineral is pyroxene or olivine, textures are granoblastic to granoblastic-

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PLATE II FIG. 8. A. Photomicrograph (crossed polarizers) showmg common texture of AI-augite pyroxenite. Large black grains near center are spmel. Orthopyroxene exsolution lamellae in clinopyroxene graIn at nght side. B. Photomicrograph (crossed polarizers) showing granoblastic texture of olivIne-nch member of Cr-diopside group. C. Photomicrograph of relic clinopyroxene with exsolved orthopyroxene in matnx of granoblastic clinopyroxene and orthopyroxene. Webstente In Cr-diopside group. FIG. 9. Large AI-augIte graIns (P) in dunite (D). Both have been deformed and partly recrystallized. Note dunite inclusions in clinopyroxene top right. FIG. 10. Clinopyroxemte vein network in dumte, Red Mountain, California alpine peridotite. FIG II. Photomicrograph of relic clinopyroxene with exsolved orthopyroxene in matrix of grano blastic clinopyroxene and orthopyroxene. Clinopyroxenite dike in Red Mountain, California alpine peridotite. Compare with Fig. 8c.

264

AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC XENOLITHS

265

polygonal (PI. II, Fig. 8b, c). Olivine and pyroxene form equigranular intergrowths, and spinel occurs as anhedral inclusions in olivine or is molded between other mineral grains. Kink bands are sporadically developed in fine-grained olivine, but are omnipresent in large olivines ((X-olivine, see JACKSON, 1968). Relic unrecrystallized diopside is characterized by abundant enstatite exsolution lamellae (PI. II, Fig. 8d), and unrecrystallized enstatite commonly has widely spaced kink bands as well as clinopyroxene exsolution lamellae. The sparse but widespread relic pyroxenes with exsolution lamellae attest to earlier hightemperature history of the Cr-diopside rocks, evidence of which has been virtually eradicated by deformation and recrystallization. Such relic pyroxenes are especially abundant in clinopyroxenites at San Carlos, Arizona. These rocks are texturally remarkably similar to the garnet clinopyroxenite from Dish Hill, California (SHERVAIS et aI., 1973), as well as to the deformed and partly recrystallized AI-augite pyroxenites.

COMPOSITIONAL TRENDS Preliminary data on mineral compositions, to be reported in detail at a later date, reveal systematic trends among different lithologies of each of the two main groups. These trends, though differing in magnitude from xenolith to xenolith, are present in all xenoliths of the AI-augite group so far investigated, but are present only in little-deformed members of the Cr-diopside group. Similar compositional variations across lithologic contacts in xenoliths of the AI-augite group are reported by BEST (1974).

AI-AUGITE GROUP Figure 12 illustrates variations in composition of olivine, clinopyroxene, orthopyroxene, and spinel with respect to the contact between AI-augite peridotite and clinopyroxenite from a single xenolith. The MgjMg + Fe ratio of olivine drops steadily in the peridotite toward the contact and reaches a minimum in the pyroxenite band. Al and Cr of the spinel vary antipathetically with Al reaching a maximum value near the center of the pyroxenite band. This change is so dramatic that it is readily seen in whole rock analyses despite the low abundance of spinel. The Al and Ti contents of clinopyroxenite rise, partly at the expense of Cr, toward the pyroxenite. The variations of Al in both pyroxenes suggest that discretion in use of the Al barometer is in order. The asymmetry of compositional variations in peridotite on opposite sides of the pyroxenite band is inferred to reflect proximity of another pyroxenite band beyond the present left edge of the xenolith before it was dislodged.

Cr-DIOPSIDE GROUP Figure 13 illustrates compositional variations of olivine, clinopyroxene, orthopyroxene, and spinel with respect to the contact between Cr-diopside peridotite and olivine websterite in a single xenolith. An olivine-rich zone separating the lherzolite from the websterite may be a "depletion zone" (BOUDIER and NICOLAS, 1972). There is no change in the MgjMg + Fe ratio of olivine and pyroxene within the peridotite, but a decrease within the websterite in keeping with the darker color of the websterite pyroxenes in hand specimen. Spinel likewise

266

H. G. WILSHIRE AND J. W. SHERVAIS

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shows no variation in AIICr within the peridotite, but shows marked changes like those of the AI-augite group within the websterite. Clinopyroxene shows an increase of Ti and decrease of Cr, and orthopyroxene an increase in AI, that begin well out in the peridotite and are enhanced in the websterite.

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13. Chemical variatIOns of olIVine, orthopyroxene, clInopyroxene and spinel With respect to the contact between peridotite and olIVine webstente 111 a sll1gIe xenolIth. Cr-dlOpslde group

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267

COMPARISON AND RELATIVE AGES OF AL-AUGITE AND CR-DIOPSIDE ULTRAMAFIC GROUPS In both the AI-augite and Cr-diopside ultramafic groups olivine-rich and pyroxene-rich rocks are associated. The AI-augite pyroxenites, however, typically form highly irregular, anastomosing bands in contrast to plane-parallel banding in the Cr-diopside pyroxenites. Olivine-rich members of both groups have metamorphic textures. The pyroxene-rich members differ in that the great majority of those in the Cr-diopside ultramafic group have unmistakable metamorphic textures, as is especially evident where they retain scattered relics of their metamorphism, in contrast to AI-augite pyroxenites whose textures are commonly ambiguous. Cr-diopside pyroxenites that are little recrystallized have textures that closely resemble those of AI-augite pyroxenites, especially those with well-developed exsolution lamellae. However, few pyroxene-rich members of the Cr-diopside group have survived recrystallization. Olivine-bearing members of the AI-augite group rarely have cumulus textures, and the distribution of spinels, dominantly irregular interstitial grains, resembles textures in the higher grades of granulite metamorphism of basic rocks (BINNS, 1964). The textures, however, contrast with those of the olivine-rich members of the group and most nearly resemble those of Cr-diopside pyroxenites that survived recrystallization. Accordingly, we consider the textures of the typical AI-augite pyroxenites to be probably igneous, perhaps modified in varying degrees by exsolution and reaction. A few members of this group have undergone solid deformation and recrystallization during which accompanying dunite was extensively recrystallized (PI. II, Fig. 9; TRASK, 1969). Chemical trends with respect to lithologic contacts are in the same direction for the AI-augite and Crdiopside groups, but are more pronounced in the AI-augite group. Direct evidence of the relative ages of AI-augite and Cr-diopside groups is uncommon. However, in a single specimen from Kilbourne Hole, New Mexico, an AI-augite pyroxenite band cuts across banded Cr-diopside peridotite. Crosscutting of metamorphic structures in Cr-diopside peridotite by AI-augite pyroxenite bands is more common. The far more prevalent metamorphism of members of the Cr-diopside group contrasts with the igneous structures and textures of the AI-augite group, and suggests that the latter group is younger. Apparent ages of members of the Cr-diopside group based on whole rock (PAUL, 1971) and internal (PETERMAN et al., 1970) Sr isochrons suggest old ages for these rocks, but scarce data on members of the AI-augite group (PAUL, 1971) do not plot on isochrons. Moreover, the AI-augite rocks appear to be isotopically similar to their host basalts (STEUBER and MURTHY, 1966) but different from the Cr-diopside group. The combination of all these differences is believed to indicate that rocks in the Cr-diopside group are generally older than those in the AI-augite group.

INTERPRETATION AI-AUGITE ULTRAMAFIC GROUP Textures of most pyroxene-rich members of the AI-augite ultramafic group suggest that these rocks are igneous whereas the textures of olivine-rich members of the group with which they are in contact are metamorphic. The structural relationships of members of the

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group are consistently such that comparatively thin bands of augite-rich rock are sandwiched by or form complex networks in olivine-rich members; as a consequence, inclusions of olivine-rich rock in pyroxenites are common. Both the textural and structural relationships of the rocks clearly indicate that augite-rich members of the group are igneous veins that have formed in previously solid, metamorphic country rock composed dominantly of olivine-rich rock. These olivine-rich rocks differ from olivine-rich members of the Crdiopside ultramafic group, which are also considered to represent metamorphic country rock, mainly in the more iron-rich composition of their minerals. Chemical data indicate that mineral compositions vary systematically with distance from the veins, becoming more like those of the Cr-diopside group farther from the veins. In view of the invariable association of AI-augite pyroxenite veins with olivine-rich members of the group, we interpret the compositional difference to be a consequence of reaction between the immediate wall rock of the veins and the magma that crystallized to form the veins. These structural and compositional relationships are similar to those in the Ronda alpine peridotite (DICKEY, 1970) and the Lanzo massif (BOUDIER, 1972; BOUDIER and NICOLAS, 1972). The great majority of authors who have described xenoliths of the AI-augite ultramafic group have ascribed all members to a cumulus process. Only JACKSON (1968), TRASK (1969), and FUSTER et al. (1970) to our knowledge recognized the metamorphic textures of olivinerich members of the group. JACKSON and WRIGHT (1970) interpret the dunites as reSidues of partial fusion that produced tholeiitic lavas, but they did not deal with rocks in direct contact with the dunites. Both WHITE (1966) and IRVING (in press) illustrate rocks in the AI-augite ultramafic group that appear to have cumulus textures, and some ultramafic rocks that are subordinate members of dominantly gabbroid suites in the San Francisco volcanic field (CUMMINGS, 1972; STOESSER, 1973) have the textures of cumulus rocks. However, such clearly cumulus textures are, in our experience, rare even among the igneous augite pyroxenites, and are altogether absent in the olivine-rich members of the AI-augite group. The absence of clear cumulus textures is apparently a general characteristic of these rocks. as the textures are almost always. where the subject is considered, assigned to the adcumulate class, that class whose cumulus origm is most difficult to establish. The augiterich anastomosing veins in olivine-rich metamorphic country rock can scarcely have been formed by a cumulate process. Indeed, it seems inescapable that the veins crystallized from magma whose composition probably has been modified by wall rock reactions and veinforming processes rather than by processes operating in a floored magma chamber. Relationships between the augite pyroxenite veins and their wall rocks are of three intergradational types: (1) where vein material is at least partly derived from the immediately adjacent wall rock, augite pyroxenite and augite lherzolite are separated by a dunite "depletion zone" (BOUDIER and NICOLAS. 1972); (2) where the wall rock is mechanically disaggregated (on scales from grain size to dimensions comparable to the size of the xenoliths), it has been saturated with through-going melt enroute to main conduits (PI. I, Figs. 2, 4). The common wehrlite and olivine clinopyroxenite members of the group may form under these conditions and represent partial melts with varying amounts of disaggregated olivine-rich wall rock; (3) where the melt invades country rock far removed from the source rock, there are no "depletion zones", and the extent of wall rock reactions depends partly on the size of the veins (these correspond to the "intrusive" dikes of BOUDIER and NICOLAS, 1972). Compositions of minerals in the wall rock of veins cover a comparatively broad range (e.g. olivine F074-88). suggesting that reactions between the wall rock and vein magma range from virtually nil to extensive.

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Pyroxene-rich members of the AI-augite ultramafic group are typically undeformed. However, some show thoroughly annealed cataclastic textures (Potrillo maar, New Mexico), others were deformed and recrystallized together with their dunite hosts (Lunar Crater, Nevada; TRASK, 1969). Still others (Dish Hill, California) are so coarsely recrystallized that identification is difficult. This suggests that such rocks are not all contemporaneous, and not all have a direct cognate relationship to the basalt that brought them to the surface. Rather, they appear to represent partial melts generated under generally similar conditions over a period of time.

Cr-DIOPSIDE ULTRAMAFIC GROUP The Cr-diopside ultramafic group has a similar variety of subtypes, although in different proportions, as the AI-augite ultramafic group. The main differences between the groups, aside from the compositions of their component minerals, are the extreme deformation and recrystallization that has affected all members of the Cr-diopside ultramafic group and the plane-parallel character of the lithologic layering. Despite the degree of penetrative deformation, relics of a former high-temperature history are preserved in the form of pyroxenes with exsolution lamellae, and in places (PI. I, Fig. 7), dunites separate pyroxenites and Iherzolites in a manner analogous to "depletion zones" in some alpine peridotites (BOUDIER and NICOLAS, 1972) (PI. I, Fig. 7), and lithic inclusions of peridotite occur in the pyroxenite as they do in the AI-augite group. Systematic chemical variations between peridotite and pyroxenite in contact in the same xenolith are like those in the AI-augite group. It is possible that pyroxene-rich members of the Cr-diopside ultramafic group also represent segregation of melts as dikes and veins like those of the AI-augite ultramafic group but at some earlier time in the history of the mantle. Subsequent solid flow could produce the plane-parallel orientation of the diopside-rich layers and their metamorphic textures (THAYER, 1963; NICOLAS et al., 1971; JACKSON and THAYER, 1972). In this way, the branching and intersecting diopside-rich bands in the San Carlos xenoliths (PI. II, Fig. 8) are interpreted as fragments of such a vein system. Their survival indicates that these rocks have not undergone as severe solid deformation as members of the Cr-diopside ultramafic group elsewhere. A lower degree of solid deformation of the San Carlos rocks is also indicated by the unusual abundance of relic deformed pyroxenes with exsolution lamellae. Clinopyroxenite vein systems of the sort we envisage occur in the Red Mountain, California alpine peridotite (HIMMELBERG and COLEMAN, 1968) (PI. II, Fig. 10), and in the Canyon Mountain, Oregon alpine peridotite (R. A. LONEY, oral communication, 1973). Furthermore, textures of the Red Mountain clinopyroxenites (PI. II, Fig. 11) are virtually identical with those of the San Carlos xenoliths (PI. II, Fig. 8d). However, clearly contemporaneous complex veining appears to be rare in alpine peridotites (A. NICOLAS, written communication, 1973), and some intersectirig pyroxene-rich layers constitute two or more systems of parallel layers of different age as observed in the Baldissero massif, Italy (ETIENNE, 1970) and in the Red Hill, New Zealand complex (WALCOTT, 1969). If vein systems like those of the AI-augite series were ever widespread in the Cr-diopside rocks, they have been destroyed by solid flow. Alternatively, structural controls on separation of partial melts may generally have produced plane parallel layering in the Cr-diopside series. More thorough comparisons of xenolith and alpine peridotite layering, distinguishing the main chemical groups, are clearly in order.

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Figure 14 illustrates om view of a possible physical arrangement of members of the AI-augite and Cr-diopside ultramafic groups before incorporation in the magma that brought pieces of them to the surface. Olivine-rich members of the AI-augite ultramafic group are interpreted as Cr-diopside peridotite modified by partial fusion and reaction. Considerably larger volumes of Cr-diopside ultramafic rocks may have contributed to the melt portions of the AI-augite ultramafic rocks than were involved in the melt-wall rock reactions. Identification of olivine-rich members of the AI-augite ultramafic group depends on reaction having produced more iron-rich compositions. Hence, peridotite that escaped the reaction but still yielded part of the melt would be identified as members of the Crdiopside ultramafic group. This may explain why olivine-rich types overwhelmingly dominate the Cr-diopside ultramafic group but occur in more nearly the same proportions as the pyroxene-rich types in the AI-augite ultramafic group.

o M-DISCONTINUITY _ _~~J-:-

~_

c

B

FIG. 14. Schematlc diagram illustrating possIble relatIOnships of cognate AI-augite pyroxenite and Cr-dlOpside ultramafic groups. (A) Mantle source zone; anastomosing feeders to main conduits. AI-augite ultramafic xenoliths with complex vein networks represent earher melts and wall rock from thIS and higher zones. (B) Gneissic mantle matenal composed of penetratively deformed Cr-dropside pendotites. Augen preserve parts of former complex vem networks. or. complex vein systems oeeur locally m roek bodIes dominated by plane parallel lithologic layenng. Dependmg on depth In mantle, these may belong to the Cr-diopside, garnetlferous, or feldspathic ultramafic groups. (C) Offshoot veinS from the main condUits penetrate crustal and mantle rock that was not Involved in the youngest melting episode. XenolIths from thiS honzon Include veinS in pendotlte of the Cr-dJOpslde and feldspathlc ultramafic group~ (D) SIll injectIOns In upper crust and wIthin contemporaneous volcanIC pIle yield cumulate dlffcrentlates that form locally Important members of xenolIth ,uite'i.

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ACKNOWLEDGEMENTS We are indebted to our colleagues N. 1. Page and T. P. Thayer for their helpful, critical reviews of the manuscript. Discussions over the years with E. D. Jackson, R. A. Binns, R. A. Loney and M. Prinz among others contributed immensely to the ideas presented here. This work was done under NASA contract W-13,130.

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