California Division of Mines Special Report 68 Igneous and Metamorphic Rocks of the Western Portion of Joshua Tree National Monument, Riverside and San Bernardino Counties, California

Pinto Gneiss

The Pinto gneiss was first named by Miller (1938). Although his discussion was restricted to an area near Twentynine Palms, the writer believes that the Pinto gneiss includes similar and apparently correlatable rocks throughout a large area in and around the western part of Joshua Tree National Monument. Some of the possible correlatives are the Berdoo series (described by Maclellan, 1936) and the Chuckwalla complex (described by Miller, 1944).

The Pinto gneiss is probably the oldest formation in the western part of the Monument. All of the igneous rocks appear to intrude the gneiss, although the age relations of some are not certain. The absolute age of the gneiss, however, is unknown.

Composition and Texture. The Pinto gneiss is a middle rank metamorphic rock characterized by plagioclase (albite to andesine), biotite, and quartz. Potash feldspar and muscovite occur in some facies, and amphibole, concentrated into thin bands of amphibolite, is also present. Rare grains of reddish brown garnet are scattered through the rock. The grain size of most minerals ranges from one-eighth to one millimeter, and some sections contain alternating one- to two-millimeter bands (parallel to the foliation) of different grain sizes. The foliation bands average from one-half to three millimeters wide and maintain a constant thickness within a given specimen. Portions of the rock mapped as gneiss up to a mile or so in diameter (and apparently randomly distributed) are almost completely devoid of foliation. Photo 4 shows a typical section of the gneiss.

Photo 4. Photomicrograph of Pinto gneiss. Quartz, the most abundant mineral, forms coarse irregular grains. Biotite forms flakes parallel to the foliation, slightly below and left of center is an aggregate of fine grained, felty, muscovite. Crossed nicols. X25.

About 90 percent of the Pinto gneiss is dark colored and characterized by the presence of biotite. An average composition is:

Plagioclase ranges from albite to andesine in different specimens and averages about An₂₀. Plagioclase is generally twinned, invariably unzoned, and forms the largest grains in the gneiss (up to two millimeters). Quartz commonly forms layers several millimeters wide and devoid of all other minerals; the borders between quartz grains are highly sutured. Biotite is pleochroic from yellow to dark brown and occurs in bands approximately one millimeter wide between bands of quartz and feldspar. Muscovite forms masses, several millimeters in diameter, of tiny, interlocking crystals; possibly these masses are pseudomorphs of some other mineral. Potash feldspar is lacking in many specimens; where it does occur, it forms small, irregular grains. The most common accessory mineral is magnetite, but rutile is present in some specimens in which the biotite appears to have been chloritized. All grains except the accessory minerals are anhedral, and most are morphologically and/or optically oriented parallel to the foliation.

Almost all of the Pinto gneiss not included in the biotitic facies consists of a light-tan, well-foliated, feldspathic, rock with the following average modal composition:

The average grain size in this light colored gneiss is slightly smaller than the grain size of the biotitic gneiss, and grain orientation is not as well developed as in the biotitic rock. The average composition of the plagioclase is An₂₀, and most of the grains are twinned and unzoned. Biotite is pleochroic from yellow to dark brown and forms very irregular flakes. The most common accessory mineral is magnetite. Except for the accessory minerals, all grains are anhedral.

In addition to the biotitic and feldspathic types of gneiss, a few thin (less than ten feet wide) bands of black amphibolite are scattered throughout the Pinto gneiss. One sample contains an estimated 65 percent of yellow to greenish-blue hornblende prisms, 30 percent untwinned, anhedral plagioclase (An₅₀), 5 percent biotite, and rare quartz, magnetite, and apatite. The average grain size is one-fourth millimeter.

The non-foliated (granitoid) portions of the rock mapped as Pinto gneiss appear to be randomly distributed throughout the formation. In general, these portions contain slightly more potash feldspar and less quartz than the rest of the gneiss but otherwise are mineralogically identical with either the biotitic or feldspathic facies of the gneiss. Foliation, where present at all, is indicated by alignment of biotite. The average grain size is 0.5 millimeter, and all grains are anhedral. An excellent example of non-foliated rock occurs south of Quail Spring.

One-half mile east of Stirrup Tank is a type of foliated feldspathic gneiss which contains only 25 to 30 percent quartz and, thus, has a composition similar to that of the non-foliated portion of the formation. Such combinations of low quartz content and well-developed foliation, however, are very uncommon.

Structure. Throughout the area the gneissic foliation shows a general north to northwesterly strike and nearly vertical dip. Both on a broad scale and in detail, however, the foliation trends parallel to the contact with the White Tank quartz monzonite in the Pinto, Hexie, and Lost Horse Mountains. Thus, in areas of intrusion of the White Tank quartz monzonite the gneissic foliation is bent away from the regional trend as if warped by the same force which caused the intrusion of the quartz monzonite.

In the Pinto and Hexie Mountains, for distances of up to a mile from the quartz monzonite contact, the gneissic foliation strikes parallel to the contact but shows extreme variation in dip. In fact, the entire gneiss in this region is contorted into nearly vertical, isoclinal folds with a width seldom exceeding 50 feet. The dip of the foliation ranges from vertical through horizontal and back to vertical in the space of a few feet. In the Lost Horse Mountains this intricate folding and variation in dip is not present, and the reason for its presence in one place and absence in another is not known.

Detailed mapping in the Lost Horse Mountains shows the presence of folded structures, although the general trend is north-south. In detail, the foliation parallels the contact with the quartz monzonite, and in the northern part of the mountains there is some indication that a regional foliation making an angle up to 30 or 40 degrees with the contact is warped into parallelism within a foot or so of the contact. In other places, however, gneiss with a regional strike at an angle of approximately 60 degrees to the contact is cut obliquely by the quartz monzonite.

One of the most striking features of the gneiss is the banding shown by the biotitic and feldspathic facies. In the Lost Horse Mountains these two types form layers 100 to 500 feet thick and generally parallel to the foliation. In places there is compositional gradation between these facies, but generally a sample of the gneiss is definitely of one or the other type.

Origin. For several reasons the Pinto gneiss is thought to be almost entirely the product of metamorphism of sedimentary or volcanic material. For instance, layers of different composition several hundred feet wide and parallel to the foliation (as in the Lost Horse Mountains) seem most easily explained by metamorphism of a layered sequence. This evidence does not distinguish between metasediments and metavolcanics.

Secondly, the difference in the composition of the plagioclase in different layers indicates formation in a chemically non-homogeneous system. If, as seems possible, this lack of homogeneity represents chemical differences in the original rock, then the parent rock was more likely sedimentary or volcanic than plutonic.

Thirdly, the average quartz content of the gneiss is approximately 50 percent. Such a high percentage suggests a sedimentary rather than an igneous parent rock.

Assuming that the Pinto gneiss formed by middle-rank metamorphism of sedimentary or volcanic rocks, it is difficult to determine the exact nature of the parent rock. The composition indicated by modal analysis of the gneiss (and assuming isochemical metamorphism) is similar to the composition of graywackes* except for a higher potash-soda ratio (K/Na averages 1 to 1.5 in the Pinto gneiss). The total silica content seems too high for normal igneous rocks but may well represent the composition of mixed sedimentary and volcanic material. No relict textures or structures, other than the large-scale layering, have been found which would aid in determining the nature of the parent material. The gneiss of quartz-monzonitic composition east of Stirrup Tank may be a true metamorphosed igneous rock, or it may represent transition into the Palms quartz monzonite or the non-foliated portion of the gneiss.

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*As given by Pettijohn (1949), p. 250.

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The mode of formation of the granitoid, non-foliated parts of the formation is not known. They may be related to the emplacement of the Palms quartz monzonite, and some geologists would probably consider them to be the product of magmatic intrusion into the gneiss. On the other hand, they may be recrystallized gneiss lying above areas of magmatic or metasomatic emplacement of quartz monzonite. But in many places there seems to be no spatial relation between the Palms quartz monzonite and the non-foliated portions of the Pinto gneiss. At contacts between the Palms quartz monzonite and the gneiss, the most common effect is the formation of migmatite and other rocks of apparently mixed metamorphic and igneous origin; at only one contact (east of Queen Mountain) is the gneiss pronouncedly recrystallized by the Palms quartz monzonite to rock similar to the large areas of non-foliated material. Thus, although the granitoid portions of the gneiss probably developed at a late stage of, or subsequent to, the metamorphism which produced the gneiss, they apparently are unrelated to the Palms quartz monzonite and may represent a separate phase of plutonic activity.

Gold Park Gabbro-Diorite

The Gold Park gabbro-diorite was first named and described by Miller (1938). The formation occurs in small, isolated outcrops scattered throughout the area investigated.

Composition and Texture. The gabbro-diorite is a dark gray, massive, coarse- to fine-grained, inequigranular rock which shows extreme textural and compositional variation. Perhaps the most common variety is characterized by hornblende prisms several millimeters long which poikilitically enclose subhedral laths of plagioclase. Another type of gabbro-diorite is fine-grained and equigranular and consists largely of plagioclase and biotite with approximately 10 percent accessory minerals.

Modal analyses of these two types of gabbro-diorite gave the following results:

Plagioclase in both rocks is normally zoned, the range being from labradorite to andesine in the coarse-grained rock and from calcic andesine to calcic oligoclase in the fine-grained rock; some of the oligoclase is antiperthitic. Hornblende in both sections mentioned above is pleochroic from yellow green to greenish blue. Biotite is pleochroic from yellow to dark brown. The identification of pyroxene in the equigranular rock is uncertain. The equigranular rock contains up to five percent colorless, low birefringent, tiny needles with hexagonal cross-sections; these needles are tentatively identified as apatite. Alteration products are common only in the coarse-grained gabbro-diorite; plagioclase is intensely sericitized, and hornblende is altered to chlorite and a mineral tentatively identified as sphene. Photo 5 shows a typical section of the fine-grained gabbro-diorite.

Photo 5. Photomicrograph of Gold Park gabbro-diorite. The major (white) mineral is plagioclase. Dark grains ore primarily biotite with some magnetite and small grains of olivine. The tiny needles are apatite (?). Plain light, X65.

Structure. No structure has been found in the gabbro-diorite, but the intense weathering of most outcrops makes detection of structure difficult.

Origin. Although the Gold Park gabbro-diorite probably formed by crystallization of a melt, some mineralogical details are difficult to explain by such a process. The presence of olivine and hornblende in one sample and the absence or scarcity of pyroxene seems to contradict crystallization according to the standard Bowen reaction series.

Palms Quartz Monzonite and Monzonitic Porphyry

The Palms quartz monzonite crops out in the mountains south of Twentynine Palms, in Queen and Lost Horse Valleys, and in the area west of Lost Horse Valley. South of Twentynine Palms the rock is typically coarse- to medium-grained and inequigranular, but in Queen and Lost Horse Valleys and west of Lost Horse Valley, the rock is finer grained and more equigranular. Miller (1938) described the Palms granite (with typical exposures south of Twentynine Palms).

The monzonitic porphyry (described by Miller, 1938) crops out around the northern periphery of the mountainous area south of Twentynine Palms. For reasons to be discussed later, the porphyry is considered to be intimately related genetically to the quartz monzonite.

The Palms quartz monzonite and monzonitic porphyry are younger than the gneiss, as is shown by dikes of each rock in the gneiss at various places. Contacts between the quartz monzonite and the gabbro-diorite are invariably so gradational that no exact age relations may be stated, although in places the quartz monzonite appears to invade and inject the gabbro-diorite. Dikes of quartz monzonite cut the monzonitic porphyry and vice versa, a further indication of the genetic relation between the two rocks. The absolute age of the Palms quartz monzonite and monzonitic porphyry is unknown.

Composition and Texture of the Palms Quartz Monzonite. The Palms quartz monzonite is a gray to brown, coarse- to fine-grained, generally massive rock which may be divided into three types: Unit A (Photo 6), a coarse grained rock as in the type area near Forty Nine Palms; Unit B (Photo 7), a finer-grained rock in the upper part of the mountains south of Twentynine Palms; and Unit C (Photo 8), a fine-grained rock in Queen and Lost Horse Valleys and the area west of Lost Horse Valley. An average composition of the formation is approximately 30 percent quartz, 30 percent potash feldspar, 40 percent plagioclase (oligoclase), and a few percent biotite and accessory minerals.

Photo 6. Photomicrograph of Unit A of Palms quartz monzonite. The major minerals are quartz (aggregate of coarse grains), a large crystal of plagioclase (center of the picture), and microcline (showing some grid twinning). All minerals are seriate from the large crystals to the smallest grains shown here. Crossed nicols. X25.

Photo 7. Photomicrograph of Unit B of Palms quartz monzonite. Major minerals are quartz (in aggregates), plagioclase (poorly twinned, altered), and microcline (showing grid twinning). Crossed nicols, X25.

Photo 8. Photomicrograph of Unit C of Palms quartz monzonite. Major minerals ore quartz (undulant), plagioclase (well twinned), and microcline (grid twinned). Crossed nicols, X25.

Unit A, the rock in Forty Nine Palms Canyon and the mountainous area to the south, is coarse-grained, very inequigranular, generally massive, and ranges from brown to gray. Foliation, caused by alignment of biotite flakes, is present in some places, but no general foliation trends could be found, even over small areas. An average modal analysis of this coarse-grained rock is as follows:

Quartz forms aggregates of sutured grains. Plagioclase ranges from An₃₀ to An₁₅, the more calcic grains generally zoned and the more sodic grains twinned. Potash feldspar and plagioclase are typically complexly intergrown, the borders between the two minerals being characterized by islands of each mineral in the other. Biotite is pleochroic from yellow to reddish-brown. Perthite, myrmekite, and fine grained, interstitial albite are common. Common blue-green hornblende is present in a few sections. Magnetite, sphene, apatite, and epidote are present in almost all sections, but zircon and allanite are rare. All of the major minerals are anhedral, and the accessory minerals are generally euhedral.

Dikes of Unit A in either monzonitic porphyry or gneiss are either fine grained and equigranular or contain phenocrysts of plagioclase in a fine-grained groundmass. The best exposure of these dikes is in the monzonitic porphyry east of Indian Cove.

In the higher parts of the mountains around and south of Forty Nine Palms is a facies of quartz monzonite slightly different from Unit A. This facies (Unit B) everywhere lies above Unit A, but the contact between the two rocks is completely gradational and very difficult to locate.

In distinction to the massive Unit A, Unit B is poorly to well foliated. Generally, the foliation is nearly vertical and parallels the northwesterly trend of the Pinto gneiss. Foliation is marked by orientation both of biotite flakes and elongate aggregates of quartz grains. Some parts are apparently massive.

Units A and B are very similar in bulk chemical composition and general textural features. The chief differences between the two rocks are:

In two separate outcrops, one in Queen Valley and the other in and west of Lost Horse Valley, is an equigranular, fine-grained quartz monzonite (Unit C). Unit C intrudes and sends dikes into the gneiss on the west side of Lost Horse Valley (where the contact is gradational through about an inch), but is clearly older than the White Tank quartz monzonite (as is shown by dikes and other contact features). Although unconnected, the two bodies of Unit C are very similar and almost certainly the same formation; their correlation with Units A and B of the Palms quartz monzonite, however, is based on the following rather tenuous evidence—first, that Unit C grades into Unit B in the eastern part of Queen Valley; and second, that as nearly as can be determined, Unit C is the same age as Units A and B.

Megascopically, Unit C is light brown and generally massive, but some specimens show a faint foliation. Modal analyses of several specimens give the following average results:

Plagioclase ranges from An₁₀ in the eastern part of Queen Valley to An₃₀ west of Lost Horse Valley and seems to show a general westerly increase in anorthite content. Quartz and potash feldspar are anhedral, but plagioclase is subhedral. Accessory minerals are uncommon magnetite is present in most specimens, zircon and apatite are rare, and sphene is absent. Textural characteristics of Unit C which distinguish it from Units A and B are fineness of grain, equigranularity, lack of intergrowths between potash feldspar and plagioclase, and absence of quartzose or biotitic aggregates.

Structure of the Palms Quartz Monzonite. Most of the Palms quartz monzonite is either massive or contains local patches of foliated rock which exhibit no general structural trend. Orientation of biotite flakes in portions of Unit B gives the rock a distinct foliation parallel to contacts with the gneissic wall rocks, and thereby, parallel to the foliation in the gneiss.

Jointing is well developed in areas of Palms quartz monzonite which overlie units of the White Tank quartz monzonite. This feature will be discussed in the section on the structure of the White Tank quartz monzonite.

Contacts Between the Palms Quartz Monzonite and Pinto Gneiss. Contacts between the Palms quartz monzonite and the Pinto gneiss range from very abrupt to broadly gradational. Most contacts are gradational through a distance from 1 inch to 10 feet, and it is generally possible to distinguish between a poorly foliated, slightly recrystallized and metasomatized gneiss, and the bordering, slightly foliated, quartz monzonite. Only rarely is there a complete mineralogical and textural transition between the two rocks over a distance greater than a few feet. As generally no accessory minerals have been added to the gneiss by the quartz monzonite intrusion, one way to distinguish between the two rocks is by the presence or absence of the accessory minerals. A distinction may also be made on the basis of the detailed textural relations between potash feldspar and plagioclase; but the discussion of both these features is outside the scope of this paper.

Composition and Texture of the Monzonitic Porphyry. The typical monzonitic porphyry is brown to gray, coarse grained, and characterized by phenocrysts of potash feldspar which make up 25 to 50 percent of the rock. These phenocrysts are pink to gray, subhedral, and commonly twinned on the Carlsbad law; they have an average length of 15 millimeters, and there is some indication that the crystals in the westernmost of the three principal porphyry bodies are slightly larger than in the other two bodies. Photo 9 shows a single crystal of potash feldspar in which inclusions comprise 50 percent of the grain.

Photo 9. Photomicrograph of monzonitic porphyry. The entire picture represents one large crystal of potash feldspar (the relatively large, gray areas) in which inclusions make up 50 percent of the grain. Crossed nicols, X25.

Accurate modal analyses of such coarse-grained rocks as the porphyry are very difficult to make. The composition given below is an estimated average of several analyses and is adjusted to correlate with field measurements of the percentage of potash feldspar.

In the porphyry, quartz forms aggregates of anhedral, sutured grains. Potash feldspar, although megascopically appearing as well formed crystals, contains abundant inclusions and has highly irregular borders intergrown with other crystals in the rock. Inclusions may make up to 50 percent of the volume of the potash feldspar crystal, and micrometric measurement shows that the total composition of the inclusions is approximately the composition of the whole rock (the inclusions even consisting partly of small grains of potash feldspar). Plagioclase in the porphyry forms subhedral laths with an average length of 2 to 3 millimeters and a maximum of 10; an average composition is An₂₀. Most sections contain hornblende (pleochroic from yellow to blue green). Biotite is pleochroic from yellow to olive green, and in sections of porphyry which contain no hornblende, clusters of biotite and accessory minerals are common. Possibly these clusters formed by alteration of the amphibole. Common accessory minerals are magnetite, apatite, sphene, allanite, and epidote. All of the monzonitic porphyry contains a groundmass of fine-grained quartz and feldspar which makes up 10 to 20 percent of the rock; the average size of most grains in the groundmass is 1/25 millimeter.

Structure of the Monzonitic Porphyry. Orientation of potash feldspar grains gives the porphyry a planar structure parallel to some contacts with the gneiss, whether or not these contacts are parallel to the gneissic foliation. Most of the porphyry away from the contacts is massive. Along one contact south of Twentynine Palms the foliation in the porphyry parallels the contact and cuts perpendicularly across the foliation in the adjoining gneiss.

Contacts Between Monzonitic Porphyry and Wall Rocks. Most contacts between the Palms quartz monzonite and the monzonitic porphyry are slightly gradational. A few very sharp contacts may be explained as the result of movement of the partially solid porphyry. The principal changes which take place upon passing from the porphyry into the quartz monzonite are: a very large decrease in the size of potash feldspar grains; a slight decrease in the size of plagioclase and quartz grains; possibly a decrease in the percentage of hornblende with respect to biotite; change from greenish to reddish biotite; change from porphyritic texture to an inequigranular seriate one. The porphyry and quartz monzonite are approximately similar in bulk composition, types of accessory minerals, and types of plagioclase.

The contact between porphyry and Pinto gneiss is almost invariably gradational, the transition being far more gradual than the transition between porphyry and quartz monzonite. Changes upon passing from porphyry to gneiss are: a decrease in grain size of all minerals; a decrease in the percentage of potash feldspar; a decrease in the percentage of accessory minerals; a decrease in the albite content of the plagioclase; an increase in the percentage of dark minerals; an increase in the quality of foliation; probably a decrease in the ratio of hornblende to biotite.

Summary of Relations Between Palms Quartz Monzonite, Monzonitic Porphyry, and Pinto Gneiss. The principal relations between quartz monzonite, gneiss, and porphyry may be summarized under three headings: distribution, composition, and grain size.

The monzonitic porphyry is roughly peripheral around the northern portion of the Palms quartz monzonite, and in many places the two rocks grade into each other. Unfortunately, the gneiss which probably exists north of the porphyry is not exposed, and it cannot be stated absolutely that the porphyry is a border between gneiss and quartz monzonite. The porphyry is, however, gradational into the gneiss in many places, and local development of the porphyry at the contact between gneiss and quartz monzonite is found in at least two places.

The compositional changes upon passing from gneiss through porphyry into quartz monzonite are: the percentage of quartz decreases from gneiss to porphyry and may increase slightly in the quartz monzonite; the percentage of potash feldspar increases continuously from gneiss through porphyry to quartz monzonite; the percentage of plagioclase increases from gneiss to porphyry and decreases slightly in the quartz monzonite; the total percentage of dark minerals decreases steadily from gneiss through porphyry to quartz monzonite; the percentage of accessory minerals increases from gneiss to porphyry and may decrease slightly in the quartz monzonite; the albite/anorthite ratio for the whole rock (including all the separate phases of plagioclase) increases steadily from gneiss through porphyry to quartz monzonite.

The significant feature concerning grain-size distribution in the gneiss, porphyry, and quartz monzonite is that potash feldspar, plagioclase, and hornblende are all larger in the porphyry than in the other two rocks. The porphyry is also the only one of the three rocks which exhibits a distinct separation between large crystals and a groundmass. The extremely large size of the potash feldspar in the porphyry compared to its size in the quartz monzonite is especially remarkable in view of the fact that the quartz monzonite contains a higher percentage of potash feldspar. Evidently, conditions in the monzonitic porphyry at the time of its formation were favorable to the growth of large grains and were totally different from conditions in either the gneiss or the quartz monzonite.

Origin of Palms Quartz Monzonite and Monzonitic Porphyry. The Palms quartz monzonite and the monzonitic porphyry are believed to be related in origin. Their similar mineralogy, the gradation between the two rocks in some places, and the distribution of the porphyry peripheral to the quartz monzonite all indicate a genetic relationship between the two rocks.

This genetic relationship is substantiated by the fact that the porphyry is compositionally intermediate to the gneiss and quartz monzonite, and partly for this reason the writer believes that the monzonitic porphyry formed by reaction between the quartz monzonite melt and the surrounding gneiss. Apparently the intrusive rock supplied potash feldspar and sodic plagioclase (An₁₅ to An₀₀), or potash and soda, to the gneiss. Other evidence for the transformation of the gneiss to porphyry is in the complete gradation between the two rocks. Sharp contacts between porphyry and both gneiss and quartz monzonite in places indicate that, at least locally, the porphyry was sufficiently mobile to intrude the other rocks. Actually, the planar structure in portions of the porphyry which appear to have undergone movement near some contacts suggests that only a small portion of the porphyry was liquid at the time of movement. Presumably, growth of crystals in the partially solid wall rock around the quartz monzonite led to the formation of some very large grains and a resultant porphyritic texture.

The exact mode of formation of the Palms quartz monzonite is in doubt. It is proposed, however, that the quartz monzonite crystallized from an intruded melt, for, though there is complete gradation between quartz monzonite and both gneiss and porphyry in many places, the transitions are generally complete within a few feet. It appears easier to explain this phenomenon by reaction between a fluid quartz monzonite and a solid wall rock than by replacement in the solid state. Also, dikes of quartz monzonite in both porphyry and gneiss indicate that parts of the quartz monzonite were mobile during at least part of its period of formation. What is more, if the quartz monzonite had formed in the same manner as the porphyry (i.e., by partial incorporation of the surrounding rocks), one would not expect the grain sizes of both potash feldspar and plagioclase to be maximum in the porphyritic rims and smaller in the center of the area of replacement; rather, the grain sizes should increase from the rim toward the center of the quartz monzonite.

It may be that the Palms quartz monzonite is partially the product of injection and crystallization of fluid material and partially the product of solid-state replacement, with the percentage of rock formed by each process unknown.

White Tank Quartz Monzonite

The White Tank monzonite (White Tank quartz monzonite) was described by Miller (1938). The formation probably represents a Jurassic intrusion.

According to Dr. D. F. Hewett (personal communication), the White Tank quartz monzonite is similar to rocks of apparent Jurassic age elsewhere in the Mojave Desert, and a pegmatite in the Cactus quartz monzonite (a possible correlative) has been dated by Hewett and Glass (1953) as approximately 150 million years old, thus Middle Jurassic.

Composition and Texture. The White Tank quarts monzonite is light brown to gray, massive, coarse to medium grained, and inequigranular. Slight textural and compositional differences occur between different intrusive masses and between different parts of the same mass, but individual hand specimens are generally homogeneous. Modal analyses give the following average composition:

Photo 10. Photomicrograph of White Tank quartz monzonite. The picture shows a grain of plagioclase, one half of which is twinned and the other half zoned. Crossed nicols, X65.

In the quartz monzonite, quartz is almost invariably in aggregates of undulant, fretted grains. Potash feldspar is generally pink, anhedral, and has an average maximum dimension of 5 millimeters. Plagioclase occurs as subhedral, twinned or zoned laths with an average length of 2 millimeters and an average composition of An₂₀. Biotite is pleochroic from yellow to brown and forms very irregular flakes, commonly in clusters with the accessory minerals. A few grains of common blue green hornblende were found in sections from one part of one intrusive body. Some portions of the quartz monzonite contain apparently primary muscovite in flakes with an average length of 1 millimeter. The common accessory minerals are magnetite, apatite, and sphene. Zircon forms pleochroic halos in some biotite flakes. Allanite is present in a few sections. Manganiferous garnet occurs as small euhedrons in the muscovite-bearing, highly albitic portions of the quartz monzonite.

Photo 11. Photomicrograph of White Tank quartz monzonite. The picture shows the final product of differentiation in the large body south of Queen Mountain. Minerals are quartz (undulant), plagioclase (twinned), and microcline (large, grid twinned, grains). Koolinization of the feldspars is intense. Crossed nicols, X25.

Myrmekite, rod and vein perthite, and graphic textures are common in the quartz monzonite. Most of the myrmekite is associated with pure albite, which rims the plagioclase laths or forms small, irregular grains interstitial to the other minerals in the rock. Plagioclase in the rod perthite is commonly of the same composition as the neighboring plagioclase laths and, therefore, has presumably formed by exsolution; plagioclase in the vein perthite may have any composition from the composition of the laths to pure albite. Graphic textures are common only in some border phases of the quartz monzonite or in the outer portions of some large potash feldspar grains.

Photo 12. Photomicrograph of White Tank quartz monzonite. Picture shows irregular myrmekite formed in the outer part of a plagioclase grain next to potash feldspar. Crossed nicols, X65.

The irregular, unoriented, intergrowths between potash feldspar and plagioclase so characteristic of the Palms quartz monzonite are not present in the White Tank quartz monzonite. The more acidic portions of the White Tank quartz monzonite do contain oriented intergrowths between sodic oligoclase and microcline, but they are not characteristic of the formation as a whole.

Two distinct trends of differentiation may be identified in the White Tank quartz monzonite. One trend leads to the formation of a rock characterized by abundant, large gray crystals of microcline intergrown with sodic oligoclase. The other trend results in the formation of a highly micaceous, albitic, rock containing small manganiferous garnets.

The first trend is illustrated by vertical differentiation in the large body south of Queen Mountain. In hand specimen the vertical change is best illustrated by the appearance of the potash feldspar crystals; in the lower parts of the rock the feldspar grains are pink, anhedral, and have an average length of 2 to 3 millimeters, whereas in the upper part of the rock the grains are gray, subhedral, and have an average length of 7 to 8 millimeters. The large gray potash feldspar crystals are intergrown with white streaks of sodic oligoclase. Microscopically, it may be seen that the overall composition of the plagioclase changes from sodic andesine at the base to sodic oligoclase at the top. Observable in both hand specimen and thin section is the fact that the biotite decreases in both grain size and percentage from the base to the top of the rock unit; biotite composes about 10 percent of the rock at the base and about 3 percent at the top. Other upward changes in the quartz monzonite include an increase in the amount of vein perthite, an increase in the amount of myrmekite and separate grains of albite, and an increase in the amount of graphic intergrowth. All changes appear perfectly gradational from the base to the top of the body. The rock in the small body east of Queen Mountain apparently represents an intermediate stage of this differentiation sequence; it contains a high concentration of large gray phenocrysts of potash feldspar but has plagioclase grains with a composition of calcic oligoclase.

The second differentiation trend is best illustrated by the formation of a micaceous eastern rim along the upper portions of the eastern contact of the quartz monzonite in Lost Horse Valley; the body of White Tank quartz monzonite between Forty Nine Palms and Queen Mountain appears to represent a continuation of this trend. The rock formed by differentiation in this sequence is characterized by books of primary muscovite up to 10 millimeters long, white, anhedral microcline grains with an average length of 2 millimeters, subhedral plagioclase laths with a composition of An₀₃, and small manganiferous garnets. Thus, differentiation has led to a decrease in the size of the microcline grains, an increase in the albite content of the plagioclase, and the formation of two minerals (muscovite and garnet) not common to the main bodies of quartz monzonite.

Structure. Almost no megascopic structure, with the exception of the widespread jointing, is to be found in the White Tank quartz monzonite. Most of the rock is massive, and no simple pattern has been discerned from the isolated patches that show a poorly developed foliation. The massive nature of the rock obtains even a few feet from the contacts.

Joints, however, are a characteristic feature of the quartz monzonite (Photos 13, 14). The most commonly developed jointing is a sub-horizontal sheeting which is present at all places in the quartz monzonite. Other joints are most commonly vertical but may have any dip from 45 to 90 degrees. Generally, the joints form a set in which the two major directions of strike intersect at a right angle. They appear to have formed synchronously, as no joint offsets any other.

Photo 13. Cap Rock, view east. Note jointing and spheroidal weathering of White Tank quartz monzonite. Photo courtesy National Park Service.

Photo 14. Typical bouldery exposure of weathered White Tank quartz monzonite, near Lost Horse Well. Photo courtesy National Park Service.

In Lost Horse Valley and the north part of Queen Valley, joints in the White Tank quartz monzonite have an identical pattern to those in the Palms quartz monzonite. Many individual joints cross the contact with no deflection. Both radial and concentric patterns are present in the quartz monzonite near contacts with the gneiss. The gneiss is generally unjointed even near the quartz monzonite.

Contacts Between the White Tank Quartz Monzonite and its Wall Rocks. Contacts between the White Tank quartz monzonite and any of its wall rocks are characterized by their abruptness. Gradation nowhere extends through more than a few inches, and most contacts are knife-edged. The intrusion of the quartz monzonite has had no visible effect on any of the wall rocks except for some slight ingestion of the Gold Park gabbro-diorite.

Near most of its contacts the quartz monzonite exhibits notable textural differences from rock in the center of the large bodies. In many places the grain size of all minerals decreases toward the border; or locally, the grain sizes of quartz and potash feldspar decrease toward the border, and the size of the plagioclase remains constant. Pegmatitic segregations are common a few feet from some contacts. The quartz monzonite at some knife-edged contacts exhibits a partially resorbed quartzose rim with an average width of one inch.

Most contacts of the quartz monzonite are apparently vertical over the small interval observable in the field. The complete three-dimensional configuration of the intrusive masses, however, is unknown.

Dikes Associated with the White Tank Quartz Monzonite. In many places the quartz monzonite forms small dikes which cut the surrounding rocks. These dikes are generally characterized by a grain size smaller than that of the associated major intrusive body, and some dikes are porphyritic. The compositional range of the dikes is the same as that of the large bodies of quartz monzonite.

The quartz monzonite is cut by abundant aplite dikes, generally less than one foot wide. These dikes are fine grained, equigranular, and massive. The aplite is generally slightly more silicic than the quartz monzonite which it cuts, but the compositional range of the aplites overlaps the compositional range of the quartz monzonite. The aplite dikes are apparently earlier than the joints which cut the quartz monzonite, for the dikes rarely follow joint planes.

Pegmatite dikes are also earlier than the joints, but wherever the age relations are discernable, the pegmatite postdates the aplite. Most of the large pegmatite dikes are zoned, with potash feldspar (and albite in some cases) near the edge of the dike and quartz in the center. Small dikes are generally unzoned. The composition of individual dikes ranges from nearly pure feldspar to nearly pure quartz.

Rarely, other types of dikes are found cutting the quartz monzonite. The various types noted are a quartz latite-porphyry, an epidote-albite dike, and a dike consisting of equal amounts of hornblende and andesine.

Origin. The White Tank quartz monzonite is believed to have formed by crystallization of an intruded magma, for textural and compositional lack of homogeneity represented by vertical variation in individual bodies and between different bodies of the quartz monzonite is most easily explained by crystallization differentiation of a magma. Rock bodies which have come to equilibrium by solid-state reactions should be far more homogeneous than the White Tank quartz monzonite. In addition, the fine-grained borders and pegmatitic segregations along contacts seem compatible only with formation by cooling of a magma. The very abrupt contacts with no relicts of wall rocks in the quartz monzonite, too, are most easily explained by injection and crystallization of a magma.

Relations Between the White Tank Quartz Monzonite and the Southern California Batholith. It might be expected that two large bodies of plutonic rock of approximately the same bulk composition, same age (late Mesozoic), and geographically separated by no more than 100 miles should exhibit roughly the same general features. And as mentioned previously, many quartz monzonitic rocks in the Mojave area are very similar to the White Tank quartz monzonite.

The individual silicic to intermediate plutons of the southern California batholith,* however, differ considerably in some respects from the individual bodies of the White Tank quartz monzonite.† The features characteristic of the White Tank but absent or at least not widespread in the exposed parts of the southern California batholith include the abundant evidence of differentiation (especially gravitative settling) in any one intrusive body; the fine-grained rims and other pronounced textural variations related to contacts (such variations are not common in the southern California batholith); and the almost complete absence of inclusions and evidence of ingestion of wall rocks.

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*For a detailed description of the northern part of the batholith, see Larsen (1948).

†Several of the distinctions mentioned here were suggested by Dr. R. H. Jahns.

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The White Tank quartz monzonite and the rocks of the southern California batholith are similar, however, in the abruptness of almost all contacts with the wall rocks, and in the conformity of foliation in the wall rocks with the outline of the contact.

The reasons for the above-noted differences between the White Tank quartz monzonite and the rocks of the southern California batholith are not known. It is pointless to say that the differences may be caused by a difference in temperature of crystallization of the minerals in the two rocks, for magmas of similar composition should crystallize over the same temperature range unless their environments of formation differ with regard to some other variable, such as pressure. Compositions of the rocks are so similar that any differences must be related to fugitive constituents of the magmas from which the rocks crystallized; but evidence of reactions involving water in the batholith, and evidence of crystallization from a highly fluid medium in parts of the quartz monzonite indicate that both rocks may well have had about the same concentration of fugitive material. A possible cause for the differences between the White Tank quartz monzonite and the rocks of the southern California batholith is a difference in depth of intrusion below the surface and a consequent difference in pressure on the magmas. Possibly the greater evidence of differentiation in single bodies of the White Tank quartz monzonite than in plutons of the batholith indicates greater depth of intrusion (and consequent slower rate of cooling) for the quartz monzonite. Fine-grained rims in the quartz monzonite bodies and their absence in the batholith suggest lower temperatures in the wall rocks around the quartz monzonite than around the batholith; these higher temperatures possibly resulted from the long sequence of intrusions which characterizes the batholith and which may have warmed the wall rocks in the area.* The White Tank quartz monzonite was not part of such a sequence, and its wall rocks, though deeper than those of the batholith at the time of intrusion, may have been cooler. Mainly by a process of elimination, one reaches the conclusion that the principal difference between the environments of formation of the White Tank quartz monzonite and the southern California batholith was depth of intrusion.

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*See Larsen (1948), p. 141.

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Granodiorite

The granodiorite, a rock that is possibly related to the White Tank quartz monzonite, crops out in an area south of Jumbo Rocks and north of the Hexie Mountains. Megascopically, it is white to light gray, massive, medium to coarse grained, inequigranular, and locally contains white phenocrysts of plagioclase up to 10 millimeters long.

A modal analysis of the granodiorite gave the following composition:

Quartz occurs as aggregates of undulant, fretted, grains or as slightly smaller (0.5 millimeter) interstitial grains. Potash feldspar occurs as anhedral grains with an average size of 0.5 millimeter. Plagioclase forms subhedral laths having an average length of 1.3 millimeter and an average composition of An₃₅. Biotite is pleochroic from yellow to dark brown, and hornblende is pleochroic from yellow green to blue green. The accessory minerals are magnetite, apatite, sphene, and zircon. Approximately one-third of the rock can be considered a groundmass of 0.25- to 0.5-millimeter grains of all minerals in the rock.

Compositionally, the granodiorite is very similar to the lowermost part of the neighboring body of White Tank quartz monzonite, but texturally there seems to be little relation between the two rocks. All of the quartz monzonite is coarser grained, has more of its quartz in aggregates, and lacks the distinct groundmass of the granodiorite. The only portion of the White Tank quartz monzonite which is texturally at all similar to the granodiorite is in a relatively fine grained (apparently chilled) zone within one hundred feet of the eastern border of the body south of Queen Mountain; the granodiorite is not related to a border of the quartz monzonite.

Thus, the granodiorite may have been derived from the same magma source as the White Tank quartz monzonite but probably was intruded at a different time. Possibly the granodiorite is a large dike injected soon after crystallization of the quartz monzonite.

Dikes

Scattered throughout the mountains around Forty Nine Palms are numerous small dikes which are too small to map on the scale used by the writer. They are rarely more than one foot thick and generally can be traced for only a few feet in outcrop. These dikes cut the Palms quartz monzonite and monzonitic porphyry, but their age relation to the White Tank quartz monzonite has not been determined. It has also been impossible to determine whether or not these dikes are related to one of the major plutonic intrusions.

Composition and Texture. Two types of dike rock occur: white to light gray, silicic dikes; and, in lesser amounts, greenish-gray, basic dikes.

The silicic dikes (Photo 15) contain about 20 percent phenocrysts, 0.5 to 1 millimeter in diameter, consisting of quartz, potash feldspar, and plagioclase. The potash feldspar phenocrysts are anhedral, may exhibit zones of slightly different crystallographic orientation, and may contain inclusions of quartz in the outer portions of the grains or along the zonal borders. Plagioclase forms subhedral to euhedral, faintly zoned and/or twinned, grains with an average composition of An₃₀. Quartz occurs in subhedral, equant grains. The groundmass consists of quartz, potash feldspar, and plagioclase (in that order of abundance). A few biotite flakes, muscovite flakes, and some opaque white powder make up the rest of the groundmass. The average grain size in the groundmass is 0.1 millimeter or less and is different in different dikes or in different layers of the same dike.

Photo 15. Photomicrograph of silicic dikes. Phenocrysts are quartz (white), plagioclase (gray), and potash feldspar (block, containing small patches of twinned plagioclase). Groundmass is a mixture of quartz and feldspar.

Photo 16. Aerial view east across Malapai Hill, a volcanic cone, composed of basalt (dark) cutting White Tank quartz monzonite (bouldery outcrop). West edge of Hexie Mountains in middle distance composed of Pinto gneiss (dark) intruded by White Tank quartz monzonite (light). Squaw Tank and Pleasant Valley just off photo at right. Photo courtesy National Park Service.

The basic dikes contain approximately 30 percent hornblende, pleochroic from yellowish green to greenish blue; 5 percent biotite, pleochroic from yellow to reddish brown; 45 percent irregularly twinned, strongly zoned plagioclase with a compositional range from An₁₀ in the center to An₃₀ on the edge; 20 percent quartz; and a little magnetite, apatite, and allanite. All grains are anhedral and average 0.1 millimeter in size (except for some plagioclase laths which are seriate from the groundmass size up to 1 millimeter). Some of the quartz occurs in aggregates of 0.5-millimeter grains. Sericitic alteration is abundant in the center of many plagioclase grains.

Structure. All of the dikes appear massive both in hand specimen and thin section.

Origin. Both the silicic and basic dikes have almost certainly been formed by intrusion and crystallization of a melt. The distinction between phenocrysts and groundmass is partly caused by chilling of a partially crystallized melt, although the inclusions of groundmass in the outer portions of potash-feldspar phenocrysts indicate that at least some of the groundmass started crystallizing before the feldspar phenocrysts stopped growing.

Basalt

Malapai Hill (Photo 16), is composed largely of black, aphanitic, olivine-bearing basalt. Similar rock is present at one place (not mapped) along the western part of the Lost Horse Mountains. No flows are associated with the basalt of Malapai Hill, and apparently it is a shallow intrusive rock. Columnar structure is commonly vertical but forms wide loops and is nearly horizontal in places. The basalt cuts the White Tank quartz monzonite and is probably quite young, but its true age is unknown.