California Division of Mines Speial Report 53 Igneous and Metamorphic Rocks of Parts of Sequoia and Kings Canyon National Parks, California

Inclusions

Inclusions in plutonic rocks are of two kinds: (1) xenoliths, or fragments of material foreign to the rock that encloses them; and (2) autoliths, or fragments that represent an earlier crystallized part of the rock that encloses them.

Recognizable xenoliths of metamorphic material and plutonic material are locally present in the Sequoia area, and the layered hornblende concentrations are probably autolithic; but the great bulk of the inclusions are not easily identified with either type of inclusion. The inclusions of doubtful origin are dark, fine-grained, and contain the same minerals as the plutonic rocks. The same type of inclusion is common throughout the Sierra Nevada, as well as in other plutonic complexes throughout the world. Some of the inclusions of doubtful origin can be shown to be xenoliths. None were conclusively shown to be autoliths.

Layered Hornblende Concentrations. Regular and irregular layers that are chiefly the result of hornblende concentrations are illustrated in photo 13. The layering, locally common, is particularly well developed in the Giant Forest pluton in the vicinity of Pear Lake, and is weakly developed locally in the Weaver Lake and Lodgepole masses. The layers in the Giant Forest pluton are autolithic, as they seem to be the result of the segregation of early formed hornblende.

Rhythmic layering has often been described in the literature. Gilbert (1906) has described similar layering in the Sierra Nevada and shows an illustration of the layering from a locality near the east boundary of the area of this report. Gilbert's illustration looks similar to photo 13. He mentions unconformities between the layers but does not describe them. Such an unconformity is shown in photo 13 where a dark layer is cut off by a wider light layer. Gilbert postulates the hornblende has settled and the layers are essentially the result of sedimentation.

Wager and Deer (1939) have described in great detail mineralogic layering in the Skaegaard intrusion in Greenland. They postulate the layering is due to the combination of settling and convection currents.

Bateman (1947) found "angular unconformities" and "cross-bedding" in banded gabbros in the east Sierra Nevada. The structures are shown by alternating felsic and mafic layers. The origin of these structures is postulated to be a combination of crystal settling and convection currents.

Hamilton (1951) has found, in the Huntington Lake area of the Sierra Nevada, "long wispy bands rich in dark minerals, especially hornblende"; some of the bands are considered to be hybrid in origin. One example indicates the hornblende concentrations are concentric layers around a hybrid rock. Hamilton considers that the hornblende crystals were formed from the associated hybrid and metamorphic rocks.

Hornblende was observed to have formed from the interaction of igneous and metamorphic rocks at one locality near the south end of the eastern pendant in the Sequoia area. Near the contact, hornblende crystals have formed from schist and have been freed into the granodiorite (see photo 4). Specimens show the hornblende has grown in the schist and has incorporated some of the groundmass material. Mechanical disintegration near the contact may have freed the crystals, or the hornblende crystals may be residuals of schist assimilation. Weight is added to the argument for the metamorphic origin of these particular hornblende crystals by the fact that the granodiorite in the area around the contact is practically devoid of hornblende. The layered hornblende concentrations, however, are not found near metamorphic or hybrid rocks, but metamorphic or hybrid rocks could have been present at a higher level which has since been eroded away. The layering probably represents the settling of hornblende, and the recurring layers are the result of alternate settling and sweeping by convection currents. The alternation of quiet conditions to permit settling, and moving magma to give mixing, would lead to the rhythmic layering present in these rocks.

Dark Inclusions. The dark inclusions show marked variations in mineralogy, grain size, size, and shape. They range from less than one inch to a few feet in diameter. Normally they are subspherical to ellipsoidal, but less commonly they have irregular outlines. The inclusions generally occur singly, but swarms are locally present (see photo 7). The inclusions are much elongated where the foliation is well developed (photo 3). The elongation indicates a certain amount of plasticity or fluidity and the variation in plasticity is well shown in photo 8, in which elongated inclusions are bent around a rounded inclusion that apparently was less easily deformed.

Plagioclase makes up slightly more than half of the dark inclusions and ranges from intermediate andesine to calcic oligoclase. Scattered plagioclase phenocrysts as large as 5 mm in diameter are common in the dark inclusions of the Giant Forest pluton. Some plagioclase phenocrysts are speckled with hornblende and biotite and have an irregular outline that suggests the plagioclase crystals have grown late by replacement. Other plagioclase phenocrysts are clear, well shaped, and were probably freed mechanically from the surrounding plutonic rock. This process has been arrested in some specimens.

Microcline and quartz are present in subordinate amounts. Dark brown biotite comprises from 5 to 20 percent of the dark inclusions. Common green hornblende comprises as much as half of some specimens. It is commonly sieve-like, and has apparently grown at the expense of the surrounding minerals.

One specimen contains irregular remnants of monoclinic pyroxene included in hornblende. Two specimens from the Big Meadow pluton show a feature that closely resembles schiller structure in the hornblende. The schiller-like structure suggests that the hornblende may in part be the result of the replacement of pyroxene.

Well-formed needle-like crystals of apatite are concentrated in the inclusions of nearly all the specimens. The maximum is 4 percent (photo 19), but many of the specimens contain more than 1 percent of apatite. Much more apatite is found in the inclusions than in the plutonic rocks. Zircon, magnetite, and sphene are present in nearly all the inclusions, sphene being slightly more abundant than in the accompanying plutonic rocks.

The texture of the Sierra Nevada dark inclusions has been variously described as " allotriomorphic" by Knopf and Thelen (1906), "hypidiomorphic" by Pabst (1928), Hurlbut (1935), and MacDonald (1941)," suggestive of a hornfels" by Grout (1937), and "crystalloblastic" by Hamilton (1951). In the area of this report, the inclusions are composed almost exclusively of anhedral crystals. The sieve-like nature of many of the larger crystals in the dark inclusions, as contrasted with the well-formed subhedral plagioclase and hornblende crystals of the plutonics, suggests a replacement origin.

A correlation is noticeable between the individual plutons and the characteristics and amount of inclusions they contain. The texture, mineralogy, and size of inclusions depend to some extent on the nature of the enclosing rock, as is shown by the following generalizations. In the Giant Forest pluton, inclusions are generally abundant, of all sizes up to a few feet across, 0.2 to 0.3 mm in grain size, and porphyritic. In the Big Meadow pluton, inclusions are less common, generally small (normally less than 8 inches in diameter, but rarely as large as 2 feet), 0.1 to 0.2 mm in grain size, and commonly nonporphyritic. In the Weaver Lake pluton, inclusions are uncommon, small, 0.1 mm in grain size, and equigranular. Inclusions were not found in the alaskite plutons, and they are normally rare in the other plutonic units, except for the Tokopah granodiorite, which contains many inclusions. The plagioclase of the inclusions in the Giant Forest pluton averages intermediate andesine, and is generally in equilibrium with the plagioclase of the pluton. The plagioclase of the inclusions of the Big Meadow and Weaver Lake plutons is generally sodic andesine. Hornblende is common in the Giant Forest and Big Meadow plutonic inclusions and generally absent in the inclusions in the Weaver Lake pluton.

The variation in type of plagioclase and amount of hornblende indicates that the more felsic rocks have the more sodic inclusions. This is to be expected, as the inclusions would tend to reach equilibrium with the enclosing rock. The variation in abundance and size of inclusions is directly related to the order of intrusion and suggests that the early Giant Forest pluton may have had access to more material that was re-made into inclusions than did subsequent intrusions. The Giant Forest pluton is in contact with metamorphic rocks for many miles, but the metamorphic rocks are not in contact with the Big Meadow, Weaver Lake, and alaskite plutons. As the present surface is only a chance section across the area, the difference in contacts may not be significant; but the fact remains that the other plutons mentioned intrude the Giant Forest pluton and may have had less opportunity to receive xenolithic material.

The alteration of added material is also controlled to some extent by the chemical composition of the material with which it is mixed. Xenolithic material that is close to the composition of the enclosing rock will probably be preserved, but the greater the chemical gradient, the more the chance of reaction. The abundance of hornblende in many of the dark inclusions, especially in the Giant Forest pluton, suggests a mafic parentage. Mafic material, presumably rich in hornblende and possibly pyroxene, if added to the Weaver Lake or alaskite plutons which are poor in dark minerals, would be subject to considerable reaction. The reactions and resultant recrystallization could also lead to the physical disintegration of the inclusion. Thus if equilibrium conditions were reached, inclusions would be rare. The rarity and small size of inclusions in the Weaver Lake pluton suggest that chemical gradient is important. Probably the decrease in numbers, size, and dark mineral content of the inclusions in the more acid plutonics is due to a combination of less access to material and the greater chemical gradient between the incoming granitic material and the added basic material of the inclusions.

The theories of origin of the dark inclusions are divided between those of autolithic origin and those of xenolithic origin. The following list gives the most common ideas on the origin of the inclusions.

1. Autolithic origin
    a. Magmatic segregation
    b. Stoping of a basic border phase

2. Xenolithic origin
    a. Recrystallization of older gabbro
    b. Recrystallization of metamorphic rocks

PHOTO 17. Recrystalized beta-quartz phenocryst (upper left) in a meta-volcanic rock north of Amphitheater Point (crossed nicols). (X 20)

PHOTO 18. Plagioclase replaced by microcline in granodiorite of the Giant Forest pluton. In the lower right center corroded plagioclase fragments are in optical continuity indicating a former large crystal (crossed nicols). (X 15)

PHOTO 19. Abundant apatite crystals in an inclusion from the Big Meadow pluton. Black crystals are biotite and the light background is plagioclase and quartz (ordinary light). (X 50)

PHOTO 20. Large, poikilitic brown hornblende crystal (dark gray) in the Elk Creek gabbro including scattered residual crystals of olivine (strong relief) and hypersthene (pale gray). The white, lath-shaped crystals are bytownite and the black patches are magnetite (ordinary light). (X 30)

Magmatic segregation involves the settling of early formed crystals, or accretion around early crystallizing centers, to form nodular aggregates. Early formed hornblende crystals have concentrated near Pear Lake, but nothing resembling the dark inclusions has formed (see photo 13). The hornblende of the inclusions is not as well-formed as the hornblende in the layered concentrations, but is commonly sieve-like and appears to have grown late in the inclusion mass. The sieve-like crystals are the ones that are noticed as phenocrysts, although most of the hornblende is in smaller anhedral crystals which are much smaller than the crystals of the enclosing rock. If segregation is to account for the inclusions the early formed hornblende should resemble the well-formed crystals of the Giant Forest pluton. Also, segregated inclusions would be subject to the same subsequent reactions as the enclosing magma. Although many of the inclusions are in equilibrium with the enclosing rock, there are such exceptions as inclusions with hornblende and sodic andesine in the Weaver Lake pluton, and abundant hornblende in inclusions in the Big Meadow pluton. This suggests the inclusions were composed of material more basic than the magma, and in some cases did not reach equilibrium.

PHOTO 21. Large poikiloblastic muscovite crystal cutting the schistosity at an angle of 35°; specimen from a large xenolith east of Potwisha Camp (crossed nicols). (X 20)

PHOTO 22. Fine-grained, dark-gray rock of the Ash Mountain complex showing the xenomorphic granular texture (crossed nicols). (X 20)

PHOTO 23. A broken plagioclase phenocryst in meta-quartz diorite porphyry north of Big Baldy. Note the prominent zoning as well as the fine mosaic groundmass inset with phenocrysts and fragments of plagioclase, hypersthene quartz and hornblende (crossed nicols). (X 25)

PHOTO 24. Embayed quartz phenocryst in meta-quartz diorite porphyry north of Big Baldy (crossed nicols). (X 25)

Stoping of a basic border phase requires that an early formed border phase be stoped by a later intrusive to form. inclusions. A paper by Pabst (1928), which includes many petrographic descriptions, as well as photographs and drawings of textures, is the only detailed description of Sierra Nevada inclusions. Pabst favors the idea of an early border phase that was first suggested for the Sierra Nevada inclusions by Gilbert (1906). Pabst covered a large portion of the Sierra Nevada in his study, and found that basic border facies are extremely rare and where they do occur, they contain inclusions themselves. Also the border facies present do not resemble the inclusions. Pabst says "the writer has not seen, in the Sierra Nevada, any rocks of the type of the autoliths described in any other form than as inclusions in granitic rocks."

In the Sequoia area no rock was found that would qualify as a basic border phase. The Potwisha quartz diorite is a more basic gradation of the Giant Forest pluton and has yielded some hand specimens that closely resemble inclusion material. Close observation of exposures, however, reveals abundant inclusions in the Potwisha pluton. To use the Potwisha quartz diorite as a basic border would present Pahst's problem; namely the presence of inclusions in the border phase necessitate another, more basic, border phase to account for these inclusions.

In the southern California batholith, inclusions are found that seem to be identical with those of the Sierra Nevada, judging by descriptions and illustrations. Hurlbut (1935) considers the dark inclusions of the southern California batholith to be the result of the reaction of the magma on included fragments of gabbro and applies the term "reaction inclusion" to the recrystallized fragments. His best evidence is the presence, in the inclusions, of hypersthene, augite, and bytownite-anorthite cores in plagioclase crystals. These minerals are common in the gabbros and are unknown in the enclosing rocks of the southern California batholith. The mineralogical evidence is supported by abundant field evidence showing mixed contacts of gabbro with quartz diorite and partially altered gabbro blocks.

Unfortunately, in the Sequoia area, there is little conclusive proof for or against this type of origin. The older gabbros are not in contact with plutonic units that contain abundant inclusions, except for a poorly exposed area close to the Potwisha Campground. The Cactus Point granite that is in contact with the Elk Creek gabbro has only scattered inclusions. The belt of outcrop of these two bodies is in an area that is thickly covered with brush, so the contact between the two formations was studied at only one locality. A foliated zone 30 to 50 feet wide between the gabbro and the granite represents a mixing and reaction of the two types. The zone is in part lit-par-lit and contains some dark inclusions. The presence of streaks and drawn-out inclusions strongly suggests some inclusions have developed from the reaction between the granite and the gabbro.

Many references are found in the literature concerning the incorporation and reconstitution of metamorphic rocks to form inclusions in plutonic rocks. In the Sequoia area incorporated metamorphic material has formed some of the inclusions. Photo 12 shows Giant Forest material that contains intimately veined schist and hornfels, which has released abundant xenolithic material into the magma. The xenolithic material is granoblastic and contains plagioclase, quartz, biotite, and hornblende. The texture and mineralogy are similar to those of the dark inclusions. Only the incorporation of some phenocrysts or the growth of sieve-like crystals is needed for the xenolithic material to be identical with the inclusions.

North of Crystal Cave a rounded fragment of foliated andesine amphibolite is present in a swarm of dark inclusions. A known xenolith in a swarm of inclusions suggests that the whole swarm is xenolithic, and that some of the other inclusions were altered so that no relic structure is now present. The amphibolite fragment would probably be stable in this environment where amphibole is abundant in the granitic rock.

The marked concentration of apatite in many of the inclusions, especially those of the Big Meadow and Weaver Lake plutons has also been observed by Knopf and Thelen (1906) in the Mineral King district. They attribute the concentration to magmatic segregation. Nockolds (1933) mentions many localities where concentrations of apatite are present in xenoliths that were mostly derived from mafic igneous rocks. Nockolds considers the apatite to represent volatiles that become fixed in the inclusion during reciprocal reaction between the xenolith and the magma. The reciprocal reactions do not occur if the same minerals are present in both the inclusion and in the magma, even though there is a difference in percentage. The schist and hornfels that have been added to the Giant Forest pluton are in mineralogical equilibrium with the magma; but if such material were added to the Weaver Lake pluton, reciprocal reactions would take place to form an inclusion approaching mineralogic equilibrium with the Weaver Lake pluton. Also, gabbroic material added to any of the plutons would require a readjustment, and more so in the more felsic masses. Apatite is more concentrated in the more felsic rocks; this may reflect the more extensive reciprocal reactions, as suggested by Nockolds.

Hurlbut (1935) lists apatite as occurring only as occasional grains in the southern California batholith. The lack of apatite concentration seems to be a contradiction of Nockolds' theory, but the inclusion-bearing rocks described by Hurlbut are quartz diorites, and may not have the high apatite concentration characteristic of the inclusions in the more felsic bodies of the Sequoia area.

The Potwisha quartz diorite has abundant streaked out inclusions and a general dark color caused by an abundance of hornblende and biotite. The nearness of xenolithic metamorphic fragments, especially the abundance of marble, suggests the differential assimilation of schist similar to that observed near the Ash Mountain Park Headquarters. The Potwisha quartz diorite may be a belt contaminated by assimilated material, darker because of the mechanical disintegration of inclusions as well as chemical reactions. The abundant drawn out schlieren inclusions in the Potwisha body also suggest that the inclusions were more fluid and more capable of being mixed with the plutonic material.

Most of the dark inclusions are essentially in equilibrium with the enclosing magma. This is especially true in the Giant Forest pluton. The constituents are almost invariably anhedral and the inclusions contain "phenocrysts" that have grown late or have been mechanically freed from the enclosing rock. In the preceding discussion on modes of origin some facts favor a metamorphic origin for some of the inclusions.

The conclusion from the present study agrees with the statement by Grout (1937): that in the absence of some unusual chemical composition or some relict structure, it is frequently impossible to determine the origin of inclusions.

Orbicular Inclusions. About half a mile southeast of Twin Lakes an elliptical inclusion swarm approximately 50 feet by 10 feet in plan is exposed. Most of the inclusions in the swarm have gray coatings, but some have no coatings at all. The inclusions are the typical dark hornblende-bearing inclusions and the matrix is biotite granodiorite. Small patches of pegmatite are found in this swarm, especially near the gray rims. The exposure near Twin Lakes was the only orbicular structure observed in the area. Elsewhere inclusions are in sharp contact with the enclosing material.

The cores of the orbicular inclusions are similar to the previously described dark inclusions—predominantly intermediate andesine with a small amount of quartz. One-third of the specimen is composed of biotite and hornblende, hornblende being the most abundant. The matrix is a common biotite granodiorite composed of intermediate andesine, microcline, quartz, biotite, and a trace of hornblende.

The rims look sugary and fine-grained in the hand specimen and are a maximum of one inch thick. Most of the rim, however, is made up of poikilitic microcline crystals as large as 3 mm across, in which are set abundant rounded quartz grains. A minor amount of biotite is present, which forms faint concentric streaks. Irregular, corroded andesine crystals are also present in the rims.

The microcline crystals of the rim are elongated perpendicular to the contact with the core. The ratio of elongation is about 2 to 1. Some elongated quartz grains also show this radiating trend. The weak radiating trend is not apparent in the hand specimen.

The contact of the rim with the core is sharp, but irregular; contact of the rim with the matrix is sharp in part, and elsewhere grades to a medium-grained pegmatitic rock which is associated with the orbicular swarm as pockety masses or as an irregular zone around the rim.

A single group of orbicules in an area of 150 square miles suggests a rather unusual set of conditions. This is especially true when the abundance of dark inclusions (the core rocks of the swarm) is considered. Another anomaly is the texture of the rim material. Though megascopically aplitic. the microcline is in large masses similar to the late microcline in the other plutonic rocks. The small patchy spots of pegmatite in the orbicular inclusion swarm are also different from other pegmatite in the area.

The rims with the concentric and radiating structure resemble orbicules as described by Sederholm (1928), Johannsen (1932), and Eskola (1938). The orbicular rocks near Twin Lakes are not as complex as those of other described localities, but they have all the general features, such as a nucleus, concentric and radiating structure, an ellipsoidal shape, and an occurrence as a group of similar bodies.

The three most probable theories of origin for rims of the Twin Lakes orbicules are: (1) a reaction between the core and the plutonic matrix; (2) some sort of successive crystallization of shells around the dark inclusion nucleus (either the rims crystallized before the matrix, or grew within an already solid or mushy matrix); and (3) selective replacement along inclusion contacts by a late, active aplitic mass.

The relatively homogenous rim could be construed as a reaction rim between the plutonic matrix and the dark inclusion core. The problem with such a theory is the reaction of a biotite-hornblende quartz diorite with a biotite granodiorite to produce a rock of granite composition. The reaction seems impossible from a chemical standpoint. Another difficulty of a reaction origin is the volume of rim material. One of the larger orbicules has a core with a volume of 14 cubic inches and a rim with a volume of 50 cubic inches. This amount of material could not develop from a core-matrix reaction, especially when the core shows no evidence of any difference from other dark inclusions of the area.

The growing of the rim by successive crystallization of concentric layers is implied by the combination of concentric and radiating structures. Similar structures are common in orbicules of other localities and also are well known in oolites, which are thought to grow layer by layer.

The major problem in the origin of the Twin Lake orbicules is the concentration of rim material and the restriction of the rims to one locality. The abundance of small patches of pegmatite and the very acidic composition of the rims, as well as a microcline habit similar to that of the late replacing microcline of the plutonic units suggest a relation of the inclusion swarm to a pocket of late aplitic and pegmatitic material. Possibly the inclusion swarm, in a mushy matrix of plutonic material, was invaded by a mass of late pegmatitic and aplitic material. The dark inclusions would serve as suitable nuclei for the silicic material of the rims. Aplite dikes are found in the area with pegmatite cores, suggesting that pegmatite crystallized last from a silicic dike. The appearance of pegmatitic material as coatings on some of the rims and also as small patches between orbicules could be explained by the later pegmatite.

The crystallizing aplite material probably in part replaced existing plutonic material. There is ample evidence elsewhere in the area of late microcline replacing earlier plutonic material. The presence of corroded, irregular plagioclase in the rims, which is identical with that of the plutonic matrix, suggests that the rims grew late, or at least filled in and partly replaced a mushy matrix. Scattered intermediate andesine in a rim composed of 65 percent microcline and 25 percent quartz strongly suggests that the andesine is a replacement residual.

The fact that no tails or stringing out of rim material is found indicates that the rims had crystallized to an extent that any later movement of the swarm moved the coated inclusions and the interstitial pegmatite as a mass. Some post-rim-formation movement may be postulated from the fact that some biotite bands in the rims are transected at a low angle by the matrix material. This implies some erosion, which may be considered analogous to the wearing off of a pebble in a stream.

Some of the dark inclusions in the swarm are apparently uncoated. The contact with the matrix of one of the barren inclusions shows a thin rim of sugary quartz and small poikilitic microcline crystals. The writer checked all the thin sections of rock from the area that showed a contact between dark inclusions and plutonic material, and in no section was there a rim of fine, granular material—merely a sharp change in grain size. A patchiness of the aplite-pegmatite in the swarm could account for some inclusions being uncoated, or a difference in the degree of crystallization around the inclusions might make some easier to coat.

The aplitic-pegmatitic material could have been intruded after the matrix and inclusion swarm were essentially solid and the contacts of matrix and inclusions might form suitable zones for replacement. The marked chemical gradient between the two types would tend to promote a chemical reaction. The replacement by a late aplite, of inclusion material, could also operate in conjunction with the previously described rim crystallization and replacement of some granodiorite.