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

Structure of the Metamorphic Rocks. The metamorphic rocks are completely enclosed in plutonic rocks. The large masses strike northwest and dip steeply. The accordance with the attitude of the roof rocks of the Sierra Nevada to the north suggests the larger metamorphic bodies are remnants of the original batholith roof. The name roof pendant is applied to such isolated roof remnants, on the assumption that they are protrusions of the roof down into the batholith. Some of the smaller bodies are parallel in attitude to the larger roof pendants, while others have a random orientation and are xenoliths—presumably roof fragments that were loosed into the batholith.

No major folds have been recognized, nor have any features been recognized for determining the tops of beds. Numerous minor folds a few feet in magnitude were observed, but no correlation with major features was found. Everywhere it was observed, the metamorphic layering, inferred as parallel to the original bedding, was parallel to schistosity. The apparent lack of repetition of the major metamorphic units suggests a homoclinal sequence with no major folds within the mapped area.

Igneous-Metamorphic Contacts. Sharp contacts are present along the east side of the largest pendant mass, but the lack of plentiful exposures does not warrant the statement that sharp contacts are the most plentiful.

Lit-par-lit injection zones are uncommon; but one such zone, particularly well developed, is exposed east of Hospital Rock camp (photos 9, 10). The presence of injection zones may reflect the relative nearness of the pendant root. On the east side of the largest pendant mass, the contact shows decreasing amounts of injection at higher elevations. The magma would have easier access into the metamorphic rocks near the broken root zone, while the nearly conformable wall rock at higher elevations would present a formidable barrier to intrusion. Near the Ash Mountain Park Headquarters the variation in sharpness of contacts with elevation can be seen on a small scale. The Generals Highway cuts across some narrow slivers of metamorphic rocks that are in sharp contact with the adjoining igneous rock. The small slivers can be traced down to the Kaweah River and they die out in depth.

Structure of the Plutonic Rocks. Planar structure is common locally in the granitic rocks. It is commonly shown by the sub-parallel orientation of discoid to ellipsoidal dark inclusions, and less commonly shown by schlieren and the parallelism of hornblende crystals. The local alignment of the inclusions in a plutonic body without alignment of the minerals within the inclusions, and the random orientation of inclusions in other parts of the plutons indicate the foliation is primary. Foliation is prominent only in parts of the Giant Forest, Tokopah, Potwisha, and Big Meadow plutons. The best example of the increase of foliation near a contact is along the east side of the largest pendant body. The foliation is pronounced in the Giant Forest pluton near the contact and becomes increasingly less prominent to the east. The prominent foliation is interpreted as a result of the pressure of the granitic rocks along the pendant barrier; how much is actually due to flow, and how much due to expansion is difficult to ascertain. The lack of lineation in the inclusions at this contact, however, suggests that flow was not a prominent feature in aligning the inclusions.

Lineation was only observed at two exposures. Even where the inclusions are extremely drawn out (photos 3, 8), the inclusions are generally shaped like pancakes and not prominently linear. A detailed statistical study of the dimensions of the inclusions, however, might disclose lineation that is not obvious in brief field examinations of outcrops.

Joints are developed to various degrees in all the granitic masses of the mapped area. Parallelism of prominent joints over considerable areas is evident on some of the aerial photographs. Photo 11 shows an exposure of prominent parallel joints near Emerald Lake. On the geologic map (pl. 1) the joint symbol was used only where prominent joints are developed over a large area. The joints do not appear to be related to individual plutons, as the most prominent joints in the eastern part of the area transect inter-pluton contacts. The joints appear to be superimposed on the whole plutonic complex and cannot be applied to the study of the intrusion mechanics of separate plutons. Likewise, Balk's (1937) contention, that the orientation of the flow movements can be determined from the arrangement of fracture systems, does not appear to be applicable in this area.

Inter-Pluton Contacts. Most of the inter-pluton contacts are sharp, where studied by the writer. The contact between the Giant Forest and the Potwisha plutons is gradational and is marked by an increase in dark minerals and inclusions in the Potwisha pluton. The Potwisha quartz diorite may represent a contaminated facies of the Giant Forest pluton. The contact of the Cactus Point granite and the Elk Creek gabbro was studied at only one exposure, and the contact was marked by a 30- to 50-foot foliated mixed zone. The mixed contact is interpreted as a zone of chemical reaction between the two diverse rock types.

The scarcity of gradational contacts may indicate that the succeeding intrusions came into relatively solid rocks. The sharp contacts also indicate equilibrium conditions between the chemically similar quartz-rich types, with little tendency for interaction.

Abundant dikes are present near some of the contacts, in particular, dikes of Big Meadow pluton material in the Giant Forest pluton. The anastamosing dikes and some isolated blocks exhibit what amounts to a mixed contact, but the individual contacts are sharp.

Mode of Intrusion of the Plutonic Rocks. The Sequoia area, though an excellent place to study some of the relations between plutons and the characteristics of individual plutons, is not well suited for a study of the mechanics of intrusion. The general massiveness of the plutonic units makes it difficult to judge the effect of later plutons on the intruded rock. The only remnants of wall and roof rocks are the pendants and scattered large xenoliths. The lack of wall rock contacts makes it particularly difficult to prove or disprove forceful injection as the mode of intrusion. The only evidence in the Sequoia area concerning forceful injection is negative. The schist shows no apparent disturbance at the contact, and there is no brecciation. The lack of brecciation is also evident in the plutonic units, but if the invaded rock was plastic at the time of intrusion displacements caused by forceful intrusion might not he evident.

A recent paper by Noble (1952) discusses the evaluation of criteria for the forcible intrusion of magma. Noble states that the structure of the surrounding schists in the Sierra Nevada does not show how much room was made, but that the internal structure of the plutons suggests forcible injection. Ernst Cloos (1933) considers as conclusive evidence of forceful injection the dome-shaped internal structures of Sierra Nevada plutons and the absence of stoped blocks and assimilated material. No domes of foliation were found in the Sequoia area; but the good foliation along the east margin of the Sequoia roof pendant the writer interprets to indicate upward movement or expansion along the pendant wall, with no necessity for forceful injection. After a passive invasion by stoping or assimilation, later movements could align minerals and inclusions while the magma was in a plastic state. Such later movements would tend to conform to the wall rock contacts.

The plutonic complexity of the Sequoia area suggests that in earlier work (Mayo, 1941) igneous contacts may have been missed, and that large areas that have been considered structural units are in actuality comprised of several granitic masses.

Assimilation along the margins of the schist pendants is best developed east of Ash Mountain Park Headquarters where small metamorphic bodies feather out into plutonic rocks, leaving wisps of weakly schistose dark material. Photos 9 and 10 show an injection zone with some assimilation.

That stoping has been active is most obviously shown by blocks of metamorphic rocks that do not conform to the attitude of the roof rocks. The largest of these blocks covers an area of approximately half a square mile. Xenoliths are present along some of the contacts and in some exposures large numbers have been liberated from lit-par-lit areas (see photos 9, 10). If the dark inclusions of doubtful origin are considered xenolithic, they also probably represent stoped material. North of Big Meadow in an area of abundant exposures, numerous blocks of Giant Forest pluton material are included in the Big Meadow pluton. One of these blocks is 100 feet across. This is one of the few areas where direct evidence was seen of one pluton stoping another. The mixed contacts resulting from the presence of many dikes may well be examples of arrested stoping, in which most of the block material is still in place.

One of the best examples of passive intrusion as opposed to forceful injection is seen on the map in the area north of Cabin Creek. Two small disconnected bodies of Big Meadow pluton material show a well-developed foliation which is coincident with the foliation of the main mass to the south. The foliation is not distorted by the intrusion of the Weaver Lake quartz monzonite as would be expected with forceful injection.