PETROLOGY OF THE SALMON MOUNTAIN STOCK,
KLAMATH MOUNTAINS, CALIFORNIA
by
JAMES JOSEPH BOARDMAN, B.S.
A THESIS
IN
GEOSCIENCES
Submltted to the Graduate Faculty of Texas Tech Unlverslty In Partlal Fulflllment of the Requlrements for the Degree of
MASTER OF SCIENCE
Approved
Accepted
December, 1985 ^- ^
f\/<^, l'^f ACKNOWLEDGMENTS
I would like to thank my advisor, Calvin Barnes, for the interest that he showed in this project from its inception to its corapletion. His encouragement, patience and professionalism brought me through this project. Critical comments of the final manuscript by Dr. Stanley Cebull and Dr. Gary Strathearn were very appreciated. Without the interest, energy, and help of Robert Gribble in the field, this particular project would never have started. Tim Horner generously donated his time and skill to do the lettering on my field map.
Jesse O'Halloran (l895-198i*) and James O'Halloran {19OI-I983) provided support for the field work of this project. Sigma Xi also provided money toward the completion of this research. Of course, the encouragement and understanding of my family throughout my graduate career is appreciated more than I can say.
ii TABLE OF CONTENTS
ACKNOWLEDGMENTS ii
LIST OF TABLES v
LIST OF FIGURES vi
I. PURPOSE AND BACKGROUND 1
Purpose 1
Location 2
Study Methods 5
Regional Geology 5
II. GEOLOGY OF THE SALMON MOUNTAIN AREA 13
Previous Work 13
Metasedimentary Rocks of
Salmon Mountain 13
Structure 17
Dikes 18
Glaciation 19
Summary 20
The Salmon Mountain Stock 20
III. ROCKS OF THE SALMON MOUNTAIN STOCK 3*+
Petrographic Descriptions 3^
IV. EMPLACEMENT HISTORY h^
Origin ^5
Mode of Emplacement '•6
Percent Crystallinity During Emplacement ^l iii Origin of Schlieren U8 Emplacement of the Rock Units 50 Dikes 51 V. PETROLOGY 52
Paragenesis 52
Chemical Composition 53
Trondhjemitic Trend ..... 55
Comparison to Other Klamath Mountain Plutons 56 Implications to Plutonism in the Klamath Mountains 62 VI. CONCLUSIONS 63
LIST OF REFERENCES 6k APPENDIX 69
IV LIST OF TABLES
1. Modal mineralogy and plagioclase An content of rocks of the Salmon Mountain stock 37
2. Chemical analyses and CIPW norms of coarse-grained rocks of the Salmon Mountain stock 5** LIST OF FIGURES
1. Plutonic belts of the Klamath Mountains 3
2. Subprovinces and major plutons of the Klamath Mountains 6
3. Geologic Map of Salmon Mountain,
California in pocket k, Outline of the shape of the Salmon Mountain stock ... 21
5. Schlieren of the Salmon Mountain stock 30
6. Modal plagioclase-quartz-alkali feldspar diagram for rocks of the Salmon Mountain stock 35 7. Plagioclase-quartz-alkali feldspar diagram for plutons of the Klamath Mountains 57
VI CHAPTER I
PURPOSE AND BACKGROUND
Purpose
The purpose of this study is to characterize the petrology, chemistry, and eraplacement history of the Salmon Mountain stock, and to discern its relation to plutonic belts lying to the north and to the south of the study area. The Salmon Mountain stock is located in a zone between calc-alkaline plutons to the north and mildly alkaline plutons to the south (Barnes, personal coraraunication). Knowledge of the petrology and chemistry of this body should indicate which group, if either, the Salmon Mountain stock belongs to. Furthermore, the
Salraon Mountain stock is oriented east-west, transverse to the regional north-south orientation of most Klaraath plutons. Studies of the emplacement history of this body may add to our understanding of rifting and terrane accretion in the Klaraath Mountains (e.g. Saleeby et al., 1982).
The Klamath Mountains are thought to be the remnants of ancient island arc systems that formed along the western margin of North
America (Irwin, 198I, I98U; Saleeby et al., 1982; Wright, 1982).
Arcuate belts of granitic to gabbroic plutons in the Klamath Mountains presumably represent magma charabers that fed these volcanic arcs.
Plutonic belts were first recognized in the Klamath province by
Lanphere et al. (1968) on the basis of sparse K-Ar dates. With the
1 2 acquisition of raore radioraetric age dates on plutons, twelve plutonic belts are now thought to be present (irwin, I98U). The Salmon Mountain
stock is located in a zone between the northern end of the Ironside
Mountain plutonic belt and the southern end of the Wooley Creek plutonic belt of Irwin (198U) (Figure l). The Ironside Mountain plutonic belt trends northwest, is raid-Middle Jurassic (about I60 to
166 my) in age, and is thought by Irwin (l98it) to have forraed prior to accretion to the Klamath province. The Wooley Creek plutonic belt trends northeast, is late-Middle Jurassic (about 163 ray)i n age, and
Í8 believed by Irwin (l981t) to be postaccretion in origin. Inforraation
gained from the study of the Salmon Mountain stock will show if the
zone between these belts is: l) transitional, 2) a member of one of these belts, 3) unrelated to either belt. Postaccretion versus
preaccreion relations in this area can then be further examined in
light of this information.
Location
The gabbroic to trondhjemitic Salmon Mountain stock is located at
Salmon Mountain, eleven miles east-southeast of Orleans, California,
in the Salraon Mountain 7.5 rainutequadrangle . Elevations in the study
area range from 1,707 meters at Red Cap Lake to 2,120 meters at the
summit of Salmon Mountain. Access is via unpaved road frora Orleans,
California, plus 3.5 miles on an iraproved trail. From'^î^lí n ^98^!°"'' "'"' °' *^' '''"""*'^ Mountains. x = study area. nriiiiTiii
rLVTIIIC lElTS
riSTUCIETICI
riEtcciniti
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40'-f Vo> *> -(fe *^ '^ *'^
^%:'*:%.''^.:\^ •<> % V Study Methods
Field studies were performed during the summer of 198î|. Mapping was completed on 1:6,000 and 1:12,000 scale enlargements of the Salmon
Mountain 7.5 minute quadrangle, using brunton compass and altimeter
methods.
Lab studies were done at Texas Tech University during the 198I+-85 academic year. Plagioclase anorthite content was determined using the
A-norraal, Michel-Levy and Carlsbad twinning methods on a flat stage petrographic microscope. All raodal analyses are based on at least one
thousand points per thin section.
Regional Geology
Subprovinces
The Klamath Mountains form an elongate north-trending geological province located in northwestern California and southwestern Oregon and occupy more than 11,000 square miles between the Cascade province to the east and the Coast Range province to the west (Figure 2). The province contains predominately eugeosynclinal sedimentary, raetasedimentary, and metavolcanic rocks that are intruded by numerous granitic plutons. Rocks of the Klamath Mountains that were involved in the late Jurassic Nevadan orogeny will be briefly discussed in this section. Overlying weakly deformed rocks occurring sparsely throughout the Klamath province, such as the Great Valley sequence at the extrerae southern part of the province, will not be considered. Figure 2. Subprovinces and major plutons of the Klamath Mountains. EK: Eastern Klamath belt, SM: Central Metamorphic belt, TrPz: western Paleozoic and Triassic belt, WJ: western Jurassic belt, CMS: Condrey Mountain Schist, AP: Ashland Pluton, VB: Vesa Bluffs, GP: Grants Pass, GB: Greyback, IMB: Ironside Mountain batholith, EP: English Peak, WCB: Wooley Creek batholith, SL: Slinkard, BMC: Bear Mountain coniplex. After Irwin (1981). study area
*''*" Of fflop 8
The Klamath Mountains have been divided into four arcuate, north- trending subprovinces, or lithic belts, separated by thrust faults that dip to the east, so that each belt is thrust over the adjacent belt to the west (irwin, 196O, 1966) (see Figure 2). From east to west, these belts are the eastern Klamath belt, the Central
Metamorphic belt, the western Paleozoic and Triassic belt, and the western Jurassic belt. These belts are differentiated on the basis of
lithology and structure, and in a general sense become progressively younger from east to west. The rocks range in age frora Ordovician to
Jurassic, as determined by radiometric dating (Lanphere et al., I968) and sparse fossil evidence (lirwin, I966; Irwin et al. 1978).
The eastern Klamath belt consists of an eastward dipping horaoclinal
sequence of eugeosynclinal clastic sediments and volcanic rocks that
exhibit variable degrees of low-grade metaraorphisra and have an aggregate thickness of as much as 15,2^0 meters (irwin, I966; Davis,
1966; Hotz, 1971)« These rocks range in age from Ordovician to
Jurassic as shown by fossil evidence (irwin, 1978, 1981) and uranium-
lead and rubidium-strontium dating methods (Lanphere et al.,1968;
Mattinson and Hopson, 1972).
The central raetaraorphic belt is separated frora the eastern Klaraath belt on the east by discontinuous elongate bodies of ultramafic rocks.
The central metaraorphic belt consists of the raetavolcanicSalrao n
Hornblende Schist and the overlying raetasediraentaryAbrara s Mica Schist
(Davis et al. , I965). Primary metamorphism of the Salmon Hornblende
Schist and the Abrams Mica Schist is believed to have occurred approxiraately 380 Ma (Devonian) (Lanphere et al., I968). 9
The western Paleozoic and Triassic belt is a complex eugeosynclinal terrane coraposed of thinly bedded cherts, argillites, metaserpentinites, mafic volcanic rocks, detrital rocks and coarsely crystalline limestone lenses. These rocks are generally metamorphosed to the lower greenschist facies, but locally attain grades as high as upper amphibolite facies near the Condrey Mountain Schist and pyroxene-hornfels facies near the margins of intrusions (irwin, 1966;
Davis, 1966; Barnes, I983, Snoke et al., I981). The age of this most extensive of the four belts is not tightly constrained, but radiolarian fossil evidence suggests a mid-Perraian to Early Jurassic age (Hotz, 1971; Irwin, 1972; Irwin and Galanis, 1976; Irwin et al.,
1977; Irwin et al., 1978). Liraestone blocks occur throughout this subprovince, and yield Pennsylvanian and Perraian fossil ages (irwin,
1972; Irwin and Galanis, 1978). These limestone blocks are thought to be exotic to this subprovince, and may have been derived frora the
eastern Klamath belt (lirwin, I98I; G. Gray, personal cormnunication).
The western Jurassic belt is in fault contact with the western
Paleozoic and Triassic belt to the east and the Franciscan terrane to the west (irwin, I96O; I96U). The Josephine peridotite and related
mafic and ultraraafic rocks, such as the Chetco intrusive complex
(Garcia, I982), comprise the base of this terrane (irwin, 1966). These
are overlain by greenschist-facies metasediraentary and raetavolcanic
rocks of the Galice and Rogue Formations, respectively (Harper, 198U;
Garcia, I982).
Sheets of ultramafic rock occur throughout the Klamath Mountain
province. These rocks are chiefly peridotites but range in composition 10 from pyroxenite to dunite (irwin, I966), are sheet-like or tabular in shape, and commonly separate rocks of different age or metamorphic grade (irwin, 1981). These ultramafic sheets typically forra conspicuous, elongate bodies along the borders of subprovinces as well as between the western Jurassic belt and Coast Range province. These relationships, combined with their serpentinized and sheared nature
(irwin, 1966), have led researchers to conclude that the ultramafic sheets are portions of dismembered ophiolite sequences (Saleeby et al., 1982; Irwin, I98U).
Terranes of the Western Paleozoic and Triassic Belt
The southern portion of the western Paleozoic and Triassic belt has been divided on the basis of lithologic, structural and age relations into north-trending terranes separated by eastward dipping thrust faults (Irwin, 1972). Frora east to west, these are the Stuart
Fork Forraation, the North Fork terrane, both of which are overlain in thrust contact by the Central Metamorphic belt, the Hayfork terrane, and the Rattlesnake Creek terrane. The Hayfork terrane is of particular interest, because it is the host terrane for the Salmon
Mountain stock.
The Hayfork terrane ranges between 15 and 25 kilometers in width
(Irwin, 1977) and attains local (structural) thicknesses of several thousand meters (irwin, 1972). Wright (1982) revised Irwin's (1977) orignal three-fold division of this terrane. He defined the eastern and western Hayfork terranes, which are separated by the Wilson Point 11
thrust. The structurally lower western Hayfork terrane consists
predominately of raid-Jurassicmetavolcaniclasti c rocks, whereas the
structrually higher eastern Hayfork is a metasedimantary raelange. The
Salmon Mountain stock lies within the eastern Hayfork terrane, which
is further described below.
The eastern Hayfork was first interpreted to be olistrotroraal by
Cox and Pratt (1973), but has since been interpreted as a raelange by
Wright (1982). As defined by Wright (1982), the eastern Hayfork is
"...a tectono-stratigraphic unit consisting of a coraplex chert,
argillite, and quartzose sandstone melange and broken forraation that
contains radiolarian cherts as young as late Triassic in age." Minor amounts of tuffaceous mudstone and tuffaceous chert are also present throughout the terrane, along with various exotic blocks such as metachert, shallow water liraestone, blueschist, and scarce fine- grained raetavolcanic greenstone (Wright, 1982). This terrane is termed a tectono-stratigraphic unit due to its chaotic nature and high degree of structural imbrication.
Plutonism
The entire Klamath Mountains province has been intruded by plutons that range in composition from olivine pyroxenite to granite.
Elongation of these plutons is subparallel to the arcuate shape of the province (Figure l). Figure 2 shows that most granitiod (i.e., interraediate to felsic rocks with a granitic texture) plutons occur in the central raetamorphic and western Paleozoic and Triassic belts.
Granitoid intrusions in the Klaraath Mountains are typically dioritic. 12
quartz dioritic, or granodioritic in coraposition (Davis, 1966), with
quartz diorite the most comraon type (Hotz, 1971). Gabbro and quartz
raonzonite represent compositional extremes found in sorae intrusions,
such as the Wooley Creek batholith (Barnes, 1983) and the Ironside
Mountain batholith (Charlton, 1979). The granitoid intrusions of the
Klamath Mountains are typically of a calc-alkaline chemical affinity
(Hotz, 1971). The Salmon Mountain stock is located in a transition
zone between calc-alkaline plutons to the north and mildly alkaline plutons to the south (Barnes, personal conmiunication).
The majority of Klamath plutons are middlé to late Jurassic in age, but range from Ordovician to Early Cretaceous (Lanphere et al.,
1968; Hotz, 1971). Lanphere et al« (1968) determined the existence of
four arcuate plutonic belts, as determined by radiometric ages. These belts have been subdivided, yielding a total of twelve belts according to Irwin (198U) (Figure l). The plutonic belts trending northeast are
thought to be postaccretion, whereas the northwest-trending belts are believed to be preaccretion, i.e., plutons emplaced, and later moved, with their host rocks, to a new location (irwin, I98U).
There are several published works on the calc-alkaline plutons found in the Klamath Mountains. A synopsis of geochemical and petrological data of Klamath plutons is given by Hotz (l97l). In addition, Vennum (1980) studied the Castle Crags pluton 80 kiloraeters to the east of Salmon Mountain, Snoke £t al^ (1982) investigated the
Bear Mountain igneous complex k8 kilometers to the north of the study area, and Barnes (1983) studied the Wooley Creek batholith 16 kilometers to the north of the study area. CHAPTER II
GEOLOGY OF THE SALMON MOUNTAIN AREA
Previous Work
Geologic studies in the Salmon Mountain area are sparse. Cashman
(1979) did detailed geological mapping of the Hayfork terrane several kilometers to the east of the study area and reconnaissance mapping of the Salmon Mountain vicinity. Charlton (1979) studied the composite
Ironside Mountain batholith and related granitic plutons, which lie to the south-southwest of Salmon Mountain. Donato et al. (1983) did geological mapping of the Orleans Mountain area, within which the study area lies. Current research in the Salmon Mountain area is
focused on the Orleans Mountain diorite, six miles to the north of
Salmon Mountain (Barnes, in progress).
Metasedimentary Rocks of Salmon Mountain
General Statement
Several varieties of metasedimentary rocks are found on Salmon
Mountain. These include chert, tuffaceous argillite, sandstone, greenstone, and blocks of recrystallized limestone. Exposure of these rocks along the Salmon Mountain summit ridge is excellent, but exposures to either side of this ridge are restricted, due to thick soil cover. These rock units are regionally metamorphosed to the
13 Ik greenschist facies; lower raiddle hornblende hornfels facies rocks are present near contacts with the Salmon Mountain stock.
Hayfork Bally Meta-Andesite
Hayfork Bally meta-andesite occurs along the Salmon Suramit trail about three kiloraeters to the northwest of Salraon Mountain. The rock occurs as gray to greenish gray float and no outcrops are present. The
Hayfork Bally raeta-andesitei s defined by Irwin (1977) as an augite meta-andesite with local interlayers of chert, tuff, and aggloraerate.
Wright (1982) defined the boundary between the eastern and western
Hayfork terranes as being the uppermost extent of the Hayfork Bally meta-andesite.
Chert
Chert is the most abundant raetasedimentary rock on Salmon
Mountain. It forms high rugged cliffs on the edges of the two cirques, and smaller outcrops on the lower eastern and northern flanks of the raountain. This chert is massive, rarely thickly bedded, and weathers to a rust brown or black color. The rock breaks with difficulty along irregular, flat surfaces, revealing a fresh light blue surface.
Bedding, where observed, is 5 to 11 centiraeters thick.
Tuffaceous Argillite
Fissile tuffaceous argillite crops out on ridges in the Salmon
Mountain area and is widespread as tabular float. This rock type is nearly as abundant as chert within the study area. Weathered surfaces 15 consist of green, black, and light brown bands that typically range from 0.1 to 3.5 centimeters thick and reach thicknesses of 5 to 8 centiraeters. The thin layers are diffuse and irregular, and are locally discontinuous or anastomosing. Sparse thin layers (less than 2 centimeters thick) of argillite are interbedded with massive chert in cliffs on the southeastern part of Salmon Mountain.
Petrographically, the tuffaceous argillite consists of lenses and bands of anhedral quartz and large, blocky anhedral plagioclase alternating with layers of stretched biotite and green subhedral laths of actinolite. Small anhedral diopside, green hornblende, and potassiura feldspar occur sparsely throughout the rock. Quartz and
feldspar grains are strained and slightly recrystallized, and quartz grains are stretched parallel to the direction of banding. Fine- grained clinozoisite is an alteration mineral.
Sandstone
Sandstone is rare on Salmon Mountain, but where present comprises
dense massive outcrops of feldspathic sub-lithic greywacke that weather to a dark buff color. These sandstones are poorly sorted and
matrix supported, with lithic grains consisting of chert (quartzite).
Angular quartz and plagioclase grains are strained and slightly
recrystallized. Rounded detrital grains of clinopyroxene and opaque
minerals are also present. The raatrixi s composed of biotite and white
mica. 16
Chert-Argillite Breccia
Chert-argillite breccia (Cox and Pratt, 1972) borders the Salraon
Mountain stock along the Red Cap Lake trail. The rock is massive and consists of a black, siliceous, argillite matrix supporting dull white
clasts of massive raetachert(quartzite ) that range in size from 0.3 to
60 centiraeters long. The rock is estimated to consist of 60 per cent
siliceous argillite, 30 per cent larger clasts (i.e., greater than ten
centimeters long) and 10 per cent smaller clasts. The shape of the
chert clasts is variable, but most are smoothly elongate or lense-
shaped with gradational boundaries. Gradational boundaries are
probably due to contact metamorphism. Zones of strong preferred
orientation of elongate and lensoidal clasts are present lopally.
Mineralogically, the argillite consists of fine-grained (less than 0.1
millimeter long) subhedral biotite, grains of white mica, and
volumetrically lesser quartz. Chert clasts are composed of
recrystallized quartz and minor amounts of biotite and white mica.
Greenstone
Metavolcanic greenstone occurs in outcrops with bedding that
ranges from 1.3 to 12 centimeters thick. The greenstone weathers to
light olive-green and black colors. Blocks of coarse-grained raarble
are ubiquitous in these rocks. Stratigraphic relations on Salraon
Mountain show that greenstone outcrops are enclosed by tuffaceous
argillite or raassivechert , suggesting that the greenstone occurs as
exotic blocks (i.e., discrete block or slab formed in an environraent 17 distinctly different from that in which it is found) in the melange
(e.g., Wright, 1982).
Llmestone Blocks
Coarse-grained recrystallized blocks of limestone occur within tuffaceous argillite and greenstone in the study area. These blocks are variable in shape, white to grey in color, and range in size from
5 to 25 centimeters long. Limestone blocks found in float were as much as three raeters long, and were probably formerly enclosed by massive chert. The stratigraphic relations suggest that liraestone blocks are also exotic blocks.
Structure
Numerous small folds deform thickly bedded chert about 200 meters southeast of the summit of Salraon Mountain (see Figure 3). The folds raeasure 1 to 2 meters from crest to crest and have fairly uniforra orientations of rainorfol d axes, lineations and major fold axes.
Lineations have an average trend and plunge of N60 W, 12 SE; fold axes have an average orientation of N77 W, 19 SE. Axial planes strike
N80°W, and are typically upright. The axial planes of the structures are aproximately parallel to the outline of the stock, a fact that suggests that the folds were formed by intrusion of the stock.
Attitudes of tuffaceous argillite are variable over distances of less than 100 meters, and the only place suitable for these measureraents is along the sunmiit ridge of Salmon Mountain. In these rocks S =S . Strikes range from N20°E to N50 E, and dips from 32 NW 18 to 38 SE. Small folds and crenulations are observed in these rocks and increase in abundance from south to north toward the Salmon
Mountain stock. The attitudes of axial planes and fold axes are generally parallel to the contacts of the Salmon Mountain stock. These
relations and the close proximity of these structures to the stock
suggest that the structures were also formed during intrusion of the
stock.
Dikes
Indian Rocks (irwin, 1972) is a dike that forms prominent oucrops
along a high ridge on the Salmon Summit Trail about 2 kilometers
northwest of Salmon Mountain. Chemical and petrographic analysis
(Barnes, personal coimnunication) shows this rock to be a porphyritic
garnet-bearing hornblende quartz dacite. It contains seriate
phenocrysts of zoned plagioclase, as well as sraaller phenocrysts of
quartz, hornblende, and garnet. The groundmass consists of plagioclase
and quartz. Because this dike is located several kilometers from
Salmon Mountain, it will not be considered further in this paper.
Numerous narrow porphyritic dikes of hornblende diorite (?)
composition cut the metasediments of Salmon Mountain. These dikes
range from 10 to 90 centimeters in width, and are black and grey on
fresh surfaces. Most of the dikes strike NU9 W to N60 W, and dip
steeply (greater than 75 degrees) to the southeast or southwest (see
Figure 3). The dikes have sharp, slightly undulose contacts with their
wallrocks, and many contain partially assimilated xenoliths of chert
or greenstone as much as 30 centimeters long. Chilled margins are 2 to 19
10 centiraeters wide, are finer grained than the interior of the dike, and contain hornblende laths aligned parallel to the walls of the dike. None of these dikes could be shown to intersect, or be intersected by, the body of the Salmon Mountain stock,
The dikes contain phenocrysts of euhedral and subhedral hornblende as imich as 1.5 centimeters long, plagioclase, and a groundmass of hornblende, plagioclase, biotite, and opaque minerals. Many hornblende crystals are hollow and some form glomerocrysts that are as raucha s 2 centimeters long. Sparse subhedral and anhedral clinopyroxene is present, sorae of which is enclosed by euhedral hornblende. Some of the dikes contain post-igneous biotite, tremolitic and actinolitic amphibole, and green hornblende, suggestive of metamorphisra to the biotite subfacies of the greenschist facies. Brittle fracturing is
observed in thin section, but with no signs of penetrative
deformation.
Two mafic dikes trending approxiraately east-west intersect the
Salmon Mountain stock near its eastern end. These dikes are black, highly weathered, less than 1 meter wide, and have sharp contacts with the stock. These mafic dikes will not be considered further, because
they are believed to be rauchyounge r than the raajorigneou s activity
on Salraon Mountain.
Glaciation
Salraon Mountain has been effected by Wisconsin (?) glaciation. The
northeast side of the raountaini s composed of two weakly developed,
rugged cirques separated by a small arete. The glaciated part of the 20 stock has glacial steps up to 12 meters high, abundant glacial striations and grooves, and several tarn lakes (see Figure 3).
Exposure of the stock within these cirques is nearly one hundred per cent. In the forested western half of the stock, exposures are limited mostly to small cliffs.
Sumraary
The raetasediraents in the Salraon Mountain vicinity comprise a
section in which sandstone, greenstone, tuffaceous argillite and
limestone are enclosed by chert. Individual blocks are continuous for
distances of less than 300 raeters. The blocks occur randoraly with no
stratal continuity of any single lithologic type. Chert is the only coherent unit in the study area. These lithologic types, their chaotic
fabric, and the presence of Hayfork Bally meta-andesite to the
northeast of Salmon Mountain indicate that the Salmon Mountain stock
is located in the eastern Hayfork terrane, as defined by Wright
(1982).
The Salmon Mountain Stock
Intrusive Relations
The Salraon Mountain stock is elongate east-west and about two and
one-half kiloraeters long. It is slightly concave to the south in raap
view (Figure k). The stock consists of numerous rock types
representing several intrusive stages. The intrusive phases are, in
order of decreasing abundance, coarse-grained rocks that range in M o o •p co c •H o! •P c 1=1 ê c r-1 Q) CQ
o (U
0) Xt m i> •P
<(-i o (1) c •H H •P
(U
•H 22
<(Uu »4-1 o o o ro o CM CS
o o o o o o CN
o .-H o f—f 23 composition from hornblende gabbro to hornblende quartz diorite, medium-grained rocks that range in coraposition from hornblende quartz diorite to biotite tonalite, fine-grained rocks that range in composition from hornblende gabbro to hornblende quartz diorite, biotite and clinopyroxene norite, and late-stage trondhjemitic intrusion breccia. Grain sizes are defined as: coarse-grained, 2 to U millimeters; medium-grained, 1 to U milliraeters; fine-grained, less than 1.5 milliraeters.
Field relations establish relative age relations for these intinisive phases. The coarse-grained rocks are the earliest intrusive phase and are intruded by irregularly shaped bodies of medÍTim-grained rock. Fine-grained rocks comprise the third intrusive phase, as they enclose or cross-cut both the raedium-grained and coarse-grained varieties. Biotite and clinopyroxene norite cross-cut the first three phases, typically as irregularly shaped blebs. Trondhjeraite intrusion breccias cut and disrupt all rock types.
Field relations were also used to elucidate magmatic conditions during eraplaceraent of the first three phases. Contacts "between these intrusive phases are irregular, undulatory, and gradational over distances of 0.3 centimeters to 3 meters. In one instance medium- grained rock was back-intruded by coarse-grained rock. At several locations the coarse-grained rock has a 2.5 centimeter-wide reaction rira at the contact with medium- or fine-grained rock. The rim is presuraably due to raetasoraaticalteratio n by the later phases. Coarse- or raedium-grained rock that is intruded by fine-grained rock typically occurs as round raasses 0.3 to 0.5 raeters in diaraeter. The fine-grained 2k host enclosing these masses contains strongly oriented hornblende laths that apparently were oriented by flow around the earlier phases.
These relations are compatible with the idea that the stock was a mobile, partially crystalline liquid during the emplacement of these
first three phases, and that each phase was emplaced singly. In addition, the large distances of gradation between phases and the undulatory nature of contacts suggest that some mixing took place between intrusive phases.
Emplaceraent of norite and tondhjeraite intrusion breccia occurred
during the middle and late stages of solidification of the stock,
respectively. Both groups cross-cut the three phases described above.
One of the norite bodies extends into the pluton frora the contact as a
sharp dike-like body 2 to 3 meters wide. However, it becomes
irregularly-shaped and disrupted inward and ultimately disappears in the center of the stock. This suggests that at the time the norite was
intruded, the stock had a mostly solid outer rind and a core that was
still liquid. The trondjemite intrusion breccias have sharp intrusive contacts with the surrounding rock, and contain large, brecciated
fragments of earlier rock types. These relations suggest that the breccia bodies were emplaced when the stock was nearly or entirely
crystalline.
Stoping
Wallrock blocks that were stoped into the Salraon Mountain stock are observed in the eastern one-half of the intrusion. In that area, most margins of the intrusion contain numerous wallrock blocks in a 25 zone that ranges from 7 to 30 meters in width. Xenoliths of tuffaceous argillite and chert within these contact zones range in size from 2 centiraeters to 2.5 meters in diameter. Blocks less than 0.75 meters in diameter are rounded or lense-shaped, and some exhibit effects of partial or nearly complete assirailation by the enclosing igneous rock.
Larger wallrock blocks are rounded in outline, but have an angular, irregular surface. Hornblende occurs in sorae of the partially assimilated blocks. Rare xenoliths of greenstone are less than 7 centiraeters long and always show effects of partial assimilation.
Rounded or very rare angular autoliths of coarse-grained and raediura- grained plutonic rocks are sparse in the gradational contact zone; these rocks show no effects of re-equilibration with their surrounding host. Larger recrystallized chert blocks (some radiolarian cherts) occur near the north-central contact of the the stock. These blocks are elongate and parallel to the stock-wallrock contact and as imich as
300 meters long (see Figure 3).
Contacts
Wallrock-stock contacts observed of the Salraon Mountain stock are vertical. This is best seen by the manner in which the stock cuts across the topography (see Figure 3).
The Salraon Mountain stock has a contact aureole 10 to 15 raeters wide. Metaraorphic rainerals in this aureole include brown hornblende, biotite, and sparse garnet and fibrolite. These rainerals indicate contact raetaraorphisra to the hornblende-hornfels facies and typically forra a fabric that is an overprint of the regional metaraorphic fabric. 26
Coarse-Graíned Gabbro and Diorite
Coarse-grained hornblende gabbro, hornblende quartz-gabbro, hornblende diorite, and biotite hornblende quartz diorite compose more than eighty percent of the exposed area of the Salmon Mountain stock.
These rocks are distinctive in outcrop and are easily distinguished
frora the other rock groups by large clots of poikilitic hornblende that enclose pyroxene. These rocks crop out as light tan and white cliffs and glacial steps in the eastern half of the stock and as
speckled black and white cliffs in the western portion of the stock.
Hornblende quartz-gabbro is restricted in occurrence to the outer 175 meters of the body.
Ovoid pegmatoidal segregations of tonalitic composition occur in thê coarse-grained rocks near the north-central contact of the Salraon
Mountain stock. Segregations are generally 5 to 15 centimeters in
diameter, but are as large as 1.5 meters in diameter. The pods have
radially-oriented hornblende laths as imich as 5 centimeters long.
Medium-Gralned Dioríte and Tonalite
Medium-grained hornblende quartz diorite and hornblende and biotite tonalite comprise scattered, discontinuous bodies that intrude the coarse-grained rocks along irregular contacts. These rocks constitute less than 5 percent of the stock, and in this grouping the tonalites predominate over the diorites. In hand saraple and outcrop
sorae of these rocks appear similar to the coarse-grained rocks, but their mafic grains are smaller. 27
Flne-Grained Gabbro and Diorlte
Fine-grained gabbro, diorite and quartz diorite form less than 1 percent of the Salraon Mountain stock, and occur as discontinuous bodies that intrude the raediura and coarse-grained rocks. Locally, the
fine-grain rock types enclose closely-spaced rounded 0.3 meter to 0.5 meter diameter blocks of the two earlier types. Several such outcrops contain less than 25 percent of the intrusive fine-grain rock. These
rocks have a distinctive appearance in the stock, due to a color index of about 50, and abundant, small (less than 2 raillimeterslong ) hornblende prisms. The hornblende prisms are strongly aligned near contacts. Irregularly-shaped mafic enclaves about k cra long with a color index of approximately 85 occur in some of these fine-grained
rocks.
Norite
Norite, biotite norite, and clinopyroxene norite occur as
discontinuous outcrops in the Salraon Mountain stock. They are fine- grained (average grain size less than 1 millimeter), porphyritic rocks that occur in hard, brittle outcrops that weather to an olive green
color. These rocks cross-cut the fine, medium and coarse-grained
rocks, and were intruded when the outer portion of the stock was
solidified, as discussed above. Some of these rocks enclose small
angular and rounded enclaves of coarse-grained hornblende gabbro and
schlieren. The major minerals in the norites are orthopyroxene,
clinopyroxene, biotite, plagioclase and olivine. Biotite phenocrysts 28 are visible in some hand samples.
Trondhjemite Intruslon Breccia
Trondhjemite intrusion breccias crop out in five locations in the eastern half of the Salraon Mountain stock (see Figure 3). The breccias
contain enclaves of wallrock and earlier intrusive material. The
largest of these dikes is as niuch as 57 meters wide, can be traced for
over 300 meters in outcrop, and is oriented subparallel to the outline of the stock (see Figure 3). The other intrusion breccias are rounded or elongate in map view. The walls are vertical or subvertical, and have sharp contacts against the surrounding stock. Phenociysts and
gloraerocrysts of biotite give the host trondhjeraite a color index of
11. Enclaves of earlier igneous material are absent in two small (less than 15 meters long) trondhjeraite dikes.
Enclave corapositions in the intrusion breccias include the coarse-, mediura-, and fine-grained rock types explained above, schlieren, chert and greenstone. The largest dike contains enclaves that range in size frora less than 1 centimeter to over 1.5 meters long, and average 0.5 to 1 raeteri n length. Enclaves in the other intusion breccias range frora less than 1 centimeter to 0.6 meters long. Enclave abundance ranges from kO per cent in the largest dike to
50 to 65 percent in the other bodies. Enclaves in all intrusion breccias are angular and rarely show signs of assimilation by the host. Several individual enclaves contain a rounded core of coarse- grained rock (as discussed above) enclosed by medium-grained rock, which is in turn enclosed by fine-grained rock. 29
Schlieren
Distribution
Schlieren layering is abundant in the Salmon Mountain stock (see
Figure 3), and ranges in size from 2 to over 15 raeterslong . Outcrops of schlieren occur from the contacts to the center of the stock, with one area in the central portion having more than 20 distinct schlieren within 250 square raeterarea . Schlieren consists of alternating bands
of light and dark rainerals(se e Figure 5). The schlieren are coarse-
grained and related to the coarse-grained rock type described above.
The thickest schlieren (dark bands greater than approximately 6
centimeters thick) dip gently and occur along the outer portions of
the stock. Much more abundant thinner schlieren are oriented within 30
degrees of vertical, dip to the southeast or southwest, and are rare
in the outer 25 meters of the stock.
Mineralogy and Structure
Dark-colored bands are composed predominately of plagioclase and
rounded grains and glomerocrysts of pyroxene. Late-stage poikilitic
hornblende encloses pyroxene. Light-colored bands have the same
raineralogy as dark colored schlieren, but a greater proportion of
plagioclase. Dark-colored layers contain slightly larger plagioclase
and pyroxene crystals than do light-colored layers. Domains of
parallel alignment of long crystal axes and of the long dimensions of Figure 5. Schlieren of the Salmon Mountain stock. Note narrow dike of raedium-grained rock that contains enclaves of banded rock (left center of picture). 31 32 glomerocrysts were observed in thin section but were rarely observed
in outcrop. Broken plagioclase grains are coramon in the schlieren.
Schlieren in the Salmon Mountain stock display variable thicknesses and typically small gradations from layer to layer. Dark- colored schlieren vary from 0.5 to 20 centimeters thick, and light- colored schlieren from O.k to ik centimeters thick. The dark bands are thicker than the light-colored bands, except where all bands are less than 0.5 centimeters thick. In these cases, the bands are of approximately equal thickness. In outcrop the bands show both sharp mutual contacts and gradation over distances as great as 0.6
centimeters. However, concentration grading is veiy rare in these
rocks. Where it occurs, 1-centimeter-thick mafic bands have a sharp
contact with a narrower felsic band on one side and a gradational
contact with a felsic band on the other. Sharp contacts all face the
same direction. These layers dip 60 to 70 degrees to the southwest.
Sedimentary or flow structures also occur in these schlieren. Low-
angle intersection of schlieren is common, as is branching of bands in
which the aggregate thickness of the branches is the same as the
original bands. Cut-and-fill structure also occurs. Sorae gently-
dipping bands display high-angle peneconteraporaneous faulting with
offsets as great as 7 centimeters. Enclaves of raedium and coarse-
grained igneous rock, and rarely of highly altered chert, are
incorporated in the schlieren at several locations in the stock. These
enclaves range frora 5 to 15 centimeters in length, and elongate ones
are parallel or subparallel to the banding. 33 Comb Layering and Orbicular Structure
Two small outcrops of corab layering occur in the Salmon Mountain
stock and are weathered to a light brown or black color. The comb
layers are 13 to 15 centiraeters thick and consist of distinct, 0.7 to
1.3 centimeter-thick bands. "Corabing" is formed by plagioclase laths
as much as 1.3 centimeters long oriented perpendicular to the contacts
between hands. Comb layering occurs as disrupted bands within the
medium-grained rock described above, which is intrusive into the
coarse-grained rock. This indicates that the layers were transported
after formation to their current location by raovement of the medium-
grained raagma.
Orbicular structures occur at two locations in the stock. The
orbicules are about 25 centiraeters in diameter and consist of two-
centiraeter-wide concentric layers of radial plagioclase and
hornblende. Orbicules have a 3-centiraeter-wide reaction rira of fine-
grained mafic minerals where they are in contact with the surroundding
rock. CHAPTER III
ROCKS OF THE SALMON MOUNTAIN STOCK
Petrographic Descriptions
Coarse-Grained Gabbro and Diorite
This group of rocks is characterized by a hypidioraorphic-granular texture with late-stage raagmatic hornblende oikocrysts enclosing glomerocrysts of pyroxene. Plagioclase exhibits discontinuous normal zoning (i.e., MacKenzie et al., 1982), is typically subhedral, and comprises 50 to 67 raodal percent of the rocks (see Figure 6 and Table
1). Plagioclase grains are as much as k raillimeters long, enclose sraall clinopyroxene anhedra, and sorae exhibit patchy zoning due to late-stage raagraatic re-equilibration. Hornblende forras interlocking subhedral and anhedral oikocrysts as much as 5 millimeters long. The hornblende is pleochroic from brown or olive brown to yellow, and encloses rounded clinopyroxene, skeletal and rounded opaque rainerals, relatively rare orthopyroxene, and sparse plagioclase. Many hornblende grains are twinned. Hornblende ranges in raodal abundance frora 9 to 25 percent. Anhedral interstitial grains and gloraerocrysts of clinopyroxene are enclosed by hornblende, and comprise frora a trace to
6 raodal percent of these rocks. Clinopyroxene grains range in size from less than 0.1 to 1.5 millmeters long. Subhedral and anhedral orthopyroxene is pleochroic frora light pink to light green or clear,
3k Figure 6. Modal plagioclase-quartz-alkali feldspar diagram for rocks of the Salmon Mountain stock. A= coarse-grained rocks, e = medium-grained rocks, 0= fine-grained rocks, D= norite, + = trondhjemite. One additional norite, two additional coarse-grained rocks, and one additional fine-grained rock plot in the plagioclase corner, and are not shown. Field labeling: 1, gabbro/diorite/anorthosite; 2, quartz-diorite/quartz-gabbro/quartz- anorthosite; 3, tonalite; U, monzodiorite/monzogabbro; 5, quartz monzodiorite/quartz monzogabbro; 6, granodiorite; 7, monzonite; 8, quartz monzonite; 9, granite; 10, syenite; 11, quartz syenite; 12, granite (leucogranite); 13, alkali-feldspar syenite; lU, quartz alkali-feldspar syenite; 15, alkali-feldspar granite; l6, quartz-rich granitoids; 17, quartzolite. Nonmenclature after Streckeisen (1976). 36 37
:!: 01 .-1 co eC s(S 3 ki t-l I .-I O i-l X o c -o; m u__ . (0 ou n 0) 01 o C U VO •o co
Quartz is poikilitic, and encloses small grains of plagioclase, hornblende, and biotite. Some of the quartz grains are strained.
Quartz coraprises from a trace to 15 modal percent of these rocks.
Apatite grains as rauch as 1.5 railliraeterslong , rare zircon, and
opaque minerals are accessory minerals. Opaque minerals occur as
rounded and skeletal grains within hornblende and biotite. Tremolitic
and actinolitic amphibole occur as alteration minerals of hornblende
and pyroxene. Iddingsite is a alteration mineral of pyroxene and
olivine (?).
Medium-Grained Diorite and Tonalite
Rocks of the medium-grained group are characterized by
hypidiomorphic-granular texture with sparse hornblende glomerocrysts.
Plagioclase comprises 51 to 56 modal percent of these rocks, and forms
seriate subhedral grains exhibiting discontinuous normal zoning.
Zoning is strong in many plagioclase crystals, and crystals are rarely
bent or broken. Quartz comprises 8 to 26 raodalpercen t of these rocks,
and forras anhedral, poikilitic grains that enclose hornblende,
plagioclase and biotite. Subhedral and anhedral hornblende is
pleochroic frora light yellow or green to light brown and forms 3 to 20
raodal percent of these rocks. Hornblende occurs as poikilitic grains. 39 rare subhedral laths, and in sraall glomerocrysts with biotite.
Hornblende encloses very sparse anhedral clinopyroxene and orthopyroxene. Bent subhedral brown biotite forms as much as 13 percent of the rocks and occurs as interstitial grains as much as one milliraeter long, raonoraineralicglomerocrysts , and glomerocrysts with hornblende. Many of the biotite grains enclose anhedral opaque minerals. Apatite, zircon, and opaque minerals are accessories; the opaque rainerals forra skeletal and rounded grains that are enclosed by biotite or hornblende. Tremolitic and actinolitic amphibole are alteration products of hornblende.
Fine-Grained Gabbro and Diorite
Fine-grained gabbro, diorite, and quartz diorite have hypidioraorphic-granular texture characterized by interstitial hornblende prisms. Subhedral hornblende is pleochroic from olive-brown or light tan to yellow and forms as much as 35 modal percent of the rocks. It has inclusions of anhedral plagioclase, sparse anhedral opaque minerals, and rare rounded clinopyroxene grains. The average grain size of hornblende is 0.6 milliraeters, but grains as rauch as 2.5 milliraeters long occur. Plagioclase forras normally zoned laths and tablets, sorae of which display strong discontinuous zoning, and comprises as much as 60 raodal percent of these rocks. Plagioclase grains are typically bent or cracked. Quartz occurs as anhedral interstitial grains and as oikocrysts around plagioclase and hornblende. Quartz content varies frora 0 to 10 percent. Biotite is a rainor constituent (less than one raodal percent) and occurs as ragged UO subhedral grains with inclusions of anhedral opaque rainerals. Apatite and zircon are accessories. Apatite occurs as prisms less than 0.1 millimeters long and as subhedral grains nearly a milliraeter long.
Norite
Biotite norite and clinopyroxene norite (classification of
Streckeisen, 1976) have porphyritic textures. Orthopyroxene forms about 17 modal percent of these rocks and occurs as subhedral phenocrysts less than 1.3 millimeters long, and equant, anhedral groundmass grains less than 0.05 millimeters in diameter. The orthopyroxene is pleochroic from light pink to clear or light green.
Subhedral plagioclase coraprises 65 to 73 raodal percent of the rocks, is normally zoned, and forms subhedral phenocrysts and groundmass grains, sorae of which enclose small anhedra of biotite, clinopyroxene, and orthopyroxene. Many plagioclase grains are broken or cracked; sorae phenocrysts are partially resorbed. Sorae grains have a norraally zoned rim around an unzoned core. Olivine forras sparse rounded anhedral phenocrysts as much as 0.5 millimeters in diameter. Olivine grains are enclosed by thin rims of anhedral opaque minerals, which are in turn surrounded by anhedral orthopyroxene grains. Clinopyroxene forms subhedral phenocrysts as much as two milliraeters long, glomerocrysts, and rare groundraass grains. Clinopyroxene phenocrysts are twinned and enclose opaque minerals and rounded orthopyroxene. Some clinopyroxene grains show slight resorption. Brown biotite phenocrysts are subhedral, range in size from O.O6 to 0.27 millimeters long, and enclose rounded opaque rainerals. Opaque minerals form over two percent Ul of the mode and occur as anhedral interstitial grains less than 0.02 millimeters in diameter and as syraplectic intergrowths with olivine and orthopyroxene. Interstitial quartz is rare, and apatite is an accessory phase. Iddingsite occurs as an álteration product of pyroxene.
The only hypidiomorphic-granular norite observed consists essentially of plagioclase and orthopyroxene crystals with a weak to strong trachytic texture. The orthopyroxene is pleochroic from light pink to light green, occurs as subhedral prisras, and forms over 11 percent of the mode. Some orthopyroxene grains have inclusions of opaque minerals and plagioclase. Subhedral plagioclase laths and tablets exhibit norraal and patchy zoning and are rarely broken.
Biotite, clinopyroxene and amphibole occur in trace amounts. Apatite and opaque minerals are accessory phases.
Trondhjemite
Trondhjemite has hypidioraorphic-granular texture. Seriate,
subhedral plagioclase laths and tablets forra UO to 60 raodal percent of
the trondhjemite. Plagioclase composition is andesine (see Table l),
making these 'calcic' trondhjemites (classification of Davis, I963).
Most plagioclase grains are norraally zoned; oscillátory zoning is
rare. Some plagioclase grains are probably xenocrystic, as evidenced
by their anhedral shape and severely altered grain boundaries. Some
plagioclase grains enclose opaque minerals. Biotite occurs as
subhedral grains and gloraerocrysts, and forms from a trace to 15 modal
percent of these rocks. Strained quartz forras anhedral interstitial U2 grains and lesser poikilitic patches, and comprises 37 to U8 modal percent of these rocks. Poikilitic quartz grains enclose plagioclase and biotite. Apatite and anhedral opaque minerals are accessory minerals. Strongly altered enclaves less than one-half miHimeter in diameter can be seen in thin section. Large enclaves (greater than seven centimeters in diameter) exhibit an altered rim about 0.35 milliraeters thick that is composed of strongly saussuritized plagioclase and actinolitic amphibole. Chlorite occurs as an alteration product of biotite, and in one saraple (see Table l) has corapletely replaced biotite.
Schlieren
Banding is faint in thin section. Seriate, subhedral and anhedral clinopyroxene forras gloraerocrysts and sparse interstitial grains with some grains more than 1.5 millimeters long. Orthopyroxene is pleochroic frora light pink to light green, and has the same habit as clinopyroxene. Orthopyroxene is size-graded in some rocks, with larger grains located in mafic bands and smaller grains located in felsic bands. Clinopyroxene and orthopyroxene grains form glomerocrysts whose long dimensions are parllel to the direction of banding. Subhedral laths and tablets of plagioclase are norraally zoned, with sorae saraples exhibiting normal discontinuous zoning and rare reverse zoning. Some schlieren samples contain broken or bent plagioclase, and rare resorbed plagioclase. Plagioclase forras as rauch as 90 percent of the felsic bands and typically raore than 55 percent of the mafic bands.
Subhedral and anhedral hornblende is pleochroic from light green or U3
olive green to olive brown and occurs as oikocrysts and rare
interstitial grains that enclose clinopyroxene, orthopyroxene, and
rounded or symplectite opaque minerals. Interstitial to rarely
poikilitic quartz forras less than five percent of these rocks.
Subhedral biotite grains comprise less than two percent of these
rocks, characteristically as glomerocrysts with hornblende and as
interstitial grains enclosing opaque minerals. Apatite and opaque
minerals are accessories. Tremolitic and actinolitic amphibole are
alteration products of hornblende.
Comb Layering
Comb layered rocks are hypidiomorphic-granular. Plagioclase is
subhedral, and exhibits normal and normal discontinuous zoning.
"Combed" plagioclase laths are about 1.6 millimeters long, taper in the same direction and display slightly curved twin planes. Numerous
sraaller plagioclase grains (less than .09 millimeters long) within the bands are oriented perpendicular to combing (i.e., parallel to bands).
Pleochroic brown to light brown subhedral and anhedral poikilitic hornblende comprises more than 30 percent of these rocks. Hornblende
is zoned frora a brown pleochroic core to a nearly clear rim.
Hornblende encloses plagioclase, quartz, orthopyroxene, clinopyroxene, apatite and opaque rainerals.Orthopyroxen e occurs as anhedral grains
0.30 to 0.U8 raillimeters in length and comprises less than two percent of these rocks and always occurs within hornblende. Clinopyroxene grains are typically 0.15 raillimeterslong , and have the same habit as orthopyroxene. Ragged anhedral biotite comprises grains less than .05 uu millimeters long, sorae of which rira anhedral opaque minerals. Biotite
occurs in trace amounts. Anhedral interstitial grains of quartz occur
in trace araounts. Apatite and opaque minerals are accessories. Opaque
minerals form symplectic intergrowths with hornblende and biotite.
Actinoltic and tremolitic amphibole are alteration minerals of
hornblende and biotite (?). Iddingsite is an alteration mineral of
pyroxene, and comprises less than 2 percent of these rocks. CHAPTER IV
EMPLACEMENT HISTORY
Origin
At the present level of exposure the Salmon Mountain stock is a plan section of a vertical or sub-vertical dike. The Salraon Mountain stock was emplaced as a dike that solidified from the walls inward as the inner portion of the body served as a conduit for magraa raoving to shallower levels of the crust. This is deraonstrated by vertical schlieren, vertical stock-wallrock contacts, and intrusive contacts between igneous phases. Areas in the stock of dense, closely spaced schlieren could represent the last unconstricted conduits passing upward through the stock. Vertical and sub-vertical schlieren from larger intrusions that differentiated from a single pulse of raagma have been reported by Wilshire (1969), Moore and Lockwood (1973),
Smith (1975), and are believed to have forraed by raagmatic flow.
However, schlieren in these bodies are limited to raarginal areas where igneous flow took place along the walls. Vertical schlieren occur throughout the Salraon Mountain stock, suggesting that the center as well as the raargins of the stock was upwardly mobile throughout the crystallization of the body.
Rocks of the Salmon Mountain stock characteristically have internal intrusive contacts. This suggests that pulses of magma were sampled frora an underlying, differentiating raagraa charaber during the
U5 U6 life of the stock. Mafic enclaves brought up by fine-grained rocks in the stock suggest that this underlying charaber was basaltic. These possibilities will be discussed in a later section.
The Salraon Mountain stock probably was eraplaced at shallow crustal levels, and raay have extended further toward the surface, possibly feeding a volcanic vent. A narrow contact aureole and intrusion breccias support eraplacraent at shallow crustal levels. The presence of comb layering and orbicular structure in the stock also imply a shallow crustal origin, where rapidly changing physiochemical conditions such as undercooling can occur (Wager and Brown, 1967;
McBirney and Noyes, 1979). Intrusion breccias in the Salmon Mountain stock probably were forraed by late-stage vesiculation of evolved magma, inqplying the occurrence of an eruptive event. Such an eruption cannot be substantiated, though, because the upper part of the stock and any eruptive products have since been removed by erosion.
Mode of Eraplaceraent
The Salraon Mountain stock was emplaced by a combination of stoping and forceful intrusion. Areas of dense wallrock contamination along some margins of the stock as well as the inclusion of large wallrock blocks demonstrates that the stock broke and assimilated wallrock during emplacement. The presence of folds adjacent to the stock with their axes and axial planes sub-parallel to stock-wallrock contacts in^plies that wallrocks were also forcefully pushed aside during emplaceraent of the intrusion. Due to the scattered occurences of both U7 of these features, it is difficult to estimate which process, if either, was predominant.
Percent Crystallinity During Eraplacement
The coarse-grained quartz-gabbro that coraprises the outer portion of the Salmon Mountain stock has been examined to estiraate the percent of ciystals present during the eraplacement of the body. The modal percent of quartz and late magmatic hornblende present in these quartz-gabbros can be used to estimate the minimum amount of melt present in these rocks during emplaceraent. Late-stage hornblende has the general reaction relation with pyroxene and plagioclase (Deer et al., 1966):
Diopside + Estatite + Labradorite + Water
Hornblende (pargasite) + Quartz.
Using the forraula weights and densities of these rainerals, it is expected that about two-thirds of the hornblende forras directly frora pyroxene, the remaining amount from melt. One-hundred per cent of the maximum araount of raodal quartz present in these rocks (15 percent) and one-third of the average amount of hornblende plus tremolite- actinolite is assummed to represent interstitial liquid present at intrusion. Assuming 15 percent overgrowth before final welding, an estiraate of 62 percent crystallinity upon emplacement is obtained.
These rocks are characteristically even-grained and phenocryst-poor.
Broken and bent plagioclase grains are common. The estiraate of 62 U8 percent crystallinity is corapatible with this evidence. This estimate applies directly only to the outer zone of the stock, which solidified
first. Inner zones of the stock where upward flow of fresh magraa was taking place could have had a lower volume of suspended crystals.
Norite, as indicated by its porphyritic texture, was probably emplaced as a crystal-poor liquid.
Origin of Schlieren
Several hypotheses have been advanced to explain the origin of mineral layering in igneous intrusions. Among these are: l) gravity accumulation (Emeleus, 1963; Ferguson and Pulvertaft, I963); 2) deposition from convection currents (Grout, 1926; Wager and Brown,
1967); 3) oscillatory processes of crystal growth and nucleation
(McBirney and Noyes, 1979); and U) flow sorting (Bhattacharji and
Sraith, I96U; BhattacharJi, I966). The vertical to sub-vertical nature of raostschliere n in the Salmon Mountain stock argues against an origin by gravity accumulation or convective currents. Sediraentary
structures and enclaves found within schlieren of the stock also oppose an origin by oscillatory crystal growth and nucleation in a
stagnant raagma. The feasibility of an origin of schlieren in the
Salmon Mountin stock by flow sorting is disscussed below.
Structures like those occurring in schlieren of the Salmon
Mountain stock have been produced in shear flow experiraents
(Bhattacharji and Sraith, I96U; Bhattacharji, 1966). These experiraents show that crystals of the sarae shape raove away frora regions of high shear stress at a rate that is proportional to their size, so that k9 large crystals are separated from smaller ones. Crystal size sorting like that seen in the Salmon Mountain stock could be produced in this raanner. Low angle cross-layering, branching and pinching-out of bands occur in the Salraon Mountain stock, and these also have also been produced in flow experiments by using flow constrictions and varying rates of flow.
It is concluded that the best explation for the formation of schlieren in the Salmon Mountain stock is flow sorting. Flow in the stock took place against a front of highly viscous or solidified magraa that advanced inward frora the walls. This flow caused seperation of larger plagioclase crystals and individual grains and gloraerocrysts of pyroxene frora sraaller grains of the sarae corapositions to forra the banding. Strong parallel alignraent of the long axes of raineral grains is sparse in the schlieren, however, and could be due to mutual interference of crystals during flow of crystal-rich magma.
The origin of thicker, gently sloping schlieren near the margins of the Salmon Mountain stock is problematical, as these features do not fit well with a hypothesis of vertical flow. These layers contain sedimentary structures, so their formation by flow sorting, convective currents and gravitational settling are possibilities. Slumping of vertical or sub-vertical layering frora the walls of the intrusion also should be considered. Gravitational settling and deposition by convective currents are not supported by the location of the layers.
Slumping of originally sub-vertical layering and horizontal flow sorting could occur when the outer part of the stock had solidified to a crystal-rich raush. Vertical layering at this point could slurap 50 without destroying the layering. Sub-horizontal flow sorting also could occur at this point, because the outer margin of the stock would be isolated frora vertical flow occurring further inward. The absence of thick vertical schlieren along the walls of the stock argues against an origin by sluraping frora the walls. It is concluded that sub-horizontal layering in the Salmon Mountain stock formed by late- stage horizontal flow sorting in a crystal-rich raush, or less possibly by slumping of originally vertical schlieren frora the walls.
Eraplacement of the Rock Units
Field and petrographic relations suggest that the Salraon Mountain stock was emplaced as a gabbroic unit. The gabbroic raagraa consisted predomiately of plagioclase and glomerocrysts of orthopyroxene and clinopyroxene upon emplacement. Later intrusions into this gabbroic body sampled a magma chamber that differentiated along a trondhjemitic trend (see Figure 6). Quartz gabbro represents orthocumulates that cooled against the walls of the intrusion, isolated from circulating magma. Gabbro further within the stock solidified in a circulating system, and so does not possess cumulate characteristics. Although medium-grained rocks, fine-grained rocks, norite and trondhjemite bear intrusive relations to the gabbro, the coraplete range of rock types present, norite-gabbro-diorite-tonalite-trondhjeraite, suggests that the Salraon Mountain stock represents a suite of rocks frora an underlying raagraa charaber. Cross-cutting relations show that these later rock units were not emplaced into the stock in a strict order from less evolved to more evolved corapositions. How these later raagmas 51 rose through the gabbro is unclear, but bouancy due to higher teraperature and lower density could have played a role.
Dikes
Porphyritic dikes on Salmon Mountain were probably eraplaced in an intrusive event unrelated to the emplaceraent of the Salraon Mountain stock. This is suggested by mineralogical differences and the orientation of the dikes. The dikes contain large euhedral hornblende phenocrysts while the raajority of the stock contains rauch smaller subhedral and anhedral late-stage hornblende. However, the overall mineralogy of the Salraon Mountain stock and nearby porphyritic dikes is similar, suggesting that they could be co-raagmatic bodies en^jlaced at different times. The orientation of the dikes, NW-SE (see Figure
3), further suggests that they are not related to the intrusion of the stock. CHAPTER V
PETROLOGY
Paragenesis
General conclusions can be made about the raineralogicalevolutio n of the Salmon Mountain rock suite (see Table l). Either olivine norite
(i.e., a composition mineralogically similar to saraple SM-UO, Table l)
or high-Al, low-K basalt can be assummed to be a parental composition
of the Salraon Mountain stock. As this parental phase evolved to forra
the coarse-grained rock types, olivine became unstable leaving
orthopyroxene and clinopyroxene as the primary mafic minerals.
Subhedral and euhedral interstitial opaque rainerals are found only in
norite; their presence in raore evolved rock types is due to incoraplete
reaction of pyroxene and olivine to yield hornblende or biotite with
enclosed anhedral opaque rainerals.Plagioclas e was a stable phase
throughout the evolution of these rocks, with compositions ranging
from sodic bytownite in norite to sodic andesine in trondhjeraite.
î^roxene reacted with melt to form late-stage hornblende in the
coarse-grained gabbro. In the medium- and fine-grained rocks,
hornblende became a primary liqiudus phase at the expense of pyroxene.
The presence of biotite in some of these rocks and its absence in
others could be due to varying P^^^ in the evolving melt. Biotite was
the sole mafic phase in trondhjemite. Quartz has a general, but
inconsistent enrichment trend in the Salmon Mountain stock. Quartz
52 53 content increases from the non-curaulate coarse-grained rocks to the medium-grained rocks, but then drops substantially in the fine-grained rocks. Quartz content increases strongly frora the fine-grained rocks to trondhjeraite, in which it occurs as a priraary liquidus phase.
Chemical Composition
Chemical analyses and CIPW normative mineralogies for two coarse- grained hornblende gabbros frora the Salmon Mountain stock are shown in
Table 2. These data indicate that these rocks are metalurainous. This alurainum enrichraent is illustrated in the analyses and in the amount of norraative feldspar present; norraative feldspar coraprises over 73 per cent of sample OMB-22. The very low amount of norraative diopside present shows that these rocks are nearly corundura-normative.
Mineralogical evidence indicates that the Salmon Mountain stock possesses calc-alkaline or calcic affinities (i.e., Peacock, 1931), and that it differentiated along a trondhjemitic trend. The absence of pigeonite as a primary or exsolved mineral phase in the stock precludes tholeiitic affinities. The absence of potassium feldspar in the stock indicates that potassium contents in the stock, and probably in its parental magraa, were low. This low araount of potassiura could impart calcic affinities to the stock, but further chemical studies are required to investigate this possibility. 5U
Table 2. Chemical analyses and CIPW norms of coarse- grained rocks of the Salmon Mountain stock.
OMB-21 OMB-22
SÍO2 52.87 U9.73 TiOp 0.68 0.66 AlpO^ 18.8U 21.91 totaí Fe 8.99 8.61 MnO 0.20 0.13 MgO U.77 3.01 CaO 8.27 9.97 NaO 2.12 2.50 KgO 0.62 0.50 P2°5 0-11 0.20 Rb 19.2 11.9 Sr U5U 658 Zr 35 25 q 8.13 2.82 co - _ or 3.79 3.06 ab 18.Ul 21.77 an U1.O9 U8.U1 lc - - ne - _ di 0.21 0.98 hy 25.06 19.5U wo - _ ol - - rat I.7U 1.67 il 1.32 1.29 ap 0.27 O.U7
Major eleraent analyses by X-ray fluorescence of fused discs, using the raethod of Norrish and Hutton (1969). Rb, Sr and Zr deterrained by X-ray fluorescence using pressed powder pellets according to the method of Norrish and Chappell (1967). Trace elements in ppra. Samples and analyses courtesy of Calvin G. Barnes. 55 Trondhjemitic Trend
Genesis
Many trondhjemites are thought to be related by crystal fractionation to a basaltic source (Arth, 1979; Barker et al. 1979).
The way in which a potassiura-poor basaltic source follows a trondhjeraitic trend has been the subject of several studies (Arth and
Barker, 1976; Barker and Arth, 1976; Arth, 1979). These studies have found that trondhjemites show a strong depletion of heavy rare-earth elements and small positive Eu anoraalies. Arth and Barker (1976) suggest that these REE patterns can be produced by fractionation of hornblende and sraaller araounts of plagioclase from a gabbroic liquid.
Such removal of hornblende will also enrich a raelt in silica (Arth and
Barker, 1976).
Discussion
Production of trondhjeraite in the Salraon Mountain stock probably occured by crystal fractionation of a potassiura-poor basaltic (i.e., gabbroic) parent to diorite, tonalite, and finally trondhjemite.
Hornblende and plagioclase, in a qualitative sense, would be the important fractionating minerals in this case. Clinopyroxene and orthopyroxene probably also fractionated to produce more evolved conipositions, but in accordance with the studies cited above, would not be critical in production of the final trondhjeraites. Hornblende and plagioclase are the priraary liquidus rainerals in the raedium and fine-grained gabbro, diorite, quartz diorite and tonalite of the 56
Salmon Mountain stock. Hornblende is absent in trondhjemite of the
Salmon Mountain stock, where biotite is the sole raaficphase . However, the absence of hornblende in these trondhjemites does not preclude their origin by means of hornblende fractionation. Experimental data
(Cawthorn and Brown, 1976) suggest that the raineralogyo f calc-
alkaline magmas is sensitive to Na/(Na+K) ratios in the melt. At low
Na/(Na+K) ratios (< 0.6) the stability field of biotite expands at the
expense of hornblende. The Na/(Na+K) ratio of the Salmon Mountain
stock might have decreased during the evolution of the stock as
potassium was concentrated in the final liquids. This would favor
biotite as the late-stage mafic mineral over hornblende. Futhermore,
Dodge et al. (1968) sugested that amphibole does not crystallize frora
magmas containing more than 70 percent SiO„, but is replaced by
biotite. This consideration is applicable to the trondhjemites of
Salraon Mountain, which have high modal quartz contents. Therefore, it
is concluded that hornblende and plagioclase fractionation from a
potassium-poor melt, in a qualitative sense, is a reasonable
explanation for the formation of trondhjemite in the Salmon Mountain
stock.
Comparison to other Klamath Mountain Plutons
A modal quartz-plagioclase-alkali feldspar diagram, such as the
one shown in Figure 7, can be used to corapare the chemical-
mineralogical trends of the Salmon Mountain stock to other Klaraath
Mountain plutons. Figure 7 illustrates the contrast between the Figure 7. Plagioclase-quartz-alkali feldspar diagrara for plutons of the Klaraath Mountains. 1 = Salmon Mountain trend; 2 = general Klamath plutonic trend (Hotz, 1971); 3 = Wooley Creek batholith trend (Barnes, 1983); U = Ironside Mountain batholith trend (Charlton, 1979). Rock fields same as in Figure 5- 58 59 strongly trondhjeraitic trend of the Salraon Mountain stock and most other itrusive bodies of the Klamath Mountains. Further comparisons of these bodies are given below.
The Wooley Creek batholith and the Ironside Mountain batholith are located to the north and south, respectively, of the Salmon Mountain stock (see Figure 2). The Wooley Creek batholith is believed to have forraed under a vertical P^^Q gradient from a basaltic parent, forraing a pyroxene-free granitic upper portion and an underlying zone of pyroxene-bearing gabbro, diorite and tonalite (Barnes, 1983). Modal potassium feldspar occurs in the felsic as well as sorae of the more raafic rocks of the Wooley Creek batholith (Barnes, I982). The Ironside
Mountain batholith is a coraposite body consisting of the Ironside
Mountain batholith and the Ironside Mountain zoned complexes
(Charlton, 1979). The main body of the pluton is zoned from biotite- hypersthene-augite diorite, quartz diorite and monzonite outer zones to an augite-biotite-hornblende quartz-monzonitic core zone (Charlton,
1979). Modal analyses from the Happy Camp Mountain pluton (part of the
Ironside Mountain batholith) lie along a trondhjeraitic trend, while rocks from the reraaining parts of the batholith lie along a monzonitic differentiation trend. Normative mineralogy from these rocks, however, indicate that the Ironside Mountain batholith fractionated only along raonzonitic trends (Charlton, 1979). Charlton (1979) concluded that the
Ironside Mountain batholith crystallized from highly potassic magmas that differentiated along calc-alkaline trends.
Examination of Figure 7 and the information given above strongly suggest that the Salmon Mountain stock is petrogenetically unrelated 60 to the Ironside Mountain and Wooley Creek batholiths. This is because
of the potassic nature of these plutons, in comparison to the Salmon
Mountain stock, which contains no raodal alkali feldspar, even at its
raost evolved stages.
Several trondhjemitic plutons occur in the Trinity Alps area
(Eastern Klamath belt) of the Klamath Mountains (see Figure l). These
intrusions include the Caribou Mountain pluton (Davis, I963), the
Gibson Peak pluton (Lipman, I963), the Canyon Creek pluton (Davis,
1963; Lipman, 1979), and other bodies described by Davis (1963).
Eraplaceraent of these bodies has been dated at 127 to lUO Ma, or Late
Jurassic to earliest Cretaceous (Davis et al. I965; Lanphere et al.,
1968). These plutons are in the Shasta Bally plutonic belt of Irwin
(198U). Most of these intrusions are roughly oval in outline, show
distinct foliation, and forcefully intruded their wallrocks (Davis et
al., 1965). The Horseshoe Lake and Gibson Peak plutons are discordant
with regional structure. Compositional zoning occurs in these bodies:
the Gibson Peak body ranges from hypersthene gabbro to tonalite
(Lipman, I963), the Canyon Creek body from mafic tonalite at its
borders to central trondhjemite (Lipman, 1979), and the Caribou
Mountain body from calcic trondhjemite to trondhjeraite (Davis, I963).
These bodies have sraooth]y gradational internal rock contacts, except
the Gibson Peak pluton, which possesses intrusive contacts (Lipman,
1963; Davis, 1963; Lipraan 1979). The raineralogy of the trondhjeraites
consists of plagioclase (andesine to oligoclase coraposition), quartz,
biotite and sparse (less than four raodal percent) alkali feldspar
(Davis, 1963; Lipraan, I963). Small amounts of hornblende occur in some 61 of these trondhjeraites (Davis, I963; Lipman, I963). It is believed that the tonalitic-trondhjeraitic bodies of the Trinity Alps form a distinctive petrographic suite and are genetically related (Lipman,
1963).
The plutons of the Trinity Alps and the Salmon Mountain stock bear several similarities. All of these plutons show distinct departures from the calc-alkaline trends of other Klamath plutons, in that they differentiated along trondhjeraitic trends. Sirailar rock suites (i.e. gabbro-diorite-tonalite-trondhjeraite) occur in these plutons. The
Salraon Mountain stock is oriented transverse to regional structure, a phenoraenon seen in sorae Trinity Alps plutons. Finally, the plutons of the Trinity Alps were implaced by forceful injection; the Salmon
Mountain stock was emplaced at least in part in this manner.
The principle difference between the Salmon Mountain stock and the
Trinity Alps intrusions is mineralogy. Trpdhjeraite of the Salmon
Mountain stock contains no alkali feldspar; most trondhjemites of the
Trinity Alps intrusions contain a few modal percent of this raineral.
The predorainately gabbroic nature of the Salraon Mountain indicates that it is raore mafic than the Trinity Alps intrusions as well. These factors could indicate that the Salmon Mountain stock originated from a melt that was more mafic and potassium-poor than those of the
Trinity Alps plutons. 62
Implications to Plutonism in the Klamath Mountains
The Salmon Mountain stock lies in a zone between the Ironside
Mountain and Wooley Creek plutonic belts of Irwin (I98U) (Figure l).
By coraparison with the Ironside Mountain and Wooley Creek batholiths,
the largest bodies in these belts, it is doubtful that the Salmon
Mountain stock is petrogenetically related to either of these belts.
Modal Q-A-P diagrams suggest that it is also not transitional between
the belts. Another possibility is that the Salmon Mountain stock is
petrogenetically and temporally related to the 127-lUO Ma Shasta Bally
plutonic belt of Irwin (198U), which contains the trondhjeraitic
intrusions of the Trinity Alps (Figure l). If this were correct, then
the Salraon Mountain stock would be Upper Jurassic to Lower Cretaceous
in age, and postaccretion according to the terrainology of Irwin
(198U).
Tonalitic-trondhjemitic plutonism in the Trinity Alps region has
been shown by radiometric dating and field evidence to have occurred
after the the Nevadan orogeny (Davis et al., 19^5; Lanphere et al.,
1968). If the Salraon Mountain stock is temporally related to these
plutons, then the stock represents an isolated (as our current
knowledge stands) occurrence of post-Nevadan plutonism to the west of
the locus of this activity in the Trinity Alps region. The Salmon
Mountain stock would, in this case, not be related to crustal
shortening and terrane imbrication during the Nevadan orogeny, or to
prior rifting (extensional) events in an island arc setting (i.e.,
Saleeby et al. I982). CHAPTER VI
CONCLUSIONS
The Salraon Mountain stock is a portion of a vertical or sub- vertical dike emplaced transverse to regional structure in the eastern
Hayfork terrane of the Klamath Mountains. The stock was an upwardly-
flowing body that solidified inward frora the walls during flow. The
stock sarapled an underlying potassiura-poor basaltic magma that
differentiated along a trondhjemitic trend. Derivation of trondhjemite
from this magma could have been due to fractionation of hornblende and
plagioclase. The Salmon Mountain stock probably is petrogenetically
unrelated to the Wooley Creek and Ironside Mountain plutonic belts
that lie to the north and south (see above). The Salmon Mountain stock
could be petrogenetically and teraporally related to the tonalitic-
trodhjemitic intrusions of the Trinity Alps area. If this is so, then
the Salmon Mountain stock is much younger than the plutonic belts that
surround it, and is probably not related to rifting events in the
Klamath Mountains. Radiometric age dates on the Salmon Mountain stock,
as well as studies on the intusions imraediately surrounding it, are
required to test these possibilities.
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PETROGRAPHIC DESCRIPTIONS
Coarse-Grained Rocks
SM-I8 is an hypidioraorphic-granular hornblende quartz gabbro.
Anhedral poikilitic hornblende as rauch as six mm in diameter encloses rounded, anhedral plagioclase and cpx grains with average grain sizes of 0.3 mm. Hornblende pleochroism: X, olive green; Y, light yellow; Z, light brown or brown. Plagioclase occurs as seriate anhedral to subhedral saussuritized grains; some grains are bent or broken.
Plagioclase displays normal and normal discontinuous zoning; some grains are resorbed. Sorae plagioclase crystals contain cpx inclusions.
Cpx only occurs as anhedral, zoned chadocrysts in hornblende. Quartz and rare microcline are interstitial, as is very rare microcline.
Accessory minerals are apatite and sparse anhedral opaque minerals enclosed by hornblende. Actinolitic amphibole is an alteration mineral of hornblende. Iddingsite comraonly replaces cpx.
SM-I5 is a gloraerocrystic biotite hornblende quartz gabbro.
Gloraerocrysts are as rauch as four mm long and consist of pyroxene, hornblende and lesser biotite. Hornblende encloses resorbed, rounded cpx, opaque minerals, and sparse plagioclase. Hornblende pleochroism:
X, olive green; Y, light yellow; Z, light brown or brown. Patchy zoning is common in hornblende. Cpx occurs within hornblende as
69 70 rounded anhedral grains and as raottled patches. Cpx also occurs as inclusions within plagioclase, and rarely as rounded interstitial grains about 0.1 ram in diaraeter. Weakly saussuritized subhedral and anhedral plagioclase occurs as tablets and laths as nnich as 2.5 mm long. Plagioclase exhibits normal and normal discontinuous zoning, and rare deflected twinning. Anhedral quartz is interstial and poikilitic around subhedral hornblende and plagioclase. Subhedral and anhedral biotite flakes have an average grain size of 0.35 mm, and some rim
opaque rainerals.Apatit e and zircon are accessory rainerals.
Actinolitic amphibole is an alteration mineral after hornblende and
biotite, and chlorite is an alteration mineral after biotite. Some
pyroxene has altered to brown iddingsite. Trace amounts of calcite
occur as a secondary mineral.
SM-20 is a hornblende gabbro with the same texture and mineralogy
as SM-15.
SM-U7 is an hypidiomorphic-granular quartz-diorite with
glomerocrysts of pyroxene enclosed by late stage hornblende and an
average grain size of 1.3 mm. Subhedral plagioclase forms normally-
zoned laths and equant grains as much as three mm long. Anhedral
hornblende occurs as ragged late-stage oikocrysts that enclose
glomerocrysts of pyroxene. One such group of hornblende grains is six
mm across. Hornblende pleochroism: X, olive green; Y, light yellow; Z,
light brown or brown. Clinopyroxene occurs as blocky, twinned anhedral
grains and glomerociysts as raucha s six mm in diaraeter. Quartz is 71 interstitial and rarely encloses biotite or hornblende grains. Brown biotite forras interstitial subhedral grains, most of which enclose anhedral opaque rainerals. Sorae biotite grains are enclosed in hornblende gloraerociysts. Apatite and anhedral opaque rainerals are accessories. Opaque minerals are always enclosed by biotite or hornblende, sorae as symplectite grains. Green actinolitic amphibole rims most hornblende grains, and chlorite is an alteration mineral of biotite.
Medium-Grained Rocks
SM-12 is an hypidiomorphic-granular hornblende tonalite.
Hornblende forras subhedral interstitial laths, equant grains, and rare glomerocrysts. Hornblende has a seriate distribution with the largest grains as rauch as 1.3 mra in diaraeter. Hornblende pleochroism: X, olive green; Y, light yellow; Z, brown or olive brown. Subhedral and anhedral plagioclase has a hiatal distribution, and comprises about 50 modal per cent of this rock. Plagioclase grains are as much as 1.6 mm long, and display norraal and oscillatory zoning. Brown subhedral biotite grains are subhedral and some are bent. Biotite forms three raodal percent of the rock, and occurs as interstitial laths and as grains in glomerocrysts with hornblende. Quartz poikilitically encloses plagioclase and rarely hornblende. Quartz coraprises 21 percent of the rock, and most grains are slightly strained. Accessory opaque minerals are sparse, anhedral, and enclosed by biotite or hornblende. Apatite is also an accessory mineral. Actinolitic and treraolitic amphibole are alteration minerals, and occur as clear or 72 light green rims around hornblende. Chlorite is an alteration mineral after biotite. Calcite is a rare secondary mineral.
SM-30 is an hypidiomorphic-granular hornblende quartz diorite.
Plagioclase exhibits normal, norraal discontinuous and patchy zoning in subhedral hiatal grains. The cores of most grains are weakly saussuritized. The largest plagioclase grains are about two raralon g and their average grain size is 1.2 mm. Subhedral and anhedral hornblende occurs as interstitial grains and as glomerocrysts as much as 3.U ram long. Hornblende pleochroism: X, olive green; Y, light yellow; Z, brown or olive brown. Hornblende glomerocrysts enclose plagioclase, opaque minerals, and rare opx grains. Subhedral brown biotite grains average 0.5 mm in length and occur as interstitial grains and in glomerocrysts with hornblende. Biotite grains are bent, and enclose opaque minerals. Poikilitic quartz encloses plagioclase, biotite, hornblende and rare opaque minerals. Anhedral, irregularly shaped opaque minerals are an accessory phase as inclusions in hornblende and biotite. Alteration rainerals include light green actinolitic araphibole after hornblende, and chlorite after actinolitic araphibole and biotite.
SM-66 is an hypidioraorphic-granular hornblende biotite tonalite.
Subhedral seriate hornblende ranges in length from 0.2 to 1.0 mra.
Hornblende pleochroism: X, olive green or greenish-brown; Y, light yellow; Z, olive brown. Subhedral seriate plagioclase exhibits normal and norraal discontinuous zoning and ranges in size frora 0.10 to 0.35 73 rara long. Subhedral biotite con )rises dark brown interstitial books as much as 1.2 mra long. Quartz is poikilitic and encloses plagioclase, hornblende and biotite. Clinopyroxene is very rare, and occurs as mottled inclusions within hornblende. Apatite and anhedral opaque minerals enclosed by hornblende are accessories. Actinolitic amphibole and chlorite are alteration products after hornblende.
Fine-Grained Rocks
SM-38 is an hypidiomorphic-granular hornblende gabbro with an average grain size of 0.3 mm. Hornblende forms subhedral interstitial laths that are pleochroic from yellow to brown or light olive brown.
Subhedral plagioclase laths are normally zoned and some are broken.
Some larger plagioclase grains contain inclusions of hornblende.
Anhedral biotite occurs in trace amounts. Clinopyroxene is very rare, and occurs as inclusions less than 0.15 raralon g within hornblende.
Apatite grains as rauch as O.8O raralon g and opaque rainerals are accessories. Opaque rainerals occur as rounded and symplectite grains in hornblende. Chlorite is an alteration mineral of biotite, and actinolitic amphibole is an alteration mineral of hornblende.
SM-36 is a hornblende diorite with the sarae texture and mineralogy as SM-38.
SM-UU is an hypidioraorphic-granular quartz diorite with average grain size of about 0.5 mra. Seriate hornblende occurs as subhedral laths and sparse anhedral oikociysts. Hornblende pleochroism: X, olive 7U green; Y, light yellow; Z, light brown or brown. Many hornblende grains are zoned from an olive brown core to an olive green rim.
Hornblende laths are as much as 2.5 mra long. Seriate plagioclase exhibits normal and normal discontinuous zoning, and ranges in size from 0.05 mm to 1 mra long. Anhedral biotite occurs in trace araounts, and locally rims opaque minerals. Quartz is interstitial, and locally encloses plagioclase. Orthopyroxene and clinopyroxene comprise sraall
(< 0.05 mm) anhedral chadocrysts enclosed by hornblende. Apatite and anhedral opaque minerals enclosed by hornblende or biotite are accessory minerals. Chlorite and actinolitic amphibole occur as alteration minerals of biotite and hornblende.
Norite
SM-UO is a porphyritic biotite norite. Plagioclase occurs as subhedral phenociysts and groundraass grains that range in size from
0.1 mm to 1.2 rara long and exhibit normal and norraal discontinuous zoning. Larger grains enclose anhedra of opx. Numerous plagioclase grains are broken or cracked. Opx forms anhedral groundmass grains about 0.03 mm in diaraeter, and less comraon subhedral phenocrysts about
0.19 mra long. Dark brown to light yellow subhedral and anhedral biotite has a seriate distribution, and locally encloses opaque rainerals. Subhedral cpx forras phenocrysts, glomerocrysts, and rare groundraass grains. Cpx grains have an average diaraeter of 1.2 mm and some phenocrysts are twinned. Olivine occurs as rounded anhedral phenocrysts with an average grain size of 0.35 mm. These grains are
Jacketed by a thin rim (about 0.01 mm thick) of opaque rainerals, which 75 is enclosed by a raosaic (about O.OU ram thick) of opx grains. Rare quartz is interstitial. Apatite and opaque minerals occur as accessoiy phases. Opaque minerals form anhedral grains with an average grain size of 0.02 mra and symplectite structures in some olivine. Rare chlorite and sparse actinolitic amphibole occur as alteration minerals. Calcite is a rare secondary mineral.
SM-63 is an hypidiomorphic-granular norite with domains of trachytoid texture of plagioclase and opx laths. Opx occurs as seriate, pleochroic pink to green subhedral laths, anhedral grains, and sparse, small glomerociysts. Opx laths are as much as 0.8 ram long, and nearly all are broken. Opx includes anhedral cpx, plagioclase, apatite and opaque minerals. Plagioclase forms seriate subhedral crystals that exhibit patchy zoning and weak saussuritization.
Hornblende occurs as green to light green poikilitic interstitial patches. Cpx occurs as rare anhedral grains less than 0.07 mm in diaraeter. Brown to light brown subhedral and anhedral biotite forras grains as rauch as 0.20 mra long. Anhedral opaque minerals are either interstitial or symplectite with opx. Apatite is the other accessory raineral. Treraolitic and actinolitic amphibole, chlorite, and white raicas are alteration rainerals after biotite and hornblende. Calcite is a rare secondary mineral. 76
Trondhjemite
SM-lU is an hypidiomorphic-granular trondhjemite with an average grain size of 0.25 mm. Crystals of anhedral, weakly saussuritized plagioclase are norraally zoned, seriate, and as rauch as 0.9 mm long.
Quartz forras strained interstitial grains and sparse poikilitic grains. Poikilitic quartz encloses sraall biotite and plagioclase grains. Brown biotite forras broken and bent subhedral and anhedral flakes, and gloraerociysts as much as 0.9 nmi in diameter. Accessory apatite, and rare opaque minerals are locally enclosed by biotite.
Radiating cholrite is an alteration product of biotite.
SM-56 is a trondhjemite with the same texture and mineralogy as
SM-lU. The average grain size is O.UO ram and all biotite has been nearly completely altered to chlorite.
Banded Rocks
SM-2U is an hypidioraorphic-granular banded hornblende gabbro.
Banding is defined by alternating mafic-rich and mafic-poor bands that are about 0.5 cm wide. Hornblende is present as olive green and brown
subhedral and anhedral grains and as glomerocrysts. Some grains are
twinned, exhibit patchy zoning, and enclose resorbed cpx, plagioclase,
and opaque minerals. Glomerocrysts are as much as three mm long and
are elongate in the direction of banding. Subhedral plagioclase laths
exhibit normal, patchy, and rare normal discontinuous zoning, and have
saussuritized cores. A few plagioclase grains are broken. Quartz is 77 interstitial. Accessory anhedral and equant subhedral opaque minerals occur within hornblende grains, as skeletal or symplectite structures with chlotite. Apatite as rauch as 0.25 ram in diaraeter is also an accessory. Chlorite and actinolitic amphibole occur as alteration minerals after hornblende and clinopyroxene.
SM-65 is a banded orthopyroxene hornblende gabbro. Opx grains are anhedral, and are pleochroic from light pink to light green. Opx forms interstitial grains and glomerocrysts comprised of interlocking poikilitic opx enclosing subhedral plagioclase and rounded opaque minerals. Opx glomerocrysts are as much as U.5 mm long. The largest individual opx grains (>1.5 mra long) are concentrated in one mafic band. Normally-zoned plagioclase forms subhedral laths, many of which are broken. Plagioclase has a seriate distribution with grain sizes ranging from 0.2 to 2.0 ram. Cpx forms ragged anhedral grains within hornblende, and rare interstitial grains in glomerocrysts of opx.
Hornblende forms subhedral and anhedral poikilitic grains that are pleochroic frora olive brown to olive green. Most hornblende grains enclose interstitial grains and gloraerocrysts of pyroxene and plagioclase. Glomerocrysts comprised solely of interlocking hornblende are rare and are as much as 3.5 ram long. Anhedral quartz and subhedral to anhedral brown biotite occur in trace amounts. Apatite is an accessory mineral, as are opaque rainerals that form rounded or symplectite grains within hornblende, opx, and biotite. Actinolitic amphibole is an alteration mineral of hornblende. Sorae actinolite is 78 enclosed by hornblende, suggesting the actinolite formed by reaction from pyroxene. Calcite is a rare secondary mineral.
Comb Layering
SM-29 is a raafic hornblende diorite displaying corabed layering of plagioclase. This rock was described in Chapter 3. KEY FOR GEOLOGIC MAP (FIGURE 3)
C T: 3 ) 500 1000 feet
1:6000
Contour Interval 80 feet
Contact ^ Strike and dip of igneous schlieren
— — Contact, approximately located CA^AJ Trondhjemite intrusion breccia
^ Strike and dip of tuffaceous argillite 4 Other trondhjemite intrusion breccia
lO Strike and dip of dike — •.— Pack trail
(A/l Average orientation of fold axes Spring ffj too small to plot individually ? U i L/ ^'^^'^^^ ^°^^ i" stock (^^ Tarn lake ^/QQQ^ Elevation contours /\ Salmon Mountain summit, el. 6,956 feet
X Sample locality, SM prefix not shown
-y^ FIGURE 3. GEOLOGIC MAP OF SALMON MOUNTAIN, CALIFORNIA