Regional Metamorphism in the Condrey Mountain Quadrangle^ North-Central Klamath Mountains, California Regional Metamorphism in the Condrey Mountain Quadrangle, North-Central Klamath Mountains, California Bv PRESTON E. HOTZ GEOLOGICAL SURVEY PROFESSIONAL PAPER 1086 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress catalog-card No. 79-600048 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-03160-4 CONTENTS Page Page Abstract _____________________________ 1 Amphibolite facies__ — __ — __ —— _ ——————— — —— 12 Introduction__________________________ 2 Foliated amphibolite ___ — _ — — _ ———— — — ——12 Greenschist facies ______________________ 2 Nonfoliated amphibolite ___________ — — _—— 14 Condrey Mountain Schist ____________________ 2 Chemical composition ___________ —— _——15 Quartz-muscovite schisL_________________ 6 Metasedimentary rocka__ —— — _ —— __ — —— ——— 15 Chemical composition _______________ 6 Age _________________________________ _ 17 Actinolite-chlorite schist ______________ 6 Mafic and ultramafic rocks______—_ — ——— — —— -17 Chemical composition ______________ 7 Serpentinite __________ ;____———— — _—17 Metamorphosed granitic (?) rock___________ 7 Gabbro and pyroxenite_____—___ ———— _————19 Age and correlation ___________________ 8 Age __________________________________ 20 Metavolcanic and metasedimentary rocks of the Plutonic rocks______-l_______-__ —— —————— 20 western Paleozoic and Triassic belt ____________ 9 Pyroxene quartz diorite and granodiorite — — __ — —— -20 Metavolcanic rocka__________________ 9 Granitic rocks _______ — _ — —— — _ —— _ —— -21 Chemical composition _____________10 Age__________________________________-23 Metasedimentary rocks.________________11 Structural and metamorphic relations —_ ——————— — —— 23 Age and correlation _________________ 11 References cited _—— — — __ — — ___ — — __ ————— 24 Stuart Fork Formation___________________ 11 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of Condrey Mountain quadrangle and vicinity_____________ — — ——— __ _______ 3 2. Geologic map of Condrey Mountain quadrangle and parts of Seiad Valley and Hornbrook quadrangles ————— — — ——— 4 3. Graphs showing Na2O and CaO relative to SiO2 for greenschist from Condrey Mountain Schist__ ——— — _-— ————— — 8 4. AFM diagram of metavolcanic rocks and amphibolite from Condrey Mountain and Hornbrook quadrangles and greenschist from Condrey Mountain Schist ______________________—___ —— __ ——— _— 8 5. Graphs showing Na2O and CaO relative to SiO 2 for metavolcanic rocks from Condrey Mountain and Hornbrook quadrangles ______________________________— — ___ —— —— — —— — 11 6. Photomicrographs of amphibolite, foliated and nonfoliated ___________________ — __ ———— —— —— - 13 Diagram of amphiboles in metavolcanic rock and amphibolite from the Condrey Mountain and Hornbrook quadrangles.________________________________—— ——— — — —— 14 Graph showing change in cation content of amphibolite at contact with serpentinite__________ —— ___ —— — __ 19 TABLES Page TABLE 1. Chemical analyses and CIPW norms of quartz-muscovite schist from the Condrey Mountain Schist.__ —— _____ — —— 6 2. Chemical analyses, CIPW norms, and modes of greenschist from the Condrey Mountain Schist——— —— _ —— __ — _— 7 3. Chemical analysis, CIFW norm, and mode of albite-quartz-muscovite-epidote schist———__ — —————— ———— ____— 9 4. Chemical and spectrographic analyses and CIPW norms of metavolcanic rock __ —— ___ —— ———— __ ——— — — __10 5. Chemical and spectrographic analyses, CIPW norms, and modes of amphibolite ________ — — —— _ ——— — — — — __16 6. Chemical and spectrographic analysis and CIPW norm of quartz-biotite phyllite—— — —— _ —— _ — _____ —— — —17 7. Chemical analyses of serpentinite and magnesian schist _________ — _____ _____________ — __ ——— — —18 8. Chemical analyses illustrating metasomatic reaction at serpentinite-amphibolite contact.——— ___ — __ — — _—-_ —18 9. Chemical analyses, CIPW norms, and modes of gabbro, pyroxenite, and pyroxene quartz diorite__ —— — — ____ ———20 10. Chemical and spectrographic analyses, CIPW norms, and modes of rocks from the Vesa Bluffs and Ashland plutons^__ —— 22 III REGIONAL METAMORPHISM IN THE CONDREY MOUNTAIN QUADRANGLE, NORTH-CENTRAL KLAMATH MOUNTAINS, CALIFORNIA By PRESTON E. HOTZ ABSTRACT A subcircular area of about 650 km2 in northern California and ing metavolcanic and metasedimentary rocks. Hornblende and southwestern Oregon is occupied by rocks of the greenschist plagioclase (An30_35) are the characteristic minerals, indicating that metamorphic facies called the Condrey Mountain Schist. This the rocks are of the almandine-amphibolite metamorphic facies. The greenschist terrane is bordered on the east and west by rocks belong­ metasedimentary rocks interbedded with the amphibolites include ing to the amphibolite metamorphic facies that structurally overlie siliceous schist and phyllite, minor quartzite, and subordinate and are thrust over the Condrey Mountain Schist. The amphibolite amounts of marble. Potassium-argon age dates obtained on facies is succeeded upward by metavolcanic and metasedimentary hornblende from foliated amphibolite yield ages of 146±4 and 148± 4 rocks belonging to the greenschist metamorphic facies. m.y, suggesting a Late Jurassic metamorphic episode. The Condrey Mountain Schist is composed predominantly of Mafic and ultramafic rocks are widespread in the amphibolite ter­ quartz-muscovite schist and lesser amounts of actinolite-chlorite rane but are almost entirely absent from the area of greenschist schist formed by the metamorphism of graywacke and spilitic vol­ facies metavolcanic and metasedimentary rocks. The ultramafic canic rocks that may have belonged to the Galice Formation of Late rocks, predominantly serpentinite, occur as a few large bodies and Jurassic age. Potassium-argon age determinations of 141 ±4 m.y. and many small tabular concordant bodies interleaved with the foliated 155±5 m.y. obtained on these metamorphic rocks seem to be incom­ rocks. The ultramafic rocks include harzburgite and dunite and their patible with the Late Jurassic age usually assigned the Galice. serpentinized equivalents. In the Condrey Mountain quadrangle, The rocks that border the amphibolite facies are part of an exten­ probably more than 90 percent of the ultramafic rocks is serpen­ sive terrane of metavolcanic and metasedimentary rocks belonging tinized. Most of the serpentinite is composed of antigorite formed by to the western Paleozoic and Triassic belt. The metavolcanic rocks metamorphism of serpentinized ultramafic rocks; it is not known include some unmetamorphosed spilite but are mostly of the whether the metamorphism was of the same episode as that which greenschist metamorphic facies composed of oligoclase (An15_20) and produced the enclosing amphibolite facies. Some of the serpentinite actinolite with subordinate amounts of chlorite and clinozoisite- has been converted to tremolite-, talc-tremolite-, and talc-carbonate epidote. The interbedded sedimentary rocks are predominantly argil- rocks. Numerous small bodies of quartz-bearing hornblende gabbro lite and slaty argillite, less commonly siliceous argillite and chert, and hornblende pyroxenite are closely associated with some of the and a few lenticular beds of marble. On the south, high-angle faults serpentinite. and a tabular granitic pluton separate the greenschist metavolcanic The age of the mafic and ultramafic rocks is uncertain, but they terrane from the amphibolite facies rocks; on the east, nonfoliated occur as pendants in granitic plutons of Late Jurassic age. The amphibolite is succeeded upward, apparently conformably, by numerous discontinuous bodies of mafic and ultramafic rock scat­ metasedimentary rocks belonging to the greenschist metavolcanic tered throughout the terrane of amphibolite facies rocks suggests a terrane. melange, and the association of serpentinite, gabbro, and pyroxenite In the southern part of Condrey Mountain quadrangle, an outlier with amphibolite and siliceous metasedimentary rocks suggests that of a thrust plate composed of the Stuart Fork Formation overlies the this assemblage may be part of a dismembered ophiolite suite. metavolcanic and metasedimentary rocks. The Stuart Fork in this Two large granite plutons, the Ashland and the Vesa Bluffs, range region is composed of siliceous phyllite and phyllitic quartzite and is in composition from hornblende gabbro and diorite to quartz diorite; believed to be the metamorphosed equivalent of rocks over which it is they contain some granodiorite, and alaskitic rock in minor amounts. thrust. In the Yreka-Fort Jones area, potassium-argon determina­ The Ashland pluton is mainly in Oregon; its southern part is in the tions on mica from the blueschist facies in the Stuart Fork gave ages northeastern Condrey Mountain and northwestern Hornbrook quad­ of approximately 220 m.y. (Late Triassic) for the age of metamor­ rangles of California. The Vesa Bluffs pluton, a south-dipping tabu­ phism. lar body in its western part, occurs between the amphibolite facies on Rocks of the amphibolite facies structurally overlie the Condrey the footwall and a hanging wall of greenschist facies metavolcanic Mountain Schist along a moderate to steeply dipping thrust fault. and metasedimentary rocks in the southern Condrey Mountain and The amphibolite terrane is