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Metamorphism in the northern Klamath Mountains, Oregon

By
M. Allan Kays
M. Allan Kays
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Published:
January 01, 1995

Geologic mapping in an extensive metamorphic belt in the northern Klamath Mountains reveals three distinct groups of lithotectonic units that are parts of two or more allochthonous metamorphic terranes (1) a western group of recrystallized greenstones and mudstones along Cow Creek; (2) a central group of serpentinized ultramafic and basic amphibolitic rocks along Elk Creek; and (3) a schistose group of largely supracrustal rock units, best exposed along Wildcat Ridge, occurring on both flanks and distal to the main group of ultramafic rocks and amphibolites. The rock units cannot be assigned unequivocal terrane status because regional correlations have not been established. Nevertheless, this research and previous work establishes that the westernmost group has metamorphic and deformational features in contrast with the central and associated distal groups of ultramafic rocks, amphibolites, and schists. These features are consistent with those recognized elsewhere in the western Jurassic belt, and in the western Paleozoic and Triassic belt, respectively. A fault termed the Cedar Springs Mountain thrust separates the western from the largely ultramafic-mafic central and distal rock groups. Small-scale structures related to deformation-metamorphism suggest that the rocks on either side of the thrust evolved independently until they were juxtaposed. Following juxtaposition by thrusting, metamorphism overlapped intrusion of calc-alkaline plutons. Late metamorphism synchronous with plutonism produced a pattern of progressive metamorphic zones subparallel to boundaries of the group of elongate plutons. The mapped pattern of metamorphic zones in metapelites includes (1) chlorite, (2) chlorite + biotite, (3) andalusite + biotite + chlorite, (4) andalusite + staurolite, and (5) staurolite + sillimanite. The large area of staurolite + sillimanite zone rocks coincides generally with the area of most abundant calc-alkaline plutonic rocks. Late regional metamorphism was probably older than a 141-Ma K-Ar cooling age for trondhjemites of the White Rock pluton, the largest of the calc-alkaline rock bodies. The pluton cooling age is consistent with a K-Ar metamorphic cooling age of 150 ± 11 Ma for amphibolites intruded by calc-alkaline rocks in the central group.

A model to help explain the evolution of metamorphic textures, structures, and assemblages in the three groups of rocks involves (1) pre-Late Jurassic formation of an ocean floor followed by its simultaneous deformation and metamorphism during accretion to western North America; (2) formation offshore of a Late Jurassic volcanic island arc and an adjacent deep basin behind the arc; (3) collapse of the arc and basin and accretion to the continental margin by underthrusting accompanied by folding and cleavage formation in accreted and continental margin rocks; (4) syntectonic intrusion of calc-alkaline plutonic rocks with simultaneous porphyroblastic recrystallization of accreted arc-basin and older rocks.

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GSA Special Papers

Jurassic Magmatism and Tectonics of the North American Cordillera

David M. Miller
David M. Miller
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Cathy Busby
Cathy Busby
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Geological Society of America
Volume
299
ISBN print:
9780813722993
Publication date:
January 01, 1995

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