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Book Chapter

A summary of critical relations in the central part of the Sierra Nevada batholith, California, U.S.A.

By
Paul C. Bateman
Paul C. Bateman
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Published:
January 01, 1983

The Sierra Nevada batholith was emplaced in Mesozoic time into strongly deformed, but weakly metamorphosed, strata of Precambrian, Paleozoic, and Mesozoic ages. The batholith is composed of many discrete plutons that are in sharp contact with one another or are separated by thin septa (screens) of older metamorphic or igneous rocks. The plutons can be grouped into formations and the formations into comagmatic suites. Within comagmatic suites, successively younger granitoids are commonly, but not invariably, more felsic, representing progressively lower temperature mineral assemblages. Compositional differences within suites have been attributed to crystal fractionation and differential upward movement of a parent magma produced during a single fusion event. However, recognition that initial 87Sr/86Sr increases in the younger and more felsic units of some suites suggests that the composition of the parent magma changed during emplacement, probably because of continuing incorporation of felsic, low-melting crustal rocks. The relative importance of crystal fractionation and of changing composition of the parent magma in the source region is not settled.

The granitoid suites are of Triassic, Jurassic, and Cretaceous ages. Triassic granitoids appear to be limited to a single extensive suite in the east-central part of the batholith. The Jurassic granitoids form a belt of scattered discrete plutons, which trends about N. 40° W. in the central Sierra Nevada. These granitoids are crossed by a N. 20° W. trending continuous belt of granitoids along the central Sierra Nevada. The optimum isotopic U-Pb age of the Triassic granitoid suite is about 210 m.y.; the Jurassic granitoids range in age from about 186 to 155 m.y. and the Cretaceous granitoids from about 125 to 88 m.y. Cretaceous granitoid suites are progressively younger eastward, but no pattern is apparent for the Jurassic granitoids. Few granitoids appear to have been emplaced in the central part of the Sierra Nevada batholith between 155 and 125 m.y. ago, an interval that includes the Late Jurassic Nevadan orogeny.

Quartz diorite, tonalite, plagiogranite, and gabbro predominate in the west, granodiorite and granite in the axial part of the batholith, and monzodiorite, monzonite, quartz monzonite, and granite in the east. The variations in the compositions of the granitoids across the central Sierra Nevada are independent of their ages. The most conspicuous chemical variation is an eastward increase in potassium. This change is accompanied by eastward increases in uranium, thorium, beryllium, rubidium, the oxidation ratio, total rare earths, and initial 87Sr/86Sr, and by decreases in specific gravity and calcium.

Seismic P-wave velocities increase downward within the crust from 6.0 km/s near the surface to 6.4 km/s at intermediate depths, and to 6.9 km/s just above the seismic Moho. The Moho is depressed beneath the high Sierra Nevada to a depth of about 50 km but rises eastward to about 35 km and westward to about 20 km. The depression in the Moho follows the axis of the Cretaceous granitoids. The measured P-wave velocity in the upper mantle is 7.9 km/s.

Negative Bouguer gravity anomalies coincide with the axis of the Cretaceous granitoids, doubtless because of the increased thickness of the batholith. The residual magnetic intensity reflects the abundance of magnetite in the rock and is highest in the core of the batholith and lowest in the western part. Both heat flow and radioactive heat generation in the exposed rocks increase eastward to the crest of the range. Farther east, heat flow continues to increase at a slower rate, and heat generation decreases. Heat-flow measurements range from a little more than 0.4 H.F.U. to about 1.8 H.F.U. A plot of heat-flow measurements against heat-generation measurements made at the same sites is linear and shows that, at zero heat generation, the heat flow would be 0.4 H.F.U. This amount of heat probably represents the mantle contribution to the heat flow in the Sierra Nevada.

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Contents

GSA Memoirs

Circum-Pacific Plutonic Terranes

J. A. Roddick
J. A. Roddick
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Geological Society of America
Volume
159
ISBN print:
9780813711591
Publication date:
January 01, 1983

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