Abstract

The Rocklin pluton is a Lower Cretaceous quartz diorite that is intrusive into Upper Jurassic plutonic (Penryn pluton) and metamorphic rocks about 30 km northeast of Sacramento, California. The Rocklin pluton varies from hornblende quartz diorite near Folsom to leucocratic, muscovite-bearing granodiorite in the quarry exposures at Rocklin. The mineral zonation is very regular and is interpreted to be the result of fractional crystallization beginning near Folsom and ending at Rocklin.

Compositions of aplite dikes and mineral assemblages in the contact aureole permit an estimate to be made of the load pressure at the rime of intrusion. Minimum pressures estimated from the dike compositions range from 0.5 to 1.5 kb. Metamorphic mineral assemblages indicate an estimated load pressure of 3 to 6 kb, with the large range due to uncertainties in the aluminosilicate triple point. Combining both estimates gives a preferred value of 2 to 3-kb load pressure for the intrusion of the Rocklin pluton.

Intrusion of the Rocklin pluton followed a period of complex deformation in the Sierra Nevada that is referred to as the Nevadan orogeny. New K-Ar analyses of hornblende and biotite confirm the results of Curtis, Evernden, and Lipson and give an intrusive age of 128 m.y. In about 15 m.y., during Late Jurassic time, sedimentary and volcanic rocks were deposited, buried, recrystallized to low-grade slates and schist, isoclinally folded, and intruded by quartz diorite plutons. This complex history has been interpreted as reflecting the collision of an island arc with the North American plate. Analysis of the radiometric dates on the Penryn and Rocklin plutons and the age of the metamorphic rocks places the time of the proposed collision at about 143 m.y. ago.

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