Age,origin, and thermal histories of some plutonic rocks from the Salinian block of California |
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Authors: | James M Mattinson |
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Institution: | (1) Department of Geological Sciences, University of California, 93106 Santa Barbara, CA, USA |
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Abstract: | U-Pb isotopic measurements on zircons from some granitic rocks of the Salinian block indicate emplacement and crystallization
of these rocks about 104 m.y. ago (mid-Cretaceous). The relatively radiogenic nature of initial Sr and common Pb in these
rocks, and the presence of an inherited component of zircon in some of them strongly suggest the involvement of continental
crust in the genesis of the magmas. Possibly the magmas were generated in a zone of melting that overlapped the boundary between
the mantle and the continental crust.
U-Pb measurements on sphene, apatite, and feldspar from the plutons, along with previously published K-Ar and fission-track
ages shed light on the post-emplacement thermal histories of the plutons. Most of the samples from the northern part of the
Salinian block (Bodega Head, Point Reyes) show relatively simple thermal histories. Evidently these plutons were emplaced
at moderate levels in the crust, crystallized, and cooled to moderate temperatures over an interval of about 10–15 m.y. Plutons
from the central Salinian block (Santa Lucia Range) show more complex and prolonged thermal histories. These plutons evidently
were emplaced at greater depths in the crust than were the plutons from the northern Salinian block. They remained at elevated
temperatures for ca. 25 m.y., then cooled fairly rapidly, probably as a result of rapid uplift and erosion. One sample from
the northern Salinian block shows an even longer span of time between emplacement and cooling.
The thermal evolution of the Salinian plutonic rocks predates major offset along the San Andreas fault zone and thus reflects
the thermal evolution of the undisrupted source terrane of the Salinian block. Isotopic measurements of the type reported
here might therefore prove valuable in correlations across the San Andreas fault zone. Moreover, detailed study of thermal
evolution within the Salinian block could shed light on major offsets within the block. |
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