Abstract

The plutonic rocks of the northern Santa Lucia Range arc calc-alkaline and consist of hornblende gabbro, tonalite-granodiorite, and quartz monzonite of probable Cretaceous age. They were emplaced as concordant plutons during regional folding and metamorphism at temperatures above 650° C and at depths of 15 to 20 km. The country rocks consist predominantly of quartzofeldspathic meta-sedimentary rocks (Sur series) of probable Paleozoic age.

The earliest major intrusions were of hornblende gabbro and leucocratic quartz monzonite. These formed homogeneous, mappable plutons and heterogeneous mixtures of gabbro and quartz monzonite with locally abundant hybrid rock. Tonahte-granodionte plutons were emplaced next and locally intruded the heterogeneous gabbro-quartz monzonite terrane. The last major intrusions were of quartz monzonite.

Contacts between quartz monzonite and hornblende gabbro vary Irom sharp to gradational, and hybrid rocks are locally developed. Most contact relations indicate that gabbro was solid when quartz monzonite magma intruded it. Structural evidence of the broad contemporaneity of quartz monzonitic and gabbroic magmas allows the possibility that some mixing of magmas furthered the development of hybrid rocks and gradational contacts. Tonalite-granodiorite plutons emplaced somewhat later may have formed by hybridization of gabbro and quartz monzonite.

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