Modification of a magmatic tonalite to produce a megacrystal granodiorite by K-metasomatism, Monterey peninsula and northern Santa Lucia Mountains, California, USA
A megacrystal granodiorite, occurring in a pluton in the Monterey peninsula of California, is gradational south to quartz monzonite and then to tonalite and trondhjemite and southeast to granodiorite and then to tonalite. A primary magmatic origin for the pluton is indicated by normally zoned plagioclase crystals and by dikes that extend into biotite-hornblende quartz diorite southeast of Point Lobos and into biotite schists south of Carmel Valley Village. Plagioclase in the tonalite lacks zonation, but toward the northwest, the plagioclase becomes increasingly zoned in the megacrystal granodiorite. Therefore, the shallowest part of the original pluton must have been near Monterey. The tonalite is undeformed but progressively becomes cataclastically broken from the southeast toward the northwest. In less than one kilometer, broken plagioclase crystals are increasingly penetrated and gradually replaced by microcline, converting the tonalite into granodiorite. K-metasomatism first creates tiny microcline islands and veins in interiors of broken plagioclase crystals. Then, the microcline grows and engulfs plagioclase remnants and other groundmass minerals. Eventually, the microcline becomes megacrysts, some as large as 4 cm long. Wartlike myrmekite commonly borders the microcline. In formerly strongly-deformed zones, the microcline megacrysts locally have parallel alignment. Some megacrysts show concentric zonation and alignments of tiny plagioclase inclusions parallel to possible crystal faces of the microcline. Thus, the four facies in the pluton, tonalite, quartz monzonite, granodiorite, and megacrystal granodiorite, which seem to have formed by magmatic differentiation, are a result of cataclasis of a former trondhjemite/tonalite pluton in which some of the plagioclase has been replaced progressively by microcline.
Plate tectonics, the segmented Laramide Slab, deformation, and large scale K- and Si-metasomatism
Deformation, caused by the down-going segmented Laramide Slab under southern California, created shear zones in the Sierra Nevada batholith, in the Mojave-Salinia arc segment, and in eastern California. In these shear zones, the plagioclase crystals in primary quartz diorite plutons were deformed and microfractured so that introduced K caused some of them to be replaced by Kfeldspar. Broken mafic silicates were also replaced by quartz. As a consequence of the Kand Si-replacements, the quartz diorites in the shear zones were converted into metasomatic granodiorite, quartz monzonite, or granite. Myrmekite is a clue to the metasomatism. In other parts of the world where K is available and where primary mafic igneous rocks are sheared, K-metasomatism that produces Kfeldspar and myrmekite can be expected.
The Oxygen Isotope Composition of Lower Crustal Granulite Xenoliths
Stable isotope information, though potentially helpful in identification of protolith material and investigation of superimposed fluid-rock interactions, are sparsely represented in the granulite literature. Here we report 86 new whole-rock 18O/16O analyses of granulite xenoliths from alkali basalt localities in North America, Europe, Australia and Asia. The data for 71 samples of igneous origin form a well-defined population with an unexpectedly high average value of +7.5 ± 1.4‰. For some granulite xenolith suites, correlations between oxygen and radiogenic isotope systems are strong, whereas for others no covariation is observed. Overall, even radiogenically primitive samples lack δ18O values appropriate to mantle-derived magmas, which are the suspected progenitors of the mafic granulites studied. We therefore suggest that the O-isotope composition of the lower crust has been modified, through the introduction of surficially-dervived 18O, on a scale not previously recognised.