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.
Myrmekite as a clue to metasomatism on a plutonic scale: origin of some peraluminous granites
In this presentation questions are raised about a melt-restite unmixing model proposed by Barton and Sidle (1994) for the myrmekite-bearing Waldoboro granite complex along the southeastern coast of Maine (USA). This complex consists mostly of granite/granodiorite gneiss and granite, but smaller masses of quartz diorite, aplite, pegmatite, leucogranite, tourmaline granite, and granite porphyry occur. A mylonite zone extends through the granite porphyry on the east side of the pluton and through wall rock consisting of mafic igneous rocks of the Benner Hill sequence. On the west side, the granite complex is bordered by paragneisses of the Bucksport Formation. The granite pluton is poorly exposed in an area covering >340 km 2 , but relationships between rock types are readily apparent in wave-cut shorelines south of Friendship and along other shorelines on islands and the mainland.
Geological, mineralogical and chemical characterization of Devonian kaolinite-bearing sediments for further applications in the ceramic (tiles) industry in La Paz, Bolivia
Clay minerals are widely distributed in Bolivia; among them, illitic clays are most common in the Altiplano where they are mined for use in the ceramic industry that has been growing in the last few years. In addition to illitic sediments, kaolinitic sediments have been recently discovered in sedimentary units in the Bolivian Altiplano. Residual ball clay occurrences in Devonian sedimentary units were studied as part of this work. Geological mapping and geophysical studies (ERT and GPR) were done for better understanding the origin of the deposits and were part of a preliminary study of the mineral potential to define the relationship with the host rock. Chemical and mineralogical techniques such as X-Ray Diffraction, Scanning Electron Microscopy and Inductively Coupled Plasma analyses were performed in samples from the studied area to verify the presence of kaolinite. Atterberg limits and behaviour of the raw material in ceramic specimens supported by chemical analyses show that this material is suitable for manufacturing tile ceramics. This study provides fundamental knowledge for deposit exploitation and future generation of an alternative source of employment for the inhabitants of Micaya.