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.
Nd-, Sr-, O-isotopic and chemical evidence for a two-stage contamination history of mantle magma in the Central-Alpine Bergell intrusion
Two different contamination processes have been identified as having been operative in the genesis of a plutonic suite: initial contamination of a mantle source, and subsequent crustal contamination of uprising partial melts from the mantle. These processes are indicated by a detailed analyses of Nd, Sr, and oxygen isotopes together with major-and trace-elements of the 32–30 Ma calc-alkaline Bergell intrusion. This intrusion is located at the suture of the Alpine continental collision zone and contains rock types capable of discriminating between mantle and intracrustal processes. A range from basaltic-andesitic dykes in the surrounding country rocks, cumulitic hornblendites, gabbros, tonalite, granodiorite and lamprophyres, to pegmatites and aplites, is exposed in this single intrusion. The results of REE modelling and isotopic compositions of the basic members suggest that the cumulates were fractionated from a picrobasaltic liquid originating by partial melting of enriched subcontinental mantle (εNd=+4). Increases in 87Sr/86Sr (0.7055) and δ18O(+6.7) in these samples relative to the mantle array and compositions of other Periadriatic intrusions are most likely the result of an initial contamination of the mantle source by dehydration or partial melting of altered subducted oceanic crust. Slight differentiation of such a picrobasaltic liquid produced the basaltic-andesitic dykes. Simultaneous fractional crystallization and contamination of the uprising magma by continental crust produced crustal isotopic signatures which increase with acidity to values of (εNd=-7.6), 87Sr/86Sr=0.716 and δ18O=+10. The crustal imprint and LREE enrichment in the dominating tonalite increase with decreasing crystallization depth which indicates that the tonalites were emplaced in several distinct batches with different degrees of contamination. Shoshonitic lamprophyres, which intruded into the partly solidified granodiorite, were generated in a deep, strongly contaminated mantle source. The posttectonic 26 Ma Novate leucogranite is not cogenetic with the main Bergell body, but rather formed from a predominantly crustal source. If the described features are indeed due to mantle source contamination processes, which are well known for volcanic arcs, it must be concluded that these may also play a significant role in the genesis of calcalkaline plutonic suites.
Archaeological Survey of Dangoyama Kofun by 3DGPR
We presented a three-dimensional ground penetrating radar (3DGPR) system for the archeological investigation in a harsh site, where we have to deal with many trees and/or great topographic variations. This system consists of a commercial GPR system with a laser positioning system. With this system, we can move the GPR antenna freely, on which a positioning detector can record accurate position for each GPR data trace. We applied our 3DGPR system at Dangoyama Kofun, a burial mound, in Fukushima prefecture, Japan. A lot of trees and bushes covered the survey area and cannot be cut. We acquired six datasets in two days in this area. We successfully imaged the boundary between the original surface of the burial mound and the covering soil clearly at about 0.3 m depth around the mound. Furthermore, we found some strong reflections appeared at 0.8 m depth in the top area of the mound, which can be from the artificial parts of the burial mound. The results would greatly aid the following archaeological excavation. In addition, we compared the non-interpolated depth slice with interpolated depth slice at same position and demonstrated that the interpolation process on threedimensional GPR data may introduce some artifacts which might mislead our interpretation.