Tectonic influences on ground water quality: insight from complementary methods

A study using multiple techniques provided insight into tectonic influences on ground water systems; the results can help to understand ground water systems in the tectonically active western United States and other parts of the world. Ground water in the San Bernardino Valley (Arizona, United States and Sonora, Mexico) is the main source of water for domestic use, cattle ranching (the primary industry), and the preservation of threatened and endangered species. To improve the understanding of ground water occurrence, movement, and sustainability, an investigation was conducted using a number of complementary methods, including major ion geochemistry, isotope hydrology, analysis of gases dissolved in ground water, aquifer testing, geophysics, and an examination of surface and subsurface geology. By combining information from multiple lines of investigation, a more complete picture of the basin hydrogeology was assembled than would have been possible using fewer methods. The results show that the hydrogeology of the San Bernardino Valley is markedly different than that of its four neighboring basins in the United States. The differences include water quality, chemical evolution, storage, and residence time. The differences result from the locally unique geology of the San Bernardino Valley, which is due to the presence of a magmatically active accommodation zone (a zone separating two regions of normal faults with opposite dips). The geological differences and the resultant hydrological differences between the San Bernardino Valley and its neighboring basins may serve as a model for the distinctive nature of chemical evolution of ground water in other basins with locally distinct tectonic histories.     

Low-grade metamorphic alteration of feldspar minerals: a CL study

ABSTRACT Two styles of feldspar alteration – carbonatization and albitization – were investigated using a cathodoluminescence (CL) technique. The nature of the alteration depends on the composition of the fluids. The infiltration of CO₂-rich fluids causes decomposition of An-rich zones in plagioclase followed by the formation of secondary calcite, albite and white mica. K-feldspar is more resistant to CO₂-induced alteration. The circulation of aqueous fluids results in decomposition of primary oligoclase into albite and clinozoisite. Secondary K-feldspar exsolved as small independent grains on the rim of the primary oligoclase, if the primary plagioclase was enriched in the orthoclase component. The fluids easily penetrate the crystals using, crystallographic plains, e.g., twinning or cleavage or simply along cracks. These migration pathways enable the fluids to enter the inner parts of the grain, which would otherwise not be affected by grain-surface alteration.     

Recent progress in the study of accessory minerals

Mineralogy and Petrology -     

Petrology of two granite types from the Tribeč Mountains,Western Carpathians: An example of allanite (+ magnetite) versus monazite dichotomy

Two granitoid series have been distinguished in the Tribeč Mountains (Western Carpathians) on the basis of the contrasting petrological behaviour of their accessory minerals. The allanite–magnetite-bearing (AM) tonalite–granodiorite–granite series typically contains dark magmatic enclaves and is produced from oxidizing, more hydrous (about 5.2 wt.% water) melts that were emplaced at depths equivalent to 350 ± 100 MPa. The monazite-bearing (M) tonalite–granodiorite–granite series contains only metamorphic xenoliths and comes from reduced and drier (about 2.3 wt.% water) melts that were probably emplaced at a shallower depth. The inferred magma properties probably reflect source rock effects. The mineralogical discrimination scheme between the AM and M series corresponds broadly to I- and S-type subdivision and may be useful not only for the granitoid bodies in the Western Carpathians but also for the whole Variscan orogenic belt.     

Accessory Fe-Ti oxides in the West-Carpathian I-type granitoids: witnesses of the granite mixing and late oxidation processes

Fe-Ti-oxides may reach hundreds ppm in I-type granitoids and close to microgranular mafic enclaves (MME) up to several thousands ppm. Western Carpathian I-type granitoids have magnetic susceptibility above 3?×?10^?4 SI units, whereas S-type granites are lower. Associated MMEs reach up to 160?×?10^?4 SI. The measurement of magnetic susceptibility in field appears a useful tool for regional mapping of I-type granites and searching enclaves. The increased contents of Fe-oxides around MME within host I-type granitoids are interpreted as result of hybridization with mafic magma. The hybridisation is manifested by occurrence of two Fe-Ti-oxide generations: (1) orthomagmatic titanomagnetite from pre-mixing stage, (2) late-magmatic magnetite of post-mixing stage. The titanomagnetites show composite textures with exsolved ilmenite. The oxybarometry (Sauerzapf et al. 2008; Ghiorso Evans 2009) yields temperatures 700?C750°C at fO₂ about NNO, and 650?C700°C below FMQ, respectively. Post-mixing pure magnetites originated from early titanomagnetite, annite and anorthite associated with titanite and apatite. The late oxidation seems to be responsible for high magnetic susceptibility of metaluminous I-type tonalites. Both post- and pre- mixing Fe-Ti oxides are locally converted to hematite.     

Pegmatites in two suites of variscan orogenic granitic rocks (Western Carpathians,Slovakia)

Summary Two rare-element (Be-Nb-Ta) granitic pegmatite populations have been observed in the Western Carpathian granitoids: (1) pegmatites with Ti- and Mg-poor mineral assemblages, and (2) pegmatites carrying Ti- and Mg-enriched phases (Nb-Ta oxide minerals, garnet, beryl). Mineral chemistry of the pegmatites reflects the primary composition of the parental granitic rocks. The first pegmatite type is derived from monazite-bearing orogenic granites (MOG), and the second from allanite-bearing orogenic granites (AOG). The MOG produced an abundance of pegmatites, whereas in the AOG group the pegmatites are less evolved and relatively scarce. The two kinds of pegmatites support the subdivision of the Western Carpathian granitoids into two principal genetic groups.

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Pegmatite in zwei Suiten variszischer orogener Granite (West-Karpathen, Slowakei)

Zusammenfassung In den Granitoiden der West-Karpathen kommen zwei Populationen von Selten-Element (Be-Nb-Ta) granitischen Pegmatiten vor: (1) Pegmatite mit Ti- und Mg-armen Mineralvergesellschaftungen und (2) Pegmatite mit Ti- und Mg-angereicherten Phasen (Nb-Ta Oxyde, Granat, Beryll). Die Mineralchemie der Pegmatite spiegelt die primäre Zusammensetzung der granitischen Ursprungsgesteine wider. Der erste Pegmatit-Typ stammt von Monazit-führenden orogenen Graniten (MOG) ab, und der zweite von Allanit-führenden orogenen Graniten (AOG). Die MOG sind für eine Vielzahl von Pegmatiten verantwortlich, während die Pegmatite der AOG-Gruppe weniger entwickelt und relativ selten sind. Das Vorkommen dieser zwei Arten von Pegmatiten unterstützt die Unterteilung der Granitoide der West-Karpathen in zwei genetische Hauptgruppen.

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With 6 Figures     

First Permian–Early Triassic zircon ages for tin-bearing granites from the Gemeric unit (Western Carpathians, Slovakia): connection to the post-collisional extension of the Variscan orogen and S-type granite magmatism

This study presents the first preliminary U–Pb zircon data on tin-bearing S-type granites from the Gemeric unit of the Western Carpathians (Slovakia). U–Pb single zircon dating controlled by cathodoluminescence suggests crystallization of the Gemeric granites during Permian to Early Triassic (303–241 Ma) time. Post-crystallization, low-temperature metamorphic overprint is reflected by partial Pb loss in zircons. These Gemeric granites are younger than the highly fractionated, S-type, tin- and rare-element-bearing leucogranites in the European Variscides. They may have resulted from partial melting, triggered by increased heat flow from the mantle below the continental crust, and most probably intruded during the post-collisional extension and initial rifting of the Variscan orogenic belt. During Alpine orogeny, the Gemeric granites were affected by a low-temperature deformation and metamorphism.     

Electron-microprobe dating of monazites from Western Carpathian basement granitoids: plutonic evidence for an important Permian rifting event subsequent to Variscan crustal anatexis

Accessory monazites from 35 granitoid samples from the Western Carpathian basement have been analysed with the electron microprobe in an attempt to broadly constrain their formation ages, on the basis of their Th, U and Pb contents. The sample set includes representative granite types from the Tatric, Veporic and Gemeric tectonic units. In most cases Lower Carboniferous (Variscan) ages have been obtained. However, a much younger mid-Permian age has been recorded for the specialised S-type granites of the Gemeric Unit, and several small A- and S-type granite bodies in the Veporic Unit and the southern Tatric Unit. This distinct Permian plutonic activity in the southern part of the Western Carpathians is an important, although previously little considered geological feature. It appears to be not related to the Variscan orogeny and is interpreted here to reflect the onset of the Alpine orogenic cycle, with magma generation in response to continental rifting. The voluminous Carboniferous granitoid bodies in the Tatric and Veporic units comprise S- and I-type variants which document crustal anatexis accompanying the collapse of a compressional Variscan orogen sector. The Variscan magmas were most likely produced through the remelting of a subducted Precambrian volcanic arc-type crust which included both igneous and sedimentary reworked volcanic-arc material. Although the 2C errors of the applied dating method are quite large and typically ᆞ-20 Ma for single samples, it would appear from the data that the Variscan S-type granitoids (333-367 Ma) are systematically older than the Variscan I-type granitoids (308-345 Ma). This feature is interpreted in terms of a prograde temperature evolution in the deeper parts of the post-collisional Variscan crust. In accordance with recently published zircon ages, this study shows that the Western Carpathian basement must be viewed as a distinct "eastern" tectonomagmatic province in the Variscan collision zone, where the post-collisional crustal melting processes occurred ~20 Ma earlier than in the central sector (South Bohemian Batholith, Hohe Tauern Batholith).