Textural evidence of magma decompression, devolatilization and disequilibrium quenching: an example from the Western Krušné hory/Erzgebirge granite pluton

We report new occurrences of “two-phase” granitic textures from the Western Krušné hory/Erzgebirge pluton (central Europe) and use crystal-size distribution data and thermodynamic modeling to interpret their crystallization conditions. The two-phase texture consists of (1) early phenocrysts of quartz, plagioclase, K-feldspar and biotite, (2) medium-grained matrix of the same phases and (3) interstitial channels and patches of a late-stage, very fine-grained matrix. The porphyritic two-mica microgranites, which host two-phase textures, occur as minor intrusions in early low-F biotite granites or as marginal parts of evolved high-F Li-mica granites. Measurements of the crystal-size distribution of quartz revealed three grain populations: (1) early phenocrysts (0.5–3.0 mm) showing partial resorption by residual melt, (2) a medium-grained population of the equigranular rock matrix (0.05–0.50 mm) that experienced minor coarsening by subsolidus annealing and (3) a fine-grained population (<0.03 mm) in the interstitial channels and patches formed during rapid devolatilization; this quartz group shows no or poor grain coarsening. All samples exhibit similar fraction of the fine-grained population (44–52%) but proportions of phenocrysts to medium-grained matrix vary significantly. Thermodynamic modeling of liquidus equilibria and experimental data in the hydrous haplogranite system require: (1) ascent of a granitic suspension (15–25% phenocrysts) under H₂O-undersaturated conditions at 25–45 bar/°C and a cooling rate of 40 J/(g kbar) in order to produce partial resorption of quartz phenocrysts and continued growth of feldspar phenocrysts, followed by (2) emplacement as discrete intrusions or bodies along pluton roof accompanied by sudden devolatilization. At the onset of matrix nucleation, disequilibrium undercooling of 70–85°C was inferred from the presence of micrographic intergrowths of quartz and K-feldspar. The two-phase granites in the Western Krušné hory/Erzgebirge pluton and in the Southeast Asian batholith form compositionally narrow groups with high-silica and moderate volatile enrichments but they differ in peraluminosity and phosphorus concentrations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Variscan Sn–W–Mo metallogeny in the gravity picture of the Krušné hory/Erzgebirge granite batholith (Central Europe)

Late Variscan wolframite (± molybdenite) and cassiterite–wolframite greisen, skarn and vein deposits occur in a close spatial association with the granites of the Krušné hory/Erzgebirge batholith (KHEB) in Central Europe. We examined the distribution of the deposits in relation to the gravity field affected by Late Variscan granites using the data from previous gravity and metallogenic studies. Late Variscan granites are differentiated into earlier biotite monzogranites (low-F granites) and later biotite or lithium mica syenogranites (high-F granites) in accordance with the previous classifications. All the outcrops of granites in the KHEB region and their hidden continuation are confined to the Bouguer anomaly contour of − 20 mGal. The Sn–W–Mo (rare metal) deposits and occurrences are within the gravity contour of − 30 mGal with the exception of the Grossschirma stratiform tin deposit in the Freiberg polymetallic ore district. We constructed a geological model based on the gravity data along two profiles across the KHEB showing the position of some rare metal deposits and of outcropping and hidden granite bodies. The models show that the overlapping of earlier and later granites is in the areas of the most intense regional gravity minima. These coincide with the Eastern Volcano-Plutonic Complex (Altenberg minimum), which encloses large volumes of felsic extrusives, microgranite dikes and granites, and the Western Plutonic Complex (Eibenstock minimum), with small volumes of felsic dikes and predominance of earlier and later granites, with no extrusives preserved. There is no distinct relationship between the masses of Late Variscan granites and the distribution and the sizes of associated W ± Mo and Sn–W deposits. We prefer the idea that rare metal mineralization was formed by hydrothermal fluids derived from outside of presently outcropping granites. It originated in two cycles: one connected with the formation of earlier granites producing W ± Mo associations and the other one associated with later granites connected with Sn–W mineralization. Mineralizing fluids were probably generated by mantle–crustal interaction in the crust near the mantle–crust boundary as also indicated by lamprophyric intrusions coeval with the Late Variscan granitic magmatism.     

Earth's surface movements in the hazardous area of Jezeří Castle,Krušné hory Mountains

Abstact The results are summarized of the correlation between the detailed geomorphological analysis and the geodynamic interpretation of the twelve high-precision levelling measurements of the Earth's recent surface movements (1983–1989) in the hazardous area of Jezeí Castle in the Kruné hory Mts. Indicated are the types and régime of these movements (Tab 1, Fig 3) which are climatic, tectonic and anthropogenous in origin. Both slightly and strongly deformed zones in the near-surface part of crystalline rocks massif were formed (Fig 1). The landform patterns (Fig 2), as well as the present-day activity of geological processes are described.     

Composition of coexisting zircon and xenotime in rare-metal granites from the Krušné Hory/Erzgebirge Mts. (Saxothuringian Zone,Bohemian Massif)

Zircon and xenotime, from two mineralogically and chemically contrasting granite suites occurring in the Kru?né Hory/Erzgebirge Mts., display extended compositional variability with respect to abundances of Zr, Hf, REE, Y, P, Th, Ca, Al, Fe and As. According to their geochemical signatures, P-rich (S-type) and P-poor (A-type) granites could be distinguished here. Both granite suites display high Ga/Al ratios (>2.6) and according to FeO_tot./(FeO_tot. + MgO) ratio can be classified as ferrous granites. Consequently, the both ratios cannot be used for discrimination S- and A-type granites. Both minerals are characterized by a variety of complex zircon-xenotime textures. They are usually strong hydrated and enriched in F. Zircon from P-rich granites displays a significant enrichment in P (up 0.24 apfu P), whereas zircon from P-poor granites has lower P and higher Y (up to 0.15 apfu Y). The xenotime-type substitution is the most important mechanism of isomorphic substitution in zircon in both granite suites. Zircon from both granite suites is typically enriched in Hf, especially unaltered zircon from P-rich granites (up to 8.2 wt. % HfO₂). However in altered zircons the Hf/Zr ratio is higher in the P-poor granites. The Hf-rich zircon from unaltered P-rich granite crystallised from low temperature granite melt, whereas altered zircons crystallised during post-magmatic hydrothermal alteration (greisenization). Xenotime from P-poor granites displays a considerable enrichment in HREE (up to 40 mol. % HREEPO₄) compared to xenotime from P-rich granites (up to 20 mol. % HREEPO₄). Xenotime compositions from P-rich granites are influenced by brabantite-type substitution, whereas for xenotime from P-poor granites the huttonite-type substitution is dominant. Unusual enrichments in HREE is significant for xenotime from P-poor granites, especially in Yb (up to 0.17 apfu Yb) and Dy (up to 0.11 apfu).