Interest rate pluton

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The Zinsnock pluton , also called Plutone della Cima di Vila in Italian , is an Oligocene intrusion in the eastern Alpine basement of South Tyrol . It consists mainly of granodiorite and tonalite . Together with other plutons, it forms part of the periadiatric intrusions that imposed on the border between the southern and central eastern Alps.

geography

The Zinsnock-Pluton, named after the 2528 meter high Zinsnock , in Italian Cima di Vila , is located in the Rieserferner Group about 10 kilometers northeast of Bruneck . The east end of the intrusion begins about 2 kilometers west of Antholz Obertal (Anterselva di Sopra). The summit of the Zinsnock is located about 5 kilometers west of Antholz Mittertal on the southwest ridge that descends from the Schwarzen Wand (3105 m). The Rieserferner Pluton is found only 4 kilometers further north.

The intrusion, which is elongated in an east-west direction, runs immediately north of the Defereggen-Antholz-Vals Lineaments (DAV Lineaments), a significant sinistral lateral displacement associated with the periadriatic seam . The pluton is around 8 kilometers long and is a maximum of one kilometer wide in a north-south direction.

geology

The intrusion had occurred in the Oligocene into the east-alpine Greenschist Facial Old Gneisses south of the Tauern Window, which form a broad, east-west-trending syncline between the Rieserferner intrusion and the DAV lineament . This Upper Eastern Alpine basement is polymetamorphic and consists of high-grade paragneiss , granitic pegmatites ( orthogneiss ), amphibolites , quartzites and marbles . The rather complex rocks were in the course of variscan mountain formation metamorphosed amphibolitfaziell already suffered then the eoalpine metamorphosis 100 million years ago at the turn Aptian / Cenomanium (broad Mylonitisierung under the physical conditions of 0.75 ± 0.15), gigapascals and 450 ± 50 ° C, as well as the late Alpine Tauern crystallization under green slate conditions with 350 ± 50 ° C. The last event for them was the contact metamorphosis induced by the Zinsnock pluton with pressures between 0.25 and 0.35 GPa and temperatures of up to 620 ° C. The contact area of ​​the intrusion (aureole) extends up to 700 meters into the adjacent rock in the north of the pluton. A special feature of the mylonites within the contact area is the static coating of microperthitic alkali feldspar pyroclasts with myrmekite , which indicates metasomatic changes in the adjacent rock.

geochemistry

The rocks of the Zinsnock Pluton are all calcareous and predominantly of the high-K type . Only a few samples are normally calcareous or shoshonite . As far as the main elements are concerned, they are geochemically very closely related to the rocks of the Rieserferner Pluton. However, there are important differences in some trace elements and also in the strontium isotopes. For example, at the ratio 87 Sr / 86 Sr, the Zinsnock pluton with values ​​between 0.7058 and 0.7113 has a significantly larger spread than the Rieserferner pluton ( 87 Sr / 86 Sr between 0.7090 and 0.7110) and overlaps hence with it only in the more highly differentiated members.

The K 2 O / Na 2 O ratio in mafic inclusions (for MgO> 3 percent by weight) fluctuates between 0.5 and 1.0, the inclusions are therefore sodium-emphasized. The mafic inclusions in the Rieserferner Pluton show a much broader spread from 0.4 to 1.8, so they are partly potassium-accentuated. The mafic inclusions of the neighboring Rensen pluton are clearly set off from this and are all below 0.6 and are strongly sodium-emphasized.

Petrology

The Zinsnock pluton is mainly composed of biotite- bearing granodiorite , with tonalite (biotite-amphibole-tonalite) as a subordinate part . It also contains fine-grained mafic inclusions, so-called MME s ( English microgrenular mafic enclaves ) and only a surprisingly few metamorphic xenolites . Rare ultramafic inclusions ( hornblendite ) appear in the tonalites. In the late stages, the intrusion was riddled with passages of granite and aplite . In the pluton there are thus three different types of magma , a mafic precursor magma, an intermediate parent magma and acidic residual melts.

Petrogenesis

As the main and trace element distributions show, these three types of magma are not comagmatic. The mineralogical and geochemical findings can be explained much better by the melting of the lower crust, which generated the intermediate granodioritic magma. The tonalites are likely to have emerged from the spatial mixing (interaction, English mingling ) of an acidic with a mafic mantle magma. The granites and aplites, on the other hand, are probably formed by fractional crystallization or are separated residual melts of a granodioritic magma that crystallized in situ.

When the granodioritic host magma was seated and crystallized, there must also have been a massive interaction between mafic and acidic magmas - recognizable by the selective enrichment of the elements rubidium , potassium and barium in the mafic enclaves. However, the light rare earths ( LREE ) are depleted here. This fact is explained by the partial crystallization of the mafic magma and an interaction of the residual melts with the granodioritic host magma. Despite the extensive exchange processes, no equilibrium of the strontium isotopes between acidic magma and enclaves was achieved. The strontium isotope composition of the late crystallized granitic and aplitic melts was influenced by fluids penetrating from the side rocks.

Age

Borsi and colleagues (1979) were able to determine an age of 29.5 million years (Upper Oligocene, Rupelian ) on biotite and total rock samples with the help of rubidium-strontium dating .

See also

literature

  • G. Bellieni: The Cima di Vila (Zinsnock) massif: geochemical features and comparisons with the Vedrette di Ries (Rieserferner) pluton (Eastern Alps - Italy) . In: New Yearbook for Mineralogy, Treatises . tape 138 , 1980, pp. 244-258 .

Individual evidence

  1. ↑ CL Rosenberg, A. Berger and SM Schmid: Observations from the floor of a granitoid pluton: inferences on the driving force of final emplacement . In: Geology . tape 23 (5) , 1995, pp. 443-44 .
  2. ↑ R. Kleinschrodt: Quartz grain structure analysis in the old crystalline south of the western Tauern window (South Tyrol / Italy) . In: Erlanger Geologische Abhandlungen . tape 114 , 1987, pp. 1-82 .
  3. ↑ NS Mancktelow include: The DAV and the Periadriatic fault system in the eastern Alps south of the Tauern window . In: International Journal of Earth Sciences . tape 90 , 2001, p. 593-622 , doi : 10.1007 / s005310000190 .
  4. ^ S. Borsi, A. Del Moro, FP Sassi and G. Zirpolli: On the age of the periadriatic Rensen massif (Eastern Alps) . In: N. Jb.Geol.Paleont. Mh. 1978, p. 267-272 .
  5. ^ B. Cesare: Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette di Ries, eastern Alps, Italy . In: Contributions to Mineralogy and Petrology . tape 116 , 1994, pp. 239-246 .
  6. ^ B. Cesare, C. Marchesi and JAD Connolly: Growth of myrmekite coronas by contact metamorphism of granitic mylonites in the contact aureole of Cima di Vila, Eastern Alps, Italy . In: Journal of Metamorphic Geology . 20, Number 1, 2002, pp. 203-213 .
  7. ↑ Keegan E. Alagna, Angelo Peccerillo, Silvana Martin and Carmelita Donati: Tertiary to Present Evolution of Orogenic Magmatism in Italy . In: Journal of the Virtual Explorer . 36 Paper 18, 2010, doi : 10.3809 / jvirtex.2010.00233 .
  8. ↑ a b G. Bellieni G. Cavazzini, AM Fioretti, A. and P. Peccerillo Zantedeschi: The Cima di Vila (Zinsnock) Intrusion, Eastern Alps. Evidence for crustal melting, acid-mafic magma mingling and wall-rock fluid effects . In: Mineral. Petrol. tape 56 , 1996, pp. 125-146 .
  9. ^ S. Borsi, A. Del Moro, FP Sassi and G. Zirpoli: On the age of the Vedrette di Ries (Rieserferner) massif and its geodynamic significance . In: Geologische Rundschau . tape 68 , 1979, pp. 41-60 .