Abstracts

Crustal modelling of a type area in Eastern Alps:

a multidisciplinary attempt

Sassi F.P., Burlini L., Cesare B., Galgaro A., Mazzoli C., Meli S., Peruzzo L., Sassi R., Spiess R.

1 INTRODUCTION

Results concerning the Eastern Alps of the Italian National Project “Petrographic and geochemical modelling of the continental crust in some italian type areas” are presented herewith, integrated by new data obtained by the authors and new perspective on future work. The type area is a NS traverse roughly passing through Brunico, and runs from the Penninic-Austroalpine boundary to the southern termination of the Southalpine basement. From North to South, the transect passes close to the villages of Lutago, Molini di Tures, S. Vigilio di Marebbe and Valdaora, along the Aurina, Tures and Badia Valleys. Le location of the transect, which is 30 km long and 10 km wide, has been carefully selected along the TRANSALP 98 transect.

2 THE RESULTS

2.1 GEOLOGICAL DATA

Most of the authors are involved in the field works for making the new geological map of Anterselva-Antholz and Dobbiaco-Toblach 1:50.000 Sheets. We produced new geological maps at the scales of 1:10.000 of all the area in which the transect falls. These new maps are supported by new geological surveys with structural field observations and new sampling of all lithologies (Cesare et al., 1997; Mazzoli et al., 1997; 2000; 2002)

2.2 PETROGRAPHIC DATA

340 samples belonging to 27 different lithologies have been studied from the petrographic point of view. Particular attention has been payed to the tectono-metamorphic evolutions of these rocks. Mineral chemistry was investigated by EMP and SEM analytical work, and results were compared with previously published data (Bellieni et al., 1981; Borsi et al., 1978; Butturini et al., 1995; Cesare, 1994a; 1994b; 1999; Cesare et al., 2002; Mazzoli and Moretti, 1998; Mazzoli et al., 1993; 2001; 2002; Mazzoli and Sassi, 1992; Moretti and Mazzoli, 2000; Neubauer and Sassi, 1993; Sassi and Spiess, 1992; 1993; Sassi et al., 1994; 2002; Sassi and Zirpoli, 1989a; 1989b; Sassi et al., 1995; Spiess et al., 2001).

2.3 GEOCHEMICAL DATA

152 samples of 3-5 kg in weight, depending on their grain size, from 20 most representative lithologies have been selected for new geochemical analyses: ICP AS analyses for major oxides and ICP MS analyses for trace elements and REE. For each rock sample, 56 elements were measured at the ActLabs (Ontario, Canada).

2.4 PETROPHYSICAL DATA

In order to better constrain the interpretation and the nature of the seismic reflectors, experimental measurements at high confining pressure (up to 300 MPa) and room temperature of the compressional wave velocity (Vp) on ten samples representative of the volumetrically most abundant lithologies along the considered profile (Easterns Alps, Italy) have been performed, at the laboratory of experimental rock deformation of ETH Zürich (Mazzoli et al., 2002).

For each sample, the speed of ultrasonic waves was measured in three mutually perpendicular directions, parallel and normal to the rock foliation and lineation (Fig. 1). The main results are: a) good agreement between the calculated vs. measured modal compositions of the considered rocks, indicating that they were presumably equilibrated at the estimated P-T conditions; therefore, the seismic properties are representative of the crustal level indicated by the thermobarometry; b) measured and calculated average Vp are in good mutual agreement, and are typical of mid crustal level (6.0 – 6.5 km/s); only the amphibolites show Vp typical of the lower crust (7.2 km/s); c) the seismic anisotropy of metapelites is very high (12–27%), both with orthorhombic and transverse isotropy symmetry; amphibolites are transversely isotropic with an anisotropy of 8%; orthogneisses and granitoids are isotropic or weakly anisotropic; d) the contacts between amphibolites and all other rock types may generate good reflections, provided that they are not steeply inclined. Although the metamorphic foliation remains steeply inclined, discordant buried sub-horizontal igneous contacts may be detected.

2.6 GEOPHYSICAL DATA

New gravimetric and GPS measurements will be taken along the transect. Furthermore, a lot of still unpublished geophysical data (Galgaro, pers. comm.) are going to be processed.

2.7 PETROVOLUMETRIC ESTIMATES

Petrovolumetric estimates required 3 different ranks of data: a) a 3-D, simplified structural model of the studied crustal portion, b) selection of what rock types are to be considered as most representative in 3-D perspective; c) quantitative estimates of the volume of each representative rock type in the 3-D model. This has been achieved starting from a set of five profiles published by Borsi et al. (1978), and revisiting and integrating them by means of further field work and with detailed new unpublished geological maps of the Cima Dura (Durrek) and of the Monte Mutta (Müten Nock) – Cima Valperna (Perntaler Spitze) – Monte Sommo (Sambock) areas. The results (Sassi et al., 2002) show that in the Austroalpine basement, paragneisses, micaschists and phyllonites are the most common lithologies. Granitoid rocks like the Casies (Gsies) and Anterselva (Antholz) orthogneisses, or the Vedrette di Ries (Rieserferner) Tonalites, are also volumetrically relevant, but their abundance varies significantly from east to west, so that relative petrovolumetric estimates of paragneisses and granitoid rocks laterally change considerably. In addition, other lithologies (marbles, amphibolites, quartzites, pegmatitic orthogneisses, “augen” gneisses and rocks belonging to the Permo-Mesozoic cover) are also present, but their volumes are much lower and their distribution is rather constant from east to west. In the Southalpine basement, the metamorphic sequences are monotonous, and are mainly made up of phyllites with minor intercalations of metaphyolites (“porphyroids”) and green schists. Overall the regional structures and rock bodies display a strong east to west orientation. Therefore, we assume a latitudinal cylindrical symmetry, and believe that rock type volumes are proportional to areas in appropriately selected profiles. Table 1 shows the petrovolumetric estimates for the two considered profiles. Each of the two selected profiles has been extrapolated to a depth of 2 Km, which is the average difference between the watersheds and the main valleys altitudes and, with lower degree of confidence, to a depth of 4 Km, respectively.

An additional consideration is that virtually no difference exists among the petrovolumetric estimates based on the profiles extrapolated at different depths (compare column 1 and 2, 3 and 4 in Tab. 1). This is clearly in relation with the structural features, characterised by roughly subvertical regional foliation and isoclinal fold planes. Only the volume estimate of the Thurntaler complex varies considerably with depth, due to the low angle boundary with the Austroalpine paragneisses.

3 CONCLUSIONS

The average composition of the main lithologies based on the new geochemical data, weighted for the petrovolumetric estimates along the selected profiles, and normalised for the oxides to the sum of the major elements including the LOI, gives an estimate of the average composition of the continental crust in the considered volume of the crystalline basement of the Eastern Alps. Because the rock samples were collected in order to also represent the compositional variability within each considered rock type, standard deviation is always relatively high, due to the natural compositional variability. Data have been compared with the chemical composition of the post-Archean average Australian shales and to the upper continental crust after Taylor and McLennan (1985). The obtained results show that the average composition of the uppermost continental crust of the considered area is more similar to that of the post-Archean Australian shales rather then to the average upper continental crust of Taylor and McLennan (1985) both in the case of average composition based on profile D extrapolated at 2 and 4 km. Therefore we suggest that metamorphic rocks building up the considered portion of the crystalline basement mainly derive from pelitic terrigenous protoliths. On the contrary, the composition calculated on the basis of profile E both at a depth of 2 and 4 km, which is considered as a granitoid-poor compositional end member in our models, significantly differs both from post-Archean shales and from the average value of the composition of the upper crust from Taylor and McLennan (1985).

4 INVITATION

The location of the transect, which is 30 km long and 10 km wide, has been carefully selected to favour a multidisciplinary modelling of the crust, integrating our new petrographic, geochemical, petrovo-lumetric, geological, petrophysical and geophysical data with CROP and TRANSALP data. Therefore, collaboration with other Research Units is welcome.

Tab. 1 – Relative petrovolumetric estimates (in percent) of the lithologies belonging to the crystalline basement south of the Tauern Windows (Eastern Alps) along the two selected profiles D and E, considering extrapolation of geological structures at depth of 2 and of 4 Km, respectively.

	Profile D (2 km)	Profile D (4 km)	Profile E (2 km)	Profile E (4 km)
Vedrette Ries Tonalites	14.78	15.19	5.28	6.50
Amphibolites	0.38	0.67	1.46	0.74
Marbles	0.10	0.11	0.78	0.66
Cima Dura Phyllonites	16.46	18.98	18.34	20.38
"Augen" Gneisses	1.04	1.43	0.42	0.18
Paragneisses-Micaschists	28.24	28.10	43.37	51.63
Orthogneisses	19.73	18.39	1.99	2.28
Devonian Limestones	0.22	0.11	0.83	0.51
Permo-Mesozoic Cover	5.46	2.71	6.97	3.18
Southalpine Phyllites	12.60	13.63	6.53	6.40
Porphyroids	0.52	0.45	1.19	1.12
Greenschists	0.46	0.24	0.52	0.47
Thurntaler Phyllites	0.00	0.00	12.33	5.95
	100.00	100.00	100.00	100.00

REFERENCES

Bellieni G., Peccerillo A. and Poli G., 1981 – The Vedrette di Ries (Rieserferner) Plutonic Complex: petrological and geochemical data bearing on its genesis. Contrib. Mineral. Petrol., 78, 145-156.

Borsi S., Del Moro A., Sassi F.P., Zanferrari A. and Zirpoli G. (1978) – New petrological and radiometric data on the alpine history of the austridic continental margin south of the Tauern Window (Eastern Alps). Mem. Sci. Geol. Padova, 32, 1-20.

Butturini F., Peruzzo L., Sassi R. and Scarperi E., 1995. Growth history of two garnet porphyroblast of the Cima Dura-Durreck Complex and its implication on the polymetamorphic evolution of this complex (Austrides, Eastern Alps). Rend. Fis. Acc. Lincei, 9, 6, 223-238.

Cesare B, Mazzoli C., Meli S., Peruzzo L., Sassi R., Spiess R. and Zane A., 1997 - Carta dei Basamenti Cristallini della Provincia di Bolzano alla scala 1:50.000.

Cesare B., 1994a - Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette di Ries, eastern Alps, Italy. Contributions to Mineralogy and Petrology, 116, 239-246

Cesare B., 1994b - Synmetamorphic veining: origin of andalusite-bearing veins in the Vedrette di Ries contact aureole, Eastern Alps, Italy. Journal of Metamorphic Geology, 12, 643-653.

Cesare B., 1999 - Multi-Stage pseudomorphic replacement of garnet during polymetamorphism: Microstructures and their interpretation. Journal of Metamorphic Geology, 1999, 17, 723-734.

Cesare B. Marchesi C. and Connolly J.A.D., 2002 - Growth of myrmekite coronas by contact metamorphism of granitic mylonites in the aureole of Cima di Vila (eastern alps - Italy). Journal of Metamorphic Geology, 20,

Mazzoli C. and Moretti A., 1998 - Alpine alteration of kyanite aggregates from Cima Valperna area, Austroalpine basement, Eastern Alps. Plinius, 20, 151-152.

Mazzoli C., Meli S., Peruzzo L., Sassi R. and Spiess R., 1997 - Note Illustrative della Carta dei Basamenti Cristallini della Provincia di Bolzano alla scala 1:50.000.

Mazzoli C., Meli S., Peruzzo L., Sassi R., Spiess R., 2000 - Litostratigrafia, magmatismo e metamorfismo nel basamento cristallino delle Alpi Orientali: una rassegna delle attuali conoscenze. Atti e Memorie dell'Accademia Galileiana di Scienze, Lettere ed Arti, CXVII, 25-95.

Mazzoli C., Peruzzo L. and Sassi R., 1993 - An Austroalpine mylonite complex at the southern boundary of the Tauern Window: crystallization-deformation relationships in the Cima Dura-Durrek Complex. IGCP No 276, Field Meeting, Messina, 27 Sept-2 Oct 1993, 30-35.

Mazzoli C., Peruzzo L., Sassi R., Spiess R. and Tinor Centi M., 2002 - Problematiche cartografiche in basamenti polimetamorfici affetti da milonisi: due casi dall'Austroalpino delle Alpi Orientali. Atlante delle Carte Geologiche Prototipali. (2 carte geologiche allegate). In press.

Mazzoli C., Sassi R. and Baronnet A., 2001 - A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path. Eur. J. Mineral., 13, 1127-1138.

Mazzoli C., Sassi R. and Burlini L., 2002 - Experimental study of the seismic properties of the Eastern Alps (Italy) along the Aurina-Tures-Badia Valleys transet. Tectonophysics, 354, 179-194.

Mazzoli C. and Sassi R., 1992 - Further chemical data on the upper Ordovician acidic plutonism in the Austrides of the Eastern Alps. IGCP No.276. Newsletter, 5, 263-277.

Moretti A. and Mazzoli C., 2000 – Record of a buried igneus body underneath the Mt. Mutta area (Eastern Alps). Plinius, Abstracts, 24, 155-156.

Neubauer F. and Sassi F.P., 1993 - The Austro-Alpine Quartzphyllites and Related Palaeozoic Formations. In: von Raumer J.F. and Neubauer F. (Eds.), The pre-Mesozoic Geology in the Alps. Springer-Verlag, 423-439.

Sassi R. and Spiess R., 1992 - Further data on the pre-Alpine metamorphic pressure conditions of the Austridic phyllitic complexes in the Eastern Alps. IGCP No. 276, Newsletter, 5, 297-307. .

Sassi F.P., Burlini L., Cesare C., Mazzoli C., Peruzzo L., Sassi R. and Spiess R., 2002 - Petrographic and geochemical modelling of the continental crust in a type area of the crystalline basement of the Italian Eastern Alps. In: F.P. Sassi (ed) Petrographic and geochemical modelling of the continental crust in some italian type areas. In print.

Sassi F.P., Neubauer F., Mazzoli C., Sassi R., Spiess R. and Zirpoli G., 1994 - A tentative comparison of the Paleozoic evolution of the Austroalpine and Southalpine quartzphyllites in the Eastern Alps. Per. Min. 63, 35-52.

Sassi F.P. and Spiess R., 1993 - The South-alpine metamorphic basement in the Eastern Alps. In: von Raumer, J.F., Neubauer, F. (Eds.), Pre-Mesozoic Geology in the Alps. Springer-Verlag, 599-607.

Sassi F.P. and Zirpoli G., 1989a - The lithostratigraphic sequence in the Southalpine basement of the Eastern Alps. In: Sassi F.P., Zanferrari A. (Eds.), Pre-Variscan and Variscan events in the Alpine-Mediterranean belts: Stratigraphic Correlation Forms. Rend. Soc. Geol. It. 12, 397-402.

Sassi F.P. and Zirpoli G., 1989b - Interpretative lithostratigraphic sequence in the Austridic Pre-Permian basement in the Eastern Alps. In: Sassi F.P. and Zanferrari A. (Eds.), Pre-Variscan and Variscan events in the Alpine-Mediterranean belts: Stratigraphic Correlation Forms. Rend. Soc. Geol. It. 12, 403-409.

Sassi R., Árkai P., Lantai Cs. and Venturini C., 1995 - On the boundary between the epimetamorphic Southalpine basement and the anchimetamorphic Paleozoic sequences of the Carnic Alps: illite crystallinity and vitrinite reflectance data. Schweiz. Mineral. Petrogr. Mitt. 75, 399-412.

Spiess R., Bertolo B., Borghi A. and Tinor Centi M., 2001 - Crustal – mantle lithosphere decoupling as a control of the Variscan metamorphism within the Eastern Alps. Australian Journal of Earth Sciences, 48-3, 479-486.

Taylor A.R. and McLennan S.M. (1985) – The continental crust: its composition an evolution. An examination of the geochemical record preserved in sedimentary rocks. Blackwell Scientific Publication, Oxford, pp. 1-312.

Wepfer, W.W., Christensen, N.I., 1991. A seismic velocity confining pressure relation, with applications. Int. J. Rock Mech. Min. Sci. and Geomech. Abstr. 28 (5), 451-456.