Tuscan Magma Province

from Wikipedia, the free encyclopedia

The Tuscan Magma Province is a more than 30,000 square kilometers claiming magma province of the late Miocene , Pliocene and Pleistocene . Their magmatic activities lasted for around 8 million years and cover the period 8.3 to 0.2 million years BP . It belongs to the circum-Tyrrhenian magma provinces, which arose as a result of crustal expansion in the ridge of the collapsing Apennine Orogen .

Geography and occurrence

The magma province is named after its main area of ​​distribution, Tuscany . It extends along the Tyrrhenisseite the Italian mainland , including the islands of the Tuscan Archipelago ( Elba , Montecristo and Giglio ) of Orciatico in the north to 40 km north-west of Rome located Tolfa . Its north-west-south-east extension is around 150 kilometers with a maximum width of up to almost 100 kilometers. The island of Capraia and the Pontine Islands are often added to it.

Geodynamic background

The western Mediterranean region is the result of a very complex geodynamic development that started BP around 35 to 30 million years ago and can be seen in the overarching context of Africa's rapprochement with Eurasia . After the general expansion phase of the Oligocene , the Corso-Sardinian microcontinent had separated from mainland Europe by 19 million years BP in the Lower Miocene and carried out a counterclockwise eastward drift during the Burdigali . As a result of the expansion of the crust, the Liguro-Provençal Basin emerged in its back, which is partly underlain by oceanic crust and can be understood as a backarc basin . During the Miocene, the eastern drift resulted in crust narrowing in the upstream Adriatic spur of the Apulian microplate with subduction in a south- westerly direction and simultaneous initiation of the Apennine nappes. At the end of the Middle Miocene ( Serravallian ) around 13 million years BP, the Corso-Sardinian microcontinent had already roughly reached its current north-south trending position.

The calcareous to ultrapotassic magmatism in the Tuscan Magmatic Province ( English Tuscan Magmatic Province or TMP for short ), which began in the late Miocene from 8.3 million years BP ( Tortonium ), migrated eastwards from Elba and Capraia to the Tuscan mainland. This development is the gradual withdrawal of subduction to the east ( English slab roll-back ) explained generellem retreat and delamination of the continental lithosphere of the Adriatic spur. In the northern Tyrrhenis, the retreat had set in motion a crustal expansion extending from Corsica to the east (in contrast to the early Miocene, however, the expansion was now located in the apron of Corsica or in the back of the Apennine arc), which in turn made anatexis and volcanism possible.

Timeframe

In the Tuscan Magma Province, volcanic and plutonic rocks are roughly in balance - with a predominance of plutonites in the Tuscan archipelago and volcanic rocks on the mainland.

Volcanites

If Capraia is counted as part of the Tuscan Magma Province, volcanic activity began at the end of the Miocene in the Messinian and the composite volcano on Capraia was formed 7.2 million years ago . Around 5.8 million years BP volcanism then changed from Capraia to Porto Azzurro on Elba (mafic walk from Monte Castello ). At the beginning of the Pliocene in the Zancleum around 4.8 million years BP, after a long break in production on Capraia, the volcanic activities awoke again and the Zenobito volcano formed , which, however, differs from its predecessor petrologically.

Around 4.5 million years BP, volcanism finally reached the Italian mainland with the rhyolite vents near San Vincenzo , in the Val d'Era with the sub-volcanic minette of Montecatini Val di Cecina , the Orendit of Orciatico around 4.1 million years BP and the corridors from olivine latite near Campiglia by 4.0 million years BP. Almost simultaneously, eruptions occurred at Tolfa, Manziana and Cerveteri , which left cathedral complexes behind. These dome complexes are trachytic to rhyodacitic in composition and are built up from massive lava flows and welded ignimbrites , but at the same time contain latitic to olivin-latitic mafic inclusions, which indicate the presence of a mafic magma component. The cathedral complexes are assigned an age of 4.3 to 1.9 million years BP. However, a new date by Villa and colleagues has now shown an age of 3.5 million years BP ( Piacenzium ) for samples from Tolfa and Manziana. Around 2.5 million years BP followed again rhyolite laves at Roccastrada .

Radicofani volcanic center

The youngest volcanic rocks of the Tuscan Magma Province lie further east along a north-northwest-south-south-east trending expansion structure ( ditch ), which was later to be filled in in the Pleistocene by the leucite- bearing volcanic rocks of the Roman Magma Province . The volcanic complex of Monte Cimino already belongs to the Pleistocene and is dated to the time interval 1.43 to 0.97 million years BP. It is made up of Lamproitic and Shoshonite rocks and overlaps with the volcanic rocks of the Radicofani volcanic center - dated 1.3 million years BP. The latter consist of a chimney and several lava flows of basaltandesitic and shoshonitic composition. This is followed by the quartz latite dome of Faggeta in the interval 1.16 to 1.10 million years BP and the lamproic olivine latite from Torre Alfina around 820,000 years BP .

The late Pleistocene volcanic complex of Monte Amiata on the southern border of Tuscany (approx. 300,000 to 200,000 years BP) already represents the transition from the Tuscan to the neighboring Roman magma province and is the result of a mixture of these two different magma components. The volcano was formed along a crevice and created some lava flows and dome structures, some with collapse phenomena. The extracted volcanic rocks are rich in potassium and have a calcareous to shoshonite character.

Volcanism began on the Pontine Islands off the south coast of Lazio around 4.2, but possibly also around 5.1 million years BP. On Ponza , rhyolite dykes and hyaloclastites are formed between 4.2 and 3.7 million years old BP . A second volcanic episode occurs around 3.0 million years BP in the center and south of the island. The last ash eruptions occur in the south around 1.0 million years BP, possibly a little earlier. Dating of volcanic rocks on Palmarola showed ages between 1.8 and 1.6 and between 1.6 and 1.5 million years BP. Ventotene Island is a remnant of ancient volcanic deposits on an earlier caldera rim . The actual Ventotene stratovolcano is far below sea level. The ages for Ventotene vary between 1.75 and 0.92 million years BP. The volcanic activity on Ventotene still extends almost to the present day, for example 150,000 years BP was determined for a pumice of the uppermost pyroclastic unit. The island of Santo Stefano is a pyroclastic-covered lava dome that sits on a flank of the sub-sea-level stratovolcano of Ventotene. Its age is given as 1.2 to 0.6 million years BP.

Anatectic rocks

Characteristic granite landscape on Montecristo

In addition to the volcanic rocks mentioned, anatectic rocks were also produced in the Tuscan Magma Province in the late Miocene and Pliocene , which are summarized under the term Tuscan Anatexis Province . These include the following intrusion bodies of granitoid composition:

The plutonites bear a characteristic geochemical signature of the S-type , which indicates a large-scale melting of supracrustal rocks. Geochemical investigations and isotope studies also show a clear mantle component that mixed with the anatectic magmas.

Petrology

Chemical composition

The following analyzes are intended to clarify the spread in the chemical compositions:

Main elements

Oxide
wt.%		 Orendit
Orciatico		 Minette
Montecatini		 Shoshonite
Cimino		 High-K calcium alkali
Campiglia		 Lime-alkali
Tolfa
SiO 2 57.79 56.86 56.39 58.39 54.37
TiO 2 1.51 1.37 1.12 0.70 0.88
Al 2 O 3 11.79 12.61 16.01 13.84 18.82
Fe 2 O 3 2.24 3.25 1.07 2.90 6.87
FeO 3.12 2.84 3.99 4.36 1.52
MnO 0.08 0.10 0.09 0.53 0.10
MgO 8.23 7.15 7.98 5.84 2.22
CaO 3.46 3.47 5.55 3.12 5.69
Na 2 O 1.31 1.20 1.27 0.64 1.43
K 2 O 8.06 7.91 5.87 6.64 4.27
P 2 O 5 0.85 0.92 0.26 0.23 0.47
LOI 1.55 2.43 0.88 2.80 3.31
Mg # 0.77 0.72 0.77 0.64 0.38
Al / (Na + K) 1.08 1.20 1.90 1.68 2.70
Al / (Na + K + Ca) 0.69 0.75 0.86 0.99 1.09
A '/ F - 0.16 - 0.10 - 0.07 0.08 0.74

The volcanic rocks of the Campanian Magma Province show a broad spectrum in their SiO 2 content , which can vary between 50 and 78 percent by weight and thus include mafic, intermediate and acidic rocks. The Al 2 O 3 -contents are also quite variable, whereby only the calcareous- alkali stones are saturated with aluminum , all others are hyp- or metaluminous. On the basis of the widely scattered K 2 O values (from 2 to 10.5 percent by weight) or the total alkalis (from 5 to 14 percent by weight), however, several clans can be separated from one another in the continuum:

  • the High Potassian Lamproit clan with associated intermediate and sour minettes
  • the Shoshonite clan
  • the potassium-rich calcium-alkali lip

As in the Corsican Magma Province, the association Lamproit-Shoshonite-calcareous limestone can also be observed in the Tuscan Magma Province, which is accompanied by a gradual decrease in the K 2 O content over time. The Lamproit clan consists of rocks oversaturated with silicon with a high MgO content (7 to 10 percent by weight) and a relatively high magnesium number (0.70 to 0.75), but low CaO (3 to 5 percent by weight) and Na 2 O (around 1, 5 percent by weight), whereby real lamproites are relatively rare. The Shoshonite clan is represented by trachy basalts and trachytes , the calcareous-alkali cliff by basaltic andesites and rhyodacites rich in potassium . The magmas of the calcareous-alkali cliff are typical of the adjoining Roman magma province; consequently, the Shoshonite clan can be interpreted as a possible mixed series, illustrated by the example of the Shoshonite by Radicofani.

The hybrid volcano Monte Amiata, seen from Montegiovi

Trace elements

Trace elements
ppm		 Orendit
Orciatico		 Minette
Montecatini		 Shoshonite
Cimino		 High-K calcium alkali
Campiglia		 Lime-alkali
Tolfa
Cr 500 380 401 418
Ni 280 140 175 145
Zn 80 90
Rb 612 768 363 267 205
Sr 577 408 427 453 527
Zr 749 491 444 166 296
Ba 1400 1370 682 1210 1107
Ce 352 206 197 61 114
Nd 193 133 99.8 27 47
Sm 26.9 23.5 15.3 5.9 8.2
Hf 21.4 13.4 13.5 3.9
Th 119 112 61.8 13 26th

In the case of trace elements, a clear enrichment of the incompatible elements can generally be recorded for the Tuscan Magma Province . In addition, there is an accumulation of incompatible elements among the individual magma clans, starting from the calcareous-alkali cliff via the Shoshonite clan to the Lamproit clan with a very high degree of enrichment up to a factor of 4 or 5. Except for the relatively constant elements Ba, Cr and Sr. Compared to mantle rocks normalized mafites of the Tuscan Magma Province show negative anomalies for HFSE , Ba and Sr. Incidentally, the patterns found are very similar to the conditions found in the Oligocene Mafites of the Western Alps , which also reveal the association between the Lamproits, Shoshonites, and calcareous alkalis. Upper crust rocks such as slate and gneiss also bear this pattern.

Isotope ratios

The following initial ratios were determined for the radioisotopes of Sr, Nd and Pb:

Isotopes Orendit
Orciatico		 Minette
Montecatini		 Shoshonite
Cimino		 High-K
calcium alkali CampIglia		 Lime-alkali
Tolfa
87 Sr / 86 Sr 0.71579 0.71672 0.71561 0.709579 0.711686
143 Nd / 144 Nd 0.512094 0.512086 0.512054 0.512202
206 Pb / 204 Pb 18.697 18.624 18.725 18.788
207 Pb / 204 Pb 15,698 15.638 15.663 15.721
208 Pb / 204 Pb 39,062 38.947 39.017 39.021

For the isotope ratios, 87 Sr / 86 Sr and 143 Nd / 144 Nd are much more meaningful than the lead isotope ratios. In the diagram 143 Nd / 144 Nd versus 87 Sr / 86 Sr, the rocks of the Tuscan Magma Province and those of the Roman Magma Province take a clear middle position between the upper crustal component and the enriched mantle component EM 2 . Compared to the Roman Magma Province, however, the rocks of the Tuscan Magma Province are much closer to the upper crustal component and were therefore more heavily contaminated by crustal rocks such as metapelite. This fact is also very nicely expressed in diagram 143 Nd / 144 Nd compared to 206 Pb / 204 Pb. In an internal comparison, the rhyolites from San Vincenzo followed by the peraluminous rhyolites from Roccastrada show the strongest crust contamination, whereas the volcanic rocks from Capraia come closest to the enriched mantle component EM 2. In the examples given, the degree of contamination increases from the lime-alkali group via the Shoshonite group to the Lamproit group.

Magmosse

The broad petrological and geochemical variety of magmas in the Tuscan magma province indicates complicated petrogenetic processes, the ultimate causes of which are still not fully understood. In particular, the great variability among the mafic magmas requires a very inconsistent mantle source region for their formation. The low contents of CaO, Na 2 O and Al 2 O 3 in combination with high concentrations of MgO, Ni and Cr in the Lamproit family result in an upper coat that has the above-mentioned depletion in its main components such as clinopyroxene . On the other hand, the high potassium content in the Lamproit clan implies the presence of phlogopite or other potassium-rich mineral phases in the source region. For this reason, Conticelli and Peccerillo (1992) proposed a phlogopite resin burgite as the parent rock for the lamproit magma . The phlogopite itself stems from metasomatic processes that simultaneously lead to the accumulation of incompatible elements and of radiogenic strontium in the mantle source region.

The most likely candidate for contamination of the Upper Mantle in Tuscany are rocks of the upper crustal area with a pelitic composition.

Compared to the Lamproit family, the Shoshonite and the lime-alkali groups have much higher contents of CaO, Na 2 O and Al 2 O 3 , but lower potassium contents and also significantly lower contents of incompatible elements. Your clad source containing clinopyroxene must therefore have undergone significantly less metasomatic changes; qualitatively, however, the absolutely comparable patterns for the incompatible elements speak for the same contamination process.

This geochemically verifiable contamination by pelitic crustal rocks obviously points to subduction as the cause, the exact timing of which remains a matter of dispute. Mafish rocks (e.g. minettes) with a comparable geochemical signature also occur in the western Alps, but where the magmatic events date back to the Oligocene . The contamination had occurred in the Western Alps during the subduction of the European plate under the Western Alpine margin. Since mafic rocks (Lamproit clan) in the Tuscan Magma Province are very similar to their western alpine representatives, a comparable contamination was assumed for them during the course of the Alpine subduction. This assumption is based on the hypothesis that Tuscany actually represents a section of the Alpine belt that was only shifted further east due to the opening of the Tyrrhenian Basin.

Individual evidence

  1. ↑ G. Serri, F. Innocenti, P. Manetti: Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy . In: Tectonophysics . tape 223 , 1993, pp. 117-147 .
  2. ↑ C. Brunet, inter alia: Migration of compression and extension in the Tyrrhenian Sea, insights from 40 Ar / 39 Ar ages on micas along a transect from Corsica to Tuscany . In: Tectonophysics . tape 321 , 2000, pp. 127-155 .
  3. ↑ M. Gasparon, G. Rosembaum, J. Wijbrans, P. Manetti: The transition from subduction arc to slab tearing: Evidence from Capraia Iceland, northern Tyrrhenian Sea . In: Journal of Geodynamics . tape 47 , 2009, p. 30–38 , doi : 10.1016 / j.jog.2008.06.004 .
  4. ^ S. Conticelli, among others: Petrology, mineralogy and geochemistry of a mafic dyke from Monte Castello, Elba Island, Italy . In: Ofioliti . tape 26 , 2001, p. 249-262 .
  5. ↑ a b S. Conticelli, among others: Trace elements and Sr – Nd – Pb isotopes of K-rich, shoshonitic, and calc-alkaline magmatism of the Western Mediterranean Region: genesis of ultrapotassic to calc-alkaline magmatic associations in a post-collisional geodynamic setting . In: Lithos . tape 107 , 2009, p. 68-92 .
  6. ^ S. Conticelli: Genesi del magmatismo alcalino-potassico dell'Italia centrale: evidenze petrologiche, geochimiche e petrologico sperimentali (doctoral thesis) . Università degli Studi di Firenze, Italia 1989, p. 404 .
  7. ↑ L. Pinarelli: Geochemical and isotopic (Sr, Pb) evidence of crust-mantle interaction in silicon melts. The Tolfa-Cerveteri-Manziana volcanic complex (Central Italy): a case history . In: Chemical Geology . tape 92 , 1991, pp. 177-195 .
  8. ^ IM Villa, O. Giuliani, G. De Grandis, R. Cioni: Datazioni K / Ar dei vulcani di Tolfa e Manziana . In: Bollettino Gruppo Nazionale di Vulcanologia . tape 5 , 1989, pp. 1025-1026 .
  9. ↑ M. D'Orazio, MA Laurenzi, IM Villa: 40Ar / 39Ar dating of a shoshonitic lava flow of the Radicofani volcanic center (Southern Tuscany) . In: Acta Vulcanologica . tape 1 , 1991, p. 63-67 .
  10. ^ S. Conticelli: Effects of Crustal Contamination on Ultrapotassic Magmas with Lamproitic Affinity: Mineralogical, Geochemical and Isotope data from the Torre Alfina Lavas and Xenoliths, Central Italy . In: Chem. Geol. Volume 149 , 1998, pp. 51-81 .
  11. ↑ A. Cadoux, DL Pinti, C. Aznar, S. Chiesa, P.-Y. Gillot: New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy) . In: Lithos . tape 81 , 2005, pp. 121-151 .
  12. ↑ M. Barboni, B. Schoene: Short eruption window revealed by absolute crystal growth rates in a granitic magma . In: Nature Geoscience . tape 7 , 2014, p. 524-528 , doi : 10.1038 / NGEO2185 .
  13. ↑ Gagnevin, D. et al .: In-situ zircon U-Pb, oxygen and hafnium isotopic evidence for magma mixing and mantle metasomatism in the Tuscan Magmatic Province, Italy . In: EPSL . tape 305 , 2011, pp. 45-56 .
  14. ↑ Dini, A. et al .: Hidden granitoids from boreholes and seamounts . In: G. Poli, et al., Miocene to Recent Plutonism and Volcanism in the Tuscan Magmatic Province [Central Italy] (Ed.): Per. Mineral. tape 72 , Special Issue No. 2 , 2003, p. 133-138 .
  15. ↑ Giraud, A. et al: Behavior of trace elements during magmatic processes in the crust: application to acidic volcanic rocks of Tuscany (Italy) . In: Chemical Geology . tape 57 , 1986, pp. 269-288 .
  16. ↑ Gagnevin, D., Daly, JS and Poli, G .: Petrographic, geochemical and isotopic constraints on magma dynamics and mixing in the Miocene Monte Capanne monzogranite (Elba Island), Italy . In: Lithos . tape 78 , 2004, p. 157-195 .
  17. ^ S. Conticelli, inter alia: Leucite-bearing (kamafugitic / leucititic) and -free (lamproitic) ultrapotassic rocks and associated shoshonites from Italy: constraints on petrogenesis and geodynamics . In: Journal of the Virtual Explorer, Electronic Edition . vol. 36 paper 20, 2010.
  18. ↑ Peccerillo, A., Alagna, KE and Donati, C .: The Radicofani volcano: a window on a complexly zoned upper mantle beneath southern Tuscany, central Italy . In: Acta Vulcanol. 2008.
  19. ↑ a b A. Peccerillo, G. Martinotti: The Western Mediterranean lamproitic magmatism: origin and Geodynamic significance . In: Terra Nova . tape 18 , 2006, p. 109-117 .
  20. ↑ S. Conticelli, A. Peccerillo: Petrology and geochemistry of potassic and ultrapotassic volcanism in Central Italy: petro genesis and inferences on the evolution of the mantle sources . In: Lithos . tape 28 , 1992, pp. 221-240 .
  21. ↑ A. Peccerillo, G. Poli, G. Serri: Petrogenesis of orenditic and kamafugitic rocks from Central Italy . In: Canad. Mineral. tape 26 , 1988, pp. 45-65 .
  22. ↑ C. Doglioni, F. Mongelli, GP Pialli: Boudinage of the Alpine belt in the Apenninic back-arc . In: Mem. Soc. Geol. It. Band 52 , 1998, pp. 457-468 .