Sisco-Lamproit
The Sisco-Lamproit is a corridor of lamproitic composition that intruded into the metamorphic Schistes Lustrés in northern Corsica in the Middle Miocene . It belongs to the Corsican Magma Province and opens up the ultrapotassian magmatism in the western Mediterranean .
Initial description
The Sisco-Lamproit was scientifically described for the first time in 1967 by D. Velde.
Occurrence
The Sisco-Lamproit is named after the Corsican community Sisco , in whose area it is located. The best outcrop is located 15 kilometers north of Bastia along the mountain side of the D 80 (former N 198) between kilometers 160 and 160.1 in the immediate vicinity of the Tour de Sacro . The Lamproite here is more or less parallel to the flat-lying cleavage in Glaukophan -führende slate and prasinites the Castagniccia series penetrated the Schistes Luster. The warehouse corridor continues inland for around 1500 meters in a north-westerly direction.
mineralogy
Act as phenocrystals:
Apatite , spinel (Cr), ilmenite , pseudobrookite and priderite are subordinate to this . Roedderite also occurs very rarely . The olivine is mostly weathered to montmorillonite . The sanidin is very low in sodium . Richterite is very rich in potassium .
Chemical composition
Main and trace elements
The chemical composition of the main and selected trace elements of Sisco lamproite is as follows:
| Oxide wt.% |
Sisco-Lamproit 07 |
Sisco-Lamproit 04 |
CIPW standard | Sisco-Lamproit 07 |
Sisco-Lamproit 04 |
Trace elements ppm |
Sisco-Lamproit 07 |
Sisco-Lamproit 04 |
|---|---|---|---|---|---|---|---|---|
| SiO 2 | 58.30 | 58.50 | Q | 4.51 | Cr | 411 | 340 | |
| TiO 2 | 2.31 | 2.27 | Or | 59.11 | Ni | 272 | 230 | |
| Al 2 O 3 | 10.80 | 10.80 | From | 10.84 | Zn | 100 | ||
| Fe 2 O 3 | 1.74 | 0.81 | On | 0.00 | Rb | 317 | 318 | |
| FeO | 3.26 | 2.42 | Ns | 1.49 | Sr | 681 | 640 | |
| MnO | 0.07 | 0.06 | Ks | 1.17 | Zr | 996 | 1040 | |
| MgO | 7.05 | 6.63 | Ac | 1.97 | Ba | 925 | 1460 | |
| CaO | 2.93 | 3.12 | Tuesday | 8.76 | Ce | 302 | 367 | |
| Na 2 O | 1.01 | 1.02 | Hy | 14.89 | Nd | 125 | 146 | |
| K 2 O | 10.40 | 10.73 | Mt | 0.00 | Sm | 16.3 | 19.1 | |
| P 2 O 5 | 0.76 | 0.67 | Il | 4.32 | Hf | 26.6 | 32.1 | |
| LOI | 2.14 | 2.09 | Ap | 1.59 | Th | 36, 8 | 37.9 | |
| Mg # | 0.76 | 0.82 | ||||||
| Al / (Na + K) | 0.84 | 0.82 | ||||||
| A '/ F | - 0.37 |
With 56 to 61 weight percent SiO 2 , the Sisco-Lamproit is an intermediate rock . Among all the Central Mediterranean lamproites, it has the highest K 2 O ( 10.2 to 11.0 percent by weight ) (and is therefore ultra-potassic), but also the lowest Al 2 O 3 content with 10.5 to 10.9 percent by weight . The content of MgO is high (6.4 to 7.1 percent by weight).
The Sisco-Lamproit is a quartz-normative rock, ie is supersaturated with silicon . The standard minerals Akmit ( Ac ), potassium metasilicate ( Ks ) and sodium metasilicate ( Ns ) as well as Al / (Na + K) <1 underline its peralkaline character.
Like all ultrapotassic rocks of the western Mediterranean, the Sisco-Lamproit also shows a strong accumulation of incompatible trace elements , but differs in its extremely high contents of lanthanum , hafnium and zirconium . The elements cesium , lead and uranium , however, are only very slightly enriched. Among the rare earths , it has a negative europium anomaly.
The gangue contains between 15 and 16 grams / ton of antimony (III) sulfide and is therefore likely to be related to the antimony dikes from Cap Corse . Very similar veins with the mineral association stibine - pyrite - marcasite with occasional vermilion and barite are also found in Tuscany , but are much younger there ( Pliocene ).
Isotope ratios
The following initial ratios were determined for the radioisotopes of Sr, Nd and Pb:
| Isotopes | Ratio value | Ratio value |
|---|---|---|
| 87 Sr / 86 Sr | 0.712298 | 0.712270 |
| 143 Nd / 144 Nd | 0.512162 | 0.512149 |
| 206 Pb / 204 Pb | 18.786 | |
| 207 Pb / 204 Pb | 15,692 | |
| 208 Pb / 204 Pb | 39.181 |
Petrology
The 1 to 2 meter thick, dark corridor was originally referred to by D. Velde as a minette (ie lamprophyrus ). More recent studies now clearly classify it as Lamproit. At the contact with the adjacent rock, the corridor shows signs of deterrence. It mostly follows the flat foliage, but can change level along the conjugate elongation fractures in the Schistes Lustrés that strike N 070 to N 080.
Petrologically, the Sisco-Lamproit belongs to the ultrapotassic rocks and is characterized by the absence of leucite and plagioclase . It is also peralkaline . In the TAS diagram , it falls into the trachyte field. Its structure is intersertal .
In the isotope diagram 87 Sr / 86 Sr compared to 206 Pb / 204 Pb, the Sisco-Lamproit occupies an intermediate position between the Tuscan Magma Province on the one hand and the Roman Magma Province on the other. Here it falls halfway between the very Sr-rich upper crust component and the enriched shell component EM 2 . The Tuscan Magma Province is closer to the crustal component, the Roman Magma Province is closer or directly to EM 2. The fact that the Lamproit does not follow the main trend of the Italian mainland is very nice in the diagram 143 Nd / 144 Nd compared to 206 Pb / 204 Pb to see where it sets itself apart from the main trend with a lower lead ratio.
Dating
The Sisco-Lamproit was dated 1978 by Civetta and colleagues using the potassium-argon method to 14.58 ± 0.2 million years BP , ie in the Langhium .
Individual evidence
- ↑ D. Velde: Sur un lamprophyre hyperalkaline potassique: la minette de Sisco (Ile de Corse) . In: Bulletin Societè Francaise Mineralogique et Cristallographique . tape 90 , 1967, p. 214-223 .
- ^ C. Wagner, D. Velde: The mineralogy of K-richterite bearing lamproite . In: Am. Mineral. tape 71 , 1986, pp. 17-37 .
- ^ S. Conticelli, RW Carlson, E. Widom, G. Serri: Chemical and isotopic composition (Os, Pb, Nd, and Sr) of Neogene to Quaternary calc-alkalic, shoshonitic, and ultrapotassic mafic rocks from the Italian peninsula: Inferences on the nature of their mantle sources . In: L. Beccaluva, G. Bianchini, M. Wilson, Cenozoic Volcanism in the Mediterranean Area (Eds.): Geol. Soc. Am., Special Paper . tape 418 , 2007, p. 171-202 .
- ↑ S. Conticelli, among others: Trace elements and Sr-Nd-Pb isotopes of K-rich to shoshonitic and calc-alkalic magmatism of the Western Mediterranean region: genesis of ultrapotassic to calc-alkalic magmatic associations in post-collisional geodynamic setting . In: Lithos . tape 107 , 2009, p. 68-92 .
- ↑ A. Peccerillo, G. Poli, G. Serri: Petrogenesis of orenditic and kamafugitic rocks from Central Italy . In: The Canad. Mineral. tape 26 , 1988, pp. 45-65 .
- ^ 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 . tape 36 , paper 20, 2010.
- ↑ L. Civetta, G. Orsi, P. Scandone, R. Pece: Eastward migration of the Tuscan Anatectic magmatism due to anticlockwise rotation of the Apennines . In: Nature . tape 276 , 1978, pp. 604-606 .