Fortunit

The Fort Unit is a rare igneous rock , which the Lamproiten is expected. The alkaline rock with a predominance of potassium occurs in the south-east Iberian volcanic province .

First description and occurrence.

The Fortunit of the Cabecicos Negros

The fortunit was first scientifically described in 1893 by Ramon Adan de Yarza . There are three known occurrences of Fortunit, all of which are located near the eponymous southeastern Spanish city of Fortuna ( Murcia province ). Including the chimney filling of the Cabecicos Negros (3.5 kilometers southeast of Fortuna) and the two passages of El Tale (3.5 kilometers east of Fortuna) and Los Derramadores (2 kilometers east-southeast of Fortuna).

mineralogy

Adan de Yarza originally described the fortunite as a trachyte variety characterized by the presence of phlogopite and the orthopyroxen bronzite . According to the redefinition of lamproites by Mitchell and Bergman in 1991, the rock is now referred to as hyalo-enstatite-phlogopite-lamproit , so it belongs to the phlogopite-lamproites .

The fortunite consists of the phenocrystals olivine and phlogopite, which are embedded in a glassy to mesocrystalline base mass of 61 percent by volume. In addition, microphenocrystals of idiomorphic clinopyroxene and skeletal sanidine appear in the matrix , which crystallized during an early devitrification phase. Orthopyroxene is a substitute for olivine. Apatite and pseudobrookite act as accessories . Phlogopite, aluminum-rich spinel as well as ilmenite and rutile have an origin as xenocrystals . Occurring nickel sulfides are post-magmatic formations. The olivine, which occupies up to 24 percent by volume, is rich in the component forsterite (Mg # = 0.91) and often contains chromium-rich spinel. Phlogopite (Mg # = 0.90) with grain sizes up to more than 5 millimeters has a volume fraction of 10 percent. The pyroxenes reach 5 percent by volume.

Chemical composition

Main elements

Oxide wt.%	Fortunit average	Fortunit bandwidth	Fortunit El Tale	Fortunit Cabecicos Negros	Fortunit Barqueros	Fortunit Salmerón	Fortunit 73A8
SiO ₂	56.05	53.36-57.51	59.14	56.4-57.1	55.32	56.88	58.77
TiO ₂	1.35	0.95-1.86	1.46	1.35-1.54	1.63	1.52	1.35
Al ₂ O ₃	11.43	9.69-13.03	12.42	11.0-11.7	11.24	9.04	11.34
Fe ₂ O ₃	2.23	0.72-3.23	4.82 (dead)	5.58-5.98 (dead)	2.25	2.29	1.31
FeO	3.31	1.31-4.82			3.40	2.79	3.49
MnO	0.08	0.01-0.12	0.04	0.07	0.07	0.08	0.10
MgO	9.27	5.20-11.81	5.96	8.93-9.23	9.48	11.56	8.05
CaO	4.21	1.41-7.01	2.71	2.60-2.78	3.59	3.03	4.71
Na ₂ O	2.10	1.35-4.07	1.30	0.87-1.16	1.25	1.64	1.25
K ₂ O	6.14	3.52-9.15	8.73	7.84-8.48	6.88	8.45	7.87
P ₂ O ₅	0.81	0.30-1.57	1.14	0.75-0.86	0.72	1.08	2.22
LOI	2.97	1.22-7.71	1.70	1.84-3.26	4.46	1.06	1.27
K / Na	1.92	1.48-1.72	4.41	4.45-6.32	3.61	3.38	4.13
K / Al	0.58	0.39-0.76	0.76	0.74-0.82	0.66	1.01	0.75
(K + Na) / Al	0.88	0.62-1.27	0.93	0.74-0.95	0.85	1.31	0.93

Source: For analysis 73A8: Wagner, C. and Velde, D. (1986)

Fortunite is an intermediate rock that is oversaturated with silicon . It is also predominantly metaluminous with (Na + K) / Al <1 (rarely also peralkaline) and potassic to ultrapotassic (with K / Na> 3). It is one of the alkaline rocks with a predominance of potassium. The main elements MgO , CaO and Na ₂ O have very low concentrations .

In the case of halides, fluorine has extremely high values of around 0.5 percent by weight - the highest concentrations ever measured for lamproites worldwide. The chlorine levels, on the other hand, are low, as are those of carbon dioxide and sulfur .

The Fortunit is a quartz normative rock ( Q = 2.27 percent by volume). The standardized feldspar components Or , Ab and An together come to 60.22, orthopyroxene ( En and Fs ) to 24.69 and clinopyroxene ( Di ) to 5.75 percent by volume. Imenite ( II ) is 2.57 and apatite ( Ap ) 1.88 percent by volume.

Trace elements

Trace elements ppm	Fortunit average	Fortunit bandwidth	Fortunit El Tale	Fortunit Cabecicos Negros	Fortunit Barqueros	Fortunit Salmerón
Cr	531	495-568	629	543-582	583	523
Ni	428	353-503	163	371-479	479	552
Zn			82.0	110.9-138.7
Rb	489	478-501	828	753-903	500	528
Sr	507	478-536	493	586-703	516	615
Zr	541	413-669	753	594-687	678	856
Ba	1164	806-1523	1587	1766-2096	1274	1722
Ce			222	195-235	248	204
Nd			141	127-152	163
Sm			25.6	25.2-30.0	26.6
Hf			20.6	15.6-17.9	19.8
Th	92	85-100	112	75.2-106	106	129

The Fortunit is clearly enriched in incompatible elements , LILE , HFSE and rare earths all have high contents. Rubidium , zirconium and thorium in particular are very high . Nickel , lead and scandium have low concentrations .

Isotope ratios

Neodymium-strontium isotope diagram with the position of the Fortunite (F) compared to other lamproites

Isotopes	Cabecicos Negros SP 044	Cabecicos Negros SP 049	Cabecicos Negros SP 050
⁸⁷ Sr / ⁸⁶ Sr	0.717689	0.717820	0.717928
¹⁴³ Nd / ¹⁴⁴ Nd	0.512039	0.512045	0.512035
²⁰⁶ Pb / ²⁰⁴ Pb		18.764
²⁰⁷ Pb / ²⁰⁴ Pb		15.706
²⁰⁸ Pb / ²⁰⁴ Pb		39.122

With the isotope ratios ¹⁴³ Nd / ¹⁴⁴ Nd is constantly low, but ⁸⁷ Sr / ⁸⁶ Sr is significantly increased and approaches the values of upper crustal rocks . With these two parameters, a correspondence with the crustal rocks in the Tuscan Magma Province and with the conditions in the Italian western Alps can be observed. In contrast to the madupites and orendites , which belong to the association of the interior of the plate, Fortunite follow a flat trend of orogenetically influenced association. In comparison with the other lamproites in southeastern Spain, their ⁸⁷ Sr / ⁸⁶ Sr values are above those of the Jumillite (J) - Fortunites are therefore somewhat more crustally contaminated - but still clearly below the Verite (V); with the Cancalites (C) they are almost indistinguishable and practically identical.

In the lead ratios (206 and 207), which overlap with the values for oceanic sediments, there is also a correspondence with the lamproites of Tuscany and the lamproic minets of the western Alps (206). The very high lead ratios of ²⁰⁷ Pb / ²⁰⁴ Pb and ²⁰⁸ Pb / ²⁰⁴ Pb are also striking . The lead ratio of ²⁰⁶ Pb / ²⁰⁴ Pb, however, is the lowest among the lamproites of south-east Spain.

Temporal development and dating

The intermontane Fortuna Basin was transgressed by the sea from the lower Tortonian (about 10 million years ago BP) and initially calcarenites, bioclastic limestone and marl sedimented. With progressive deepening followed 500 to 600 meters thick turbidites and marls, the Fortuna marls . From the Messinian onwards, in connection with the salinity crisis of the Mediterranean, evaporites were deposited, which were bordered by reefs and fluvial sediments at the edge. The final pelvis development up to the Pliocene then took place detritically.

The Fortunitganges of Derramadores and El Tale intruded below the Lower Gypsum into the Fortuna marl. Their intrusion age should correspond to the Ar-Ar age of 7.1 ± 0.11 million years BP found by Kuiper and colleagues. However, the Cabecicos Negros fortunites penetrated stratigraphically nearly 200 meters higher and intruded at the base of the Ramblada Salada gypsum. Since the intrusion occurred at the same time as the sedimentation, they are inevitably much younger than the tunnels that only ascended to the Fortuna marl. They may have the age of 6.16 ± 0.3 million years BP found by Bellon and colleagues (1983) using the K-Ar method.

Individual evidence

↑ Adan de Yarza, R .: Roca eruptiva de Fortuna (Murcia) . In: Boletin de la Commissión del Mapa Geológico de España . Vol. 20. Madrid 1893, pp. 349-353 .
↑ Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum Press, New York 1991, ISBN 0-306-43556-X .
^ Wagner, C. and Velde, D .: The mineralogy of K-richterite-bearing lamproites . In: American Mineralogist . Volume 71, 1986, pp. 17-37 .
↑ Conticelli, S. et al .: 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 .
↑ Kuiper et al .: ⁴⁰ Ar / ³⁹ Ar for the lamproite volcanism of Cabezos Negros, Fortuna basin (Eastern Betics, SE Spain) . In: Palaeogeogr. Paleoclimatol. Palaeoecol.
↑ Bellon, H. et al.: Chronologie du magmatisme néogène des Cordillères bétiques (Espagne méridionale) . In: Bull. Soc. Géol. Ms. Band 7 (25) (2) , 1983, pp. 205-217 .

[1] Adan de Yarza, R .: Roca eruptiva de Fortuna (Murcia) . In: Boletin de la Commissión del Mapa Geológico de España . Vol. 20. Madrid 1893, pp. 349-353 .

[2] Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum Press, New York 1991, ISBN 0-306-43556-X .

[3] Wagner, C. and Velde, D .: The mineralogy of K-richterite-bearing lamproites . In: American Mineralogist . Volume 71, 1986, pp. 17-37 .

[4] Conticelli, S. et al .: 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 .

[5] Kuiper et al .: ⁴⁰ Ar / ³⁹ Ar for the lamproite volcanism of Cabezos Negros, Fortuna basin (Eastern Betics, SE Spain) . In: Palaeogeogr. Paleoclimatol. Palaeoecol.

[6] Bellon, H. et al.: Chronologie du magmatisme néogène des Cordillères bétiques (Espagne méridionale) . In: Bull. Soc. Géol. Ms. Band 7 (25) (2) , 1983, pp. 205-217 .