Cabecicos Negros

38.160277777778 -1.0936111111111Coordinates: 38 ° 10 ′ N , 1 ° 6 ′ W The Cabecicos Negros are small contiguous volcanic structures in the province of Murcia , which are located about 3.5 kilometers southeast of Fortuna near the village of La Matanza . They are built from the very rare Fortunite lamproite rock .

description

View over one of the Cabecico passages towards Fortuna

The volcanic constructs of the Cabecicos Negros (also Cabezos Negros or Cabezitos Negros - in German black heads ), culminating at 151 to 154 msnm , may have emerged from a single mine passage . Fúster's assumption of a conveyor chimney (neck) with a diameter of 400 meters cannot be upheld by any evidence. In the north, massive, low-bubble, superficially emerged lava flows can be seen, which form peperites on their outer skin - which suggests contact with water-rich sediment. In the center, several lava flows interlock at different levels with marl-rich sediment and are simultaneously traversed by several irregular, more or less vertical tunnels. We may be dealing with the remains of a crater wall. The much lower, flattened and irregularly shaped southern part consists of an intrusive, holocrystalline, vertical corridor and a storage corridor . The corridor, which has risen to a shallow level, has a bulbous, glassy edge zone, which indicates its very rapid cooling in the water-rich sediment. The storage corridor has evidently spread only a few meters below the unconsolidated sediment, as indicated by pillow structures.

The host rocks of the volcanic center are lutites that occur stratigraphically below the Rambla Salada gypsum in the upper portion of the Chicano cycles (outgoing tortonium ).

geology

In addition to the Cabecicos Negros, two other sites with Fortunit tunnels are known in the neogene Fortuna Basin , which are stratigraphically almost 200 meters lower and are therefore older. These are Los Derramadores 2 kilometers east-southeast of Fortuna and Le Tale 3.5 kilometers east of Fortuna. Other dike rocks can be found in the neighboring Mula . The Sierra de la Pila near Fortuna is also of volcanic origin.

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 (which was only to reach its maximum towards the end of the Messinian), evaporites were deposited, which were bordered by coral reefs and fluvial sediments at the edge. The final pelvis development up to the Pliocene then took place detritically.

mineralogy

The Cabecicos Negros fortunite has the minerals olivine , phlogopite , clinopyroxene and apatite as phenocrystals . Alkali feldspar , orthopyroxene , priderite and opaque minerals can also be found in the glassy matrix. Alkali feldspar can also be formed secondarily.

Chemical composition

Main and trace elements

Oxide wt.%	Fortunit SP 044	Fortunit SP 049	Fortunit SP 050	Trace elements ppm	Fortunit SP 044	Fortunit SP 049	Fortunit SP 050
SiO ₂	56.4	57.1	56.8	Cr	582	543	567
TiO ₂	1.38	1.35	1.54	Ni	371	430	479
Al ₂ O ₃	11.0	11.5	11.7	Zn	113.8	138.7	110.9
Fe ₂ O ₃	5.60 (dead)	5.58 (dead)	5.98 (dead)	Rb	765	903	753
FeO				Sr	586	703	649
MnO	0.07	0.07	0.07	Zr	620	687	594
MgO	8.93	9.06	9.23	Ba	2096	1841	1766
CaO	2.60	2.78	2.69	Ce	195	235	228
Na ₂ O	0.87	1.16	1.04	Nd	127	152	149
K ₂ O	8.34	7.84	8.48	Sm	25.2	30.0	29.0
P ₂ O ₅	0.86	0.75	0.86	Hf	17.9	16.6	15.6
LOI	3.26	2.26	1.84	Th	75.2	106	101
Mg #	0.79	0.79	0.78
K / Na	6.32	4.45	5.36
K / Al	0.82	0.74	0.78
(K + Na) / Al	0.95	0.74	0.93

The Fortunit of the Cabecicos Negros is an intermediate rock that is oversaturated with silicon . He is also predominantly metaluminos with (K + Na) / Al <1 and ultrapotassic with K / Na> 3. It is clearly enriched in incompatible elements , LILE , HFSE and rare earths all have high levels. Rubidium , zirconium and thorium in particular are very high . The main elements CaO and Na ₂ O as well as the trace elements lead and scandium have very low concentrations .

Isotope ratios

Isotopes	Fortunit SP 044	Fortunit SP 049	Fortunit 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, an approximation of the crustal rocks in the Tuscan Magma Province and the conditions in the Italian western Alps can be observed. With the lead ratios (206 and 207), which overlap with the values for oceanic sediments, there is also an approximation to the lamproites of Tuscany and the lamproitic minets of the western Alps (206). Also striking are the very high lead ratios of ²⁰⁷ Pb / ²⁰⁴ Pb and ²⁰⁸ Pb / ²⁰⁴ Pb (highest values among the lamproites of south-east Spain). The lead ratio of ²⁰⁶ Pb / ²⁰⁴ Pb, however, is the lowest among the lamproites of south-east Spain.

Age

The Fortunite of the Cabecicos Negros intruded at the transition from the Chicano cycles to the Ramblada-Salada gypsum, which, however, has not yet been clearly established by magnetostratigraphy . There are two interpretations: either the base of the gypsum formation correlates with the polarity zone C3An2n, which is younger than 6.56 million years BP, or it is one cycle lower at polarity zone C3Bn, which is younger than 7.1 million years BP.

Radiometrically, Bellon and colleagues (1983) found an age of 6.16 ± 0.3 million years BP for the Cabecicos Negros using the K-Ar method. Kuiper and colleagues found a much older, probably more reliable Ar-Ar age of 7.1 ± 0.11 million years BP. In a revision (2006) the same authors found an even higher value of 7.71 ± 0.11 million years BP. This would mean that the intrusion of Fortunit along the left-shifting Alhama-de-Murcia fault already occurred in the upper Tortonium and that the evaporites in the Fortuna Basin had also formed before the beginning of the Messinian. This higher age consequently correlates magnetostratigraphically with the polarity zone C4n.

Conservation and Global Geosite

The Cabecicos Negros have been under nature protection since 1917. Because of their petrological interest, they were proposed by the Instituto Geológico y Minero de España as a Global Geosite of international relevance. It bears the designation VU003 - volcanic center of the Cabecitos Negros near Fortuna and ranks under the geological category of Neogene and Quaternary volcanism of the Iberian Peninsula .

Web links

Commons : Cabecicos Negros volcanic cone - Collection of images, videos and audio files

Individual evidence

↑ Fúster, JM et al .: Las rocas lamproiticas del SE de España . In: Estud. Geol. Band XXIII , 1967, p. 35-69 .
↑ Elisabet Playà and Domingo Gimeno: Evaporite deposition and coeval volcanism in the Fortuna Basin (Neogene, Murcia, Spain) . In: Sedimentary Geology . tape 188-189 , 2006, pp. 205-218 .
↑ 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 .
↑ Dinarès-Turell, J. et al .: Paleomagnetic chronology of the evaporitic sedimentation in the Neogene Fortuna Basin (SE Spain): early restriction preceding the Messinian Salinity Crisis . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 154 , 1999, pp. 161-178 .
↑ 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 .
↑ Kuiper, KF et al .: ⁴⁰ Ar / ³⁹ Ar for the lamproite volcanism of Cabezos Negros, Fortuna Basin (Eastern Betics, SE Spain) . In: Palaeogeogr. Paleoclimatol. Palaeoecol.
↑ Kuiper, KF et al .: Revised isotopic ( ⁴⁰ Ar / ³⁹ Ar) age for the lamproite volcanism of Cabezos Negros, Fortuna Basin (Eastern Betics, SE Spain) . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 238 , 2006, pp. 53–63 , doi : 10.1016 / j.palaeo.2006.03.017 .

[1] Fúster, JM et al .: Las rocas lamproiticas del SE de España . In: Estud. Geol. Band XXIII , 1967, p. 35-69 .

[2] Elisabet Playà and Domingo Gimeno: Evaporite deposition and coeval volcanism in the Fortuna Basin (Neogene, Murcia, Spain) . In: Sedimentary Geology . tape 188-189 , 2006, pp. 205-218 .

[3] 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 .

[4] Dinarès-Turell, J. et al .: Paleomagnetic chronology of the evaporitic sedimentation in the Neogene Fortuna Basin (SE Spain): early restriction preceding the Messinian Salinity Crisis . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 154 , 1999, pp. 161-178 .

[5] 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 .

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

[7] Kuiper, KF et al .: Revised isotopic ( ⁴⁰ Ar / ³⁹ Ar) age for the lamproite volcanism of Cabezos Negros, Fortuna Basin (Eastern Betics, SE Spain) . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 238 , 2006, pp. 53–63 , doi : 10.1016 / j.palaeo.2006.03.017 .