Cerro Negro de Calasparra

Cerro Negro de Calasparra
Columnar segregation in the Cerro Negro de Calasparra
height	385  msnm
location	Murcia Province , Spain
Coordinates	38 ° 14 '35 "  N , 1 ° 43' 3"  W Coordinates: 38 ° 14 '35 "  N , 1 ° 43' 3"  W.
rock	Lamproit
Age of the rock	Miocene ( Messinian )

The Cerro Negro de Calasparra is a small, flattened, conical volcano of the Messinian , which rose in marl of the Upper Miocene . The now horn-shaped, in north-south direction 400 meters long and 225 meters wide volcanic remnant is located near Benefudre on the C-3314 to Paradores, around 2 kilometers northwest of Calasparra ( Murcia province ). It has a crater 80 meters in diameter. The Jumillit or Cancalit that builds it up belongs to the Lamproites of the Southeast Iberian volcanic province . The interior of the volcano shows beautiful columnar discharge of lava rock .

geology

The intrusion of the Cerro Negro de Calasparra occurred in the immediate vicinity of the right- shifting Socovos Fault , on which several lamproite tunnels are lined up and which should have made the rise of the magma possible. This N 120 trending lateral shift with an offset of 60 kilometers is a very deep structure in the prebetic , on which, in addition to the lamproites, diapirs of the Triassic and mineralization of copper and zinc can be recorded.

In general, the intrusion has bulged the host rocks so that they collapse in the immediate vicinity at 30 ° from the center. In the south the influence of the Socovos Fault is already noticeable, the angle of incidence increases and even N 120 striking, steep upheavals can be observed. As a result, volcanic agglomerates were dragged synclinally and even show overturned storage conditions.

description

The first description of the Cerro Negro de Calasparra - in German Black Hill of Calasparra , alluding to the dark gray to blackish color of the compact, reddish weathered rock - was made in 1929 by Fallot and Jérémine. Fallot later (1945) interpreted the structure as a lava flow . It was not until 1966 that Fúster and Sagredo realized that a central, intrusive spur was surrounded by pyroclastic breccias.

Since the Cerro Negro was mined especially for road construction because of its resistant material, its interior can now be seen clearly. Clearly recognizable is the tuff ring , which rises around 20 meters above the base level and whose stratification slopes outwards at 18 to 24 °. In the footwall it consists of yellowish solid Lapillituffen which are poorly stratified and sorted. The majority of the lapilli consist of juvenile, angular or rounded, bulged, generally very low-bubble lamproitclasts. Miocene claystones and siltstones of the internal prebetic function as host rock inclusions. The inclusions show metasomatic changes in the contact area. In the hanging wall , 8 to 10 meters of thin to laminated fine tufts appear.

The central part of the Cerro Negro is in the range of the former chimney (from a central plug engl. Plug ) filled with concentric of columnar secretion. This plug once towered over the tuff ring, with the pillars running upwards in the now no longer existing upper part. The lower part obtained, however, shows a reverse bending of the columns downwards. Such a fracture formation resulting from the cooling of the magma indicates the former tuber or bud shape of the intrusion. In the interior of the central conveying area, at the contact zone between the plug and the surrounding volcanic plastic, several storage tunnels can be observed that penetrate the tufa layers and deform them.

interpretation

The volcaniclastic deposits of the person lying on the Cerro Negro de Calasparra point to a highly concentrated pyroclastic density flow , which emerged from a collapsing phreatomagmatic eruption column and can possibly be associated with the initial opening of the production chimney. The upper volcaniclastic series is likely to have originated from a relatively low-concentration, "wet" gas cloud ( surge ). The intrusion of the central plug took place late, clogging the chimney and sending storage tunnels and duct-like intrusions into the volcanic plastic of the tuff ring.

The phreatomagmatic eruptions at Calasparra were likely caused by the presence of a shallow aquifer in the Miocene sediments. The raised tuff ring is not particularly large and was therefore created by a limited volume of magma. After the groundwater supply was exhausted, only low-concentration pyroclastic density currents could be emitted. Eventually the lamproic magma filled the crater interior as a plug-like extrusion. As with the Cancarix volcano , the early magma filling had only a low crystal content, whereas the later mines were rich in crystals. Due to the increased viscosity of the magma, only a plug could be pressed out in the final phase (for a current example, see the whaleback plug on Mount Saint Helens ).

The whaleback on Mount St. Helens - a lava plug of much larger dimensions

Even if the Cerro Negro has a very unusual chemical composition, its development - comparable to normal mafic magmas - follows the Mitchell-Bergman model :

• initial blasting out of the crater
• Formation of phreatomagmatic sequences
• Lava flow and / or plug formation in the chimney.

During the first two stages there is an enormous loss of volatile substances ( gases , steam ), so that in the final phase the degassed magma, due to its increased viscosity and higher crystal content, can only flow out slowly or as a highly viscous plug in the chimney gets stuck.

Petrology and Mineralogy

The Lamproit of the Cerro Negro de Calasparra is a porphyry volcanic rock. It has similarities to Cancalit , but is slightly richer in potassium and iron .

Olivine and phlogopite appear as phenocrystals in a microcrystalline matrix . The matrix also contains diopside , phlogopite, amphibole , apatite and orthopyroxene . Olivine can also be present as a xenocrystal. In addition, porphyry rock fragments with a glass matrix are rich in bubbles . Secondary chalcedony formed in the blisters .

Chemical composition

Main and trace elements

Oxide wt.%	Cerro Negro	Cerro Negro 72A7	Standard mineral	CIPW standard 72A7	Trace element ppm	Cerro Negro
SiO 2	56.2	56.61	Q	2.90	Cr	434
TiO 2	1.64	1.61	Or	43.37	Ni	470
Al 2 O 3	9.5	7.96	Ac	5.57	Zn	88
Fe 2 O 3	5.3 (dead)	2.12	Ks	3.08	Rb	737
FeO		3.13	Tuesday	5.29	Sr	558
MnO	0.07	0.08	En	28.87	Zr	849
MgO	12.0	12.45	Fs	2.85	Ba	1883
CaO	2.69	2.97	Mt	0.28	Ce	263
Na 2 O	0.78	0.75	Ilm	3.05	Nd	173
K 2 O	9.9	9.21	Ap	2.92	Sm	33
P 2 O 5	1.12	1.23			Hf	23.6
LOI	0.87	2.50			Th	108
K / Na	30.03	8.08
K / Al	1.13	1.25
(Na + K) / Al	1.17	1.41

Source for analysis 72A7: Wagner, C. and Velde, D. (1986)

The present rock analyzes show an extremely ultrapotassic (K / Na >> 3), intermediate alkaline rock with peralkaline character (since (Na + K) / Al> 1). It is characterized by very high contents of MgO and K ₂ O , whereas Na ₂ O is clearly depleted.

The normative quartz content ( Q ) shows silicon saturation . The standard minerals Akmit ( Ac ) and potassium metasilicate ( Ks ) underline the peralkaline character of the rock.

In the trace elements, as with all lamproites, the incompatible elements are enriched, especially among the LILE rubidium , but also the HFSE thorium , cerium , zirconium , hafnium and the rare earths neodymium and samarium . Very high concentrations are also found in the six-coordinate nickel .

The original address of the rock as Jumillit by Fallot and Jérémine can not be maintained because Jumillit substantially lower than mafic rock SiO ₂ content has. Rather, it is very similar to Cancalit, with which it almost corresponds chemically (exception: low chromium values). The presence of orthopyroxene ( enstatite ) suggests the same conclusion, since orthopyroxene is essential for Cancalit.

Dating

Dating by Pérez-Valera and colleagues in 2013 using the Ar-Ar method showed an age of 7.1 million years BP . The intrusion thus took place at the beginning of the Messinium.

Individual evidence

1. ↑ Rodriguez Estrella, T .: Contribución de la Hidrogeología al conocimiento tectónico en el sureste español . In: II Simp. Nac. De Hidrología . Pamplona.
2. ↑ Fallot, P. and Jérémine, A .: Sur la presence d'une varieté de jumillite aux environs de Calasparra (Murcia) . In: CR Acad. Sci. tape 188 , 1929, pp. 800 .
3. ^ Fallot, P .: Estudios geológicos en la Zona Subbética entre Alicante y el río Guadiana Menor . In: CSIC . Madrid 1945, p. 719 .
4. ↑ Fúster, JM and Sagredo, J .: Estudio petrologico de las rocas lamproiticas de Calasparra . In: Congreso del manto superior . Madrid 1966, p. 4-19 .
5. ↑ Fisher, RV and Schmincke, H.-U .: Pyroclastic Rocks . Springer, Heidelberg 1984, p. 474 .
6. ↑ Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum Press, New York 1991, pp. 447 .
7. ↑ Seghedi, I. et al .: Miocene lamproite volcanoes in south-eastern Spain - an association of phreatomagmatic and magmatic products . In: Journal of Volcanology and Geothermal Research . tape 159 , 2007, p. 210-224 , doi : 10.1016 / j.jvolgeores.2006.06.012 .
8. ^ Wagner, C. and Velde, D .: The mineralogy of K-richterite-bearing lamproites . In: American Mineralogist . Volume 71, 1986, pp. 17-37 .
9. ↑ Pérez-Valera, LA et al .: Age distribution of lamproites along the Socovos Fault (southern Spain) and lithospheric scale tearing . In: Lithos . tape 180 , 2013, p. 252-263 .