Cabezo Negro de Zeneta
| Cabezo Negro de Zeneta | ||
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 The Cabezo Negro de Zeneta from the north |
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| height | 203 msnm | |
| location | Murcia Province , Spain | |
| Coordinates | 37 ° 59 '52 " N , 0 ° 57' 55" W | |
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| rock | Lamproit | |
| Age of the rock | 8.1 million years BP ( Tortonium ) | |
The Cabezo Negro de Zeneta , southeast of Zeneta , is an important volcano in the Murcia region . Its lamproit rock belongs to the southeast Iberian volcanic province . It originated in the Tortonian BP 8.1 million years ago .
Geological introduction
The volcanic complex of the Cabezo Negro de Zeneta - in German Schwarzkopf von Zeneta , so named because of its dark lamproite rock - lies in the transition area between the neogenic, post-orogenic collapse basins of Bajo Segura and Murcia-Cartagena . The intruded basin sediments belong to the Torremendo Formation and consist of marls , intermediate sandstones and local conglomerates . They could be assigned to the Tortonium (11.6 to 7.25 million years BP) by means of planktonic foraminifera . These originally pelagic basins were noticeably flattened at the beginning of the Torton and finally dried out with the precipitation of evaporites - even before the actual salinity crisis of the Messinium began .
description
The Cabezo Negro is located about 4 kilometers east-southeast of Zeneta, about 15 kilometers east-southeast of Murcia and is in the immediate vicinity of the border with the province of Valencia . The volcano forms a north-facing, 1000-meter-long and about 500-meter-wide pod, the steep south and west side of which slopes almost vertically, partly with pillars. It reaches 203 in the center, 190 in the west and 150 msnm in the east . Its dark, name-giving lamproite rock stands out clearly from the light-colored Miocene sediments along its base.
stratigraphy
The stratigraphic sequence on the Cabezo Negro can be broken down as follows (from hanging to lying ):
- Fault breccia
- massive volcanic rocks with sediment inclusions
- massive volcanic rocks
- volcanic sedimentary breccia ( peperite )
Dikes of massive volcanic rocks can cut through the entire sequence.
Volcanic sedimentary breccia
The volcanic sedimentary breccias are to be interpreted as typical peperites and they represent the most powerful and most widespread unit on the Cabezo Negro. They are always found at the base above the Miocene sediments and are in turn overlaid by the massive volcanic rocks. They are neither stratified nor do their blocks show any adjustment. Their thickness varies between 40 meters in the west to 15 meters in the east. Their block content is not constant, in the west it is 30 to 60%, but in the east and in the hanging walls with a predominant sediment matrix it is only 5 to 15%. The block size is not uniform either, but varies from centimeters to meters. The volcanic blocks are bleached inside, but their edges do not show any signs of deterrence. As phenocrysts can be phlogopite and biotite , in a dark, fine grained matrix are embedded seen. The surrounding sediment matrix is extremely fine-grained, of yellow-orange color and emerged from fossil-containing, planktonic foraminifera containing marls.
Massive volcanic rocks
The massive volcanic rocks overlap the peperite concordantly, undulating surfaces are evened out. They are dark gray in color, compact and structureless and build up the main mass of the volcanic building, including the summit regions. They contain up to 0.5 millimeter large phenocrystals of phlogopite and biotite, occasionally also decomposed crystals of olivine and clinopyroxene . Associated with this facies is an extensive lava flow with flow layers parallel to the base, which falls flat in the south of the volcanic building, but steepens to the north to the vertical.
Massive volcanic rocks with sediment inclusions
This facies occurs in the east and in the center of the volcanic structure. The lenticular inclusions consist of marls, as in the peperites, but are not diffusely distributed, but in layers with banks up to 50 centimeters thick. Repeated bench sequences are found in the recumbent position, but here they are only 15 centimeters thick. Their frequency decreases towards the hanging wall, but sediment packages up to half a meter thick can still be found here and there. Their spatial position ranges from horizontal to vertical.
Fault breccia
The two massive volcanic facies are affected by faults where breccias from massive volcanic blocks could form in situ. These breccias are often crossed by secondary hydrothermal, network-forming quartz veins. The steep faults strike in the central part of the Cabezo Negro N 160.
In addition, steep clefts that strike the N 170 run at irregular intervals through the entire volcanic building. In the east, vertical corridors following the northeast direction have penetrated.
interpretation
The Cabezo Negro de Zeneta can be explained in three evolutionary steps. The volcanic sedimentary breccia (peperite) formed during an initial intrusion phase in the shallow marine environment. They are the product of phreatomagmatic explosions in the contact area magma / unconsolidated sediment, either on the edge of intrusions or below leaked lava flows. During the second intrusion phase the massive volcanic rocks formed over the Peperites. Their sediment inclusions, which become smaller and smaller towards the hanging wall, indicate a general decrease in the phreatomagmatic explosive force. The third intrusion phase took place subaerically and relatively calmly. Two massive, columnar dome structures without internal inclusions slowly emerged from the sea and released the already mentioned lava flow from their interior. Towards the end of the magmatic activities, corridors then emerged again based on weak zones. After solidification and cooling, the two dome structures were finally victims of the brittle deformation and of faults and fissures.
mineralogy
The lamproite rock of the Cabezo Negro is porphyry with a hypocrystalline structure. It is affected by secondary alteration processes. In it, the following minerals occur both as sprinkles and as components of the basic mass:
- Olivine (mostly decomposed) - 5 percent by volume
- Clinopyroxene - 8 volume percent ±
- Phlogopite and biotite - 47 percent by volume
- Sanidine - 32 percent by volume
Accessory minerals and secondary formations take up the remaining 8 percent by volume, including apatite , magnetite , zircon and monazite . Furthermore, minerals such as orthopyroxene , sillimanite , aluminum-rich spinel , plagioclase and xenocrystals of quartz appear that are atypical for lamproite .
Dating
According to Soria and colleagues (2005), the intruded basin sediments come from the Middle to Upper Miocene. For the Torremendo formation in question, Soria and colleagues (2008) give the time interval from the Upper Tortonium to the Lower Messinian.
Duggen and colleagues (2005) were able to date the lamproit intrusion using the Ar-Ar method on phlogopite to an age of 8.08 ± 0.03 million years BP.
Individual evidence
- ↑ a b Soria, JM et al .: The Messinian - early Pliocene stratigraphic record in the southern Bajo Segura Basin (Betic Cordillera, Spain). Implications for the Mediterranean Salinity Crisis. In: Sedimentary Geology . tape 203 (3-4) , 2008, pp. 267-288 .
- ↑ Lancis, C. et al.: Nannoplankton biostratigraphic calibration of the evaporitic events in the Neogene Fortuna Basin (SE Spain) . In: Geobios . tape 43 (2) , 2010, p. 201-217 .
- ↑ Cambeses, A. and Scarrow, JA: Ultrapotassic volcanic centers as potential palaeogeographic indicators: The Mediterranean Tortonian salinity crisis, southern Spain . In: Geologica Acta . Vol. 2, No. 3 , 2013, p. 295-310 , doi : 10.1344 / 105.000001860 .
- ↑ Cambeses, A .: Characterization of the volcanic centers at Zaneta and La Aljorra, Murcia: Evidence of Minette formation by lamproite-trachyte magma mixing (diploma thesis) . University of Granada 2011, p. 249 .
- ^ Soria, JM et al: The stratigraphic record of the Messinian salinity crisis in the northern margin of the Bajo Segura Basin (SE Spain) . In: Sedimentary Geology . tape 179 , 2005, pp. 225-247 .
- ↑ Duggen, S. et al .: Post-collisional transformation from subduction to intraplate-type magmatism in the westernmost Mediterranean: evidence for continental-edge delamination of subcontinental lithosphere . In: Journal of Petrology . tape 46 , 2005, pp. 1155-1201 .