Garnet-Cordierite Dacite

The NeogeneVolcanic Province (NVP) is located in the Betic Cordillera, which forms the western termination of the Alpine orogen in the Mediterranean region, along with the Moroccan Rif (Fig.1). The area is characterized by a thinned ithosphere, a consequence of the opening of the Alboran Domain in the late Tertiary. The upwelling of asthenospheric mantle caused an anomalous heat flow, still present in the area, with partial melting of the lower crust and subsequent eruption or emplacement of large volumes of magmas of variable composition, ranging from basaltic andesites to rhyolites, between 15 and 2 Ma.

In the same area, a fault system (CPM: Carboncras-Palomares-Alhame de Murcia Fault System) was active as sinistral strike-slip faults at least since the Late Miocene, leading to the juxtaposition of disparate crustal segments. To the east of this fault system the lithosphere is thinner (about 22 km thick), denser and characterized by higher heat flow values than to the west; this indicates a mantle thermal anomaly under the area of thinner crust.
Outcrops of calc-alkaline to shoshonitic rocks are widespread along the fault system, in an area from Mar Menor in the NE to the depression of Nijar, close to Almeria, in the SW. The few available geochronological data on these rocks suggest an age of about 6.5-8.5 Ma.

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Fig.1: Geographical location and schematic tectonic elements of the main edifices of the Miocene volcanics of the NVP within the Alboràn Domain (after Lòpez-Ruiz & Rodrìguez Badiola, 1980).



El Hoyazo volcano

The El Hoyazo of Nijar (Fig.1 and Fig.3) is the remains of a volcano edifice (with a volume of erupted volcanics of c. 1 km3) surrounded by a reef complex forming a perfect example of fossil atoll (Fig.4).
At present, the volcanic structure is deeply eroded, forming a crater-like depression (Fig.3). The finegrained dacite has a phenocryst assemblage made of plagioclase, biotite, quartz, cordierite, ilmenite and scarce sillimanite, as well as abundant xenocrysts of almandine-rich garnet (Fig.5 and Fig.7), cordierite, hornblende, orthopyroxene, quartz, plagioclase and spinel (that represent dismembered metapelitic xenoliths).

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Fig. 2: General view of El Hoyazo hill and Rambla de Las Granatillas (in the center).



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Fig.3: "Cráter de la Granadilla" or "El Hoyazo" crater. Note the surroundin reef complex at the top of the crater.



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Fig. 4: Genesis of El Hoyazo volcano: 1) submarine volcanism and accretion of the volcanic edifice. 2) subaerial volcanic phase. 3) End of the volcanic activity and formation of coral reef (in red). 4) Formation of an coral atoll. 5) erosive phase and formation of a crater-like structure (do to the erosion of volcani material).



Crystalline fragments (Fig.6 and 8) in El Hoyazo lava are medium to coarsegrained granulite facies rocks that range in size from a few tens of centimetres down to single xenocrysts. These are interpreted to be the result of fragmentation and dispersion in the host lava, possibly because of explosive emplacement. They have been characterized by Zeck (1968, 1970), who gave detailed petrographic descriptions and distinguished several types:

• Grt-Bt-Sil gneiss (the most abundant type).
• Spl-Crd hornfels.
• Qtz-Crd rocks.
• Schists-quartzites.
• Mafic inclusions.

Mafic, gabbroic to basaltoid enclaves display sharp contacts with host dacite, and show a very variable mineralogy composed of Pl+Bt±Hbl±Opx±(Cpx)±Cumm±Qtz±glass. Textural evidences indicate that many of the inclusions were intruded in the host magma as essentially liquid blobs.

Mineral chemistry and textural complexity strongly suggests a relatively prolonged history of intrusion of several mafic pulses into the dacite magma. It is proposed that repetitive intrusions of hot, hydrous mafic magmas in a partially crystallized dacite magmatic system may have induced rejuvenation, remelting, mobilization, and finally convective overturn of the magma chamber, resulting in the final homogenization of the host magma and dissagregation of previous mafic enclaves. A last batch of hydrous, andesitic magma injected in the dacite magma chamber may have caused the eruption.

The Grt-Bt-Sil and Spl-Crd enclaves are strongly depleted in Si and enriched in Al and Fe with respect to common metapelites such as those exposed in the adjacent Betic Cordillera. They have been interpreted as the residue after extraction of 35-60 wt% of rhyolitic glass from these metapelites. Available geothermometric determinations on the enclaves of El Hoyazo indicate equilibration at 850 ± 50 °C and 5.5-7 kbar with a possible further heating event at >900 °; ages obtained from zircon and monazite of the enclaves and their host dacite indicate that anatexis took place at c. 9.7 Ma, while the eruption has been dated at c. 6.3 Ma.

One important aspect of the crustal enclaves of El Hoyazo is that they show evidence of having been intensely deformed during partial melting and melt extraction. In most cases, the foliation appears to be syn-anatectic. Most garnet occurs in these rocks as equidimensional, euhedral crystals. However, in a few Spl-Crd enclaves, garnet grains display elongated and elliptical shapes within layers with a pronounced foliation.

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Fig. 5: Dacite with red garnet crystals and black metapelitic xenolith.



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Fig. 6: Dacite with black garnet rich metapelitic xenolith.



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Fig. 7: Dacite with red garnet crystals.



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Fig.8: Metapelitic xenolith made of Plagioclase-cordierite-garnet-biotite-sillimanite.



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Fig.9: Garnet rich sand in the "Rambla de las Granatillas" placer deposit.



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Fig.10: Garnet rich sand in the "Rambla de las Granatillas" placer deposit.





Bibliography



• Alvarez-Valero, A.M., Cesare, B., & Kriegsman, L. M. (2005). Formation of elliptical garnet in a metapelitic enclave by melt assisted dissolution and reprecipitation. Journal of Metamorphic Geology, 23(2), 65-74.
• Alvarez-Valero, A. M., & Kriegsman, L. M. (2007). Crustal thinning and mafic underplating beneath the Neogene Volcanic Province (Betic Cordillera, SE Spain): evidence from crustal xenoliths. Terra Nova, 19(4), 266-271.
• Alvarez-Valero, A. M., & Waters, D. J. (2010). Partially melted crustal xenoliths as a window into sub-volcanic processes: evidence from the Neogene Magmatic Province of the Betic Cordillera, SE Spain. Journal of Petrology.
• Jose Miguel Alonso., Jose Maria Calaforra., J. Gisbert., Angela Vallejos (2015): EL HOYAZO DE NÍJAR: ARRECIFES Y VOLCANES. Guía Geológica de Campo. Editor: Sociedad Geológica de Espana.


Garnet crystal surrounded by a groundmass containing biotite (brown), quartz and feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
Photo
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Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
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Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
niar(3).jpg

Garnet crystal (isotropic) surrounded by a groundmass containing biotite, quartz, feldspar (I order gray) and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 2x (Field of view = 7mm)
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Garnet and cordierite crystals surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
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Garnet and cordierite (twinned) crystals surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 2x (Field of view = 7mm)
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Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
niar(8).jpg

Garnet and cordierite (colorless) crystal surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
niar(9).jpg

Garnet and cordierite (twinned) crystal surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 2x (Field of view = 7mm)
niar(10).jpg

Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
niar(11).jpg

Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
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Garnet crystal surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 2x (Field of view = 7mm)
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Biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
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Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 2x (Field of view = 7mm)
niar(15).jpg

Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
niar(16).jpg

Garnet crystal surrounded by a groundmass containing biotite (brown), quartz (resorbed), feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
niar(17).jpg

Garnet crystal (isotropic) surrounded by a groundmass containing biotite, quartz (resorbed), feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
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Cordierite crystal (colorless) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
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Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
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Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL (with Compensator plate) image, 10x (Field of view = 2mm)
niar(21).jpg

Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
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Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
niar(23).jpg

Cordierite crystal (colorless) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
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Cordierite crystal (twinned) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
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Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
niar(26).jpg

Garnet crystal surrounded by a groundmass containing biotite (brown), quartz, feldspar (colorless) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
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Biotite (brown), quartz (resorbed), feldspar (with sieve texture) and glass. Dacite from El Hoyazo, Niar, Spain. PPL image, 10x (Field of view = 2mm)
niar(27).jpg

Cordierite crystal (with cyclic twinning) surrounded by a groundmass containing biotite, quartz, feldspar and glass. Dacite from El Hoyazo, Niar, Spain. XPL image, 10x (Field of view = 2mm)
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Cordierite crystal (with cyclic twinning). Dacite from El Hoyazo, Niar, Spain. XPL image, 20x (Field of view = 1mm)
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Cordierite crystal (with cyclic twinning). Dacite from El Hoyazo, Niar, Spain. XPL (with Compensator plate) image, 20x (Field of view = 1mm)
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Cordierite crystal (with cyclic twinning). Dacite from El Hoyazo, Niar, Spain. XPL image, 20x (Field of view = 1mm)
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Sillimanite-rich xenolith. PPL image, 10x (Field of view = 2mm)
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Sillimanite-rich xenolith. PPL image, 10x (Field of view = 2mm)
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Cordierite-spinel (green) xenolith. PPL image, 10x (Field of view = 2mm)
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Cordierite-spinel xenolith. XPL image, 10x (Field of view = 2mm)
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Cordierite-spinel (green) xenolith. PPL image, 20x (Field of view = 1mm)
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Cordierite-spinel (green) xenolith. PPL image, 20x (Field of view = 1mm)