Achala Batholith

The Achala Batholith (Spanish: batolito de Achala) is a group of plutons in the Sierras de Córdoba in central Argentina. With a mapped surface of over 2500 km2 it constitutes the largest group of intrusions exposed in the Sierras Pampeanas.[1][2] The oldest reference to the batholith dates to 1932.[1]

Host rock and structural setting

Plutons intrude into older migmatites, metamorphic rocks of sedimentary and volcanic protoliths.[1][3] Some specific intruded rock types are: biotite-bearing tonalitic gneisses, amphibolite, marble and quartzite.[1]

In the large-scale the intrusion is parallel to the schistosity of the older rocks. However, at smaller scales the intrusion seems to disregard schistisity.[3] Host rocks are altered contact metamorphism and associated fluids. Alteration is seen in the occurrence of marble and minerals such as vesuvianite and humite, both of which are high on fluorine. This alteration is thought to also have affected the batholith itself.[4]

Lithology and alteration

Most rocks of the batholith are monzogranites[2] but granodiorites and tonalites do also occur.[4] The central parts of the batholith are fully grantic and include leucogranite.[1] Grain size vary from coarse to fine and at places the rocks are porphyritic. Dykes of lamprophyre and nephelinite that occur in the region are associated to the batholith.[1]

Pegmatites and aplites, albeit not voluminous, are recurrent in the batholith.[4] The pegmatite of Las Tapias in the southwestern part of the batholith make up Argentina's "most important" beryllium deposit. Other pegmatites of the batholith have been mined for quartz, feldspar and beryl plus lesser amounts of columbite and tantalite.[1] More important deposits is the tungsten that can be obtained from skarns with scheelite and wolframite-bearing quartz veins. Skarns are associated to marble and amphibolite host rock.[1]

The whole batholith has been subject to deuteric alteration that replaced rock's biotite with muscovite and in general depleted biotite in iron, magnesium and titanium.[4] Plagioclases have been affected to a much lesser degree by alteration having have slight compositional changes along the crystal rims or limited transformation into muscovite. This alteration has also led to overall loss of alcalis (potassium, sodium) from the rock.[4] A number of fractures of the batholith show greisen alteration with quartz, sericite, fluorite and tourmaline. No metallic deposits are known from these greisens.[5]

The Achala Batholit contains unusual enclaves of biotite and apatite that form layers in the intrusion.[6]

Origins of magmas

The batholith plutons intruded and cooled 370 million years ago in the Devonian Period.[7][2]

Geochemical characteristics indicate the granites are A-type granites and peralumineous (alumineous A-type).[2] These characteristics are interpreted to reflect a mixed origin for the magmas with sources both in the mantle and in the crust. Crustal sources would have contributed to the magmas by melting under anhydrous conditions, with such conditions being allowed by a large undetermined heat source.[2] The magma formed and cooled in the aftermath of an orogeny and qualify thus the group of post-orogenic intrusions.[1]

Four igneous suites of rocks with geochemical affinities make up the batholith, these area the Achala, El Condor, Champaqui, Characato and Cumbresita suites. The Achala suite has by far the largest extent making up c. 70% of the surface area of the batholith.[4] The five suites represents different magmatic episodes.[4] Suites differ in associated metals as well as biotite chemistry.[4] Chemical variations along each suite are concordant with the fractional crystallization model of igneous differentiation.[4]

References
  1. Lira, Raúl; Kirschbaum, Alicia M. (1990). "Geochemical evolution of granites". In Mahlburg Kay, Suzanne; Rapela, Carlos W. (eds.). Plutonism from Antarctica to Alaska. Geological Society of America Special Paper. 241. pp. 67–76.
  2. Rapela, C.W.; Baldo, E.G.; Pankhurst, R.J.; Fanning, C.M. (2008). The Devonian achala batholith of the Sierras Pampeanas: F-rich alumineous A-type granites. VI South American Symposium on Isotope Geology. San Carlos de Bariloche, Argentina.
  3. de Patiño, Marta G.; Patiño Douce, Alberto E. (1987). "Petrología y petrogénesis de batolito de Achala, provincia de Córdoba, a la luz de la evidencia de campo". Revista de la Asociación Geológica Argentina (in Spanish). XLII (1–2): 201–205.
  4. Demange, Michel; Alveres, Juan O.; Lopez, Luiz; Zarco, Juan J. (1996). "The Achala Batholith (Cordoba, Argentina): a composite intrusion made of five independent magmatic suites. Magmatic evolution and deuteric alteration". Journal of South American Earth Sciences. 9 (1/2): 11–25. Bibcode:1996JSAES...9...11D. doi:10.1016/0895-9811(96)00024-7.
  5. Lira, Raul; Ripley, Edward M.; Españón, Adriana I. (1996). "Meteoric water induced selvage-style greisen alteration in the Achala Batholith, central Argentina". Chemical Geology. 133 (1–4): 261–277. Bibcode:1996ChGeo.133..261L. doi:10.1016/S0009-2541(96)00077-0.
  6. Dorais, Michel J.; Lira, Raul; Chen, Yadong; Tingey, David (1997). "Origin of biotite-apatite-rich enclaves, Achala batholith, Argentina". Contributions to Mineralogy and Petrology. 140 (1): 31–46. Bibcode:1997CoMP..130...31D. doi:10.1007/s004100050347. S2CID 129621632.
  7. Dahlquist, Juan A.; Alasino, Pablo H.; Bello, Carina (2014). "Devonian F-rich peraluminous A-type magmatism in the proto-Andean foreland (Sierras Pampeanas, Argentina): geochemical constraints and petrogenesis from the western-central region of the Achala batholith". Mineralogy and Petrology. 108 (3): 391–417. Bibcode:2014MinPe.108..391D. doi:10.1007/s00710-013-0308-0. S2CID 128877732.

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