Geologic and Petrographic Features of the Jaboncillo Phonolite, Sierra de Valle Fértil, Province of San Juan, NW Argentina

GÓMEZ, RICARDO; CASTRO DE MACHUCA, BRÍGIDA ; LOPEZ, MARIA GIMENA; PALACIO BALDERRAMO, GLADIS

São Pablo

Simposio; VI Simposio de Vulcanismo e Ambientes Associados.; 2015

Universidad de São Pablo

Alkaline volcanism associated with Triassic extensional regime in the Sierra de Valle Fértil, Western Sierras Pampeanas, is little known and the only existing geological reference is due to Mirré (1976), who recognized a non-orogenic alkaline continental association of alleged Triassic age composed by alkali-olivine basalts - basaltic trachyandesites - phonolites - trachytes - alkali rhyolites, overlying or intruding the lower Paleozoic crystalline basement. The Jaboncillo Phonolite (30 °41´27.24" S - 67 °39´36.84" W) is located in the central sector of the hill and its outcrops, subcircular in plan, cover a restricted area of about 40 m2. It is a dome-shaped body of phonolite within a confining wall of volcanic breccia which has intruded the crystalline basement with sharp contacts. Two distinct sections are clearly distinguished. The lower section, approximately 50 m thick, is constituted by a polymictic matrix-supported volcanic breccia, with angular to subangular fragments of heterogeneous size (0.5 cm - 2 m). Clasts of the volcanic breccia are from the host rocks (mainly metagabbros), and also juvenile (cognate) fragments, immersed in a fine-grained olive-gray phonolitic matrix. The upper section, about 20 m thick, corresponds to a steep-sided approximately cylindrical body of phonolite. The phonolite is pale greyish-brown in color and fine-grained; it presents seriate porphyritic texture. Phenocrysts (0.1 - 1.5 mm) of feldspar (ranging from sodic sanidine to anorthoclase) are euhedral and show a combination of Carlsbad and tartan twinning. Some phenocrysts exhibit microperthitic intergrowths and/or fish-tail and chevron-shaped twins. Microphenocrysts (0.05 - 0.1 mm) are composed largely of alkali feldspar (mostly sanidine) and Na-pyroxene (aegirine). Pyroxene is euhedral, dark- green in color and strongly pleochroic; it normally shows hour-glass sector zoning. The groundmass consists of aligned laths of alkali feldspar up to 0.05 mm in length, pyroxene, and equant nepheline crystals. Accessory minerals are apatite, zircon and opaques. Abundant zeolites (mostly analcime) are the main secondary products. Based on field and petrographic observations, the Jaboncillo Phonolite may be interpreted as a shallow level intrusion (volcanic plug) that has been revealed by erosion of a larger volcanic structure. The lower section could represent a first violent eruptive stage with formation of the explosive breccia, closely followed by a stage of dome-building by infilling of phonolite magma which cemented the breccia and sealed the volcanic vent. The geochemistry of the phonolite confirms its petrographic classification and its alkali character with average Na2O + K2O = 9.16, and points to a genesis related to intraplate magmatism (continental rifting). These features reaffirm the relationship between the Jaboncillo Phonolite and the extensional event that took place during the Triassic in the Western Sierras Pampeanas region. The Jaboncillo Phonolite would represent the last stages of the alkaline magmatism related to this tecto-magmatic setting.