Petrography and U-Pb Zircon Geochronology of Geological Units of the Mesa de Cocodite, Península de Paraguaná, Venezuela

David Mendi ¹, Marvin Baquero ², Elson P. Oliveira 3, Franco Urbani ^{1, 5}, Jesús Pinto ⁴, Sebastián Grande ¹, Víctor Valencia ⁶

¹UCV, Escuela de Geología, Minas y Geofísica, Caracas, Venezuela

²PDVSA-INTEVEP, Urb. Santa Rosa, El Tambor, Los Teques 1201, Miranda, Venezuela

³Universidad Estadual de Campinas, Barao Geraldo Campinas, Sao Paulo, Brasil

⁴ENAP-Magallanes, José Nogueira 1101, Punta Arenas 6200988, Chile

⁵FUNVISIS, Calle Mara, El Llanito, Caracas, Venezuela

⁶WSU, School of Earth and Environmental Sciences, Pullman, USA

Several continental crust units crop out in The Mesa de Cocodite, central Paraguaná Peninsula, Northwestern Venezuela, including a newly mapped quartz-feldspar gneiss that intruded (or is intruded by?) the El Amparo Pluton, a major low-graded metamorphic unit of Permian age. It is unconformably overlying by Late Jurassic phyllites of the Pueblo Nuevo Formation. All these units are cross-cut by narrow dykes. This contribution focuses on the petrography and LA-ICP-MS U-Pb zircon dating of the igneous units, with the aim of constraining magmatism and its tectonic significance in the area.

The quartz-feldspar gneiss consists of albite (average 57%), bluish and smoked quartz (40%), muscovite-chlorite (2%), epidote (1%) and zircon (1%). The enclosing El Amparo Pluton is typically a coarse-grained and thick-banded metagranodiorite, containing andesine (35%), quartz (28%), hornblende (12%), epidote (10%), K-feldspar (7%), biotite (6%), chlorite (1%), titanite (1%) and zircon (1%). The younger dykes consist of fine-grained, porphyritic hornblende diorites that contain numerous hornblende phenocrysts. The main minerals are andesine (65%), hornblende (25%), quartz (5%), and chlorite (5%). Accessory minerals include zircon, epidote and opaques. A felsic gneiss xenolith collected from the dykes showed a modal composition of quartz (50%), K-feldspar (27%), chlorite (15%), epidote (5%), albite (3%) and zircon.

Zircon population in the quartz-feldspar gneiss displays a discordia trend, ranging in age from 1050-750 Ma but mostly around 950-900 Ma, which may represent a peak high-grade metamorphism in the area. The El Amparo Pluton provides a concordant Permian age (271.3±6.5 Ma), which is comparable with previous reported U-Pb ages in titanite. Because the porphyry dykes cross-cut all the units in the area, they should be post-Late Jurassic in Age; however, all dated zircons from the dykes are in the range of 1200-750 Ma. The absence of younger ages can be attributed to either formation during a relatively low temperature magmatic event, which generated very narrow younger zircon rims only and thus undetectable with the technique used, or a completely lack of newly generated outer zones due to quick cooling. The emplacement age for the dykes may be early Cenozoic, as for similar basement rocks from the Gulf of Venezuela and the Guajira Peninsula.

A felsic-gneiss xenolith enclosed in the dykes contains inherited Proterozoic zircons (1200-950 Ma) with rims of Permian age (270.2±4.1 Ma). Thus, the inherited older ages detected in the feldspar-gneiss unit, the later dykes, and the xenolith indicate the occurrence of Grenvillian crust in Northwestern South America. The Permian ages obtained in the Paraguaná Peninsula are similar to ages reported for the Mérida Andes, Santa Marta Massif, Perijá Range, Orchila Island, northern part of the Maracaibo Basin (Toas Island), northwestern Falcon Basin. Therefore, the Permian magmatic event may have been related to an active margin during the construction of Pangaea, caused by the subduction of oceanic crust under the northwestern corner of South America.