Ages and cooling history of the Early Cretaceous Caleu pluton: testimony of a switch from a rifted to a compressional continental margin in central Chile

Journal of the Geological Society, Mar 2005 by Parada, Miguel A, Féraud, Gilbert, Fuentes, Francisco, Aguirre, Luis, Et al

The Caleu pluton is emplaced into a thick Early Cretaceous volcanic-sedimentary arc succession deposited in a subsiding basin (Fig. 2; Vergara et al. 1995). This succession includes the Ocoite Group (Aguirre et al. 1989) and the Las Chilcas Formation (Thomas 1958). At the latitude of Santiago (33°S) the Ocoite Group is represented by a c. 15 km thick succession, tilted to a dip of 30-40°E. Its lower third (the Valanginian to Hauterivian Lo Prado Formation) consists of marine and continental volcanic sedimentary rocks, limestones, and a bimodal succession of dacitic ignimbrites and interbedded basalts. The central part of the group is the c. 5-10 km thick (see Vergara et al. 1995) Hauterivian to Barremian Veta Negra Formation, which is made up of subaerial porphyritic basalts and basaltic andesites with high-K to shoshonitic affinity (Levi & Aguirre 1981; Aberg et al. 1984; Levi et al. 1988; Vergara et al. 1995). The uppermost part consists of continental flow-breccias of basaltic andesite to andesite composition and sedimentary clastic intercalations. The Veta Negra Formation has been affected by very low- to low-grade burial metamorphism in an extensional geodynamic setting (Aguirre et al. 1999).

^sup 40^Ar/^sup 39^Ar ages of about 119 ± 2.4 Ma have been obtained on primary plagioclase in basaltic flows of the Veta Negra Formation at the Bustamante Hill section, 35 km south of the Caleu pluton (Aguirre et al. 1999). In the vicinity of the Caleu pluton, the Las Chilcas Formation consists of a succession of limestones, red sandstones, andesites, rhyolitic tuff intercalations and thick strata of coarse volcano-sedimentary breccias and conglomerates, which conformably overlies the Veta Negra Formation. Unlike that formation, the Las Chilcas Formation does not exhibit a widespread metamorphic mineralogy attributable to very low-grade metamorphism, although the coarse conglomerates contain abundant very low-grade metavolcanic rock clasts. A mid-Albian (c. 105 Ma) age has been recorded from marine planktonic microfossils found in limestones at the lower levels of the Las Chilcas Formation (Martinez-Pardo et al. 1994). Recently, Wall et al. (1999) have obtained zircon U-Pb ages in felsic volcanic rocks of the lower section of the Las Chilcas Formation in the 109.6 ± 0.2 to 106.5 ± 0.4 Ma range. These ages are older than the corresponding whole-rock K-Ar ages of 101 ± 3 and 100 ± 3 Ma (near the Bustamante Hill section) and plagioclase K-Ar age of 95 ± 3 Ma (near the Caleu pluton) obtained in lavas of the upper stratigraphie levels of the Las Chilcas Formation (Wall et al. 1999). A biotite K-Ar age of 91 ± 4 Ma has been obtained for the Caleu pluton (Gana et al. 1996) indicating that it represents a magmatic event contemporaneous with the deposition of the upper section of the Las Chilcas Formation.

Geology of the Caleu pluton

The Caleu pluton, located in the Coastal Range of central Chile c. 40 km NW of Santiago, has a subrectangular shape (Fig. 3), good exposures and >1400 m of vertical relief. The western margin of the pluton is the contact with the Early Cretaceous La Campana gabbro stock and with felsic volcanic rocks of the Valanginian-Hauterivian Lo Prado Formation. The southern and northern margins of the pluton intrude the Veta Negra Formation, whereas its eastern margin is well defined only along a nearly north-south fault contact with rocks of the lower section of the Las Chilcas Formation. Pressures of emplacement of the pluton of c. 2.0 kbar were calculated using the Al-in-hornblende geobarometer in more than 10 representative samples containing quartz, plagioclase, K-feldspar, hornblende, biotite, sphene and magnetite (Parada et al. 2002).