Convergent strategy to date metamorphic minerals in sub-greenschist facies metabasites by the 40Ar/39Ar method

Quantifying the time interval between deposition of volcanic materials in a subsiding basin and the generation of very low-grade metamorphic assemblages in those same materials due to burial is a scarcely explored subject (see erg et al., 1984; erg, 1985; Aguirre et al. 1999). This is mainly due to the reduced number of mineral phases compositionally adequate to apply methods such as the 40Ar/39Ar and K-Ar, in addition with the small amount in which these phases are commonly found. Among them, adularia, sericitic micas, and celadonites are the best known examples of K-bearing secondary minerals present in metabasites in sub-greenschist facies. In the Coastal Range of central Chile, the Early Cretaceous volcanism is typically represented by the Veta Negra Formation. This unit is mainly composed by a pile of c. 10.000 m thick of basaltic andesites and andesites (Levi, 1969; Levi et al. 1982; Vergara et al., 1995), with a porphyrytic texture characterized by abundant and large (up to 2 cm) plagioclase (An56Ab40Or4 to An70Ab28Or2) crystals, subordinate augite (Wo38En45Fs17), titanomagnetite, and small amounts of totally altered olivine. The flows of the Veta Negra Formation were deposited in a subsiding intra-arc or back-arc basin ( erg et al. 1984; Vergara et al. 1995) and affected by very low-grade metamorphism in the prehnite-pumpellyite facies. The metamorphic minerals are found in different habitats (metadomains) among which: (i) partially or totally replaced primary minerals; (ii) altered glassy material from the groundmass; (iii) amygdale filling; (iv) veinlets and microfractures. Secondary (metamorphic) feldspars appear: (1) replacing primary phenocrysts and microliths of plagioclase and, (2) filling open spaces, e.g. amygdales and veinlets. In (1) they correspond to pure albite resulting from albitization processes and to K-feldspar (adularia) as patches inside totally albitized plagioclase phenocrysts; in this last case sericite flakes accompany adularia. In (2), adularia (Or97Ab3) is abundantly present filling amygdales in low-variance assemblages together with pumpellyite, chlorite, low albite and scarce sericite (Morata et al., 1997). The time elapsed between volcanism and metamorphism has been recently investigated in flows of the Veta Negra Formation at the Bustamante Hill area, c. 25 km west from Santiago (Aguirre et al., 1999). A plateau age of 119.4 ±1.2 Ma was obtained from transparent crystals of primary Ca-plagioclase from a lava and is thougt to represent the best estimation of the age of the volcanism. Crystals of adularia from amygdales in two different flows displayed flat and concordant age spectra and a plateau age at 93.1±0.3 Ma which is taken as the closest approximation to the thermodynamic equilibrium reached during this metamorphic event. Two partially sericitized, milky-white plagioclase single crystals from two flows, displayed low apparent ages at medium temperatures, intermediate between those of fresh transparent Ca-plagioclase and adularia. A clear relationship between apparent ages and the 37ArCa/39ArK ratio shows that the low-intermediate temperature apparent ages are the result of a mixture of pure plagioclase and younger sericite and/or K-feldspar as observed in the plagioclase under the microscope. The results above suggested a convergent methodological approach to the age determination of the metamorphic events in these type of rocks which consists in dating adularia and sericite co-existing as part of the same mineral parageneses. To test this approach 40Ar/39Ar analyses were carried up in samples from basic flows at the Cord de Chacana area, some 80 km north from the Bustamante Hill area, belonging to the same Veta Negra Formation. Four samples, CHAC8, CHAC18, CHAC19 and CHAC22, in stratigraphic ascending order, were taken along a c. 2000 m profile of the Cord de Chacana. CHAC18 and CHAC19 correspond to different levels of the same flow. CHAC18 and CHAC22 are separated by about 60 m in the stratigraphic pile whereas the stratigraphic distance between CHAC8 and CHAC19 is of about 900 to 1000 m (au)