ADAKITE-LIKE SIGNATURE IN THE LOWER MEMBER OF THE COMBIA FORMATION (LATE MIOCENE) AT THE SOUTHERN OF AMAGÁ SUB-BASIN, NORTHWEST COLOMBIA

Abstract

In this study, we report chemistry of samples of the volcaniclastic deposits from the Lower Member of the Combia Formation and the intruding sub-volcanic bodies. These rocks are located at the southern of Amagá sub-basin of the Cauca River, Colombia. Most of the samples correspond to a medium-K calc-alkaline andesitic-dacitic suite (SiO2 =59, 04 -62,86 wt. %), and only one is a toleiitic basalt SiO2 =50,6 wt%. The samples have high LILE/ HFSE, Ba/LREE, Sr/Nd ratios and Nb and Ta relative depletion, typical of magmas in a subduction system. The REE patterns show that two samples have high MREE/HREE and LREE/HREE ratios (Gd/Yb >3,9 and La/Yb > 19,7) in comparison with the rest of the samples. Most of the samples have an adakite-like signature characterized by SiO2 >56 wt%, Al2 O3 >15 wt%, MgO < 3 wt%, Na2 O between 3,5 to 7,5 wt%, K2 O/ Na2 O ~ 0.42, high HREE fractionation, lack of Eu anomalies , Yb ≤1,8 ppm, Y < 18 ppm, high Sr contents (>400 ppm), Sr/Y > 20 and La/ Yb > 20. Variations of the radiogenic isotopes from four sub- volcanic bodies are small and their values: 87Sr/86Sr = 0,704201 - 0,704293; 143Nd/144Nd= 0,512849 - 0,512907 are indicative of melts from a mantle-derived source, although an interaction with slab melts is not discarded, and the adakitic magmas were generated at the base of the crust. The volcanic sequence described in this study is a segment of the Miocene magmatic arc extending along the northern Andes. According to our results, in the Lower Member of the Combia Formation, at the southernmost Amagá sub-basin sector, it must be included the intruding sub-volcanic bodies as an adakitic suite.