The geochemistry and geochronology of Early Jurassic igneous rocks from the Sierra Nevada de Santa Marta, NW Colombia, and tectono-magmatic implications
The Sierra Nevada de Santa Marta in NW Colombia is an isolated massif at the northernmost end of the Andes chain near the boundary with the Caribbean plate. Previous geologic mapping and K-Ar dating have shown that Jurassic plutonic and volcanic units make up a large part of the Santa Marta Massif (SMM). These rocks have been considered to be part of a Jurassic magmatic arc extending from NW Colombia to northern Chile, but without any geochemical basis for comparison. This paper reports on a geochemical and Sr-Nd-Pb isotope study of the Jurassic rocks in the SMM and provides 12 new U-Pb zircon ages from in-situ laser ICP-MS dating. The plutonic and volcanic units span a range from 45 to 78 wt.% SiO2, with a dominance of intermediate to felsic compositions with SiO2>57 wt.%. They classify as calc-alkaline, medium to high-K, metaluminous rocks with trace-element features typical for arc-derived magma series. In terms of their major and trace-element compositions, the SMM Jurassic units overlap with contemporary plutonic and volcanic rocks from other regions of the Central and Eastern Cordilleras of Colombia, and confirm an arc affinity. The new U-Pb ages range from 176 ± 1 Ma to 192 ± 2 Ma (n=12), with most between 180 and 188 Ma (n=7). The initial Sr isotope ratios (at 180 Ma) are between 0.7012 and 0.7071 (n=29), with 3 outliers attributed to mobilization of Rb and/or Sr. Nearly all samples have negative εNd(180) values between −10.3 and 0.0 (n=30), the two exceptions being only slightly positive (1.1 and 1.9). Measured Pb isotope ratios fall in a narrow range, with 206Pb/204Pb from 18.02 to 19.95, 207Pb/204Pb from 15.56 to 15.67 and 208Pb/204Pb from 37.76 to 39.04 (n=28). In the regional context of previous studies, these results confirm early Jurassic ages and an arc affinity for the widespread magmatism exposed in the eastern and northeastern Colombian Andes. We also note patterns in the distribution and composition of magmas. The magmatic activity in the Central Cordillera tends to be younger than in the Eastern Cordillera and is spatially more restricted to the vicinity of regional fault systems. In terms of composition, Jurassic igneous rocks in the Eastern Cordillera have systematically lower εNd(180) values than those from the Central Cordillera, whereas the Pb isotope ratios overlap. We ascribe the Nd isotope variations to heterogeneity in the mantle source and/or degree of crustal contamination, whereas the Pb isotope ratios are crust-dominated and similar throughout the region. The spatio-temporal and compositional evolution of Jurassic magmatic rocks in the Northern Andes reflect the major plate kinematic eadjustment between the Triassic and the Early Jurassic in the proto-Andean margin.