The quaternary volcanic belt of the southern continental margin of South America: Transverse structural and petrochemical variations across the segment between 38°S and 39°S

Abstract

In the segment between 38°S and 39°S of the southern South American continental margin volcanic belt, the volcanoes of the Quaternary “orogenic arc” may be divided into two groups: the first forming the volcanic front, which is located within the Cordillera Principal along a trend oriented 15° east of north; the second east of the volcanic front, occurring upon an uplifted precordilleran block that is east of the Bío-Bío/Aluminé fault system, trends west of north, and merges with the Cordillera Principal near 38°S. Quaternary “back-arc” plateau lavas, consisting of basalts erupted from small cones, occur in the valley east of this precordilleran block. Quaternary intra-arc and back-arc extension appear to be occurring in this section (38–39°S) of the southern South American volcanic belt and perhaps as far north as 34°S. South of 39°S, intra-arc extension is absent and the orogenic arc is restricted to the Cordillera Principal. Differences in the structural characteristics of the arc north and south of 39°S may be due to the change in age from older to younger oceanic lithosphere being subducted beneath, respectively, the two regions. Basalts erupted from both Quaternary arc and back-arc centers located east of the volcanic front between 38°S and 39°S have higher K 20, Rb, Ba, and LREE contents, and higher La/Yb but lower Ba/La ratios, than basalts erupted along the volcanic front. 87Sr/ 86Sr ratios for arc lavas from volcanic centers both within and east of the volcanic front range from 0.7038 to 0.7041 independent of their Si0 2 content, suggesting that more silicic compositions developed from basalts by crystal-liquid fractionation without significant crustal assimilation. Back-arc plateau basalts from the region 38–39°S have Sr isotopic compositions similar to those of the arc volcanoes. Basalts from both the arc and the back-arc centers between 38°S and 39°S may form by partial melting of a large-ion-lithophile-element-enriched mantle source similar to the source of mantle-xenolith-bearing Quaternary alkali plateau basalts from southern Patagonia (south of 39°S), further modified by the addition of alkali elements derived from subducted oceanic lithosphere.