BASALTIC ROCKS FROM THE ANDEAN SOUTHERN VOLCANIC ZONE: COMPARISON OF SYSTEMATIC ALONG-STRIKE AND SMALL SCALE GEOCHEMICAL VARIATIONS AND THEIR ORIGINS

The Andean Southern Volcanic Zone (SVZ) has many well-established systematic along-strike, across-strike, and small-scale variations in the geochemistry of volcanic rocks, and work by numerous researchers over the past 40 years has addressed the origin of those variations. Along-strike variations may be linked to the greater thickness and/or age of the subarc continental crust and mantle-lithosphere in the Northern SVZ (NSVZ; 33^oS-34^o30'S) and Transitional SVZ (TSVZ; 34^o30'S-37^oS) compared with the Central SVZ (CSVZ; 37^oS-42^oS) and/or to along-strike changes in the age and character of the subducting Nazca plate. For basaltic rocks, some outstanding along-strike geochemical trends that have been noted are: 1) higher ⁸⁷Sr/⁸⁶Sr, and lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf at volcanic centers in the NSVZ and TSVZ compared with the CSVZ; 2) higher La/Yb at volcanoes in the NSVZ and TSVZ compared with the CSVZ; 3) higher values for time-sensitive subduction tracers such as ¹⁰Be/⁹Be and ²³⁸U/²³⁰Th at volcanic centers in the CSVZ compared with the TSVZ and NSVZ; and 4) a lack of correlation between these geochemical and location parameters and trace element ratios considered to be typical subduction-zone signatures, such as high Th/La, Ba/La and low La/Nb compared with MORBs. Within the CSVZ, these same geochemical patterns are repeated on a small scale when the geochemistry of basalts from major stratovolcanoes and nearby small eruptive centers located along the Liquine-Ofqui Fault Zone are compared. Basalts from small eruptive centers have relatively higher La/Yb, lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf compared with those from stratovolcanoes. ¹⁰Be/⁹Be is lower and ²³⁸U/²³⁰Th closer to secular equilibrium, and La/Nb and Th/La and other subduction zone tracers are not correlated. Basalts from the small eruptive centers diverge from the along-strike pattern in having slightly lower rather than higher ⁸⁷Sr/⁸⁶Sr compared with stratovolcanoes. Existing and new models for along-strike changes in the SVZ have been evaluated as possible explanations for the similar geochemical variations between fault-controlled small eruptive centers and stratovolcanoes in the CSVZ. Possible overarching models for these geochemical variations in basaltic volcanic rocks of the SVZ will be presented.