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The tectonic setting of Northwestern South America is very complex due to interaction between the Nazca, Caribbean, and South America plates and several oceanic terranes. Of particular interest is the Nazca plate slab geometry change since the Mid-Miocene, and its tectonomagmatic effects on the overlying South American Plate. Several studies indicate that the modern Nazca slab is torn into two segments at 5.5oN in Colombia, the so-called "Caldas tear", where the northern segment dips at a shallow angle while the southern segment dips at a steep angle. This slab geometry difference is manifested in magmatic activity where only the region with steep slab shows typical subduction zone volcanism, while the one with flat slab lacks such activity. However, due to the limited number of geochronological data, the timing of this slab shallowing and tearing remains poorly understood. We conducted an extensive 40Ar/39Ar geochronology study on post mid-Miocene volcanic rocks in Colombia to study the temporal evolution of Nazca plate geometry and its effects on magmatism in Colombia. We find evidence of continuous magmatism north and south of the Caldas tear from 10.5 Ma to 6.4 Ma, with peak activity between 9-8 Ma. This is followed by a ~4 million year magmatic hiatus, until the resurgence of magmatism south of the Caldas tear as monogenetic domes at 2.1 Ma, and continuous volcanic activity in modern composite volcanoes since 1.1 Ma. Results of this study support the presence of a complex subduction system beneath Colombia where the northern segment slab has been flat since ~6.4 Ma ago, and the southern segment has re-steepened at ~2.1 Ma. This study showcases that the Nazca slab geometry was the most important factor driving magmatism in Colombia since the mid-Miocene. 

The tectonic setting of Northwestern South America is very complex due to interaction between the Nazca, Caribbean, and South America plates and several oceanic terranes. Of particular interest is the Nazca plate slab geometry change since the Mid-Miocene, and its tectonomagmatic effects on the overlying South American Plate. Several studies indicate that the modern Nazca slab is torn into two segments at 5.5oN in Colombia, the so-called "Caldas tear", where the northern segment dips at a shallow angle while the southern segment dips at a steep angle. This slab geometry difference is manifested in magmatic activity where only the region with steep slab shows typical subduction zone volcanism, while the one with flat slab lacks such activity. However, due to the limited number of geochronological data, the timing of this slab shallowing and tearing remains poorly understood. We conducted an extensive 40Ar/39Ar geochronology study on post mid-Miocene volcanic rocks in Colombia to study the temporal evolution of Nazca plate geometry and its effects on magmatism in Colombia. We find evidence of continuous magmatism north and south of the Caldas tear from 10.5 Ma to 6.4 Ma, with peak activity between 9-8 Ma. This is followed by a ~4 million year magmatic hiatus, until the resurgence of magmatism south of the Caldas tear as monogenetic domes at 2.1 Ma, and continuous volcanic activity in modern composite volcanoes since 1.1 Ma. Results of this study support the presence of a complex subduction system beneath Colombia where the northern segment slab has been flat since ~6.4 Ma ago, and the southern segment has re-steepened at ~2.1 Ma. This study showcases that the Nazca slab geometry was the most important factor driving magmatism in Colombia since the mid- Miocene. 

Abstract Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes.