The Central Andean margin, where the name andesite originated, is the type locality for arc andesites erupted through thick continental crust. The <25 Ma mid to high K2O andesites erupted from 25.5°S to 28.2°S exhibit a large variation in trace element and isotopic ratios, reflecting formation over an evolving slab, a crust thickening to 65–75 km and a frontal arc that migrated c. 45 km eastward at 8–3 Ma. Andesites at 28–26.8°S have the most variable and extreme heavy rare earth element (REE), high field strength element (HFSE) and Ba/La ratios and wt% Na2O, with the highest values in those erupted as the frontal arc migrated and the slab shallowed to the south. The required garnet-bearing, feldspar-free residue is generated in both the thick crust and the mantle wedge, into which crust was injected in a peak of forearc subduction erosion as the arc migrated. Andesites at 25.5–26.8°S, east of the Puna plateau under which the slab shallowed at 18–7 Ma and then steepened as lithospheric delamination occurred, generally lack extreme REE and HFSE ratios. Their upper crust-like features reflect eruption in a mixed stress regime and incorporation of westward-flowing radiogenic crust from a region of extensive deep crustal melting to the east. AbstractThe Central Andean margin, where the name andesite originated, is the type locality for arc andesites erupted through thick continental crust. The <25 Ma mid to high K2O andesites erupted from 25.5°S to 28.2°S exhibit a large variation in trace element and isotopic ratios, reflecting formation over an evolving slab, a crust thickening to 65–75 km and a frontal arc that migrated c. 45 km eastward at 8–3 Ma. Andesites at 28–26.8°S have the most variable and extreme heavy rare earth element (REE), high field strength element (HFSE) and Ba/La ratios and wt% Na2O, with the highest values in those erupted as the frontal arc migrated and the slab shallowed to the south. The required garnet-bearing, feldspar-free residue is generated in both the thick crust and the mantle wedge, into which crust was injected in a peak of forearc subduction erosion as the arc migrated. Andesites at 25.5–26.8°S, east of the Puna plateau under which the slab shallowed at 18–7 Ma and then steepened as lithospheric delamination occurred, generally lack extreme REE and HFSE ratios. Their upper crust-like features reflect eruption in a mixed stress regime and incorporation of westward-flowing radiogenic crust from a region of extensive deep crustal melting to the east.

The Nambija gold district, southeastern Ecuador, consists of oxidized skarns developed mainly in volcaniclastic rocks of the Triassic Piuntza unit, which occurs as a 20-km-long, north-trending, contact-metamorphosed lens within the Jurassic Zamora batholith. High gold grades (10–30 g/t) are accompanied in most mines by very low Fe, Cu, Zn, and Pb sulfide contents. The skarn is constituted dominantly by massive brown garnet (mean Ad38). Subordinate pyroxene-epidote skarn developed mainly at the margins of brown garnet skarn bodies. Mostly idiomorphic and more andraditic garnet (mean Ad45) occurs in blue-green skarn formed as a later phase, in places with high porosity, at the transition with vugs and discontinuous dilational type I veins. The last garnet generations are mainly andraditic and occur largely as honey-yellow to red-brown clusters and cross-cutting bands (mean Ad84). As typical for other skarns developed in volcaniclastic rocks, mineral zoning is poorly defined.The retrograde overprint is weakly developed, commonly fails to alter the prograde minerals, and is mainly recognized in mineral infilling of structurally controlled (N10°–60°E) vugs and up to several-centimeter-wide type I veins, as well as interstices in blue-green skarn. Retrograde minerals are milky quartz, K-feldspar, calcite, chlorite, and hematite, ±plagioclase, ±muscovite, plus minor amounts of pyrite, chalcopyrite, hematite, sphalerite, and gold. Vugs and type I veins are cut by thin (1–2-mm) throughgoing type II veins that show similar orientations and mineralogy. Native gold is associated with retrograde alteration, mainly in the irregular vugs and type I veins, and subordinately in interstitial spaces and throughgoing type II veins. It is not observed in sulfide-rich type III veins, which cut the previous vein generations.High-temperature (up to 500°C) and high-salinity (up to 60 wt % NaCl equiv) inclusions in pyroxene represent the best approximation of the fluid responsible for a significant part of the prograde skarn stage. Such a highly saline fluid is interpreted as the result of boiling of a moderately saline (~8–10 wt % NaCl equiv) magmatic fluid at temperatures of ~500°C. Moderate-to low-salinity fluid inclusions (20−2 wt % NaCl equiv) in paragenetically later garnet as well as in epidote and quartz from vugs and type I veins may represent later, slightly lower temperature (420° −350°C) trapping of similar moderately saline fluids with or without some degree of boiling and mixing. The similarity of salinities and homogenization temperatures in late garnet, epidote, and quartz fluid inclusions is consistent with the apparent continuum between the prograde and retrograde skarn stages, as illustrated by the general lack of prograde mineral alteration, even at the contacts with retrograde fillings.Gold deposition, together with that of small amounts of hematite, chalcopyrite, and pyrite, took place during fluid cooling in the retrograde skarn stages but not during the last retrograde alteration, as indicated by the absence of gold in the sulfide-rich type III veins. The abundance of gold-bearing samples with high hematite/sulfide ratios and generally low total sulfide contents suggests high oxygen fugacities during gold deposition. The northeast structural control of vugs and type I veins, compatible with regional northeast-striking structures, in part with a dilational character, suggests that skarn formation, including gold deposition in the retrograde stage, took place under conditions of tectonic stress.Minimum Re-Os ages of 145.92 ± 0.46 and 145.58 ± 0.45 Ma for molybdenite from type III veins are compatible with skarn formation and gold mineralization during Late Jurassic magmatism. A genetic relationship with felsic porphyry intrusions that cut the Jurassic Zamora batholith and crop out near several gold skarns is suggested by a published hornblende K-Ar age of 141 ± 5 Ma for a felsic porphyry in the northern part of the Nambija district. Furthermore, the minimum Re-Os ages of ~146 Ma are just slightly younger than the published K-Ar ages (154 ± 5, 157 ± 5 Ma) for the Pangui porphyry copper belt about 70 km north of Nambija. AbstractThe Nambija gold district, southeastern Ecuador, consists of oxidized skarns developed mainly in volcaniclastic rocks of the Triassic Piuntza unit, which occurs as a 20-km-long, north-trending, contact-metamorphosed lens within the Jurassic Zamora batholith. High gold grades (10–30 g/t) are accompanied in most mines by very low Fe, Cu, Zn, and Pb sulfide contents. The skarn is constituted dominantly by massive brown garnet (mean Ad38). Subordinate pyroxene-epidote skarn developed mainly at the margins of brown garnet skarn bodies. Mostly idiomorphic and more andraditic garnet (mean Ad45) occurs in blue-green skarn formed as a later phase, in places with high porosity, at the transition with vugs and discontinuous dilational type I veins. The last garnet generations are mainly andraditic and occur largely as honey-yellow to red-brown clusters and cross-cutting bands (mean Ad84). As typical for other skarns developed in volcaniclastic rocks, mineral zoning is poorly defined.The retrograde overprint is weakly developed, commonly fails to alter the prograde minerals, and is mainly recognized in mineral infilling of structurally controlled (N10°–60°E) vugs and up to several-centimeter-wide type I veins, as well as interstices in blue-green skarn. Retrograde minerals are milky quartz, K-feldspar, calcite, chlorite, and hematite, ±plagioclase, ±muscovite, plus minor amounts of pyrite, chalcopyrite, hematite, sphalerite, and gold. Vugs and type I veins are cut by thin (1–2-mm) throughgoing type II veins that show similar orientations and mineralogy. Native gold is associated with retrograde alteration, mainly in the irregular vugs and type I veins, and subordinately in interstitial spaces and throughgoing type II veins. It is not observed in sulfide-rich type III veins, which cut the previous vein generations.High-temperature (up to 500°C) and high-salinity (up to 60 wt % NaCl equiv) inclusions in pyroxene represent the best approximation of the fluid responsible for a significant part of the prograde skarn stage. Such a highly saline fluid is interpreted as the result of boiling of a moderately saline (~8–10 wt % NaCl equiv) magmatic fluid at temperatures of ~500°C. Moderate-to low-salinity fluid inclusions (20−2 wt % NaCl equiv) in paragenetically later garnet as well as in epidote and quartz from vugs and type I veins may represent later, slightly lower temperature (420° −350°C) trapping of similar moderately saline fluids with or without some degree of boiling and mixing. The similarity of salinities and homogenization temperatures in late garnet, epidote, and quartz fluid inclusions is consistent with the apparent continuum between the prograde and retrograde skarn stages, as illustrated by the general lack of prograde mineral alteration, even at the contacts with retrograde fillings.Gold deposition, together with that of small amounts of hematite, chalcopyrite, and pyrite, took place during fluid cooling in the retrograde skarn stages but not during the last retrograde alteration, as indicated by the absence of gold in the sulfide-rich type III veins. The abundance of gold-bearing samples with high hematite/sulfide ratios and generally low total sulfide contents suggests high oxygen fugacities during gold deposition. The northeast structural control of vugs and type I veins, compatible with regional northeast-striking structures, in part with a dilational character, suggests that skarn formation, including gold deposition in the retrograde stage, took place under conditions of tectonic stress.Minimum Re-Os ages of 145.92 ± 0.46 and 145.58 ± 0.45 Ma for molybdenite from type III veins are compatible with skarn formation and gold mineralization during Late Jurassic magmatism. A genetic relationship with felsic porphyry intrusions that cut the Jurassic Zamora batholith and crop out near several gold skarns is suggested by a published hornblende K-Ar age of 141 ± 5 Ma for a felsic porphyry in the northern part of the Nambija district. Furthermore, the minimum Re-Os ages of ~146 Ma are just slightly younger than the published K-Ar ages (154 ± 5, 157 ± 5 Ma) for the Pangui porphyry copper belt about 70 km north of Nambija.

The late Mesozoic to early Tertiary Magallanes foreland basin in southernmost Chile formed when regional shortening related to the Andean orogeny caused the inversion of the earlier Rocas Verdes back-arc basin into a retroarc foreland basin during the Late Jurassic. This inversion, the orogeny, and the onset of deep-water sedimentation is recorded by the ~1000 m thick Punta Barrosa Formation (Turonian-Coniacian), characterized by 40-150 cm thick, generally medium-grained, tabular, turbiditic sandstone beds that alternate with abundant shale, sandy slurry flow and debris flow deposits. Sedimentation style changed during deposition of the overlying Cerro Toro Formation (Coniacian-Campanian), which consists of thick successions of thin-bedded mudstone and sandstone turbidites and debris flow deposits interrupted by thick (up to 400 m) deep-water channel complexes. The channels are filled with conglomeratic turbidity current deposits, conglomeratic slurry flow deposits, and thick-bedded turbidite sandstone. Sediment transport was dominated by a 5-8 km wide axial channel belt with southward directed paleocurrents. Potential tributary channel complexes, possibly confined behind a local structural high, seem to have funneled sediment into the axial channel. In contrast, the overlying Tres Pasos Formation (Campanian) comprises marine slope deposits that represent southward progradation into the basin. The transition from slope deposits into shallow-marine sediments is recorded by the Dorotea Formation. The Magallanes Basin remained a site of deep-water sedimentation for at least 15 m.y., from the Turonian through the Campanian. The sedimentation style of the deep-water basin fill varied significantly throughout the three formations and the basin fill eventually shoaled upward into shallow-marine and deltaic deposits. The sculpting of the outcrops by recent glaciation provides superb exposure of the basin fill, making the Magallanes Basin an excellent field area to study deep-water depositional processes as well as foreland basin evolution.