Field Guide Stops 21-30

 

The third leg of our trip launched out of San Pedro de Atacama and climbed up into the land of calderas and ancient supervolcanoes.

Stops 21-30

Stop 21: The City of San Pedro de Atacama

San Pedro de Atacama

 22°55'15.27"S,    68°12'9.94"W -
Elevation: 2428m (7968 ft)

From El Tatio, take the B-245 South all the way to San Pedro de Atacama. 

 Paso Tres Campground

While there we stayed at the Paso Tres Campground

 

The Swimming Pool at Paso Tres

Which has a swimming pool!

 

The market at San Pedro de Atacama

The town had a great marketplace with intersting merchants...

 

The team enjoys a meal in San Pedro de Atacama

and good food!!

The Church at San Pedro de Atacama  

The town also features a significant archeological museum, the R. P. Gustavo Le Paige Archaeological Museum, with a large collection of relics and artifacts from the region as well as the the Church of San Pedro which is a National Monument.

Stop 22: Tocanao Quarry

 

THE TOCANAO QUARRY

23°11'28.38"S, 67°59'29.04"W

There is a 30-40 m thick exposure of the Tocanao ignimbrite, 4.49 Ma the first phase of the Atana Formation at the Toconao quebrada. The quarry has become a national monument, and they now charge 1500/person and do not welcome geologists without permission. It is necessary to go to the Tocanao city official’s office with passports and verification of employment (business card) showing that you are a geologist in order to receive permission. All communication is done in Spanish.

The Tocanao ignimbrite within the quebrada contains a relatively crystal poor (< 10%) and lithic-poor matrix. Devitrified pumice clasts are randomly distributed throughout roughly 30% of the deposit and show distinctive elongated, tube-like vesicles and spherulites as evidence for devitrification. The pumice and the matrix contain copper-colored biotites and plagioclase as well as lithic enclaves from an older ignimbrite unit, possibly the Pujsa dated at 5.6 Ma.

The top of the flow has a cemented sillar texture that is quarried as construction material known locally as “Piedra Blanca”. This vapor-phase altered texture was created by fusion processes that thermally indurated pieces of ash together and by the escape of hot gases through the conduit that form the elongated vesicles.

At the Toconao quebrada the ignimbrite is in contact with the underlying plinian fall deposit. The fall is mantling the surface of the underlying Pujsa ignimbrite, 5.6 Ma and has also incorporated some lithics from the Pujsa. The flow deposit as well as the fall layer are related to the La Pacana caldera that is roughly 20-30 km from this location. It is possible that both the Tocanao member and the subsequent Atana ignimbrite were the result of the eruption from the La Pacana caldera.

You can drive down from the quarry itself and into the national monument, which is essentially a canyon comprised of ignimbrite. There is a nice trail that leads you along the edge the canyon along the deposit. In the distance you are able to see the Purico ignimbrite directly overlying the Toconao. The Atana/Talabre ignimbrites are not exposed here, which makes this the most distal part of the Purico deposit. Also, as you continue up the trail, you can see the Pujsa exposed beneath the Tocanao. If you take the trail down the canyon, it leads you to a river and beautifully lush vegetation. 

Stop 23: Corrales Blancos

Corrales Blancos

S22 55.792; W68 04.438

Recharged from our stay in San Pedro de Atacama, we headed east toward the Purico complex. With Dr. Schmitt navigating we found our way to the Corrale Blancos outcrop. The Corrales Blancos outcrop is a large exposure of the Lower Purico ignimbrites. This may have been the most dangerous location of the trip due to potentially live land mines that have been transported down the wash. The lower Purico ignimbrite has been divided into two sections that will be discussed seperately in the following paragraphs.

Figure from Schmitt et al. (2001). Note that

only section A is exposed at Corrales Blancos.

Lower Purico Ignimbrite I

LPI contains crystal-rich dacitic pumice, crystal-rich inclusions, and banded pumice. The crystal-rich dacite represents the juevunile material from the eruption, the crystal-rich inclusions have been speculated to represent side-wall crystal cumulates, and the banded pumice appear to be an incomplete mixture of the crystal-rich dacite and more mafic material (andesite), possibly recharging the system. Systematic size and distribution changes up section in this unit are problematic, however, mafic material seems to increase up section. Dacitic pumices from LPI and LPII are indistinguishable and contain plagioclase, hornblende, quartz, biotite, and possible orthoyroxene in a glassy vesicular groundmass. Crystal-rich inclusions are also similar in LPI and LPII and contain plagioclase, hornblende, quartz, and biotite in a crystalline/glassy groundmass. It has been proposed that the crystal-rich inclusions represent side-wall cumulates (de Silva et al., 1989c)

Image of previously retired land mine.

 

Image of LPI.

Lower Purico Ignimbrite II

LPII displays greater compositional variability than LPI due to increased proportions of mafic material. LPII contains crystal-rich dacite pumice, a yellow pumice, crystal-rich inclusions, banded pumice, andesitic pumice, and rhyolite pumice. The base of LPII differs from LPI physically due to the presense of pyroclastic surge deposits and a crystal-poor rhyolite fall deposit.

Geochemical studies of the LPII ignimbrite show that the rhyolite fall deposit can not be explained as a fractionation product of the crystal-rich dacite, but instead appears genetically related to the andesitic pumice (Schmitt et al., 2001). The dacite pumice and crystal-rich inclusions are indistinguishable from those in LPI, see LPI for modes. The rhyolite pumice at the base of LPII is crystal-poor with the only significant crystals being plagioclase (some hornblende and biotite may also be present in trace amounts). Detailed geochemical analyses show that rhyolite is not a fractionation product of the dacite, but instead a liquid fractionated from the andesite (Schmitt et al., 2001; Grove and Nolin, 1986). The andesitic pumice in LPII contains plagioclase, clinopyroxene, and orthopyroxene in a glassy groundmass. Groundmass in the andesite pumice is ~50% less vesicular than the dacite pumice (Schmitt et al., 2001).

Image of LPII. Note the plinian fall deposit at the bottom of the section.

Image of different populations of pumice collected at Corrales Blancos.

Although the upper Purico ignimbrite (UPI) doesn't crop out here, it's an oppurtune time to discuss the differences between the lower and upper flow units. The UPI is white, well to moderately sorted ignimbrite containing both flattened white pumice and fiamme in a glass and crystal-rich groundmass. The UPI is easily distinguishable from the lower Purico ignimbrites by the fact that it's welded and contains fiamme.

Stop 23.5: Silapeti

 

THE SILAPETI GROUP

23° 9'43.38"S, 67°57'22.38"W

On our drive to the Silapeti Group located to the northeast of Tocanao, Lazcar Volcano (#0353) is visible in the distance. Fumaroles were seen actively degassing on Lazcar in November of 2009. Licancabur, a stratovolcano is also visible from the road (#0354).

The southern section of the ignimbrites is most complete in the gorges or quebrada around Silapeti. Five ignimbrite units are exposed that comprise the Silapeti Group:

Silapeti

 

Purico (1.35 Ma)

Talabre (2.17 Ma)

Atana (4.09 Ma)

Tocanao (4.49 Ma)

Pujsa (5.87 Ma)

Once you reach Silapeti, there is a noticeable, clean trail that leads you down the canyon and provides a great opportunity to the see the complete section of ignimbrites. It is also a great idea to begin the hike in the morning in order to arrive at the base of the canyon by midday for some shade and a nice lunch break by the river.

 

The top layer of the quebrada is the uppermost member of the Silapeti Group, the Purico ignimbrite, 1.35 Ma. The Purico is comprised of Lower Purico 1, Lower Purico II and the Upper Purico (see following descriptions of Purico). The main volume of the ignimbrite is the typical non-welded, crystal-rich ignimbrite. The Purico contains large pumice clasts that are often concentrated in “rafts” or lenses. Minerals include course-grained hornblende, quartz, small copper-colored biotite and plagioclase. The matrix is white/pinkish, very ashy and has a devitrified glassy exterior. The Purico Ignimbrite was deposited during the late-stages of the APVC formation.

The Talabre ignimbrite, 2.17 Ma is a rhyolite, homogenous, well-indurated unit. The matrix is fine-grained, crystal-poor (10-15%) and contains plagioclase and copper-colored biotite. Tubular pumice and lithic clasts are common. The deposit is extremely thin (<5 m) and is only exposed as wedges within the canyon. The unit has been subjected to intense vapor phase alteration. When the flow was deposited, the material was very gas-rich and as the gases escaped, the crystals were altered. The gas rich nature of the ignimbrite gives it a sillar texture. The source area for the Talabre ignimbrite is a small caldera now occupied by the Salar de Aguas Calientes in the southern La Pacana caldera.

 

The team surveys Silapeti

The Atana formation is directly beneath the Tocanao ignimbrite and consists of two units: the younger Atana ignimbrite and the older Tocanao ignimbrite. Both members were likely the result of the eruption from the La Pacana caldera. The Tocanao is thought to represent the volatile-rich cap of the magma chamber, which when erupted, actually caused the dome to collapse. When the caldera forms, Plinian eruptions are less likely, which explains why the Atana pyroclastic flow followed the Tocanao with no fall layer deposited between them.

The Atana ignimbrite, 4.09 Ma is the next layer of rocks that are visible on the walk into the quebrada. The unit is indurated due to vapor phase alteration. The ignimbrite is dacitic and contains abundant crystals including cristobalite or quartz, plagioclase, altered biotite and hornblende. The Atana lacks abundant pumice clasts and has relatively small lithics (~5 cm). The Atana was likely erupted from the bottom of the La Pacana magma chamber, which explains its crystal-rich nature.

the base of the Silapeti Group

The Tocanao, 4.49 Ma is very similar to the exposure in the Tocanao Quarry (Stop 22). It is a crystal-poor rhyolite that sounds flat when it is hammered into. Little silky pumices are distinctive, elongated clasts that have been formed by vapor-phase alteration. The pumices are easily removed by erosion, leaving holes in the deposit and giving the ignimbrite a spongy texture. They contain plagioclase and biotite crystals. Abundant tubed pumices are found at this location. The Tocanao was likely erupted from the top of the La Pacana magma chamber, which explains its crystal-poor nature.

 

The Pujsa ignimbrite, 5.87 Ma is the lowermost unit of the Silapeti Group.The deposit is a purple-gray, crystal-rich (55-60%) dacite that is over 40 m thick in this area. The base of the Pujsa has not yet been found. The unit is less indurated and less matrix supported than the other ignimbrites in the canyon. It contains large (~20 cm) crystal-rich pumices that are welded to the matrix and are often found in concentrated lenses near the tops of the flows. The matrix contains plagioclase, quartz, biotite and hornblende crystals. The Pujsa sits above the basement, which explains the abundant angular lithic clasts of basement lithologies. Lithics of ignimbrite material are also apparent, which may be from older ignimbrites that are not exposed. The Pujsa ignimbrite may represent an early eruption from the La Pacana caldera.

Stop 24: Purico Complex (Overview)

 

The 1.3 My Purico complex is a ~circular apron of ignimbrites and summit domes overlying older ignimbrites from the La Pacana caldera (4.5-4.1 My). The summit domes overly the ignimbrites and can be characterized as flat "torta" domes (Dome D) or conical Pelean domes (Cerros Purico, Negro, Putas, Chascon, Aspero, and El Cerrillo). The ages of the domes are not available. However, Chascon, Aspero, and El Cerrillo are all post-glacial and therefor less than 0.5 M.

Figures from Schmitt et al. (2001).

Dome D is compositionally similar to the Purico ignimbrite and probably represents the same magma degassed, whereas, post-glacial domes commonly contain mafic enclaves and show signs of magma mixing. These post-glacial domes are of particular interest because the magmatic components (i.e. mafic enclaves) within the domes may be the components responsible for the formation of the APVC. See following stops for more information on the individual domes.

Purico Dome

Image is the Cerro Purico dome

 

Stop 25: Aspero

Cerro Aspero

S23 04.836 W67 42.166

 

Leaving Cerro Purico we headed West to Cerro Aspero. Cerro Aspero is a post-glacial (<0.5 My) dome erupted after the Purico ignimbrites (1.3-1.0 My). Hawkesworth et al. (1982) found that mafic enclaves at Cerro Aspero were basaltic-andesite in composition, and therefore  represent the most primitive compositions found in the Purico Complex.

Aspero Dome

Image of the Cerro Aspero dome.

The host rock at Cerro Aspero, similar to the other domes of the Purico Complex is a crystal-rich dacite. Multiple mafic enclaves were collected at this location. The enclaves are fine grained, dark gray, and porphyritic. Crystals include quartz, plagioclase, and hornblende. Quartz crystals are relatively large (~10 mm) and have green reaction rims of clinopyroxene. The quartz crystals along with some of the plagioclase may not be primary. The crystals are sitting in a groundmass of glass, plagioclase, and hornblende.

Mafic enclave in Cerro Aspero dacite.

Rocks from Aspero also have large megacrysts of plagioclase that may represent the granitic basement beneath the Purico Complex.

Stop 26: Chascon

Chascon

S23 01.847 W67 41.938

 

Chacson Dome

The Chascon dome is another post-glacial dome (<0.5 My) erupted after the Purico ignimbrites (1.3-1.0 My). The bulk of Chascon is crystal-rich dacite. However, mafic enclaves are very prevalent (up to 20%).

Chascon Dome

The Chascon dome is dominated by porphyritc crystal-rich (30-35% xls) dacite containing plagioclase (<1-15 mm), quartz ( <1-4 mm), hornblende (2-15 mm), and biotite (<1-5 mm) in a glassy groundmass. Rocks from Chascon also contain plagioclase megacrysts (>25 mm), andesitic enclaves, crystal-rich enclaves, and rhyolite. Plagioclase megacrysts may represent basement rocks which have interacted with the APMB. Samples CH09013-DHB (mafic enclave) and CH014-DHB (host) collected at this location.

Stop 27: Dome D

Dome D

S22 57. 0137 W67 42. 5709

 

Leaving the post-glacial domes behind we then headed back east to Dome D. Dome D is a pre-glacial dome that yields similar ages and compositions to the Purico ignimbrites, and is thought to be the same magma that formed the Purico ignimbrites just degassed. Dome D lavas are crystal-rich containg plagioclase, hornblende, quartz, and biotite in a glassy groundmass, and are ~60% less vesicular than the purico ignimbrite (Schmitt et al., 2001). Mafic enclaves were found in Dome D lavas, which would be expected if Dome D is genetically  related to the Purico ignimbrite.

 

Stop 28: La Pacana Caldera

La Pacana caldera

S23 13.191, W67 27.963

 

 

The La Pacana caldera is one of the largest calderas on earth and has been the source of two major ignimbrites in the past 5.5 My. The first activity at La Pacana was the eruption of the ~5 My Toconao ignimbrite, followed the eruption of the caldera-forming Atana ignimbrite (Lindsay et al., 2001). Following the emplacement of the ignimbrites, continuing volcanic activity created a large resurgent dome. Activity continued until ~1.6 My, producing a series of silicic domes on the margin of the resurgent block, and within the caldera moat. The silicic domes are crystal-rich dacites, and are similar to the Atana ignimbrite in composition. The silicic domes likely represent degassed Atana magma (Lindsay et al., 2001).

Figures from Lindsay et al. (2001).

 

 

 

The Ignimbrites

The Toconao a crystal-poor rhyolitic ignimbrite with an estimated volume of ~180 Km3. The ignimbrite can be sub-divided into two facies, a lower non-welded and non-indutrated facies containing abundant tube pumice, and an upper phase that is clearly indurated. See stop 22 for more details.

 

The Atana ignimbrite crystal-rich dacite that occur as both outflow and intracaldera fill and has a total volume of ~2500 Km3. The Atana ignimbrite is typically welded and show various degrees of devitrification. The Atana outflow is typically ~30-40 m thicks and sits atop pyroclastic surge and soft ash deposits. See stops 31 and 33 for more details.

Stop 29: Cerro Bola

Cerro Bola

23°13.213'S, 67°27.889'W

 

Taking a break from the Purico complex, we travel into the La Pacana Caldera. Build on top of the resurgent dome of the caldera, Cerro Bola is the last large silicic dome we will see on our tour. Like most of the previous lava domes we have seen in our time on the APVC, Cerro Bola takes on the class “torta” shape. Around the flanks of the dome the fallen blocks seem to take quite a beating from the intense winds that dominate the area. Incredible yardangs and hoodoos can be seen all over the area, many taking on amazing and complex shapes, be sure to check it out.

 

Amazing wind erosion

 

Cerro Bola seems to have undergone a few different stages in its formation. The first stage was a series of pyroclastic flows that can now be seen at the base of the dome and in some places around its flanks. These flows are fine grained, ash rich and matrix supported. Pinkish in color, abundant lithics are commonly found with phenocrysts on quartz, plagioclase and hornblende. Obsidian can be found weathering out of this deposit, some pieces quite remarkable.

 

 

 

Petrographic and mineralogic details

Quartz from Bola

 

 

The second stage is the effusive dome building stage, where lavas of dacitic composition we erupted onto the floor of the La Pacana caldera. Like all the other lava domes of the area, Bola is extremely crystal rich (generally over 60% crystals). Mineralogically, the lava displays phenocrysts of plagioclase, quartz, and hornblende is a glassy, microvesicular matrix, which also leads to a somewhat friable texture.

 

 

 

 

 

 

 

Casey on top of Bola Dome Bob looking down from Bola Dome
Bob on Bola

 

 

After all that climbing... we were a bit tired!

Domed out?

Stop 30: Filo Delgado Ignimbrite

 

THE FILO DELGADO IGNIMBRITE

23°2'58.34"S, 67°30'18.70"W

As you continue to drive east on the international road toward Argentina, the Atana ignimbrite changes its dip direction. Its dominant dip direction was west and it suddenly it dips to the east, suggesting that the road crosses over the La Pacana caldera.

There is an obvious exposure along the international road that is a great introduction to the deposits that dominate the northeast region of Chile. When looking to the south, the lowermost unit is the Atana ignimbrite, 4.09 Ma. Above the Atana is a fall deposit that is overlain by the Tara ignimbrite, 3.68 Ma. The Filo Delgado ignimbrite is the uppermost unit exposed.


The Filo Delgado ignimbrite is younger than Purico (1.35 Ma) and is the youngest unit of the La Pacana caldera. It is exposed on the east side of the La Pacana caldera and is stratigraphically related to the Upper Purico that is exposed on the west side of the caldera.

 

 

The Filo Delgado is a thin crystal-poor unit that contains welded pumices and distinctive crystal-poor, obsidian fiamme. The ignimbrite is characterized as lenticulite, because the matrix is crystal rich but the obsidian-like lenses have minimal crystal content. Also the entire ignimbrite is welded and has a blocky texture.