Columnar jointing in vapor-phase-altered, non-welded Cerro Galán Ignimbrite, Paycuqui, Argentina

Abstract

Columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits. However, the non-welded Cerro Galán Ignimbrite at Paycuqui, Argentina, contains well-developed columnar joints that are instead due to high-temperature vapor-phase alteration of the deposit, where devitrification and vapor-phase crystallization have increased the density and cohesion of the upper half of the section. Thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures, above 630°C, but the lack of welding textures indicates temperatures below the glass transition temperature. In order to remain below the glass transition at 630°C, the minimum cooling rate prior to deposition was 3.0×10 −3 -8.5×10 −2°C /min (depending on the experimental data used for comparison). Alternatively, if the deposit was emplaced above the glass transition temperature, conductive cooling alone was insufficient to prevent welding. Crack patterns (average, 4.5 sides to each polygon) and column diameters (average, 75 cm) are consistent with relatively rapid cooling, where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer. The presence of regularly spaced, complex radiating joint patterns is consistent with fumarolic gas rise, where volatiles originated in the valley-confined drainage system below. Joint spacing is a proxy for cooling rates and is controlled by depositional thickness/valley width. We suggest that the formation of joints in high-temperature, non-welded deposits is aided by the presence of underlying external water, where vapor transfer causes crystallization in pore spaces, densifies the deposit, and helps prevent welding.

High-temperature emplacement of the Cerro Galán and Toconquis Group ignimbrites (Puna plateau, NW Argentina) determined by TRM analyses

“…Using the heat transfer model of Carslaw and Jaeger (1959), Bardot (2000) estimated that clasts sized between 2 and 10 cm, initially at ambient temperature, will reach thermal equilibrium with the medium after 10 2 -10 4 s, respectively, i.e., within hours. Minimum estimates of cooling rate prior to emplacement reported in Wright et al (2011a) vary between 3×10 −3 and 8.5×10 −2°C /min, based on the lack of welding textures. Since the analyzed clasts between 2 and 10 cm in diameter are completely remagnetized in the Cerro Galán ignimbrite, we can argue that the minimum emplacement temperature required to remagnetize the clast at the cooling rates calculated by Wright et al (2011a) varies between 621°a nd 635°C.…”

“…Minimum estimates of cooling rate prior to emplacement reported in Wright et al (2011a) vary between 3×10 −3 and 8.5×10 −2°C /min, based on the lack of welding textures. Since the analyzed clasts between 2 and 10 cm in diameter are completely remagnetized in the Cerro Galán ignimbrite, we can argue that the minimum emplacement temperature required to remagnetize the clast at the cooling rates calculated by Wright et al (2011a) varies between 621°a nd 635°C. We can constrain the T dissipation path of the Cerro Galán eruption first assuming that the initial temperature was the magmatic temperature (T m ), which is inferred by Fe-Ti oxide geothermometry to be maximum of 790°C (cf.…”

“…4a, and sections height and distances from caldera rim are reported in Table 3, for each site. At Paycuqui, five sites were sampled at each of two sections (CG0201-CG0205 and CG0281-CG0285; see also Wright et al 2011a). Finally, three sites were sampled in the CGI subunits at the distal locality of Tacuil (sites CG0420, CG0430, and CG0440).…”

A Petrological Study of the Mudeungsan Tuff Focused on Cheonwangbong and Anyangsan

“…또한 용결구조는 화산쇄 설물이 쌓이고 탈유리화가 진행되는 과정 속에서 대 부분이 사라질 수 있다. 이러한 이유로 무등산응회암 전역에서 용결구조가 관찰되지 않는 것으로 생각된다 (McPhie et al, 1993;Wright et al, 2011). 탈유리화는 밀도와 응집력을 증가시켜 주상절리 형 성을 더욱 촉진시킬 수 있다 (Wright et al, 2011).…”

“…이러한 이유로 무등산응회암 전역에서 용결구조가 관찰되지 않는 것으로 생각된다 (McPhie et al, 1993;Wright et al, 2011). 탈유리화는 밀도와 응집력을 증가시켜 주상절리 형 성을 더욱 촉진시킬 수 있다 (Wright et al, 2011). 무등산응회암은 하부(450 m)에서 더 탈유리화 된 것 으로 보이나 주상절리는 주로 상부(700 m 이상)에서 형성되어있어 탈유리화 정도가 무등산의 주상절리 형 성에 미치는 중요한 요인은 아닌 것으로 보인다.…”