Patricia Sruoga

Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: Analogy to the Middle Proterozoic in North America?

A vast region of upper Paleozoic to Middle Jurassic (300-150 Ma) silicic magmatic rocks that erupted inboard of the Gondwana margin is a possible Phanerozoic analogue to the extensive Middle Proterozoic (1500-1350 Ma) silicic magmatic province that underlies much of the southern mid-continent of North America. Like the North American rocks, the Gondwana silicic magmas appear to be melts of crust that formed about 200-300 m.y. earlier. In the North American case, this older crust formed and was accreted to the continent during a major period of crustal formation (1700-1900 Ma), whereas in the Gondwana case, the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the Paleozoic. In both cases, basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older (and previously melted) crust. The similarities between the North American and Gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas, ponded at the base of the preheated crust, caused extensive crustal melting that inhibited upward passage of the basalts. In both cases, silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes, and before breakup of the supercontinent. By analogy with the polar wander curves for Gondwana, the granite-rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles.

Processes Controlling Porosity and Permeability in Volcanic Reservoirs from the Austral and Neuquén Basins, Argentina

Volcanic rocks develop primary and secondary porosity and permeability, depending on both their lithology and the sequence of processes involved in their formation. Primary processes (welding, deuteric crystal dissolution, gas release, flow fragmentation, and crystal shattering) may lead to high porosity and permeability, the best example of which is a nonwelded ignimbrite with well-developed gas-pipe zones. Secondary processes (different types of alteration), however, tend to decrease primary porosity. However, certain secondary processes, such as dissolution and hydraulic fracturing, may contribute to enhance total porosity and permeability. These conclusions were developed through a systematic review of reservoir quality in volcanic rocks, integrating lithology and process interpretation with petrophysical data. The said information was taken from selected cores of volcanic rocks from the Serie Tobfera unit in the Austral Basin and the Precuyano unit in the Neuqun Basin, Argentina. A clear understanding of both primary and secondary processes may serve to predict the quality of volcanic reservoirs and could be used as a guide for oil and gas exploration and development in many parts of the world.

Permo-Triassic leucorhyolitic ignimbrites at Sierra de Lihue Calel, la Pampa Province, Argentina

Abstract The Lihue-Calel ignimbritic plateau in central Argentina represents the easternmost extension of the Permo-Triassic Choiyoi rhyolitic province. The rocks of this pampean plateau are chiefly pyroclastic, with minor intrusives and lavas. Rhyolites are a high-silica, high-potassium, slightly peraluminous type depleted in compatible elements (Sr, Ba, Eu) and variably enriched in incompatible elements (Rb, Cs, U, Th, Y, Nb, Ta). REE patterns are flat (La/Yb = 0–10), showing conspicuous negative Eu anomalies. The geochemistry of the ignimbrites reflects two processes: (1) fractional crystallization leading to a very high degree of differentiation, and (2) the activity of a residual F-enriched fluid phase during the last cooling stages. In this respect, they resemble the topaz rhyolites of the western United States. In both cases, the silicic magmas were erupted in an extensional tectonic setting, subsequent to a previous compressive episode of crustal thickening and uplifting. Thus, the silicic rocks of Lihue-Calel exhibit particular geochemical features that distinguish them from subduction-related calc-alkaline types and from the peralkaline rocks of anorogenic intraplate regions.