November 11. Cretaceous flood basalts and amethyst

Two great episodes of volcanism took place during the Cretaceous, one early in the period, and one at the end. We’ll talk more about the second one near the end of the month, but today’s topic is the first one. 

In South America about 128 to 138 million years ago, thick basaltic flows poured into the Parana Basin. The Parana Basin was a persistent low-lying zone on the South American continent, located in southwestern Brazil and adjacent areas, where the Parana River flows today. In some ways it was similar to the intracratonic basins of North America, such as the Williston, in that they were initiated early in the Paleozoic, probably during Ordovician time. But the Parana Basalts definitely make it a different beast from the sedimentary Williston Basin. 

Present-day South Atlantic showing features related to Tristan Hotspot and South Atlantic Ocean opening.

The volcanism was most likely related to rifting that marked the onset of active sea-floor spreading as the South Atlantic Ocean began to form. One reason we suspect this is that there is a conjugate pile of volcanic rocks on the African side, in Namibia and Angola, where it is called the Etendeka Province. Furthermore, there are two lines of seamounts, underwater volcanoes, that extend from the coasts of both South America and Africa to the Mid-Atlantic Ridge. Those seamounts, called the Walvis Ridge on the African side of the Atlantic and the Rio Grande Rise on the South American side, trace back essentially to the Mid-Atlantic Ridge at Tristan da Cunha, a volcanic island whose magma source is thought to be the Tristan Hotspot.

Tristan da Cunha is pretty much just like Iceland, which also straddles the Mid-Atlantic Ridge and a hotspot. The difference between tiny Tristan da Cunha and massive Iceland is that the Tristan hotspot has been active since the Cretaceous whereas the Iceland one is quite recent. The two ridges that extend away from the Mid-Atlantic Ridge, and the volcanic piles on land in the Parana and Entedeka Provinces, reflect the episodic volcanism from the Tristan hotspot as the Atlantic Ocean opened.

Some estimates put the Parana-Entedeka’s biggest eruptions, about 132 million years ago, as some of the largest volcanic events in earth history.

The Parana flood basalts in Rio Grande del Sul province, Brazil, are famous for their beautiful amethyst-lined tubes and geodes. Amethyst is purple quartz whose color is related to trace amounts of iron in the silica crystal framework, together with some degree of natural radiation. The occurrences in the Parana are unusual in their intense color and uniformity. The geode formation was probably a two-stage process, with the openings forming initially in the original molten rock because of blobs of material that was unable to be incorporated into the basalt magma. Later, the cavities were the sites of deposition of quartz tainted by iron and other elements, including some rare earth elements. The entire process of deposition of the amethyst probably took place over a time span on the order of 40 million years in the early to mid-Cretaceous.

Today, these amethysts make Brazil the leading producer of gem-quality amethyst in the world, and of course the huge amethyst-lined pipes are famous around the world too.

—Richard I. Gibson

Largest volcanic eruptions 

Amethyst tubes

Map (public domain) from NOAA (annotated by Gibson)