The Llanite Dike

Mineral Monday + Tectonic Tuesday.  Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.

More generally, the rock is a rhyolite porphyry – rhyolite meaning pretty high in silica (a granite-like composition) and formed at or near the surface of the earth, and porphyry meaning it has two grain sizes – a fine matrix, with larger crystals of quartz (and microcline feldspar) suspended in that matrix. This implies that there were two periods of cooling, one at deeper depths where it took the larger crystals a longer time to cool (and grow), followed by a later, quicker period of cooling, so the matrix crystallized so fast the grains are very small, but the larger, older grains are still there within the matrix.

All that cooling happened about 1,093,000,000 years ago (almost 1.1 billion) during a time called the Grenville Orogeny (orogeny means mountain-building) when what is now central Texas was amalgamated to the main part of the North American continent. The llanite was probably an aspect of the intrusions of the Town Mountain Granite, which has similar age but crystallized at greater depth. It’s part of a long belt that extends with some discontinuity to central Tennessee, through Kentucky and Ohio, then northeast across Ontario, Quebec, and into Labrador. Rocks now in southern Scandinavia were part of the Grenville mountain belt, the result of a collision between continental masses that was assembling the supercontinent Rodinia over a long period of time, from about 1,250 million years ago to 980 million years ago.

The llanite, in the form of a narrow dike, intruded older rocks toward the end of the Grenville Orogeny. The mountain belt continued into Mexico, and rocks of similar age are found in Australia and Antarctica as well as South America today. Exactly how those rocks fit into the big picture is still debated, but one version of the assembled continent of Rodinia is in the comments.

At left, one reconstruction (others exist) of Rodinia about 750 million years ago, just before it began to break up. “Rodinia” is from Russian for “motherand,” or “to give birth,” alluding to this continent’s early place in the rifting-collision cycles (called Wilson Cycles with respect to ocean basins, for J. Tuzo Wilson) that have followed. Even so, Rodinia was probably preceded by at least one earlier supercontinent, named Columbia – but that’s debated.

 

The llanite dike intrudes older Precambrian metamorphic rocks called the Valley Spring Gneiss, which have a lot of magnetite in them. The Valley Spring Gneiss is dated to about 1,375,000,000 years ago or older.

The phenocrysts (“showing” or “visible” crystals) of blue quartz in the llanite are supposed to be “beta quartz,” a high-temperature, higher symmetry form of quartz that can crystallize only above 573ºC – in fact, it cannot even exist at surface conditions and pressures, so all “beta quartz” is actually a pseudomorph (“false form”) of regular quartz that has formed as the original beta quartz cooled. This little crystal (not from Llano) is probably one such pseudomorph. (Technically, since I knew you wanted even more jargon, it’s a paramorph rather than a pseudomorph, because while the crystal structure has changed, the chemistry has not.) The essentially perfect hexagonal symmetry of this crystal marks it as a beta-quartz shape, versus the lower (trigonal) uneven symmetry typically displayed by normal quartz. It’s possible for normal trigonal quartz to have equally developed faces that appear fully hexagonal, but it’s unusual.

An extension of the Mid-Continent Rift?

In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.

All that is interesting, I guess, but the thing has much broader implications. If it is a dike – which is likely in my opinion – that suggests that it formed at a time when extension, pulling apart, was the dominant stress in this area. Dikes can form under compression, but it’s a lot easier for them to intrude if the rocks are pulling apart, opening up cracks into which magma can force itself.

The northeast-southwest orientation is also intriguing, because it points pretty much dead on at a possible branch of the Mid-Continent Rift, a pull-apart feature that runs from Kansas through southeast Nebraska, northeast across Iowa, up into Minnesota, and into Lake Superior. It’s a 1.1-billion-year-old break in North America – a break that failed to completely dismember the continent, but just formed a long narrow trough filled in many places with dense, magnetic basalt. Kind of like the Red Sea today, but not as linear.

This dike in the Texas panhandle isn’t trivial – it’s at least 45 miles (70 kilometers) long. There are additional similar features on trend with it in Kansas. My interpretation is that it represents a far away expression of the extension and intrusion related to the Mid-Continent Rift System. This possible relationship is shown in a map below.

The depth to these rocks in the panhandle is probably only 3500 or 4000 feet, but to my knowledge there is no drilling to those depths in this area, so we don’t actually have rocks to validate this interpretation. But I’d bet a beer that you’d find a reversely polarized dike of basalt or similar lithology, containing a decent amount of magnetite, that solidified around 1.1 billion years ago.

—Richard I. Gibson

November 19. Volcanoes in Texas

Eighty million years ago, while the Laramide Orogeny was getting underway in western North America, central and south Texas were parts of the shallow carbonate bank that developed on the North American side of the Gulf of Mexico. The Gulf had begun to form during the Jurassic as Yucatan pulled away from what is now Texas and Louisiana. These Cretaceous rocks are part of the Gulf Coastal Plain that we talked about a few days ago. 

Remember the rudists? The tubular clams a meter high that trapped sediment to help form reefs? They grew in this area too. It was overall a quiet, shallow sea, perhaps something like today’s Florida Shelf. The setting was not one where you’d expect volcanoes, but that’s what we got. 

In a long, linear zone east of a line from Waco to Austin and on south to San Antonio and Uvalde, Texas, there’s a string of more than 200 little volcanoes that erupted into the carbonate sediments. They’re basically piles of volcanic ash together with some basaltic flows, and some of them today are resistant enough to form mounds on the land surface. One well-known example is at Pilot Knob, not too far from Austin. Pilot Knob is about two miles across. 

Most of these volcanic bodies are buried in the subsurface and have no expression that we can see on the land, but most of them are basaltic and contain a lot of magnetite. This gives them a distinct expression in a magnetic map. That characteristic was useful in oil exploration, because the volcanics and the surrounding carbonate rocks contain oil in 35 or 40 of the known volcanoes. Something like 50 million barrels of oil have been produced from several accumulations since the first field was discovered in the 1910s.

Magnetic map of Uvalde and Medina Counties, Texas (from USGS). Most of the little pimple-like bumps represent igneous plugs of Cretaceous age.

The margin of the old Texas Craton, the Precambrian core of this region, is called the Balcones Escarpment, a topographic feature that follows a fault zone separating the older rocks of central Texas from the Cretaceous and younger rocks of the Gulf Coastal Plain. The Balcones Escarpment is followed by Interstate 35 from Waco to Austin to San Antonio, just about the same zone where the volcanic centers are found.

It’s not completely clear why these volcanoes erupted into this tectonically quiet region. Probably the best explanation is that there was a pulse of extension, pulling apart, that opened deep-seated faults and fractures through which the magma rose. The weakest zone was along the old break between the strong craton and the stretched crust to the south and east that was pulled and stretched by Yucatan’s departure in the Jurassic. The magmas are not all basaltic, which makes it difficult to say, for example, that they simply came from the mantle – there must have been some melting of other rocks and mixing of magmas to get the kinds of igneous rocks we see.

—Richard I. Gibson

Cretaceous volcanism in South Texas and oil 

Pilot Knob

Magnetic modeling  

Magnetic map (USGS OFR-02-0049)   and this one