L. García Cacho

Roque Nublo caldera : a new stratocone caldera in Gran Canaria, Canary Islands

Abstract An inferred elliptic structure 5 × 4 km in diameter, has been identified as the caldera that brought the Roque Nublo Pliocene (5-3.5 Ma.) volcanic cycle in Gran Canaria to an end. This buried caldera, whose morphological expression is no longer evident, is now filled with almost 400 m of interbedded lacustrine sediments and nephelinite lava flows, ashes and tuffs. Mapping of the postcaldera sedimentary layers, which comprise debris-flow lobes, turbiditic sandstones and muds, has permitted the paleogeographic reconstruction of a saucerlike volcanic depression. This has been confirmed by a gravimetric survey, whose data clearly define a double gravity minimum in the area of the inferred caldera. Some postcaldera vents are located near the caldera rim, but most of them define a NW-SE line, across the caldera, which apparently marks an important tectonic lineation. The paucity of remains of Roque Nublo breccias in the caldera infilling suggests a powerful explosive terminal phase for the stratocone, after which the center of the residual edifice collapsed to form the caldera. The very early resumption of activity in the caldera is not a definite proof for a resurgent activity in the area. Whereas the Roque Nublo Formation was formed after a quiescent period of almost four million years on Gran Canaria, this event was soon followed by the third and last cycle of volcanic activity on the island.

A large volcanic debris avalanche in the Pliocene Roque Nublo Stratovolcano, Gran Canaria, Canary Islands

Abstract During the second magmatic cycle of Gran Canaria, between 5 and 3 Ma, the Roque Nublo Stratovolcano (RNS) achieved its almost total development, reaching a height of 2500–2600 m and spreading about 100 km 3 of products over a surface area of at least 250 km 2 . In the last growth stage of the RNS, and probably in connection with the paroxysmic genesis of the Roque Nublo explosive Caldera, a sector of the volcanic edifice suffered a gravitational collapse of significant importance. The data reported here show that the collapse was accompanied by a great avalanche: the Roque Nublo Debris Avalanche Deposit (RNDAD) composed mainly of block facies. The last explosive emissions of the RNS were restricted to ignimbrites and some explosive breccias that locally overlap the avalanche deposits. The features of the outcrops along the course of the avalanche make it possible to distinguish between the proximal, intermediate and distal deposits. The debris avalanche (3–4 km 3 ) moved SW then S, and finally SSE. Part of the deposit, mainly formed by large megablocks, remained very near the source area. Halfway along the course, the avalanche was channelled by broad and steep paleovalleys. In the most distal areas, the RNDAD opens into a fan-shape, reaching a distance of 28–30 km. A simplified kinematical physical model is used to understand the movement of the RNDAD. The model is based on the speculative reconstruction of the RNS edifice, based in turn on its scarce peripheral remains. An arbitrary avalanche path was selected, with a total travel distance from the source area (top of the stratovolcano) of 28 km. With these assumptions it is possible to calculate the speed at each point ( V m = 90 m/s) and other parameters that show that the RNDAD is very similar to other well-known volcanic avalanches.

Major-element geochemistry and mineralogy of the Huichapan Caldera, Hidalgo, Mexico

Abstract The Huichapan Caldera, a quasi-circular structure ≈8.5 km in diameter, is located in the central part of the Mexican Volcanic Belt. It was formed at about 4.2 Ma by eruption of several flows of San Francisco and Don Guinyo ignimbrites. X-ray fluorescence spectrometry was used to analyze major elements in 28 rock samples. Chemically, these rocks vary from basalts to rhyolites and belong to the calc-alkaline and high-K calc-alkaline series. Common minerals were analyzed (electron microprobe), and mass balance estimates using least-squares approximations were carried out stepwise for rocks and their minerals. These calculations permit us to propose a general fractional crystallization model for the evolution of magmas in the Huichapan Caldera, according to which the most differentiated rhyolitic magma represents about 10% of the parental basaltic magma.

Chemical variations in biotites during prograde metamorphism, Sierra de Guadarrama, Sistema Central, Spain

Abstract With increasing metamorphic grade, the biotites in the metapelites of the Sierra de Guadarrama show an increase of Al IV , Ti, Mn and K, and a decrease of Si and Al VI contents. On the contrary, the biotites in meta-arkoses show increasing Si, Mn and K and decreasing Al IV and Ti contents. The increase of Al IV (and consequently the decrease of Si) of the biotites in the metapelites could have been conditioned by increase of temperature, whereas the decrease of Al IV of the biotites in the meta-arkoses could be related with modal percentage of plagioclase. The variation trends of Mn and K can be related with resorption processes of garnets and progressive breakdown of muscovite, respectively.

The garnets of the eastern area of the Sierra de Guadarrama, Sistema Central, Spain

Abstract Electron-probe microanalysis of a series of garnets in metapelitic rocks of the chloritoid staurolite, kyanite and sillimanite metamorphic zones, eastern area of the Sierra de Guadarrama, Sistema Central, Spain, manifest the well-known cryptozonation commonly observed in these minerals, with MgO and FeO increasing and MnO and CaO decreasing from the center to the outer rim of the crystals. The differences in composition of the garnets, from one metamorphic zone to another, is mainly a result of small differences in composition of the host-rock, since: (1) the amounts of MnO in the garnet are controlled by the amounts of SiO 2 , Al 2 O 3 and FeO present in the host-rock; and (2) the percentages of MnO and MgO of the parent-rock influence in some way the concentration of CaO in the garnet, and those of MnO, Al 2 O 3 and CaO influence the concentration of FeO. Nevertheless, the amount of FeO in the garnet is finally controlled, due to the diadochy, by the concentration of MnO + CaO in this mineral.