Magmatic breccia; Prado de las Pozas, Gredos

 

The detailed map below was carried out by undergraduate students (1) of Geology at the University of Huelva during a field-work training course in igneous and metamorphic rocks in Gredos (Spanish Central System), guided by Antonio Castro and Carmen Rodríguez Ruiz de Almodóvar, in July 2013.

3 m

For a geological location of this area in the Avila Batholith click hereGredos_batholith.html
 

Prado de las Pozas is a high and flat area at 2000 m altitude in the Gredos Mountains, Avila province. The outcrops of basic rocks extend over a small area about 600 m across. It is a large breccia formed by irregular, angular and globose bodies of massive diorite invaded by the host monzogranite and aplite-pegmatite veins

High resolution pictures can be download from flickr. Click herehttp://www.flickr.com/photos/99495443@N08/9369091571/lightbox/
 

To make the detailed map we used two guides (cords; see arrows) separated by 1 m distance and marked at 1 m length. Each student is drawing to scale (1:10) in a reticulated paper the relations and contacts of 1 squared meter area. After a band is finished, the cord is shifted 1 m ahead and the new band is mapped out. Finally, all individual sections are put together into a single transparent paper. To do the final map, the students have to discuss about the criteria used in distinguishing rock types and structures to match each band into a unified map. Variations in grain size, crystal orientations, fractures, transitional contacts, etc. are among the most relevant observations transferred to the map. The work requires a high concentration. Observations are forced to define carefully types of rocks and variations. This a good exercise for mapping igneous rocks over a larger km scale. In this case, mapping of the outcrop ids essential to interpret the apparently complex relations between diorites and host granite.

Now we have the map and go to the interpretation

Important observations on the map


  1. 1.Diorites form fragmented globular bodies that were dismembered from a major globular intrusion

  2. 2.Fine-grained zones are irregularly distributed near the contacts with the host monzogranite

  3. 3.Aplite and pegmatite veins are concentrated in the fine-grained zones

  4. 4.Flow structures (orientations of K-feldspar megacrysts) are embracing the diorite globular bodies

  5. 5.Fine-grained enclaves (autoliths) from the margin facies are present in the medium-grained diorites

  6. 6.Aplites and pegmatites are transitional to the host monzogranite

  7. 7.Ocelar quartz crystals, showing a corona of mafic minerals, are common

We show here several pictures on these observations

Sierra de Gredos
Prado de las Pozas
 

Quartz ocelli surrounded by a corona of mafic minerals (Hornblende and biotite). These are interpreted as xenocrysts that were captured by the intruding diorite into a crystal rich granite. The interpretation of these ocelli as xenocrysts is the most widely acepted. However, some of them are bigger than normal crystals in the host granite. The elongated ocelli on the right hand photo is polycrystalline and 2 cm length. Another possible origin is fragmented quartz veins... ? These ocelli are enigmatic, and present in many other similar complexes (e.g. in Gerena in Seville).

Enclaves of fine-grained marginal facies enclosed by medium-grained diorites. These are interpreted as autoliths, i.e. fragments from the margins that were eroded and captured by the intruding magma

The fine-grained zones close to the contacts against the host granite are interpreted as chilled margins. The diorite magma is frozen against the colder granite magma. In the right hand photo, we may observe the veins of aplite and pegmatite intruding back to the fragmented diorite. These are filled with fluids (water-rich magma) coming from the residual melt in the host granite magma.

Cartoon showing a hypothetical magma chamber in which a solidification front is formed. The crystal mush of the margins is intruded in a near-solid state by dikes of mafic magma dragging by magmatic erosion crystals from the host that are incorporated as xenocrysts. Hypothetical locations of pictures are labeled in boxes.

Here is a fragment of text from a recent paper about the implications of synplutonic intrusions in relation with the formation of a solidification front in plutons



/...Synplutonic intrusions of intermediate magma are common feature in calc-alkaline batholiths. They may be fragmented to form mafic microgranular enclaves (Vernon, 1984; Pitcher, 1987; Didier and Barbarin, 1991). Occasionally, they may appear as irregular bodies or magma blobs with lobate and crenulated contacts with the host granite (Fig. 7a). They have unequivocal features of coexistence as magmas. An intriguing feature is the presence of rounded Qtz crystals rimed by a corona of magmatic Hbl (Fig. 7b). These are interpreted as xenocrysts trapped by the mafic magma during intrusion into the felsic host magma. Paradoxically, Qtz is a late phase in the host granite, his presence implying a high crystal content that can be close to the rheological threshold (60-70 vol% crystals; (Vigneresse et al., 1996). However, the felsic magma hosting the synplutonic dikes behaved as crystal-poor fluid, which is evidenced by the liquid-liquid contacts (Fig. 7a). The inference is that Qtz crystal entrapment was previous to the arrival of the mafic magma to the final level of emplacement. The passage of the mafic magma through a crystal-rich zone of the felsic chamber or pluton is a necessary condition to account for the presence of Qtz xenocrysts. Xenocryst entrapment at the contact of synplutonic dikes (Fig. 7c) and crystal mingling zones in areas of high crystal contents (Fig. 7d) are common features in calc-alkaline plutons, giving account of mechanical interaction between a crystal-poor mafic magma and a crystal-rich granitic host. This crystal-rich zone may correspond to the outer part of the pluton, supporting the existence of a solidification front. Accordingly, the mafic magmas crossed the solidification front and were finally emplaced into a low-crystalline zone of the magma chamber (Fig. 7e). In parallel with this observation is the need for a rigid host able to accommodate by brittle fracturing mafic dikes of magmas that are denser than the granite host.  Once the mafic magmas have crossed the rigid carapace of the pluton, where magma was almost completely crystallized, they are emplaced as magma blobs in a crystal-poor area inwards of the pluton. In summary, the presence of Qtz xenocrysts in synplutonic intrusions of mafic magmas are indicating the existence of a more crystalline zone, possibly close to the walls of the host silicic pluton.../


Castro, A., 2013. Tonalite-Granodiorite suites as cotectic systems: A review of experimental studies with applications to granitoid petrogenesis. Earth Science Reviews, 124, 68-95 DOI: http://dx.doi.org/10.1016/j.earscirev.2013.05.006.

 

The map of the outcrops at Prado de las Pozas represent one of the lobes of a large (>500 m diameter) body of diorite magma that was intruding into a monzogranite magma. During cooling of the whole granite intrusive complex (granite and diorite), the diorite becomes solid before the host granite. For instance, at the temperature of 800 °C the diorite may be a solid rock but the host granite may have a 50 vol% liquid. The rigid parts of the diorite lobes (the chilled margins) behave as a rigid body (rock) and are fragmented and intruded by residual liquid of the host granite magma. These are the veins of aplite and pegmatite. Before to reach the level of emplacement, the diorite traversed a high crystalline zone of the monzogranite, where xenocrysts of quartz were trapped (also plagioclase). Quartz xenocrysts are a proof about the presence of a crystal-rich zone in the pluton. This is the expected solidification front, from which the pluton is crystallizing from the walls inwards. A near-solid region in the granite magma is a condition needed to allocate the intrusion of a more dense magma by pressure gradient imposed by fractures opened in the near-solid margin of the pluton. This is drafted in the cartoon below.

On the implications of the quartz ocelli in the diorites of the Gredos massif, Scarrow et al. (2009), pag 63 say:

(...The presence of coarse quartz xenocrysts derived from the granodiorite in the appinite stock border facies (Fig. 2D) indicate that the granodiorite must have been partially solidified with a significant crystal fraction when the basic magmas were emplaced....).

J.H. Scarrow, J.F. Molina, F. Bea, P. Montero (2009) Lithos 110 (2009) 50–64

Different aspects of the breccia-like relations observed in the complex. Some veins contain turmaline (left hand photo)

Flow structures defined by K-feldspar megacrysts in the host monzogranite