7th Lherzolite Meeting, SpainNow that the deadline to register for the 7th Orogenic Lherzolite conference is fast approaching, I will start a thread showing different aspects of the rocks we will see on the pre-conference excursion. #Lherzolite2024 #Geoscience #CaboOrtegalComplex
Let's start with the mantle rocks. In the Cabo Ortegal complex they consist mainly of serpentinised amphibole-bearing harzburgites, dunites and massive pyroxenites, outcropping as rock bodies ranging in size from hundreds of metres to a few kilometres.
Let's look at a few examples. Mantle rocks are mainly found in three massifs called Limo, Herbeira and Uzal/Ouzal. The one we see here is that of Limo, consisting mainly of serpentinised harzburgites. The distance from the top to the sea is 500 metres.
The Cabo Ortegal mantle section is well known for the large amount of pyroxenite it contains, particularly the Herbeira massif below. Pyroxenites are high-MgO pyroxenites that outcrop as layers with sharp contacts alternating with peridotites. The distance from top to the sea is 609 m.
It was in the Herbeira massif that a complete column of the mantle section was established in the classic paper ‘Evidence for a Heterogeneous Upper Mantle in the Cabo Ortegal Complex, Spain’ published in Science in 1989 by Girardeau and co-workers.
The following is an outcrop example of the unit with massive pyroxenites alternating with dunites. In particular, massive websterites alternating with dunites. In some areas the thickness of the pyroxenite layers exceeds one metre.
Romain Tilhac (2018) identified four pyroxenite types: dunite-lensed (type 1), massive websterites (type 2), foliated websterites/clinopyroxenites (type 3), and Opx-rich pyroxenites (type 4). Here is an example of alternating Cpx and Opx-rich websterite from another PhD thesis (García-Izquierdo, 2005)
Some pyroxenite layers are folded, the most common being completely rootless folds, usually with thickened hinges.
Many of the pyroxenites have garnet and amphibole, with the garnet entering after spinel. There are some spectacular examples of veins with huge (centimetric) garnets and chlorite associated with these garnet pyroxenite zones.
The occurrence of PGE-enriched chromitites in the Cabo Ortegal peridotites is also common. There are two types high-Cr (>0.6) and low-Cr (<0.6). Here are some examples:
Peridotites are primarily amphibole-bearing spinel harzburgites with up to 30% of Mg-hornblende. They are strongly deformed and serpentinized (up to 70% in extreme cases). Chlorite is also very common. Below is an example of harzburgite with chlorite aggregates defining the mineral lineation.
We're picking up where we left off yesterday, now talking about high-pressure granulites and other related ultramafic rocks of the Cabo Ortegal Complex. #CaboOrtegalComplex #granulites
Why do we talk about high-pressure granulites at all? 🤔 Because these are granulites that contain garnet and lack orthopyroxene. Opx-free granulites are rare, found in about 10% of granulitic terranes worldwide according to Harley (1989).
As pressure increases, garnet forms from Opx and plagioclase. Opx disappears because there is usually more Plag than Opx. See the schematic PT diagram below created by Pattison (2003).
Cabo Ortegal granulites are characterized by the lack of Opx, a high garnet content, and very variable contents of Cpx, Amph and Pl. Also, they present a wide compositional variation from ultramafic to acidic compositions.
Thermobarometric data indicate that the Cabo Ortegal granulites reached pressures of 1.8 GPa and temperatures >750°C. A sub-facies called high-P migmatites peaked even higher: 1.9-2.3 GPa and 815-865°C (Beranoaguirre et al. 2020).
The most common granulites are those of mafic composition, usually referred to as 'layered granulites' because of the strong layering produced by the contrast between mafic and plag-rich layers (LS tectonites). They look like this:
Intermediate compositions are garnet-rich, often lacking Cpx, and show clear evidence of widespread partial melting.
The more acidic compositions (trondhjemitic gneisses) can be cutting the granulitic layering (see image below). The dark rock in the image is a pyrigarnite (explanation on this below!)
Wait! You mentioned ultramafic facies above, what about them? These are bi- or tri-mineral rocks (Grt-Cpx-Amph), with a Pl+Qtz content <5%. They range from Grt-Cpx-rich rocks, called pyrigarnites, to those mainly composed of amphibole (hornblendites).
By the way, the term pyrigarnite was first defined in these rocks by Vogel (1967). Although today this metamorphic term is discouraged.
Below are some examples of thin sections. They vary from Grt-rich (15-XY) Grt-Cpx pyrigarnites, Grt-Cpx-Amph (30B-XY), Amph-Cpx (90%) (12-XZ, 70% Amph, Grt<2%) or predominantly Amph (30A-XZ).
The main outcrop of these rocks is a band of tens to hundreds of metres in contact with the peridotites that outcrop in the Uzal/Ouzal massif, both forming a spectacular fold that places the peridotites on top of the ultramafic granulites.
Besides these main types, there are other volumetrically much smaller or exotic rock types within the granulites. As an example, these calcium-rich granulites including pyrigarnite blocks taken from the PhD thesis of Puelles (2005).
And that's it for today. We will continue this thread in the coming days, describing the Cabo Ortegal eclogites. #CaboOrtegalComplex
We're picking up where we left off last day, today talking about the eclogites of the Cabo Ortegal Complex. #CaboOrtegalComplex #eclogites
The eclogites of Cabo Ortegal form a continuous band, about 20 kilometres long and at least 100 metres thick, forming crests that stand out in the landscape. The lighthouse of Cabo Ortegal rests on these eclogites.
Eclogite conditions peaked at ~1.7-2.3 GPa with a thermal maximum between 770-800 °C (Mendia 2000) in both common and Ky-bearing eclogites (Mendia, 2000).
In the Cabo Ortegal Complex, eclogites are always sandwiched between Opx-free garnet granulites to the west and high-P paragneisses to the east. The paragneisses, in fact, contain retrogressed eclogite blocks.
According to Mendia (1996), this eclogite band comprises three types of eclogite: common eclogites (volumetrically dominant), kyanite-bearing eclogites, and the least common ferro-titaniferous eclogites.
Common eclogites vary from massive, lacking lineation and showing weak foliation, to highly deformed (mylonitic LS-tectonites). Garnets typically range in size from 1 to 3 mm, but can exceptionally exceed 2 cm. Here are some examples.
The deformed facies are typically enriched in zoisite and amphibole (Zo-Amph eclogites) relative to their precursor, which is indicative of high pressure wet deformation. Here some examples:
Ky-bearing eclogites are lighter in colour, contain garnets and Cpx ranging in size from a few millimetres to several centimetres (exceptionally up to 6 cm) and are always strongly deformed.
Although difficult to see at outcrop scale, the eclogites also developed tight folds that can be deduced from geological mapping, but also visible in some cases at the scale of hundreds of metres.
John Faithfull 🌍🇪🇺🏴🧡✊🏻✊🏿