1. INTRODUCTION
1.3 E NVIRONMENTS CONTAINING PERIDOTITES
Peridotites consist mainly of olivine, clinopyroxene (cpx) and orthopyroxene (opx) in variable amounts. Based on their composition the peridotite can be divided into harzburgite, dunite, wehrlite and lherzolite (Figure 1).
Figure 1. Scheme for classification off peridotites and pyroxenites, the upper part is for peridotite classification, modified after (Streckeisen, 1974).
Peridotites can be found in different environments (
Table 1), at slow spreading ridges, fore- arc systems, mountain belts, passive margins and transform faults.
3 Table 1. Showing the different environments containing peridotites with examples of localities and the peridotite classification at each environment. The different environments will be further described in the text.
Tectonic environment Some localities/examples Peridotite classification Slow spreading ridges Gakkelridge, SWIR Harzburgite and lherzolite
Fore-arc Mariana Trench, Hahajima
seamount
Harzburgite and dunite
Transform faults Vema and Garret transform fault
Harzburgite and lherzolite
Rifting Red sea Lherzolite
Alpine mantle peridotite Caledonides, the Alps, The Pyrenees
Commonly
lherzolite(Pyrenees, Alps), but also dunitic/harzburgitc (Caledonides)
Ophiolites Oman, Semail Harzburgite and lherzolite
with transformation zone with dunite
In order to evaluate the origin of the meta-peridotites studied in Stølsheimen, an overview of the tectonic environments where peridotites are likely to be found is presented below.
1.3.1 Peridotites at slow/ultra-slow spreading ridges
An ultra-slow spreading ridge is characterized by a low rate of magma production along the ridge and lack of transform faults. Mantle is placed on the seafloor continuously over large areas. The spreading rates are observed to be less than 12mm/year, but the mentioned characteristics are seen at ridges with spreading rates up to 20mm/year (Dick et al.,, 2003).
Serpentinized peridotites have been found at slow/ultra-slow to intermediate spreading ridges (Figure 2) several places and show similar features to Alpine peridotites and peridotites found in the lowermost portion of the ophiolite sequences (Dick et al.,, 1984). There are very few
4 peridotite occurrences at the sea floor in proximity of the fast spreading ridges, the reason for this is the high volcanic activity and the more steady state magmatic upwelling at fast
spreading ridges than at slow (Hekinian, 2014). Peridotites found at or near spreading ridges are referred to as abyssal peridotites. The abyssal peridotites are commonly coarse grained tectonites with porphyroblastic texture. They frequently show strong alteration due to hydration, and serpentinization alters commonly between 20-100% of the peridotites olivine and pyroxene content (Dick, 1989).
Figure 2. Showing the localities of peridotite along mid ocean ridges together with hotspots and their location on land. Notice that the peridotite located on land is named ophiolites in some cases this is thought to not be true see chapter on hyperextension (Hekinian, 2014)
The Gakkel ridge is an ultra-slow spreading ridge with spreading rates between 6-12mm/year.
The ridge extends for 1800 km from the North coast of Greenland to the North coast of Siberia (e.g. Hellebrand et al.,, 2002). The ridge show very deep rift valleys and transform faults are absent. Abyssal peridotites from this ridge have been altered due to serpentinization.
There have also been found cores of spinels that are surrounded by alteration rims. The cores are believed to be remnants of primary minerals (Hellebrand et al., 2002).
5 The Southwest Indian-Ocean ridge (SWIR) is another example of an ultra-slow spreading ridge, with spreading less than 20mm/year. The minerals, found in abyssal peridotite from this ridge, are mainly lizardite, chlorite, carbonates and magnetite together with small amounts of talc, pyroxene and olivine (Zeng et al.,, 2012).
1.3.2 Peridotites in fore-arc environments
Serpentinized peridotite has been found by dredging and drilling in the fore-arc environment.
The peridotites can be situated in seamounts that have been serpentinized or on the landward edge of the trench (Okamura et al.,, 2006). In the lower Köli nappe, serpentinites occurs above and below meta-volcanic rocks (Trouw, 1973). This observation has been interpreted to indicate that the serpentinite could occur as hydrothermal intrusions in former fore-arc
environments (Grimmer and Greiling, 2012).
The Tonga trench, located in the Southwest Pacific, was the first place where peridotites were observed in the nearshore flank of the trench. The peridotite was observed to be fresh in some places, the minerals observed were olivine, opx, and small amounts of serpentine, the
peridotite was classified as a dunite (Fisher and Engel, 1969).
The Hahajima seamount, located in the fore-arc system of the Izu-Bonin arc, show sepentinized peridotite with harburgitic and dunitic lithologies in association with altered gabbros, dolerites and basalts. The degree of serpentination is extensive, and all the data from this locality show between 80-100% serpentinization. Chromite can be observed as cores with magnetite rims, cpx is seen in small amounts also opx and amphibole are observed (Okamura et al., 2006).
The peridotites, from the fore-arc system, are seen to have some things in common. They are most often seen to have a harburgite-dunitic lithology; they have a high degree of melt extraction and having high Cr# in the spinels (Arai, 1994).
1.3.3 Peridotites at transform faults
The transform fault represent the tectonic active parts of fracture zones, the fault makes a discontinuity and disrupt the linearity of spreading centres, and creates depressions separating two spreading ridge segments (e.g. Hekinian, 2014).
6 Peridotites have been found at transform faults, e.g. the Vema fracture zone in the North Atlantic (Cannat et al.,, 1991) and the Garret transform fault near the East pacific rise (Hébert et al.,, 1983).
The Vema fracture zone is characterized by a 10-20 km transform valley floor filled with sediments. The valley floor is bordered by two steep transform faults. Dredge hauls in the Northern flank of the Sema transverse ridge show the presence of basalts, amphiboles, serpentinized peridotites, dolerites and gabbros (Cannat et al., 1991).
The peridotites found at the Garret transform faults were classified as harzburgite, and showed medium grained nodules of peridotite located in a foliated serpentine matrix. The harzburgite nodules were seen to have diopside replacing enstatite located in a matrix of partly serpentinized olivine. Also Cr-rich spinels were observed (Hébert et al., 1983).
1.3.4 Peridotites in the Red Sea
As seen in the previous sections the peridotite found at the mentioned tectonic environments are fully or partly serpentinized and therefore it is not straight forward using them to get information on the mantle, which these rocks originated. At the Zabargad island in the Red Sea on the other hand some of the peridotites are especially well preserved with very little serpentinization.
The Zabargad Island is thought to be an uplifted part the Red Sea lithosphere. There are three groups of peridotites present at the island; a protogranular spinel lherzolite, amphibole
peridotite and plagioclase peridotite. Small outcrops of dunite and wehrlite have also been observed. Together the three peridotite bodies cover 1,5 km2 of the 5km2 island. Other lithologies on the island consist of metamorphic rocks probably of Precambrian age. Dolerite and basaltic intrusion and dikes in the peridotite and metamorphic units, and several
sedimentary rocks like reefs and beach deposits (See Figure 3 for geologic map) (Bonatti et al.,, 1986).
The mantle peridotites at this island are thought to be the result of the emplacement of mantle peridotite diapir within the Africa-Arabian continental crust during the early stages of rifting in the Red Sea (Nicolas et al.,, 1987). The distribution of three different peridotite phases is
7 evidence for small scale heterogeneities in the mantle together with small scale
metamorphism and metasomatic processes. The fo-number of olivine together with the Mg-number implies that the Red Sea peridotite is similar to the one found in ocean basins. Based on that information two theories on the origin of the peridotite has been proposed; either as ophiolites or as uplifted upper mantle, the first theory is unlikely since none of the classical characteristics of ophiolites can be found at the Zabargad Island. The first peridotite that got uplifted probably was the result of thermal upwelling that is known to happen in rift zones (Bonatti et al.,, 1981). At the time this complex on Zabargad was described (before 1990) the concept of hyperextension was not known and alternative models for exhumation of these rocks have not been suggested later. There has however, been suggested that this rocks have been exhumed in transform submarine fracture zones that is located North of the island. The fracture zone caused differential movement between the central Red Sea rift and the Northern Red Sea lift and caused the exhumation of the peridotite (Marshak et al.,, 1992).
Figure 3. Geologic map over Zabargad Island. The different symbols represents; 1: limestone from young reef, 2: limestone from old reef, 3: conglomerate and breccias, 4: evaporates, 5: Zabargad sedimentary formation, 6: metamorphic group, 7: peridotite 8: Basalt and dolerite intrusion and dikes, 9: nickle mineralization 10: faults. Figure from Bonatti et al. (1981)
It is interesting to note that these peridotites are intimately associated with old gneisses, reef and metamorphic rocks as well as young limestone and conglomerates and sandstones eroded from the exhumed rocks. All these rocks have been mixed together without compressional tectonics in a mountain belt.
8
1.3.5 Alpine mantle peridotite massifs
These peridotites range in size from a few meters to several km. The Ronda massif is an example of an orogenic peridotite massif in Spain that cover approximately 300km2 peridotite (Suen and Frey, 1987). Most Alpine mantle peridotites are not that extensive. Observations show that they are intermediate in size and range from 100 meter to a few km. It is also common to find several Alpine peridotites in the same area. The orogenic peridotite massifs are frequently lherzolites (Figure 1) with composition equilibrated in the plagioclase, spinel or garnet field (Bodinier and Godard, 2003).
Based on mineral assemblage, pressure and temperature (P-T) conditions orogenic peridotite massifs can be divided into three categories (Bodinier and Godard, 2003):
High pressure (HP)/ Ultra High Pressure (UHP) massifs
Peridotites from this group are found in many high pressure terrains in mountain belts e.g. the Western Gneiss Region (WGR) in the Caledonides. The peridotite can be of any of the
classification mentioned (Figure 1) and they commonly are associated with garnet, and therefore garnet peridotites. The garnet peridotites from such terrains show many similarities, they are strongly depleted and cut by veins of garnet pyroxenite and in some cases associated with eclogite. This group can be further divided (Brueckner and Medaris, 2000).
Prograde peridotite: peridotite thought to originate from the supra-subduction mantle wedge.
The peridotites are exhumed to the surface when continental rocks underthrust in the mantle due to continent-continent collision. The continental rocks are more buoyant and then starts to rise. At this point the peridotite will intrude the slab, and get garnet bearing through
subduction and prograde metamorphism (Bodinier and Godard, 2003). An example of this category of peridotites can be found in the Western Alps, the Alpe Arami (Green II et al.,, 2010). Also the peridotites found in the Seve Nappe complex in Sweden have been suggested to be of this origin (e.g. Brueckner et al.,, 2004, Brueckner and Van Roermund, 2007).
Relict peridotite: This category of peridotites consists of old subcontinental lithosphere that shows no evidence of subduction processes. The creation of this group of peridotites is still unclear. The garnet peridotites observed in the WGR have been suggested to be of this origin
9 (Brueckner, 1998). The different assignment of the WGR peridotites shows that their origin is still unclear.
Intermediate pressure (IP) massifs
Peridotites in this category are equilibrated in the spinel field. This category of peridotites commonly show preserved mantle structures and for this reason data from IP massifs are preferred to use for geochemical studies of Alpine peridotites . The mineralogy is also often well preserved since IP peridotites have not been altered by prograde recrystallization in the garnet or retrograde plagioclase peridotite field. Examples of this category of peridotites can for example be found in the Ronda massif in Spain and the lherz peridotite in the Pyrenees (Bodinier and Godard, 2003).
Low Pressure (LP) massifs
Consist of commonly fertile lherzolite that was exhumed during continental rifting and exposed as denudated mantle on the seafloor at passive continental margins (Bodinier and Godard, 2003). This group of peridotite is commonly found in the Western parts of the Alps where they form a belt between the Alpine and the North Appenine arc. Ophicarbonated and oceanic sediments are regularly found juxtaposed with the serpentinized peridotite. The peridotites frequently have strong alteration due to hydrothermal activity, and frequently peridotites are situated in cores surrounded by serpentinite (e.g. Beltrando et al.,, 2014).
1.3.6 Exposures of solitary Alpine mantle peridotites/serpentinite
The Pyrenees
The peridotites in the Pyrenees consist of approximately 40 bodies ranging from a few metres to 3 km across. The peridotites can be found in an
approximately 400 km long, but only a few km wide zone that is located parallel to the
Figure 4. Simplified map showing the locations of alpine mantle peridotites in the Pyrenees mountain belt from Lagabrielle et al. (2010)
10 North Pyrenean fault zone (Fabriès et al.,, 1991) (Fig. 2). Most of the peridotites exposed in this zone are well preserved with little serpentinization. The Lherz peridotite body, which is the type locality for lherzolite show characteristics that imply it is of subcontinental origin (Lagabrielle and Bodinier, 2008)
Based on mineralogy and geothermobareometry the peridotites in the Pyrenees can be divided into the Eastern massifs (EP) and the central Western massifs (CWP). The CWP massifs shows an abundance of coarse grained structures and the peridotites consist of spinel
lherzolites with a large amount of cpx (Fabriès et al.,, 1998). All the massifs in this part of the Pyrenees have been affected by hydrothermal alteration, this alteration result in
serpentinization. The degree of serpentinization varies from one massif to another; some are completely serpentinized while some others are only slightly affected. Cores of relict olivine have been found in mesh textures of serpentine. The opx has commonly been replaced by pseudomorps often by talc and actinolite amphibole (Fabriès et al., 1998).
The EP massifs consist of layered spinel lherzolites with harzburgite layering, the cpx content in these massifs are low. Also spinel websterite can be found. The borders of the massifs are often brecciated, but inside the lherzolite it can be found areas of rather fresh peridotite (Burnham et al.,, 1998).
The Alps
The peridotites seen in the Western parts of the Alps probably had their origin in the Tethys Ocean (Manatschal and Müntener, 2009). They all underwent a great amount of
serpentinization, and thus consist mainly of antigorite and small amounts of magnetite
(Barnes et al.,, 2014). The serpentinites are juxtaposed to different rock types including meta- pillow-basalt, meta-gabbro and meta-sediments (Beltrando et al., 2014). At the borders of the different lithologies reaction rims due to metasomatism can be witnessed, e.g. in the Piedmont unit where talc, tremolite and carbonate can be seen at the borders of the ultramafic bodies.
No relict fresh peridotite is found at this locality, the olivine and the opx has been altered to brucite and antigorite (Beltrando et al.,, 2012).
The Caledonides
The ultramafic rocks found in the Caledonides can be found as cumulates in layered intrusions, Alpine peridotites and as detrial serpentinites (e.g. Qvale and Stigh, 1985). The
11 Alpine-type peridotites are most abundant, and can be seen as; ultramafic associated with ophiolites and solitary Alpine peridotites. The solitary bodies lay in metasedimentary sequences and are lens-shaped. The contact zone is commonly highly deformed. In South Norway, the unit with abundant solitary meta-peridotites have been referred to as a melange (Andersen et al., 2012, Corfu et al.,, 2014)
Based on mineralogy and textures the solitary Alpine mantle peridotites can be divided into three groups; category one can be found in rocks of Lower Paleozoic age. They may have primary olivine, cpx, opx (rare) and chromite, but they are often fully serpentinized. Type two consists of polymetamorphic meta-peridotites. The minerals present are olivine, enstatite, carbonates, talc and amphiboles. The host rock is medium to high grade metamorphic. Type three occurs in gneisses, and the peridotites have a metamorphic mineral assemblage
consisting of olivine, opx and small amounts of chromite. Cpx, amphibole and chlorite can also be present (Moore and Qvale, 1977).
The Alpine mantle peridotites can be seen as massifs with low Al-content with relict metamorphic structures found in harzburgitic, dunitic or as layered bodies thus with a relict cumulate structure (Qvale and Stigh, 1985).
In the Caledonides most of the modern studies on meta-peridotites have been on the high- and ultra-high pressure rocks in the WGR and the Seve Nappe Complex. Less attention has been paid to the melange, and they have mostly been regarded as remnants of ophiolites and not much studied until recently (Beinlich et al.,, 2010) and this work.