Download Alpine Granites

Alpine Granites
Alps Excursion 2013
Jacqueline Engmann (294225)
Applied Geosciences (M.Sc.)
Supervisor: Prof. Urai & Prof. Littke
Table of Contents
Abstract ................................................................................................................................................... 3
1.
Introduction .................................................................................................................................... 3
2.
Geometry and Emplacement of the Intrusions ............................................................................... 4
2.1
2.1.1
Aar Massif ........................................................................................................................ 5
2.1.2
Gotthard Massif ............................................................................................................... 6
2.1.3
Kreuzberg-Iffinger-Brixen Pluton..................................................................................... 7
2.2
3.
Variscan Intrusions .................................................................................................................. 4
Alpine Intrusions...................................................................................................................... 9
2.2.1
Bergell Pluton ................................................................................................................ 10
2.2.2
Adamello Batholith ........................................................................................................ 11
2.2.3
Rieserferner Pluton ....................................................................................................... 13
Conclusion ..................................................................................................................................... 14
References ............................................................................................................................................. 15
2
Alpine Granites
Abstract
Most of the granitic intrusions in the Alps can generally be divided into Variscan (Mesozoic) and
Alpine (Cenozoic) intrusions. This paper will focus on the geometry and the factors of emplacement
of the Aar and Gotthard Massifs (part of the External Crystalline Massif) and the Kreuzberg-IffingerBrixen pluton on the one hand and the Bergell, Adamello and Rieserferner plutons on the other.
While the former represent an example for Variscan intrusions, the latter constitute Alpine
intrusions. The Alpine plutons are associated with the activity of the Tertiary Periadriatic Fault
System, an orogen-parallel dextral transpressive belt.
1.
Introduction
This paper will mainly focus on the granite intrusions that were examined during the Alps Excursion
2013. In this context the intrusions can be subdivided into the Variscan granites (Chapter 2.1) to
which the Aar and Gotthard Massifs and the Kreuzberg-Iffinger-Brixen pluton belong and the Alpine
granites (Chapter 2.2), which are represented in this paper by the Bergell, Adamello and Rieserferner
intrusions.
The formation of a granitic intrusion involves four stages: generation, segregation, ascent and
emplacement. Granitic melts are generated by partial melting of crustal rocks, where heat is
advected into the crust from the underlying hotter mantle by basaltic magmas. The melts are
transported by two processes, the segregation (fractionation) and long-scale ascent. The
fractionation occurs mostly within the source region, while the ascent takes place through the
continental crust to the site of final emplacement. Transport and final emplacement can occur due to
different mechanisms, such as ballooning or stoping. During a ballooning process an asthenospheric
plume ascends through the lithosphere and inflates like a balloon due to density differences, when it
gets closer to the surface. Stoping is a process of magma emplacement, where the magma chamber
rises, because the roof material breaks into the chamber until the roof collapses, creating space for
the rising magma (PETFORD ET AL., 2000)
3
2.
Geometry and Emplacement of the Intrusions
2.1
Variscan Intrusions
The Aar and Gotthard Massifs are part of the
External Crystalline Massifs (ECM), which belong
to the Helvetic domain and are limited in the east
by the Frontal Penninic Thrust (DEBON ET AL.,
1999). The External Massifs formed during the
Variscan orogeny as a part of the HelveticMoldanubian terrane of the internal zone of the
Variscides (DEBON ET AL., 1999). The Argentera,
Pelvoux, Belledonne and Aiguilles Rouges/Mont
Blanc intrusions are part of the External Massifs
and belong to the Western Alps, while the Aar
and Gotthard Massifs represent the ECM in the
Central Alps (PFIFFNER, 2010) (Fig. 1). The former
intrusions are generally surrounded by Mesozoic
sediments and occur as dome-like structures of
crystalline basements. The Massifs are arcFig. 1: External Crystalline Massifs in the Western and
shaped, which is most likely due to the influence
Central Alps (http://origin-ars.els-
of the Variscan orogeny (DEBON ET AL., 1999). The
cdn.com/content/image/1-s2.0-S1631071308002782-
oldest plutons of the External Crystalline Massifs
gr1.jpg)
were emplaced during the Lower Carboniferous (350-330 Ma). They are K-rich, part of the calcalkaline series and of S-type origin, i.e. the magma is derived from continental crust (GRATZER ET AL.,
1993). The plutonic bodies are referred to as Late Variscan intrusions since they intruded the crust
pile after the collision of Gondwana with Baltica (PFIFFNER, 2010). Furthermore they are associated
with a Late Variscan strike-slip regime. During the Upper Carboniferous (320-290 Ma) the
emplacement of alkaline-calc-alkaline plutons occurred due to extension (PFIFFNER, 2010). The
plutons of the External Massifs occur as bodies of numerous shapes, such as ellipses to almost linear
forms. Their sizes range from <1km2 up to 550 km2 and generally reveal sharp contacts to the country
rocks (DEBON ET AL., 1999).
4
2.1.1 Aar Massif
According to DEBON ET AL. (1999) the Aar and Gotthard massifs consist of Precambrian metamorphic
units as well as of Precambrian to Palaeozoic sediments interlayered with volcanic and ultramafic
rocks. Most areas underwent a polymetamorphic history displaying a complex pre-Variscan and
Variscan evolution. The crystalline rocks of the basement are only weakly affected by alpine nappes
tectonics and originate from basement blocks of the European continental crust. Both massifs are
strongly deformed thrusting in northward direction (LÜTZENKIRCHEN, 2011).
Fig. 2: Aar massif displaying the Tödi granite in the east (green) and the Central Aar granite in the core (red) (modified
after DEBON, 1999)
The Aar massif extends over 110 km in northeast direction, cropping out in the eastern part of the
Bernese Alps and the Lepontine Alps, between Leukerbad to the west and Tödi to the east
(SCHALTEGGER, 1990). The basement rocks are mainly composed of gneisses, schists and amphibolites
featuring a main metamorphic overprint of Variscan age that reached amphibolite facies conditions.
These units are separated by mylonite zones (DEBON ET AL., 1999). After the Variscan orogeny the
basement rocks were intruded by Permian granites; the Aar granites. In a late phase of the Alpine
orogeny a thrust fault brought the basement to the surface, uplifting the Aar massif in the form of a
large elongated dome structure (SCHALTEGGER, 1990). According to DEBON (1999) and references
therein three intrusive suites can be distinguished within the Aar massif; the shoshoniticultrapotassic, the high-K-calc-alkaline and the calc-alkaline to sub-alkaline granitic suites. Two
plutonic members of these types will be presented in the following.
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The Tödi granite belongs to the shoshonitic-ultrapotassic suite comprising a surface area of less than
1 km2 (Fig. 2). The pluton is a fine grained, strongly deformed porphyritic granite of Carboniferous
age (~333 Ma) that intruded into clastic sediments (~350-340 Ma) in a graben to the eastern part of
the Aar massif (DEBON ET AL., 1999). These metasediments were overprinted by a
contactmetamorphism during the emplacement of the pluton. Additionally they underwent a
regional metamorphism and folding, the age of these processes, however, is still unknown. Around
310 Ma ago dykes formed within the Tödi granite, about 10 Ma later the pluton was exhumed and
covered by late carboniferous volcaniclastics (PFIFFNER, 2010).
The Central Aar granite belongs to the calc-alkaline suite and comprises a surface area of 550 km2
(Fig. 2). It forms the magmatic core of the Aar massif (SCHALTEGGER, 1990) and is composed of coarse
to fine grained, massive to strongly foliated granodiorites and granites (DEBON ET AL., 1999). The
Central Aar granites were emplaced at a depth of less than 10 km due to stoping processes resulting
in discordant contacts. It shows remnants of the pendant roof at a height of about 3500 m a.s.l..
During the emplacement the pluton intruded basement and volcaniclastic rocks resulting in a
contactmetamorphism followed by an alpine greenschist facies metamorphism (SCHALTEGGER, 1990).
2.1.2 Gotthard Massif
Fig. 3: Gotthard massif. UGZ=Urseren-Garvera-Zone, NPZ=Nufenen-Piora-Scopi-Zone, CG=Cristallina Granite,
GC=Gamsboden Granite, FG= Fibbia Granite, RG=Rotonda Granite (LÜTZENKIRCHEN, 2011)
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The Gotthard massif extends over 80 km parallel to the Aar Massif in W-E and about 10 km in N-S
direction. The Aar and Gotthard massifs are separated by the “Urseren-Garvera”-zone, which defines
the northern boundary of the Gotthard massif (Fig. 3). It consists of metamorphosed and deformed
Carboniferous to Mesozoic sedimentary cover rocks of the Tethys Ocean (DEBON ET AL., 1999). The EW striking Nufenen-Piora-Scopi-zone (Fig. 3), consisting of Mesozoic rocks, displays the southern
boundary of the Gotthard massif (LÜTZENKIRCHEN, 2011). The massif can be divided into four major
Variscan granite intrusions: Fibbia, Cristallina, Gamsboden, Rotondo granite (Fig. 3). These four
intrusions can be divided into two groups concerning the structural and chronological data; an older
group, which is composed of deformed rocks and a younger group consisting of massive rocks that
display only weak deformation (DEBON ET AL., 1999). This leads to the assumption that there had to be
a Variscan deformation phase between the emplacement of the first group and the intrusion of the
younger group, which is less affected by a Variscan compressional deformation (LÜTZENKIRCHEN,
2011). The Fibbia granite (~299 Ma) with a surface area of 8 km2 consists of coarse grained
porphyritic syenogranites and belongs to the older group, as well as the Gamsboden monzogranite
(~301 Ma) with a surface area of 13 km2 and the Cristallina calc-alkaline granodiorite. The Rotondo
syenogranite (~295 Ma) on the other hand can be ascribed to the younger group. It has a surface
area of 26 km2 and intruded in a late phase of the Variscan orogeny undergoing an amphibolite facies
overprint (LÜTZENKIRCHEN, 2011). According to LÜTZENKIRCHEN (2011) the Mesozoic cover of sediments
of the Gotthard massif was almost entirely detached during the Tertiary Alpine compression phase
(38 Ma), leaving behind the metasedimentary zones of the Nufenen-Piora-Scopi and the UrserenGarvera. In the lower Miocene, the Gotthard and later the Aar massifs were backfolded, rotating the
main tectonic boundaries, foliation and shear zones into a subvertical orientation (LÜTZENKIRCHEN,
2011).
2.1.3 Kreuzberg-Iffinger-Brixen Pluton
This pluton is divided into three plutonic bodies situated along the Periadriatic Lineament, a dextral
transpressive fault system that is discussed in more detail in the paper “The Periadriatic Lineament
and the role of strike slip faulting in Alpine tectonics” by Bernadette Bastian.
The Brixen pluton comprises a surface area of 180 km2 and extends over 30 km in E-W and 10 km in
N-S direction, from Pens to Pustertal to the west (Fig. 4). In Pens the Brixen pluton transitions into
the Iffinger pluton (GRATZER ET AL., 1993). The Brixen intrusion comprises a medium grained, felsic
granodiorite of Late Variscan, Permian age (281 Ma). Its emplacement occurred along the JudikarienPustertal-Line to the north, which is part of the Periadriatic Lineament (EXNER, 1976). It creates a
tectonic boundary to the north and northwest of the plutonic body, while the southern margin still
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Fig. 4 Kreuzberg-Iffinger-Brixen pluton (modified after POMELLA ET AL., 2012)
displays the initial contacts of the intrusion consisting of characteristic minerals (andalusite,
cordierite, corundum, sillimanite) from a contact-metamorphism. The intrusion is the boundary
between the Eastern and the Southern Alps and separates the crystalline basement of the Eastern
Alps from the Brixen quartzphyllite, in which the pluton intruded (EXNER, 1976).
The Iffinger pluton extends over 20 km in NE-SW and 3 km in NW-SE direction along the Giudicarie
Lineament (Fig. 4). It is of granodioritic composition of Permian age (291 Ma), similar to the Brixen
pluton. In the NW the plutonic body is confined by a tonalitic gneissic zone. The initial contact of the
intrusion with the Eastern Alps still exists in some places (EXNER, 1976).
The Iffinger pluton transitions in the SE ino the Kreuzberg intrusion. It is located in the Ultental, SW
of Meran and extends over 8 km in the same direction as the Iffinger pluton (Fig. 4). It is limited by
the Giudicarie Line to the west and the Völlaner fault to the east. On both sides of the intrusion the
plutonic contacts still exist (GRATZER ET AL., 1993). The pluton consists of mostly medium to coarse
grained Permian granodiorites (275 Ma) and rarely granites and diorites. In the contact zones of the
intrusion, however, the granodiorite and granites become fine grained (EXNER, 1976).
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2.2
Alpine Intrusions
The formation of the Alpine granites is
strongly connected to the Periadriatic
Fault System (PFS). The syntectonic
ascent and emplacement of the plutons
took place during a transpressive regime
along fracture planes in the vicinity of the
PFS (ROSENBERG, 2004). The intrusions are
accompanied by dykes and veins and
formed during the Middle Eocene (42
Ma) to the Lower Oligocene (28 Ma).
Except for the Adamello batholith (42
Ma), the emplacement took place almost
simultaneously over a short period of
time (34-28 Ma) (Fig. 5).
Fig. 5: Age of Periadriatic Plutons (ROSENBERG, 2004)
The intrusions are exposed over 700 km along the Periadriatic Fault System, which occurred to be an
active dextral strike-slip and a thrust fault during the Tertiary (PFIFFNER, 2010). The largest Periadriatic
granite intrusions are represented by the Biella, Bergell, Adamello, Rieserferner, Karawanken and
Pohorje plutons. They are plutons of calc-alkaline composition and mainly of I-type origin, i.e. the
magma is derived from source rocks of igneous composition (GRATZER ET AL., 1993). Magmatism
occurred during continental collision and generated Cr-, Ni-, Sr-, Nd- enriched melts in the
lithosphere in 40 to 50 km depth (PFIFFNER, 2010). According to ROSENBERG (2004) the magmas were
channeled from the base of the thickened continental crust into the narrow mylonitic belt of the
Periadriatic Fault System. The PFS acted as a pathway for the ascending magma and covered vertical
length of 20 to 40 km. Hence, the linear alignment of the plutons at the surface is not caused by a
linear source region at depth. Final emplacement of the intrusions occurred by extrusion from the
PFS into the adjacent country rocks (ROSENBERG, 2004). In some places the magma ascended more
than 30 km resulting in the emplacement of plutons at shallow depths of 5 to 10 km (PFIFFNER, 2010).
The formation of diorite, tonalite, granodiorite and granite melts are most likely due to the process
of mantle melting mixed with partial melting of the mafic lower crust, followed by a fractionated
crystallization. Most of the plutons are of tonalitic and granitic composition, while gabbros, diorites
and granites are less present (PFIFFNER, 2010).
9
The surrounding dykes are of similar composition as the plutonic bodies and are situated about 50
km around the intrusions along the Periadriatic Fault System. Hence, the melts of the lithospheric
mantle were not limited to the vicinity of the plutons, but occured over a wider area (PFIFFNER, 2010).
The reason for the melting of the lithosphere in the first place is still controversially discussed.
According to VON BLANCKENBURG & DAVIS (1995) it has been part of the breaking down process of the
subducted Euopean Plate resulting in the formation of a gap or interstice. Supposedly this led to an
ascent of asthenospheric mantle filling this gap, resulting in a heating-up and melting of the
lithosphere of the Adriatic Plate. However, according to PFIFFNER (2010) this model does not entirely
explain the prevalence of the dykes over a wide setting.
2.2.1 Bergell Pluton
Fig. 6: Bergell pluton displaying the tonalite feeder zone, the granodiorite complex, the smaller Novate intrusion in the
west, the ophiolites as well as the PFS (modified after BUCHER, 1977)
The Tertiary Bergell pluton is located in the SE of the Swiss Alps and comprises a surface area of
about 50 km2. The pluton is nearly concentrically zoned, exposing granodiorite with its characteristic
large K-feldspar minerals in the core and tonalite at the margin (Fig. 6). To the west of the Bergell
pluton (22-25 Ma), a smaller granitic intrusion, the Novate granite (17 Ma), is exposed (Fig. 6)
(BUCHER, 1977). The first step of the intrusion’s formation was the concordant emplacement of
tonalite in a pre-existing alpine nappe entailing a contactmetamorphism featuring temperatures of
around 650-700 °C and pressures of approx. 2-3 kb at the contact. The southern margin of the Bergell
pluton, consisting of a 40 km long steep tabular body of tonalite, is termed the tail and is situated
10
sub-parallel to the PFS (Fig. 6). This tail belongs to the Periadriatic mylonite zone and is considered to
represent the feeder zone of the pluton (ROSENBERG, 2004). After the emplacement of the tonalitic
feeder zone, the intrusion of the central granodiorite occurred, resulting in a boudinage and mineral
alignment within the tonalite and country rocks (BUCHER, 1977). The pluton formed due to ballooning
processes resulting in mostly concordant contacts between the granodiorite and the country rocks
(ROSENBERG, 2004). The final emplacement occurred, after ascent along the PFS, in two stages. At first
the magmas were emplaced to the north of the PFS, along an active, slightly inclined nappe contact
forming a pull apart structure, followed by the formation of a foliation. Secondly, during large scale
regional N-S shortening synmagmatic folding at the base of the intrusion and its country rocks
occurred, simultaneously to the ballooning at the top (ROSENBERG, 2004). The uppermost part of the
exposed pluton was intruded below the base of the Austroalpine nappes, whose temperature was
less than 300 °C at the time of emplacement (ROSENBERG, 2004) with a crystallization temperature of
the pluton of around 800 °C (BUCHER, 1977). Later both the intrusion and the country rocks were
overprinted by a regional metamorphism featuring temperatures of about 450-550 °C and pressures
of around 2-3 kb. During a following uplift they underwent a retrograde metamorphism (BUCHER,
1977). Post-intrusive tilting and erosion of the Bergell massif resulted in the exposure of a 12 km
deep crustal section, located between the eastern and western part of the pluton (ROSENBERG, 2004).
2.2.2 Adamello Batholith
Fig. 7: Adamello Granite (Picture by Jacqueline Engmann, 2013)
11
The Adamello batholith (
Fig. 7) is located in northern Italy forming the largest
Periadriatic Tertiary intrusive complex in the Alps (PFIFFNER, 2010). It is bordered by two major alpine
tectonic faults; in the north by the late- to post-magmatic Tonale Line and to the east by the Miocene
Giudicarie Line (Fig. 8), which are both segments of the PFS (HANSMANN ET AL., 1990). Its interior part
is crossed by only a few late- to post-magmatic fault zones (SCHALTEGGER ET AL., 2009). Adamello
magmas were emplaced during Middle/Late Eocene to Early Oligocene (PFIFFNER, 2010). From north
to south the entire batholith can be subdivided into four different plutonic bodies; the Presanella,
Avio, Adamello and Re di Castello (Fig. 8). The individual plutons are composite bodies displaying
varying structural relationships among each other and their adjacent rocks (ROSENBERG, 2004). They
were emplaced over a period of around 14 Ma, successively from the oldest units in the south (~42
Ma) to the youngest in the north (~28 Ma) (SCHALTEGGER ET AL., 2009). The emplacement of
predominantly tonalitic rocks in the southern Alpine Variscan basement and its non metamorphic
Permian-Triassic sedimentary cover rocks resulted in a contact-metamorphic aureole (HANSMANN ET
AL.,
1990). The intrusion took place due to ballooning and stoping processes, the former resulting in
concordant, compositionally zoned plutons with concentric foliation patterns, the latter eventuating
in discordant, kilometer-scale stoped blocks (ROSENBERG, 2004). According to SCHALTEGGER ET AL. (2009)
the
87
Sr/86Sr and
18
O/16O ratios as well as the concentrations of incompatible elements (U, Cs, K)
Fig. 8: Adamello batholith divided into the Presanella, Avio, Adamello and Re di Castello. In the north the Tonale Line
and in the East the Giudicarie Line (Schaltegger, 2009)
12
increase northwards reaching peak values in the Avio pluton, pointing to increasing crustal
contamination during the interval of magmatism. Therefore SCHALTEGGER ET AL. (2009) deduce from
that an assimilation fractional crystallization model, meaning that larger amounts of lower to middle
crustal material will progressively mix with fractionating mantle derived magmas. Furthermore the
plutons define a calc-alkaline fractionation trend (SCHALTEGGER ET AL., 2009). The southern part of the
Adamello batholith has no spatial relationship to the Periadriatic Lineament, apart from this the
pluton was emplaced prior (~42 Ma) to its activity. However, the northern and northeastern parts of
the batholith are in direct vicinity to the PFS and yield intrusion ages that match the activity of the
Lineament (ROSENBERG, 2004). The lithologies cannot be continuously traced from the southern
plutons northward to the northern. For the mentioned reasons Rosenberg (2004) concludes that the
southern intrusions most likely ascended independently of the PFS.
2.2.3 Rieserferner Pluton
Fig. 9: Rieserferner pluton (6) to the north of the DAV and in the south of the Tauern Window (TF) (GRATZER, 1993)
This pluton is situated in the south of the Tauern Window and to the north of the DefreggerAntholzer-Valser Line (DAV), which is a sinistral transpressive branch of the PFS (Fig. 9). The intrusion
expands over 40 km in E-W and about 4.5 km in N-S direction. At the northern boundary it dips
shallowly to the north, while its southern margin becomes steeply dipping as it gets closer to the PFS,
dipping parallel to the foliation of the DAV (GRATZER ET AL., 1993). The intrusion mostly consists of
calc-alkaline granodiorites, coarse to medium grained tonalites and sporadically granites and diorites
(GRATZER ET AL., 1993, STEENKEN ET AL., 2000). During the middle Oligocene (~31 Ma) the syntectonic
emplacement occurred along the north side of the DAV (STEENKEN ET AL., 2000) at a depth of 12-15 km
due to buoyancy-driven uplift (ROSENBERG, 2004). According to ROSENBERG (2004), the DAV is
considered to be the feeder zone of the Rieserferner pluton. The intrusion can be divided into two
cores; the Rieserkern in the east and the Rainwaldkern to the west, separated by a synform with a NS striking axial plane (GRATZER ET AL., 1993). In the western part occurred a slight migmatisation, while
13
the eastern part displays a sharp tectonic contact between the pluton and the country rocks, which
consist of mica schists and amphibolites overprinted by a contact metamorphism (GRATZER ET AL.,
1993).
3.
Conclusion
The intrusion cycles of the Variscan and Alpine granites have different geotectonic environments,
because Variscan plutons show a higher content in K, Rb, Sr, Ti than Alpine plutons. For the granitic
melts of the Variscan intrusions the melts mainly had to be of S-type origin meaning continental crust
as magma source. That is assumed because of the high K/Rb ratios and high Al contents of the
Variscan intrusions. The Alpine intrusions, on the other hand, are of I-type origin, so the magma is
derived from source rocks of igneous composition (GRATZER ET AL., 1993). Furthermore the
emplacement of the Alpine plutons was mainly dependent on the activity of the Periadriatic
Lineament and its branches.
14
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