Alpine Granites

Alps Excursion 2013

Jacqueline Engmann (294225)

Applied Geosciences (M.Sc.)

Supervisor: Prof. Urai & Prof. Littke

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Table of Contents

Abstract ................................................................................................................................................... 3

1. Introduction .................................................................................................................................... 3

2. Geometry and Emplacement of the Intrusions ............................................................................... 4

2.1 Variscan Intrusions .................................................................................................................. 4

2.1.1 Aar Massif ........................................................................................................................ 5

2.1.2 Gotthard Massif ............................................................................................................... 6

2.1.3 Kreuzberg-Iffinger-Brixen Pluton ..................................................................................... 7

2.2 Alpine Intrusions ...................................................................................................................... 9

2.2.1 Bergell Pluton ................................................................................................................ 10

2.2.2 Adamello Batholith ........................................................................................................ 11

2.2.3 Rieserferner Pluton ....................................................................................................... 13

3. Conclusion ..................................................................................................................................... 14

References ............................................................................................................................................. 15

3

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-Iffinger-

Brixen 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)

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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 Helvetic-

Moldanubian 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 arc-

shaped, which is most likely due to the influence

of the Variscan orogeny (DEBON ET AL., 1999). The

oldest plutons of the External Crystalline Massifs

were emplaced during the Lower Carboniferous (350-330 Ma). They are K-rich, part of the calc-

alkaline 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).

Fig. 1: External Crystalline Massifs in the Western and

Central Alps (http://origin-ars.els-

cdn.com/content/image/1-s2.0-S1631071308002782-

gr1.jpg)

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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 shoshonitic-

ultrapotassic, 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 E-

W 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 Urseren-

Garvera. 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 Judikarien-

Pustertal-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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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).

Fig. 4 Kreuzberg-Iffinger-Brixen pluton (modified after POMELLA ET AL., 2012)

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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).

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).

Fig. 5: Age of Periadriatic Plutons (ROSENBERG, 2004)

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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

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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)

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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 87Sr/86Sr and 18O/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)

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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 Defregger-

Antholzer-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 N-

S striking axial plane (GRATZER ET AL., 1993). In the western part occurred a slight migmatisation, while

14

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.

15

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