Fakultät für Geowissenschaften - Digitale Hochschulschriften der LMU

In the South Indian basement, several crustal-scale amphibolite facies shear zones occur between high-grade

metamorphic units with a different geological history: the EW-trending Moyar Shear Zone (MSZ) is a zone of

predominantly dip-slip transport separating the Archaean Dharwar Craton in the north from the late Archaean

Nilgiri Block in the south. The NE-SW-trending, dextral-transpressive Bhavani Shear Zone (BSZ) constitutes

the southern boundary of the Nilgiri Block in its western part and bounds the southern Dharwar Craton further

east. South of the BSZ, the high-grade metasediments and metaintrusives of the Maddukarai region are separated

from the 0.6 Ga-metamorphic Madurai Block by the EW-trending dextral Palghat Shear Zone (PSZ). MSZ, BSZ

and PSZ are regarded as parts of the prominent Cauvery shear system. The N-S-trending sinistral Kollegal Shear

Zone (KSZ), which transects the Dharwar Craton, is cut off by the Cauvery shear system.

These shear zones play an important role in reconstructing the position of India within the East Gondwana

terrane assembly. A combined Sm-Nd, Rb-Sr and U-Pb isotopic study was carried out on granulite remnants,

amphibolite facies (mylonitic) gneisses and pre-, syn- und postmetamorphic intrusives in order to examine the

tectonometamorphic evolution of the MSZ, BSZ, PSZ and KSZ.

Whole rock data

The majority of relic and retrogressed granulites from the MSZ (TDM 2.3–3.1 Ga) and BSZ (TDM 2.6-2.9 Ga)

show late Archaean average crustal residence ages. With respect to their LIL-, Nb-depleted tonalitic nature and

Nd-model ages, these rocks resemble the neighbouring Nilgiri granulites. Mylonitic gneisses and granulite

remnants from the BSZ yielded a Sm-Nd-whole rock-age of 2520 ± 150 Ma (εNd(t) +0.6; MSWD = 2.2), which

is interpreted as protolith age. I-Type granites and tonalites, which intruded the MSZ ~620 Ma ago (87Sr/86Sri

0.7039), gave unusually young Nd model ages of 1.8-1.9 Ma suggesting derivation from a Mid- to

Neoproterozoic upper mantle source, presumably with minor contribution of an older crust. An allochthonous

quarzite (TDM 3.3 Ga) of the BSZ is regarded as counterpart of the Archaean Sargur group, which is exposed in

schist belts of the Dharwar Craton. As suggested by geochemical features (LIL-, Nd-rich granitoids) and

Neoarchaean-Palaeoproterozoic Nd-model ages (TDM 2.3-3.0 Ga), the PSZ-gneisses show affinity to the adjacent

Madurai Block. Sheared orthogneisses from the KSZ show Mesoarchaean average crustal residence ages (TDM

3.2-3.3 Ga) typical for Dharwar Craton gneisses.

Mineral dating on granulites

Mineral age data of relic granulites from the MSZ, BSZ and PSZ provide evidence for the metamorphic

precursor history of the shear zone rocks: gnt-plag-px-granulites from low-strain domains yielded Sm-Ndgarnet-

whole rock ages of 2355 ± 22 Ma (εNd(t) -1.4) for the MSZ and 2329 ± 19 Ma (εNd(t) -2.0) for the BSZ,

both recording late-stage Palaeoproterozoic granulitisation of the rocks and corresponding with garnet data from

the Nilgiri Block. Correlated low εNd-initial values reflect the short time span between crustal genesis and garnet

crystallisation. Further Sm-Nd mineral data from BSZ-granulites are between 1275 ± 10 Ma and 1106 ± 48 Ma

(garnet/plagioclase-whole rock-pairs; εNd(t) –5.8 to –25.4), indicating a Mesoproterozoic metamorphic imprint.

A charnockite from the southern BSZ, which is interpreted as a separate lithological unit, yielded a reproducable

Sm-Nd-garnet-whole rock age of 1705 ± 11 Ma (εNd(t) –12.4), presumably recording late Palaeoproterozoic

metamorphism.

Mineral dating on gneisses and younger intrusives

Amphibolite facies rocks with younger fabrics yielded Neoproterozoic to early Palaeozoic mineral age data for

the MSZ, BSZ and PSZ:

Sm-Nd mineral ages from gneiss-mylonites imply a first stage of early Pan-African shearing in the MSZ ~745

Ma ago (garnet/plagioclase-hornblende-pairs: 743 ± 13 Ma, 747 ± 75 Ma) and in the BSZ ~730 Ma ago (garnetwhole

rock-pair: 726

In der hier vorgestellten Arbeit wurde gezeigt, daß das weltweit beobachtete Ph¨anomen

geringer Intensit¨at der nat¨urlichen remanenten Magnetisierung von etwa 20 Ma alten

Ozeanbasalten und das dadurch verursachte Minimum bei den Amplituden der ozeanischen

Magnetfeldanomalien auf die Tieftemperaturoxidation der Titanomagnetite zu

Titanomaghemiten zur¨uckzuf¨uhren ist.

Dazu wurden magnetische und mineralogische Untersuchungen an etwa 100 durch

das Deep Sea Drilling Project bzw. Ocean Drilling Program erbohrten Ozeanbasalten

durchgef¨uhrt. Die Proben decken einen Altersbereich von 0.6 bis 135 Ma ab und stammen

haupts¨achlich aus dem Atlantischen und Pazifischen Ozean.

Tr¨ager der Magnetisierung bei den meisten Proben ist je nach Alter der Ozeanbasalte

Titanomagnetit oder Titanomaghemit. Um den Oxidationsgrad der Titanomaghemite

zu bestimmen, wurden die davon abh¨angigen Parameter Curie-Temperatur und Gitterkonstante

gemessen sowie Mikrosonden-Analysen durchgef¨uhrt. Der Oxidationsgrad wird

durch den von 0 (nicht oxidiert) bis 1 (vollst¨andig tieftemperaturoxidiert) variierenden

Oxidationsparameter beschrieben.

Es wurde nachgewiesen, daß die S¨attigungsmagnetisierung der Ozeanbasalte (gemessen

bei Raumtemperatur) in gleicher Weise wie die Intensit¨at der nat¨urlichen

remanenten Magnetisierung (NRM) mit dem Alter variiert und bei 10 bis 40 Ma alten

Ozeanbasalten ein Minimum aufweist. Dieses Ph¨anomen wird durch die fortschreitende

Tieftemperaturoxidation der Titanomaghemite verursacht, bei der Fe-Ionen bevorzugt

aus den Oktaederpl¨atzen des ferrimagnetischen Gitters auswandern und verbleibende

Fe2+-Ionen zu Fe3+-Ionen oxidiert werden. Die S¨attigungsmagnetisierung der ferrimagnetischen

Titanomaghemite ist gleich der Untergittermagnetisierung der Fe-Ionen auf

Oktaederpl¨atzen abz¨uglich der antiparallelen Untergittermagnetisierung der Fe-Ionen auf

Tetraederpl¨atzen und nimmt deshalb mit der Tieftemperaturoxidation ab.

Bei 10 bis 40 Ma alten Proben wird f¨ur die Titanomaghemite eine fast vollst¨andige

Oxidation beobachtet. Der Oxidationsparameter erreicht hier einen Wert von ≈ 0.8. Bei noch ¨alteren Proben findet vermutlich eine Diffusion der Fe-Ionen aus den Tetraederl

¨ucken in die Oktaederl¨ucken statt. Dies k¨onnte die beobachtete Zunahme der

S¨attigungsmagnetisierung im Altersbereich von 40 bis 130 Ma bei ungef¨ahr gleichbleibendem

Oxidationsgrad der Titanomaghemite erkl¨aren.

Zur weiteren Charakterisierung der Titanomaghemite wurde die Temperaturabh¨angigkeit

der S¨attigungsmagnetisierung MS(T) bestimmt. Der Verlauf der MS(T)-Kurven

h¨angt von der Zusammensetzung der Titanomaghemite ab und zeigt deshalb eine

Abh¨angigkeit vom Alter der Proben. Die MS(T)-Kurven von Ferrimagnetika werden nach

N´eel (1948) in verschiedene Typen eingeteilt, die sich jeweils aus dem Unterschied der

Temperaturabh¨angigkeiten ihrer Untergittermagnetisierungen ergeben. Proben aus allen

Altersbereichen zeigen ein Maximum bei MS(T) oberhalb des absoluten Nullpunktes der

Temperatur. Bei einem Teil der 10 bis 40 Ma alten Proben wird außerdem beiT <-180◦C

die Untergittermagnetisierung der Fe-Ionen auf den Tetraederpl¨atzen gr¨oßer als die der

Fe-Ionen auf den Oktaederpl¨atzen. Diese Proben verhalten sich nach dem sogenannten

N´eel N-Typ. Die anderen Proben entsprechen dem N´eel P-Typ.

Bei den 10 bis 40 Ma alten Proben liegt das Maximum der S¨attigungsmagnetisierung

oberhalb der Raumtemperatur, w¨ahrend bei den Proben aus den anderen Altersbereichen

das Maximum der S¨attigungsmagnetisierung unterhalb der Raumtemperatur liegt.

Dieses Ph¨anomen wurde genutzt, um den Einfluß der Tieftemperaturoxidation auf

die NRM direkt zu untersuchen. Dazu wurde ein Magnetometer gebaut, mit dem die

NRM zwischen Raumtemperatur und 600◦C gemessen werden kann. Die Messungen

ergaben, daß sowohl die Temperaturabh¨angigkeit der NRM wie auch der S¨attigungsmagnetisierung

in gleicher Weise von dem Alter der Proben abh¨angen. Insbesondere

zeigen Proben aus dem Altersbereich 10 bis 40 Ma

The fragmentation of highly viscous magma by rapid decompression is a young

technique. For the first time an experimental tool permits the analysis of the influence

of pressure, temperature and texture on the fragmentation of magma. The physical

range of the experiments is close to the natural conditions of dome collapse.

The experiments were performed at temperatures up to 950°C. The initial pressure

difference in the range of 10 to 250bar is in good agreement to the natural conditions.

In addition to the analysis of the fragmentation threshold, the rapid decompression

experiments permit analysis of the primary particle distribution above the

fragmentation level of an eruption. Further analyses were aimed at the influence of

crystals on the particle distribution.

An experimental set-up was used to investigate the speed of the propagating

fragmentation wave, initiated by the rapid decompression of porous magma samples.

The fragmentation speed experiments were performed at ~20°C using two dynamic

pressure transducers. A new technique was designed to extend the temperature

range of the fragmentation speed experiments up to 950°C.

Next to the fragmentation experiments a comparison of analytical methods for the

characterisation of the experimental pyroclasts was necessary. The applicability of

the characterisation method to both experimental and fieldwork was of major interest.

The experiments were performed on Dacite samples form Mt. St. Helens

(Washington, USA), Fugendake (Unzen volcano complex, Japan) and basaltic

Andesites of the 1994 eruption of Merapi (Java, Indonesia).

The experimental investigation, the methodological and technical development are

scope of the presented dissertation.

The assumption that stationary hotspots underlie the Earth’s lithospheric plates has been

most important in the development of the theory of plate tectonics. According to the fixed

hotspot hypothesis seamount trails are formed by volcanism penetrating the lithospheric

plates whilst moving over ”hotspots”of upwelling mantle. In turn, the azimuths and age

progressions of seamount trails can be used to quantify plate motions with respect to an

independent reference frame of hotspots in the mantle. Also, assuming fixed hotspots, the

direction of characteristic remanent magnetization in the basalts acquired during cooling

should always be the same. Even if due to plate motion the products of the hotspot are

located far away from the position of the hotspot itself, paleomagnetic studies on the

basalts must always provide the position of the hotspot itself. Recently the question

arose, why a hotspot with its origin deep in the mantle would not get advected in the

convecting mantle of the Earth. - In this thesis a possible motion of the Kerguelen hotspot

in the southern Indian Ocean and of the Louisville hotspot in the Pacific has been studied.

The Kerguelen hotspot is active since approximately 117 Ma. Since then it formed

the Kerguelen Plateau and the Broken Ridge in the southern Indian Ocean as well as the

Ninetyeast Ridge, which is the hotspot track going north up to India, and the Ramajal

Traps in India. Drilling into basement rocks of Broken Ridge and the Kerguelen Plateau

was aim of the Ocean Drilling Program, Leg 183, from December 1998 to February 1999.

Eight sites have been drilled. In seven of the sites also the sediments have been recovered.

In this thesis, a possible motion of the Kerguelen hotspot has been studied by

determining its paleolatitudes. First, basalts from the Kerguelen Plateau have been

studied paleomagnetically to compare the paleolatitudes with the latitude of the hotspot

itself. Basement from a drillsite on the central Kerguelen Plateau (Site 1138) and of a

site on the northern Kerguelen Plateau (Site 1140) were suitable for a determination of

paleolatitudes. A sufficient number of independent lavaflows has been penetrated and

sampled there to properly average out paleosecular variation, an important requirement

for determining paleolatitudes. The characteristic magnetization from the subaerial Site

1138 with AA- and Pahoehoe lava and of the submarine Site 1140 with its pillow basalts

is carried by magnetite and titanomagnetites and -maghemites and consists of a single

remanence component with sometimes a small viscous overprint, that could easily be

removed during demagnetization. Stepwise demagnetization in an alternating field and

stepwise heating of the specimens provided the inclination value of the characteristic

magnetization very precisely with small error. Conversion of the mean-site inclination

into the paleolatitude of a site provided a latitude of λ = 43.6◦S (max.: 47.8◦S; min.:

37.9◦S) for Site 1138 on the central Kerguelen Plateau and a latitude of λ = 35.8◦S (max:

43.0◦S; min.: 28.9◦S) for Site 1140 on the northern Kerguelen Plateau. In Site 1136 on the southern Kerguelen Plateau only two lava flows have been sampled. Therefore

paleosecular variation could not be averaged out properly. Site 1142 on the Broken

Ridge has been tilted and deformed tectonically after its formation, as was found from

seismic explorations prior to drilling, and the inclination of the magnetization could

therefore not be used for a determination of paleolatitudes. Compared to the latitude of

the Kerguelen hotspot at 49◦S, the paleolatitudes of the central and northern Kerguelen

Plateau are further north. This result agrees with previous paleomagnetic studies on the

southern Kerguelen Plateau and the Ninetyeast Ridge, where paleolatitudes have been

found that indicate also a formation north of the present-day hotspot position. This

difference indicates a southward movement of the hotspot since the Cretaceous relative

to the

Verschiedene kalkschalige rezente Foraminiferen werden hinsichtlich ihrer Wandstruktur, des lamellaren Gehäusebaus und der Zahnplattenstruktur untersucht.Die taxonomische Wertigkeit der Zahnplatte wird kritisch diskutiert und ihre funktionsmorphologische Rolle postuliert.