Martin Thöni

Magmatism and metamorphism in the Ladakh Himalayas (the Indus-Tsangpo suture zone)

Abstract Ladakh (India) provides a complete geological section through the northwestern part of the Himalayas from Kashmir to Tibet. Within this section the magmatic, metamorphic and geotectonic evolution of the northern Himalayan orogeny has been studied using petrographic, geochemical and isotope analytical techniques. The beginning of the Himalayan cycle was marked by large basaltic extrusions (Panjal Trap) of Permian to Lower Triassic age at the “northern” margin of the Gondwana continent (Indian Shield). These continental type tholeiitic basalts were followed by a more alkaline volcanism within the Triassic to Jurassic Lamayuru unit of the Gondwana continental margin. Lower Jurassic to Cretaceous oceanic crust and sediments (ophiolitic melange s.s.) accompany the Triassic to Cretaceous flysch deposits within the Indus-Tsangpo suture zone, the major structural divide between the Indian Shield (High Himalaya) and the Tibetan Platform. So far, no relic of Paleozoic oceanic crust has been found. Subduction of the Tethyan oceanic crust during Upper Jurassic and Cretaceous time produced an island arc represented by tholeiitic and calc-alkaline volcanic rock series (Dras volcanics) and related intrusives accompanied by volcaniclastic flysch deposits towards the Tibetan continental margin. Subsequent to the subduction of oceanic crust, large volumes of calc-alkaline plutons (Trans-Himalayan or Kangdese plutons) intruded the Tibetan continental margin over a distance of 2000 km and partly the Dras island arc in the Ladakh region. The collision of the Indian Shield and Tibetan Platform started during the middle to upper Eocene and caused large-scale, still active intracrustal thrusting as well as the piling up of the Himalayan nappes. The tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle (Spongtang klippe). In the High Himalayas the tectonic activity was accompanied and outlasted by a Barrovian-type metamorphism that affected Triassic sediments of the Kashmir-Nun-Kun synclinorium up to kyanite/staurolite grade and the deeper-seated units up to sillimanite grade. Cooling ages of micas are around 20 m.y. (muscovite) and 13 m.y. (biotite). Towards the Indus-Tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist, prehnite-pumpellyite to zeolite grade. Remnants of possibly an Eo-Himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone.

Some new aspects of dating eclogites in orogenic belts: Sm-Nd, Rb-Sr, and Pb-Pb isotopic results from the Austroalpine Saualpe and Koralpe type-locality (Carinthia/Styria, southeastern Austria)

New Sm-Nd, Rb-Sr, and Pb-Pb isotope data on eclogites and metagabbros from the Austroalpine Koralpe and Saualpe basement nappes of the eastern Alps are presented. These rocks are encased in polymetamorphic gneisses and micaschists that yield tNdCHUR ages of between 1.04 and 1.81 Ga. ϵ0Nd values from seven eclogite whole rocks range between +7.0 and +10.8; 147Sm144Nd is close to modern DM In a 208pb206Pb diagram all samples plot very close to the MORB field. Most analyses of the major mineral components, garnet, clinopyroxene, zoisite/epidote, phengite, amphibole and rutile, show disequilibrium in all three isotopic systems. Internal Sm-Nd and Rb-Sr mineral isochron ages range between 53 and 151 Ma. A minimum age of around 100 Ma is estimated for the crystallization of the high-P paragenesis garnet + omphacite + zoisite + kyanite + amphibole + quartz + rutile ± phengite + accessories, on the basis of these results. Later thermal overprint, fluid activity, and retrogression during exhumation of the eclogites involved (re-)crystallization of amphibole and garnet, thus leading in part to geochronologically poorly interpretable isochrons, without strict time significance. The last (eo-Alpine) thermal climax, involving static (re-)crystallization of garnet, staurolite and kyanite within the eclogite host rocks, is defined by concordant Sm-Nd and Rb-Sr isochrons on garnet, white mica, and staurolite at around 90 ± 3 Ma. Biotite Rb-Sr ages from these rocks range between 57 and 92 Ma. Plagioclase, pyroxene, and whole rock, analyzed from a relic gabbro core that shows continuous transition into eclogite from the southern Koralpe, yielded a Sm-Nd isochron of 275 ± 18 Ma, and an initial 143Nd144Nd ratio of 0.51271 ± 2 (ϵtNd = + 8.4 ± 0.5). This age is interpreted to date primary magmatic crystallization, thus setting also an uppermost time limit for eclogite metamorphism in the study area. The same outcrop yields a Sm-Nd isochron age for garnet and whole rock from the eclogitized gabbro of 93 ± 15 Ma; clinopyroxene from the same assemblage, however, lies clearly off this isochron. Whereas the 93 Ma figure may be regarded as a lower age limit for the eclogite event, an upper age limit of ca. 150 Ma may also be inferred for this metamorphism on the basis of these results. Taking the isotopic data from both Saualpe and Koralpe together, two basically different processes may be responsible for the data scatter and the partly unrealistically young isochrons in the Saualpe eclogites: 1. 1) Incomplete isotopic resetting, even on the grain scale, during the eclogitization of the igneous rocks, leaving cpx partly as a closed system. 2. 2) Ongoing crystallization (primarily of amphibole and/or garnet) after the peak of high-P metamorphism, probably combined with the introduction of a fluid phase with a strongly different isotopic signature from the eclogite host rocks. The results show that the Alpine evolution within the Austroalpine domain began very early (already in Permian times), with continental fragmentation, crustal thinning, and oceanic magmatic activity, close to the northern border of Gondwana. Ongoing extensional processes led to extensive production of basaltic melts close to or within a province of disrupted and strongly reduced continental basement. With the onset of collision at the western end of the Tethys ocean in Upper Mesozoic times, basement and young ocean floor were involved in Alpine subduction and, finally, in nappe tectonics, in forming the “root zone” of the present Austroalpine basement nappes.

Sm–Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm–Nd chronometry

Interpretation of Sm–Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm–Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite. Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of 147Sm144Nd is 0.15–2.5 in garnet from metabasites, 0.12–3.03 in metapelite garnet and 0.66–7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and SmNd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low SmNd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm–Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm–Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve SmNd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors. Sm–Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps. (a) A Variscan high-P event is documented in metabasites from the northern–central Otztal basement around 360–350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345–330 Ma. (b) A long-lived, Permian to Triassic event (285–225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist. (c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100–85 Ma). At cooling rates of 20–30 °C/Ma (exhumation rates: 3–5 km/Ma), the Sm–Nd closure temperature (Tc) for mm-sized garnet in these rocks is estimated at 650–680 °C.

The early Palaeozoic magmatic event in the Northwest Himalaya, India: source, tectonic setting and age of emplacement

In the High Himalayan Crystalline Series of Northwest India, numerous peraluminous granites intruded the metasediments of the late Proterozoic to early late Cambrian Haimanta Group. Nd and Sr isotope systematics confirm that they were derived from heterogeneous crustal sources. New geochronological data from two plutons range in age from late Precambrian to early Ordovician: single zircon U-Pb dating yielded an age of 553 ± 2 (2σ) Ma for the Kaplas granite, whereas mineral Sm-Nd isotope systematics define a crystallization age of 496 ± 14 (2σ) Ma for the tholeiitic mafic rocks in the Mandi pluton, where evidence of magma mingling documents a close association between mafic and granitic melts. The end of this period of magmatic activity coincides with the depositional gap below the Ordovician transgression, caused by surface uplift and erosion, that is an important feature in the stratigraphy of the Northwest Himalaya. In Spiti, the transgression of the Ordovician basal conglomerates on a normal fault indicates pre-Ordovician extensional faulting. Therefore, the early Palaeozoic magmatic activities in the Northwest Himalaya could be correlated with a late exten- sional stage of the long-lasting Pan-African orogenic cycle which ended with the formation of the Gondwana supercontinent.

Proterozoic crustal evolution in the NW Himalaya (India) as recorded by circa 1.80 Ga mafic and 1.84 Ga granitic magmatism

Abstract Single zircon dating of the Rampur metabasalts of the Larji–Kullu–Rampur window in the Lesser Himalayas yielded an evaporation age of 1800±13 Ma. The zircon age is considerably younger than the previously published whole rock Sm–Nd age of 2510±90 Ma, suggesting that the Sm–Nd age may be geologically meaningless and that the Sm–Nd whole rock array may have resulted from mixing. In the NW Himalaya, there is also evidence for extensive silicic melt generation in the Paleoproterozoic. Zircons from a metarhyodacite in the Larji–Kullu–Rampur window yielded an evaporation age of 1840±16 Ma, which we interpret as the minimum age of magmatism. The Main Central Thrust granitic mylonites are interpreted as the basement of the Neoproterozoic Haimanta Group metasediments. Together with the granitic rocks from the Lesser Himalaya, they were derived from pre-existing continental crust prior to 1.84 Ga. The Nd depleted mantle model ages are in the range of 2.6–2.4 Ga, suggesting a contribution of Archean crust. A recycled Archean component is also documented by a 2.9 Ga domain in one of the zircons.

Eo-alpine eclogitisation of Permian MORB-type gabbros in the Koralpe (Eastern Alps, Austria): new geochronological, geochemical and petrological data

Abstract New petrological, geochemical and isotopic data (SmNd, RbSr) from (meta)-gabbros, eclogites and metapelites from the southern Koralpe are presented. Pl-Cpx gabbros (mostly olivine-free) that have preserved their magmatic minerals to varying extent occur as relics within eclogites. Transformation of gabbro into eclogite is documented in different stages ranging from macro- to microscale. Based on textural characteristics, the eclogites are classified into four groups (types 1–4). Typical eclogite parageneses are Grt + Omp + Zo ± Ky + Qtz ± Amp ± Phe + Rut. The metapelitic eclogite host rocks are composed of Qtz + Ms + Bt + Pl + Grt ± Ky. P-T estimates for both metabasites and metapelites range at 1.8-2.0 GPa/600–650°C. SmNd mineral isochrons (Pl-Cpx) for the gabbroic eclogite precursors give ages of between 247 ± 16 and 255 ± 9 Ma. Their initial Nd and Sr isotopic compositions are clearly within the range of modern depleted mantle values. Minerals from type-1, metagabbroic eclogites are in clear Nd isotopic disequilibrium. Pervasively recrystallized type-3 eclogites yield Grt-WR or Grt-Omp-Zo-Amp-WR ages of 97 ± 5, 101 ± 7 and 109 ± 8 Ma. The metapelitic eclogite host rocks (mica schists) yield Nd DM (depleted mantle) model ages of 1.44-1.63 Ga. Grt-WR isochron ages from these rocks are 86 ± 1, 87 ± 4 and 89 ± 3 Ma. The new data further support the Permian/Early Mesozoic rifting event for this part of the Austroalpine basement, which is related to initial oceanization at the western end of the Alpine Tethys. During Alpine plate convergence pre-Alpine continental basement and oceanic crustal fragments where subducted to depths of c. 60 km. The most likely time span for the high-pressure peak is c. 100 ± 10 Ma. Rapid exhumation in the Upper Cretaceous is inferred from mineral cooling ages between 80 and 60 Ma.

Geochemistry and tectonomagmatic affinity of the Yungbwa ophiolite, SW Tibet

The Yungbwa ophiolite is a thrust sheet of about 800 km2 tectonically overlying an Upper Cretaceous melange south of the Indus–Tsangpo Suture Zone in SW Tibet. Both units have been thrust over the sedimentary series of the Tibetan Tethys zone in the course of the India–Eurasia collision. Harzburgite and clinopyroxene-poor lherzolite are the dominant lithologies. Occasional gabbronoritic and basaltic dikes crosscut the mantle tectonites, but plutonic and volcanic sections are notably absent. The majority of basaltic dikes are tholeiitic and similar to N-MORB with respect to LREE depletion, ratios of diagnostic trace elements and Sr and Nd isotope systematics. Sm–Nd isotope data of three tholeiitic samples yielded an isochron age of 147±25 Ma (MSWD=0.59), and an initial e(t)Nd value of +8.8. In addition, magnesio-hornblende in a tholeiitic basaltic dike yielded a 40Ar/39Ar age of 152±33 Ma. The mineral chemistry of the spinel–peridotites is consistent with an origin in a tectonic setting similar to abyssal peridotites. The nearly chondritic initial 187Os/188Os value of a Re-poor orthopyroxene concentrate matches the composition of abyssal peridotites. The strongly LREE depleted trace element signature of the peridotite clinopyroxenes resembles that of clinopyroxenes from ocean floor peridotites. The Nd isotopic composition of a clinopyroxene concentrate is extremely depleted (143Nd/144Nd=0.514420), indicating an early Jurassic time of depletion (TDM=187 Ma) and suggesting that this clinopyroxene represents residual mantle material. In contrast, the bulk peridotites are characterized by convex-downward REE patterns, coupled with low 143Nd/144Nd ratios and elevated 87Sr/86Sr ratios. We suggest that these geochemical signatures resulted from secondary processes during or after emplacement of the ophiolite complex.

Relations between metamorphism and orogeny in a typical section of the Indian Himalayas

A comprehensive geological and petrological investigation has been undertaken in an area of about 10 000 km2 in the Indian Himalaya (S-Lahul, Himachal Pradesh). The development of mineral assemblages in metamorphic rocks of medium grade is considered to be a dominant Alpine event, although almost exclusively Paleozoic and Precambrian rocks have been involved. The Barrowian type of this metamorphism, ranging from the anchi- to the sillimanite zone, took place under the elavated T-gradient of about 4°C/100 m. It is suggested that “normal” geothermal conditions prevailed only in the outermost zone of this orogenic belt. In the Late Tertiary this metamorphic series has been moved as a huge nappe upon the Lower Himalaya. During this process a unique feature of reverse metamorphism has been formed. It can be shown that this feature was caused by a combination of metamorphism and very rapid tectonic movements.

Origin of eclogites from the Austroalpine Ötztal basement (Tirol, Austria): geochemistry and SmNd vs. RbSr isotope systematics

Geochemical and combined Sm-Nd vs. RbSr isotope data from the Otztal metabasite suite point to production of gabbros and basalts in an extensional regime in Late Cambrian times. The source of these magmas and cumulates had a chemical and isotopic composition similar to MORB. The time of intrusion is constrained by SmNd mineral isochrons (clinopyroxeneplagioclase) of relictic gabbro occurrences within the eclogite-amphibolite series in the time span between 530 and 521 Ma. Subsequently, the basic eclogite precursors suffered three metamorphic stages. The imprint of the “Caledonian event” is unclear. During the Hercynian cycle, the basic magmatic rocks were transformed into eclogites at surprisingly high P-T conditions of ∼27 kbar/730°C. The time of this subduction event is given by mineral isochrons (garnet, clinopyroxene, zoisite, whole rock) as ∼ 370–340 Ma, with a most probable time span for the pressure peak between 360 and 350 Ma. During subsequent exhumation and pressure release the eclogites were partly transformed into amphibolites. Alpine overprint during a greenschist facies-grade metamorphism caused further and partly intense retrograde alterations in the metabasic suite.

Mantle–crust interactions during Variscan subduction in the Eastern Alps (Nonsberg–Ulten zone): geochronology and new petrological constraints

Abstract We have analyzed the Sm–Nd and Rb–Sr whole-rock and mineral isotope systematics of garnet peridotites and associated eclogites and migmatitic gneisses from the Nonsberg–Ulten zone of the Eastern Alps. The garnet peridotites include coarse-grained varieties, characterized by well-preserved to slightly modified mantle geochemical signatures, and finer-grained varieties enriched in amphibole and LILE. Hydration of some of the most strongly deformed, fine-grained peridotites by crustal fluids caused isotopic disequilibrium between the peridotite minerals, preventing accurate age determinations. The coarse-grained peridotites, the eclogites and the country migmatitic gneisses yield garnet–whole-rock and garnet–clinopyroxene Sm–Nd ages that indicate for all rock types an isotopic homogenization event at ca. 330 Ma. The similar ages suggest that all rock types shared a common history since the incorporation of the peridotites in the crust, and constrain the garnet-facies metamorphism of the peridotites, as well as partial melting of the crust, to an episode of crustal subduction at the end of the Variscan orogenic cycle.