G. Hoinkes

Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40 Ar/ 39 Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40 Ar/ 39 Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at � 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until � 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography. 2002 Elsevier Science Ltd. All rights reserved.

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.

Effect of grossular-content in garnet on the partitioning of Fe and Mg between garnet and biotite: An empirical investigation on staurolite-zone samples from the Austroalpine Schneeberg complex

In contrast to Ferry (1980) (XCa)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InKD= -1.7500 (±0.0226) + 2.978 (±0.5317)XCaGt-5.906(±2.359)(XCaGt)2 Temperatures derived by the Ferry and Spear (1978) calibration using “chemistry — corrected”KD values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (WFeMg= −133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (WMgCa —WFeCa) at metamorphic conditions of ∼570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures.

SHRIMP U–Pb zircon and Sm–Nd garnet ages from the granulite-facies basement of SE Kenya: evidence for Neoproterozoic polycyclic assembly of the Mozambique Belt

The Taita Hills–Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20–30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630–645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850–960 Ma was derived from zircon cores. Sm–Nd garnet–whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630–645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560–580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.

Petrologic constraints for eoalpine eclogite facies metamorphism in the austroalpine Ötztal basement

SummaryMetabasites of the southern Ötztal basement hitherto mapped as amphibolites, were identified as eclogites. Primary mineral parageneses are tschermakitic to pargasitic green amphiboles, omphacite (Jd40), garnet II (Gr20–30) Py10), phengite (Si3.5), zoisite, rutile and quartz. Al—pargasite (≈20 wt% Al2O3) rims between garnet and omphacite are interpreted as retrograde reaction products.Retrogression of the eclogite parageneses reflecting decreasing pressure and increasing temperature conditions are: Symplectites of diopside and plagioclase after omphacite, Al-and Na-poor green amphiboles, grossularite-poor garnet III surrounding garnet II partly with atoll textures and symplectites of biotite and plagioclase replacing phengite. Continuation of retrogression with decreasing temperature conditions is indicated by actinolitic amphiboles and albite-rims between amphibole II and quartz.A pre-eclogitic metamorphic stage is only recorded by garnet I cores of discontinuous grossular-poor composition. Minimum pressure and temperature conditions of the eclogite stage derived from Jd-content in omphacite and the gt-cpx-geothermometer were 11–12 kbar and 500–550 °C. Maximum temperature conditions of the posteclogitic stage were between 600 and 650°C. Occurrence of these eclogitic metabasites as lenticular interlayers with ortho- and paragneisses indicate high pressure metamorphic conditions within the whole rock-sequence. This interpretation is confirmed by the occurrence of phengite-rich micas in orthogneisses indicating pressures of ≈ 11 kbar. S econdary chemical changes of these phengites to muscovite-rich compositions again show the decreasing pressure conditions in the southern Ötztal basement after the eclogite stage. The age of the eclogite stage is interpreted as Eoalpine due to the following arguments:1.The eclogites show concordant, tectonically undisturbed contacts to the encasing orthogneiss-metapelite series. This points to a common history during the last metamorphic evolutionary stage.2.Continuous readjustment from high to intermediate pressure conditions is observed in both, eclogites and acid country rocks.3.Isotopic results from the wider study area exclusively yield Cretaceous mineral ages. Rb-Sr data an eclogite phengites (texturally clearly correlated with the high-P stage) and thin whole rock slabs of layered eclogites are in agreement with a dominant post-Hercynian crystallization history, following a continuous high-P/low-T to low-P/high-T loop.ZusammenfassungMetabasite aus dem südlichen Ötztalkristallin, die als Amphibolite kartiert sind, wurden als Eklogite erkannt. Der primäre Mineralbestand setzt sich aus pargasitischem bis tschermakitischem, grünem Amphibol, Omphacit (Jd40), Granat II (Gr20–30 Py10), Phengit (Si3.5), Zoisit, Rutil und Quarz zusammen. Al-Pargasitränder (≈20 Gew.-% Al2O3) zwischen Granat und Omphacit werden als erste retrograde Reaktionsprodukte interpretiert.Auf retrograde Umwandlung der Eklogitparagenesen bei fallendem Druck und steigender Temperatur weisen folgende Bildungen hin: Symplektite aus Diopsid und Plagioklas nach Omphacit, Al- und Na-armer, grüner Amphibol, Grossular-armer Granat III um Granat II (teilweise als Atollgranat) und Symplektite aus Biotit und Plagioklas nach Phengit. Aktinolithischer Amphibol und Albitränder zwischen Amphibol 11 und Quarz geben weitere Hinweise für andauernde retrograde Umwandlung bei der Druckentlastung.Ein prä-eklogitisches Metamorphosestadium ist nur durch Granat 1-Kerne mit Grossular-armer Zusammensetzung belegt. Die minimalen PT-Bedingungen für das Eklogitstadium wurden aus dem Jd-Gehalt in Omphacit und dem Granat-Clinopyroxen-Geothermometer abgeleitet; sie liegen bei 11–12 kbar und 500–550 °C. Die maximalen Temperaturen für das posteklogitische Stadium betragen 600–650 °C. Das ungestörte Auftreten dieser eklogitisierten Metabasite als Linsen und Lagen in einer Ortho- und Paragneisabfolge weist darauf hin, daß die gesamte Serie eine Hochdruckentwicklung mitgemacht hat. Diese Interpretation wird durch Phengit-reiche Hellglimmer aus benachbarten Orthogneisen mit Druckwerten um 11 kbar gestützt. Die sekundäre chemische Veränderung des Chemismus dieser Phengite zu Muskowit-reicheren Zusammensetzungen weist auf abnehmende Druckverhältnisse nach dem Eklogitstadium im südlichen Ötztalkristallin hin. Das Alter der Eklogitmetamorphose wird auf Grund folgender Argumente als frühalpidisch vermutet.1.Die Eklogite weisen konkordante, tektonisch ungestörte Kontakte zu den Orthogneis-Metapelit-Hüllgesteinen auf. Das belegt eine gemeinsame Geschichte für das letzte metamorphe Entwicklungsstadium.2.Sowohl in den Eklogiten als auch in den sauren Hüllgesteinen ist eine kontinuierliche Anpassung von Hochdruck- zu intermediären Druck-Verhältnissen zu beobachten.3.Isotopenanalysen aus dem weiteren Untersuchungsgebiet ergaben ausschließich kretazische Mineralalter. Rb-Sr-Daten an Eklogit-Phengiten (die texturell eindeutig dem Hochdruckstadium angehören) sowie Rb-Sr-Ergebnisse an Eklogiten mit Feinlagenbau belegen im wesentlichen eine postvariscische Kristallisationsgeschichte. Für die frühalpidische Entwicklung kann somit ein kontinuierlicher PT-Pfad von “Hoch-P/Niedrig-T-” zu “Niedrig-P/Hoch-T-Bedingungen” konstruiert werden.

Evolution of the Ötztal-Stubai, Scarl-Campo and Ulten Basement Units

The Austro-Alpine basement, between the Tauern window to the E and the Western Alps to the W, treated in this contribution may be divided into three subunits due to differing lithology and metamorphic history. These three subunits, the Otztal-Stubai-, the Scarl-Campo- and the Ulten basement, are divided by two major tectonic lines, the Schlinig line (OtztalStubai/Scarl-Campo) and the Peio line (Scarl-Campo/Ulten). Although each of the three units consists of quartzo-feldspatic and pelitic metasediments, acid to intermediate metamagmatites, in addition to metabasites and metacarbonates, the dominant rock types of the Otztal-Stubai-, the Campo- and the Ulten-unit are metasediments. The Scarl unit however is mainly composed of acid metamagmatites. Ages of the acid magmatic protoliths are most probably Variscan in the Scarl unit and Caledonian in the Otztal-Stubai unit.

Contrasting Eoalpine P-T evolutions in the southern Koralpe, Eastern Alps

SummaryThe Koralpe crystalline complex and the Plankogel unit represent two lithologically distinct units within the Koralpe region of the southeastemmost Austroalpine crystalline basement. The Eoalpine P-T evolution of these two units is derived from new petrographical data. The Plankogel unit and the Koralpe crystalline complex show markedly different P-T evolutions during the early stages of the Eoalpine event. The rocks of the Koralpe crystalline complex experienced eclogite facies conditions with minimum pressures in the range of 15–16 kbar and temperatures in excess of 700°C. At the same time the Plankogel unit resided in a shallower environment at pressures of 10–11 kbar and temperatures of less than 600°C. The tectonic emplacement of the Plankogel unit into its present position on top of the Koralpe crystalline complex took place after the eclogite facies event in a relatively shallow crustal level. After their juxtaposition the Koralpe crystalline complex and the Plankogel unit were affected by a common amphibolite facies metamorphic overprint. The distinctly different P-T evolution during the early stages of the Eoalpine event and a common history at later stages imply that major tectonic processes were operative in this part of the Austroalpine crystalline basement during the Cretaceous. Such processes may have involved subduction of oceanic and continental lithosphere which may have lead to significant crustal shortening within the Austroalpine basement.ZusammenfassungDas Koralpenkristallin und die Plankogelserie stellen zwei unterschiedliche lithologische Einheiten in der südlichen Koralpe des ostalpinen Kristallins dar. Die P-T Entwicklung dieser beiden Einheiten während der Eoalpinen Metamorphose wurde anhand neuer petrographischer Daten abgeleitet. Das Koralpenkristallin und die Plankogelserie zeigen deutlich unterschiedliche P-T Entwicklungen in einem frühen Stadium der Eoalpinen Metamorphose. Die Gesteine des Koralpenkristallins waren eklogitfaziellen Bedingungen mit Mindestdrucken im Bereich von 15 bis 16 kbar und Temperaturen von über 700°C ausgesetzt. Die Plankogelserie verweilte zur gleichen Zeit in einem relativ seichten Niveau bei Drucken von 10 bis 11 kbar und Temperaturen unterhalb 600°C. Die Platznahme der Plankogelserie in ihrer heutigen Position im tektonisch Hangenden des Koralpenkristallins erfolgte nach dem eklogitfaziellen Ereignis in einem relativ seichten Krustenniveau. Nach ihrer Vereinigung erfuhren die beiden Einheiten eine gemeinsame amphibolitfazielle Überprägung. Die markant unterschiedlichen P-T Entwicklungen in einem frühen Stadium der eoalpinen Orogenese und die gemeinsame Entwicklung in einem späteren Stadium können als Hinweis auf eine umfangreiche tektonische Aktivität in diesem Teil des ostalpinen Grundgebirges in kretazischer Zeit gewertet werden. Diese Tektonik bestand eventuell in einer Subduktion von ozeanischer und kontinentaler Lithosphäre, und kann zu einer signifikanten Krustenverkürzung im Ostalpinen Kristallin geführt haben.

TiO2 exsolution from garnet by open-system precipitation: evidence from crystallographic and shape preferred orientation of rutile inclusions

We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt<111> or describes cones with opening angle 27.6° around Grt<111>. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with microchemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as “open-system precipitation” (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of Fe2+ to Fe3+ and transfer from the dodecahedral to the octahedral site just vacated by

Mineralreaktionen und Metamorphosebedingungen in Metapeliten des westlichen Schneebergerzuges und des angrenzenden Altkristallins (Ötztaler Alpen)

{\text{Ti}}^{ 4+ }: 6\,{\text{M}}^{ 2+ }_{ 3} {\text{TiAl}}\left[ {{\text{AlSi}}_{ 2} } \right]{\text{O}}_{ 1 2} + 6\,{\text{M}}^{ 2+ }_{ 2, 5} {\text{TiAlSi}}_{ 3} {\text{O}}_{ 1 2} = 10\,{\text{M}}^{ 2+ }_{ 3.0} {\text{Al}}_{ 1. 8} {\text{Fe}}_{0. 2} {\text{Si}}_{ 3} {\text{O}}_{ 1 2} + {\text{M}}^{2+} + 2 {\text{e}}^{-} + 1 2\,{\text{TiO}}_{ 2} .