Project

Electron microprobe (EMP) Th-U-Pb monazite dating

Goal: Development and application of the in-situ CHIME Th-U-Pb isochron dating method of monazite by the electron microprobe.

Date: 1 January 2007

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Bernhard Schulz
added a research item
The concepts of progressive and polyphase deformation have been widely applied by structural geologists to explain complexly deformed rocks, particularly for ductile conditions. Interpreting complex structural patterns as the result of progressive deformation is mainly based on structural and kinematic evidence, applying the Ockham's razor principle: single-phase progressive deformation is the simplest and thus most plausible explanation in many cases. Processes such as strain partitioning and localization are commonly considered to explain such patterns. In this contribution, guidelines to distinguish between progressive and polyphase deformation processes are presented, mainly based on a critical discussion of advantages and pitfalls of P-T-D-X-t (pressure – temperature – deformation – composition – time) data. Such information not only allows for a robust reconstruction on the timing and rates at which deformation operates, but also provides insights into the progressive or polyphase evolution of pressure-temperature conditions and fluid flow. For example, complex structural patterns are common in transpressional and transtensional settings, particularly (but not only) for non-steady progressive deformation, which seems to be the rule in nature. Consequently, assessing the structural and microstructural context is a must, particularly because analytical data commonly record only distinct stages in the protracted evolution of deformation. CPO data are useful to disentangle complex structural histories, particularly in the case of changing deformation mechanisms and related conditions. For petrochronology, it is thus highly relevant to understand equilibrium conditions and mechanisms of intracrystalline deformation and (re)crystallization of dated minerals, in order to properly link the obtained ages with specific deformation stages and mineral reactions. In a similar way, structural facies characterization is critical to properly interpret geochronologic data linked to ductile and brittle deformation. Though poorly explored, microstructural information of fluid inclusion data is a valuable tool to understand the role of fluids in deformation-assisted settings. The recognition of phases is ultimately related to their tectonic significance and, therefore, may not be easy in cases of overlapping tectonic processes (e.g., subduction during a post-collisional phase) and transitional stages that may not necessarily imply a dramatic change in the deformation pattern (e.g., post-orogenic collapse and intracontinental rifting). https://authors.elsevier.com/c/1g2-92weQpJdt
Bernhard Schulz
added a research item
Monazite in lithoclasts of suevite impact breccia in the Nördlinger Ries (Bavaria, Germany) and its Th-U-Pb dating by electron probe microanalysis Bernhard Schulz, Jan-Michael Lange, Joachim Krause, Dana Czygan Abstract In the Lehenberg (Lehberg or Limberg North) quarry in the NW part of the Megablock Zone of the Nördlinger Ries impact crater, granite and micaschist lithoclasts occur in a polymict suevite impact breccia. The lithoclasts display the petrographic characteristics of the shock metamorphism scale, as cavities filled with diaplectic glass, decorated planar elements in quartz grains, and severly kinked mica. In petrographic thin sections, monazite grains were detected by scanning electron microscope based automated mineralogy methods of spectral mapping. In backscattered electron imaging (BSE), monazite revealed the typical crystal shapes, and internal Th zoning and distribution pattern as known from igneous and metamorphic crystallization. Intragrain signs of shock metamorphism in a minority of monazite grains are strictly straight and parallel crack pattern resembling lamellae structures. Monazite mineral chemistry and bulk chemical Th-U-Pb ages were investigated by electron probe microanalyser (EPMA). The igneous and metamorphic monazites display contrasting and typical mineral-chemical properties. Metamorphic monazites follow strictly the cheralite substitution trend in Th + U vs Ca coordinates. Igneous monazite in an alkalifeldspar granite has the highest Y2O3 contents (~2 wt%) among all studied samples. In ThO2* vs PbO coordinates the monazite data define isochrones. Micaschist lithoclasts yielded 328 ± 3 Ma, 326 ± 6 Ma and 324 ± 5 Ma, interpreted to represent the thermal peak and post-peak age of metamorphic monazite crystallization. The 328 ± 5 Ma age of igneous monazite in the alkalifeldspar granite in contact to micaschist is interpreted to date the crystallisation of a synmetamorphic anatectic melt. This contrasts the 313 ± 3 Ma monazite crystallization age in a post-tectonic monzogranite. No indications of bulk Pb loss in monazite by shock metamorphism have been observed. The EPMA Th-U-Pb monazite ages from the lithoclasts match data from granites and meta-psammopelites in the outcropping pre-Mesozoic basement in the Western Bohemian Massif and the Black Forest. They confirm that the bottom of the Nördlinger Ries impact crater is situated in crystalline basement rocks belonging to the Moldanubian Zone. Kurzfassung Im Steinbruch am Lehenberg (auch Lehberg oder Limberg-Nord) im NW-Teil der Megablock-Zone des Nördlinger Rieskraters kommen Granit- und Glimmerschiefer-Lithoklasten in einer polymikten Suevit-Impaktbrekzie vor. Die Lithoklasten zeigen die petrographischen Merkmale der Stoßwellenmetamorphose-Skala, wie Einschlüsse mit diaplektischem Glas, Quarzkörner mit dekorierten planaren Elementen und stark geknickte Glimmer. In petrographischen Dünnschliffen wurde Monazit durch Spektralkartierung mit einem automatisierten Rasterelektronenmikroskop detektiert. Im Rückstreuelektronenbild (BSE) zeigt Monazit die für magmatische und metamorphe Kristallisation typischen Kristallformen sowie die internen Zonierungen und Verteilungsmuster von Th. Nur wenige Monazitkörner zeigen die für Stoßwellenmetamorphose typischen Internstrukturen wie scharf parallel angeordnete Risse die Lamellen bilden. Mineralchemie und Th-U-Pb-Alter der Monazite wurden mit der Elektronenstrahlmikrosonde bestimmt. Magmatische und metamorphe Monazite zeigen unterschiedliche Zusammensetzungen. In Th + U vs Ca Koordinaten folgen die metamorphen Monazite dem Cheralith-Substitutions-Trend. Magmatischer Monazit im Alkalifeldspat-Granit hat die höchsten Y2O3-Gehalte (~2 wt%) der untersuchten Proben. In ThO2* vs PbO Koordinaten definieren die Monazitanalysen Isochronen. In den Glimmerschiefer-Lithoklasten liegen die Th-U-Pb-Monazitalter bei 328 ± 3 Ma, 326 ± 6 Ma und 324 ± 5 Ma und werden als Alter der Metamorphose-Maximaltemperatur und nachfolgender Abkühlung interpretiert. Die Monazit-Isochrone von 328 ± 5 Ma im Alkalifeldspat-Granit im Kontakt zum Glimmerschiefer wird als Kristallisationsalter einer synmetamorphen anatektischen Schmelze interpretiert. Sie unterscheidet sich deutlich vom jüngeren Kristallisationsalter des posttektonischen Monzogranits mit seinen 313 ± 3 Ma alten Monaziten. Es ergeben sich keine Anzeichen für Verlust von Gesamt-Pb in Monazit durch die Stosswellenmetamorphose. Die Elektronenstrahlmikrosonden-Th-U-Pb-Alter der Monazite in den Lithoklasten gleichen denen von Graniten und Metapsammopeliten im prä-mesozoischen Basement der westlichen Böhmischen Masse und des Schwarzwalds. Dies belegt dass im Untergrund des Rieskraters die kristallinen Basementgesteine der Moldanubischen Zone anzutreffen sind. Keywords: suevite impact breccia, micaschist, granite, monazite, microstructures, Th-U-Pb age dating, Moldanubian Zone Schlüsselwörter: Suevit-Impaktbrekzie, Glimmerschiefer, Granit, Monazit, Mikrostrukturen, Th-U-Pb-Altersbestimmung, Moldanubisches Basement
Bernhard Schulz
added a research item
This contribution examines the strain pattern of the Archean Ntem Complex, located in the northwestern part of the Congo Craton (southern Cameroon). We carried a detailed field study coupled with aeromagnetic map for the two main greenstone belts (Sangmelima and Nyabisan), focusing on the geometry of structures as well as kinematics, and identifying several lithological facies with distinct petrological characters. The time constraints of magmatism, metamorphism and deformation were obtained by U-Pb LA-ICP-MS dating on zircon and EPMA U-Th-Pb dating on monazite. The Nyabissan Greenstone Belt is dominated by sub-vertical transpressive shear zones with steeply plunging stretch lineations. There are vertical regional fabric domains with sub-horizontal foliation domains suggesting a strain partitioning at a regional scale. Different generations of syntectonic mafic and felsic intrusions attest to long-lasting progressive deformation. In the Sangmelima granite-greenstone belt (SGB), charnockites and TTG suites formed between~ 3155 and 2850 Ma. The gabbro intrusion was dated at 2866±6 Ma. Migmatization of the TTG-gneiss, coeval with a sub-horizontal shortening synmetamorphic D1 event, is dated around 2843-2820 Ma using U-Pb LA-ICP-MS on zircon and chemical U-Th-Pb EPMA dating on monazite. Syn-kinematic monzogranite emplaced 2838±6 Ma ago. D2 is associated with the F2 folds and C2 shear zones coeval with the second anatexis period between ~2788-2722 Ma marked by the emplacement of high-K granite that peaked at ~2750 Ma. Ages of migmatisation and syn-kinematic granites (2843 to 2722 Ma) suggest a long-lasting tectonic process in the Sangmelima area. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D3 and ANC protracted cooling between ~2670-2550 Ma. A synthesis of all results leads to the reconstruction of the general structure for the Ntem Complex, from the Paleo and Neoproterozoic to the Archean domains. In addition, a new model of Archean geodynamic evolution in the NW Congo Craton from 3.32 Ga to 2.55 Ga is proposed.
Bernhard Schulz
added a research item
Garnet-bearing metapelites in the Helvetic and Austroalpine pre-Mesozoic polymetamorphic basement are characterised by pressure-temperature path segments reconstructed by microstructurally controlled geothermobarometry, and the Th-U-Pb monazite age distribution pattern revealed by the electron probe microanalyser (EPMA). In the Helvetic Aiguilles Rouges Massif and the Austroalpine Oetztal-Stubai basement to the NW an Ordovician-to-Silurian high temperature event preceded a pressure-dominated Carboniferous metamorphism. In the Austroalpine basement units to the south of the TauernWindow, the maximal pressures of the Carboniferous amphibolite-facies metamorphism range from 12 to 6 kbar. The decompressional P-T path segments signal a transition to low pressure conditions. A subsequent high pressure overprint is restricted to the Prijakt Subgroup unit in the Schobergruppe and documented by Cretaceous monazite crystallisation at 88 +- 6 Ma. In the Austroalpine Saualpe basement to the SE, a distinct early Permian metamorphism which started at low pressures of ~4 kbar/500 °C and reached maximal 6 kbar/600–650 °C predated the intrusion of Permian pegmatites. Permian monazite crystallised in line with the intrusion of pegmatites. Corona microstructures around the Permian monazites indicate retrogression previous to a Cretaceous high pressure metamorphism. That way, pressure-temperature-time paths resolve the spatial and temporal evolution in the polymetamorphic Alpine basement prior to the Tertiary collision.
Bernhard Schulz
added a research item
Erinpura Granites form the basement for the Neoproterozoic Malani Igneous Suite in NW India. Based on this study, the Erinpura granites can be divided into Erinpura-East (gneissic fabric), a belt parallel to the southern sector of the South Delhi Fold Belt, and Erinpura-West (magmatic fabrics). EPMA dating on monazites gives a time frame of 890 to 860 Ma for crystallization of both types. The geochemically homogeneous peraluminous S-type granites with ε Nd values of −2.1 to −10.8 are interpreted as melting products of Archaean crust. REE pattern follows the pattern of the average continental crust, but Erinpura-East samples are more fractionated with steeper HREE depletion indicating melting in a deeper crustal level. A thermal pulse between 835 and 820 Ma constrains the timing of deformation in granite-gneisses during uplift along thrust planes, coeval with shear-bound exhumation of high-grade metamorphic rocks and initiated by delamination of the lower crust in this southern sector of the South Delhi Fold Belt. This is in contrast to the northern sector of the SDFB with arrested orogenic development and without considerable delamination or erosion of the lower crust. Latest movement related to the 200 Ma tectono-magmatic history overlaps with initiation of rifting during the Malani igneous event. A change from S-type to A-type granites and shift of isotopic signatures to ε Nd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process. ARTICLE HISTORY
Bernhard Schulz
added a research item
The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO 4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.
Bernhard Schulz
added 2 research items
The Conlara Metamorphic Complex, the easternmost complex of the Sierra de San Luis, is a key unit to understand the relationship between the late Proterozoic-Early Cambrian Pampean and the Upper Cambrian-Middle Ordovician Famatinian orogenies of the Eastern Sierras Pampeanas. The Conlara Metamorphic Complex extends to the east to the foothills of the Sierra de Comechingones and to the west up the Río Guzmán shear zone. The main rock types of the CMC are metaclastic and metaigneous rocks that are intruded by Ordovician and Devonian granitoids. The metaclastic units comprise fine to medium-grained metagreywackes and scarce metapelites with lesser amounts of tourmaline schists and tourmalinites whereas the metaigneous rocks encompass basic and granitoids rocks. The former occur as rare amphibolite interlayered within the metasedimentary rocks. The granitic component corresponds to a series of orthogneisses and migmatites (stromatite and diatexite). The CMC is divided in four groups based on the dominant lithological associations: San Martin and La Cocha correspond mainly to schists and some gneisses and Santa Rosa and San Felipe encompass mainly paragneisses, migmatites and orthogneisses. The Conlara Metamoprphic Complex underwent a polyphase metamorphic evolution. The penetrative D2-S2 foliation was affected by upright, generally isoclinal, N-NE trending D3 folds that control the NNE outcrop patterns of the different groups. An earlier, relic S1 is preserved in microlithons. Discontinuous high-T shear zones within the schists and migmatites are related with D4 whereas some fine-grained discontinuous shear bands attest for a D5 deformation phase. Geochemistry of both non-migmatitic metaclastic units and amphibolites suggest that the Conlara Metamorphic Complex represents an arc related basin. Maximun depositional ages indicate a pre- 570 Ma deposition of the sediments. An ample interval between sedimentation and granite emplacement in the already metamorphic complex is indicated by the 497 ± 8 Ma age of El Peñon granite. D1-D2 history took place at 564 ± 21 Ma as indicated by one PbSL age calculated for the M2 garnet of La Cocha Group. D3 is constrained by the pervasively solid-state deformed Early Ordovician granitoids which exhibits folded xenoliths of the D1-D2 deformed metaclastic rocks. Pressure-temperature pseudosections were calculated for one amphibolite using the geologically realistic system MnNCKFMASHTO (MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3). Peak metamophic conditions (M2) indicate 6 kbar and 620 °C. Late chlorite on the rims and in cracks of garnet, along with titanite rims on ilmenite and matrix plagioclase breaking down to albite suggests that the P-T path moved back down. Monazite analyses yield isochron Th–U–Pb ages ranging from 446 to 418 Ma. The oldest age of 446 ± 5 Ma correspond to a migmatite from the Santa Rosa Group. Monazites in samples from the La Cocha and the San Martin group crystallized at decreasing temperatures, followed by the 418 ± 10 Ma low-Y2O3 monazites in one sample of the la Cocha Group that was also obtained from a migmatite, and would likely mark a later stage of a retrograde metamorphism New CHIME monazite ages presented here likely represent post-peak fluid assisted recrystallization that are similar to amphibole and muscovite cooling ages. Therefore the monazite ages may represent a re-equilibration of the monazite on the cooling path of the basement complex.
In this contribution, we present the stratigraphy of the igneous and metamorphic rocks of the Taquetrén Range, a sector located in the southernmost margin of the North Patagonian Massif (42°42′ 00′′ S - 69°30′00′′ W). Its igneous and metamorphic basement is composed of the newly defined “Lagunita Salada Igneous-Metamorphic Complex” (LSIMC), “Paso del Sapo Plutonic Complex” (PSPC) and “Sierra de Taquetr´en Plutonic Complex” (STPC). The LSIMC comprises gneisses, schists, amphibolites and migmatites, which share a S1–S2 penetrative foliation with a mean orientation of 300°–330°/40°–60° NE. Based on mineral paragenesis, metamorphic conditions of these rocks are the result of Barrovian-type metamorphism in the upper amphibolite to granulite facies. EPMA Th–U–Pb ages of monazites display two isochron main populations at 379 ± 5 Ma and 323 ± 5 Ma, which suggest long-term high-temperature conditions for the region between Late Devonian and Carboniferous times. The Complex is intruded by concordant tonalites, granodiorites, porphyric granites and minor pegmatites and felsic dykes, which are grouped in the PSPC. Both the LSIMC and PSPC are intruded by unfoliated peraluminous granitoids grouped in the STPC. Based on field and microstructural data, the pervasive foliation identified in the PSPC was caused by processes ranging from magmatic flow to solid-state deformation, indicating a syntectonic emplacement. Zircon U–Pb analysis by LA-ICP-MS in the PSPC shows two distinguishable groups with concordia ages of 314.1 ± 2.2 Ma and 302.8 ± 2.2 Ma, interpreted as the crystallization and subsequent deformation age respectively, related to protracted high-strain conditions. The outcrops in this area represent an almost full tectonic cycle encompassing from medium-high grade metamorphic rocks and syn-tectonic intrusions to posttectonic intrusions, therefore configuring a key locality for the analysis of North Patagonian Paleozoic evolution. Moreover, based on the compilation of U–Pb zircon ages, a ~20 My magmatic gap period (360-340 Ma) is recognized in the southwestern margin of the North Patagonian Massif coeval with amphibolite-granulite facies metamorphism in different sectors of the Central Patagonian Igneous-Metamorphic Belt, presenting thus important implications for the tectonic evolution of the area.
Bernhard Schulz
added a research item
The Hamadan high-grade metapelites in the northwestern part of the Sanandaj–Sirjan zone, Iran, show a polymetamorphic evolution with relics of a garnet-bearing metamorphic mineral assemblage (M1), a contact metamorphic overprint (M2) related to the emplacement of the Middle to Late Jurassic Alvand composite pluton and a Buchan-type regional metamorphic event (M3) marked by 40Ar/39Ar ages in the 80–70 Ma range that is associated with penetrative ductile deformation producing a foliation and a thermal overprint onto the M2 assemblages. The M1 event is exclusively preserved as small garnet grains and mineral inclusions contained therein, incorporated into M2-stage cordierite porphyroblasts. Distinct metamorphic zones are developed over a region of ~ 600 km2, which are partly correlated with distance to the composite pluton: zones (1) cordierite + K-feldspar hornfels, and (2) andalusite ± cordierite hornfels that surround the Alvand composite pluton at a distance of up to 5 km. These two zones are clearly related to M2 metamorphism associated with pluton emplacement. Zones (3) staurolite schist, (4) andalusite schist, and (5) sillimanite schist are found outside of the contact aureole and are considered to be the result of regional M3 metamorphism in the eastern part distant to the Alvand composite pluton. Conventional thermobarometry shows that temperatures in the area vary between ~ 560 and 660 °C for zones 1 and 2 and ~ 490 and 690 °C for zones 3–5. Phase equilibria modelling in the MnNCKFMASHT system indicates two distinct isobaric prograde paths at low pressures, at ~ 2.7 kbar for zones 1 and 2 and slightly higher pressures of around 3.5–5.5 kbar for zones 3–5. U–Th–Pb monazite geochronology revealed overlapping ages of 168 ± 11 Ma and 149 ± 19 Ma in the hornfels (1 and 2) and schistose (3–5) zones, respectively. These ages are similar to the intrusion age of the Alvand composite pluton (153.3 ± 2.7 to 166.5 ± 1.8 Ma) and are interpreted to reflect heating due to the emplacement of the composite pluton (M2 contact metamorphic event). However, 40Ar/39Ar dating of white mica and amphibole yielded plateau ages ranging from 80 to 69 Ma over the entire transect. The formation of schistosity in zones 3–5 postdates the intrusion and is thus related to M3 metamorphism. The white mica fabric indicates formation of the foliation during M3 garnet growth, which is followed by local retrogression of garnet to chlorite during exhumation. Consequently, the 40Ar/39Ar white mica and amphibole ages likely indicate reheating during M3 to more than ca. 500 ± 25 °C (argon retention temperature in amphibole). These data establish the occurrence of a Cretaceous, Buchan-style regional metamorphic event that had not been firmly identified before. Subsequent Late Cretaceous exhumation of the Hamadan complex with its high-grade metapelites is due to extension along the Tafrijan–Mangavi–Kandelan fault, which represents a major ductile low-angle normal fault. Metamorphic temperatures coupled with mineral ages from this and published work suggest a fast stage of cooling with a rate of ~ 6 °C/Ma during exhumation after M3 metamorphism.
Bernhard Schulz
added a research item
Monazite dating in metapelites is an emerging method to investigate polymetamorphic areas. A protocol for Th–U–Pb dating of monazite by electron microprobe was adopted for a JEOL JXA-8530F. It was applied to the Variscan and Early-Alpine metamorphic Austroalpine Oetztal-Stubai Complex (OSC). In the Alpeiner Valley in the Stubai region, the Schrankogel complex is the eastern succession of the Central Metabasite Zone. In this part, metabasites are alternating with metapelites. In 4 samples from micaschist lenses, dominantly Carboniferous monazite isochrone ages at 335 ± 4 Ma, 320 ± 4 Ma; 319 ± 4 Ma and 319 ± 4 Ma were obtained. The micaschist samples with diverse modal compositions and variable bulk rock Ca contents of calculated assay, display distinct monazite microstructures, as quantified by automated SEM-MLA (mineral liberation analysis) routines. Clusters of small monazite could indicate new crystallization and yielded isochrones at 313 and 304 Ma. In contrast, corona structures of apatite and allanite around large monazites with isochrones between 350 and 315 Ma suggest a decomposition during decreasing temperature. Garnets in metapelitic assemblages display growth zonations with low pyrope contents in the cores and pyrope-rich rims. A prograde metamorphism with high pressure amphibolite-facies peak conditions at ~ 12 kbar and ~ 680 °C, and a post Pmax path with decompression to 4 kbar and 640–600 °C was estimated from the micaschists and from zoned Ca-amphiboles in retrogressed amphibolitized eclogites. The P–T path entered the monazite stability field during the decompression. This signals a Carboniferous age of the metamorphism. A minor population in one sample is composed of sporadic Permian single monazite ages. A Cretaceous monazite population is lacking. In the wide parts of the Austroalpine basement with Carboniferous-to-Cretaceous mica mixing ages, monazite age populations allow to discriminate a distinct Permian metamorphic event.
Bernhard Schulz
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Combined field structural analysis with in situ electron probe microanalysis Th‐U‐Pb monazite dating, petrologic, and microstructural data provides a reconstruction of the pressure‐temperature‐deformation‐time (P‐T‐D‐t) path of the Gondwanide basement of the North Patagonian Cordillera. For samples from the Challhuaco hill, the timing of development of the metamorphic S2 foliation and associated L2 lineation and tight to isoclinal F2 folds is constrained by monazite ages of 299 ± 8 and 302 ± 16 Ma during peak metamorphic conditions of ~ 650 °C and 11 kbar, achieved during prograde metamorphism and progressive deformation. Metamorphism and deformation of metamorphic complexes of the North Patagonian Andes seem to record Late Paleozoic crustal thickening and are coeval with metamorphism of accretionary complexes exposed further west in Chile, suggesting a coupled Late Devonian‐Carboniferous evolution. Instead of the result of continental collision, the Gondwanide orogeny might thus be essentially linked to transpression due to advancing subduction along the proto‐Pacific margin of Gondwana. On the other hand, second generation of monazite ages of 171 ± 9 and 170 ± 7 Ma constrains the timing of low‐grade metamorphism related to kink band and F3 open fold development during Jurassic transtension and emplacement of granitoids. Finally, a Cretaceous overprint, likely resulting from hydrothermal processes, is recorded by monazite ages of 110 ± 10 and 80 ± 20 Ma, which might be coeval with deformation along low‐grade shear zones during the onset of Andean transpression.
Bernhard Schulz
added a research item
The Saxothuringian Zone in the eastern part of the Variscan orogen is composed of autochthonous and allochthonous domains. The dating of metamorphic events in the Saxonian Granulite Massiv (SGM), and the Münchberg and Frankenberg Massivs, and Erzgebirge Nappe Units is critical for resolving the geodynamic evolution during the Variscan orogeny. In-situ chemical Th-U-Pb monazite dating by electron microprobe has demonstrated its potential to resolve polyphase metamorphism. Monazite is abundant in cordierite-sillimanite-garnet-gneisses (kinzigites), but rare in the quartz-feldspatic kyanite-garnet-gneisses of the Saxonian Granulite Massiv. Monazite at 10-200 µm in foliated matrix, enclosed in garnets has been detected by automated SEM-MLA (Mineral Liberation Analyser) methods in thin sections. An electron microprobe (EMP) Th-U-Pb monazite dating routine, was performed with JEOL-8530F, producing 100 - 200 single analyses per sample. Energy dispersive x-ray mapping GXMAP of automated SEM-MLA was used for semiquantitative identification of garnet zonation patterns [2]. Quantitative chemical compositions of garnet and related plagioclase, biotite and cordierite were then measured by EMP for geothermobarometric estimates by cation exchange and net transfer reactions. Garnets in the kinzigites from Mohsdorf in the central part and the SE margin of the SGM display Ca- and Mn-rich cores, and Mg-rich rims of prograde metamorphism. The P-T paths started at ~550 °C/6 kbar and reached maximum pressures at 700 °C/8 kbar. Maximum temperatures were approached at 750-800 °C/4 kbar. A significant decrease of temperatures at low pressures was recorded by decreasing Mg contents in the garnet rims. ThO2*-PbO isochrones of monazites enclosed in the kinzigite garnets range between 340±8 to 330±4 Ma and appear to be slightly older as the matrix monazites (331±5 to 317±5 Ma). As P-T conditions from garnet-bearing equilibria match the stability field of monazite, this may bracket the period of garnet crystallisation. In garnet-free granulites and gneisses, considerably older metamorphic monazite with isochrone ages at around 358±8 Ma, and monazite populations at 407-440 Ma have been detected. The post-340 Ma monazite populations match zircon and Ar-Ar data as summarised in [2]. Also the 335±2 Ma monazites from a discordant granite match earlier results from such rocks. References [1] Schulz, B. (2017) Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps, Austria), resolved by automated SEM methods and EMP-Th-U-Pb monazite dating. J. Metam. Geol., 35:141-163. [2] Franke, W., Stein, E. (2000) Exhumation of high-grade rocks in the Saxo-Thuringian Belt: geological constraints and geodynamic concepts. Geol. Soc. London Spec. Publ., 179, 337-354.
Bernhard Schulz
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Shear zones play a major role in the deformation of the crust at a variety of scales, as expressions of strain localization during orogeny and rifting, and also as reactivated structures. They influence the geometry and evolution of orogenic belts and rifts, crustal rheology, magma ascent and emplacement, and fluid flow. Consequently, assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere. The interpretation of thermochronologic data from shear zones is, however, not straightforward. In the first place, closure temperatures depend on a number of factors (grain size, cooling rate, mineral composition and pressure, among others). On the other hand, deformation-related processes such as dynamic recrystallization, neocrystallization and fluid circulation seem to be crucial for isotopic systems and, thus, the obtained ages cannot be solely interpreted as a function of temperature in sheared rocks. For this reason, geochronologic data from shear zones might not only record cooling below closure temperature conditions but may also be affected by neo- or recrystallization, fluid-assisted deformation and inheritance of the protolith age(s). In order to robustly reconstruct P-T-ε-t paths of long-term crustal-scale shear zones, structural, microstructural and petrologic data from mylonites need to be integrated with ages from different thermochronometric systems. In addition, geochronologic data from associated intrusions and adjacent blocks can provide further irreplaceable constraints on the timing of deformation and its regional implications. One of the most challenging aspects that future lines of investigation should analyze is the quantitative evaluation of so far poorly explored aspects of isotopic diffusion, particularly the coupling with deformation processes, based on natural, theoretical and experimental data. Future works should also investigate the role of strain partitioning and localization processes in order to constrain the timing of deformation in different parts of a shear zone or in different branches of anastomosing shear zone networks.
Bernhard Schulz
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Bernhard Schulz
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The Saxothuringian Zone in the eastern part of the Variscan orogen is composed of autochthonous and allochthonous domains. The dating of metamorphic events in the various allochthonous domains (Erzgebirge, Münchberg Massif, Frankenberg Massif) is critical for resolving the complex geodynamic evolution. The dating of metamorphism is hampered by various methodical aspects. Zircon is highly refractory at greenschist and amphibolite facies conditions. Apart from mass-bias problems, the LA-ICPMS isotopic analysis of metamorphic minerals has insufficient spatial resolution in most fine-grained metamorphic rocks. Although restricted to metapelites and leucogranitoids, the in-situ chemical Th-U-Pb monazite (Th, U, Si, LREE, Y, Ca)PO4 dating by electron microprobe has demonstrated its high potential to resolve polyphase metamorphism [1]. The method is based on the premise that monazite inherits negligeable amounts of common Pb and that the radiogenic Pb is retained due to very low diffusion rates even at high T (Montel et al. 1996). Monazite crystallises at greenschist to granulite-facies conditions in Ca-poor aluminous pelites and is also an accessory phase in peraluminous granitoids. Monazite in various microstructures and at grain sizes of 10 - 50 µm, rarely up to 80 µm, has been detected by automated SEM methods in polished thin sections from phyllites, garnet micaschists and orthogneisses. After detailed BSE imageing of the monazite grains, the electron microprobe Th-U-Pb monazite dating routine was performed with a JEOL-8530F, enclosing the analysis of HREE. Garnet micaschists in the Liegendserie of the Münchberg Massif yielded Th-U-Pb monazite ages between 390±8 Ma and 377±6 Ma, which were calculated from isochrons defined by a single population. There are also garnet micaschists which display a dominant population at 379±5 Ma and a minor one at 312±13 Ma, interpreted as a metamorphic overprint. An exceptional age of 498±19 Ma in a garnet micaschist could be explained by a regional contact metamorphism, as a biotite orthogneiss yielded monazite of 503±12 Ma, interpreted as a magmatic crystallisation age. The Devonian monazite ages match the K-Ar hornblende data from various metabasites in the Münchberg Massiv [3]. In the Frankenberg Massiv the orthogneiss samples yielded monazite ages between 515± 11 Ma and 507±17 Ma. The monazite age data from the Frankenberg phyllites range at 365±8 Ma, thus matching also the Late Devonian. References [1] Schulz, B. (2017): Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps, Austria), resolved by automated SEM methods and EMP-Th-U-Pb monazite dating. - J. Metam. Geol., 35: 141-163. DOI 10.1111/jmg.12224 [2] Montel, J.-M., Foret, S., Veschambre, M., Nicollet, C., Provost, A. (1996): Electron microprobe dating of monazite. - Chem. Geol., 131: 37-51. [3] Kreuzer, H., Seidel, E., Schüssler, U., Okrusch, M., Lenz, K.-L., Raschka, H. (1989): K-Ar geochronology of different tectonic units at the northwestern Margin of the Bohemian Massif. - Tectonophysics, 157: 149-178.
F. Finger
added a research item
Low-temperature thermal events of Permian (ca. 265 Ma) and Triassic (ca. 215 Ma) age that predate medium-grade regional metamorphism were identified using high spatial resolution field emission-scanning electron microscopy-energy dispersive X-ray (FE-SEM-EDX) U-Th-Pb dating of uraninite microcrystals in basement rocks of the Tauern Window, Eastern Alps. Three novel points of generic geochronological importance are raised in this study. First, uraninite can be meaningfully dated with FE-SEM-EDX methods, with moderate precision. Second, uraninite is geochronologically robust, even at microcrystal scale, and can survive at least medium-grade metamorphic overprint without being reset. Third, uraninite microcrystals are powerful tools for identifying and dating discrete low-temperature thermal events in orogenic belts. Dating of uraninite microcrystals should be considered an important complementary geochronological method in the study of polymetamorphic rocks.
Bernhard Schulz
added an update
Project goal
Development and application of the in-situ CHIME Th-U-Pb isochron dating method of monazite by the electron microprobe.
Background and motivation
Development and application of the in-situ CHIME Th-U-Pb isochron dating method of monazite by the electron microprobe to metamorphic rocks in orogenic domains. Configuration of analysis protocols for EMP dating with the JEOL-8530 FEG instrument and other instruments manufactured by JEOL. Application of SEM automated mineralogy methods for detection and evaluation of monazite in various rock types. Application of EMP monazite dating and mineral chemistry for evaluation of ore deposits.
 
Bernhard Schulz
added 3 research items
The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulites as well as lower grade gneisses and mylonites. The complex is distributed over an area ofca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometric calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulites based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 ºC and pressures between of 5.0 and 9.9 kbar and a retrometamorphic path characterized by near-isobaric cooling. Part of the granulites was affected by anatexis. The melting of felsic granulites resulted in the generation of pegmatites and two aluminous lithotypes. These are:
The Capelinha Formation (Macaúbas Group) consists of a lower quartzitic unit with metamafic intercalations and an upper metapelitic sequence. It occurs in a complex tectono-metamorphic sector of the Araçuaí orogen, where post-collisional collapse-related structures superimposed collisional structures. The garnet-bearing assemblages started crystallization in the collisional deformation stage that formed the main regional foliation around 570 Ma. Garnet porphyroblasts display a well-developed growth zonation of Fe-Mg-Ca-Mn and show, from core to rim, increasing almandine and pyrope contents in contrast with decreasing grossular and spessartine contents. Mineral relations and microstructures provide criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The P-T values calculated from cores to rims of garnets, aligned along clockwise trends, resulted in increasing temperatures (from 500 ºC up to 620 ºC) under decompression conditions (from 8.0 kbar to 4.5 kbar). The Th-U-Pb dating of homogeneous monazites by electron microprobe revealed a recrystallization period at around 490 - 480 Ma. These ages can be related to the tectono-thermal event associated with the gravitational collapse, constraining the youngest time limit for metamorphic processes in the Araçuaí orogen.
This paper focuses on pressure- and temperature path analyses in paragneisses from a metasedimentary succession (MSS) in the south western Araçuaí orogen. The sampling area is limited by the Abre Campo shear zone (West) and the Rio Doce Magmatic Arc (East). This region is rich in ortho-derived metamorphic basement rocks (Mantiqueira and the Juiz de Fora complexes) and para-derived metamorphic rocks, including paragneisses interlaid by quartzites, which form the MSS. The sampled rocks are mainly compost of quartz, plagioclase, garnet, K-Feldspar, orthopyroxene and sillimanite. Measurement spot profiles through garnet porphyroblasts show a certain zonation characterised by decrease in pyrope and the increase in almandine from the core to the rim. This implies retrograde growth. Metamorphic conditions are of high amphibolite- to granulite facies with maximum pressure of ca. 6 kbar and maximum temperature of ca. 700°C. The data suggest that the cores of the garnet porphyroblasts of MSS started to grow during the final deformation stage of the Araçuaí orogen (south western part); the event of decompression is captured in the rims of the porphyroblasts and related to the gravitational collapse in the Cambrian. Keywords: P-T path, metasedimentary succession, geothermobarometry, Araçuaí orogenResumo: RECONSTRUÇÃO DE TRAJETÓRIAS P-T EM PARAGNAISSES GRANADÍFEROS NEOPROTEROZÓICOS DA UMA SUCESSÃO METASSEDIMENTAR NO SUDOESTE DO ORÓGENO ARAÇUAÍ, MINAS GERAIS, BRASIL. Este estudo foca analises de caminhos de pressão e temperatura em paragnaisses de sucessão metassedimentar (SMS) da região sudoeste do Orógeno Araçuaí. A área de amostragem é limitada pela zona de cisalhamento de Abre Campo a oeste e pelo Arco Magmático Rio Doce a leste. A região é rica em rochas metamórficas de alto grau ortoderivadas (complexos Mantiqueira e Juiz de Fora do embasamento) e paraderivadas; incluindo paragnaisses intercalados a quartzitos, que formam o SMS. As rochas amostradas são compostas por quartzo, plagioclásio, granada, feldspato potássio, ortopiroxênio e sillimanita. Perfis de pontos de análise em porfiroblastos de granada mostram certo zoneamento caracterizado pela redução de piropo e por aumento de almadina do núcleo para borda; isto implica em crescimento de cristal sob condições de metamorfismo retrógrado. Condições metamórficas são de fácies anfibolito alto a granulito com um máximo de pressão de ca. 6 kbar e com temperatura máxima de ca. 700°C. Os dados sugerem que porfiroblastos de granada da SMS têm nucleação e início de crescimento durante o final da etapa deformacional principal compressiva do Orógeno Araçuaí (região sudoeste do orógeno); o evento de descompressão captado em bordas dos porfiroblastos foi relacionando ao colapso gravitacional no Cambriano. Palavras-chave: Trajetórias P-T, sucessão metassedimentar, geotermobarometria, Orógeno Araçuaí
Bernhard Schulz
added 4 research items
Automated SEM-EDS methods in support of EMP-monazite dating and P-T path reconstruction in the polymetamorphic garnet micaschists of the Austroalpine Saualpe Eclogite Unit Polymetamorphic micaschists from the Austroalpine Saualpe Eclogite Unit display complex microstructural and mineral-chemical relationships. Numerous complete thin sections were studied by automated scanning electron microscopy with spare phase search (SPL) for monazite and energy dispersive spectral mapping (GXMAP) of garnet. The spectral maps allow to resolve garnet semi-quantitative Fe-Mg-Mn-Ca zonation trends in various combinations and to define locations of electron microprobe analyses. Two garnet porphyroblast generations and several monazite populations have been revealed in the low-Ca and high-Al-metapelites. The EMP Th-U-Pb monazite dating identified low-Y Cretaceous (80-100 Ma), and high-Y Permian (250-270 Ma) and Carboniferous (310-320 Ma) age groups which are variably distributed in the samples. Coronas of apatite and allanite around large Permian monazites signal a retrogressive stage. Garnet 1 porphyroblasts enclosing mica, plagioclase and quartz display increasing XMg and constant XCa at decreasing Mn contents. They crystallised during a M1 prograde metamorphism at increasing pressure and temperature up to ~650 °C/6 - 8 kbar. Carboniferous and Permian monazite crystallised along the margin of garnet 1. This microstructure in combination with the retrogressive monazite coronas suggest a Carboniferous-to-Early-Permian age for the M1 event, not yet reported from the unit. The M2 event with garnet 2 postdates the corona formation around Permian monazites. Garnet 2 displays complex zonations trends with high Mg and Ca contents at always low Mn contents. This can be sorted into trend Grt2-1 with increasing XCa at decreasing XMg, then trend Grt2-2 with increasing XCa at increasing XMg, and finally Grt2-3, with decreasing XCa at increasing XMg. Garnet 2 crystallised at the well-known Cretaceous eclogite event (Thöni et al. 2008). Maximum temperatures at 750 °C/14 kbar were passed during decreasing pressure. Cretaceous monazites then crystallised in large grains and also in satellite structures (Finger et al. 2016). The two prograde metamorphic events in the Saualpe Eclogite Unit are related to continental collisions under different thermal regimes. Finger F, Krenn E, Schulz B, Harlov DE, Schiller D, 2016. American Mineralogist 101, 1094-1103. Thöni M, Miller C, Blichert-Toft J, Whitehouse, MJ, Konzett J, Zanetti A, 2008. J. Metam. Geol. 26, 561-581.
Allanite-fluorapatite reaction coronas around monazite are abundant in metamorphic rocks. We report here special cases where a new generation of “satellite” monazite grains formed within these coronas. Using examples from different P-T regions in the eastern Alps, we examine the origin and the petrological significance of this complex mineralogical association by means of the electron microprobe utilizing Th-U-Pb monazite dating and high-resolution BSE imaging. Satellite monazite grains form when a monazite-bearing rock is metamorphosed in the allanite stability field (partial breakdown of first generation monazite to fluorapatite plus allanite), and is then heated to temperatures that permit a back reaction of fluorapatite plus allanite to secondary satellite monazite grains surrounding the remaining original first generation monazite. Depending on the whole-rock geochemistry satellite monazites can form under upper greenschist- as well as amphibolite-facies conditions. In each of the three examples focused on here, the inherited core monazite was resistant to recrystallization and isotopic resetting, even though in one of the samples the metamorphic temperatures reached 720 °C. This shows that in greenschist- and amphibolite-facies polymetamorphic rocks, individual grains of inherited and newly formed monazite can and often will occur side by side. The original, inherited monazite will preferentially be preserved in low-Ca, high-Al lithologies, where its breakdown to allanite plus fluorapatite is suppressed. Conversely, a medium- or high-Ca, monazite-bearing rock will become particularly fertile for secondary monazite regrowth after passing through a phase of strong retrogression in the allanite stability field. Based on this knowledge, specific sampling strategies for monazite dating campaigns in polymetamorphic basement can be developed.
Bernhard Schulz
added a project goal
Development and application of the in-situ CHIME Th-U-Pb isochron dating method of monazite by the electron microprobe.
 
Bernhard Schulz
added 6 research items
The pre-Mesozoic, mainly Variscan metamorphic basement of the Col de Bérard area (Aiguilles Rouges Massif, External domain) consists of paragneisses and micaschists together with various orthogneisses and metabasites. Monazite in metapelites was analysed by the electron microprobe (EMPA-CHIME) age dating method. The monazites in garnet micaschists are dominantly of Variscan age (330–300Ma). Garnet in these rocks displays well developed growth zonations in Fe–Mg–Ca–Mn and crystallized at maximal temperatures of 670°C/7kbar to the west and 600°C/7–8kbar to the east. In consequence the monazite is interpreted to date a slightly pressure-dominated Variscan amphibolite-facies evolution. In mylonitic garnet gneisses, large metamorphic monazite grains of Ordovician–Silurian (~440Ma) age but also small monazite grains of Variscan (~300Ma) age were discovered. Garnets in the mylonitic garnet gneisses display high-temperature homogenized Mg-rich profiles in their cores and crystallized near to ~800°C/6kbar. The Ordovician–Silurian-age monazites can be assigned to a pre-Variscan high-temperature event recorded by the homogenised garnets. These monazite age data confirm Ordovician–Silurian and Devonian–Carboniferous metamorphic cycles which were already reported from other Alpine domains and further regions in the internal Variscides. KeywordsOrdovician–Variscan–External Massiv–Garnet metapelites–EMP monazite age dating–P–T path–Polyphase metamorphism