Direct dating of mid-crustal shear zones with synkinematic allanite:new in situ U-Th-Pb geochronological approaches applied to the Mont Blanc massif

International audienceDating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U-Th-Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA-ICP-MS U-Th-Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common-Pb component. The use of measured Pb-isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common-Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite

Timing and duration of partial melting and magmatism in the Variscan Montagne Noire gneiss dome (French Massif Central)

International audienceUnravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation. The Montagne Noire Axial Zone (MNAZ) is a migmatitic dome located within the Variscan orogen in the southern French Massif Central. The timing of the main thermal event that was responsible for intense partial melting is still highly debated. In this study we present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) age data on micaschists, migmatites and granites that clarify the P–T–t-D evolution of the MNAZ. Structurally controlled samples were collected in order to constrain the timing of metamorphism, migmatization and plutonism regarding the main structural pattern D1, D2 and D3. D1 and D2 correspond to nappe stacking and dextral transpression, respectively. D3 is related to vertical shortening and coaxial thinning with a preferential NE–SW- to E–W-directed stretching. LA-ICP-MS analyses on the syntectonic Anglès, Soulié and Martys granites yielded U–Th/Pb monazite ages of 305 ± 1.5, 306 ± 1.9 and 314 ± 2 Ma, respectively. Five migmatitic rocks sampled in the eastern and central Espinouse area yielded in situ ages ranging between 312 ± 2 and 301 ± 2 Ma. Along the dome envelope, two garnet–staurolite-bearing micaschists near Saint-Pons-de-Thomières village gave in situ U–Th–Pb ages of 312.1 ± 2.1 and 309.0 ± 3.1 Ma. A fine-grained gneiss with a D3 fabrics in the eastern dome envelope yield a 208Pb/232Th mean age at 305.7 ± 3.9 Ma. All ages obtained in this study for the micaschists, migmatites and granites range between 315 and 301 Ma. We interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust. The onset of partial melting occurred at ca. 315 Ma that marked the beginning of transpressional deformation D2. Based on structural and petrological studies, our new U–Th–Pb results suggest that (1) partial melting may have started at ca. 315 Ma and lasted 15–10 Myr and (2) D2 et D3 developed between 315 and 300 Ma and were synchronous. D1 deformation ended at 315 Ma. The onset and duration of D1 related to nappe stacking and crustal thickening is still uncertain

Complete Alpine reworking of the northern Menderes Massif, western Turkey

This study focuses on the petrology, geochronology and thermochronology of metamorphic rocks within the northern Menderes Massif in western Turkey. Metasediments belonging to the cover series of the Massif record pervasive amphibolite-facies metamorphism culminating at ca. 625–670 °C and 7–9 kbars. U–Th–Pb in situ ages on monazite and allanite from these metapelites record crustal thickening and nappe stacking associated with the internal imbrication of the Anatolide–Taurides platform during the Eocene. In addition, new 39Ar/40Ar single muscovite grain analyses on deformed rocks were performed in three localities within the northern Menderes Massif and ages range from 19.8 to 25.5 Ma. These mylonites may be related to both well-known detachments, Simav to the north and Alaşehir to the south, which accommodate Oligo–Miocene exhumation of the Menderes core complex. U–Th–Pb data on monazite grains (22.2 ± 0.2 Ma) from migmatites emplaced within the Simav detachment confirm these ages

Ternary feldspar thermometry of Paleoproterozoic granulites from In-Ouzzal terrane (Western Hoggar, southern Algeria)

International audienceThe In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9–12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT).In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin.The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism

Relationships between the occurrence of accessory Ge-minerals and sphalerite in Variscan Pb-Zn deposits of the Bossost anticlinorium, French Pyrenean Axial Zone: Chemistry, microstructures and ore-deposit setting

The presence of unique accessory Ge-minerals (containing up to 70 wt% Ge) is a widespread phenomenon in Pb-Zn deposits of the Variscan Pyrenean Axial Zone (PAZ). Such a mode of occurrence is, however, rare worldwide with germanium more typically occurring as a trace component of sulfides, notably sphalerite (<3200 ppm), or in coal deposits (<5570 ppm). The PAZ Pb-Zn deposits are thus an excellent target to unravel the processes and key factors controlling formation of mineralization highly concentrated in Ge. Three Pb-Zn deposits are studied here: Argut-dessus; Pale Bidau; and Pale de Rase. All three are located in Late Ordovician rocks within the PAZ Bossost anticlinorium. The main mineralization (Type 2) is localized in epigenetic veins with different geometries relative to host rocks and deformation. Textural analysis of sphalerite mineralization shows evidence for both recrystallization and deformation. Sphalerite is associated with muscovite or graphite and is the unique host for the discrete Ge-minerals in the studied samples. Brunogeierite (GeFe2O4) and argutite (GeO2) occur almost universally at sphalerite grain boundaries. Laser ablation inductively coupled plasma-mass spectrometry analysis of sphalerite and brunogeierite show that sphalerite is relatively depleted in Ge (mean ∼13 ± 3 ppm) and other trace elements such as Sb (∼21 ± 2 ppm), Cu (∼153 ± 23 ppm) and Ga (∼127 ± 10 ppm) compared to brunogeierite (29–30 wt% Ge), which is also richer in Ga (∼2406 ± 270 ppm) and Cu (∼2438 ± 550 ppm). Collectively, these observations on this exceptional example of Ge enrichment highlight the role of oxidizing, moderate temperature (∼<450 °C) metamorphic fluids on the partitioning of Ge between discrete Ge-minerals and their sulfide hosts. Comparison with other Ge-deposits worldwide supports the hypothesis that low grade metamorphism and deformation may play a key role in the genesis and concentration of Ge-mineralization, especially in low-grade metamorphic Kipushi-type deposits in which discrete Ge-minerals are located immediately adjacent to recrystallized sulfides

Structural Control on the Formation of Pb-Zn Deposits: An Example from the Pyrenean Axial Zone

International audiencePb-Zn deposits and specifically Sedimentary-Exhalative (SEDEX) deposits are frequently found in deformed and/or metamorphosed geological terranes. Ore bodies structure is generally difficult to observe and its relationships to the regional structural framework is often lacking. In the Pyrenean Axial Zone (PAZ), the main Pb-Zn mineralizations are commonly considered as Ordovician SEDEX deposits in the literature. New structural field analyzes focusing on the relations between mineralization and regional structures allowed us to classify these Pb-Zn mineralizations into three types: (I) Type 1 corresponds to minor disseminated mineralization, probably syngenetic and from an exhalative source. (II) Type 2a is a stratabound mineralization, epigenetic and synchronous to the Variscan D1 regional deformation event and (III) Type 2b is a vein mineralization, epigenetic and synchronous to the late Variscan D2 regional deformation event. Structural control appears to be a key parameter in concentrating Pb-Zn in the PAZ, as mineralizations occur associated to fold hinges, cleavage, and/or faults. Here we show that the main exploited type 2a and type 2b Pb-Zn mineralizations are intimately controlled by Variscan tectonics. This study demonstrates the predominant role of structural study for unraveling the formation of Pb-Zn deposits especially in deformed/metamorphosed terranes

Rare metal concentration associated to recrystallization: example from the Pyrenean Axial Zone

International audienceGermanium is often found in trace concentration in non-deformed sphalerite (ZnS). However, locally in sphalerite from the Variscan Pyrenean Axial Zone, the presence of Ge-minerals (oxides, sulphides or chloritoids with up to 70% wt Ge) is remarkable. The rise question is thus what are the chemical and/or mechanical processes that control rare element contents of sulphides which have undergone deformation and metamorphism? In order to answer this question, we document microstructural and chemical heterogeneities in sphalerite, performed with EBSD (electron backscatter diffraction) coupled to LA-ICPMS in situ analysis. Deformation could induce recrystallization of primary sphalerite. Recrystallized domains contain low Ge contents (1-50 ppm Ge) whereas porphyroclastic sphalerite grains present higher Ge concentrations (up to 550 ppm Ge). Geminerals (up to 70 wt% Ge) are exclusively hosted in the Ge-poor recrystallized domains. We propose that Germanium was removed from the primary sphalerite crystal lattice during sulphide recrystallization and subsequently concentrated in Ge-minerals, leaving behind a Ge-depleted fine-grained recrystallized sphalerite matrix. Numerous sulphide ores enriched in rare elements similar to Pyrenean deposits may present recrystallization features like deformed Mississippi-Valley-Type, Kipushi-type or Volcanic-Hosted-Massive-Sulphide deposits. Consequently, the potential of this type of deposits shall be evaluated and we suggest that exploration methods must integrate precise textural and microstructural information coupled with trace element in situ and mapping techniques

Rare metal concentration associated to recrystallization: example from the Pyrenean Axial Zone

International audienceGermanium is often found in trace concentration in non-deformed sphalerite (ZnS). However, locally in sphalerite from the Variscan Pyrenean Axial Zone, the presence of Ge-minerals (oxides, sulphides or chloritoids with up to 70% wt Ge) is remarkable. The rise question is thus what are the chemical and/or mechanical processes that control rare element contents of sulphides which have undergone deformation and metamorphism? In order to answer this question, we document microstructural and chemical heterogeneities in sphalerite, performed with EBSD (electron backscatter diffraction) coupled to LA-ICPMS in situ analysis. Deformation could induce recrystallization of primary sphalerite. Recrystallized domains contain low Ge contents (1-50 ppm Ge) whereas porphyroclastic sphalerite grains present higher Ge concentrations (up to 550 ppm Ge). Geminerals (up to 70 wt% Ge) are exclusively hosted in the Ge-poor recrystallized domains. We propose that Germanium was removed from the primary sphalerite crystal lattice during sulphide recrystallization and subsequently concentrated in Ge-minerals, leaving behind a Ge-depleted fine-grained recrystallized sphalerite matrix. Numerous sulphide ores enriched in rare elements similar to Pyrenean deposits may present recrystallization features like deformed Mississippi-Valley-Type, Kipushi-type or Volcanic-Hosted-Massive-Sulphide deposits. Consequently, the potential of this type of deposits shall be evaluated and we suggest that exploration methods must integrate precise textural and microstructural information coupled with trace element in situ and mapping techniques

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

International audienceIn Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension