Philippe Goncalves

Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 µm3, U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phase may have formed as a result of fluid activity at 500 Ma, and the phases amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data.

Electron-microprobe age mapping of monazite

Abstract High resolution X-ray maps of Th, U, Pb, and Y in monazite can be used to construct age maps, which reveal the continuous spatial distribution of ages in a single grain of monazite. The age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique, the insights it can provide into monazite geochronology in general, and its limitations compared to electron microprobe (EMP) quantitative dating and other in-situ geochronologic techniques. Age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques, and they are a valuable aid for rigorously locating in-situ analytical points. Age mapping also can be used to better understand the behavior of the U-Th-Pb system in monazite during metamorphism, deformation, and fluid-circulation events. Age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale, like a now-healed fracture not visible in back-scattered electron (BSE) images or young domains less than 5 μm in width located inside an older core. In both cases, using age maps as a template for locating in-situ analysis points will minimize the peril of age mixing and erroneous geological interpretations. In addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite, age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization, and thus, even more powerful applications of the monazite geochronometer. The AgeMap program is available in a Windows version and can be downloaded from the internet at the following address: http://www.geo.umass.edu/probe/agemap.

Finite strain pattern in Andriamena unit (north-central Madagascar): evidence for late Neoproterozoic–Cambrian thrusting during continental convergence

This paper deals with the late Neoproterozoic–Cambrian tectonic evolution of a part of north-central Madagascar, which is characterized by the occurrence of a mafic-ultramafic sequence (the Andriamena unit) overlying a gneissic-granitic basement. The finite strain pattern has been determined by carrying out a SPOT satellite image analysis, structural mapping of specific areas and kinematic analyses of shear zones. Structural investigations reveal the presence of two superposed finite strain patterns, D 1 and D2. The D1 event is related to the emplacement of the Andriamena unit on the top of the gneissic-granitic basement. The western contact between these units is a major mylonitic zone characterized by a non-coaxial strain regime consistent with a top-to-east displacement. We suggest that the Andriamena unit originated as a lower crustal fragment of a middle Neoproterozoic continental magmatic arc related to the closure of the Mozambique Ocean. This fragment was thrusted onto the gneissic-granitic basement after 630 Ma, i.e. the age of emplacement of characteristic stratoid granites found only in the lower unit. The D2 event is related to east-west horizontal shortening mainly accommodated by F2 upright folds. In-situ electron microprobe dating of monazites from the Andriamena unit constrains the age of the D1 and D2 events between 530 and 500 Ma under amphibolite to granulite-facies conditions (5–7 kbar, 650–700 ◦ C). The eastward thrust emplacement of the Andriamena unit (D1) followed by the horizontal shortening (D2) are ascribed to the same Cambrian tectonic regime (i.e. east-west convergence). Such D1–D2 bulk strain pattern has been recognized throughout Madagascar and at various structural levels of the crust: in the lower crust in Southern Madagascar and in the uppermost crustal level in the SQC unit (central Madagascar). The D1–D2 event is interpreted to result from the continental convergence of the Australia–Antarctica block and the Madagascar, India, Sri Lanka block during the final amalgamation of Gondwana.

Thrusting and sinistral wrenching in a pre-Eocene HP-LT Caribbean accretionary wedge (Samaná Peninsula, Dominican Republic)

AbstractThe North Caribbean margin is an example of an oblique convergence zone where the currently exposed HP-LT rocks are systematically localised close to strike-slip faults. The petrological and structural study of eclogite and blueschist facies rocks of the peninsula of Sarnana (Hispaniola, Dominican Republic) confirms the presence of two different metamorphic units. The former diplays low metamorphic grade (Santa Barbara unit), characterized by the assemblage albite - lawsonite (7.5 ± 2 kbar and 320 ± 80 °C). The latter (Punta Balandra unit), thrust over the first unit towards the NW, and is characterized by the occurrence of blueschist and eclogite facies assemblages (13 ± 2 kbar and 450 ± 70 °C), within oceanic metasediments. The isothermal retrograde evolution occurred in epidote-blueschist facies conditions (9 ± 2 kbar and 440 ± 60 °C). The late greenschist facies evolution is contemporaneous with conjugate NW-SE extension and E-W strike-slip faulting. This late extension is for regional dome an...

Thermodynamic Modeling and Thermobarometry of Metasomatized Rocks

Determining the P-T conditions at which metasomatism occurs provides insight into the physical conditions at which fluid-rock interaction occurs in the crust. However, application of thermodynamic modeling to metasomatized rocks is not without pitfalls. As with “normal” metamorphic rocks, the main difficulty is to select mineral compositions that were in equilibrium during their crystallization. This essential task is particularly difficult in metasomatized rocks because it is often difficult to distinguish textures produced by changes in P-T conditions from those caused by fluid-rock interactions and associated changes in bulk composition. Furthermore, the selection of minerals in equilibrium in metasomatized rocks is made difficult by the great variability of scale of mass transfer (see Chaps. 4 and 5), and therefore equilibrium, which varies from micrometer- to hand-sample or larger scale, depending on the amount of fluid involved and the fluid transport mechanisms (e.g. pervasive or focused). Finally, another major limitation that is discussed in detail in Chap. 5, is that fluid composition coming in or out of the rock is unknown. Since fluid is a major phase component of the system, neglecting its impact on the phase relations might be problematic for thermobarometry. Despite these pitfalls, we describe in this contribution examples where thermobarometry has been apparently successfully applied. We emphasize that pseudosection thermobarometry is particularly suitable for metasomatized rocks because the effects of mass transfer can be explored through P-T-X phase diagrams. Application of thermodynamic modeling to metasomatized rocks requires (1) detailed mineralogical and textural investigation to select appropriate mineral compositions, (2) essential geochemical analyses to define the relative and absolute mass changes involved during the metasomatic event(s), and (3) forward modeling of the effects of mass transfer on phase relations.

Strain partitioning along the anatectic front in the Variscan Montagne Noire massif (southern French Massif Central)

We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.

Le pointement de péridotite à grenat-spinelle de La Croix-Valmer (Maures centrales): un cumulat d'affinité océanique impliqué dans la subduction éohercynienne ?

Abstract A garnet-spinel metaperidotite body has been found in the leptyno-amphibolitic complex of the central pan of the Maures Massif (Hercynian Belt, south eastern France). Based on bulk rock and mineral chemistry, a mantle tectonite origin can be ruled out for this rock. Instead, it appears to be an igneous cumulate that crystallized at low P, in the olivine-plagioclase stability field, and was later brought down to mantle depths (P = 16–18 kbar, T = 850–860 °C). Although Sm-Nd isotopes preclude a strictly comagmatic origin, the clear oceanic affinity of neighbouring metagabbros (e Nd 500 + 7.7 and + 8.7) suggests a very evolved extensional tectonic setting, having led to the break-up of the continental lithosphere and the formation of a rifted margin. The high pressure metamorphic overprint recorded by the peridotite is interpreted as reflecting the subduction of a passive margin, at the final stage of consumption of an oceanic domain. This event probably occurred in Early Hercynian times, as documented elsewhere in the west european Hercynides.