Christian Nicollet

Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India

Monazites of five samples (one leptynitic garnet–biotite gneiss, one khondalite, one augen gneiss and two pegmatites) from the central and northern part (Ponmudi Unit) of the Kerala Khondalite Belt (KKB) in southern India were analyzed with the electron microprobe dating technique. Monazites in the augen gneiss and the pegmatites yield grain sizes between 200–800 μm, Th abundances are rather low (<10 wt.%) and the distribution of Th, U and Pb within single grains is fairly homogeneous. Contrasting to this, monazites in the leptynitic gneiss and the khondalite are small (<150 μm). They often display very complex Th–U–Pb patterns and contain high Th concentrations up to 20 wt.%. The statistical treatment of individual ages from the investigated samples revealed three populations of Lower Proterozoic (∼1.9 Ga), Upper Proterozoic (∼580 Ma) and Ordovician age (∼470 Ma) as well as Mid Proterozoic ages between 0.8–1.7 Ga which are not regarded to be of geological significance. Lower Proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites. They fairly agree with Sm–Nd model ages for similar rocks of the KKB and give a minimum age for first monazite growth or complete homogenization. The prominent Pan-African population with mean values between 540 and 580 Ma is present in the leptynitic gneiss, the khondalite and the augen gneiss and in line with published isotope ages for the KKB. The Ordovician population finally marks the emplacement of granitic pegmatites subsequent to the Pan-African high-grade metamorphic event. There is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of Lower Proterozoic ages. While these are abundant in the former, often rimmed by Upper Proterozoic ages, they are completely absent in the latter. It appears unlikely that Lower Proterozoic ages were completely reset during a Pan-African event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites. It is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites. Thus, the Upper Proterozoic age of 605±37 Ma calculated for the investigated sample approximates the time of magma emplacement, which slightly precedes the peak stage of Pan-African high-grade metamorphism in the KKB, and of monazite crystallization from the granitic melt. A characteristic feature of the investigated monazites is the resetting of Lower Proterozoic and Pan-African ages to significantly younger values due to partial Pb loss. Monazites not shielded by other minerals (e.g., garnet) suffered selective mobilization of Pb along fractures or at their rims while Th and U concentrations remained almost unchanged. The results presented in this study indicate that this was mainly due to fluid-rock interaction. It is concluded that thermal diffusion of Pb even at the suggested temperatures of 900°C only had minor influence on the Th–U–Pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed (∼750°C).

Strain pattern and late Precambrian deformation history in southern Madagascar

This paper examines the thermo-mechanical evolution of the lower crust, in Precambrian times, with an example from southern Madagascar. The finite strain pattern is derived from the study of satellite images complemented by field structural analysis. The finite geometry reflects the superposition of two distinct finite strain patterns, D1 and D2. The geodynamic significance of the D1 event remains unclear. However, the D2 finite strain pattern is partitioned between a network of kilometric vertical shear zones and folded domains, and is consistent in the Mozambique belt with east‐west shortening in a transpressive regime.The metamorphic conditions associated with the two finite strain fields show that rather uniform, high temperatures were ubiquitous in southern Madagascar. By contrast, regional pressure differences were controlled primarily by the major shear zones (D2). Chronological constraints obtained from monazite (U‐Th‐Pb electron microprobe dating) indicate ages of 590‐530 Ma for the D1 event and 530‐500 Ma for the D2 event. The two distinct strain fields D1 and D2 are continuous in time.The late pan-African transpressional tectonic regimes, contemporaneous with granulitic facies metamorphic conditions, resulted in strong strain partitioning between shear zones and folded domains. The D2 transpression was efficient and pervasive enough to exhume pieces of the lower crust. The amount of exhumation was controlled by the D2 strain gradient (amount of pure shear with respect to simple shear).

Lithospheric tectonic structures developed under high-grade metamorphism in the Southern part of Madagascar

AbstractThis paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear ...

Granulite microfabrics and deformation mechanisms in southern Madagascar

Optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite- to high-amphibolite facies quartzo-feldspathic rocks in southern Madagascar. The microstructures of coarse-grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt. The extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep, in conjunction with stress-controlled intracrystalline diffusion. In the studied rocks, the considerable volume of quartz is not interconnected even at high strain. The lack of its interconnectivity in coarse-grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load-bearing framework structure at high-temperatures. In dynamically recrystallized granulite, the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity. We predict an important decrease in strength of quartzo-feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz.

Fluid flow vs. scale of shear zones in the lower continental crust and the granulite paradox

The external or internal origin of CO2 and H2O in marbles and metabasites from a large-scale granulite section is related to their structural setting: outside shear zones, within major shear zones (≈ 25 km wide by >340 km long), or within minor shear zones (<10 km wide by <150 km long). Outside shear zones, marbles have isotopic compositions similar to their protholiths values and metabasites record mantle δ13C signatures. Marbles from major shear zones show little variation of δ18O but are depleted in 13C down to −3‰ due to exchange with mantle-derived carbon. In addition to an earlier input of mantle carbon related to mafic magmatism, major shear zones act as conduits for mantle carbon influx. They are probably mantle rooted. Oxygen was buffered by the crust. Marbles and their related skarns from minor shear zones were subjected to devolatilization reactions and water-rich infiltration from crustal sources with no evidence for a mantle carbon contribution. Large isotopic variations at the metre scale in the shear zones reflect the heterogeneous distribution of fluid flow associated with heterogeneous deformation and contrasting petrologic features and permeabilities. These geochemical-structural relationships reconcile the conflicting arguments on the origin of fluids during granulite genesis.

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.

Extension syn-convergence, poinçonnement vertical et unités métamorphiques contrastées en bordure ouest du Grand Paradis (Alpes Franco-Italiennes)

Résumé À la bordure occidentale du massif du Grand Paradis (Alpes occidentales), la zone Piémontaise est constituée des unités océaniques des schistes lustrés et de l’unité continentale du Grand Paradis. Une étude métamorphique nous permet de préciser et de discuter des discontinuités de pression et des conditions de rétro-morphose de trois unités piémontaises déjà distinguées antérieurement. L’unité supérieure des schistes lustrés (LS) métamorphisée dans le faciès des schistes bleus (9.5 ± 2 kbar, 340 ± 30 °C). L’unité inférieure des schistes lustrés (LI) métamorphisée dans le faciès des éclogites (12.5±3 kbar, 480±50 °C). L’unité continentale du Grand Paradis (GP) métamorphisée dans le faciès des éclogites de plus haute pression (12 à 20 kbar, 500 ± 50 °C). L’ensemble des unités enregistre une décompression dans les conditions du faciès amphibolite à épidote, et donc avec une légère augmentation tardive de température, pour les unités LI et GP, et dans celles des schistes verts pour l’unité LS. L’étude des inclusions fluides dans les fentes de tension et dans les plans de cisaillement, en parallèle à l’élude des paragenèses rétromorphiques permet de montrer que c’est tardivement (4 ± 1 kbar, 400 ± 50 °C) que l’histoire tectonomélamor-phique devient commune à l’ensemble des unités. L’étude du champ de la déformation finie met en évidence une tectonique en extension qui débute en conditions ductiles dans le faciès amphibolite à épidote pour LI, et schiste vert pour LS, et se poursuit en conditions fragiles. La déformation ductile se traduit par des trajectoires en dômes et bassins de la foliation, avec une partition entre des domaines en aplatissement au cœur des dômes et des domaines en cisaillement simple en bordure des dômes, au contact entre les différentes unités. La déformation fragile correspond au continuum plus tardif de la déformation extensive ductile. Cette tectonique extensive n’est qu’en partie responsable des sautes de pression entre les trois unités étudiés. Elle correspond à l’accommodation, en surface, au poinçonnement vertical des unités de haute pression, en contexte de convergence, par l’écaillage progressif de la croûte européenne à l’avant du butoir mantellique apulien.

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...

Nano-petrographic investigation of a mafic xenolith (maar de Beaunit, Massif Central, France)

The thermal history of a mafic xenolith from the Beaunit maar (Massif Central, France) is reconstructedatthe basisofa transmissionelectronmicroscopystudy. The protolith isa meta-microgabbro(opx- 1,cpx-1, pl-1)sampledinthelowercontinentalcrust(T=870-970°C,P 0.7-0.8 GPa).The incorporation inthe basalticmagmaproducedfivereactionsaround orthopyroxene: opx-1 ficpx-2fi cpx-3(augite+highpigeonite) fi liq fi cpx-4. The final reaction is the transformation of residual cpx-3 (augite + high pigeonite) into cpx-5 (augite+lowpigeonite).The calculationofthetimerequiredforeachtransformationyieldsaminimumresidence time of the enclave in the host magma of 16 hours and a magma ascent velocity of 1.8 km.h -1 . Exsolutions are produced by pressure decreaseas the xenolith is brought up to the surfacein the host basalt. Fracturesobserved in primary minerals are interpreted as a consequence of xenolith shocks against the wall of the magma conduit.

Metamorphism and Tectonics in Southern Madagascar: An Overview

Abstract The scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern Madagascar. After a presentation of the main geophysical features of the Malagasy crust, the brittle and the ductile strain patterns are established at a crustal scale. The Pan-African metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime.