First paleomagnetic and 40Ar/39Ar study of Paleoproterozoic rocks from the French Guyana (Camopi and Oyapok rivers), northeastern Guyana Shield

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Abstract

In order to understand the Paleoproterozoic geographic evolution of the Guyana Shield, paleomagnetic and 40Ar/39Ar investigations were carried out on granitoids and volcano-sedimentary rocks from the Oyapok and Camopi rivers (French Guyana–Brazil frontier). Scanning electronic microscope, thermomagnetic and isothermomagnetic experiments show that magnetite is the main magnetic remanent carrier in most of the samples. The metavolcano-sedimentary rocks (Paramaca) show a weak magnetization and scattered magnetic directions. Therefore, no reliable magnetic component could be isolated from these samples. Samples taken from tonalite and meta-ultrabasite rocks yield a characteristic magnetic direction, carried by subautomorphous magnetite, that is well defined and distinct from that of the present Earth field and that of nearby Jurassic dikes. A virtual geomagnetic pole (VGP) deduced from this probable primary remanence was calculated, namely pole OYA, λ=28.0°S, φ=346.0°E, N=5, k=31.9 and A95=13.8°. Four 40Ar/39Ar ages, ranging from 2052 to 1973 Ma, were obtained from amphiboles and biotites of tonalite rocks, showing a relatively slow cooling rate of ca 4.8+2.6/−2.1°C Ma−1. The linear extrapolation of this cooling rate to the magnetite unblocking temperature (540 to 580°C) yields a magnetization age of 2036±14 Ma for pole OYA. Pole OYA differs significantly from available paleomagnetic results from Venezuela of the West Guyana Shield dated at 2000±10 Ma. This difference may indicate an important latitudinal movement of the Guyana Shield between 2036 and 2000 Ma with a velocity of 9±7 cm/year.

Introduction

Paleomagnetism, for decades, has provided important constraints on the geodynamic history of the Earth. Most of the available data (77%) were obtained on Phanerozoic rocks (Global Paleomagnetic Database version 3.3; GPMDB, 1998). Only 7% concern the Paleoproterozoic period, which occupies 20% of Earth history. Recent paleomagnetic data concerning the Fennoscandian Shield (Torsvik and Meert, 1995, Fedotova et al., 1999; Mertanen et al., 1999) and North American Shield (Buchan et al., 1996) yielded new paleogeographic constraints between these two zones during this period. Most of these investigations were conducted on intrusive rocks and were associated with U/Pb dating (Buchan et al., 1996, Fedotova et al., 1999, Mertanen et al., 1999). The magnetic remanence age in quickly cooled volcanic rocks and dikes is close to the U/Pb age, but in slowly cooled Precambrian terrains the age of acquisition of thermal remanent magnetization (TRM) cannot always be directly defined by one isotopic method. The slow cooling rate is generally attributed to gradual uplift and erosion. In order to estimate the magnetic remanence age more precisely, the combined study of the paleomagnetism with 40Ar/39Ar thermochronology is important and sometimes critical (Berger, 1979, Costanzo-Alvarez and Dunlop, 1988, Briden et al., 1993).

In the framework of a multidisciplinary BRGM (Bureau de Recherche Géologique et Minière) geological mapping project of the French Guyana Territory in collaboration with Orleans Institut of Geosciences (ISTO), UMR-CNRS 6527 and the CPRM (Brazilian geological survey), we carried out two field trips in 1997 and 1998. This study presents new time-calibrated paleomagnetic data from the northeastern part of the Guyana Shield. The 40Ar/39Ar data allow one to evaluate the cooling rate for the central Oyapok/Camopi zone at about 2 Ga for the first time. Paleogeographic and tectonic implications of paleomagnetic results are then discussed with previous studies from the northwest part of the Guyana Shield (Fig. 1, Onstott and Hargraves, 1981, Onstott et al., 1984, Onstott et al., 1989)

Section snippets

Regional and local geology

The Guyana Shield is composed of a narrow Archean belt (Imataca Complex in Venezuela) and of granite–greenstone belts (2.2 to 2.0 Ga), formed during the Transamazonian tectonothermal event (Fig. 1; Montgomery and Hurley, 1978, Teixieira et al., 1989). Two major strike-slip fault zones cross the Guyana Shield: (1) the Guri fault zone (GFZ) separates the Archean belt from the Paleoproterozoic series; (2) the Pisco Jurua fault (PJF) is located on the western border of the Central Guyana Granulite

Magnetic mineralogical analysis and petrographic study

To characterize magnetic mineral compositions, we applied the following methods on representative samples: reflection microscopy (Olympus BX60) at the geological laboratory of Université d'Orléans; scanning electronic microscopy (SEM, JEOL) at Ecole Supérieur de l'Energie et des Matériaux (ESEM) in Orléans; thermomagnetic experiments using a CS3 apparatus coupled with a KLY-3S kappabridge (AGICO, Geofysica) at the joint BRGM/Université d'Orléans Laboratoire de Magnétisme des Roches (LMR).

Magnetic mineralogy

Petrographic observations in transmitted light show a considerable amount of subautomorphous grains of magnetite (Fig. 3a), with xenomorphous grains of ilmenite, pyrite in tonalite and meta-ultrabasite. However, in Paramaca rocks, elongate ilmenite consists of the principal ferri-oxide and no magnetite grains were observed (Fig. 3b). The saturation field for isothermal remanent magnetization (IRM) is 0.3 T (Fig. 4a) for tonalitic rock. The magnetic saturation intensity for tonalite samples is

Paleomagnetic results

A pilot study was carried out on several specimens using both thermal and alternating magnetic field (AF) demagnetizations with a Pyrox furnace and an automated three-axis tumbler AF demagnetizer (LDA-3, AGICO geofysica), respectively. About 10 (AF, 1 to 100 mT) to 15 (thermal, ∼20 to 595°C) progressive steps were applied to the demagnetization procedure.

40Ar/39Ar results

Analytical data for four dated specimens are given in Table 2. Fig. 7 shows age and 37ArCa/39ArK ratio spectra for minerals from B107 (Fig. 7a and b) and site GN (Fig. 7c and d).

Amphibole single grains (green grains) yield a very flat spectrum for the B107 and GN specimens (Fig. 7a and b), after a sharp decrease in age at low temperature. The corresponding 37ArCa/39ArK ratios remain constant throughout the flat section of spectra (Fig. 7a and b) and indicate that pure amphiboles were analyzed.

Age of the magnetic remanence

The significant difference of 47 Ma in 40Ar/39Ar ages observed in amphiboles and biotites for site GN in the central Oyapok zone shows that the GN tonalite rock cooled down from about 500–550°C to about 250–350°C at about 2020±4 Ma and 1973±4 Ma, respectively, according to the proposed biotite and amphibole K/Ar blocking temperatures (Harrison, 1981, Harrison et al., 1985). It is important to notice that the compositional effect in hornblende K/Ar closure temperature (Dahl, 1996) suggests a

Conclusion

The paleomagnetic and geochronological data from this and previous studies lead to the following conclusions.

(1) Subautomorphous magnetite is the principal magnetic remanence carrier for tonalite and meta-ultrabasite from our sample collection. In the majority of the greenstones (Paramaca formation) the ferromagnetic minerals are rare or absent.

(2) Two magnetic components were obtained, one from the Paramaca rock (P) and one from tonalite, meta-ultrabasite rocks (OYA). Unstable Paramaca

Acknowledgements

This study was supported by the French geological survey (BRGM) French Guyana mapping project and by a French region Centre PhD grant. Dr P. Rossi, Dr C. Delors, Dr D. Lahondere, Dr M.T. Lins Faraco, Dr J.M. Carvalho, Dr O. Monod, and Dr M. Vidal are thanked for their contribution in sampling and discussions. The constructive suggestions proposed by J. Meert and I.G. Pacca are very much appreciated for improving the manuscript. We also thank B. Henry and M. Le Goff for their help in hysteresis

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