It is widely accepted that radiometric ages determined on metamorphic minerals from orogenic belts reflect their cooling history rather than their primary crystallization [1-5]. Cooling histories obtained using different radiometric techniques on cogenetic minerals from a single sample often include zircon and apatite fission track ages as low-temperature bounds to a temperature interval of several hundred degrees [6-15].

Fission-track ages were determined on cogenetic minerals from syenite rocks of the Sushina hill, TPSZ. Sushina hill in TPSZ lies within Singhbhum Group (SG) of rocks. The North Singhbhum Mobile Belt (NSMB) in its northern margin has a tectonic boundary with the Chhotanagpur Gneissic Complex (CGC) along the TPSZ [16-22]. Singhbhum Shear Zone (SSZ) is located 40 km south of the TPSZ. At places, TPSZ passes either through rocks of SG or CGC. Singhbhum orogenic cycle experienced three major phases of deformation. CGC underwent four phases of deformation [23-28]. TPSZ witnessed reactivation near 500 Ma due to overthrusting and suffered rapid exhumation near 600 Ma [29-34]. TPSZ was thus affected by the intense deformation in the area adjoining this shear zone. With this background of geological events, it was thought that fission track dating on cogenetic minerals might help unravel the uplift history of syenite rocks of the Sushina hill.

The samples for this study were processed in the laboratory of the Geological Survey of India, Kolkata, after obtaining permission from the Director General, GSI, Kolkata, West Bengal. The samples were prepared using standard separation, grinding and polishing techniques [35]. All the samples were prepared for the external detector method. AFT mounts were etched with 70% HNO₃ at room temperature for 30 s. Zircons were mounted in PFA Teflon. Zircons were etched in KOH-NaOH eutectic etchant [36-39] at 215°C on Spinot digital hot plate for ∼8 hrs. The sample was placed in 48% HF for 2 hrs to clean up grains. After etching, mica sheets were firmly attached on the sample mounts. The samples were irradiated in the thermal facilities of FRMII at Garching, Germany together with dosimeter glass IRMM-540R (15 ppm). Mica sheets were etched using 48% HF at room temperature for 19 min [40-43]. The fission tracks were counted under a total magnification of 1000X. The calibrated area of one grid is 0.64×10^-6 cm². Durango apatites were used as the age standard mineral, which was provided by Prof. Barry Paul Kohn, University of Melbourne, Australia. FT age of zircon was determined using equation without zeta value. Zeta calibration was not performed on zircon because of the unavailability of age standard minerals [44-48].

Fission track age determinations were made on 15 apatite, and 18 zircon separates from syenite rocks from the Sushina hill shown in Figure 1. The mean fission track ages of apatite and zircon are 535.25 Ma, and 970 Ma respectively as shown in Table 1.

Figure 1: Location map of the Sushina hill.

Table 1: Results of AFT analyses : ages calculated using dosimeter glass IRMM-540R with 15 ppm U, zeta = 250, irradiated at FRMII, calibrated by traditional zeta approach and external detector method for apatite sample SAP, N=Number of grains, ρ – track densities given in 10⁶ tr cm^-2, ρ_d – dosimeter track density, N_d – number of tracks counted on dosimeter, ρ_s(ρ_i) – spontaneous (induced) track densities, N_s(N_i) – number of counted spontaneous (induced) tracks, P(² ?) – probability for obtaining ²? value for n degrees of freedom, where n=no. of grain – 1, Neutron flux(Φ) for zircon sample SZR = 1.75×10^15 neutron/cm².

The uplift rate has been calculated according to the equation:

(1)

Where, Cooling rate= ( T₁-T₂) ÷ (A₁-A₂)

T₁, T₂ = Closure temperatures of cogenetic minerals

A₂, A₂ = Mean FT ages of cogenetic minerals

Average geothermal gradient of the order of 30°C/km has been adopted. Closure temperatures for apatite and zircon have been adopted 110°C and 240°C respectively.

FT age of apatite sample namely SAP has been calculated according to the equation [49]:

(2)

Where, is the surface density of etched spontaneous fission racks, ρ_i is the surface density of etched induced fission tracks, and G is the integrated geometry factor of etched surface. λ_d =1.55×10⁻¹⁰ yr⁻¹ = total decay constant of ²³⁸U, Z = calibration factor based on EDM of fission-track age standards.ρ_d = induced fission-track density for a uranium standard corresponding to the sample position during neutron irradiation.

FT age of zircon sample namely SZR has been calculated according to the equation without zeta value [50]:

(3)

Where, Φ = Neutron flux

The large age errors, e.g. 14.52% are found in sample SAP. As already known, low Uranium samples present a problem because of low induced track densities [51-55]. The P (X²) test was performed to measure the uranium variation in the samples. A value of P(X²) larger than 5% means that the grains are assumed to be a single age. Sample SAP failed the X² test, which may indicate bimodal distributions for the sample.

By applying apatite fission track analysis, the possible uplift rate of the Sushina hill was attempted to be revealed, which could be reflected by an offset age of two cogenetic minerals (Table 2).

Table 2: Cooling and uplift rate of the Sushina hill.

The largest age error (14.52%) occurs in sample SAP. This high error is most likely due to a very low uranium concentration (0.68 ppm). As already known, low uranium samples place limits on how robust the ages could be. In low uranium samples, an exact match between the areas counted in the grains and the mica is often hard to achieve. An adjustment by eye is difficult and subjective because the outline of the induced tracks on the mica does not reflect the shape of the analyzed grain. In this study, closure temperatures for apatite and zircon have been adopted 110°C and 240°C respectively. In reality, the closure temperature concept cannot be straight forward applied.

For determining zircon FT age, zeta calibration was not performed. It places limit on precise calculation of FT age. Syenite rocks of the Sushina hill in TPSZ uplifted at the rate of 9.97 m/Ma in the range from 535 Ma-970 Ma.

I thank Prof. Barry Paul Kohn, “University of Melbourne”, Australia for his overall support and advice throughout my time as a Ph.D. student. He also sent me age standard minerals (Durango apatite) for zeta calibration, free of charge. Without his contribution, my work would have never been possible.

I thank Prof. Richard Ketcham “University of Texas”, U.S.A., who kindly reviewed my AFT models and provided valuable guidance for the improvement of the models.

I am highly indebted to the Director General of Geological Survey of India, 27, J.L. Nehru Road, Kolkata – 700 016, for his kind permission to perform my work in the laboratory of G.S.I, Kolkata.

I thank Mr. Partha Nag, Officer-in-Charge, WBMTDC, Purulia for his dedicated help with the field work.

I thank Mr. T. Ray Barman, Ex-Scientist, G.S.I, Kolkata, for his constructive advice.

Many thanks are due to the entire family of G.S.I., Kolkata, for their help and encouragement.

I thank the entire team of FRMII, Garching, Germany for providing me with use of the irradiation facility, free of charge.