Uplift History of Syenite Rocks of the Sushina Hill, Tamar Porapahar Shear Zone (TPSZ), Purulia: Constraints from Fission-track Ages of Two Cogenetic Minerals

Fission-track ages of cogenetic minerals namely apatite


Experimental Procedure
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% HNO 3 at room temperature for 30 s. Zircons were mounted in PFA Teflon. Zircons were etched in KOH-NaOH eutectic etchant [36][37][38][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][41][42][43]. The fission tracks were counted under a total magnification of 1000X. The calibrated area of one grid is 0.64×10 -6 cm 2 . 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][45][46][47][48].

Interpretation and Results
Fission track age determinations were made on 15apatite, 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.
The uplift rate has been calculated according to the equation:

Uplift rate=
Cooling rate Geothermal gradient (1) Where, ( ) ( ) 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]: 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.

Conclusion
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.
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][52][53][54][55]. The P (X 2 ) test was performed to measure the uranium variation in the samples. A value of P(X 2 ) larger than 5% means that the grains are assumed to be a single age. Sample SAP failed the X 2 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).    I thank the entire team of FRMII, Garching, Germany for providing me with use of the irradiation facility, free of charge.