6.5 Geochronology

Several leucogranitic samples have been selected for monazite U-Pb and muscovite Ar/Ar geochronology. The location of these samples is shown in figure 6.9. The monazite samples where collected and separated by J.C Vannay. Analyses were made by F. Bussy. The muscovite samples were collected, separated and analysed by the author.

The U-Pb analyses were made at the Royal Ontario Museum, Canada. Monazites have been isolated using conventional heavy liquid and magnetic separation techniques. Analysed crystals were selected on the basis of their high transparency, lack of inclusions and cracks, and euhedral shape. A slight air-abrasion was applied to some fractions to reduce possible surface-correlated lead-loss (Krogh, 1982). Monazite crystals were dissolved overnight on a hot plate using 6M HCL in Savillex^, capsules. Chemistry was carried out using the standard techniques outlined in Krogh (1973), using a mixed ²⁰⁵Pb/²³⁵U spike, except that the capsule and column size were reduced by a factor of 10. Measurement and correction procedures follow the approach described in Bussy and Cadoppi (1996). Errors are quoted at the 95% confidence level.

The ⁴⁰Ar/³⁹Ar analyses were made at the Université de Lausanne following the analytical procedure given in Cosca et al. (1992). Samples, together with standards, were irradiated in the TRIGA reactor in Denver, USA. Gas was incrementally extracted using a low blank, double vacuum resistance furnace and analysed on a MAP 215-50 mass spectrometer. Analyses were corrected for blank, mass discrimination, isotopic decay, and interfering Ca-, K- and Cl-derived isotopes. For mass 40, blank values ranged from 4 x 10^-15 moles below 1350°C to 9 x 10^-15 moles at 1650°C. Blank values for masses 36-39 were below 2 x 10^-17 moles for all temperatures. Isotopic production ratios for the TRIGA reactor were determined from analyses of irradiated CaF and K₂SO₄. Correction for the neutron flux was determined using the international standards MMHB-1 and HD-BI.

The monazites from two samples of a leucogranite pluton from the Gumburanjun Intrusion Complex have been analyzed for U-Pb geochronology (Table. 6.2 and fig. 6.13). The geochemical analyses of these two samples V1(115) and V2(107) is given in table 6.1 and they have been differentiated from the three other chemical analyses in the various composition diagrams. One analysis (V1) was performed on the first sample and six (V2 to V7) on the second one. V1 to V5 yield ²³⁵U/²⁰⁷Pb ages ranging between 22.0 ± 0.2 and 22.5 ± 1 Ma. Except for the concordant analysis V2, these analyses plot slightly above Concordia indicating small excess ²⁰⁶Pb, as has been inferred for some young Himalayan monazites (Schärer, 1984). XRF data indicate whole-rock Th/U ratios close to 1 for the studied leucogranite samples, which yields concordant or near concordant corrected ages using the calculation procedure of Schärer (1984). The discordant analyses V6 and V7 both indicate older ²³⁵U/²⁰⁷Pb ages compared to the other analyses from the same sample (V2 to V5). V7 plots below Concordia and yields a ²⁰⁷Pb/²⁰⁶Pb age of 131 Ma. This feature unequivocally indicates the presence of an inherited component, as observed in some other Himalayan leucogranites (e.g. Harrison et al., 1995; Edwards and Harrison, 1997). Because the multiple-grain analysis V6 may also be affected by inheritance, we consider these two analyses as unreliable. V1 to V5 indicate an average ²³⁵U/²⁰⁷Pb age of 22.2 ± 0.2 Ma, which we consider as the best estimate for the age of cooling of the leucogranite through the closure temperature of monazite (725±25°C). This result is comparable to U-Pb data from central Zanskar, 150 km to the NW of the studied area, where crustal anatexis and leucogranite intrusion in the foot wall of the ZSZ occurred between 22 and 19 Ma (Noble and Searle, 1995).

An upper intercept age of 499±235 Ma. was obtained from analyses V7. Similar upper intercept ages of 463±13 and 476±12 Ma were obtained by Noble and Searle (1995) on monazites and zircons from leucogranites of north-west Zanskar (upper Suru valley). These ages compare well with the ages of the Cambro-Ordovician orthogneisses in the HHCS of Zanskar. Indeed, a 495±16 Ma age was calculated for the Kade orthogneiss (Frank et al. 1975) and other orthogneisses in Zanskar yield Zircon ages of 472 Ma. (Pognante et al., 1990). These ~ 500Ma orthogneisses belong to the late Pan-African event.

A total of ten muscovite ⁴⁰Ar/³⁹Ar cooling ages (Table 6.3, fig. 6.15 ) were obtained from both deformed dikes (Z1 and Z2) and undeformed dikes (Z7 to Z10) intruding the base of the ZSZ, as well as from the leucogranitic plutons (Z3 to Z6). The deformed dikes yield cooling ages around 22 - 21 Ma, whereas the undeformed dikes consistently indicate younger ages around 19.8 - 19.3 Ma. The plutons yield muscovite cooling ages between 20.4 - 19.5 Ma.

In figure 6.14, we have plotted the five relevant monazite U-Pb ages and the four muscovite ⁴⁰Ar/³⁹Ar ages from the Gumburanjun leucogranite, as well as biotite and muscovite Rb/Sr cooling ages from the same granite (Ferrara et al., 1991), against the estimated blocking temperatures of these minerals. The muscovite Z3 and the monazites analyses V2 to V5 come from the same sample, which allows us to calculate a cooling rate of 165 °C / m.y. between 725 °C and 420 °C. A cooling rate of 95 °C / m.y. between 510 °C and 330 °C can be calculated in the same way with one of Ferrara's samples. The arrow in figure 6.14 outlines the overall cooling trend of the Intrusion Complex leucogranitic plutons.