TECTONICS, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE LADAKH BATHOLITH, TRANS - HIMALAYA

Abstract

The Himalaya is a unique example of young continent to continent collision. The geodynamics of the Himalaya has been described in terms of its large-scale continental convergence along the suture zones. During Mesozoic the Tethyan oceanic crust, attached to the Indian Plate, has subducted beneath the Eurasian Plate and collided against the latter along the Indus Tsangpo Suture Zone (ITSZ) and Shyok Suture Zone (SSZ) during Cenozoic. This collision has caused intense crustal shortening and deformation either along southerly migrating thrusts viz. The Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT) as well as large scale strike slip faults in Tibet. The area of study incorporates the Trans-Himalayan Batholithic complex lying to the north ofthe Indus Tsangpo Suture Zone (ITSZ) in the Ladakh district of Indian Territory. The batholithic complex has been variously called as the Kohistan Batholith and Ladakh Batholith in the northwest, the Kailas tonalite and Gangdese pluton in Tibet and the Lohit Batholith in Arunachal Pradesh. The Ladakh Batholith occurs as a WNW-ESE trending linear belt of about 600 km long and 30-80 km wide with about 3 km of exposed thickness and forms a part of the calc-alkaline Andean-type magmatic pluton. Within this batholith, three crucial sections have been selected for lateral and temporal variation in geochemistry and geochronology. The sections are (i) Leh-Khardung La section, (ii) Kharu-Chang La section and the (iii) Lyoma-Hanle section; the first one being the westernmost section and the last being the extreme easternmost section within the batholith with the Kharu-Chang La section in the middle. The work undertaken in the present thesisaims at the objective of looking ino (a) the lateral petrological and geochemical variations through the three crucial sections mentioned above. (b) the depth of emplacement of the Ladakh Batholith by using Al content in Hornblende geobarometer. (c) constraining the age of crystallization of the Ladakh Batholith using Rb-Sr dating technique on Thermal Ionization Mass Spectrometer and (d) the cooling and echumation history of the Batholith.by integrating the Rb-Sr biotite ages and the Fission Track apatite and zircon ages. In the present study, systematic work has been carried out for temporal and lateral variations of petrography and geochemistry, along three sections of the Ladakh Batholith, viz. Leh-Khardung La, Kharu-Chang La and Lyoma-Hanle. For this purpose 53 samples have been selected from these three sections(Leh- Khardung La: 30 samples; Kharu-Chang La: 13 samples and Lyoma-Hanle: 10 samples). The petrographic studies (Chapter 3) clearly indicate three main types of rocks-diorite (16 samples), granodiorite (27 samples) and granite (10 samples). Similar picture has emerged from the Quartz-Alkali Feldspar-Plagioclase (QA- P) normative calculation plot of Streicksen (1976). The major oxide plots, in particular the Si02 vs (K20+Na20) plot indicates its sub-alkaline, Si02 vs K20 plot indicates medium to high K- bearing rocks and AFM plot indicates its calc-alkaline character. In the discrimination Rb vs (Y+Nb) and Rb vs (Yb+Ta ) plots of Pearce et al. (1984) the batholith falls in the Volcanic Arc Granite (VAG) field. The normalized spiderplots and REE plots show enrichment of LREE-LILE and depletion of HFSE which are characteristic of subduction-related magmatism (Saunders et al., 1980; Holm, 1985). Typical signature of negative Nb anomaly for subduction zone environment (Wood et al., 1979) is also characterized by the spiderplot of the samples from the Ladakh Batholith. Similar character of REE pattern has been reported by Honneger et al. (1982), Dietrich et al. (1983), and Ahmad et al. (1998) from the study of the Ladakh Batholith from different sections. in Based on the Al-content in hornblende geobarometer, the batholith appears to have been crystallized at depth between 14.27 and 7.3 km over a vertical section of around 2 km with diorite indicating lower crustal depth around 14.30 km and granodiorite around 7.35 km. When the crustal depths are plotted against the elevation, a good correlation between crustal depths with elevation has been obtained indicating that the diorites at lower elevation have crystallized first. Also, if the total depth difference is considered, it comes out be~7 km whereas the elevation difference is ~ 2 km. It can be inferred that this body has undergone either magmatic compression or denudation by tectonic removal within this section. For obtaining the crystallization age as well as biotite cooling ages, Rb-Sr systematics on 21 whole rock and 7 whole rock-biotite pairs from the Ladakh Batholith have been attempted. All 87Rb/86Sr and 87Sr/86Sr values have been determined in newly- established TIMS Laboratory at IIT Roorkee. The 87Rb/86Sr and 87Sr/86Sr ratios show a fair amont of spread and make it very difficult to obtain an isochron. But on close observation four points from the Kharu-Chang La section give an isochron age of 61.59±0.05 Ma with initial 86Sr/87Sr ratio of0.70417±0.000006 and MSWD value of0.92 depicting a good quality fit. Seven biotite fractions separated from these samples analyzed in the same way as the whole rock give a range of ages from 36.71+0.05 to 52.38+0.05 Ma. Fission track dating of the Ladakh Batholith on 30 apatite and 3 zircon samples from three sections show a good correlation of FT-apatite age with elevation except in Lyoma- Hanle section and provide invaluable data on the exhumation rate of the batholith. The ages range from 9.21 Ma to 25.35 Ma. However, FT zircon ages are 41.73Ma, 43.37 Ma and 31.71 Ma from west to east and show younging towards east. FT zircon and apatite ages from 3 widespread sections through the Ladakh Batholith reveal its exhumation history through low temperature geotherms of 220+25°C and 110±10°C around 43.37±3.66 Ma and 25.35+2.57 Ma, as is evident from the oldest FT zircon and apatite ages. Extremely good correlations IV have been established between the elevation and age for FT apatite in two profiles of the Ladakh Batholith and provide a very slow average exhumation rates of 0.11 mm/yr between 25.35 Ma and 9.21 Ma, irrespective of any assumed values for geothermal gradient, annealing temperatures and present-day temperatures. When the Rb-Sr biotite cooling ages Ft zircon and Ft apatite ages are clubbed together with the available published data of different themochronometers complete cooling picture of the Ladakh Batholith comes out. It shows the cooling trend from crystallization temperature ofabout 750°C to 110°C and to present. Comparison of exhumation history of the continental lithosphere of the UHP Tso Morari terrain in the south, which has subducted to a depth of about 100 km around 53 Ma with the part of the Ladakh Batholith reveals that the later has exhumed as a piggy-back sequence along north-dipping ITSZ. Since its emplacement around 60 Ma till 43 Ma exhumation has been calculated at a rate between 0.55mm/yr in the initial stages to 4.42 mm/yr subsequently. The Ladakh Batholith has crossed 220+25°C geotherm at~ 43 Ma, as is indicated by average FT zircon age and 110+10°C at around -25 Ma -the oldest apatite age from the batholith. In comparison to the available FT zircon and apatite ages from any other tectonic units of the Himalaya, the Ladakh Batholith reveals an almost uniform and slow exhumation pattern between -25 to 9 Ma at an average rate of about 0.1 lmm/a. Exhumation paths of the Tso Morari and the Ladakh Batholith are almost identical since - 53 Ma and older when compared to the exhumation of the Higher Himalayan Crystallines further south.