LOW TEMPERATURE DATING OF HIGH MOUNTAIN ROCKS:(U-Th)/He AGES FROM HIGHER HIMALAYAN SAMPLES, EASTERN NEPAL

Recently the (U|Th)/He method on apatite has been applied to studies of cooling and exhumation in mountain belts in order to place constraints on the low temperature cooling history (Wolf et al., 1997; House et al., 1998). Because the closure temperature of the method is 65～75℃, it provides useful information regarding the exhumation of rocks through the upper few kilometers of the earth's crust. When the (U|Th)/He method is coupled with methods such as 40 Ar/ 39 Ar with higher closure temperatures (210～500℃), a cooling history can be reconstructed over a wide range of temperatures. In eastern Nepal, in the Dudh Kosi valley a suite of samples from the Main Central Thrust (MCT) to the middle of the Higher Himalayan section was previously dated using 40 Ar/ 39 Ar on K|feldspar, biotite, muscovite, and hornblende (Hubbard and Harrison,1989). We have applied the (U|Th)/He method to apatite from these same samples. The structurally highest site, Ngozumba, is in the middle to upper part of the Higher Himalaya in the Gokyo valley. At Ngozumba previous 40 Ar/ 39 Ar ages include: muscovite at (16 6±0 4)Ma (closure temperature (CT)350℃), biotite at (16 8±1 4)Ma (CT 300℃), and K|feldspar at (15 5±1 8)Ma (CT 250℃).The apatite age for an augen gneiss from this location was (6 3±3 8)Ma suggesting slower cooling for that part of the range during the last 15Ma. Structurally below Ngozumba, in the MCT zone, 40 Ar/ 39 Ar ages include:(20 9±0 2)Ma for hornblende (CT 500℃), (12 0±0 2)Ma for muscovite, and (8 0±0 2)Ma for K|feldspar (CT 220℃). The apatite age obtained for a biotite schist at this location was (4 6±0 2)Ma and is consistent with relatively constant cooling since 12Ma. In between the MCT and the Ngozumba site samples were collected near the settlement of Ghat that yielded anomalously young 40 Ar/ 39 Ar ages:(7 7±0 4)Ma for muscovite, (7 5±0 6)Ma for biotite, and (3 6±0 2)Ma for K|feldspar (CT 210℃). The apatite age is consistent with these young ages at (0 8±0 1)Ma. Plausible explanations for the young ages at this site include: post|MCT age movement on a small|scale shear zone or late|stage movement of hydro|thermal fluids through this localized zone. The MCT zone 40 Ar/ 39 Ar ages were used to constrain a one|dimensional thermal model aimed at understanding rates of exhumation (Hubbard et al., 1991). The model yielded exhumation rates of 0 7～1 9km/Ma for the MCT zone region. The (U|Th)/He age for apatite from this region is consistent with the model results. By coupling the (U|Th)/He ages with the 40 Ar/ 39 Ar ages in this region we can determine that exhumation processes have acted at a relatively constant rate during the last 15～20 million years (with the possible exception of the Ghat location). Major fault activity likely pre|dates this time period, or has had little affect on the cooling history.