Metamorphism and exhumation of the NW Himalaya constrained by U–Th–Pb analyses of detrital monazite grains from early foreland basin sediments

Abstract

Single detrital monazite grains from the Dharamsala and Lower Siwalik Formations (early to mid-Miocene continental foreland basin sediments in NW India) have been dated by two techniques; isotope dilution thermal ionization multicollector mass spectrometry (ID-TIMS) and laser ablation plasma ionization multicollector mass spectrometry (LA-PIMMS). The results give U–Th–Pb isotopic ages of c. 400–1300 Ma and 28–37 Ma and suggest that the source of detritus shed from the uplifting Himalayan mountains and captured in the foreland basin included (1) the protolith to the High Himalayan Crystalline Series (HHCS), i.e. rocks unaffected by the Himalayan metamorphism, (2) Cambro-Ordovician granites and (3) HHCS affected by the M 1 phase of Barrovian metamorphism (Eo-Himalayan) related to the Indo-Asian collision. Deposition of the Dharamsala Formation was coeval with M 2 sillimanite grade Himalayan metamorphism and crustal melting. The youngest monazite ( c. 28–37 Ma) ages imply that Indian plate rocks, having experienced the earliest Himalayan metamorphic event which occurred within 10–20 Ma of collision were exhumed, eroded and deposited within c. 10–20 Ma of metamorphism. This indicates a minimum cooling rate of between 60 and 40°C Ma −1 for the period 30–20 Ma. After 20 Ma our study suggests no change in source area and that this same sequence, comprising both metamorphosed and unmetamorphosed rocks, was supplying detritus and being progressively incised by erosion for at least a further 8 million years.

Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup, eastern Himalaya, Darjiling District, India

“…Naturally, we cannot fully exclude an autocyclic fluvial sedimentary origin for the observed soft sediment deformation features, but in each case, we have been able to at least argue, on the basis of field evidence, for a preferential seismic origin. The Siwalik sedimentation was taking place during and after the uplift and exposure of the GHC, which served as the source of Siwalik sediments (DeCelles et al 1998;White et al 2001) between 12 -1 Ma (Sangode et al 1996;Meigs et al 1995). The approximate time of the deposition of Middle Siwalik is ~ 11 -7 Ma (Yin, 2006 Many authors have worked on the relation between soft sediment deformation structures and earthquake intensity (Tasgin and Turkmen, 2009 and references within).…”

“…This paper aims to examine soft sediment deformation structures from fluvial deposits within the Neogene Middle Siwalik Subgroup of eastern Himalaya, Darjiling district, India. The source of sediments of the Siwalik basins is mainly from the rocks of the Greater Himalayan Crystalline Complex (GHC) to the north (DeCelles et al 1998;White et al 2001). Keeping in view the syn-tectonic deposition of Middle Siwalik sediments within the foreland basin of the Himalayan orogenic belt (Moores and Fairbridge, 1997) during the uplift of GHC in the Middle and Late Miocene (Negi, 1998;Tabata, 2004;Yin, 2006 and references therein), a strong case can be made for a predominantly seismic origin for the penecontemporaneous deformation structures in the Middle Siwalik sequence.…”

Evolution of the Himalayan Paleogene foreland basin, influence of its litho-packet on the formation of thrust-related domes and windows in the Eastern Himalayas – A review

“…U-Th-Pb ages of single monazite grains determined by White et al (2001) range from 1300 to 400 Ma and 37 to 28 Ma from the Dharmsala and Lower Siwalik beds in NW India. Monazite age of 37 Ma is from the basal part of the Dharmsala Group and ages of 27 to 28 Ma in the upper part of the group.…”

Predicting radioactive accessory mineral dissolution during chemical weathering: The radiation dose at the solubility threshold for epidote-group detrital grains from the Yangtze River delta, China

“…The tracer solution, consisting of a mixture of natural Tl ( 205 Tl/ 203 Tl = 2.3871), enriched 233 U, 209 Bi and 237 Np (concentrations of~1 ppb per isotope), and transported in a mixed Ar-He carrier gas, was employed to correct for instrumental mass bias (e.g., Cox et al, 2003;Chew et al, 2011). The tracer solution, together with the rastering, substantially reduce laser-induced U-ThPb fractionation (e.g., Li et al, 2001;White et al, 2001;Cox et al, 2003;Horstwood et al, 2003;Košler, 2008;Košler et al, 2008;Chew et al, 2011). Time-resolved data acquisitions (120-180 s experiments) consisted of~20-30 s measurement of the Ar-He gas blank and aspirated tracer solution prior to introduction of ablated material.…”

“…Although many studies of highly reactive accessory minerals during chemical weathering address the dissolution of carbonate (e.g., Garrels and Mackenzie, 1967;Mast, 1992;Velbel, 1992;Blum et al, 1998;White et al, 1999White et al, , 2001White et al, , 2005Taylor et al, 2000a,b;Mavris et al, 2010) and phosphate minerals (e.g., Probst et al, 2000;Aubert et al, 2001Aubert et al, , 2002Aubert et al, , 2004Blum et al, 2002;Oliva et al, 2004;Hausrath et al, 2011;Price et al, 2013), the solubility of the calc-silicate minerals amphibole (e.g., Berner et al, 1980;Berner and Schott, 1982;Oliva et al, 2004), sphene (Braun et al, 2009), and epidote-group (e.g., Watson, 1917;Oliva et al, 2004;Price et al, 2005aPrice et al, ,b, 2008 are also reported. Sphene and epidote-group minerals typically host the radioactinides 232 Th, 238 U, and 235 U and are used as geochronometers (e.g., Tilton and Grunenfelder, 1968;Tucker et al, 1987Tucker et al, , 2004von Blanckenburg, 1992;Barth et al, 1994;Catlos et al, 2000;Oberli et al, 2004;Chew et al, 2011;Spencer et al, 2013;Smye et al, 2014).…”