zircon U-Pb geochronlogical and Ar-Ar thermochronlogical data in Gyaca county, NE Himalaya
The data include zircon U-Pb age of a intermediate dike (Table S1-sample STV01-mendeley), and 39Ar-40Ar thermochronology result of mica quartz phyllite (Table S2-Ar-Ar data-mendeley) from the Gyaca mélange zone in Gyaca county, NE Himalaya. Sample STV01 was collected from the intermediate dikes and parrlleled to the replaced cleavages of folds, while sample DX2085 was collected from the matrix of Gyaca mélanges near the dikes. The intermediate dikes intruded the Gyaca mélange along the foliation S2 (axial plane cleavage) and remoulded each other, indiacating the syntectonic relationship and the deformation timing of folding. Fourteen zircons among twenty two dated grains obtained from sample STV01 (Appendix 2) show concordance of >95% yielded weighted mean 206Pb/238U age of 55.29±0.54 Ma (MSWD=0.85), suggesting that the folding deformation happened nearly simultaneous in the Early Eocene. Steps 2 to 5 of muscovite 39Ar-40Ar data from sample DX2085 yileded a plateau age of 55.78±0.14 Ma (81.73% of the 39Ar gas release, MSWD=2.38) and an inverse isochron age of 55.62±0.15 Ma (MSWD=4.2). The sub-particle rotation recrystallization and bulging recrystallization of quartz particles from sample DX2085 indicate a deformation temperature range of 380°~420°, exceeding the closure temperature of muscovite(~350°), which implies the 39Ar-40Ar age can be used to constrain the timing of the contraction deformation in the Gyaca mélange zone. The positive flower structural style in the upper Triassic tectonostratigraphy in NE Himalaya implies intense contraction deformation and contemporaneous folding between the Gyaca mélange zone and the Langjiexue fold-thrust belt. Combined with the regional geological background, consequently, the dramatical contraction deformation in the NE Tethyan Himalaya should begin after the Early Cretaceous, and it’s most likely to take place in the Early Eocene (55~56 Ma) through the India-Asia collision rather than in the Early Cretaceous by the collision of the “Longzi block” and an intra-oceanic arc.
Steps to reproduce
Zircons of sample STV01 were selected at the Regional Geological Survey of Hebei (Langfang), and zircon dating was performed by the LA-ICP-MS method at Milma Lab, China University of Geosciences. The NewWave 193UC ArF excimer laser was used to denudate samples, and the Angilent 7900 quadrupole plasma mass spectrometer was used to measure the ion signal intensity. Standard material NIST SRM 610 was used to optimize the ICP-MS instrument and an external standard for the determination of trace elements. The 40Ar/39Ar dating was carried out in the reactor of China Institute of Atomic Energy and lasted for 24 hours . Mica 40Ar/39Ar was determined in the Key Laboratory of Isotope Geology, Ministry of Land and Resources, Institute of Geological Sciences of China. The graphite furnace were carefully analyzed using step-heating experiments, with each step heating for 10 minutes and purification for 20 minutes . The mass spectrometry was performed on a multi-receiver Noble Gas Mass Spectrometer (GV Helix MC), with 20 sets of data collected per peak. All the data were reverted to the time zero value before mass discrimination correction, atmospheric argon correction, blank correction and interference element isotope correction. System blank level: M/E at 40/39/37/36 is less than 6×10-15mol, 4×10-16mol, 8×10-17mol and 2×10-17mol respectively. The correction coefficients of interfering isotopes generated during neutron irradiation were obtained by analyzing irradiated K2SO4 and CaF2, and their values were: (36Ar/37Ar0)Ca=0.000278, (40Ar/39Ar)K = 0.001182, (39Ar/37Ar0)Ca=0.000852. The decay constant for 40K is 5.543× 10-10A-1. The age error is expressed as 1σ.