Appendix 2: Analytical procedures
Samples were prepared for analysis by grinding in an agate mill. Major elements were analyzed in the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University. Fused glass beads were analyzed by an ARL ADVANT XP+ X-ray fluorescence spectrometer. The analytical precision for all major oxides by XRF is estimated to be better than 1%. Total loss on ignition (LOI) was measured on powders that had been pre-dried at 110°C for at least 12 h. Powders were ignited at 950°C in air for 1 h, before cooling in a silicagel desiccator and reweighing.
Trace element analyses were performed on a Finnigan Element ICP-MS at Institute of Geology and Geophysics, Chinese Academy of Science. Basaltic bulk rock powder (50mg) was digested with sub-boiling distilled HF-HNO3 in a tightly sealed SAVILLEX™ 15 ml PFA vial by heating to 200 °C on a hot plate for 72 hours, then evaporating until nearly dry. Rhyolitic rock power (50mg) was digested with the same reagent, but in a flat top 15ml Teflon vial enclosed in a high pressure steel bomb, by heating to 200°C in a blast dry oven for 72 hours, in order to completely decompose zircons as well as other acid-resistant minerals. In order to drive off residual HF the sample was dried to a very small volume, and 1ml of concentrated HNO3 (67 wt%) was added and evaporated to nearly dry again. Finally the nearly dried down sample was redissolved in 2 ml 10% HNO3 at 150°C overnight. The final solution was prepared by adding 2% HNO3 with internal indium standard and diluted to 30 ml. A part of final solution was then analyzed on the ICP-MS. Calibrations were set using international and internal rock reference material (eg: BCR-1, BHVO-1, JR-2 and GB-2).
Chemical separation for Rb-Sr and Sm-Nd isotope analysis were conducted at the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University following procedures described in Chen et al. (2000). Rock samples for Rb-Sr and Sm-Nd isotopic analysis was dissolved in a tightly sealed SAVILLEX™ 7 ml PFA vial after being spiked with 87Rb, 84Sr, 149Sm and 150Nd tracers prior to HF+HNO3 (with a ratio of 2:1) dissolution. Rubidium, Sr and rare earth elements were separated on quartz columns with a 5 ml resin bed of Bio-Rad™ AG 50W-X12, 200 - 400 mesh. Sm and Nd were separated using 1.7ml 2-ethylhexyl hydrogen 2-ethylhexyl phosphate (trade name P507 in China) impregnated Teflon™ powder as cation exchange medium. Procedural blank was monitored by Pb contents, which were <50 pg in more than 100 blank samples analyzed over a two years period. Isotopic ratios were measured using a multi-collector Isoprobe-T™ thermal ionization mass spectrometer (TIMS) at Analyzing Center of Beijing Institute of Nuclear Engineering. 143Nd/144Nd ratios were corrected for mass fractionation by normalization to 146Nd/144Nd=0.7219. Typical within-run precision (2σ) for Nd was estimated to be ±0.000015. The measured values for USGS rock standard BCR-2 were 0.512628±10 (2σn, n=5) during the period of data acquisition.
Zircon crystals were extracted by crushing and a combination of heavy liquid and magnetic separation techniques. These separates were analysed by two different techniques.Individual crystals were handpicked and mounted, along with several pieces of the standard zircon Temora 2, and R33 (Black et al. 2004) in epoxy resin discs, and polished to expose the grain’s cores.
Cathodoluminescence (CL) imaging for four LA-ICP-MSsamples was carried out at the Electron Microscopy Unit, the Australian National University, on a HITACHI S2250-N scanning electron microscope working at 15 kV, ∼60 μA, and ∼20 mm working distance. While CL imaging for SHRIMP II sample NK1 was conducted in Beijing with a PHILIPS XL30 SEM in PKU, operating at 15 kVand 120μA.
Zircon U–Th–Pb analysis for sample NK1 was conducted using the SHRIMP II ion microprobe housed at the Beijing SHRIMP Center, following analytical procedures as described by Williams (1998). The measured 206Pb/238U ratios of zircon samples were corrected using the reference zircon Temora 2 (age: 417Ma) and the U, Th and Pb concentrations were calibrated against the reference zircon SL13 (U concentration 238μg/g, Th/U=0.09; age: 572Ma). All ages have been calculated from the U and Th decay constants recommended by Steiger and Jäger (1977). Reported ages represent 238U/206Pb data that have been corrected for common Pb using the measured 204Pb (Compston et al. 1984). The data were processed using software SQUID 1.0 and ISOPLOT (Ludwig 2001), to calculate the weighted mean ages at 95% confidence level and 2σ error.
Zircon populations from 4 rocks were analyzed in round-robin style in a single analytical session using the method described in Bryan et al. (2004) except for an updated laser+cell system. The laser used is an ArF, 193 nm wavelength, excimer laser (COMPex 110) fired at 5 Hz, at energies yielding a measured power of about 60 mJ. The zircons were ablated with a 28 micron beam within an in-house built “Helix” cell connected to the Agilent 7500S ICP-MS via a signal smoothing device. Temora 2 was the primary standard. Analyses included in mean age populations are more than 95% age concordant with respect to the 207Pb/235U and 206Pb/238U, and have uncertainties on the ratio used for dating less than twice that expected from counting statistics. The resulting weighted mean 206Pb/238U age for R33, a standard known to be 419.23±0.39 Ma (Black et al. 2004) was 417.4±5.0 Ma for all 20 analyses yielding an MSWD of 1.52. This result employs a 2% uncertainty in the measured 206Pb/238U for Temora 2 propagated in quadrature to the uncertainties of each individual analysis. As 204Pb cannot be accurately corrected due to a large isobaric interference from Hg, no 204Pb based correction can be applied, and instead a 208Pb based correction is tested in each case to see if populations become more concordant. This round-robin analysis style was employed to maximize the precision of the relative ages.