Appendix A. Geochronology and geochemistry analytical procedures
Zircon characterization
Zircons were imaged under cathodoluminescence (CL) and back-scattered
electrons (BSE) using a JEOL 5600 scanning electron microscope (Fig. A1; Hanchar and
Miller, 1993). CL images of all dated zircon grains are shown in Figures A1-A5.
SHRIMP-RG analytical procedures
Minerals, concentrated by standard heavy mineral separation processes and hand
picked for final purity, were mounted on double-stick tape on glass slides in 1 x 6 mm
rows, cast in epoxy, ground and polished to a 1 μm finish on a 25 mm diameter by 4 mm
thick disc. CL images were used to identify internal structure, inclusions and physical
defects. The mounted grains were washed with 1N HCl or EDTA solution (if acid
soluble) and distilled water, dried in a vacuum oven, and coated with Au. Mounts
typically sat in a loading chamber at high pressure (10-7 torr) for several hours before
being moved into the source chamber of the SHRIMP-RG. Secondary ions were
generated from the target spot with an O2- primary ion beam varying from 4-6 nA. The
primary ion beam produced a spot with a diameter of 20-40 microns and a depth of 1-2
μm for an analysis time of 9-12 minutes. Nine peaks were measured sequentially for
zircons (the SHRIMP-RG is limited to a single collector, usually an EDP electron
multiplier): 90Zr216O, 204Pb, Bgd (0.050 mass units above
248
204
Pb), 206Pb, 207Pb, 208Pb, 238U,
Th16O, 254U16O. Autocentering on selected peaks and guide peaks for low or variable
abundance peaks (i.e. 96Zr216O 0.165 mass unit below 204Pb) were used to improve the
reliability of locating peak centers. The number of scans through the mass sequence and
counting times on each peak were varied according to sample age and U and Th
concentrations to improve counting statistics and age precision. Measurements were
made at mass resolutions of 6000-8000 (10% peak height) which eliminated all
interfering atomic species. The SHRIMP-RG was designed to provide higher mass
resolution than the standard forward geometry of the SHRIMP I and II (Clement and
Compston, 1994). This design also provides very clean backgrounds and combined with
the high mass resolution, the acid washing of the mount, and rastering the primary beam
for 90-120 seconds over the area to analyzed before data is collected, assures that any
counts found at mass 204Pb are actually Pb from the zircon and not surface contamination.
In practice greater than 95% of the spots analyzed have no common Pb. Concentration
data for zircons are standardized against zircon standard R33 (419 Ma, quartz diorite of
Braintree complex, Vermont, John Aleinikoff, pers. comm.) which are analyzed
repeatedly throughout the duration of the analytical session. Data reduction follows the
methods described by Williams (1997) and Ireland and Williams (2003) and uses the
Squid and Isoplot programs of Ken Ludwig (Ludwig, 2001; 2003). Between U-Pb
analyses and Ti-in-zircon analyses the zircon mounts were repolished and recoated with
gold.
Trace element chemistry of zircon
Trace element patterns of zircon can provide information about the conditions
under which zircon crystallized (e.g. Rubatto, 2002). A comparison of Himalayan-aged
zircon trace element patterns in leucogranites deformed by the STDS both pro-ward
(outer STDS) and retro-ward (inner STDS) of the study area from Bhutan showed that
Miocene-aged zircon rims in outer STDS leucogranites are characterized by a wide range
of negative Eu anomaly (Eu/Eu* = 0.4-0.002) and a slight depletion of heavy rare earth
elements (REEs) (Yb/Gd = 80-0.6), while zircons in leucogranite from the inner STDS
are characterized by a smaller Eu anomaly (Eu/Eu* = 0.5-0.04) and no depletion of heavy
REEs (Yb/Gd 100-10) (Kellett et al., 2009). The zircon rims from Sikkim are
intermediate between those two groups, having smaller Eu anomalies but a slight
depletion of heavy REEs (Fig. A6). This “intermediate” trace element pattern character
of zircon is interesting considering the range of leucogranite petrologies and zircon
morphologies represented, from in situ leucosome to aplite to cordierite-bearing
leucogranite. The smallest Eu anomaly was observed in SK 58, the aplite dyke (Fig.
A6c). The greatest depletion for heavy REEs was observed in SK 56, the leucosome
(Fig. A6a). The leucosome was clearly sourced from its adjacent garnet-bearing host
gneiss. Thus it is possible that partial melting of the garnet-bearing source rock which
left garnet in the residual phase caused the melt to be depleted in heavy REEs (Kellett et
al., 2009). It is also possible that zircon may have competed for heavy REEs during
crystallization with another phase such as xenotime.
Monazite characterization and LA-ICP-MC-MS analytical procedures for U-Th-Pb
geochronology and trace element geochemistry.
Monazite grains were identified in situ by reconnaissance mapping of Ca, Ce, P
and Y in polished thin sections using the JEOL 8200 electron microprobe housed at
Dalhousie University in Halifax, Canada. Targeted grains were individually mapped at
high resolution (15 kV, ~200 nA) for Th, Nd, Y and U. Back-scattered electron (BSE)
images were also collected at low magnification to illustrate the textural context of the
grains.
U-Th-Pb geochronology concomitant with trace element data of monazite in
polished thin sections was conducted using a Photon Machines (San Diego, USA) 193
nm ArF Excimer laser ablation system equipped with a two-volume “HeleX” ablation
cell that facilitates rapid transfer to the mass spectrometers connected via a split stream to
both multi-collector Nu Plasma and single collector AttoM inductively-coupled plasma
mass spectrometers operated at the University of California, Santa Barbara (UCSB). The
collector arrangement on the Nu Plasma at UCSB allows for simultaneous measurement
of 232Th and 238U on high-mass side Faraday cups equipped with 1011 ohm resistors and
208
Pb, 207Pb, 206Pb and 204Pb on four low-mass side ETP discrete dynode electron
multipliers. Each analysis began with two cleaning shots followed by 30 s ablation of a
7.2 μm diameter spot at a frequency of 3 Hz and 4.84 J/cm2 fluence. BSE images and
elemental maps guided spot selection. Primary reference material “Stern” monazite
(512± Ma ID-TIMS U-Pb age) (reference?) was used to monitor and correct for mass
bias, Pb/U and Pb/Th fractionation. Two secondary reference standards, “FC-1”
monazite (55.6±1.3 Ma ID-TIMS age) (Horstwood et al., 2003) and “44069” monazite
(424± Ma ID-TIMS age) (Aleinikoff et al., 2006) were analyzed concurrently to monitor
data accuracy. Trace element data were normalized to reference standard “44069”. One
Stern, one FC-1 and one 44069 were analyzed for every 4-5 unknowns. Overall
uncertainties achieved on the secondary reference material throughout the analytical
period were 3% (2σ) for U-Pb data and 4% (2σ) for Th-Pb data. All quoted uncertainties
include contributions from the external reproducibility of the standard for 206Pb/238U and
208
Pb/232Th ratios. For common-Pb correction purposes, a 207Pb/206Pb ratio of 0.824 was
used. This ratio has been measured in feldspar grains (which contain no radiogenic Pb)
from a GHS sample of similar composition and monazite ages, which averaged
0.824±0.004 for 5 measurements on different grains. Data reduction of age and trace
element data was done using Iolite software (www.iolite.org.au) run within the Igor Pro
host environment, and age data were plotted using Isoplot (Ludwig, 2003).
40
Ar/39Ar thermochronology analytical procedure
McDougall and Harrison (1988) described the principles and method of 40Ar/39Ar dating.
Coarse, pristine muscovite grains were hand picked from material crushed by a jaw
crusher. The separated mica concentrates were individually wrapped in aluminum foil,
and then stacked in an aluminum irradiation canister. Interspersed among the samples
were five to seven aliquots of the flux monitor, Fish Canyon tuff sanidine, which has an
apparent K-Ar age of 28.205 ± 0.046 Ma (Kuiper et al. 2008). The canister was irradiated
with fast neutrons in the nuclear reactor at McMaster University in Hamilton, Ontario,
Canada. At Dalhousie University, a double-vacuum tantalum resistance furnace was used
to carry out the step-heating. Isotopic analyses were made in a VG3600 mass
spectrometer using both Faraday and electron multiplier collectors to measure the
abundance of 39Ar for 40Ar/39Ar and 36Ar/39Ar ratios, respectively. Errors are reported at
the 2σ level and include the uncertainty in the irradiation parameter, J, but do not
incorporate uncertainty in the assumed age of the flux monitor.
Apatite fission track thermochronometry method and analytical procedure
AFT samples were processed and analyzed by Isabelle Coutand at Dalhousie
University (Halifax, Canada); standard magnetic and heavy liquid mineral separation
procedures were used. Apatite aliquots were mounted in araldite epoxy on glass slides,
ground and polished to expose internal grain surfaces, then etched for 20 s in 5.5M
HNO3 at 21 °C to reveal spontaneous fission tracks. All mounts were prepared using the
external-detector method (Hurford & Green, 1983). Samples and CN-5 glass standards
were irradiated with thermal neutrons in the Oregon State University reactor. After
irradiation, the low-U muscovite detectors that covered apatite grain mounts and glass
dosimeter were etched in 40%HF for 45 min at 21 °C to reveal induced fission tracks.
Samples were analyzed using a Kinetek computer-controlled stage driven by the FTStage
software (Dumitru, 1993) attached to a Zeiss Axioplan microscope. Counting was
operated dry at a magnification of x1000 by I. Coutand at Dalhousie University, for a
Zeta value of 370 ± 5 (weighted mean zeta calibration factor based on the analyses of 25
IUGS age standards (see Hurford, 1990) for the dosimeter glass CN5).
Between 14 to 20 grains per sample were selected and analysed. Horizontal
confined track-lengths and their angle with the crystal’s c-axis could be observed and
measured in one sample only (SK55). When available, up to four Dpar measurements
were averaged from each grain to characterize their specific annealing properties
(Donelick et al., 1999; Ketcham et al., 1999).
(U-Th)/He thermochronology of apatite
Analytical procedure AHe
Sample preparation and He measurement was performed at Dalhousie University
in Halifax, Canada. Individual apatite grains were selected from separates on the basis of
size, morphology and lack of inclusions. Grains were hand-picked using a stereoscopic
microscope at 100 x magnification under cross-polarized light to screen for fluid and
solid inclusions. Selected crystals were measured and photographed in three different
directions for alpha-ejection correction. Each crystal was then packed into a Pt foil
envelope. Fragments from a single, large, crushed Durango apatite crystal packed in Pt
envelopes served as laboratory standards and empty Pt envelopes were included for blank
measurements. Crystals, Durango standards and empty Pt foils were loaded in a 25-hole
stainless steel planchet and placed into the UHV extraction system behind a Supra-Sil
fused silica viewport. Individual packets were heated for 5 min. at 1050°C by a defocused Quantronics model 117 NdYAG laser operated in continuous mode at 1064 nm
to extract the 4He. The sample temperature was controlled by measuring the Pt
temperature with a 2-color optical pyrometer which regulated the laser output power.
Each grain was then re-heated for 5 minutes at 1050 °C to check for incomplete 4He
extraction and potential presence of undetected zircon micro-inclusions. If the percentage
of re-extraction exceeded 0.15%, the grain was discarded. The 4He abundance was
measured by isotope dilution on a Pfeiffer Prisma QMS200 quadrupole mass
spectrometer using a calibrated 3He spike. The 4He extracted from the samples was
purified before admission to the mass spectrometer by retention of condensable gases in a
CTI Cryogenics liquid He cryogenic cold trap operated at 37 K.
U, Th and Sm measurement were performed at the Geology Department of ETH,
Zurich. Apatite and platinum packages were opened and transferred to vials, where 233U,
236
U, 230Th, 149Sm and 147Sm standards were added. Apatites were digested in 5% HNO3
for 24 hours on a hotplate at 90 ºC and U, Th and Sm concentration were measured using
a quadrupole ICP-MS.
Figure Captions
Figure A1. CL images for dated zircons from sample SK 56.
Figure A2. CL images for dated zircons from sample SK 57.
Figure A3. CL images for dated zircons from sample SK 58.
Figure A4. CL images for dated zircons from sample SK 60.
Figure A5. CL images for dated zircons from sample SK 74.
Figure A6. Trace element chemistry of zircons
Figure A7. U-Pb zircon ages plotted against Ti-in-zircon temperatures
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