Fig. 3. Photographs of the studied outcrops showing the contact
between garnet lherzolite and (a) garnet-bearing granitic gneiss and (b)
garnet pyroxene, and (c) garnet lherzolite.
Zircon U–Pb dating of the garnet lherzolites has yielded peak
metamorphic ages of ca . 501 Ma (Zhang et al., 2005) andca. 498 Ma (Wang et al., 2011), and for felsic granulite ofca . 493 Ma (Zhang et al., 2005) and mafic granulite of ca .
500 Ma (Dong et al., 2018). These ages for the Yinggelisayi terrane are
consistent with those of HP-UHP rocks from the Jianggalesayi and
Danshuiquan areas (Liu et al., 2012, 2015; Gai et al., 2022).
3 Methods
Samples were cut normal to the foliation and parallel to the lineation,
and made into doubly polished thin-sections. The mineral compositions
were determined with an electron microprobe (JXA-8230) at the State Key
Laboratory of Continental Dynamics, Northwest University, Xi’an, China.
The instrument was operated at an accelerating voltage of 15 kV, a beam
current of 10 nA, a beam diameter of 1 μm, and with 20 s count times.
The natural mineral and synthetic standards were supplied by SPI
Company. The trace element compositions of the olivines were determined
by laser ablation–inductively coupled plasma–mass spectrometry
(LA-ICP-MS) at the State Key Laboratory of Continental Dynamics,
Northwest University, Xi’an, China. A laser beam diameter of 43μm and 10
Hz repetition rate were used for the analyses. The data were processed
with ICPMSDataCal software using 29Si as the internal
standard (Liu, 2011). The crystallographic preferred orientations (CPO)
of olivine was measured by electron backscatter diffraction (EBSD) using
a scanning electron microscope (SEM JEOL 6380) at the State Key
Laboratory of Geological Processes and Mineral Resources (GPMR), China
University of Geosciences, Wuhan, China. The EBSD data were processed
using HKL Channel 5 software. The sample surface was inclined at 70° to
the incident beam and the EBSD patterns were collected on a phosphor
screen. The experimental conditions were as follows: accelerating
voltage = 15 kV; working distance ~ 20 mm; spot size =
5.0. The J-index was calculated to estimate the fabric strength of the
samples using uncorrelated grain pairs analyzed from the EBSD data
(Skemer et al., 2005). The olivine and enstatite water contents were
measured by Fourier transform infrared (FTIR; Nicolet 6700) spectroscopy
at the China University of Geosciences, Wuhan, China. Unpolarized
transmitted light was used to collect the FTIR spectra. Prior to
analysis, the samples were made into doubly polished sections with a
thickness of 150–250μm, and heated at 120°C for 24 h to eliminate water
on the surface of the sections. The water contents were calculated using
the calibration method described by Bell et al.
(2003)
and Hans & Joseph. (2006).
4 Sample descriptions and metamorphic stages
4.1 Petrography and mineral chemistry
The garnet lherzolite consists of olivine (~40%),
orthopyroxene (~20%), clinopyroxene
(~20%), garnet (~8%),
amphibole
(~5%),
spinel
(~2%), and minor accessory minerals such as serpentine
and magnesite. The garnet lherzolites have a clear porphyroblastic
structure, and garnet, olivine, orthopyroxene, and clinopyroxene
porphyroblasts comprise ~30% of the samples. Detail
descriptions of each mineral are given below.