evidence for palaeo-tethyan oceanic subduction within central qiangtang, northern tibet
TRANSCRIPT
Lithos 127 (2011) 39–53
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Evidence for palaeo-Tethyan oceanic subduction within central Qiangtang,northern Tibet
Yan Liu a,⁎, M. Santosh b, Zhong Bao Zhao c, Wen Chao Niu c, Gen Hou Wang c
a Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, Chinab Department of Interdisciplinary Science, Faculty of Science, Kochi 780–8520, Japanc School of Earth Sciences and Mineral Resources, China University of Geosciences, Beijing 100083, China
⁎ Corresponding author.E-mail address: [email protected] (Y. Liu).
0024-4937/$ – see front matter. Crown Copyright © 20doi:10.1016/j.lithos.2011.07.023
a b s t r a c t
a r t i c l e i n f oArticle history:Received 21 November 2010Accepted 31 July 2011Available online 7 August 2011
Keywords:Garnet blueschistPetrologyMetamorphic historyCold slab subductionTethysNorthern Tibet
The mechanism of formation of blueschist–eclogite belts and their space-time distribution are important inunderstanding the tectonics associated with convergent plate boundaries. Here we investigate the garnet-bearing blueschists from Rongma area of central Qiangtang in northern Tibet. Themineral assemblage in theserocks is characterized by porphyroblastic garnet set within a matrix of fine-grained amphibole, white mica,epidote, chlorite, albite and quartz with accessory rutile, titanite and apatite. The garnet porphyroblastsexhibit core and rim portions, and the cores carry abundant inclusions of Na amphibole, quartz and rutile, aswell as rhomb-shaped inclusions of paragonite and epidote which are interpreted as pseudomorphs afterlawsonite. The rims are characterized by coarse-grained inclusions of epidote as well as the absence ofparagonite and epidote aggregates, clearly suggesting that the transition from garnet core to rim marks ametamorphic transformation from lawsonite- to epidote-stability field. The Mn content of the garnetporphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend. In thematrix, we identify two stages of Na amphibole rimmed by Na-Ca amphibole and albite. Retrograde chlorite isrimmed by fine-grained biotite. Based onmicrostructural observations and pseudosectionmodelling, we tracethe P–T path for the Rongma garnet blueschist from 1.92 GPa and 490 °C (lawsonite eclogite field) to about1.68 GPa and 535 °C (epidote eclogite field), marking an initial increase in temperature and decrease inpressure. This stage is followed by a decrease of pressure through the blueschist facies down to P–T conditionsof about 0.6 GPa and 530 °C. In combination with previous work including the available isotopic age data, theP–T path obtained in the present study suggests the deep subduction of palaeo-Tethyan oceanic crustbetween southern and northern Qiangtang blocks, supporting the model that the blueschist belt defines thelocation of the palaeo-Tethyan suture zone within northern Tibet. T-X(O) pseudosection modelling furtherreveals that the transformation of lawsonite to epidote occurred under high O contents. Our results provideimportant clues to better understand the subduction-related late Permian to early Triassic magmas innorthern Tibet.
11 Published by Elsevier B.V. All r
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
The Himalayan orogen is one of the youngest mountain belts onEarth and is a world standard for continent–continent collision tostudy the tectonic processes associated with mountain building, aswell as convergent margin tectonics including subduction- andaccretion-related processes (e.g., Bhattacharyya and Mitra, 2009;Zhang et al., 2010). The Tibetan region is composed of four continentalblocks that, from north to south, are the Songpan–Ganzi, Qiangtang,Lhasa and Tethyan Himalayan terranes, which are separated by the
ig
Jinsha, Bangong–Nujiang, and Indus–Yarlung Zangbo suture zones,representing Paleo-, Meso-, and Neo-Tethyan oceanic relicts, respec-tively (e.g., Yin and Harrison, 2000). In northern Tibet, aprominentN1000-km-long blueschist-bearing metamorphic belt di-vides the Qiangtang terrane into the northern and southern Qiangtangblocks and was designated as the central Qiangtangmetamorphic belt(Fig. 1). Recent studies reported lawsonite-bearing blueschist fromthe Hongjishan area (Lu et al., 2006) and eclogites from Gemo area (Liet al., 2006a, 2006b; Zhang et al., 2006a) within this belt (Fig. 1). Thewidespread occurrence of blueschist–eclogite rocks in this regionsuggests that the central Qiangtang metamorphic belt is among thelargest high-pressure metamorphic belts on Earth (e.g., Pullen et al.,2008).
The mechanism of formation and characteristics of the centralQiangtang metamorphic belt are thus critical to understanding thefirst-order crustal structure and accretionary history of northern Tibet
hts reserved.
30 N°
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Qaidam basin
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men
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Songpan-Ganziflysch complexLate Triassicgranitoids
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3 Shuanghu area 4 Gangma area
0 300km
Amdo gneisses
working area
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enarretiznaG-napgnoS
Fig. 1. Simplified geological map, showing high-pressure metamorphic rocks and Mesozoic igneous rocks within northern Tibet. IYS: Indus-Yarlung Zangbo suture zone,BNS: Bangong-Nujiang suture zone, LS: Longmu Co-Shuang hu suture zone, JS: Jinsha suture zone, KLS: Kunlun suture zone. NQB: northern Qiangtang block. SQB: southern Qiangtangblock. Amdo gneisses after Kapp et al. (2003) and Liu et al. (2009). Songpan-Ganzi flysch complex after Kapp et al. (2003). Late Triassic granitoids after Kapp et al. (2003), Zhang et al.(2006c), Xiao et al. (2007), Xiong et al. (2006) andHuang et al. (2007). Late Triassic rhyolite–dacite after Zhai and Li (2007). Late Permian–Early Triassic andesites in Zado area after Li et al.(2007) and our unpublished data, in Tuotuohe area after Li and Li (2006) and in Lugu area after Wu and Lan (1990). High-pressure complex in Hongjishan area after Lu et al. (2006), inGemoarea after Li et al. (2006a) and Zhang et al. (2006a), in Shuanghuarea after Kapp et al. (2003) and Zhang andTang (2009) and inGangma area after Kapp et al. (2003) and Pullen et al.(2008).
40 Y. Liu et al. / Lithos 127 (2011) 39–53
and the evolution of Tethys (e.g., Kapp et al., 2003; Pullen et al., 2008;Zhang et al., 2006a, 2006b). However, the origin of this belt is highlydebated and has been variously interpreted as: (1) an in situ palaeo-Tethyan suture zone between the northern Qiangtang block ofCathaysian affinity, and the southern Qiangtang block of Gondwanaaffinity (e.g., Li, 1987; Li et al., 1995; Li and Zheng, 1993; Zhang, 2001;Zhang et al., 2006a, 2006b); (2) an early Mesozoic mélangeunderthrust at low-angle from the Jinsha suture zone about 200 kmto the north, and subsequently exhumed within the Qiangtang block(Kapp et al., 2000; 2003); and (3) a continental collision zonebetween the Qiangtang block and a palaeo-Tethyan arc terrane(Pullen et al., 2008). The history of this metamorphic belt is thusfundamental to evaluating the various models, and for a betterunderstanding of the development of Tibetan plateau and Tethyanhistory. The previous estimates of pressure–temperature (P–T)conditions of this large metamorphic belt were largely made byconventional thermobarometers and show a wide range of values. Forexample, the pressures estimated from eclogites in the Gemo area arearound 20–25 kbar (Li et al., 2006a; Zhang et al., 2006a), whereasthose derived from garnet-bearing blueschists in Rongma andGangma areas show 10–15 kbar (Kapp et al., 2003). Moreover, themetamorphic evolution of this high-pressure belt is still poorlyconstrained.
In this study, we re-investigate the metamorphic history and P–Tconditions of the garnet blueschist from the Rongma area within thecentral part of the central Qiangtang metamorphic belt (Fig. 1) on thebasis of detailed petrological studies and P–T isochemical diagrams(pseudosection modelling). In combination with previous work, ourresults provide new insights into history of the central Qiangtang
metamorphic belt, and on the pre-Cenozoic tectonic evolution ofnorthern Tibet as well as Tethys.
2. Geological setting
Recent investigations including extensive geological mappinghave led to the inference that the central Qiangtang metamorphicbelt is a tectonic mélange comprising garnet–phengite–quartz schist,marble, quartzite, pillow basalt, gabbro, eclogite, garnet blueschists,garnet amphibolite and minor chert (e.g., Kapp et al., 2003; Li et al.,2006a, 2006b; Lu et al., 2006; Pullen et al., 2008; Zhai et al., 2007;Zhang et al., 2006a). This belt separated the warm-water faunas ofCathaysian affinity (similar to the Southern China block) in the northfrom the cold-water faunas and glaciomarine deposits of Gondwanaaffinity in the south (e.g., Fan, 1985; Li, 1987; Li and Zheng, 1993;Wang andMu, 1983). Thus, this belt has been regarded as a prominentpaleo-Tethyan suture zone, the Longmu Co-Shuang Hu suture zone(LS in Fig. 1), within northern Tibet (e.g., Li, 1987; Li and Zheng, 1993;Zhang, 2001; Zhang et al., 2006b). Recent Lu–Hf isotopic dating of theeclogite from Gemo area within this belt (Fig. 1) yielded an age of244 ±11 Ma, which was interpreted as the age of high-pressuremetamorphism within the central Qiangtang metamorphic belt (e.g.,Pullen, et al., 2008; Zhang and Tang, 2009). In addition, late Permian toearly Triassic basalts and andesites, as well as extensive late Triassicgranitoids, rhyolites and dacites have been recently identified withinthe southern and northern Qiangtang blocks, respectively (Fig. 1 andreferences listed in the legend). Some of the late Triassic granites,rhyolites and dacites are peraluminous and were generally attributedto substantial crustal remelting within northern Tibet (e.g., Huang et
41Y. Liu et al. / Lithos 127 (2011) 39–53
al., 2007; Kapp et al., 2003; Xiong et al., 2006; Zhai and Li, 2007). Someof these rocks show adakitic affinity, and were interpreted as theproducts of partial melting of thickened crust (e.g., Pullen et al., 2008;Xiao et al., 2007; Zhang et al., 2006c).
Within the present study area in Rongma (Figs. 1 and 2), thepresence of blueschist has been known for a long time (Hennig, 1915).Recent geological mapping in the scale of 1: 250,000 has confirmedthe occurrence of the blueschist reported in early studies (e.g., Kapp etal., 2003; Li, 1987; Pullen et al., 2008) and show that mafic,sedimentary and blueschist-bearing metamorphic slices outcropwithin the Rongma area (Fig. 2). The mafic slice consists of pillowlava and gabbro, whereas the sedimentary slice includes Ordovician toDevonian meta-siltstone, phyllite and minor marble, late Carbonifer-ous glaciomarine siltstone and sandstone, Permian limestone, lateTriassic conglomerate and sandstone as well as Tertiary red sandstoneand conglomerate (Li et al., 2006b). Within the late Carboniferousglaciomarine deposits, meta-basalts with a U–Pb age of 312 Ma areobserved (Li et al., 2006b, Fig. 2). Rhyolite and dacite generallyoutcrop within the late Triassic sedimentary rocks (Li et al., 2006b). Inaddition, early Triassic pyroxene diorite dykes and late Triassicgranitoids intrude into the late Carboniferous sedimentary rocks(Fig. 2).
The blueschist-bearingmetamorphic slices comprisemetamorphicmafic rocks andmetasedimentary rocks. The former normally occur aslenses of various sizes in the latter (Fig. 3a). The metamorphosedmafic rocks include blueschist, garnet blueschist and garnet biotiteamphibolite, whereas the metasedimentary rocks are dominated bygarnet phengite quartz schist (Fig. 3a). Within large mafic lenses, thecore portions are normally garnet blueschists, which are typicallyrimmed by garnet biotite epidote amphibolites. Garnet-free blues-chists coexist with marble. The metamorphosed mafic slice isseparated from the other slices by low-angle ductile faults with anormal sense (Kapp et al., 2003; Li et al., 2006b). Within theblueschist-bearing metamorphic slice, Permian limestone occurs as
86°30´ 86°45´
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45
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42 20
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O-D
870903
Fig. 2. Sketch geological map of Rongma area, modified after Kapp et al. (2003), Li et al. (2(2008) and other age data after Li et al. (2006b).
a dome, which is also separated from the country rocks by normalfaults (Fig. 2), suggesting that the blueschist-bearing metamorphicrocks structurally overlie the Permian sedimentary rocks.
3. Analytical methods
Mineralogical and textural observations were made using aoptical polarizing microscope. Chemical compositions, X-ray map-ping and BSE images of minerals were acquired by a JEOL 8100electron microprobe (EMP) equipped with four wavelength disper-sive (WDS) spectrometers at the Institute of Geology, ChineseAcademy of Geological Sciences and Institute of Geology andGeophysics, Chinese Academy of Sciences, Beijing, respectively.The beam current and acceleration voltage were 10 or 20 nA and15 kV, respectively. Counting times for element concentrationswere 20 s at the peak and on the background. Natural and syntheticstandards were adopted for the calibration of the EMP. Matrixcorrections were made using the ZAF program supplied with theJEOL software. X-ray maps of garnet porphyroblasts and matrixminerals were prepared by stepwise movement of the thin sectionunder the electron beam of the microprobe and/or moving theelectron beam, respectively. These X-ray images were subsequentlyanalysed using computer software. Representative mineral compo-sitions are listed in Table 1. Mineral abbreviations in this study areafter Whitney and Evans (2010) except Ca–Na amphibole (Ca–NaAmp), Ca amphibole (Ca Amp) and sodic amphibole (Na Amp).
4. Petrology
In this study, we examined ten representative samples of themaficlenses including garnet blueschists, garnet biotite amphibolites andgarnet-free blueschist. We also investigated twenty samples of thegarnet phengite schists and marble from Rongma area, centralQiangtang under the microscope. The detailed petrological
87°00´33°00´
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220-223 MaAr-Ar
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O-D meta-siltstone, phyllite andminor marble
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late Carboniferous glaciomarinedeposits and metabasaltslate Permian limestone and sandstone
Permian pillow lava and gabbro
blueschist-bearing metamorphic complex
late Triassic conglomerate, sandstone,rhyolite and dacite
Tertiary red sandstone and conglomerate
late Triassic granitoids
mafic intrusionsnormal fault; red circuleon hanging wallsense-unknown fault
thrust fault; arrow showsfootwall
sample for isotopic dating
Lu-Hf223.4±4.5Ma
Up shows isotopic datingresult and bottom isdating method
garnet blueschist for this study
006b), Pullen et al. (2008) and our own observations. The Lu–Hf age after Pullen et al.
a bNN
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GGrrtt
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EEpp
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CChhlll
Chl
d
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Ep
BtChl
garnet phengitequartz schistgarnegarnet phengitphengitequartquartz schisschist
garnet-bearingmafic lensgarnet-bearingmafic lens
Fig. 3. (a) Field photograph showing a garnet glaucophane blueschist (garnet-bearing mafic rock) occurring as a large lens in the garnet phengite quartz schists. (b) Photomicrographof porphyroblasts and matrix in the studied garnet glaucophane blueschist. (c) Photomicrograph of the studied garnet core shown in (b) showing numerous fine-grained inclusionsin the garnet core portions. (d) Photomicrograph of the studied garnet glaucophane blueschist showing coarse-grained chlorite in the matrix was rimmed by fine-grained biotite.
Table 1Representative mineral compositions in the garnet blueschist from Rongma, northern Tibet.
Garnet Na amphibole Na–Ca amphibole White micas Epidote
Core Core rim #1 #2 #3 #4 #5 #6 Core Rim #7 #8 #9 Core rim
SiO2 36.95 37.32 37.77 54.52 54.03 55.00 48.50 50.75 53.94 51.49 50.14 47.22 47.42 36.30 36.06 37.35TiO2 0.20 0.17 0.09 0.03 0.06 0.11 0.00 0.08 0.00 0.24 0.16 0.2 0.02 0.10 0.11 0.20Al2O3 19.54 19.76 20.70 6.38 6.85 10.80 5.73 4.10 1.74 24.33 26.18 27.51 38.71 21.95 22.41 25.32MnO 9.80 3.64 1.07 0.04 0.21 0.00 0.23 0.16 0.15 0.02 0 0.02 0.05 0.73 0.56 0.23FeO 24.67 29.25 30.93 15.76 17.91 10.66 19.73 16.48 14.77 4.18 4.06 6.44 1.06 13.10 12.57 9.67MgO 0.84 1.3 2.32 10.10 7.84 9.43 10.65 13.33 14.54 3.95 3.48 2 0.14 0.02 0.03 0.07CaO 7.72 8.59 7.74 1.22 0.68 0.36 6.68 9.46 9.37 0.02 0.05 0.05 0.26 21.99 21.63 22.24Na2O 0.04 0.02 0.04 6.52 6.83 7.22 3.31 2.34 1.95 0.24 0.55 0.47 5.65 0.00 0.01 0.01K2O 0.00 0.00 0.00 0.00 0.00 0.14 0.13 0.04 9.4 9.6 9.9 0.15NiO 0.00 0.00 0.00 0.00 0.00 0.03 0 0 0 0.00Total 99.76 100.07 100.66 94.57 94.41 93.58 94.97 96.83 96.54 93.87 94.22 93.81 93.46 94.19 93.38 95.09Si 2.996 3.002 2.997 7.846 7.893 7.876 7.179 7.377 7.776 3.495 3.402 3.275 3.063 2.981 2.977 2.992Ti 0.012 0.010 0.005 0.003 0.007 0.012 0.000 0.009 0.000 0.012 0.008 0.010 0.001 0.006 0.007 0.012Al 1.867 1.873 1.936 1.082 1.179 1.823 1.000 0.702 0.296 1.947 2.094 2.249 2.947 2.124 2.180 2.390Mn 0.673 0.248 0.072 0.005 0.026 0.000 0.029 0.020 0.018 0.001 0.000 0.001 0.003 0.051 0.039 0.016Fe3+ 0.122 0.106 0.067 1.001 0.858 0.285 1.495 0.878 0.693 0.900 0.868 0.648Fe2+ 1.550 1.862 1.985 0.896 1.330 0.991 0.947 1.125 1.087 0.237 0.230 0.374 0.057Mg 0.102 0.156 0.274 2.167 1.707 2.013 2.350 2.889 3.125 0.400 0.352 0.207 0.013 0.002 0.004 0.008Ca 0.671 0.740 0.658 0.188 0.106 0.055 1.059 1.473 1.447 0.001 0.004 0.004 0.018 1.935 1.913 1.909Na 0.006 0.003 0.006 1.819 1.934 2.004 0.950 0.659 0.545 0.032 0.072 0.063 0.708 0.000 0.002 0.002K 0.000 0.000 0.000 0.026 0.024 0.007 0.814 0.831 0.876 0.012Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0 0 0XFe 51.8 61.9 66.4XMg 3.4 5.2 9.2XMn 22.5 8.3 2.4Xca 22.4 24.6 22.0
The structural formula unitwas calculated from the garnet analysis based on 8 cations and 24 valencies. XFe=100Fe2+/(Fe2++Mg+Ca+Mn), XMg=100Mg /(Fe2++Mg+Ca+Mn),XMn=100Mn/(Fe2++Mg+Ca+Mn), XCa=100Ca/(Fe2++Mg+Ca+Mn). The structural formulaunit for amphibolewas computeredafter Leakeet al. (1997). The analyses forwhitemica and epidote were calculated to 21 valencies for the four- and six-fold coordinated cations, and 25 valencies, respectively. All Fe in the epidote are regarded as ferric iron. #1 Fe-richglaucophane as inclusion in garnet (Figs. 4 and 5b), #2, early Fe-rich glaucophane (Fig. 7), #3, late Fe-poor glaucophane around the early Fe-rich glaucophane (Fig. 7), #4, ferri-barroisite,#5, ferrian barroisite, #6, winchite, #7, low-Si phengite coexisting with chlorite and albite as a fine vein in the garnet (Fig. 5), #8, paragonite inclusion in the garnet (Fig. 4a and e), #9,epidote inclusion at garnet rims (Figs. 4a, f and 5b).
42 Y. Liu et al. / Lithos 127 (2011) 39–53
43Y. Liu et al. / Lithos 127 (2011) 39–53
observations in this study have confirmed the previous work (e.g.,Kapp et al., 2003) that the metamorphic rocks do not show any majorvariations in their mineral assemblages. Therefore, a representativegarnet blueschist (sample 870903, for location see Fig. 2) close to thesample 6-30-99-2D studied by Kapp et al. (2003) was selected for thedetailed petrological study and subsequent P–T isochemical diagrams
a
c dMn
Nae
500 um
500 um
500 um 5
Ep
Ep
Chl
ChlAb
Ab
Ep
I
II
III
Ep
500 um
500 um
500 um 5
Ep
Ep
Chl
ChlAb
Ab
EpEp
2
21
4150
60
80
31
13412010591
76614732
17
Fig. 4. BSE images (a) and X-ray maps (b–f) showing compositional pattern of a partially corrsolid red line in (a), electron microprobe analyses were performed as shown in Fig. 5, and lilarge size in Fig. 6(a) and (b), respectively.
in this study. The mineral assemblage in this sample comprisescorroded garnet porphyroblasts up to 2 mm in diameter set in amatrix of fine-grained amphibole, white micas, epidote, chlorite,albite, biotite, quartz as well as accessory rutile, titanite, calcite andapatite (Fig. 3b, c and d). Euhedral chlorite grains in the matrix arerimmed by fine-grained biotite (Fig. 3d).
b
f
Fe
Mg
Ca
500 um
500 um
00 um
500 um
500 um
00 um
148
346321
272
247
198173
297
223
250220190160131101714112
600
528457386
315248172101
30
oded garnet porphyroblast and its inclusions in the studied garnet blueschist. Along thested in Table 1. The dashed boxes marked I & II, in (a) and (e) refer to objects shown in
Ep
EpEp
Ep
Ep
Pg
Pg
PhPg
Rt
Rt
EpChl
Ab
Gln
Chl Ab
Ab
Ep
Qz
Gln
a
b
Ep
EpEp
Ep
Ep
Pg
Pg
PhPg
Rt
Rt
EpChl
Ab
Gln
Chl Ab
Ab
Ep
Qz
Gln
100 um
100 um
Fig. 5. BSE images showing inclusions in the garnet porphyroblast. The garnetporphyroblast was corroded by albite, chlorite, low-Si phengite (compositions inTable 1) and epidote. Within the garnet rims, the coarse-grained epidote inclusionscarry numerous inclusions of quartz, glaucophane and rutile.
pyrope almandine
spessartine grossular
0.8
0.7
0.6
0.4
0.5
0.3
0.2
0.1
Rim Core Rim
Fig. 6. Compositional profile for garnet porphyroblast from the Rongma blueschist. Forthe position of these analyses, see the red line marked in Fig. 4a. Representativeanalyses are listed in Table 1.
44 Y. Liu et al. / Lithos 127 (2011) 39–53
The garnet porphyroblasts display distinct core and rim domains(Fig. 3b and c). The core portions host numerous fine-grained inclusions,mainly Fe-rich glaucophane, chlorite, epidote, rutile and quartz (Table 1,Figs. 3b, c, 4a, e and 5). More importantly, fine-grained rhomb-shapedparagonite coexistingwith epidote and quartz is distinguishedwithin thecore portions (Table 1, Figs. 4a, e and 5a). These mineral aggregates withwell-preserved relict prismatic forms (Fig. 5) are interpreted here aspseudomorphs after lawsonite, following similar textures reported fromother areas (e.g., Ghent et al., 2009; Tsujimori et al., 2006;Wei et al., 2009).In contrast, the rim portions are more clean, and contain coarse-grainedepidote inclusions (Figs. 3b, c, 4 and5).Within the coarse-grainedepidote,fine-grained inclusions of Fe-rich glaucophane, quartz and rutile are alsoobserved (Fig. 5b). However, such fine-grained aggregates of epidote andparagonite are absent in the rim portions (Figs. 4 and 5). Therefore,microstructural observations in this study provide solid evidence to inferthat the transition from garnet core to rim marks a metamorphictransformation from lawsonite- to epidote-stability field. The garnetporphyroblasts are further corrodedbyveins offine-grained aggregates ofchlorite, epidote, albite and low-Si phengite (Figs. 3b, c, 4 and 5 andTable 1).
The garnet porphyroblasts are concentrically zoned (Figs. 4 and 6 andTable 1). The core with numerous inclusions ranges in composition fromAlm52–53Grs21–23Prp3–5Sps22–24 to Alm61–64Grs22–25Prp4–8Sps8–5,whereasthe rim portions have compositions of Alm65–67Grs22–24Prp8–10Sps1–2(Table 1). The compositional zoning in the garnet is thus characterized by(1) Mn enrichment in cores which decreases toward rims; (2) littlechange in Ca from core to rim; and (3) general increase of Fe andMg fromcore to rim (Table 1, Figs. 4 and 6).
The matrix amphibole in these blueschists grew in several stagesas inferred from the two different glaucophane compositions in thecore, and Ca–Na amphibole coexisting with albite and chlorite in the
rim (Fig. 7). The second generation of glaucophane (Gln II)surrounding the early glaucophane (Gln I) is poorer in Fe and richerin Al than the early one (Table 1and Fig. 7). Moreover, the early Fe-rich glaucophane (Gln I) has the same composition as that of theinclusion phase within garnet (Table 1). The Ca–Na amphibole is alsoenriched in Fe compared with the late glaucophane (Gln II) and it is abarroisite according to the classification of Leake et al. (1997) (Table 1and Fig. 7).
The potassic white mica in the matrix exhibits a weak composi-tional zoning. From core to rim, the Si per formula unit (pfu) of themica decreases from 3.49 to 3.40 (Table 1). Within the vein consistingof chlorite and albite, the Si pfu of the mica is, however, as low as 3.27(Table 1, Fig. 5). The epidote in the matrix shows concentric zoningranging from 0.868 Fe3+ pfu to 0.648 Fe3+ pfu (Table 1). The rutile inthe matrix is partially replaced by titanite. Secondary calcite veins arealso observed.
From themineral assemblages andmicrostructural characteristics ofthe Rongma garnet blueschists, we identify four metamorphic stages(Table 2). Stage I is characterized by the core domains of the garnetswhich show enrichment in Mn and host inclusions of quartz, rutile,chlorite and former lawsonite (now replaced by epidote and para-gonite). Phengite with the highest Si content belongs probably to thisstage. Garnet rim portions enriched in Fe and Mg, epidote with thehighest Fe3+ content, phengite of the high Si content, rutile, chlorite andquartz belong to stage II. The matrix Fe-poor and Al-rich Na amphibole,epidotewith high Fe3+ content, phengite, rutile, chlorite and quartz areattributed to stage III. The late minerals of Ca–Na amphibole, albite,titanite, quartz, low-Si phengite, low-Fe3+ epidote, calcite, chlorite andthe late stage biotite are related to stage IV (Table 2).
5. Pseudosection modelling
5.1. Model system and approaches
Pseudosection modelling is currently regarded as one of the mostpowerful approaches to acquire thermobarometric information onmetamorphic rocks because of this technique providing a frameworkto interpret both textural information and mineral compositions interms of P–T history (e.g., Powell and Holland, 2008). Pseudosectionmodelling for oxidizedhydrous systems, such as lawsonite- and epidote-bearing blueschist, has been greatly advanced by recent improvementsin solid-solutionmodels for Fe3+- and/orhydrousminerals, suchas sodicpyroxene (Green et al., 2007), amphibole (Diener et al., 2007) as well asbiotite (Tajcmanová et al., 2009). The knowledge of metamorphicevolution of lawsonite- or epidote-bearing blueschists or eclogitesworldwide has been therefore improved greatly (e.g., Groppo et al.,
GGlnI
GlnII
GlnI
GlnI
GlnII GlnII
Bar
Ab
BarChl
Ab
20um20 um20 um
a
(d
Na
80757065615651474237332823191495
Na
230217204191179166153140128115102897764513826
Fe
807570655550454035302520151050
Ca
20 um
20 um30 um
b
c d
Fig. 7. BSE image (a) and X-ray maps (b–d) showing zoning of amphibole in the studied garnet blueschist. The amphibole exhibits two stages of glaucophane, rimmed by late Na–Caamphibole, albite and chlorite. Gln I: early glaucophane, Gln II: late glaucophane.
45Y. Liu et al. / Lithos 127 (2011) 39–53
2009; Groppo and Castelli, 2010; Wei et al., 2009). We have thereforeused this approach to derive detailed information on the metamorphicevolution of the garnet blueschist sample 870903 from Rongma areawithin northern Tibet because it hosts abundant Fe3+- and/or OH−-bearing minerals, such as lawsonite pseudomorph, epidote and Fe3+-rich sodic amphiboles (Tables 1 and 2). A fixed bulk-rock compositionwas used to model the P–T pseudosection using the free energyminimization approach with the Perplex software package (Connolly,1990; 2009, updated April, 2010), which includes the newest solidsolution dataset and a suite of programs for calculating phase diagrams
Table 2Summary of metamorphic stages of the garnet blueschist from Rongma, northern Tibet.
Minerals Stage I Stage II
Garnet Alm52→64Grs21-23Prp4→8Sps23→5 Alm64→67Grs2Pyroxene Sodium clinopyroxene Sodium clinopyrLawsonite + (pseudomorph)Apatite + +Quartz + +White mica 3.49 Si p.f.u. 3.40 Si p.f.u.Amphibole Fe-rich glaucophane ?Epidote 0.900 Fe3+ p.f.u.Ti-minerals Rutile RutileAlbiteChlorite + +BiotiteCarbonate
The compositions of garnet and albite refer to the molar contents of end-members. +, present
and thermodynamic equilibria. Fluid phase for this modelling wasassumed tobepureH2O.Moreover, thepureH2Owas considered to be inexcess because water saturation is essential to consider lawsonitestability (Clarke et al., 2006). Additionally, SiO2 was always assumed tobe in excess in the modelling. Therefore, the P–T pseudosection for thesample 870903 was calculated in the closed system K2O–Na2O–CaO–MgO–MnO–FeO–Al2O3–TiO2–SiO2–H2O–O in the P–T range 0.6–3.0 GPaand 450–650 °C, using internally consistent thermodynamic dataset ofHollandandPowell (1998; revised 2002) forminerals and aqueousfluid.The Compensated Redlich-Kwong fluid equation of state (Holland and
Stage III Stage IV
1-23Prp8→10Sps5→1oxene
++ +3.40 Si p.f.u. 3.27 Si p.f.u.Fe-poor glaucophane Winchite Barroisite0.868 Fe3+ p.f.u. 0.648 Fe3+ p.f.u.Rutile Titanite
An=0–2+ +
+Calcite
, p.f.u., per formula unit; ? unsure mineral.
Table 3Effective bulk compositions used for calculation of pseudosection (wt.%).
SiO2 Al2O3 TiO2 Fe2O3 FeO CaO MgO K2O Na2O MnO P2O5 LOI Total O
Original composition 57.70 12.09 1.13 5.23 6.23 5.68 5.29 0.58 2.78 0.11 0.20 2.33 99.34Entire rock (a) 59.60 12.48 1.16 11.30 5.87 5.47 0.60 2.87 0.11 0.50Garnet core isolated (b) 59.90 12.41 1.17 11.30 5.85 5.51 0.60 2.90 0.02 0.50
The entire rock (a) is an unfractioned bulk composition, acquired by using the original composition minus the contents of loss of ignition (LOI) and P2O5, and then normalized to 100.The garnet core isolated (b) is acquired by the entire rock (a) subtracting the garnet core compositions. For further information see text.
46 Y. Liu et al. / Lithos 127 (2011) 39–53
Powell, 1991; 1998)wasusedduring thepseudosectionmodelingwith aview to employ themost updated version. The phases considered in thecalculations and references to the solid solution models used arefollowing: biotite (Tajcmanová et al., 2009), garnet, chlorite, epidote andtalc (Holland and Powell, 1998), sodic pyroxene (Green et al., 2007),mica (Auzanneau et al., 2010; Coggon and Holland, 2002), plagioclase(Newton et al., 1980), stilpnomelane (Massonne and Willner, 2008),clinoamphibole (Diener et al., 2007), ilmenite and magnetite (AndersenandLindsley, 1988). It shouldbepointedout that a small amountof ferriciron occurs in garnet (Table 1), Fe3+-free solutionmodel of Holland andPowell (1998) was, however, preferably used for the pseudosectioncalculations with respect to the Fe3+-bearing model of White et al.(2007) in this study. The latter model was developed in the peliticNCKFMASHTO system. Authors themselves emphasized that theintroduction of the endmember thermodynamics and a–x relationshipsfor spessartine probably gave unreliable results if used in conjunctionwith their model. As garnet cores in the studied sample 870903 areconsiderably richer in Mn than in Fe3+, the influence of Fe3+ on thestabilization of garnet was, therefore, neglected in this study.
5.2. Fractionation effects on the bulk-rock compositions
The whole-rock composition of the sample 870903, for whichthe pseudosection was computed, was determined by the tradi-tional wet chemical titration method at Langfang, China and theresults are listed in Table 3. However, as mentioned above, insample 870903, the zoned garnet porphyroblasts are enclosed in afine-grained matrix (Figs. 3b, c and 4). This sample, therefore, hadprobably experienced a progressive chemical fractionation as aresult of preferential sequestration of some elements in the garnetcore during porphyroblast growth, as also shown by previousstudies from other areas (e.g. Groppo et al., 2009; Marmo et al.,2002; Wei et al., 2009). A single pseudosection calculated on thebasis of the bulk-rock composition is, therefore, inadequate tomodel the entire evolution of the rock (e.g., Groppo et al., 2009;Wei et al., 2009). The fractionation effects on the bulk compositionof the sample 870903 should be calculated following theapproaches proposed by Groppo and Rolfo (2008). As mentionedabove, the garnet porphyroblasts possess distinct core and rimportions (Figs. 3b, c and 4). Image analysis has been used tocalculate the modal abundances of garnet cores at 1 vol.%, and then,the average compositions of garnet core was acquired based on themicroprobe analyses (Table 1). The entire bulk-rock compositioncorresponds to the equilibrium composition during the progradegrowth of the garnet core, whereas the bulk-rock compositionminus the garnet core composition (Table 3), which gives theequilibrium composition during the growth of garnet rim. Twopseudosections have been modelled for the two equilibria, and theresults are shown in Fig. 8.
Fig. 8. (a) P–T pseudosection for the sample 870903 calculated in the closed system K2Ocomposition (Table 3). Amp in this pseudosection is Ca-Na amphibole. Ca-Amp is Ca amphiborespectively. For further information see text. (b) P–T field of phengite contoured with isoplethe XFe, XMg and XMn. XFe=100Fe2+/ (Fe2++Mg+Ca+Mn), XMn=100Mn/(Fe2++Mg+composition (Table 3). (c) Compositional isopleths for garnet rim portions, using the bulkcontoured with isopleths for Si a.p.f.u., as well as modal content of garnet shown by isopleth
5.3. Estimation of oxygen content in the modelled bulk compositions
In order to perform the pseudosection modelling at theaforementioned system, the FeO(total) and O contents of thesample 870903 should be known first. In this study, the originalFe2O3 content of the whole rock had been converted into FeO andO contents as following: FeO (total)=FeO+0.8889⁎Fe2O3 andO=0.1111⁎Fe2O3, respectively (Table 3). The O content here isperhaps an over-estimate because this sample was affected bysurface weathering processes including oxidation. In addition,post-crushing and pre-analysis oxidation may also be significant.Nevertheless, it places a maximum value on the range of possibleO content of the rock during metamorphism at least. In addition, inorder to explore the effect of variable O content on the phaserelations, two T-X(O) pseudosections have been calculated at17 kbar and 20 kbar, 450–650 °C, respectively, following themethods of Connolly (2005). The results are given in Fig. 9.
5.3.1. Results for pseudosection modelling
5.3.1.1. Pseudosection (a): unfractionated bulk composition. Theunfractionated bulk composition (Table 3) was used to model theprograde evolution of the sample 870903, which corresponds to thegrowth of garnet cores. The calculated pseudosection and garnetcompositional isopleths are shown in Fig. 8a and b, respectively. Themodelled mineral assemblages and compositions at specific P–Tconditions are listed in Table 4. The XFe, XMn and XMg isopleths ofgarnet cores modelled in the pseudosection intersect each others in anarrow P-T region (Fig. 8b). Moreover, the modelled Si (a.p.f.u.)contents of phengite in the omphacite+chlorite+phengite+garnet +lawsonite+rutile+quartz field are in the range of 3.4 and3.5 (Fig. 8b and Table 4), coherent with the measured Si contents inphengite of stage I (e.g., 3.49 a.p.f.u., Tables 1 and 2). These clearly reflectthat our computed results are reasonable. The garnet cores, therefore,mainly grew in the lawsonite stabilityfield fromT=480 °C, P=19 kbar,through T=490 °C, P=19.5 kbar, and to T=510 °C, P=19 kbar(Figs. 8a and b). The chlorite, Fe-rich sodic pyroxene, lawsonite,phengite, rutile and quartz are stable at these P-T conditions (seeTable 4), which are compatible with the assemblage inferred for thestage I (Fig. 8and Table 2). These minerals could have been trapped inthe coreduring thegrowthof thegarnet (Figs. 3b, c, 4 and5). It shouldbepointed out that Fe-rich glaucophane has not been modelled at thisstage (see Fig. 8and Table 4), although it occurs in the core of garnet(Figs. 3c, 4 and 5b). TheNa-pyroxene, predicted to be stable at these P–Tconditions, has not beendistinguished yet.We, thus, postulated that theFe-rich sodic amphibole (Gln I) is a relict phase formed prior to stage I. Itis older than garnet core portions. The sodic pyroxene was successivelyreplaced by Ca–Na amphiboles or, more likely, by albite, chlorite andepidote.
–Na2O–CaO–MgO–MnO–FeO–Al2O3–TiO2–SiO2–H2O–O, using the unfractionated bulkle. 90%, 70%, 50%, 30% and 10% greys are fields of six, seven, eight, nine and tenminerals,ths for Si a.p.f.u. in phengite, as well as garnet core portions contoured with isopleths forCa+Mn) and XMg=100 Mg/(Fe2++Mg+Ca+Mn), using the unfractionated bulkcompositions from which garnet cores have been subtracted (Table 3). (d) Phengites, using the bulk compositions from which garnet cores have been subtracted (Table 3).
Amp Ep Ph Pg Grt Rt QzAmp Ep Ph Pg Grt Rt Qz
Amp Ep Ph Grt Rt QzAmp Ep Ph Grt Rt Qz
2
Amp Ep Pl Ph Ilm
Grt Rt Qz
Amp Ep Pl Ph Mt Grt Rt Qz
Bt Ca-Amp Pl
IlmMt Grt Qz
Bt Ca-Amp Pl
IlmMt Grt Qz
Bt Ca-Amp Pl IlmMt Grt Rt Qz
Bt Ca-Amp Pl Mt Grt Rt Qz
Bt Ca-Amp Pl Mt Grt Rt Qz
zQtRtrGtMlPpEpmAtB
Bt Amp Ep Pl P
h Mt Grt Rt Qz
3
4
Omp Amp EpPh Grt Rt QzOmp Amp EpPh Grt Rt Qz
6
Amp Chl Ep Ph Pg Grt Rt Qz
Bt Amp EpPh Grt Rt Qz
Bt Amp EpPh Grt Rt Qz
8
Bt Amp Chl EpPl Mt Grt Rt Qz
1011
Bt Amp Chl Ep
Ph Grt Rt QzBt Amp Chl Ep
Ph Grt Rt Qz
Bt Amp ChlEp Ab Rt QzBt Amp ChlEp Ab Rt Qz
12
13
12
13
14 1517
18
1920
22
21
27
25
26
16
28
29
30
31
490 570 650610530
8.4
10.8
13.2
15.6
18
6
Omp Chl Ph Grt Lws Rt QzOmp Chl Ph Grt Lws Rt Qz
Omp Chl E
p Ph GrtLws Rt Q
z
Omp Chl Ep PhPg Grt Rt Qz
zQt
Ry
KtrG
hP
pm
Op
mO
hP
trG
yK
tR
zQ
Omp Amp PhGrt Ky Rt QzOmp Amp PhGrt Ky Rt Qz
Omp Tlc PhGrt Ky Rt QzOmp Tlc PhGrt Ky Rt Qz
3
33
Om
pE
pT
lcP
hzQt
Ry
Ktr
GO
mp
Ep
Tlc
Ph
Grt K
yR
t Qz
Omp Tlc Ph Grt Lws Rt Qz Omp Tlc Ph Grt Ky Lws Rt Qz
5
6
78
Om
pChl Ep
PhzQt
RyKtr
G
10
9
Omp Amp Ep Ph Pg Grt Rt Qz
Omp Amp Ep Ph Grt Ky Rt Qz
20.4
22.8
Omp Chl EpPh Pg Rt QzOmp Chl EpPh Pg Rt Qz
P (kbar)
T (oC)
T (oC)
24
11
2
1
23
32
11
25 Am p Ep Pl Ph Ilm Mt Gr t Rt Qz26 Bt Am p Ep Pl Ph Ilm Mt Grt Rt Qz27 Bt Ca-Amp Ch l Pl Ph Mt Gr t Rt Qz28 Bt Am p Ch l Ep Gr t Rt Qz
2 Om p Ch l Ep Ph Gr t Ky Lw s Rt Qz
1 Om p Ch l Ep Ph Pg Gr t Lw s Rt Qz
3 Om p Am p Tl c Ph Gr t Ky Rt Qz4 Om p Am p Ch l Ep Ph Gr t Ky Rt Qz
10 Om p Ch l Ep Tl c Ph Gr t Ky Rt Qz
5 Om p Ep Tl c Ph Gr t Ky Lw s Rt Qz6 Om p Ch l Tl c Ph Gr t Lw s Rt Qz7 Om p Ch l Ph Gr t Ky Lw s Rt Qz8 Om p Ch l Ph Gr t Ky Lw s Rt Qz9 Om p Ch l Ep Tl c Ph Gr t Ky Lw s Rt Qz
13 Am p Ep Pl Ph Gr t Rt Qz
15 Bt Am p Ep Pl Ph Ilm Gr t Rt Qz
12 Am p Ch l Ep Ph Gr t Rt Qz11 Om p Am p Ch l Ep Ph Pg Gr t Rt Qz
18 Bt Ca-Amp Ch l Pl Mt Gr t Rt Qz17 Bt Ca-Amp Pl Ph Mt Gr t Rt Qz
14 Bt Am p Ep Pl Ph Gr t Rt Qz
16 Bt Am p Ch l Ep Pl Ph Mt Gr t Rt Qz
20 Bt Am p Ch l Ep Mt Gr t Ab Rt Qz19 Bt Am p Ch l Ep Pl Mt Gr t Ab Rt Qz
22 Bt Am p Ch l Ep Gr t Ab Rt Qz21 Bt Am p Ch l Ep Mt Ab Rt Qz
23 Om p Ch l Ep Ph Pg Gr t Ky Rt Qz24 Om p Am p Ep Ph Pg Gr t Ky Rt Qz
29 Bt Am p Ch l Ep Mt Gr t Rt Qz30 Bt Am p Ch l Ph Ep Gr t Rt Qz31 Bt Am p Ch l Ph Ep Mt Gr t Rt Qz32 Om p Ch l Ep Ph Pg Gr t Ky Lw s Rt Qz33 Om p Am p Ch l Ep Ph Pg Gr t Ky Rt Qz
4
a
b
5
7
44
4
48
50
7
86
15
1752
19
21
23
17
25
27
52
5458
Garnet out
95
862
58
64
3.3
3.4
3.53.6
490 530 570
P (
kbar
)
25.2
22.8
20.4
18
XMn with values15
XFe with values
XMg with values8
58
garnet core
3. 3 Si p.f.u. of phengite
with values
47Y. Liu et al. / Lithos 127 (2011) 39–53
2
4 6
8
10
1214
55
50
65
60
65
45
4035
8
1010
60606
1418
5
43
2
1
Garnet out
Garnet out
8
12
XMn with values5
XFe with values
XMg with values8
60
garnet rim
18
15.6
13.2
10.8
8.4
6490 530 570 610 650
T(oC)
490 530 570 610 650T(oC)
P (
kbar
)P
(kb
ar)
18
15.6
13.2
10.8
8.4
6
4.52.7
0.9
7.2
9.010.7 12.5
3.4
3.3
phengite out
phengite out
3.1
3.2
3.2
3.0
3.0
3.2
3.1
3.0
3.0 Si (p.f.u.) of phengite with values
2.7garnet modal content (vol%) withvaluesP-T field of garnet rimP-T path for the retrograde stage
garnet out
garnet out
c
d
Fig. 8 (continued).
Fig. 9. T-X(O) pseudosection for the sample 870903 calculated at 20 kbar (a) and 17 kbar (b), respectively, using the unfractionated bulk composition (Table 3). 90%, 70%, 50%, 30%and 10% greys are fields of five, six, seven, eight and nine minerals, respectively.
48 Y. Liu et al. / Lithos 127 (2011) 39–53
49Y. Liu et al. / Lithos 127 (2011) 39–53
Table 4Calculated modemineral compositions at the given P–T conditions. The results relate tothe P–T pseudosection of Fig. 8.
Phase Content/vol.%
Composition (p.f.u. or molar fraction)
480 °C, 1.9 GPaChlorite 22.22 Si: 2.99, Mg: 2.78, Fe2+: 2.19, Mn: 0.01Phengite 5.28 Si: 3.44, Mg: 0.28, Fe2+: 0.17, Na: 0.01Garnet 0.55 XFe: 52.4, XMg: 4.97, XMn: 21.3Na-clinopyroxene 27.51 Na: 0.63, Fe3+: 0.34, Fe2+: 0.16, Si: 2.00, Al:
0.29, Ca: 0.37, Mg: 0.21Lawsonite 14.55Rutile 0.80Quartz 29.08
490 °C, 1.92 GPaChlorite 21.99 Si: 2.99, Mg: 2.81, Fe2+: 2.17Phengite 5.31 Si: 3.42, Mg: 0.26, Fe2+: 0.16Garnet 0.88 XFe: 56.9, XMg: 5.8, XMn: 15.6Na-clinopyroxene 27.6 Na: 0.63, Fe3+: 0.34, Fe2+: 0.16, Si: 2.00, Al:
0.29, Ca: 0.37, Mg: 0.21Lawsonite 14.3Quartz 29.12Rutile 0.8
540 °C, 1.68 GPaEpidote 7.89 Fe3+: 0.57Chlorite 20.16 Si: 2.97, Mg: 3.01, Fe2+: 1.96Phengite 5.71 Si: 3.24, Mg: 0.15, Fe2+: 0.09, Na: 0.1Paragonite 4.35 Si: 3.00, Na: 0.97, Al: 3.00Garnet 3.49 XFe: 66.0, XMg: 9.0, XMn: 0.9Na-clinopyroxene 26.87 Na: 0.58, Fe3+: 0.28, Fe2+: 0.17, Si: 2.00, Al:
0.30, Ca: 0.42, Mg: 0.25Quartz 30.70Rutile 0.83
Mineral composition data were re-calculated by considering: a) garnet: XFe, XMg, andXMn values; b) chorite: all iron contents attributed to the daphnite end member;c) epidote according to Fe3+ content. d) white micas and Na-clinopyroxene: cations performula unit.
50 Y. Liu et al. / Lithos 127 (2011) 39–53
5.3.1.2. Pseudosection (b): unfractionated bulk composition minus thegarnet cores. The whole-rock composition subtracting the garnetcores (Table 3) was used to model the growth of garnet rim. Thetopology of this pseudosection is very similar to that of pseudosec-tion (a), and thus, the details of the pseudosection (b) are notpresented in this study. The XFe, XMn and XMg isopleths of garnetalso intersect in a narrow P–T region (Fig. 8c, Table 4), suggestingthat the garnet rim grew at T=535 °C and P=16.8 kbar. At the P–Tconditions, omphacite, chlorite, epidote, paragonite, rutile, garnet,quartz are stable, whereas lawsonite is not stable (Fig. 8a, Table 4).These could account for the inclusions of coarse-grained epidoteand the absence of lawsonite within the rim portions of garnet.Therefore, the pseudosection modelling shows that the growth ofgarnet in the garnet blueschist marks the transition from law-sonite- to epidote-stability field, which is consistent with thepetrological observations mentioned above (Figs. 3b, c, 4 and 5).
5.3.2. Retrograde stageThe strong overprint at stage III likely coincided with the eclogite to
blueschist transformation which probably occurred during an event ofdecompression at temperatures of about 530 °C (Figs. 8a and d). The P–Tpath for this stage crosses the garnet isomodes toward decreasing values(Fig. 8d), suggesting that the garnet did not grow during the retrogradestage (Fig. 8d). In addition, Na–pyroxene was totally decomposed andreplaced by Ca–Na amphibole, albite, epidote and chlorite (Fig. 8a). Thesetransformations could account for the complete disappearance of sodicpyroxene, the formation of new Ca–Na amphibole around the earlyglaucophane aswell as chlorite, albite and epidote surrounding the garnetin the studied rock (e.g., Figs. 3, 4, 5 and 7). The blueschist facies overprintcould have proceeded to pressures down to 0.8 GPa or even less, toaccount for the decrease of the phengite Si content (Tables 1, 2 and 4,
Fig. 8d), the increasing contents of Ca-Na amphibole and the presence ofbiotite around chlorite (Fig. 3d).
5.3.3. T-X(O) pseudosection modelling resultsThe unfractionated bulk composition (Table 3)was used to derive two
T-X(O) pseudosections (Figs. 9a and b). The stability field of lawsonite-bearing assemblage is relatively insensitive to O content, as shown inFigs. 9a and b,whereas the stability field of epidote-bearing assemblage isrelatively sensitive to oxygen contents (Figs. 9a and b). At 20 kbar, theepidote-bearing assemblage occur at a narrow range of temperaturebetween 550 and 570 °C and high oxygen contents (X(O)N0.4 wt.%)(Fig. 9a). At low pressure this Fe3+-bearing mineral occur at a slightlylarger range of temperature andO content (Fig. 9b). This suggests that thestability field of epidote-bearing assemblage was enlarged along theregion of decreasing pressure (see Figs. 9a and b). Moreover, the maineffect of decreasingO content is the progressive disappearance of epidote,suggesting that temperature of equilibrium for stage II decreases slightlyto about 510 °C along with the decrease in O content (Fig. 9b). The P–Tconditions inferred for stage II, should therefore be considered asmaximum estimates in this study. In addition, the Fig. 9b, furtherconstrains on the minimum possible equilibrium temperature for stage II(ca. 510 °C) and minimum O content (ca. 0.21 wt.%) are also inferred.
6. Discussion
The P–T field of blueschist facies rocks is bounded on the low-Pside by greenschist and pumpellyite–actinolite facies, on the high-Tside by epidote–amphibolite facies, and the high-P and high-T sidesby eclogite facies, respectively (see Ota and Kaneko, 2010 for a recentreview). Among the rocks belonging to these metamorphic facies,blueschists and eclogites manifest lithospheric plate subduction,because, these rocks, especially blueschists, require unusually coldupper mantle geotherms which are only found today in subductionzones (e.g., Kadarusman et al., 2010; Omori et al., 2009). Therefore,the space-time distribution of blueschist–eclogite belts have beenregarded as markers of paleo-subduction zones and are hence criticalin the context of tectonics associated with convergent plateboundaries, and the evolution of continents (Isozaki et al., 2010).
In the present study, the petrologic and microstructuralinvestigations of the garnet- and glaucophane-bearing blueschistfrom the Rongma area in northern Tibet, together with thermo-dynamic modelling and P–T estimation suggest that the rockexperienced distinct stages in the metamorphic cycle, passingfrom the lawsonite stability field (stage I) to the epidote stabilityfield (stage II) followed by lower P–T retrogression (stage III andIV) (Fig. 10, Table 2). The lawsonite- and epidote-bearing mineralassemblages (Table 2) belong to the lawsonite–eclogite andepidote–eclogite fields, respectively (Fig. 8a). The geothermalgradient for the lawsonite-stable and epidote-stable stages areabout 7.1 and 9.6 °C/km, respectively (Fig. 10). Such lowgeothermal gradients are typical for deep subduction of a coldoceanic crust along a continental margin. Regionally, the lawsoniteblueschist and epidote eclogite outcrop in the Hongjishan area (Luet al., 2006) and Gemo area (Li et al., 2006a, 2006b; Zhang et al.,2006a), respectively, to the west of the present study area (seeFig. 1). These units are further indicative of deep subduction of acold palaeo-oceanic crust between northern and southern Qiang-tang blocks (Fig. 1). Here we interpret the Lu-Hf age of 244±11 Ma from the epidote eclogite in Gemo area (Pullen et al., 2008)as the metamorphic age of epidote-stable stage (stage II) (Fig. 10).The Lu–Hf age 223.4±4.5 Ma of the garnet glaucophane withinthe present study area (Pullen, et al., 2008) is consistent withnumerous amphibole Ar–Ar age data (e.g., Kapp et al., 2003; Li etal., 2006b). All of these age data are, therefore, considered to markthe timing of retrogade metamorphism, or the exhumation ages ofthe garnet blueschist within the Rongma area (Fig. 2).
6
10.8
15.6
20.4
25.2
30
450 490 530 570 610 650
o
T (oC)
P (
kbar
)
o
amphibole out
amphibole out
Ca-Na amphibole
Ca amphibole
Na amphibole
stage I (7.1 C/Km)
stage II (244±11 Ma)9.6 C/Km
stage III ( 220 Ma)
Fig. 10. P–T evolution of the studied blueschist in the context of P–T field of amphibole, acquried in the pseudosection calculation of Fig. 8a. Age data for the P–T evolution are afterPullen et al. (2008), Kapp et al. (2003) and Li et al. (2006b). For further information see text. The classification of amphibole is based on the Na content of M4 site of amphibole. Nacontent=2-Ca content. If Nab0.5, then it is Ca amphibole. If 0.5bNa b1.5, then it is Ca–Na amphibole. If NaN1.5, it is Na amphibole.
51Y. Liu et al. / Lithos 127 (2011) 39–53
Several studies have revealed that lawsonite is a high-densityhydrous mineral (11 wt.% H2O) stable at high pressure and lowtemperature and the transformation from lawsonite-stability toepidote-stability fields leads to the release of water in the subduction
N
Paleo-Tethyan Ocean
N Qiangtangblock
+ ++ + +
+
+
+++
++
++
+ ++
+
+
++
++
++
a
+ +
+ ++
+ + +
ooo
+++
++
+
+
+
C
N
bQiangtang
oo
~~~~~~~~~++
++++
++
Qiangtang terrain
Fig. 11. A schematic cross-section displaying the tectonic evolution of northern Tibet from l(2009). (a) During Permo-Triassic times, the oceanic crust of the Palaeo-Tethys was subducteBangong-Nujiang Ocean in the south (Sengor et al., 1988). (b) During late Triassic, the southPalaeo-Tethyan Ocean resulting in thickening and partial melting of the lower crust. The fincentral Qiangtang as shown in the Fig. 1.
channel (e.g., Okamoto and Maruyama, 1999; Peacock and Wang,1999; Poli and Schmidt, 2002; Schmidt and Poli, 1994; Tsujimori etal., 2006; Wei et al., 2009). The presence of free water in thesubduction channel triggers the partial melting of mantle wedge
Amdo block
Bangong-Nujiang Ocean
S Qiangtangblock
+
+
onglomerateBangong-Nujiang Ocean
Oceanic crust
Accretionary wedgemetamorphic belt
oo o o o o
ate Palaeozoic to late Mesozoic times, modified from Sengor et al. (1988) and Liu et al.d between the northern and southern Qiangtang blocks triggered by the opening of theern Qiangtang block was accreted to the northern Qiangtang block after closure of theal collision and slab break off led to the extrusion of the metamorphic belt within the
52 Y. Liu et al. / Lithos 127 (2011) 39–53
above the subduction channel, and, thus generates island arc volcanicrocks (e.g., Maruyama et al., 2009; Peacock and Wang, 1999; Poliand Schmidt, 2002; Tsujimori et al., 2006; Wei et al., 2009).Moreover, T- X(O) pseudosection in this study has revealed thatthis mineral transformation occurred at high O content (Fig. 9). The latePermian to early Triassic andesites and dacites occurring within thenorthern and southern domains of the Qiangtang blocks (Fig. 1, andreferences listed in the legend to Fig. 1) could be related to meltsproduced by the cold subduction of oceanic crust (e.g., Peacock andWang, 1999; Wei et al., 2009). Therefore, our observations incombination with previous work confirm the subduction of an oceanplate between northern and southern Qiangtang blocks during latePermian to early Triassic times (Fig. 11). Thus, our study supports themodel of the central Qiangtang blueschist-bearing metamorphic beltbeing an in situ palaeo-Tethyan suture zone (e.g., Li and Zheng, 1993; Liet al., 1995; Zhang, 2001). Themodel of low-angle subduction proposedby Kapp et al. (2003) is, however, inconsistent with our newobservations. Furthermore, based on recent investigations (see Fig. 1,and references listed in the legend to Fig. 1), we present a modifiedmodel for the tectonic evolutionofnorthTibet in this study (Fig. 11).Ourmodel emphasized cold subduction of palaeo-oceanic crust beneath thenorth and south Qiangtang blocks, respectively, and thus, triggering offthe island arc volcanism within the north and south Qiangtang blocksduring late Permian to early Triassic times (Figs. 1 and 11).
Our results also carry important implications on the origin ofthe extensive late Triassic magmatism within northern Tibet.During the closure of the palaeo-Tethyan Ocean between northernand southern Qiangtang blocks in the Permo-Triassic times,triggered by the opening of Bongong-Nujiang ocean in the south(e.g., Groppo and Rolfo, 2008; Liu et al., 2009; Sengor et al., 1988),the high-pressure metabasites from Hongjishan, Gemo, Rongma,and Shuang Hu areas were generated by deep subduction andsubsequent exhumation of the palaeo-Tethyan oceanic crust(Figs. 1 and 11). At the same time, within the northern andsouthern Qiangtang blocks, subduction-related late Permian toearly Triassic andesites and dacites were emplaced (Fig. 11).Subsequently, the southern Qiangtang block was accreted to thenorthern Qiangtang block and probably underthrust leading tocrustal thickening. This process triggered extensive partial meltingof the lower crust to define the final lithological architecture of theQiangtang terrain during late Triassic times (Fig. 11). Meanwhile,the uplift of Qiangtang terrane and intense erosion generated thelate Triassic molasse sandstone (Fig. 11). These tectonic events areremarkably similar to those of the collision of India and Asiaforming the Himalaya during Cenozoic times.
7. Conclusion
The garnet blueschist from the Rongma area records a tectonichistory involving cold subduction of an palaeo-oceanic plate betweennorthern and southern Qiangtang blocks during Permo-Triassic times.This eventwas followedby the collisionbetweennorthern and southernQiangtang blocks, leading to crustal thickening and generation of melts,characterizing the final stage of the formation of the Qiangtang terrainduring the late Triassic times. The transformation of lawsonite-stabilityto epidote-stability field has occurred at high oxygen fugacity, implyingthat thewater released by thismineral transformationwas fundamentalto the generation of the arc magmas within north Tibet during Permo-Triassic times.
Acknowledgements
Profs. H.-J. Massonne, J.A.D. Connolly and I. Buick, as well as Dr. C.Groppo are greatly thanked for kind help with the thermodynamicmodelling. Review comments from Drs. Alicia Lopez Carmona and ChiaraGroppo, as well as valuable editorial comments and encouragement from
Prof. Ian Buick greatly improved the early versions of manuscript. Thiswork was financially supported by China Geological Survey(1212010818014, 1212011121271) and Institute of Geology, CAGS(J0816).
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