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The Luliangshan garnet peridotite massif of the North QaidamUHPM belt, NW China a review of its origin and metamorphicevolution

S. G. SONG,1 Y. L . NIU,2 L . F . ZHANG1 AND K. BUCHER31MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University,Beijing 100871, China ([email protected])2Department of Earth Sciences, Durham University, DH1 3LE Durham, UK3Institute of Mineralogy and Geochemistry, University of Freiburg, Albertstrasse 23b, D-79104 Freiburg, Germany

ABSTRACT The Luliangshan garnet peridotite massif is an ultramafic complex in the North Qaidam UHPM belt,NW China. The strongly layered complex comprising garnet-bearing dunite, garnet-harzburgite,garnet-lherzolite and garnet-pyroxenite and garnet-free dunite, occurs together with eclogite embeddedin various continental gneisses. The geological setting, the internal structure, bulk-composition, rareearth elements, isotopic and mineral composition data show that the garnet peridotite derives from amiddle Ordovician Alaskan-type layered sub-arc cumulate intrusion of ascending mantle wedge melts.An abyssal peridotite protolith can be excluded. During the Ordovician-Silurian continental collision,thickening and foundering, the Luliangshan peridotite complex was exposed to ultrahigh pressures(UHP) reaching 5.5 GPa possibly >6 GPa at temperatures of 900 C (perhaps up to 1000 C)corresponding to a depth of 200 km. The extreme pressure conditions have been derived fromthermobarometry using mineral compositions of the garnet peridotite assemblages, but they aresupported by a wealth of decompression-induced mineral exsolutions in UHP minerals and by diamondinclusion in zircon. The Luliangshan garnet peridotite has experienced four stages of retrogradeoverprint during exhumation that lasted into the Devonian: (i) decompression-induced unmixing of theUHP minerals; (ii) garnet kelyphitisation; (iii) amphibole overprinting and (iv) serpentinization.Hydrous minerals occurring within peak metamorphic assemblage represent pseudo-inclusions, that isreaction products of reactions related to various stages of decompression and cooling rather thanprograde inclusions during porphyroblast growth.

Key words: Alaskan-type ultramafic cumulates; continental subduction and exhumation; garnetperidotite; metamorphic evolution; North Qaidam.

INTRODUCTION

Garnet peridotites occuring together with eclogite ingneisses of the continental crust in orogenic belts arecharacteristic of continental-type deep subduction.They are widespread within ultrahigh-pressure meta-morphic (UHPM) belts resulting from continentalcollision (Medaris & Carswell, 1990), thickening andfoundering of continental material. This geodynamicsetting and their ultra-deep origin (>6 GPa) in someUHP terranes (Yang et al., 1993; Dobrzhinetskayaet al., 1996; Green et al., 1997; Bozhilov et al., 1999;van Roermund & Drury 1998; van Roermund et al.,2000, 2002; Song et al., 2004, 2005a,b; Zhang et al.,2000; Spengler et al., 2006) makes garnet peridotites animportant window to the geodynamic processes ofcontinental subduction, collision, exhumation andcrustalmantle interaction on a regional scale withglobal geodynamic implications.

The Luliangshan garnet peridotite massif is a largeultramafic body in the North Qaidam UHPM belt, a400-km-long continental collision belt of the EarlyPalaeozoic at the northern edge of the Tibetan Plateau.It was first reported by Yang et al. (1994) and furtherstudied by Song et al. (2004, 2005a,b, 2007) and Yang& Powell (2008). Mineral exsolution lamellae ofrutile + two pyroxene + sodic amphibole in garnetand ilmenite + Al-chromite in olivine (Song et al.,2004, 2005a) as well as the presence of a diamondinclusion in a zircon, suggest that this peridotite bodyexperienced ultraUHPM at depths in excess of 200 km.

New data are presented for the Luliangshan garnetperidotite that further support ultra-deep subductionof the peridotite and suggest a complex protolithassemblage including sub-arc continental lithosphericharzburgite and primitive arc melt cumulate, ratherthan serpentinized abyssal peridotite (Yang & Powell,2008).

J. metamorphic Geol., 2009, 27, 621638 doi:10.1111/j.1525-1314.2009.00848.x

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GEOLOGICAL SETTING

The North Qaidam UHP metamorphic belt is adistinctive continental subduction and exhumationorogenic belt of Palaeozoic age in Northwest China. Itextends from Dulan northwestward to Yuka for400 km, and it is offset for up to 400 km by the AltynTagh Fault, a large NE-striking sinistral strike-slipfault (Fig. 1). The fault cuts through blocks of latePrecambrian crystalline basement of similar age (e.g.Wan et al., 2001; Song et al., 2006). Further to thenorth is the Qilian terrane with an Early Palaeozoicoceanic-type suture (i.e. see Song et al., 2006, 2009aand references therein). The Qaidam block is locatedsouth of Precambrian basement units. It represents aMesozoic intercontinental basin underlain by a Pre-cambrian basement (e.g. Song et al., 2006).

The North Qaidam UHP belt mainly consists of atypical continental-type subduction-zone setting.Coesite and diamond inclusions in zircon and garnet

suggest UHP conditions that are similar to othercontinental-type UHPM terranes elsewhere in theworld (Yang et al., 2001, 2002; Song et al., 2003a,2005b). Zircon SHRIMP dating reveals that the HPand UHP metamorphic ages of the North QaidamUHP belt range from 460 to 400 Ma (Song et al.,2003b, 2005a, 2006; Mattinson et al., 2006, 2009;Zhang et al., 2008a,b; Chen et al., 2009). Conse-quently, parts of the Precambrian basement of theQaidam block have been subducted and metamor-phosed in the Upper Ordovician to Lower Devonianperiod.Within this Palaeozoic orogenic belt two distinct

types of peridotites can be distinguished: (i) the Luli-angshan peridotite complex (Figs 1 & 2) consists ofgarnet peridotite derived from ultramafic cumulatesfrom a continental arc and (ii) the Shaliuhe serpenti-nized harzburgite from UHP metamorphosed earlyPalaeozoic ophiolite sequences in the Dulan Terrane(Song et al., 2007, 2009b; Zhang et al., 2008a).

(b)

(a)

Fig. 1. (a) Geological overview and location map of the Luliangshan garnet peridotite body. (b) Geological map of the garnetperidotite (after Song et al., 2004).

6 22 S . G . S O N G E T A L .

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PETROGRAPHY, WHOLE-ROCK AND MINERALCOMPOSITIONS

The Luliangshan garnet peridotite forms a large massifbody 500800 m in size within an eclogite-bearingquartzofeldspathic gneiss terrane of the North QaidamUHPM belt. The peridotite is a layered ultramaficcomplex in which four rock types have been recognized(Song et al., 2005b, 2007): (i) garnet-free dunite; (ii)garnet-bearing dunite and harzburgite; (iii) garnetlherzolite and (iv) garnet pyroxenite.

Coarse-grained dunite comprises strongly serpenti-nized olivine (>95 vol%) with minor orthopyroxene(Opx) and Cr-rich spinel (Spl). It occurs as up to 10 mthick brown-coloured bands within black-colouredgarnet lherzolite and garnet-bearing dunite (Fig. 2a,b).Locally thin dunite bands alternate with garnet duniteand garnet lherzolite layers (Fig. 2c) on the scale ofsome centimetres. The fine banded structure resemblesthat of Alaskan-type ultramafic cumulates (Himmel-berg & Loney, 1995; Scheel et al., 2005).

Garnet-bearing dunite and garnet harzburgite with acoarse-grained equigranular texture contains 90%olivine (Fo0.9060.920) and variable amounts of garnet(Grt), Opx and clinopyroxene (Cpx). The rocks formlayers varying in thickness from 10 cm to 2 m withingarnet lherzolite or as rhythmic bands that graduallychange from garnet-bearing dunite to harzburgite togarnet lherzolite (Fig. 2c). Garnet is porphyroblasticand Mg-rich with 6975 mol% pyrope, 1118%almandine, 38% grossular, 0.82.0% spessartine and36% uvarovite. Fine-grained Cr-spinel occurs as aproduct of decompression-induced breakdown of ear-lier high-Cr garnet and pyroxene.

Garnet lherzolite clearly dominates with 7080vol.% the peridotite massif. The different peridotitesoccur as band and layers of varying thickness(centimetresmetres) and the bands usually show sharplithological boundaries. Garnet lherzolite bands orolivine-websterite bands are present depending onpyroxene abundances at various scales. garnet-duniteor garnet-harzburgite and garnet-lherzolite display

(a) (b)

(c) (d)

Grt-increasing

Fig. 2. Photographs showing field occurrences of various rock types of the Luliangshan garnet peridotite massif. (a) Massive garnet-free dunite (d) interlayered with garnet lherzolite (GL); (b) banded structure of dunite; (c) rhythmic variation of garnet content ingarnet peridotite from harzburgite to lherzolite, garnet-free dunite occurs as interlayers; (d) garnet lherzolite interlayered with garnetpyroxenites.

L U L I AN G S H A N GA R N E T P E R I D O T I T E M AS S I F 62 3

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apparent inter-layering on different scales determinedby modal variation of olivine and pyroxene (Fig. 2c).The layering is probably inherited from a primaryigneous layering as seen in layered cumulates. The mainconstituent minerals are garnet, olivine, Opx, Cpx andminor Cr-rich spinel. Garnet pyroxenite is a minorcomponent and occupies

garnet crystals display flat patterns from core to rim.Sharp changes of pyrope and almandine proportionstowards the rims are an effect of late kelyphitisation.

Clinopyroxene is generally diopside-rich in all rocktypes. Cr2O3 in Cpx from garnet lherzolite and garnet-bearing dunite varies from 0.6 to 1.6 wt%, whereasCpx in garnet pyroxenite is Cr-poor (Cr2O3 = 0.180.30 wt%). XMg varies from 0.991 in the garnet-bearingdunite to 0.916 in the garnet lhe