Chiang Mai J. Sci. 2008; 35(2) 283

Pressure-Temperature Estimates of theMetamorphosed Sedimentary Rocks on the EasternSide of Khao Phai, Immediate West of Ban ChoengChai, Wang Pong, Phetchabun Province, ThailandYuenyong Panjasawatwong* [a], Boontarika Srithai [a] and Udomporn Wipakul [b][a] Igneous Rocks and Related Ore Deposits Research Unit, Department of Geological Sciences, Faculty ofScience, Chiang Mai University, Chiang Mai 50200, Thailand.[b] S.C. Mining Co., Ltd., 1013 Phaholyothin Road, Samsennai, Phayathai, Bangkok 10400, Thailand.*Author for correspondence; e-mail: scgli005@chiangmai.ac.th

Received: 5 June 2007Accepted: 9 January 2008.

ABSTRACTMeta- clastic sedimentary rocks and calc-silicate rocks from Khao Phai, immediate

west of Ban Choeng Chai, Wang Pong, Phetchabun were studied for their lithologyand petrography, and thus lead to pressure-temperature estimates of metamorphism.The meta- clastic sedimentary rocks show a hornfelsic texture, with mineral constituents ofquartz, plagioclase, K-feldspar, sillimanite, andalusite, cordierite, biotite, muscovite, magnetiteand/or monazite/zircon in variable proportions. They are either non-porphyroclastic orporphyroclastic. The calc-silicate rocks have non-porphyroblastic and porphyroblastic textures,and are made up of quartz, plagioclase, K-feldspar, clinopyroxene, garnet, phlogopite, epidote,vesuvianite, carbonates, magnetite, hematite/iron hydroxide and/or Fe sulfides. The pressureand temperature of metamorphism estimated from major informative metamorphic mineralsin some rock samples, i.e. quartz and sillimanite/andalusite for meta-clastic sedimentary rocks,and quartz and clinopyroxene for calc-silicate rocks, are in ranges of 2-3 kbar and 546-607oC,respectively. The obtained pressure-temperature condition is in agreement with amphiolitefacies metamorphism, inferred from overall metamorphic mineral assemblages. The heat forthis metamorphic event is very likely to have been released from Triassic(?) microdiorite/microgabbro or Triassic(?) granitic rocks in the vicinity.

Keywords: Khao Phai, thermal metamorphism, pressure-temperature estimates, mineralassemblages, meta- clastic sedimentary rock, calc-silicate rock.

1. INTRODUCTIONKhao Phai is the name of a mountain

with a maximum elevation of 607 m, locatedto the south of Ban Wang Sai Thong and tothe west of Ban Choeng Chai, Wang Pong,

Phetchabun (Figure 1). It is appeared in a1:50,000 topographic map sheet 5141 I(Amphoe Wang Pong), prepared by the RoyalThai Survey Department on the basis of map

Chiang Mai J. Sci. 2008; 35(2) : 283-293www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

284 Chiang Mai J. Sci. 2008; 35(2)

information up to 1997. According to thegeological map (Figure 2) that is modifiedfrom Panjasawatwong [1] and Noila et al. [2],Khao Phai is composed of Permianlimestone, sandstone, siltstone, claystone andtuff, with minor conglomerate, mudstone andshale. The Permian strata are unconformablyunderlain by the ?Carboniferous strata thatinclude sandstone, siltstone, claystone and tuffwith minor conglomerate, mudstone andshale. Both the Permian and the ?Carbo-niferous sequences have been intruded by?Triassic microdiorite/microgabbro and then?Triassic granitic rocks (granite, granodioriteand tonalite), giving rise to thermallymetamorphic rocks in part. In addition, thesedimentary sequences have partly experiencedsilicification and sericitization, while themicrodiorite/microgabbro and diorite/gabbro have partly undergone potassicalteration, leading to the occurrences of micro-syenite, monzonite and monzodiorite/

monzogabbro.The project has been undertaken in order

to estimate pressure and temperature ofthermal metamorphism that took place onthe eastern side of Khao Phai, immediate westof Ban Choeng Chai, i.e. at grid reference47QQU021087 (Figures 1 and 2), usingmineral assemblages in informative metamor-phosed sedimentary samples.

2. MATERIALS AND METHODSThe rocks in the area around grid

reference 47QQU021087 are Permian sedi-mentary strata and Triassic(?) microdiorite/microgabbro that have undergone thermalmetamorphism. They include marble, calc-silicate rock, metamorphosed sandstone andargillaceous rock, and microdiorite/micro-gabbro. Of these, only marble and calc-silicaterocks occur as outcrops, while the others arein situ float rocks. Calc-silicate rocks, andmetamorphosed sandstone and argillaceous

Figure 1. Map showing the topographic features of Khao Phai and vicinity, and the studylocation.

* Study location 300 Contour line in meter School Paved and unpaved provincial roadsx 570 Spot elevation in meter

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Quaternary sediments 1 km

?Triassic granite, granodiorite and tonalite

?Triassic diorite/gabbro and microdiorite/microgabbro

?Permo-Triassic sedimentary and pyroclastic rocks

Permian sedimentary and pyroclastic rocks

?Carboniferous sedimentary and pyroclastic rocks

Q

Gr

Gb/Di

PTr

P

C

rocks are regarded in this project as theinformative samples of which metamorphicmineral assemblages can give pressure andtemperature ranges. Accordingly, a numberof these metamorphosed sedimentarysamples (4 samples of sandstone and argil-laceous rocks, and 4 samples of calc-silicaterocks) were collected to make thin sectionsand then study under the petrographicmicroscope. Identification of minerals usingan X-ray diffraction technique was alsoperformed to assist in clarifying minerals.

3. RESULTS3.1 Metamorphosed Sandstone andArgillaceous Rocks3.1.1 Lithology

The studied metamorphosed clasticsedimentary rocks include sample numbersWST_01, WST_02, WST_15 and WST_16.These rocks are fine-grained and dense, andshow a hornfelsic texture. They have a darkgray color with many tiny black specks, whereweathering has taken place, they turn orangeto brown. Some samples have parallel/

Figure 2. Geological map of Khao Phai and vicinity that is modified from [1] and [2]. Alsoshown is the study location.

N

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subparallel, thin to very thin, light grayinterlayers. The light gray layers may containelongate lenticular-shaped, dark gray rocks ofwhich long axes are also parallel/subparallelto layering. The samples may have Fe-sulfideminerals in minor amount but may containnumerous, tiny fracture-infilling, light gray tomedium gray minerals. They are non-magneticand non-calcareous.

3.1.2 PetrographyGeneral Features

Under the microscope, the studiedsamples have textural and compositionaldifferences, and therefore their generalfeatures will be separately described in thefollowing paragraphs.

Sample number WST_01 has grain sizeslargely in a range of 0.1-0.2 mm across andconsists largely of quartz, with subordinatesillimanite, small amounts of biotite, magnetite,cordierite, andalusite and plagioclase, andoccasional monazite/zircon. The minorparallel/subparallel, thin to very thin, light graylayers in this sample are compositionally similarto the major dark gray rock, but for theirproportions. The light gray layers are almosttotally made up of sillimanite. Almost all theconstituents of the sample, excludingmonazite/zircon that are inherited from theprotolith, are interpreted to be metamorphicin origin.

Sample number WST_02 displays aslightly porphyroclastic texture, consisting ofporphyroclasts (grain sizes 0.05-0.15 mmacross) of quartz, plagioclase and K-feldsparin the matrix (grain sizes largely 0.01-0.02 mmacross) of quartz, plagioclase, K-feldspar,biotite, magnetite and monazite/zircon. Theporphyroclasts and the matrix constituentsare inferred to be sedimentary relicts andmetamorphic minerals, respectively. Tinyveinlets of sericite have been occasionallyobserved.

Sample numbers WST_15 is very fine-grained, with grain sizes largely of 0.01-0.06mm across. It comprises abundant sillimanite,subordinate quartz, and small amounts ofcordierite, opaques (magnetite and Fe sulfides),biotite, muscovite and trace plagioclase. Theoccurrences of quartz, sillimanite, plagioclase,cordierite, muscovite and biotite are sup-ported by X-ray diffraction analysis. All themineral constituents in the sample areconsidered to have a metamorphic origin.Tiny veinlets of quartz are present in thesample in sporadic amount.

Sample number WST_16 has very similarpetrographic features to sample numberWST_16, except for the lack of Fe sulfidesand the trace occurrence of K-feldsparpatches. The K-feldspar patches evidently havea replacement origin and might have formedafter metamorphism. The crystals of quartz,sillimanite, plagioclase, cordierite, muscoviteand biotite have also been detected by an X-ray diffraction technique. As observed in handspecimen, tiny fractures sealed by quartz areabundantly present.

Features of Individual ConstituentsQuartz shows anhedral outlines (Figures

3a-3d) and commonly forms clusters. Theymay have triple-junction and/or sutured grainboundaries.

Feldspars, either plagioclase or K-feldspar,are largely anhedral to subhedral. Theplagioclase crystals have An-content rangingfrom 33-62 (andesine-labradorite) on thebasis of a petrographic technique, and arelabradorite (sample numbers WST_15 andWST_16) by an X-ray diffraction technique.They are variably replaced by sericite, biotite,quartz and/or magnetite (partly altered tohematite/iron hydroxide). The K-feldsparcrystals are all microcline.

Sillimanite is mainly fibrolite (Figures 3band 3e) with colorless to brown color in

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ordinary light; the rest shows subhedral toeuhedral outlines (Figures 3a and 3e) and iscolorless in ordinary light. All the varieties ofsillimanite commonly form clusters. The lightgray interlayers as shown in hand specimenof sample number WST_1 are fibrolitesthat show a preferred orientation parallel/subparallel to layering (Figure 3b).

Andalusite and cordierite largely have

subhedral to euhedral outlines (Figures 3a, 3cand 3d). The andalusite crystals displaycolorless to pale pink pleochroism andcommonly has a poikiloblastc texture. Thecordierite crystals are colorless and partlyreplaced by yellowish green pinite.

Biotite (Figure 3d) and muscovite largelyoccur as anhedral to subhedral flakes. Thebiotite flakes show pale to dark brown

Figure 3. Photomicrographs of metamorphosed sandstone and argillaceous rock (samplenumber WST_01) in ordinary light displaying (a) quartz, sillimanite, andalusite and cordierite,(b) quartz, sillimanite and magnetite, (c) quartz, andalusite and cordierite, (d) quartz, andalusite,cordierite and biotite, and (e) prismatic sillimanite and fibrous sillimanite.

(a) (b)

(c) (d)

(e)

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pleochroism and may form as secondaryclusters in association with magnetite. Theseflakes have experienced variable degrees ofalteration. Their alteration products includechlorite and/or magnetite.

Magnetite occurs as anhedral to subhedralcrystals (Figure 3b), whereas monazite/zirconas euhedral crystals.

3.2 Calc-silicate Rocks3.2.1 Lithology

The calc-silicate rocks present in this studyare sample numbers WST_03, WST_07,WST_12 and WST_14. The first three samplesare non-porphyroblastic, while the last one isporphyroblastic. The non-porphyroblasticsamples are greenish gray in color (brownishwhen weathered). In the porphyroblasticsample, the porphyroblasts are brown garnetwith sizes up to 1 cm across. Their matrix isvery fine-grained, with a greenish gray color(brownish when weathered). The studiedcalc-silicate rocks may have tiny, pinkish grayand milky white veinlets in small amount. Theyshow no sign of magnetism and do not reactwith diluted hydrochloric acid.

General FeaturesThe studied samples show different

textures and mineral constituents as those ofmetamorphosed sandstone and argillaceousrocks, and for the sake of convenience,their general features under the microscopeare separately described in the followingparagraphs.

Sample number WST_03 showsbimodal-sized distribution, i.e. made up ofcoarser grains (0.05-0.40 mm across) and finergrains (<0.05 mm across). The proportionof coarser grains to finer grains is close tounity. Almost all the coarser grains are quartz,the remainder is plagioclase. The finer grainsare largely quartz and clinopyroxene, withminor plagioclase, garnet, titanite and

carbonates. Veinlets of quartz, clinopyroxeneand/or carbonates have been observed insmall amount. The coarser quartz andplagioclase grains are most likely to have beenderived from its protolith, whereas the othersare inferred to be metamorphic minerals.

Sample number WST-07 has very similarfeatures to sample number WST_03, excludinggrain sizes, and their proportions and mineralconstituents. The coarser and finer grains havesizes varying from 0.04 to 0.50, and less than0.04, respectively. The coarser grains are moreabundant relative to the finer grains and almosttotally comprise quartz; coarser plagioclase,K-feldspar and clinopyroxene grains are rare.The finer grains have similar constituents tothe coarser grains. Of these finer grains, quartz,and clinopyroxene are the major constituents.Again, the coarser grains other than clinopy-roxene are most likely to be the relicts ofprotolith. The coarser clinopyroxene grainsand all the finer grains are regarded asmetamorphic minerals.

Sample number WST_12 has grain sizesin a range of 0.03-0.13 mm across and iscompositionally layered. Individual layers areconstituted by colorless amphibole, vesuvia-nite, clinopyroxene, plagioclase, quartz, garnet,phlogopite, carbonates, titanite, magnetite,hematite/iron hydroxide and Fe sulfide indifferent proportions. In general, the majorconstituents of this sample are quartz andclinopyroxene, and subordinate vesuvianite.Almost all the mineral constituents areinterpreted to have formed by metamor-phism, except for quartz grains that have beenpartly formed by silicification.

Sample number WST_14 is made up ofgarnet porphyroclasts and inequigranularmatrix. The matrix (grain sizes <0.05 mmacross) is by far more voluminous than theporphyroblasts, and consists dominantly ofquartz, with subordinate garnet andclinopyroxene, small amounts of epidote and

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plagioclase, and rare magnetite and hematite/iron hydroxide. All the minerals present areinferred to have a metamorphic origin.

Features of Individual ConstituentsIn similar manner to the metamorphosed

sandstone and argillaceous rocks, the individualmineral constituents of individual sampleswill be described together in the followingparagraphs.

Quartz is all anhedral, and commonlyshows bimodal distribution. The coarserquartz crystals occur as isolated crystals, andclusters with triple-junction or sutured grainboundaries. The finer quartz crystals also formclusters without triple-junction and suturedgrain boundaries.

Plagioclase and K-feldspar crystals largelyhave subhedral outlines. Measurement of An-content of plagioclase using a petrographictechnique gives the values of 38-66 (andesine-labradorite). The plagioclase crystals may bevariably replaced by sericite and clay mineral.The K-feldspar crystals are all microcline.

Clinopyroxene crystals are very pale greenin color and are largely anhedral to subhedral.They occur either as isolated crystals or asclusters, and commonly as inclusions inamphibole and vesuvianite. Veined clinopy-roxene has larger grain sizes relative toclinopyroxene of the host rock.

Amphibole is largely anhedral to sub-hedral, and commonly occurs as large crystals(Figure 4c) with inclusions of clinopyroxene,vesuvianite and titanite.

Figure 4. Photomicrographs of calc-silicate rocks in ordinary light displaying (a) inclusionsof quartz and clinopyroxene in a garnet porphyroblast (sample number WST_14), (b) quartz,clinopyroxene and garnet in the matrix (sample number WST_14), and (c) clinopyroxene,amphibole and vesuvianite (sample number WST_12).

(a) (b)

(c)

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Vesuvianite crystals (Figure 4c) are pinkiskin color, and largely display anhedral tosubhedral outlines. They may contain clinopy-roxene inclusions.

All of the generations of garnet haveanhedral outlines. The garnet porphyroblastscontain inclusions of clinopyroxene (Figure4a), quartz (Figure 4a), epidote, plagioclase andmagnetite (partly replaced by hematite/ironhydroxide).

Phlogopite flakes are largely subhedral,whereas epidote grains are anhedral. Titanite,carbonates, magnetite, hematite/iron hydroxideand Fe sulfide occur as irregular anhedralpatches.

4. CONCLUSION AND DISCUSSIONThe area on the eastern side of Khao

Phai, immediate west of Ban Choeng Chai,Wang Pong, Phetchabun is underlain byPermian sedimentary strata, comprisinglimestone, sandstone, siltstone, claystone andtuff, with minor conglomerate, mudstone andshale. The sequence was intruded by ?Triassicmicrodiorite/microgabbro and then ?Triassicgranitic rocks (granite, granodiorite andtonalite). The Permian strata and themicrodiorite/microgabbro have been ther-mally metamorphosed in part. The resultsof thermal metamorphism give rise to theoccurrences of metamorphosed sedimentaryrocks and microdiorite/microgabbro. Thestudied samples in this project are part of thethermally metamorphosed sedimentary rocksof which mineral assemblages can yieldpressure and temperature of metamorphism.These samples include metamorphosedsandstone and argillaceous rocks, and calc-silicate rocks.

Almost all the metamorphosed clasticrocks presented are fine-grained, hornfelsic,non-porphyroclastic rocks and may containparallel/subparallel, thin to very thin, light grayinterlayers; very few show hornfelsic and

porphyroclastic textures. All of the studiedsamples have a dark gray color; orange tobrown on weathering surfaces. Petrographi-cally, the hornfelsic, non-porphyroclasticsamples are composed of quartz, plagioclase,K-feldspar, sillimanite, andalusite, cordierite,biotite, muscovite, magnetite and/ormonazite/zircon in different proportions. Theporphyroclastic sample contains quartz,plagioclase and K-feldspar as porphyroclasts(grain sizes of 0.05-0.15 mm across), andquartz, plagioclase, K-feldspar, biotite,magnetite and monazite/zircon as the matrixconstituents. The metamorphic mineralassemblages in the studied hornfelsic samplesare quartz + plagioclase + biotite + magnetite

sillimanite/andalusite cordieritemuscovite microcline Fe sulfides. Theinferred metamorphic mineral assemblagesimply that the rocks have experienced lowamphibolite facies metamorphism [3].

In hand specimen, the studied calc-silicaterocks are almost all non-porphyroblastic andfine-grained, with a greenish gray color(light brown on weathering surfaces) andoccasionally porphyroblastic, with browngarnet porphyroblasts in the greenish gray,fine-grained matrix. Under the microscope,the calc-silicate rocks are, in fact, inequigranular,and consist of quartz, plagioclase, microcline,clinopyroxene, garnet, phlogopite, epidote,colorless amphibole, vesuvianite, carbonates,titanite, magnetite, hematite/iron hydroxideand/or Fe sulfides. The matrix constituentsof porphyroblastic rock are mineralogicallysimilar to the equigranular rocks but forthe absence of microcline, phlogopite,amphibole, vesuvianite and Fe sulfides. Themetamorphic mineral assemblages in thecalc-silicate samples are quartz +clinopyroxene + plagioclase + garnet phlogopite carbonates epidote colorlessamphibole, vesuvianite titanite magnetite, hematite/iron hydroxide Fe

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sulfides. These mineral assemblages, again,are characteristic of low amphibolite faciesmetamorphic rocks [3].

Of the studied, metamorphosedsandstone and argillaceous rocks, samplenumbers WST_01, WST_15 and WST_16can provide temperature and pressure ofmetamorphism since they contain indexminerals, i.e. sillimanite, andalusite andcordierite. The major abundances of quartz

and sillimanite signify that the rocks are in theAl2O3 - SiO2 - H2O (herein ASH) system.Accordingly, the calculated stability fields ofkyanite, andalusite and sillimanite in the ASHsystem (Figure 5) have been used in theestimation of temperature and pressure. Theformation of aluminosilicate minerals in thissystem from lower to higher temperatures [4]are represented by equations as follows:

Figure 5. Pressure-temperature diagram illustrating the equilibria for aluminosilicate minerals(sillimanite, andalusite and kyanite), and the isograds and mineral zones in the system Al2O3 -SiO2 - H2O, and talc, tremolite, diopside and forsterite in the system CaO - MgO - SiO2 - H2O- CO2 (after Bucher and Frey [4]).

Al2Si2O5(OH)4 + 2 SiO2 = Al2Si4O10(OH)2 + H2O kaolinite quartz pyrophylliteAl2Si4O10(OH)2 = Al2SiO5 + 3 SiO2 + H2O Pyrophyllite sillimanite quartz

and/orandalusite

The metamorphic minerals in the calc-silicaterocks are compositionally variable, reflecting

different chemical compositions of theirprotoliths. The sample numbers WST_02 and

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WST_03 are of particular interest because theirmajor metamorphic constituents are quartzand clinopyroxene, i.e. they are significantly inthe CaO - MgO - SiO2 - H2O - CO2 (hereinCMS-HC) system. The sample numbersWST_15 and WST_16 are in the morecomplex systems as revealed by the majoroccurrences of garnet and/or vesuvianite.

The presence of clinopyroxene and theabsence of tremolite and forsterite suggestthat the diopside-in and the forsterite-inisograds and mineral zone boundaries(Figure 5) are required for pressure-temperature estimates since the reactionsinvolved [4] may be

CaMg(CO3)2 + 2 SiO2 = CaMgSi2O6 + 2 CO2 dolomite quartz diopside3 CaCO3 + Ca2Mg5Si8O22(OH)2 = CaMg(CO3)2 + 4 CaMgSi2O6 + H2O + 2 CO2 calcite tremolite dolomite diopside3 CaCO3 + 2 SiO2 + Ca2Mg5Si8O22(OH)2 = 5 CaMgSi2O6 + H2O + 3 CO2 calcite quartz tremolite diopside11 CaMg(CO3)2 + Ca2Mg5Si8O22(OH)2 = 13 CaCO3 + Mg2SiO4 + 9 CO2 + H2O dolomite tremolite calcite forsterite3 CaMg(CO3)2 + CaMgSi2O6 = 4 CaCO3 + 2 Mg2SiO4 + 2 CO2 dolomite diopside calcite forsterite

The occurrences of aluminosilicateminerals in the metamorphosed clastic rocksas major sillimanite and minor andalusite, oras sillimanite alone imply that the pressure-temperature conditions of metamorphismare on and close to the univariant sillimanite-andalusite curve. The positions of thesillimanite-andalusite univariant line and thediopside-in and forsterite-in isograds andmineral zone boundaries lead to a deductionthat the studied rocks have experiencedcontact metamorphism, operated at apressure-temperature condition of 2-3 kbarand 546-607oC. The heat source for metamor-phism might have been derived from eitherthe ?Triassic microdiorite/microgabbro or the?Triassic granitic rocks (granite, granodioriteand tonalite). The occurrences of biotite clotsin both the microdiorite/microgabbrosamples [5] and the studied metamorphosedclastic samples signify that the granitic rockstook part in the metamorphic event.

ACKNOWLEDGEMENTThe Igneous Rocks and Related Ore

Deposit Research Unit, Department ofGeological Sciences, Faculty of Science,Chiang Mai University has supported thisproject. The financial support from AkaraMining Co., Ltd. is gratefully acknowledged.Special thanks is also extended to ananonymous reviewer who made fruitfuldiscussion and comments on the paper.

REFERENCES[1] Panjasawatwong Y., compiler, Geological

map of the Wang Pong – Chon Daen area:Department of Geological Sciences,Faculty of Science, Chiang MaiUniversity, scale 1:50,000 (unpublished),2006.

[2] Noila C., Saisuvan R. and Wipakul U.,Geology of the Ban Wang Sai Thong Area,Sappoeb Subdistrict, Wang Pong District,Phetchabun Province, B.S. Field Report,

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Department of Geological Sciences,Faculty of Science, Chiang MaiUniversity, Thailand, 2005.

[3] Miyashiro A., Metamor phism andMetamorphic Belts, London, George Allen& Unwin Ltd., 1975.

[4] Bucher K., and Frey M., Petrogenesis ofMetamor phic Rocks, 6th Edn., Berlin,

Springer-Verlag, 1994.[5] Wipakul U., Alteration of Rocks Collected

from the Area around Grid Reference47QQT024085, Wang Pong District,Phetchabun Province, B.S. Report ofIndependent Study, Department ofGeological Sciences, Faculty of Science,Chiang Mai University, Thailand, 2006.