PhysicsoftheEarth and PlanetaryInteriors, 74(1992)263—277 263ElsevierSciencePublishersB.V., Amsterdam

Magnetismof SouthMongolianMiddle Paleozoicophiolites

Alexei N. DidenkoInstituteofPhysicsof theEarth, RussianAcademyofScience,B. Gruzinskaya10, Moscow,123810, Russia

(Received2 January1992; accepted10 February1992)

ABSTRACT

Didenko,A.N., 1992. Magnetismof SouthMongolian Middle Paleozoicophiolites.Phys.EarthPlanet. Inter., 74: 263—277.

This paperpresentsnew paleomagneticdata from SouthMongolia. Middle Paleozoicrocks were studied from threedifferent tectoniczoneswithin theSouthMongolian fold belt. From north to souththesearethe Gobi—Altai zone(in termsof paleotectonicposition, the ‘north’ shelfof the paleobasin),the Trans-Altai zone (ophiolite complexes)and the SouthGobia zone(the ‘south’ shelf). Severalpaleomagneticfield tests, includingconglomerate,bakedcontactandfold tests,werecarriedout to determinemagneticstability. In addition,principal componentanalysisof the stepwisethermaldemagnetiza-tion data was used.The ophiolitesare characterizedby threeancient components.One magneticcomponentis pre-foldinganddirectedto the southwith a very gentleinclination (declination(D) 180—210°,inclination(I) = —6~~+ 2°,K 120).Two post-folding magneticcomponentswere observedalso.The first post-foldingmagneticcomponenthas D = 282.4°,I = — 36.4°,k 30.3, and thesecondhas D = 318.6°,I 33.0°,k= 60.5. The final magneticcomponentis very close to thePermianpaleomagneticdirection expectedfrom the North Chinaplatform. The first post-foldingcomponentof magnetiza-tion may havebeenacquiredduring the Variscanaccretionin Early Carboniferoustime whenthe basinwas closed. TheSouth Mongolian paleo-oceanwas located in equatoriallatitudes nearthe Tarim block, the SouthChina platform andprobablythe North China platform in the Devonian.

1. Introduction Baikalian (Riphean)—Indosinian (Late Paleo-zoic—Early Mesozoic) ophiolitic belts mark su-

The Paleozoicand Early Mesozoic history of tures betweenthe Siberia andNorth Chinaplat-the amalgamationof the Asian continent has forms (Dobretsov,1990).A majority of the struc-beenreportedby variousworkers,basedon pale- tures originated from closed oceanicbasins, in-omagneticand geologic data (Lin et al., 1985a, cluding the Paleoasian(Caledonianbelts), paleo-Adamiya et a!., 1986b; Zhao et al., 1990). The Tethys-1, Uralian (Hercynianbelts), and paleo-reconstructionscontainedin thesereports have Tethys-2basins(Khain, 1979; Zonenshainet a!.,demonstratedthat blocks that were distantfrom 1990).each other in Vendian—Cambrian time ap- It is not possibleto reconstructthe previousproachedto form a Pan-Asiancontinent by the positions of thesepaleobasinswithout detailedUpper Triassic. paleomagneticinvestigations,which would allow

The suturezonesbetweenthe collageof Asian estimatesto bemadeof the sizesof thesebasinsblocks are representedby ophiolitic and/orglau- andthe determinationof the apparentpolarwan-cophaneschistbelts. Representativeexamplesof der (APW) path of the various ophiolitic blocks

before their obductiononto sialic blocks (Diden-ko andPechersky,1988,1989).The presentpaper

Correspondenceto: A.N. Didenko, Instituteof Physicsof the has the aim of examining such a problem asEarth, RussianAcademy of Science, B. Gruzinskaya 10, appliedto the Middle Paleozoicophiolitic belt ofMoscow, 123810, Russia. South Mongolia.

0031-9201/92/$05.00© 1992 — Elsevier SciencePublishersBy. All rights reserved

264 AN. DIDENKO

2. The regional geologicaldata 90° 110°I I

The Variscanfold belt canbe tracedfor 200050°

km from Hingan, in the east, to Junggar,in thewest. The Variscan fold belt separatesthe Cen-tral and North Mongolian Caledonidesto theChina) in the south.Ruzhentsev(1985),Ruzhent- 45

north from the Precambriansialic block (North 45

sev et a!. (1987) and Ruzhentsevand Badarch(1989) inferred that the South Mongolian

tflfll Ufflffl!2~3 ~Variscidesformed in a paleobasinwith a rifto-genic structurethat evolved from Ordovician to Fig. 1. Tectonicsketch map of Mongolia (basedon map ofCarboniferous when continental collision de- Ruzhentsevetal. (1987),with simplification). 1—Caledonidesstroyedthe SouthMongolianocean. of Central and North Mongolia; 2—4—Variscidesof South

South Mongolia can be subdivided into the Mongolia (2—Gobi—Altai zone; 3—Trans-Altai paleo-oc-

following tectoniczones:an axial Trans-Altai pa- eanic zone; 4—South Gobia zone); 5—Indosinian zone.

leo-oceanic(?)zoneof ophiolites,volcanic rocks Numbers on the map: A—Shine—Djinst massif: B—Gour-van—Saykhanridge; C—Dzolen ridge; D—Ulan—Del—Ula

andvolcanoclasticsedimentaryunits,andthe Go- massif.

bia—Ahai andSouth Gobia zones,locatedin thenorth and south respectively,which formed in 1987).TheDzolenandGourvan—SaykhanRangescontinentalmarginsettingsfringing the paleo-oc- servedas aprincipaltargetfor thispaleomagneticean(Fig. 1) (Ruzhentsev,1985;Ruzhentsevet al., study, which focuses primarily on the pillow

TABLE 1

Petromagneticpropertiesof ophiolite rocksfrom Gurvan—Saykhanand Dzolen

Sample Rock J,, x e3 K X e5 Q,, Hcr ~s ~rs /J~ T~ ~1st~ ~rst /~st

no. type (Am’) u.SI (mT) (A mm kg’) (%) (°C) (%) (%)gb-2b Diabase,endo 333 6207 0.13 425 4.36 7 595 76 —

gb-3v Diabase,center 452 4647 0.24 275 4.1 7 600 90 5gb-3g Silic., exo 111 454 0.61 680 1.3 23 600/675 92 18gb-3g Diabase,endo 330 2926 0.28 860 2.29 18 600 104 14gb-5b Diabase,endo 5571 5179 2.7 855 3.0 24 610 94 14gb-6v Basalt,pillow 1823 3963 1.16 720 4.14 22 625 91 17gb-8g Diabase,endo 223 2110 0.27 520 2.66 8 595 87 —

gb-iDe Silic., exo 552 1487 0.93 745 2.58 27 605 101 21gb-be Diabase,endo 426 2246 0.48 810 0.88 20 610 71 24gb-13a Silic., exo 240 4257 0.14 990 1.68 29 605 110 13gb-13a Diabase,endo 420 3597 0.29 840 1.71 43 600 131 27dz-ld Silic., exo 4313 839 12.9 1200 0.67 25 605 100 —

dz-4v Diabase,endo 467 2400 0.5 710 1.67 12 590 77 —

dz-5g Diabase,center 425 3466 0.3 690 2.92 15 600 80 13dz-7b Diabase,center 1026 3033 0.9 1125 2.57 22 595 72 —

dz-8v Diabase,endo 47 943 0.1 1030 0.65 29 595 53 34dz-12a Basalt,pillow 650 2819 0.6 490 1.34 25 600 108 23dz-12v Silic. 20 71 0.7 490 0.07 — 675 124 —

dz-13e Diabase,dike 1 36 0.1 530 0.03 1 pm 111 —

dz-14a Basalt,pillow 171 224 2.0 1150 0.27 46 600 48 —

dz-15a Diabase,endo 1 23 0.1 1150 0.02 3 pm 128 —

Silic.—silicastone;endo—endocontact;exo—exocontact;.1,,—natural residualmagnetization;K —magneticsusceptibility;Q,, —

Koenigsbergerfactor; Hcr —residual coercivity; J~’~ —specific saturationmagnetization(initial value and after heating to600—670°C);J~,J~—specificsaturationremanence(initial value and afterheatingto 600—670°C);1~—Curiepoint; pm—para-magnetic.

MAGNETISM OF SOUTH MONGOLIAN OPFIIOLITES 265

basaltsand andesite—basalts,diabasicdikes andsills, and cherts and conglomerates.All unitssampledare from the Berkheulinianand Gour- ~‘°

van—Saykhanianunits of Givetian—Famennian(Middle—Late Devonian) age (Eenzhin, 1983;Ruzhentsevet al., 1987). In addition, samples ~

were also obtained from the Gobi—Altai and ~

South Gobia zones.Upper Devonian andesite— 0.5

basalts(Ruzhentsevet a!., 1987) from the Sine— -. -.

Djinst mountains(the Tsagan—Khalgyn—Tsakhirmassif)werecollectedfrom the Gobi—Altai zone. -

Silurian—Devonian volcanogenic—sedimentaryrocksfrom the Timutinian series(Sinitsyn, 1956) 300 500 vho T, °C

of the Ulan—Del—Ula massif were collected in Fig. 2. Representativethermomagneticcurves of saturationthe SouthGobia zone. magnetization.1—Diabase,samplegb-5b; 2—bakedsiliceous

rock, samplegb-3g;3—unbakedsiliceousrock, sampledz-12a.

3. Petromagnetic and paleomagneticdatalower in somecases;Table 1) andreducedmagni-

3.1. Petromagneticproperties tudeof the Hcr also.

Such behavior is typical for ophiolitic rocksTo determinean optimal approachfor mag- (Pechersky and Tikhonov, 1988; Didenko and

netic cleaningof the studiedrocks, a numberof Pechersky, 1989) undergoing secondaryalter-petromagneticcharacteristics(which make it pos- ation; namely,heterophasicalterationsof originalsible to infer the composition,structureandcon- titanomagnetitesfollowed by low-temperatureox-centration of the ferromagneticfraction), have idation (in the caseof igneousrocks), and trans-beenidentifiedon sampleduplicates(Table 1). formationof original hematiteand/orhydroxides

The temperaturerelationshipsof saturation into magnetite,as a result of high temperaturesmagnetization(~‘~)and residual saturation (~rs~ (600—700°C)under redoxconditions, followed byindicate that cation-poormagnetitewith a Curie its low-temperatureoxidation (in the case ofpoint at 590—625°Cis the principal magneticcar- bakedsediments).rier (Table 1, Fig. 2). In addition to the mag- In a favorablesituation,it may be possibletonetite, maghemite (Fig. 2, samples gb-5b and distinguish two types of magnetization that9b-3g) in igneous and baked sedimentaryrocks formedapproximatelysynchronouslywith thefor-and hematite(Fig. 2, sampledz-12v) in unmeta- mation of oceanicrocks derivedfrom: (1) mag-morphosedsedimentariesare also present.The netite resultingfrom a heterophasicalterationofpresenceof maghemitedemonstratedby the J~—t° original titanomagnetitewhich usuallyoccursfol-relationship(rapid J~increasenear200°Cand a lowing eruption of igneousrocks, and (2) mag-sharp decreasewithin the interval 400—450°C; netite resultingfrom heatingof silica-bearingsed-Fig. 2, sample9b-5b), and by high initial magni- imentaryrocksby intrusions.tudesof such parametersas ~rs/~s and residual Natural residualmagnetization(Jo) and mag-coercivity (Hcr) (Table 1). Laboratoryheatingof netic susceptibility(K) magnitudesvarywidely insampleshas shown decreasinggrain stressesbe- all studiedsamples.However, the Koenigsbergerlow about200°C(Bolshakov et al., 1987) and the ratio (Q~)>1, mostly governscasesof contact-destructionof maghemitebelow about400—450°C. metamorphosedsediments(exocontact)and theThe heating results in a significantly reduced chilling part of intrusions(endocontact)andpro-magnitudeof the ~rs (by nearly a factor of two vides evidenceof a predominantlyresidualmag-

266 A.N. DIDENKO

netizationover an inducedin situ magnetization has beenusedfor this purpose.The data (from(Fig. 3(a)). nine or ten heatings)were analyzedusingvector

The magneticstability of bakedsedimentshas diagrams(Zijderveld, 1967) andprincipal compo-also beendemonstratedby the J~/J~ratio (Fig. nentanalysis(Kirschvink, 1980),usinga program3(b)) for blocking temperaturesgreater than written by T. Torsvik. The relative agesof the460°C.For overtwo-thirds of the examinedbaked resultingpaleomagneticdirectionsweretestedus-samples,the J~~/J~magnitudeexceeds0.3 after ing threemethods.heatingat 460°C,whereasthe averageJ~5/J~of The conglomeratetesthasbeenappliedto theother rocks is less than 0.3. The J~residue is intraformationalconglomeratesfrom the upperfrequentlyeven lower than 10% after heatingto partof the Berkheoulinianstratasection(D2—D3)460°C.This temperaturehas been chosenbe- (Ruzhentsevet al., 1987). Clasts are predomi-causeone would expectat this temperaturethe nantly of basic, intermediateand, more rarelycomplete destruction of such high-magnetic acidiccomposition.The UpperBerkheoulinianisphasesasmaghemite. also composedof polymictic sandstonescontain-

ing fragmentsof limestonesandorganicdetritus3.2. Paleomagneticdata (coralsand crinoids),which suggestsformation in

low latitudes.All sampleshavebeenanalyzedusingthermal Pebblesfrom conglomerateswere sampledat

demagnetizationwithin the rangeof 20—600°C, two distant sites, and threecompositionallydis-and unbakedsedimentswere demagnetizedat tinct pebblegroupswereselected:(1) basalticandtemperaturesup to 640°C.An electric furnace andesite—basaltic;(2) siliceous; (3) siltstone—installed within a five-shield mumetal screen sandstonecomposition.Chert clastsresembleex-(compensationexternal field is about 20—15 nT) ocontact zone samples in terms of both color

TABLE 2

Paleomagneticdatafrom a conglomeratetest

T(°C) Site 1 Site 2

Silicastone Basalt Basalt Bakedsilicastone(n= 9) (n = 9) (n = 16) (n = 6)

D I D I D I D I(R) (R) (R) (R)

20 343 46 351 3 266 —19 286 —36(8.42) (5.39) (11.16) (3.35)

300 317 4 335 41 280 —35 284 —44(7.48) (4.46) (12.08) (3.84)

Differential 5 62 7 58 264 —27 286 —36(20—300) (8.45) (5.18) (11.81) (3.86)360 316 —11 331 26 284 —37 279 —46

(7.99) (3.81) (12.06) (3.91)420 320 —23 316 32 285 —35 283 —46

(8.00) (4.31) (12.02) (3.88)480 310 —19 316 12 287 —36 293 —50

(7.14) (3.10) (12.01) (3.74)540 301 —23 297 —48 286 —35 290 —46

(6.99) (2.82) (11.67) (3.22)

D, I—declination andinclination in geographicsystem,respectively;R—vectorsum; n—numberof samples.

MAGNETISM OF SOUTH MONGOLIAN OPHIOLITES 261

(from cherry to black) and magnetic susceptibility (three- to five-fold increase relative to unbaked cherts).

Statistical analysis of magnetization directions has been used to identify the systematic compo- nents. Site 1 samples reveal two components (Ta- ble 2): (1) related to the modern field (differential vector for the interval 20-300°C) and (2) NW- SE-oriented, with a 30-35” declination (basalts, 420°C). The sum of these vectors exceeds the critical Rayleigh number (Mardia, 1972). Site 2

tgJ, ?.- a

-3 -

-4-

A- 1 T- 2

-5. +- 3

.- 4

.

(a) I

-4 -3 QK

6Om

‘.. 50 L-l b

. . .

(b)

0.2 016 6.8 Jnt

Jno

Fig. 3. (a) Plots of intensity of NRM (_I,) for individual

samples against the magnetic susceptibility (K), with resulting

slope lines indicating the values of the Koenigsberger ratio

(Q,). (b) Distribution of ratio of intensity of NRM after

heating to 460°C (J,,r) to _I,. l-Baked siliceous rocks; 2-

samples from endocontact intrusive bodies; 3-basalts and

andesite-basalts; 4-sedimentary rocks.

samples displayed a west-east-oriented natural remanent magnetization (NRM) component with 20-40” negative declination (Table 2). The Site 1 summary vector tends to follow the same direc- tion.

For all three pebble groups, a trend of de- creased summary vector has been observed below the 480-540°C cleaning; however, its insignificant magnitude is related only to pebbles of burned cherts and Site 1 basalts.

The baked contact test was found to be appli- cable only to ophiolitic rocks (a total of 14 bodies from the Gourvan-Saykhanjan and Berkheoulin- ian complex) and Timurtinian series of the South Gobia zone (two bodies only).

The above stability of exocontact samples has also been demonstrated by the vector analysis data obtained after thermal cleaning. Three in situ directions are distinguished on the Zijderveld diagram (Fig. 4, sample dz-4g): (1) that along the 20-200°C differential vector with D = 353” and Z = 68”, which is very close to the contemporane- ous field direction at the selection point and is probably related to magnetically viscous pro- cesses; (2) that along the 300-420°C differential vector with D = 277” and Z = - 21”; (3) that along the 480-560°C linear area of the diagram passing through the origin, with D = 237” and Z = -53 (in the geographic system). The high-temperature component accounts for about 90% of the total magnetization.

The behavior of the NRM of a thermally clear sample from the same body’s endocontact is dif- ferent; more than 90% of the total magnetization is oriented at D = 5”, Z = 58” (Fig. 4, sample dz-4v). The remaining 10% is a west-east-di- rected component for temperature below 400- 500°C. However, this may not be recognized and is similar in orientation (D = 241”, Z = - 46”) to the highest-temperature stable component of the baked sediment.

Positive results from the baked test were also produced by samples from a sill within the Gour- van-Saykhan ridge (Fig. 51, from both endocon- tact and exocontact zones and from unbaked sediment at 15 m distance from the sill roof. .Z, values measured from both endocontact and exo- contact samples reveal (Fig. 5, samples gb-3b and

268 A.N. DIDENKO

gb-3v) similarities in stability, andarealso similar other two high-temperaturemagnetic compo-to those from the Dzolen ridge describedabove. nentsare attributedexclusively to unbakedsedi-

The only differenceis in the behaviorof the ments.west—east-orientedmedium-temperaturecompo- The bakedcontacttestwasalso applied to thenent. The unbakedsediment sample from the SouthGobia Timurtinian series,wherethe direc-Gurvan—Saykhanridge shows anotherJ~picture tion of the high-temperature.1,, componentofunderthe temperatureclearance(Fig. 5, sample baked sedimentsis similar to the high-tempera-gb-3zh). Its J~behavioris very complicatedon ture componentfrom intrusive andesite—basaltictheZijdervelddiagram,andfour componentsmay bodies.In contrast,all J~componentsof unbakedbe identified. The first two are the component sedimentsare different from those of andesite—near the contemporaneousfield (D = 3330, I = basalticbodies.86°)andobservedbetween20 and300°C,andthe It is difficult to apply the fold test in thewest—east-orientedcomponent (D = 299°, I = Gobi—Altai zone rocks, becauseof very minor

— 32°) observedbetween 300 and 420°C.The variationsof bedding dips within the examined

(c)

3~0

3

~ + E / 2

\~_~

~ ~ E

(a) -/ 2~300

D~-

~flo 2

(b) ~oc 300 500 T°CFig. 4. Representativethermaldemagnetizationdiagramsof J~for two samplesfrom oneintrusive body (exocontactsampledz-4gandendocontactsampledz-4v).(a)Zijderveld diagram:solid (open)symbolsrepresentprojectionson horizontal(vertical) plane.(b)Polarnet projection of directions of magnetizationfor samesamples:solid (open) symbolsrepresentprojections on the lower(upper)hemisphere.(c) The respectivenormalizedthermaldecaycurvesof J~for the samespecimens.

MAGNETISM OF SOUTHMONGOLIAN OPI-HOLITES 269

andesite—basalticstrata (Table 3). We use the for Timurtinian unbaked sediments is beyondlinear magnetizationarea in the high-tempera- doubt(Kg/Ks = 2).ture zoneas the characteristicmagnetizationfor As shownabove,threeancientdirections(apartthoserocks. from the componentalong the contemporaneous

Application of the fold test in other study field) havebeendistinguishedin ophiolitic rocks:areasdetectedbothpre- andpost-foldingmagne- (1) the first component(D = 318°,I = 39°)hastization components.For the South Gobia zone, beenobservedin conglomeratebasaltic pebblesthe clustering of the high-temperaturecompo- and from the differential vector of some of thenent for andesite—basaltsandbakedsedimentsin igneousrocks(Tables2 and3). The post-foldingstratigraphiccoordinatesis twice that within the character of this component is well demon-geographicalcoordinates(Table 3). In contrast, strated,asorientationsimilarities arepresentinthe post-foldingcharacterof the magnetization the geographicsystemonly.

a’

270(+~ ~ ~24~~N(oV~9

~

1.0 ~

200 £io~ ~00

Fig. 5. Representativethermal demagnetizationdiagrams for samplesfrom one intrusive body (exocontactsamplegb-3v andendocontactsamplegb-3b) and sampleof unbakedsiliceous rock at 15 m from the intrusive body (gb-3zh). (For furtherexplanation,seeFig. 4.)

270 AN. DIDENKO

TABLE 3

Meanpaleomagneticdirections

Site, Bedding Beforecleaning After cleaning Ageoftyperocks Ang D

8 ‘g D8 I~ D. component

(K/a95) (K/a95) (K/a95)

DzolenrangePillow basalts 160—210 40— 90 318 1 283 —57 329 —19 C1N 13, n = 20 (<3/—) (35.7/6.5) (12.2/11.1)Aleurolites 140—165 55— 70 103 47 101 42 114 —9 C1n = 7, N= 2 (60.4/7.9) (54.5/8.2) (42.8/9.3)Basalt from dikes 170—220 10— 20 345 26 290 —49 273 —25 C1N = 14, n = 20 (3.2/21.1) (3.3/20.6) (15.5/9.5)Bakedcontacts:endocontacts 340—360 60— 75 299 41 234 —46 208 —2 D2_3n = 14 (<3/—) (14.6/10.8 (25.6/8.0)exocontacts 340—360 60— 75 239 —47 223 —50 209 3.5 D2_3n = 13 (13.4/10.6) (19.3/8.8) (41.8/6.0)

GurvansaikhanrangePebbles1:Basalts 45 125 351 53 316 32 340 —19 P2n = 6 (8.1/20.0) (3.2/32.8) (3.2/32.8)Aleurolites 45 125 343 46 301 —23 300 22 C1n = 9 (13.8/12.6) (5.6/14.3) (5.6/14.3)Pebbles2: 35 89 270 —23 284 —38 265 16 C1n = 22 (<3/—) (3.4/16.2) (3.4/16.2)Pillow basalts 15— 20 80— 90 357 32 312 —19 266 —33 C1N = 7, n 18 (<3/—) (5.1/29.6) (10.2/19.8)Basalts(300—400°C) 322 42 P2n = 17, N = 2 (5.6/14.3)Pillowbasalts 345— 15 70— 85 320 26 288 —32 239 —28 C1n = 5, N= 1 (6.9/23.8) (21.2/17.0) (13.3/21.8)Tuffites 35— 55 95—125 3 55 319 —36 285 24 C1n = 11, N = 1 (7.0/15.9) (4.1/20.9) (3.5/22.6)Silicastones 345— 5 80— 90 347 —43 8 —83 188 —15 D2_3n = 6, N = 6 (7.6/20.7) (19.6/15.5) (39.7/10.8)Pillowbasalts 345— 15 70— 85 16 26 97 —84 178 —14 D2_3n = 14, N = 2 (<3/—) (10.5/12.9) (15.6/10.3)Baked contacts:Endocontacts 340— 15 75— 90 7 37 151 —70 177 —3 D2_3n= 7 (<3/—) (21.9/11.3) (50.4/7.5)Exocontacts 340— 15 75— 90 97 —64 125 —78 179 —12 D2_3n = 7 (12.3/16.3) (35.4/8.9) (58.3/6.9)

Tsagan— Khalgyn— Tsakhirrange (northshelf)Andes—basalts 45— 50 80— 90 344 —1.5 330 —67 253 —8.5 D2_3N 9, n = 17 (5.5/24.1) (54.5/7.1) (54.5/7.1)

Ulan — Del—Ula massif(southshelf)Andes—basalts, 200—340 50— 75 245 70 207 4 229 42 S—DBakedcontacts (<3/—) (4.6/25.2) (9.1/16.5)

N=8, n=18Sandstones 340— 10 50— 60 5 43 352 —9 333 —69 D1_2n = 15 (32.7/11.9) (44.4/10.2) (20.2/15.3)

N, n—numberof sites and samples;Az, Ang—azimuthand angleof bedding,respectively;D, I—paleomagneticdeclinationandinclination in geographic(g) and stratigraphic(s) systems; K, a95—precisionparametersof Fisher’s statistics;P—Permian;C—Carboniferous;D—Devonian; S—Silurian.

MAGNETISM OF SOUTH MONGOLIAN OPHIOLITES 271

(2) The second component (D = 282°,I = from a dike, and dz-2, unbakedchert) to that— 36°)hasbeenobservedin a numberof locali- componentwhich showsa slight predominanceofties within the Dzolen and Gourvan—Saykhan densityof distribution in the geographicsystemridges.In five casesout of seven,it clustersbetter (Table 3). We may term that componenta syn-in geographiccoordinatesand thereforeits post- foldingone. -

folding character is clear. Magnetic dikes and (3) The third component,detectedfrom thepillow lavas within the Dzolen and Gourvan— high-temperaturecomponentof the Dzolen andSaykhan ridges show decreaseddispersion in Gourvan—Saykhanendocontactand exocontactstratigraphiccoordinates(Table 3). In addition,a samples,as well as on non-magneticpillow lavassmall part of the Dzolensedimentsshow a direc- and siliceous sedimentsfrom the Gurvan—Sayk-tion almost anti-parallel (Fig. 6; dz-12a, diabase han ridge, is probably of pre-folding origin, as

N

300

\\\~ _-6O~. N~ Up

\\ + E A/

V

0.5 ~ V.-.

D2—12e~100 300 £00 T, C £,Dovc.’n

Fig. 6. Representativethermaldemagnetizationdiagramsfor samplesof unbakedsiliceousrock (dz-2)anddiabasefrom thecenterof a dike (dz-12a). (For further explanation,seeFig. 4.)

272 AN. DIDENKO

intra- andinterlayeringclusteringof this compo- tai zone as determinedfrom the high-tempera-nent is higher in the stratigraphiccoordinates ture componentis + 4°±4°in D3, which is verythanin the geographicalcoordinates(Table3). close to the Upper Devonian position of the

Tarim and SouthChina plates(Lin et al., 1985a,b; Lin, 1987; Bai et al., 1987; Li, 1990).

4. Datareviewandcomparison Thus,we believe that the primary magnetiza-tion datesfrom close to the formation of the

4.1. The Gobi—Altaizone(‘northern’ shelf) rocks, andthat the componentwas acquireddur-ing a reversepolarity epoch. If so, this suggests

According to Ruzhentsev and coworkers about40°northwardmotion of the Gobi—Altai(Ruzhentsev et al., 1987; Ruzhentsev and block and 75°clockwiserotation. This is compa-Badarch, 1989), the andesite—basalticflows of rable with the motion of the Tarim block, fromthat zoneareof Upper Devonianage, basedon the Upper Devonianto the present(Bai et al.,their relationship with the faunally well-dated 1987; Li, 1990).volcanogeniccarbonatesectionof the Dginsetiansubzone. However, the origin and age of the 4.2. The Trans-Altaipaleo-oceaniczonemagnetizationthat we have analyzedfor theserocksrequiresomefollow-up studies.The follow- The rocksanalyzedfrom the Trans-Altaizoneing pointsshouldbe noted: have allowed a four-fold distinction of compo-

(1) the rocks contain only two magneticcorn- nents,of both pre- and post-foldingorigin. Theponents;the first is nearthe expectedPermian least stable componentis removedbetween200direction (Pruner, 1987) and disappearsafter and 300°Cand hasa mediandirection equalto400°C,andthe secondis ahigh-temperaturecorn- that of the dipole part of the contemporaneousponentwhich does not coincide with detected geomagneticfield (D = 0°,I = 61°)at the sam-metachronouscomponentsin the geographicco- pling locality.ordinates; Above 400—420°C,some samplesof igneous

(2) the observedpaleolatitudeof the Gobi—Al- rocksandbasalticpebblesfrom theBerkheulinian

TABLE 4

Summaryof Paleomagneticdata

Rocksections Locality D I K Polarity VGP A95

Latitude Longitude Latitude Longitude

OphiolitesBasalts,con- 43.7 103.0 318.6 39.0 60.5 N+R 49.6 354.2 11.7

glomeratesMagmaticand 43.6 102.9 282.4 —36.4 30.3 R+N 5.3 169.9 8.5

sedimentaryrocksDzolen,baked 43.5 102.8 209.2 2.4 120 N —38.2 64.5 3.9

contactsGourvan—Saykhanbaked 43.7 103.0 179.8 —6.5 123 N —54.2 99.8 5.3

contacts,pillow lavas

SouthshelfSedimentaries 42.3 103.5 352.2 —9.0 44.4 N? 42.6 294.2 7.3Bakedcontacts, 42.3 103.5 229.1 42.2 9.1 R? 9.4 239.3 15.9Andesite—basalts

North shelfAndesite—basalts 44.3 99.3 252.7 —8.5 54.5 R 15.4 198.5 5.1

K—Fisher’s precisionparameter;VGP—virtual geomagneticpole; A95—semi-angleof coneof 95% confidenceof VGP.

MAGNETISM OF SOUTH MONGOLIAN OPHIOLITES 273

intraformational complex contain a magmatic pre-Permianrocks is well known (Didenko andcomponentthat hasdirection in geographiccoor- Pechersky,1989), and, according to geologicaldinates which are similar to the paleornagnetic data, the final folding phasesin that region wasdirection for the Central Mongolian Permian completedduring the pre-Permian(Ruzhentsevrocks (Pruner,1987) and the North China plate et al., 1987; RuzhentsevandBadarch,1989).Thus,(Lin et al., 1985a;Zhaoet al., 1990) (Fig. 7). The a highly probableposition of the block duringUpper Paleozoicremagnetizationof CentralAsia UpperPaleozoicis about22 ±7°N,andtheblock

0

330 30

/30o 0,, 7~P~ T 60

/~~lwA

7f ± 90\~ I V I

p2 13 \3t~

1<3 2~3 — I

r~3 j

(I —,P~ T r p -,

6 � ~bC ~

—I--- ~210 ~ ~

•1 ~Fig. 7. Pole positionsand their relationto polespreviouslydetermined.Pole data are taken for Siberia (Khramov et at., 1985),North and SouthChina(Lin et al., 1985a,b; Lin 1987; Zhaoand Coe,1987; Zhaoet al., 1990),Tarim (Bai et al., 1987; Li, 1990)and Mongolia (Pruner,1987). Numbersof the net: 1—UpperPermian(?)pole for remagnetizedconglomeratesand basaltsfromGourvanand Dzolen ridges;2—Early Carboniferouspole for rocksfrom Gourvanand Dzolen ridges; 3a, 3b—Middle—UpperDevonian polesfor Dzolen and Gourvanridges respectively;4—UpperDevonianpole for andesite—basaltsfrom Shine—Djinstmassif; 5,6—polesfor rocks from Ulan—Del—Ula massif,Lower—Middle Devonian(?)and Silurian—Devonian,respectively.Solid(open)symbolsrepresentprojectionson the lower (upper)hemisphere.

274 AN. DIDENKO

was an integralpart of North China andCentral ever, two sites (Table 3; diabasesof dikes fromMongoliaat that time. Dzolen, and part of a pillow lava from Curvan)

The magnetizationvector for many igneous havesimilardirectionsin the stratigraphiccoordi-andsedimentaryrocksandanintermediatedirec- nates,although they have a significantly lowertion of conglomeratesafter the 460—540°Cclean- declination(—25—35°)as well. We postulatethating form a single clusterin thegeographiccoordi- the paleomagneticdirection is of syn-folding on-nate system(D = 270—290°,1= — 40—50°).How- gin, and correlate it with Variscan accretion,

24$

3Z8

‘I08 I

248 -

328

I 1 I

:~ S. Ch

I I ~I

328 ~

I I I I I I

-“~‘0 -20 0 20 4’o 60 80 , degree

Fig. 8. Movement of South Mongolia relative to surroundingcontinental mass. ø0~—angulardistancebetweenthe place of

samplingand observedpole for theserocks; e —angulardistancebetweenthe placeof sampling and pole of comparedplate.Circles—Dzolenand Gourvan—Saykhanridges;triangles—Shine—Djinstmassif;squares—Ulan—Del—Ulamassif.

MAGNETISM OF SOUTH MONGOLIAN OPHIOLITES 275

which occurred in the early Carboniferous (2) A componentsynchronouswith the forma-(Ruzhentsevet al., 1987). However, uncertainty tion of andesite—basalticflows and their bakedexists in the estimateof paleolatitude,as we fail host sedimentshas been detectedin the strati-to takeinto a properaccountthe tectonic move- graphiccoordinatesanddatedas Silurian—Lowermentsfor five siteswithin the geographicsystem. Devonian, i.e. referred to the time when theIndeed,the averageinclination for thosefive sites upper-slopeturbiditiesof the SouthGobiamicro-is about40°,which is evenslightly morethan that continent (Timurtinian series) had formedfor the Permiancomponent.Two paleomagnetic (Ruzhentsev,1985; Ruzhentsevet al., 1987). Andirections in the stratigraphiccoordinatespossi- interpretationbased on the observeddirectionbly reflect thepaleolatitudinalposition (15 ±7°N) (D = 229°,1 = 42°)is not veryeasy;however,thebetter. The resultant angleof rotation since the possibleoccurrenceof the North Chineseplateincollision started to the presentday is 90°,andis the southernhemisphereduring Lower—Middlein the sameclockwisedirection asfor the Gobi— Paleozoic, as proposedby Lin et al. (1985b) isAltai rocks. preferred. Therefore, the detected component

From our point of view, data which are reli- may be interpretedas that acquiredin a reverseable and close to the original (D2_3) havebeen polarity magneticzoneat 24 ±9°in the southernacquired through studying exocontactand endo- hemisphere.contactrocks of the Dzolen and Gourvan—Sayk-han ophiolitic complexes. Two positive stability 4.4. Relativemotionstests(contact and fold tests), and, in part, con-glomerate tests, have been possible there. We Figure 8 illustratesthe motion of the localitiessuggestthat the ophiolites acquiredtheir magne- investigatedin this study, relativeto surroundingtizations(Table 4) near equatoriallatitudes(I ± continentalmasses.A trendof approachtowards4°Nfor Dzolen and 3 ±3°Sfor Gourvan—Sayk- the North Chinaplate,and in the oppositedirec-han)within a normalpolarity magneticzone.The tion relativeto the SouthChinaplate,is distinct.fact marking the coincidenceof the difference As far as Tarim is concerned,we may concludebetweenpaleomagneticdeclinationof that corn- that the relative motion for the studiedMongo-ponent for DzolenandGourvan—Saykhanridges, hanblockswassimilarto that of Tarim. However,on the one hand, and strike azimuths of their by the beginning of the Upper Permian,whichstructures(30°)in modern coordinates,on the was probablythe time of directioncollision withotherone. the North China plate, their rotation direction

had changedto the oppositedirection (counter-4.3. The South Gobiazone(‘southern’ shelf) clockwise).The distancebetweenSiberia andthe

studied South Mongolia zonesafter the MiddleTwo ancient magnetizationcomponentshave Paleozoicremainedlarge, thoughcontinuedrela-

been distinguishedby studying rocks from this tive convergenceis apparent.zone:

(1) A metachronouscomponent has beenfound, from studyof thesedimentaryrocksin the 5. Conclusionsgeographiccoordinates;its ageis Early—MiddleDevonian, i.e. the time of folding activity and (1) The Devonian Trans-Altai volcanogenic

granitic intrusions in the region (Sinitsyn, 1956; and volcanogenicsedimentaryseries(the SouthRuzhentsevet al., 1987). Another interpretation Mongolian Variscanpaleo-ocean)acquiredtheiris possible,and the directionof that component magnetizationnearthe equator.The position of(D = 352°,I = — 9°)is close to that found by P. the northern continental margin (Gobi—AltaiPruner for Gobi—Maundach Carboniferous zone)during Middle Devonianwas 4 ±4°N,andtuffites (Pruner, 1987) located350 km away to- that of the southernone (SouthGobia)was 4 ±wards the NE from our selectionpoint. 5°S.

276 AN. DIDENKO

(2) During the Middle—UpperDevonian,Car- Sun, D.J., 1987. Late Paleozoicpolarwanderpathfor theboniferousand Permian,both paleo-oceanicand Tarim platform and its tectonic significance.Tectono-

shelf complexeshad moved northwardswith a physics, 139(1/2): 145—153.Bolshakov, V.A, Gapeev, A.K. and Yasonov, P.G., 1987.synchronousclockwise rotation of their major Piezochemicalresidual magnetizationas result of alter-

structures.The rotation of the shelf elementsis ation coercive forceof samplerocksfrom zoneshypergen-

estimatedto be50—90°,andthat of the paleo-oc- esis (in Russian). Izv. Akad Nauk SSSR,Fiz. ZemI., 9:

eanicallochthonsas much as180°. 5563.

(3) The direction of motion of South Mongo- Didenko, A.N. and Pechersky,D.M., 1988. Paleomagnetismof Middle Paleozoic rocks, ophiolite complexesof Alayha,relativeto the surroundingcontinentalmasses, range(in Russian).Geotectonics,4: 56—68.

hasbeenidentified. TheSouth Mongolianpaleo- Didenko, A.N. and Pechersky,D.M., 1989. Direction and

ocean,in the Middle—UpperDevonian, was lo- intensityof the geomagneticfield in the Middle Devoniancated near Tarim, and the South and North(?) and Lower Ordovician: southernMugodjary ophiolites

China platforms.At the sametime, the distance (Urals). Phys.Earth Planet. Inter., 58: 289—306.Dobretsov,N.L., 1990. The problemof tectonicsof ophiolite

between South Mongolia and the South China belts in CentralAsia, SouthSiberaand North China. In:

platehad gruaduallyincreased(in the latitudinal N.L. Dobretsovand B.A. Litvinovsky (Editors),The Prob-

direction) while that between the former and lems of Magmatism and Metamorphismin East Asia.

Siberiadecreased. Nauka,Novosibirsk, pp. 7—25.Eenzhin,G., 1983. South Mongolian Hercynian geosyncline

(rangeDzolen—Mandal—Obo)in Early Devonian (in Rus-sian). Geotectonics,4: 87—99.

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Kirschvink, J.L., 1980. The least-squaresline and plane andAcademyof Sciencejoint SovietMongolianteam. the analysisof paleomagneticdata. Geophys. 1., 62 (3):

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