GEOLOGICA BALCANICA, 20. I, Sofia, Febr. 1990 , p. 67-84.

Structural state and geochemical characteristics of potassic feldspars from the Central Rhodope metamorphic complex

R umyana ArnaudovaJ Zlatka ChernevaJ Elena Stancheva

Geological Institute, Bulgarian A cademy of Sciences, 1113 Sofia

( Accepted for publication June 12, 1989)

P . ApHayiJoBa, 3. l.fepHeBa, E. CmaH•teBa- CmpyKmypHoe cocmoRHUe u zeoxuMu~tecKafl xapaKmepucmuKa KaAuef!bl x noAe6b/X umamoB 6 J.temaJ~tOpcpu~tecKOM KOJitnAeKce l.{eHmpaAbH.bl x PoiJon. YlccJieJJ:oBaHbi KaJIHeBbre noJieBbre wnaTbr HJ MeTaMop<jJHTOB (aM<jJH60JIHTOBoii ¢au.nn) B UeHTpaJibHhiX Po.nonax Me>KAY AOJIHHaMH p. Bbr'la H p. YeneJiapcKa . YCTaHOBJieHhi opTOKJiaJbl, npoMe>KyTO'IHbie MHKpOKJIHHbl H MaKCHMaJibHbie MHKpOKJIHHbl , KOTOpble pacnpe,!leJieHbl B CTpaTnrpa<jJH'IeCKOM pa3pe3e CJie)l.yiOW.HM o6pa30M: I) OpTOKJia-3bl - B MeTaMop<jJHTaX HII>KHeii '!aCTH pa3pe3a (Ill11pOKOJibiKCKal!, Bb1p6oBCKal!, J!HCKOBCKal!, noce­CTpHMCKal! CBHTbl), KOTOpbre 33TpOHYTbl npou.eccaMH MHfMaTH3aU.HII; 2) OpTOKJiaJbl, nepeXO.Ll!IW.He B npoMe>KyTO'IHble MHKpOKJIHHbl - B ,llo6pOJibiKCKOH CBHTe; 3) npoMe>KyTO'IHble H MaKCHMaJibHble MHKpO­KJIIIHbl - B MeTaMop<jJIITaX BepXHHX ropH30HTOB pa3pe3a (Da'!KOBCKall CBHTa). CTpyKTypHble H3MeHeHHll KaJIIIeBb!X noJieBbiX wnaTOB - nOBbiWeHIIe CTeneHH ynOpll,!lO'IeHHOCTH, BKJIIO'IIITeJibHO li B rpaHIIIJ.aX MOHOKJI HHHOH CHMMeTpHH , OT 6oJiee rny60KHX K 6onee BepXHHM CTpaTHrpa<jJII'leCKHM ypOBHSlM- o6yc­JIOBJieHbl npe>K,!le BCero pa3JIH'III!IMH B TeMnepaTypHbiX YCJIOBIIliX o6pa30BaHIIS! HJIH 60Jiee n03,!lHIIX npeo6pa3oBaHHH KaJIHeBbiX noJieBblX wnaTOB. 113MeHeHIIe TeMnepaTypHbiX YCJIOBIIH, yCTaHOBJieHHOe no CTPYKTYPHOMY cocTostnuro Ka JI HeBbiX noJieBbiX wnaToB, noATBep>K.naeTcH 11 pe3yJibTaTaMH, noJiy'leHHbiMII no rpaHaT-6HOTU TOBOMY reoTepMOMeTpy: 800-600°C - ,!lJI H MeTaMop<jJHTOB CBHT, 33TpOHYTbiX MHfM3TH-3aU.IIeii; 550-500°C - ,!lJISl MeTaMOp<jJHTOB ,Uo6paJibiKCKOH CBI!Tbl II 450-400°C - ,!lJIII MeTaMop<jJHTOB 13a'!KOBCKOH CBIITb[ .

.UaHHbre o co.nep>KaHHH Ba, Sr 11 R b B nccJie.nyeMbrx MeTaMop<jJuTax H KaJil!eBbiX noJieBbiX wnaTax noKa3biBaiOT, '!TO HX pacnpe,!leJieHHe B 60JibWeH CTeneHII OTpa>KaeT nepBH'IHOe, ,!lOMeTaMop<jJII'IeCKOe noae.nen11e 3JteMeHTOB B nopo.nax. B JienTHTOH.LlHbiX rueiicax 13a'!KOBCKoii CB HTbl H B KaJIIIeBbiX noJieBbiX wnaTax B HIIX Ha6nro.naeTcH •reTKO Butpa>KeHHOe ¢paKU.IIOHHposaHIIe Rb no oTHoweHIIIO K Ba H Sr, xapaKTepuoe )!.Jist KHCJibiX MarManPrecKHX )I.H<jJ<jJepeHU.HaTOB, •rTo yKaJbiBaeT 11a ux MarMaTii'tecKoe npoHc­XO>K)I.etnr e.

Abstract. Potassic feldspars from the Central Rhodop e amphibolite-facies metamorphic rocks between the Rivers of Vaca and Cepelarska have been st udied . They comprise orthoclases, interme­diate microclines and maximum microclines di stribut ed regularly across the stratigraphic section, as follows: I) orthoclases dominate the metamorphic rocks in the lower part of the section (the Siroka­Laka, Varbovo, Ljaskovo and Posestrimo Formations) affected by migmatization processes; 2) ortho­clases grading into intermed iate microclines prevail in the incumbent Dobral ak Formation, and 3) intermediate and maximum microclines characterize the B ackovo Formation higher up the section. The potassic feldspar structural changes from the lower to the upper stratigraphic levels with ordering increasing even among the monoclinic feldspars reflect primarily the temperature variations across the section at the time of formation or during later peri ods of thermal action . The temperature va­riations derived from the study of potassic feldspar structural states are compatible with estimates based on the garnet-biotite geothermometer: 800-600°C for the metamorphic formations affected by migmatization; 550-500°C for the Dobralak Formation; and 450-400°C for the Backovo Formation.

The evidence gained in this study on the distribution of Ba, Sr and Rb both in the potassic feld­spars and in the host metamorphic rocks reveals it as reflecting largely the primary premetamorphic relations between these elements. The Backovo Formation I epti toi d gneisses and the constituent po­tassic feldspars show a well defined fractionation of Rb against Ba and Sr similar to that in acid mag­matic differentiates, thus suggesting a magmatic origin.

67

I ntrod uct ion

It is known that the structural sta te of potass ic feld5p<lrs prO\·icles information abou t thE' conditions of their formation or of later structural transformations. bec<luse of whi ch it ha s been increasi ngly 11 sed in petrogenetic studiE's of i gn~..ous and m!:'tamorphic rocks.

Thi s paper reports a study of potassic feld spars irom the Central Rhodop e meta­morphi c rocks (including hornbl end e- biotit e, biotite, h\o-mica, and aplitoid gneisses and pegmal.ites) between the Rivtrs of Vaca and Cepelarska. Analyzed specimens co\·er practically the whole stratigraphic section of the metamorphic r.o mplex (Figs 1 and 2) <lllrl almost all rock v<l ri eties be2ring potassic feldspar, buth migmatizerl and non­migmatized . The potass ic felf1spar structura l characteristics (triclinicit~·. structura l ordering of AI and Si) and the di stributi on of Ba. Sr and Rb as t ypical trace-element constituents have been used to interpret t!lr conditions of potassic feldspar formation and the effect':' of later processes on their structural state .

X-ray studies

Pota :o:s ic feldspar SC! mples werE' extr<~dell from rock sp-=ci mens (0.300- 0.500 kg) by crushing, electromagnet ic separation, flotati on and fina l heavy- I iquid separation of sievecl 0.063-0.125 and 0.125-0.250 mm samples.

Structural types were det ermined from measu rements of the204, 060 and 1.3 1 (I J I, resp ) reflections on powder diffractogra m<. following th e method of the three refl ec­tions of Wright (1968). The t'Xpress method of A cj) o H 11 1r a et al. (1976) ,,·as used to calculate the occupancy of tetrahedral si tes by aluminum .

Siroka-Ldka, Varbovo and Ljaskovo Formations . Potass ic feldspar constituents of the migmatized gneisses (including hornblend e- biotitE>, ba nded biotite, augen­banded biotite, and two-mica varieties) are orthoclases of very similar structural characteristics . The 2T; site occupancy by AI varies in th e narrow range of 0.78 to 0.82 (Table I, Figs 2 and 3). Potassic feldspars irom the leucosome of the gneisses, a pi itoid gneisses and so me pegmaiites show s imi l.1r characteristics (TablE: 1) . Tlw single exception is a potassic feld spar sa mple (9G4, Tabk 1) from a cross pegmatitE' cutting biotite gneisses of the Varbovo formation t o the west of the village of Cepelare (Fig. I) which is an intermediate microcline having X -ray tricl inicity ( ~) of 0.43, and a I u mi nu m 2T 1 site occupanLy of 0.84 (Table I, Fig . 3) .

Posestrinw Formation. Potass ic feldspar~ from the migmatizecl hornblende-biotite and bi otite (augen, augen-banded and bandeu) gneisses and the assoc iated pegmatit cs nre al5o orthoclases nry close to those from the Siroka -U\ka, Varbovo and Ljasko\'o Formations in th.:ir structur(!) characteristics (Al in 2T 1 ranges between 0.76 and 0.84; Tabl e 1, Figs 2 and 3). Deviati ons have been found only in few sa mples from pegmatites crosscutting the metamorphic rocks near the village of Mihalkovo (Fig. 1) . These are structural ly alien intermediate and maximum microclines . One of the samples (222b) is an intermediate microcline of low triclinicit y, 0.32 (Table I); its diffractogram sho,,·s a broadt:necll31 ref lection with no splitting (Fig. 4). The other two samples are a mix ­ture of senral triclinic phases showing a very broau, diffuse ~ ~~ 1 reflection (224c, Fig. 4) and a mixture of monoclinic a nd tri cl inic (maximum microc.:line) phases dominated by th e monoclinic one (220h, Fig. 4).

Dolm:. /6.k Formation. Pota ss ic feld spars from the Dob ra lak Formation gneisses an d pegmatites are orthoclases grading locally into intermediate microclines . Compared to the potass if feldspar< from th e lower form<dion s th ey arc of more varied structures. :\luminum occupancy of 2T 1 s ites ranges from 0.72 to 0.8G in the monoclinic ya r ieties , and from 0.78 tu 0.88 in th e- tricl inic ones (Tabk l , Fig;; 2 and 3). The triclinic p ota s~ i c feld spars h<l\'e lo\\' tri cli nicity \ ' c.ilut' ~ , ~-0.12-0 .. 36; in a s ingl e sampl e !273•, Table l)

68

CJ~ OIITIJz ~3 E:2], S s t"...;..::::j5

~7 t--?g~ .j:,29g

Fig. I. Map showing the location of the specimens studied with reference to the lithostratigraphic scheme of )1\. VI a a H o a et a\. (1984): I - young sed imentary and volcanic rocks: 2-8 - meta­morphic rocks of the Central Rhodope Metamorphic Group -Formations: 2 - Dobrostan , Javrovo, Sitovo and Bjala-Cerkva; 3- Backovo ; 4- Dobralak: 5- Posestrimo; 6- Ljaskovo; 7- Cepe­lare; 8- Vftrbovo and Siroka-Uka ; 9 - potassic feldspar s: a - orthoclases; b - maximum and intermediate microclines; c - orthoclases and microclines

69

Lithoatratigraphio aQbdivia!on &!ter Ivanov, !. et al. ( 19&4) •

J'OBIU!IOI - clo:AiDMt t7pe of rook•

DOB!OS~AI - ~ble11

JATR01U - oalo11ah1•t• and aic• eehi•t•

SITOfo - marble•

:BJAL! CEHi:VA - lr.)'&ni h­and ataaroli t-be&.riDC eohi•t•

"' BACIOVO - leptitoid gnei••••

#o

DDRRALil - biotite atJd tvo-aica gnel••••

POSESTRlMO - gaeiaaea, IIObiata, SAIOlel, a.u.{'h : bcli tea

LJ!SKOVO - biotite gn•i••••• aaphibolite•

CEP&LARE - amphibolite•, ga~~et-kyanite acbi•t•, gneinaea, aarblea ,. VARBOTO -biotite, two-aica and

ltno"'-1 auw .c poU..Ido ~~ ........

IC&.r Ce }lotaaeic !elU,aH

aaxiaua aicrocline 6-0,78-1,00 1nterac4iate alcrocline • - o, ')6-0,68

ir.tuwed.iate a!crocline 6 - 0. 12- '-' ' 56

orthoclaae

orthoclaae

aplitoid gno1•••• orthoclase

~IRO~A - ho~blend._ biotit• and biotite gn•i••~• ortboclaae

lli'•tocr- or .U/~1 order in PG~••ic feldapar•

12 10 8 fi 4

2 Q .• L .. J

1.00 09 2 a.&4 0.7.2

e 6 I.

2 o ........ ~.-

1.00 0.9~ 0.&4 8 ., 6 j

(,~ 2 ;

6 L

2 s ~\.00........_,...0.=92 1).81,

A l T 1(o)•Aiy1(M)

Fi g. 2. H ys togra ms of AI occupancy of the 2T1 t etr ahedral s ites in potassic felds pars (!1- triclinicity)

it i :; as high as 0.56. Diffractogrnms shO\\ sl ightly broad er 131 reflect ions \\ i t h no spli t ­ting. The samp le 152b, belonging to a pegmat i te crosscu tt ing biotite gneisses northen st of the v ill age of ,\ \i ha l koYo. is an excepti on; i t r eprl'"sents <~ mixtu re oi monoc l i nic an d t r iclinic phases in approx i ma t el y ;:qun l amou nt s.

70

Tabl e 1

Al occupancy of tetrahedral sites (T1 (o), T1 (m), T2 (o), T3 (m)} in potassic feldspars f rom Central Rhodope metamorphic rocks

No of Petrographic type I Tricli- I T1 (o) T1 (m) I T1 (o) + I Tt (o)=

sample nicity I

T1 (m) T2 (m)

2 ____ 1_ 3 __ j- 4 5 ---- I 6 I 7

Siroka-Utka Formation 51 a hornblende-bioti te gneiss 0.00 0.40 0.40 0.80 0.10

Vttrbovo Formation 360b "eyes• in biotite gneiss 0.00 0.39 0.39 0.78 0.11 369b pegm atoid leucosome 0.00 0.39 0.39 0.78 0.11 391 a biotite gneiss 0.00 0.39 0.39 0.78 0.11 3933 biotite g neiss 0.00 0.39 0.39 0.78 0.11 3943 biotite gneiss 0.00 0.39 0.39 0.78 0. 11 580 aplitoid gne iss 0.00 0.43 0.43 0.86 O.Q7 581 aplitoid gneiss 0.00 0.39 0.39 0.78 0.11 937 bioi ile granite-gneiss 0.00 0.41 0.41 0.82 0.09 940 pegmatite 0.00 0.41 0.41 0.82 0.09 952 a pi itoid leucosome 0.00 0.41 0.41 0.82 0.09 953 pegmatoid leucosome 0.00 0.41 0.41 0.82 0.09 960 aplitoid zone in pegmatite 0.00 0.40 0.40 0.80 0.10 961 pegmatoid zone in pegmatite No 960 0.00 0.40 0.40 0.80 0.10 964 pegmatite 0.43 0.64 0.20 0.84 0.08

Ljaskovo Formation 294b pegmatite 0.00 0.39 0.39 0.78 O.ll

Posestrimo Formation 102a biotite gneiss 0.00 0.40 0.46 0.80 0.10 102b pegmatite 0.00 0.40 0.40 0.80 0.10 I04a biotite gneiss 0.00 0.38 0.38 0.76 0.12 104b pegmatite 0.00 0 .36 0.36 0.72 0.14 !05b biotite gneiss 0.00 0 .39 0.39 0.78 0.11 Ill b biotite gneiss 0.00 0.38 0.38 0.76 0.12 I14a biotite gneiss 0.00 0.40 0.40 0.80 0 .10 !14c amphibole-biotite gne iss 0.00 0.37 0.37 0.74 0.13 133a biotite gneiss 0.00 0.39 0.39 0.78 0.11 133c pegmatite 0.00 0.38 0.38 0.76 0.12 146a biotite gne iss 0.00 0.40 0 .40 0.80 0.10 146d biotite gneiss 0.00 o.40 0.40 0.80 0.10 146e a pliloid gnei ss o.oo 0.40 0.40 0.80 0.10 183c pegmatite 0.00 0.38 0.38 0.76 0.1 2

220b pegmati te {0.00 {0.43 {0.43 e .86 {O.o7 0.76 0.82 0.04 0.86 0.07

222b pegmati te 0.32 0.57 0.24 0 .81 0.095 224c pegmatite 0.85 0.90 0.02 0.92 0.04

Dobralttk Formation

9 bioti te gneiss 0.15 0.48 0.32 0.80 0.10 11 bioti te gneiss 0.21 0.54 0.30 0.84 0,08 14 two-mica gneiss 0.00 0.42 0.42 0.84 0.08

152b pegmatite {0.00 {0.43 {0.43 {0.86 {O.o7 0.88 . 0.88 0.00 0.88 0.06

157b pegmatite 0.36 0.59 0 .21 0.80 0.10 270b biotite gneiss 0 .32 0.59 0 .27 0.86 O.Q7 272a two-mica gneiss 0.00 0.40 0.40 0.80 0.10 272c biotite gneiss 0 .00 0.39 0.39 0.78 0.11 273a aplitoid gneiss 0.56 0.69 0.13 0.82 0.09 273b bioti te gneiss 0.19 0.54 0.30 0.84 0.08 274 bioti te gneiss 0.00 0.43 0.43 0.86 0.07

7l

Table I (continued)

2 3 4 5 6 7

275a biotite gneiss 0.12 0.48 0.36 0.84 0.08 276& biotite gneiss 0.00 0.42 0.42 0.84 0.08 334a biotite gneiss 0.17 0.50 0.32 0.82 0.09 334b aplitoid gneiss 0.22 0.54 0.32 0.86 0.07 335h pegmatite 0.00 0.40 0.40 0.80 0.10 336& aplitoid gneiss 0.00 0.37 0.37 0.74 0.13 337b aplitoid gneiss 0.17 0.48 0.30 0.78 0.11 337C biotite gneiss 0.00 0.36 0.36 0.72 0.14 337d pegmatite 0.00 0.39 0.39 0.78 0.11 338a biotite gneiss 0.00 0.41 0.41 0.82 0.09 338at •eyes• In No 338a 0.00 0.39 0.39 0.78 0.11 J38b aplitoid gneiss 0.32 0.60 0.28 0.88 0.06 340a biotite gneiss 0.00 0.39 0.39 0.78 0.11 340b biotite gneiss 0.24 0.52 0.26 0.78 0.11 340c pegmatite 0.00 0.40 0.40 0.80 0.10 983 a1pitoid gneiss 0.00 0.39 0.39 0.78 0.11 984& muscovite gneiss 0.05 0.42 0.36 0.78 0.11 984b muscovite gneiss 0.19 0.52 0.30 0.82 0.09 985 aplite-pegmatoid vein 0.00 0.39 0.39 0.78 0.11

Bat kovo ForllUJtion

8 1eptitoid gneiss 0.78 0.87 0.07 0.94 0.03 69a leptitoid gneiss 0.90 0.95 0.01 0.96 0.02 81 leptitoid gneiss 0.95 0.95 0.01 o.g6 O.O'.l

414 1eptitoid gneiss 0.80 0.88 0.06 0.94 0.03 252h leptitold gneiss 0.78 0.85 0.03 0.88 0.06 2548 1eptltoid gneiss 0.68 0.81 0.11 0.92 0.04 255 8 leptitoid gneiss 0.78 0.86 0.04 0.90 0.05 277 1eptltoid gneiss 0.81 0.90 0.02 0.92 0.04 278 leptltold gneiss 0.83 0.88 0.06 0.94 O.D3 327& biotite gneiss 0.58 0.75 0.15 0.90 0.05 327h leptitoid gneiss 0.90 0.94 0.00 0.94 0.03 395& 1eptltoid gneiss 0.53 0.76 0.18 0.94 0.03 395b leptltoid gneiss 0.68 0.82 0.12 0.94 0.03 610 leptitoid gneiss 0.99 0.99 0.00 0.99 0.005 6lla two-mica gneiss 0.98 0.96 0.00 0.96 0.02 613 1eptitoid gneiss 0.98 0.99 0.00 0.99 0.005 614 leptltoid gneiss 1.00 1.00 0.00 1.00 0.00 615 two-mica gneiss 0.98 0.99 0.00 0.99 0.005 616 1eptitoid gneiss 0.93 0.96 0.00 0.96 0.02 618 leptltoid gneiss 0.96 Q.98 0.00 0.98 0.01 621c leptitold gnelas 0.91 0.95 0.01 0.96 0.02 622 two-mica gneiss 0.80 0.89 0.05 0.94 0.03 623 leptitold gneiss 0.88 0.92 0.00 0.92 0.04 624 two-mica gneiss 0.86 0.92 0.04 0.96 0.02

976 leptitold gneiss {0.00 {0.43 {0.43 {0.86 {0.07 0.87 0.88 0.00 0.88 0.06

917 leptltoid gneiss {0.00 e·42 {0.42 {0.84 {0.08 0.44 0.82 0,02 0.84 0.08

978 two-mica gneiss {0.00 e·40 {0.40 {0.80 {0.10 0.78 0.76 0.04 0.80 0.10

981 & two-mica gneiss 0.36 0.59 0.21 0.80 0.10

Tricliniclty- A = 12.5 [d(l31 t (d131)] (after Go I d smIth & Lav e s, Analysts: N. Zotov, N. Durceva, T. Bojadzieva, A. Georgleva

1954)

72

T2o•T2m

·----- -·-·-.--t ----I f I I I' I I .O.l

0.7

• + •: • I •

·+:.: · ~ i • • Mj

...,___::""'--''----"0,"-'8:<..;;~.:.:.· 2___::0,"-'7 v<-=O.:.:...l--=c0,"'-'6"'-"0.:.:...4 ___::0,'-'-'sl'-='O.,:._S ___::0.""4.'-".0'"-6 ___:oO.::_,l •<=;•0."-7 --=c0.2,_.,..:c,O·c:..e -'"0.1 0,9 • T,m

Fig. 3. AI occupancy of tetrahedral sites in alkali feldspars (after St e w a r t & Wright, 1974) Potassic feldspars from metamor­phic rocks belonging to t he formations: 1 - Ba ckovo; 2 - Dobra­lak; 3 - Varbovo and Posestrimo; 4- fi eld of the monoclinic feld­spars belonging to the Siroka-Uka, Varbovo, Ljaskovo , Posestrimo end Dobra! i\k Formations

)I. )0° 29. )J•

Fig. 4. Diffractograms of potassic feldspar sampl es from pegmatites (220b, 222b, 224c)in the Mlhalkovo area and from gneisses (976, 977, 978) in the Skobelevo area

Opt ica l mi croscopy of gnei sses from thi s for ma ti on also shows the structural hete­rogeneit y of potass ic feldspars. Bes ides the undul atory patchy ex tincti on t ypi ca l of feldspars in metamorphi c rocks, some thin sections a lso show a vague reti cu late t exture resembling microcline cross- hat ching. There are exampl es , however , of indi stinct re­ti cul a te textures seen in potassic feldspars whi ch produce monoc linic X-ray patt erns.

73

The ava il able evidence s hows th<lt tri c!inic structures are t y pi ca l mostl y of th ose metamorphic series which, according to the current lithostratigraphi c schemes ()K. VI H a­HoB et a l., 1984; K o :>K y x a p o B, 1984), lie in the upp ermost levels of the Dob­ra lak Formation close to the Ba ckovo Formation lepti toid gne isses.

Bacfwz•o F ormatit ·tL Gneisses of thi s formation have been found to contain max i­mum microclines, intermed iate microcline", as well as mixed triclinic and monoclinic phasl's. Th e most common vari eti es are maximum microclines oi high X-ray triclinicit y values,~ - 0.78-1.00 (Table I. Figs 2 and 3), and a well defined pair of 131-13 1 reflec­ti ons. Structurally, the intermedi at e microclines (~ - 0.53-0.68; AI occupancy of 2T 1 s ites: 0.92-0.94; Table I) are fai rl y s imilar lo the maximum ones. Potassic feldspars with so me content of monoclini c phases (samp les 976, 977, 978; Table I, Fig. 4) have been found only in the leptitoicl gneisses from the Backovo Formation western outcrops near the vi ll age of Skobelevo; the triclinic phases in those sa mples consist of maxi mu 111

mi crocline (97G, 978) and intermedi ate mi crocline (977). Potassic feld spar is not co mmon in the metamorphic roc ks over lying th e Bac kovo

Forma~ion. It is sporadica ll y present only in some of the gneisses belonging to the Bj a la-Cerkva Formation and we have not succeeded in collecting sufficient liUantities to permit analysis. Under the microscope, however, thi s potassic feld spar shows well­defined microcline cross- hatching which suggests th e presence of triclini c ~tructur e' .

The observed distribution of the \Rrious _, tructural types of potRssic feldspar in the roc ks of th e strati graphic sedi\1n "ludi ed shows that ordcr:ng increase-; even ins ill e the group of monoclinic polymnrphs f;-om the lo\\ er to the upp er stratigraphic leYt·h (Fig. 2) . Rec koning that the tran,formatic>tl t emperature of monoclinic to tri clinic pota ssic fE-l ds par is abou t .500°C, it see ms justified to interpret thi s trPnd of structural cha11ges as reflecting l~rgel y the vari Rti ons i11 the temperatl!r e conditions of formatioll or of some lat er thermal action.

In th e formntions ;1ffected hy migmatiza ti on (fh iO' Siroka-Ulka, Varbovo, Lj askO\'O and Posestrimo Formations), the fin al t emperat ure of crystallization exceeded 500°C, and th<:> equilibri um rnodificRtion of potass ic feld<;p?r wRs the monoclinic one. ·1 hi s is inconsistent with th e presence of triclinic polymorphs in so me ?egmatites bel onging to th ese formations but no conclusive exp lanation c,f the fRet has been found ~o far. Since a ll four find s of triclini c feldspars come from cross pegm"tites, it may be assumed tha t they are relati vely l::lter products fo rmed at increased potential s of volat ile compo­nents. Furthermorr, it may well be tha t the monocliJLic potass ic feldspars su ffered st ruct ural tra nsforma!ions into tridi nic ones as the result of later host rock deform R­tions invol v ing lluid emanations. Such a mechRnism may ex plain the structural he­t erog~ne ity of th<:> three fe]t\ <; par samp les from th e vicillity of the village of _Mihalko vo where th e metamorphic host rocks are tectonicall y reworkecl, and in pl i:l ces hydroth <:' r­mallv altered, too.

in the rocks oi the Dobra lak Formation, the monoclini c potas.;;ic feldspars show better ord ering and a wider range of monoclinic po!ymorphs as compared to the ortho­ciRses from the migmatized zones, and the triclinic vflri eti es a re of IO\\. triclinicity , their stru ctural characteristics (AI occupancy of 2T 1 s ites) being s irnihr to those oJ monocli nic fe ldspars. AI! t1i s indica tes formation a1 temperatures very close to the transition (.SOOcC) of th e monoclinic to the triclinic modificati on. At such transiti on<ll t emp erRiures, e\en t he s li ghtest changes in the regime of volati les, chiefly \\" a ~ e r , may tri gger the transform ati on of monoclinic polymorphs into triclin! c cnes , especial ly if t ect oni c ceformatiOilS are inv olved . The fine reticulate tPxturt: , observed under the microscope in so me potassic felds pars from thi s format ion, may be regarded as 2n inci­pient mi crocline cross-hatcherl te xt ure betray ing transform ation of monoclinic feldspar5 into tric!i nic ones. The ca se::, of X-rRy p ::lttern~ showing only rnonoc1iilic sy mmetry may be du e either to the s mall content (<5 %) of thi s low-tric lini c potassic feldspar in the samJ)le examined, or to the presenl~e oi the inc ipient triclinic domains of Put­n i s & ;\-\ c Conn r 1 1 11980) produ cing the vague reticul ate tex ture seen !n thin

74

sections . T he above authors ha ve shown tha t monoclinic feldsp ars can a tt ain some degree of order on the bc.5 is of their mo nocl inic sy mmetry , i.e. "the process of inver<; ion from ort hoclase to rnicroc line involves the loca l gro'l!.th of ; he regions of differing A I s it e preference" (Pu t n i s & M c Con ll e I I, 1980, p . 169) . In such cases , the reticu iRte tex t ure is not identical with the microcline cro<>:;- hatching , i . e. it i<> not the result of fi ne-scale microc 1ine twinning , no pha sC' inter faCl''i are formed. nor symmet ry is changed.

The potass ic feldspar equilibrium modifica t ion in the B<lcko\·o Formation me­t a morp hic rocks is the tric l ini c one which indica t es t em peratures of formation lower thRn 475°C. The p r e~er.ce of structural ly het erogeneous potass ic feldspars in the Skobe­levo area is most l ikel y <l l.! f t o eleyated temp era tur e<; during the migrni'ltizRti o r; whi ch . according to )1\. Y! B RHo B et a l. ( 1985), has a ff ected th e metRmorphic series from t he west ernmost ou tcrops of th e formati on .

Geothermomet ry st udies

The t empera t ure di stribu t ion across th e strati grap hi c seque11ce studied. as deri ved irom t he str uctu ra l cha racter isti cs of potass ic feldspars, is susta ined hy th e pub l ished rnine­ra!- t hel rnometry da t a (B e p r H .1 o E , 196() ; K o z h u k h a r o v t't a !., 1977; K ocTo R d Rl., 1986) as \\'e)l as by ou r own geothermomet ry resul t ::. u<>i ng th e gamet­hi ot!t e pair .

The chemi ca l analyses of biotitt:> and garnet sam pl es used in thi s study are gi,·elt in Tab les 2 and 3. Garnet s show no zoni ng a nd are of more or less identi ca l compos ition

T a b l e 2

Chemical composition (weight 9o ) of biotites f rom Central Rhodope metamorphic rocks (X-ray micro­analysis on a Jeol Superprobe 733 instrument )**

Sample No I

Na20 ! ( ) number of Si0 2 Ti03 Al 20:~ FeO I

MnO MgO CaO K20 analyz ed grains

I _______ ___ j

Vdrbovo Formation

581 (3) 35.48 2.24 17.38 20.73 1.15 8.22 0.14 1.03 9.07 951 (3) 36. 18 2.96 16.23 19.45 0.58 9.91 0.10 1,03 8.95 960 (3) 35.26 3.20 17.09 21.16 0.69 8.20 0.08 0.82 8.93

Posestrimo Formation 104• (3) 35.35 3.28 15.70 21.25 0.44 9.25 0.10 0.89 9.09 104•1 (2 ) 35.44 2.90 15.69 21.83 0.50 9.21 0,07 0 .52 9.24 JOSe {3) 35.96 2.65 16.27 19.44 0.4 1 10.52 0. 17 0.98 8.98

Dobraltlk Formation 272c (I)* 38.81 2.59 16.5 1 18- 13 0.29 8.76 0.00 0,00 9.88 337C (.3 ) 35 .01 3.34 17.74 19.52 0.43 8.98 0-20 1.04 9.23

Backovo Formation 25 4• (I)* 37.83 2.25 17.20 20.22 0.46 7'3 1 0.12 0.00 9.98 395a (lr 37.55 1.07 16.54 18.37 0.41 9.79 0.03 0.00 9.85 622 (3 37.33 1.47 16.38 15.47 0.81 13.31 0.19 0.9 1 9.53 977 (4) 37.31 2 11 16.03 16.00 0.92 12.56 0.13 1.05 9.24

Bjala-Cerkva Formation 269a ( I)* 37. 11 2.40 16.77 20.14 0 .. 19 9.45 0.00 0.00 10.64

* Analys es made at t he Moscow State University. '!"'' Major oxides are calculated to 100% including 4.SH20 . Analyst: H. Neikov.

·75

Table 3

Chemical composition (weight %) of garnets from Central Rhodope metamorphic rock (X-ray micro­analysis on a Jeol Superprobe 733 instrument )**

Sample No () number of SiOa Ti02 Al20 3 FeO MgO MnO CaO

analyzed grains

V arbovo Formation

SRI (2) 35.76 0.05 20.04 21.60 1.91 19.37 1.27 951 (3) 36.69 0.02 21.00 31.00 3.70 4.56 3.02 960 (3) 37.47 0.11 2 1.32 28.06 4.96 2.54 5.54

Posestrimo Formation

104• (3) 36.95 0.09 20.38 23.05 3.05 8.36 8.11 104•1 (2) 37,38 O.OR 21.43 22.45 4.44 6.98 7.23 lOSe (4) 36.84 0.05 20.90 22.76 3.30 9.33 6.82

Dobralak Formation

272c (2f' 37.25 0.04 2 1.3 1 28.07 5.54 0.4 1 7.34 272c (2 )* 38.10 0.02 20.45 17.22 0.84 11.22 10.50 337c (2 ) .16.60 0.06 21.11 22.77 2.38 11.70 5.38

Backovo Formation

254a (2)'!' 38.47 0.01 20.51 17.29 0.60 10.86 11.16 .19S a (2)* .18.60 0.01 20.60 13.70 0.48 13.71 11.50 622 (7) 37.37 0.18 20.50 12.72 1.62 15.61 12.01 977 (2) 36.38 0.06 20.68 11.69 2.48 26.41 2.20

Bjala-Cerkva Formation 269a (2)* 40.97 0.01 21.40 21.03 1.20 6.68 7.58

" Analyses made at the Moscow State University. *" Analyst: H. Neikov .

acros!-- t he individual ho,; t rock specimens (Ta ble 3). A si ngle specimen (272b) of a biot it e gneiss from the Dobralak Formation has beeli found to host two kinrls of garnet of dif­ff:'rtnt chemica i compos iti olls (Table 3) which have accordi ngly yielded cliffere11t t empe­ratures. With iew excepti ons (951 - leucoso me, 960 - pegmatite, and 272c - bi ot it e' gneiss), garnets show hi gh /\·\nO cont ent s, the CaO va :ues being also hi gh in most gra ins (Tab le 3). :\ s known from fJUbli shE'd stud ies (0 e p l£ y 1<, 1967; 198 1; f .~ e 6 oR u IL ­

K H i1 et al., 1977; H o d ges & Spear. 1982). the increased .~\n and Ca cont ents in garnet di st urb the ideal di stributi on of Mg <mel Fe among coex isting ga rnet and biotite !e;tdi ng t o app reciable uncert ainties in the ca lculat ed t emperature va lu es. That is why in such cases we report esti mat es rat her t hnn the ~xact temperature valu es ca lcu­lat ed after n e p l{ y !{ (1967, 1981).

On the dia gram of Mg di stri bution in coexisting garnet a nd bioti t e in coordinat E's X~1~-- X~g (af ter r .'1 e 6 o B 11 U i\ H ~i . 1977), the ga rnet-biotitt• pai rs in thi s <; tud y

Plot in two fields (Fi g 5): one field in the interval 800-600°C wh ich groups t oget her the analyses from the Varbovo and Posest rimo formation ' . whNe th e processes of mi gmatiz(lt :on a re mo:>t pronounced , and (l nother fie ld of t 2mperatures lower than 5_50-400cC which ou tlines th e grou p of analyses fron i the Dobralak, Bac kovo and Bjala­Cerha Formati om ; th e Backovo and Bjala-Cerk v:1 metamorp hi c roc ks pl ot at the lowC''t t em peratures (450-400cq. As already reported, one gnei ss S(l mple from the Dobral<'\k Formation hac; been found to contain h\·o t ypes of garnet which differ app re­ciab ly in their content of FeO, MgO nnd M110 (Table 3). Th t: cleri\·ed tempera tures of

76

forma:ion a r ~ abou t 780 i:tnd 490-c, respecti ,·ely. Comparison ,,·ith the published mi­nera l-t h<~rmomet r y cb la aJtd with our res ults on the structural state of potassic feldspars in th is !urllla!ion indica t es that th e ga rnet of lower MgO content is the eq uilibrium product in th at particu!ar metamorphic env iron ment wherea s the other t ype i::, a relict ph a ~ e.

Distribution of Ba, Sr, and Rb

Ba. Sr and l( b cc.!llknts hi:tve been tlet ermined by .X-ra y spcclri:t l a!lal ysis \\illl a relative f.'JTor of <1bout J(; l!,. Barium ana ly::,is in potass ic f<::ldspars fo ll owed the so-calltd indi ­rect method fo r bari um concentrations hi gher than 2UOO !Jpm Jeyeloped by G. Pal1a:-;otuv and J. h: otorova (A p J! a y J. o B <' t a! , 1988). The anal yzed sa rr.ples of metamorphic rocks and pot a ·s ic ff'l dspars be lo ng mostly to the Varbovo, Posestri mo, Dobralak a nd Backo,·o Formations.

Siroka- Lalw ond Vclrbovo Fot111ations. The ana lyzed samvle::> of bi otit e gneises am! constituent potass ic feldspars from t he Varbovo Formati on show R marked ver­s istenc v in U1e contents and distributi on of Ba , Sr and Rb (Ta ble 4). The overall mean contents and element ra ti os (Ba -- 745 pp m, Sr - 245 ppm. Rb - 215 ppm, Ba Rb - 3.6: Table 5) are dose to the al ready pub lished values for thi s formation (4 e p 11 eRa eta!. , 1!:187) . 1 he ti1eai t v e~ Ju es in the ccmstituenl potassic ft> ldspars are : Ba - 2 11 5 pptr., Sr - 380 p!J rn , R.b - - 425 ppm, Ba. Rb - 5. 1.

The migma t ic leucosomt has h igher Ba contents ( 1474 pnrn ott the average) and higher Ba · Rb va lues (7 .2 on the average) than th e gne isses due largely to the grea ter contf•nt of potassic feld~par. As compared t o th e pot:J ss!c feldspars from i ht: gne isses , those from the migmatic bodil's show a certain tendency to concent ra te Ba with respect to Rb , \~h i c h i" particul arl y pronounced in the pota·:.;, ic feldspar sa mpl es (38 Jb and 38 Jc) from th e Siroka U 1ka leucosomt• (Tahle 4).

x Gr Mg

o.a ·

0,6 -

01. -I

0,2

+ I 1::>. 2 . ] • f. 0 5

0.2 OJ. 0,6 o.a xBi Mg

Ba,g/t

10000

sooo

2000

1000

;o

--' ' 1 1

\

' ' ' ><

'

20

•

•• •

AI

' •l ' ' ' " 3 ....

' ---' ' -· -· 5 ' ' ····· 6

' - 7 '~

1 I ___ __ __ ,.

" J(

X

)0 40 so 60 Or, '1.

Fig. 5. Temperature conditions of mineral formation (after r JI e 6 o s H u K 11 ii, 1977) derived by using the garnet-biotite geothermometer XG~ s. X~ Formations: 1 - Varbovo; 2- Posestrirrw ; 3- Dobra lak; 4- Ba ckovo; 5 - Bjal a-Cerkva

Fig. 6. Ba conten t in potass ic fel dspars ve rsus normative orthoclase (Or) in the rocks 1 - high-Ba pegma tites from t he Posestrimo Formation; Varbovo Formation: 2- leucosome, 3 -apli toid gnei sses; field s of gneisses from the formations: 4 - Ba ckovo, 5- Dobral ak, 6- Posestrimo, 7 - Var bovo

77

Tabl e 4

Ba, Sr and Rb contents (ppm) in metamorphic rocks and constitumt potassic feldspars I

Me ta morphic rocks I Potassic: feldspars I

Sample. No ~-Ba Ba Sr Rb Ba /Rb Sr Rb BajRb

2 3 4 5 I 6 7 8 9

Siroka-Ldka Formation

:!8 I l> 1154 .396 136 8.5 3390 6:37 :!06 11.1 38J c :3415 522 231 14.8 5340 807 .156 19.0

Vdrbovo Formation

28:2 752 259 197 3.8 2620 42:2 483 5.4 28;l 732 206 2-10 3.1 2100 323 399 5.3 299 11 65 225 251 5.2 2370 306 422 5.6 360• 762 287 183 4.2 2400 447 397 6.0 360 1.> 1408 338 273 4.2 2004 467 ·104 5.0 369b 2047 307 307 6.7 :2805 359 381 7.4 385c 374 150 304 1.2 1620 261 520 3.1 392a 788 255 240 3 .. 3 2300 480 448 5.1 :392b 1-118 182 20:J 7.0 2500 404 515 4.9 393 3 603 327 189 3.2 2167 435 -Ill 5.3 394a 1251 29 1 178 7.0 2996 427 348 8.6 580 34 ·H 432 0.1 297 76 897 0.3 581 23 60 237 0.1 197 91 51 8 0 4 895 80 :2 13 267 0.3 369 117 986 0.4 896 559 188 228 2 4 11 42 309 505 2.3 9.37 442 269 143 3. 1 1415 396 29.3 4.8 940 895 210 483 I. 952 669 178 14-1 4.7 2800 226 31-1 8.9 953 1770 196 17:2 10 .. 1 31 80 293 374 8.5 960 439 284 188 2.3 1814 422 410 4.4 961 110 2:38 151 2.7 1144 ;l61 395 :LQ 964 514 174 490 1.0

L jaskovo Formation

294b . :230 121 280 0.8 700 171 161 1..5 522a 131 0 409 148 8.8 3300 701 299 11.0 52 ·1a 1603 378 147 10.9 3260 5:l3 321 10.1 540 146:! 57 1 130 11.2 3670 733 299 12.2

Posestrimo Formation !Q1 a 452 272 11 2 4.0 3173 412 290 !0.9 I0:2a 516 206 15-1 3.4 181 9 325 3-11 5.3 102h 625 471 151 4.1 1940 347 316 6.1 104a 741 415 112 6.6 4791 1025 238 20.1 104b 3000 485 155 19.4 6000 720 271 22.1 105 b 614 449 13:! -1.6 -1700 540 269 17.5 133a 123:2 436 57 21.6 5Y33 760 156 38.0 133c 1026 630 69 14.9 4673 742 196 23.8 13-le 1370 9-13 69 19.8 9550 11 70 293 32.6 1-'36b 316 77 789 0.4 146e 684 213 11 8 5.8 2202 304 277 8.0 183c 1119 185 120 9.3 3493 12 291 12.0 220b 201 142 145 1.4 994 195 492 2.0 227. b 605 126 566 1.1 224C 565 187 621 O.Y

Dobraldk Formation

14 545 21 2 25 1 2.1 191 9 321 584 3.3 \52 b 229 126 171 1.3 7 10 180 490 1.5

78

Tab 1 e 4 (continued )

2 3 4 5 6 7 8 9

157b 2000 609 186 10.7 81 79 746 269 30.4 270b ·103 185 289 lA 1860 270 553 3.4 272a 397 120 220 1.8 1249 191 423 3.0 272C 625 197 165 3.8 2939 2H 325 9.0 273a 76 57 305 0.2 490 105 725 0.7 273b 401 228 202 2.0 1537 346 400 3.8 334• 202 62 296 0.7 967 85 675 1.4 334b 121 37 330 0.4 2·17 40 610 0.4 336• 353 97 120 2.9 1867 181 265 7.0 337b 65:2 151 141 4.6 1800 252 301 10.0 337c 813 220 126 6.7 4379 371 297 14.7 337d 3154 289 275 11.5 338a 914 217 129 7.1 4704 3-12 267 17.6 338• 1 5468 354 296 18.5 338b 630 158 104 6.1 323·1 220 317 10.2 340" 838 278 136 6.2 7020 413 255 27.5 340b 776 151 132 5.9 4224 168 297 14.2 340C 720 246 IH 4.1 3553 426 412 8.6 983 2792 623 11 2 24.9 7520 812 184 40.9 984a 660 170 92 7.2 984b 569 198 92 6.2 636 475 144 4.4 985 1158 371 220 5..3

Backovo Formation

8 225 82 204 1.1 895 173 462 1.9 o l 185 58 377 0.5 19-l 37 1380 0.1

414 230 95 115 2.0 593 168 695 0.8 :252b 70 108 196 0.4 71 -l 258 508 1..1 254a 71 97 283 0.2 673 194 648 1.0 255a 450 148 289 1.6 1388 251 547 2.5 277 168 114 415 0.4 668 267 1032 0.6 278 109 44 481 0.2 3·16 72 937 0.4 327• 595 270 160 3.7 ;~234 220 317 10.2 ,'J27u 817 231 202 4.0 1527 36:3 383 4.0 395• 244 126 380 0.6 853 229 948 0.9 395ll 130 66 477 0.3 485 94 1096 0..1 610 77 57 320 0.2 784 128 872 0,9 611 a 158 251 190 2.4 2560 329 348 7..1 613 165 124 361 0.5 707 211 90.3 0.8 61-1 70 47 102 0.2 479 66 1057 0.4 615 526 179 311 1.7 1626 197 579 2.8 616 99 2 1 543 0.2 439 51 1-1 98 o.:l 617 101 Ill 278 0.4 309 193 523 0.6 618 70 32 355 0.2 317 95 1031 0.3 621 C 227 40 441 0.5 7.:t8 97 873 0.8 622 772 120 189 3.8 2860 302 368 7.8 623 145 35 217 0.7 978 109 460 2. 1 624 249 89 251 1.0 1367 211 -1 96 2.8 976 121 55 :387 0.3 565 120 818 0.7 977 213 84 382 0.6 634 11 5 695 0.9 981 a 503 93 245 2.0 1442 123 597 2..1 978 335 129 311 1.1 127·1 198 768 1.7

Samples belong to petrographic types as described in Tabl e 1: No lOl a-hornblende-biotite gneiss; Nos 360a, 510 - biotite gneisses; Nos 282, 283, 385c, 392a - two-mica gneisses; No 895 - aplitoid gneiss; Nos 299, 522•, 524•, 896- biotite granite-gneisses; o 392b- leucosome: No 381 b- applitoid leucosome; No 381c- pegmatoid leucosome.

Analysts: G. Panajotov , M. Levacka, A. Najdenova.

79

The three elements ~how entirely different concentrations in the aplitoid gneisses­(Tab:e 4, Fig . 6). The h:gher Rb contents than those of Ba and Sr both in the rocks and in their potassic feldspars determine the fairly low values vf the B.a 1Rb (0 .2, and 0.4 resp ecti vt-ly, on the average) and Sr•Rb ratios (0.3 nnd 0.1) . These geochemical re­lationships, typical uf the gran!tuid magma leucocratic differentiates , and the fairly uniform distribution of Ba, Sr and Rb in potass ic feldspars from eat:h gneiss variety sustain the interpretation of LJ e p 11 en a ct a l. (1987) who argue magmatic origin of th p meta morph!c rods studied .

Ljaslwuo Formation . The Ljas kovo biotite gneisses and grCJnite-gneisscs have higher Ba and Sr, a nd !own Rb contents than the Varbovo gneisses, the averagt• value~ being: Ba - 1460 ppm, Sr - 455 ppm , Rb - 140 ppm, Ba/ Rb - 10.3 (Table 5). Correspon­dingl y, the constituent potassic feidspars also show higher Ba (3410 ppm) and Sr (655 ppm), and !ower Rb values (305 ppm); Ba ' l~b -- 11 . 1. In contrast to the gnies<>€S , a pegmatite veill (294b), which crosscuts the metamorphic rocks to the northwest of Cepe­tare, has very low content of Ba (230 ppm) <lnd very low Ba 'Rb ratio (0 .8) combined with lower Ba (700 ppm) and Sr (174 ppm), ann higher Rb (461 pJ-im) concentrations in the constituent potass ic feldspar as compared to that in the migmatic bodies .

Posrstrimo Fornwtir)ft. Compared to the Varbovo gneisses, the biotite gneisses of thi s formation have similar Ba (705 ppm on the average) aPd Sr (330 ppm), and lower Rb (115 ppm) concentration~ (Table 4 and 5). Their potassic feldspars, however, show higher BCJ contents (1430 ppm) and correspondingl y higher Ba ' Rb values (!2.4) than the Varbo\'o ones, which is due to their IT1Uch lo\ver amount in the ana lyzed gneisses. This is apparent from the comparison of the amount of normati ve orthoclase (Or} pre­sent in th(· gneisses \\ ith the Ba contents of potass ic feld spars (Fig. 6).

TableS

Mean Ba, Sr in Rb contents (ppm) in metamorphic rocks and constituent potassic feldspars

Petrographic type (number of analyzed Mean rock samples Potassic feldspars

_______ s_a_m_p_le_s> _______ __,___B_a___,_!_s_r __!_I_R __ b_~IB_a,_R~ - Ba _l Sr r-;~---~;; V drbovo Formation

Biotite and two-mica gneisses and granite­gneisses ( 1 0)

Leucosome in biotite and two-mica gneisses (4) A plitoid gneisses (3)

745 245 215 147S 21S 205

45 105 310

Ljaskovo Formation

3.6 211S 380 7.2 2820 320 0.2 290 95

Biotite gneisses and granite-gneisses (3) 1460 455 140 10.3 3410 6S5

Biotite gneisses (6) Pegmatltes of high Ba content (5) Pegmatltes of low Ba content (4)

Biotite and two-mica gneisses (12) Aplitoid gneisses of high Ba content (3) Aplitoid gneisses (2)

Aplitoid gneisses (21) Biotite and two-mica gneisses (7)

80

Posestrimo Formation

705 330 115 6.1 3770 560 1430 605 115 12.4 5130 760

Dobralllk Formation

600 185 180 54S 135 120 100 45 315

Backovo Formation

185 100 340 505 165 23S

630 14S

3.3 2855 295 4.5 2300 220 0.3 370 70

0.5 675 ISS 2.1 2070 235

425 5.1 395 7.4 800 0.4

305 11.1

260 14.5 275 18.6 615 1.0

385 7.4 295 7.8 665 0.6

835 0.8 465 4.4

Pegmatite ana lyses for Ba, Sr and Rb fall into h\·o groups. The first group includes mostly bedded pegmatites (described as migmatic ones by >K. J.1 B a HoB et al., 1984. vi. 1-1 B a H o B, 1989) which , similar to the leucosome of the migmatic gneisses, show high Ba (1430 ppm) Cl11d Sr (60S ppm) . and low Rb (115 ppm) contents . The three ele­ments are distributed in a simila r manner in the constitu ent potass ic feldspars, too : Ba -· 5130 ppm, Sr -- 760 ppm, Rb - 275 ppm, Ba /Rb - 18.6. The seco11d group includes pegmatites crosscutting the gneisses . They <Jnd their potassic feldspars show low Ba (200 ppm and 630 ppm) and Sr (140 ppm anc ll45 ppm) and high Rb (!45 ppm and 492 ppm) Lontents with correspondingly low Ba 'Rb values (1.4 and 1.0) , \vhich is typi ca l of pcgmatit es regarded as difierenti(lt es of granitoid lll ag mas.

The processes of migmatization in the metamorphic rocks studied, involving most!y an alkali feldspa r mobilization (4 e pHeBa et al., 1987). cau-;ed some redistribution of the constituent trace elenwnts . It is most pronounced in the localities of di stinct migmatic segregations of leucosome. The original presenLe of potassic feldspar in the gneisses is the primary c:JUse for the formation of rnigmatic products enriched in po · ta ss ic feldspar and, correspondingly, in barium. The Ba, Sr and Rb di stri bution in the newly formed migmatic potassic feldspar depends on their contents in the feldspars of the originCl! gneiss. This is vali rl both for the leucoso me formed in situ in the gneisses of the Varbovo Formation, and for the pegmatites of high barium content and high Ba iRb values from the Posestrimo Formation . Pota ssic feldspars from the migmatic bodies and from the gneisses are of sirni!ar compos ition. Arl.vanct:d migmatic processes may result ill differentiation and transfer of mobilized matter with formation of pegma ­tites whose geochemical characteristics resemble those of the pegmatites produced by differentiation of granitoid magmas. Thus, it seems justifit:d to assume a relation by origin from a common source between the two groups of pegmatites in the Posestrimo Formation. an infert::nce sustained by the evidence of the radio-i sotope and mine-ralo­gical studies on pegmatites from thP same area (A p Hay .u o s et al.,).

Donralak Formation. The mean Ba, Sr and Rb contents in the biotite and two-mica gnei sses and the constituent potassic feldspars are fairly close to those in the Varbovo gneisses and feldspars (Tables 4 and 5). However, Ba analyses of potassic feldspars show considerable scatter (9G7-7020 ppm) . In most cases (where the Ba concentrations in mean rock sa mples are similar), the differences are due to the different amounts of potassic feldspar present in the rocks (A p 11 a y .'!. o B et al., 1988) as illustrated on the diagram of the orthoclase content of gnei :,ses vers us Ba in potass ic feldspars (Fi g. 7).

Ill contrast to the already di scussed aplitoid gneisses from the migmatized forma­tions, most sa mples of the Dobrcdak a pi itoid gneisses and constituent potassic feldspars resemble the biotit e and tv.·o-mica gneisses in their Ba. Sr and Rb compos ition~. The potClssic feldspars are of high Ba contents (2300 ppm), showing also high Ba /Rb va lues (7.4). Di screpanci es from this t ypicCl! di5tribution are found in some gneisses from the area between the Zdrav ec mo:.Jntain shelter and the Bja!Cl -Cerkva mountain resort (27Qb, 2723 , 273b), and near the vi ll age of Plocnik (334 8 , 334b). Compared to the majority d gneisses in the formation. the gneisses and potasseic feldsvars from those localities contain more Rb, it s concentrations reachirg up to 330 ppm and 675 ppm respectively, and their Ba /Rb values are relatively low, 2.0 and 3.8 respectively (Table 4) . In thi s respect they resemble the Backovo leptitoid gneisses

Along the ea:-tern bank of the Vaca River (157, Fi g. I) and further east, near the village of Bojkovo (337d, 34Qc, Fig. 1) , there are oblique pegmatites resembl ing the so-ca lled rnigmatic pegmatites in th e: Posestrimo Formation in their Ba, Sr and Rb contents of the mean rock sa mples and the constituent potassic fe ldspars (high Ba a11d low Rb, Table 4) .

Backovo Formati(•n . The leptitoid gneisses and their potass ic feldspars here are characterized by high Rb (340 ppm and 835 ppm , respectively) and low Ba and Sr contents (185 ppm and 675 ppm Ba. resp . . 100 ppm and 155 ppm Sr; Table 5). They define the low Ba.Rb and Sr/Rb values both of the rocks (0.5; 0.8) and of the feldspars

6 Geologica Balcanica, 20. I 81

B<l,g/t

10000

sooo

~ 1000

soo .

lO 20

.•1 A2

40 Or,%

DJ

Bo,g/t.

10000

5000

1000

soo

)(

"' ~ rx-.----. .... ,. I X _X ' I X x· X X',, \ X X '

\ X X A ', \ ' \~ X ~)

--------11-

10 20 ) 40 or,•fo

Fig. 7. Ba content in potass ic feld spars versus normaliye orthoclase (Or) i n th e Dobralak Forma tion rocks 1- host gneisses; 2- apliloid gnei sses; 3- pegmalit rs

Fig. 8. Ba content in potassic feldspars versus normative orthoclase (Or) in th e rocks Ba ckovo Formation : 1 - biotite and two-mica g neisses, 2- lepliloid gneisses ; Dobralak Formation: 3 - biotite and two-mica g nei sses, 4 - apl iloid gnei sses

(0.3; 0.2) , as is t ypical of aci<~ igneous rocks. Th is spE'cifi c d istribut ion of t il e th ree ele­ments st ill pers i~ts in th e leptitoid gnei:,sE·s and potass ic fe!u spar' fro m t he westernmost ou tcrops of th is form <li ion (at thf· quarry near the Yili Age of Skobelevo, ~am ples 97fl, 977 , 978, Tabl e 4, Fi g. 1) whi ch presumab ly suf fere d so me mi g mat izat ion .

The two-mi ca and biotit e gneiss inter layers , occ ur ring mostl y nea r th E' form ati on bounda r ies, differ from the host leptitoid g ne!s~es in their Ba. Sr and Rb contt•nts. Ba concentrations in th e me3!l r ock~<> n;ples an d in tile potassic fel dspars (5()5 ppm a ttd 2070 ppm, resp.) as well as t hose of Sr ( 165 pprn and 235 ppm) are hi gher , and th t' Rb ones (2:35 ppm and 465 ppm) are lower. On th e d i <~gram of th e orthocl ase content of rocks n rsus Ba iP pot ass ic lelclspars (Fi g. 8) , th ese ,g neisses pl ot near th e ;Jl ready descri­bed biotite a nd two-m ica gnei sse" of th e Dobraiak Forma ti on from lhe Zdra n c and Pl oc nik localiti es. All these peculiariti es of th e Ba and Rb di stributions in th e Ba cko\' o and Dobralft]; gneisses rai se so me quest ions concerning th e boundary hehn'en th e t,,.o formations :1 nd the proporti ons of para- ;md ortho-rocks in th~ m t o a nswer which ac:d i­tiona l datc1 are needed .

The pers istent innease of Rl.J as compared lo Ba ;111d Sr both in t he mean roc k sa mpl es and in th e potassic feldspars of the typica l Bac kO\ o 1ep tit oid gneisse., is con­s ish ·nt "ith the patt ern of fracti onation and distributi on of th e th ree elements in aci d igneous rocks produced by m<lgmatic diffe renti at ion. J\'\o~ t lik ely, the Ba. Sr and Rh di stribut ion pattern refltd s prim ar ~ · , premetarnorphi c fea tmes of the or igi na l rocks :mel ha s not changed much during th t processes of rPgiona l meta mor phi s m a nd mig ma­tizati on . 8 <1 . Sr and Rb beha,·iour in th e leptitoid gnei sses of t he B;1ckovo Form ati on is ana logous t o t hat in th e aplito id gneis-;e' of th t V;;l rbuvo Form ati on whi ch 11nderwent int ense migmatizati oJI ye t retai ned th eir geoc lwm ic;J! fea t ures of igneo us rods.

82

Conclusion

The tetllperct turc regime profile across the meta morph k sequ ~:: nce during its formation, deriYeu ill thi s stud y from the structural chara cteri st ics of potass ic feldspars ctnd by mincrCll thermometry, i · consistent \\ ith the progres ·iye trend of metamorphic pro­cesses in the Central Rhodope ~\t · consisting in an increCl. e of the grade of metamor­phi m downward the cross-section, a Cllready de. cribed by a numbtr of at:thors (K o­* y x a p o B, 1984; /K. H B a 11 o s eta!., 1984 ; I( ocTo B eta!., 1986). The meta­morphic rocks studi ed were formed under amphibolite faci~s conditions including the sillimanite-a! mandi ne-musco\"ite sub facies, kya ni te- muscovite-quartz ·ub fac ies Cllld staurolit e-quartz subfacie tK ocTo B et a!., 19 . 6). The lO\\er formations. re­ferred to t he s illimanite-almandine-muscovite and kyanite-muscuvite-quartz subfa­cies, are affected by iater processes of migmati za t ion ()K. II sa 11 u B d a!., 198,1; K o .tK y x ct p o s, 198-1) . These processes of migmatization and granitizcttion have been interpreted by K ocTo u et a!. (1986) as a second, anatectic-pegmatitic stage of an integral metamorphic pruces, the inferred maximum T- and P-\alue:- Juring it bt·ing about 675 ·C and 6.5 kbar, rtspediYely. At such tempt:ratures, the equilibrium modification of pota:.s ic fddspar is the monoclinic one. orthoclase. The upper form a­tions. metamorphosed unde:.- staurolit l'! -qumtz su bfaci es conditions, contain triclinic potass ic feidspars. J\\igmatization has had a limited ef fect on these formations. In the 13ackovo one, fur instance, migmatization phenomena haYe been reported ()!(. I I B a­ll o s eta!., 1985) only from th~ \\~?stern part of the area studied here tnear SkobeleYo and Krici m), and their effect has been ddt>cled by us in the structural modifications of potass ic feldspars.

'I he Ba, Sr and Rb db tribution palt em , as suggested by 4 c p 11 e sa eta!. (1987), most likely reflects their original premetamorphic di ·tribution in the rocks and has not changed appreciably during the regional metamorphism and the subsequent migma­tization. Th~ same i~ Yalid ior the potassic feld -pars which on the whole retain their original ge:ochemical charact eristics rega rdl ess of some local structural changes caused by migmat iza ti on. Exa mp les of such devia ti ons are the potass ic feldspars from the Backovo Formation iept itoid gneis.;es near the \'i llage of SkobeleYo and from t he Var­bo\'o Formation apli toid gneisst-s .

On the basis of our structural, geothermomctric and geochemicai stud ies of the Central Rhodope mtla morphic roc ks, we may conclude the fol lowir.g:

- The monoclinic structurt- of potassic feldspa rs in the migmatized gneis ·es (T> 500 C) is the resu lt of migma t ization irrespedin of their original <;tructura l stat es .

- The pre ence of monoclinic potassic feldspars in the Dobral ak Format ion gneisses IT.-50u·C) may be du e both tu regional nwtamorphi~m and to thermal action of the later migmatization whic h affected the formations lower down thr sedion.

- The formation uf triclinic potassic feldspar~ in the Ba ckuvo Formation gnc i!>srs (T<475cq i-; the resu lt of regional mdamurphism.

- The Ba, Sr and Rb distribut ion pa~terns in the metamorphic rocks and potas ic feldspars stud ied reflt?ds largely the premdamorphic distribution of these elements ill the origi na l rocks.

-- Th t.:: observeu fractionation of Rb with H'spect to Ba a nd Sr in the B21 ckovo Forma t ion leptitoid g-n~is t:: · and constituent potassic fddspar<; is consistent wi th the Rb behaviour in acid magmatic differt>ntiate - and is thus indicati\'e of an igntous orig:n of the host rocks

Translated by /. Vesselinov

83

References

Ar naudov, V., Amov, B., Baldjieva, Ts., Pavlova, M. 1990. Tertiary migmatic pegmatites in the Central Rhodope crystalline sequence. - Geologica Bale. (in press) .

Goldsmith, J. R ., Lav es, F . 1954 . Themicrocl i ne-sanidinestabilityrelati ons.-Geo­chim . Cosmochim. Acta, 5; 1-19.

H o d g e s, K . V ., S pear , F. S. 1982. Geothermometry, geobarometry and the Al 2Si0& triple point at Mt. Moosilauke, New Hampshire. - Amer. Miner ., 67; 1118-1134.

K o z h u k h a r o v, D., K o z h u k h a r ova, E., G r o z dan o v, L. 1977. Temperature and pressure during the metamor phism of the precambrian complex from the Rhodope massif. - Geologica Bale. , 7, 3; 103-116.

Put n is, A . , McConn e II, J. D . C. 1980. Princip les of Mineral Behav iour, B lackwell Sci en­tific Pub!., O xford .

Stewart, D., Wrigh t, T. 1974. Al /Si order and symmetry of natural alkali feldspars, and the relationship of strained cell parameters to bulk compositi on . - Bull. soc. fr . Mineral. Cristallogr. , 97; 356-377 .

W r i g h t, T. 1968. X-ray and optical study of at kal i feldspar. II An X-ray method for determining the composition and structural state from measurement of 2 values for three refl ections . -Amer. Mineral ., 53, 1/2; 88-104 .

A pH a y .n 0 B, B ., A pH a y .n 0 B a, P . , n aHa il 0 T 0 B, r . 1988. Pa3npe,neJJeHHe Ha 6apHH B KaJJHeBHTe cpeJJ.llWnaTH H reHe3HC Ha Ka JJHeBOcpeJJJlWOaTOBHTe BOp"bCJJeUH B rpaHHTOHJlH OT IQ}f(Ha D'hJJrapHH . - Cn. E7>A2. zeoA. iJ-so. 49, 2; 81 - 92.

A <Po H H H a, f., ill M a K 11 n, 5., MaKar on, B . 1976. 3KcnpeccHJ>~il MeTO.ll onpe.neJJeHH!I ynop!i,llO'leHHOCTH MoHoKJIHHHbtx 11 TPIIKJIH HH btX KaJJHeBbiX noJJenbtX w naroa . - jJoKA. A H CCCP, 231, Z; 449- 452 .

B e p f H Jl 0 B, B. 1960. n eTpOJJO}f(KJI H3CJJ e,!lBaHH !I Ha KpiiCTBJI HHHftTe WHCTH OT ceaepHHTe CKJIOIIOBe Ha U.eHTpanHHTe H 3ana,nHHTe Po.nonH . - H38. Feo.1 . UHcm. , EA H. 8; 223-269.

r JJ e 6 o B H u K H A, B . A. 1977. MHHepaJJbHble <PaUHH KaK KpHrepHH oueH KH napaMCTpoa MeTa­MOpQJH3Ma. - B : TepAta- u 6apoAtempuR Atema,llopc/JwteCIWX nopoiJ. Jl . , H ay Ka; 207 c.

11BaHoB, )1(., MocKoBC KII, C., KoJJ'!eaa, K ., .ll.HMOB, .D.., KJJai\H, Jl . 1984 . reonorH'leCKOe CTpOeHHe l.I.eHTpaJJ bHbi X Po,llon. I . JlHTOCTparurpa<JlH'leCKOe pac'!JieHeHIJe H oco6eHHOCTH pa3pe3a MeTaMOpQJH'IeCKHX nopO.!l, B cesepHbiX '!aCT!IX I.J.eHTpaJibHbiX Po,non. -Geologica Bale. , 14, I ; 3-42.

11BaHoa, )I(., MocKoBCKH, C., .ll.HMOB, .D. ., Ko JJ '!eaa , K., K.TiaiiH, Jl. 1985. reonorH'leCKoe cr poeHH e U.eHTpanbHbiX Po,llon. I I. CTpyKTYPHbte nocJte,lloBaTeJJbHOCTH a Cl!HMeTaMOpQJH'IeCKOH 3BOJIIOUHH I.J.eHTpaJJbHOpOJlOilCKOH MeTaMOpQJH'IeCKOH rpynn bl. Geologica Bale., 15, 3; 3-32.

11 13 a H 0 B, 11. 1989. Bop xy reOXI!MHHTa H reHe3HCa Ha MHrMBTII'lHHTe ncrMaTIITH OT I.J.eHTpaJt!IJITe Po,llontl. - Feoxu.M., AtUHepaA. u nempoA., 26.

11 ll a H 0 B, 11 ., .lJ. a He B a, Jl., C T a H 'I e B a, E. 1984. f eOXHMH !I na pe,llK03eMHHTe eJJeMeHTH a rpaHHTHH nerMaTHTH OT IQ}f(Ha Bonrapu!! . - FeoxuAt. , .MUHepaA. tt nempoA., 19; 3-14.

K o }I( y x a p o 8, Jl.. 1984. JlHTOCTpaTurpaQJHH .llOKeM6pHiiCKHX MeTaMop<Jlti'lCCKHx nopo.n Po.noncKo i! cyneprpynnbl B l.I.eHTpan bHbiX Po,llonax . - Geologica Bale., 14, I; 43-88.

K 0 c T 0 B, 11., r p 0 3 Jl a H 0 8, Jl., neT p y c e H K 0, C., K p "b c T e 8 a, M ., p a w K 0 B a, .ll.. 1986. CnH- n nocrMeraMop<JlH n MIIHepaJtu3aUHH 8 I.J.eHTpaJIHHTe Po)lonu. - FeoxuAt., .l!uHepaA. u nempo11. , 20-21; 25-48.

n e p 'I y K, JI . JI. 1967. DHOTHT·rpaHaTOBbiH reoTepMoMeTp. - lloK.A. A H CCCP, 117, 2; 411 - 414. n e p 'I y K, JI. JI. 1981. KoppeKUH!I 6HOTHT- rpaHa TOBOfO Tep MoMerpa .!l.J!H CJty'laH H30MOp<jJHJMa

Mn;=Mg +Fe 8 rpaHaTe. - .aoKA. AH CCCP, 256, 2; 441 - 442 . c T e <P a H 0 B a, M. 1980. n empoxU.l!UJl ,\fa(!,lfamwtecKU X ltopoi} . M ., MHp; 402 c. 4 e pH e a a, 3 . , C T a H 'I e a a, E., T a Jl }I( e p, JI . 1987. reoxH MHH MHrMaTH3Hpo8aliHbJX

opTorHeifco8 H3 ceaepHbtX CKJI OH08 illHpOKOJtbJKCKoil peKII a I.J.eHTpaJt bHbiX PoAona x. -Geologica Bale . , 17, 5; 41 - 63.

84