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Ш “ . ” -

1 – 99, 2007

ANNUAIR DE L’UNIVERSITE DE SOFIA “ST. KLIMENT OHRIDSKI” FACULTE DE GEOLOGIE ET GEOGRAPHIE

Livre 1 – GEOLOGIE Tome 99, 2007

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THE VARSHILO PLUTON IN STRANDJA MOUNTAIN – NEW MINERALOGICAL AND GEOCHEMICAL DATA SUPPORTING ITS

PETROLOGICAL EVOLUTION

BORISLAV K. KAMENOV

Chair of Mineralogy, Petrology and Economic Geology e-mail: b.kamenov@gmail.com

Borislav K. Kamenov. THE VARSHILO PLUTON IN STRANDJA – NEW MINERALOGICAL AND GEOCHEMICAL DATA SUPPORTING ITS PETROLOGICAL EVOLUTION

Initial studies of the Varshilo pluton exposed in Strandja Mountain concluded that magmas followed

different petrochemical trends – an iron-enrichment with little silica enrichment until the final stages of crystallization and a high silica-enrichment one. The new presented mineral and geochemical data confirm the old speculations with more convincingness. The vast cumulative set of modal and chemical analyses worked out the following rock units within the pluton: I – pyroxenites, II – gabbro, III – gabbrodiorites, IV – monzodiorites and quartz-monzodiorites, V – quartz-diorites, VI – granodiorites, VII – aplites. The essential features of the main rock-forming minerals are present and they are consistent with the differentiation of two parental magmas – basic and intermediate. Textural and chemical evidences support the magma-mingling and magma-mixing phenomena.

The extent of the fractionation, magma-mixing and fluid influences are figured out of the geochemical plots and general mass balance calculations. REE and MORB-normalized patterns support the island-arc geodynamic setting and subduction-related origin of the Varshilo magma. The main mechanisms of the magmatic evolution are modeled successfully by general mixing calculations – fractional crystallization, magma mixing, cumulative segregation and fluid involvement. The observed geochemical variations of the trace elements are modeled almost perfectly using published partition coefficients for coexisting minerals and liquids in the pluton. It was established that LIL-elements Rb, La, Ce, Sm, Yb, Th and U had been introduced in amounts exceeding the necessary ones if only fractional crystallization would be realized. Magma-fluid relations and crystallization in an open system in the magma chambers are offered in the interpretations.

The proposed model fits well into the revealed three geochemical trends of the magma evolution: I – cumulative trend of imperfect mineral separation leading to pyroxenite at the end of the process; II – tholeiitic trend of iron-enrichment in the basic magma units; III – calc-alkaline trend in the intermediate magmas. The hybrid origin of the rock unit IV (quartz-monzodiorites and monzodiorites) was grounded more confidently.

The new results could be stimulating for rising new ideas about this key-case complex multiphase Upper Cretaceous pluton in Bulgaria.

Key words: Nomenclature, rock-forming minerals, geochemistry, magma mixing and fractional crystallization

(MFC), island arc magmatic setting

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INTRODUCTION

A typical representative of the Late Cretaceous island arc magmatism in Bulgaria outcropping in the northern marginal Strandja area is the so called Vurshilo Pluton (Batandjiev, 1962; Vassilev et al., 1965). The unusually wide petrographic variety of the pluton, its complex differentiated structure and the ore mineralization connected with it attracted the attention of the Bulgarian geologists in the years between 1946 and 1971, but only piecemeal attempts have been made to elucidate its petrological evolution. The iron and copper occurrences genetically related to such evolution remained with obscure significance and the inspection works during the beginning of the 80-ties years of 20 century did not provoke reviving of new prospecting operations. However, the petrological peculiarity of the pluton turned it into a particular target for petrological research and that is why between 1980 and 1987 the author carried out a detailed inspection of the field relationships of the rocks, remapped the massif and took many new geochemical samples from the rocks. Students of the ex-chair of Petrography in the Faculty of Geology and Geography, Sofia University “St. KХТmОnt OМСrТНsФТ” tooФ part in some of the field and laboratory work The graduation papers of Koleva (1981), Hristov (1983), Bogdanov (1984), Todorov (1984) and Bachev (1987) devoted to different aspects of mineralogy and geochemistry of the plutonic rocks stimulated new ideas for its petrological evolution to be raised.

On the basis of the collected abundant new petrochemical data an attempt for application of the mathematical methods of the Factor Analysis has been made (Kamenov, Andreev, 1989) and some original ideas of the origin of the pluton were supported. Parts of the geochemical new collected data were interpreted by Kamenov and Ivchinova (1986). Plagioclases and potassium feldspars in the plutonic rocks were studied in more details (Kamenov et al., 1988), but the considerable portion of the new petrological information and the argumentation of the genetic hypotheses are still unpublished.

The present paper aims at filling this gap up to certain degree and the new factual material for the pluton to reach to our geological circles. Simultaneously, due to the unusually comprehensive new banc of mineralogical, petrographical and geochemical data I hope that the accents on the magmatic evolution of this pluton could advertise it as a key case and different interpretations to be tried.

CONCISE MINERALOGICAL AND PETROGRAPHICAL CHARACTERISTIC

OF THE ROCKS

The Vurshilo Pluton was intruded into the marginal area between the Pre-Upper Cretaceous Basement and the lower cover formations of the Upper Cretaceous Series in the region (Fig. 1). It cuts the allochtonous Triassic metasediments of the Veleka tectonic unit and the Jurassic sediments, as well as the rocks of the Grudovo and Michurin Groups (Petrova et al., 1980). The contact metamorphic aureole around the pluton is wide and it includes hornfelses, thermally altered sandstones, quartz-sericite schists, marbles and calc-silicate skarns. The southern contact to the metasedimentary rocks is sharp dipping 35o to 60o to south.

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The pluton is made up by several closely located medium in size (2-5 km in diameter) and considerable number of smaller intrusive bodies separated by contact metamorphosed rocks, all arranged within a wider elongated in East-West zone. It is most likely that the individual plutonic bodies on the contemporary erosion level might be connected in depth in a common larger plutonic body. Endocontact zones are xenoliths abundant. Numerous dykes having dominant WSW-ENE strike occurred in the country Upper Cretaceous volcano-sedimentary rocks. Most often they are basalts and basalt andesites in petrographical composition. The dykes should be understood as subvolcanic channels draining the Upper Cretaceous magma underground centers. Much rarest are the postplutonic dykes cutting the plutonic bodies.

The petrographical composition of the pluton includes the following plutonic rock units: I – pyroxenite; II – gabbro; III – gabbrodiorite; IV – monzodiorite and quartz- monzodiorite; V – quartz-diorite; VI – granodiorite and VII – aplite.

Pyroxenite unit is represented by irregular small bodies (up to the first several hundred meters in diameter) within the gabbro rocks. They are widely distributed in the northwestern part of the pluton. Uneven distributed pyroxenite segregation several centimeters to 1-2 meters in diameter are very often met in the gabbro outcrops. The transitions to the gabbro are gradual but the contacts with the more-leucocratic gabbrodiorites are sharp. The structure is massive and the texture is coarse-grained. The mineral composition consists of mainly clinopyroxene (30-85 volume %, diopside type with Wo50-46, Mg# 0.90-0.80) and amphibole (10-45 %). Minor rock-forming minerals are plagioclase (1-15 % with An90-85), olivine (0-7 %), and orthopyroxene (0-8 %). Accessories are magnetite, ilmenite, chalcopyrite, platinum, millerite, pentlandite and titanite. The latemagmatic amphibolization is irregularly manifested. The modal relationships (Fig. 2) support the nomenclature of the following rock varieties: olivine websterite, plagioclase-bearing amphibole clinopyroxenite, gabbropyroxenite, plagioclase-bearing pyroxene hornblendite.

Gabbro unit is the most wide-spread in the plutonic exposures. Even-grained gabbro and poikiloophitic textures predominate. Sometimes the structure shows expressive taxitic stratification but the massive varieties are more often observed. The transitions to gabbrodiorite are difficult to be traced on the field. The principle rock-forming minerals are plagioclase (23-66 %, An86-68) and clinopyroxene (1-45 %, augite and diopside, Wo41-48, Mg# 0,72-0,81). Amphibole is irregularly present (2-45 %, tschermakite variety, Mg# 0.72-0.92). Minor minerals are olivine (0-5 %), potassium feldspar (0-5 %), quartz (0-2 %) and nepheline (0-6 %). It is rarely that one observes orthopyroxene in insignificantly quantity. Apatite, magnetite, ilmenite, spinel, titanite, chalcopyrite and millerite are the usual accessories.

The modal composition (Fig. 3) allows the following varieties to be distinguished: olivine-bearing common gabbro, pyroxene-amphibole gabbro, amphibole gabbro and nepheline-bearing gabbro. The transitions between all these varieties are gradual.

Plagioclases are developed in two generations (PlI=An86-75; PlII=An70-65) and pyroxenes (Table. 1) started their crystallization later than them. Magmatic amphibole (Table 2) includes poikilitic plagioclase and often became overgrown with postmagmatic actinolite-tremolite amphiboles. Opaque ore minerals (1-10 %) are present in two phases – early one crystallized showing insignificant participation of small euhedral grains and a late one in prevailing amounts forming sideronitic aggregates.

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Magnetite (2.5-10 %) is low-Ti variety (TiO2 = 1.10-3,25 %; Mn = 0.50-1.85 %; Cr – 720-1130 ppm; V – 2500-2800 ppm; Ni – 58-282 ppm; Co – 30-73 ppm; Sc – 20-25 ppm). The unit-cell parameter (a) is determined as 8.393 (average from 5 samples). Some typical geochemical ratios for magnetite crystals studied are the following: Ni/Co=1.90-4.0; Cr/Co=6-20; V/Cr=2.50-3.60; Ti/Cr=15-25; KK

V=7.50-10.00; KKCr=15-

45, where KK is the coefficient of concentration of the elements V and Cr for the magnetite. REE concentrations are analyzed on separated magnetite samples from artificial heavy concentrates (Table 3).

40

60

70

55

5050

35

50

5040

6045

30

3535

70

25

3035

30

ъ

.

0 1 2 km 4 km

(K )2

st-cmp

(K )2

con

ъ (K )2

cen-tur

Ю (J )2

( ) (Pz)

, ъ

50

К

К

Fi g. 1. Schematic geological-petrographical map of the Varshilo pluton, composed by the author

and C. Dabovski, using also unpublished data for the country rock complexes by E. Vasilev. 1 – diorite porphyrites; 2- contact aureole; 3- granodiorite; 4 – quartz-diorite and monzodiorite; 5

– hybrid gabbrodiorite; 6 – gabbro; 7 – pyroxenite; 8 – dykes; 9 – strike and dip; 10 – normal fault; 11 – reverse fault; 12- thrust fault; 13- Michurin K2 Group; 14 – Grudovo K2 Group; 15 – Varshilo K2 Group; 16 – Jurassic sediments; 17 – Allochthonous metasediments; 18 – Paleozoic granites.

Gabbrodiorite unit rocks are exposed in the marginal parts of the gabbro bodies

adjacent to the quartz-monzodiorite and quartz-diorite exposures. Their outcrops have irregular outlines and are comparatively small and difficult to map in the field. The structure is massive and the textures are often porphyritic and coarse-grained. The multiple small-grain mafic inclusions gave rise of motley spotted view of these rocks. The modal varieties (Fig. 3) are amphibole-, pyroxene-amphibole-, quartz-bearing pyroxene-biotite-amphibole gabbrodiorites and monzogabbrodiorite.

The principal minerals are plagioclase (20-75 vol. %, PlI = An70-60, PlII = An50-40) and amphibole (magnesio-hornblende and magnesio-hastingsite, Mg# 0.80-0.75). Minor minerals are clinopyroxene (0-15 %, augite, Wo0.77-0.81Mg#0.75-0.79); biotite (0-4 %);

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quartz (0-6 %); potassium feldspar (1-8 %, high orthoclase to low sanidine with Or95-90 and up to 30 % cryptoperthitic exsolved albite). Accessory minerals are apatite, magnetite (1.5-5%) and titanite. Several mineral associations in non-equilibrium between them are observed in the gabbrodiorite: (i) coarser-grained to porphyroid consisted of basic plagioclase (PlI), clinopyroxene and amphibole and (ii) smaller grained built up by intermediate plagioclase (PlII), quartz, biotite and potassium feldspar. The presence of interstitial nepheline together with quartz, of complex spotted appearances and some features of magma corrosion, of oscillatory and reverse zoning in the plagioclases and sometimes in the amphiboles as well reveals a hybrid origin of these rocks.

Px Hb

Pl

1 2

3

45

Fig. 2. Modal nomenclature of mafic rocks from Varshilo pluton in the plagioclase-pyroxenes-amphibole classification scheme of IUGS (Le Maitre et al., 1989).

Fields: 1 – plagioclase-bearing amphibole pyroxenites, 2 – plagioclase-bearing pyroxene hornblendites, 3 – pyroxene-amphibole gabbro, 4 – amphibole gabbro, 5 – common gabbro

Quartz-monzodiorite and monzodiorite unit is distinguished close to the contacts

of quartz-diorites and granodiorites with gabbro and gabbrodioritic bodies. The rocks of this unit form irregularly in shape and relatively small bodies. Indistinct porphyroid appearance due to the larger clinopyroxene grains and the glomeroporphyritic ovoid clusters of large-scale amphiboles are typical. The following varieties are discriminated: amphibole-, biotite-amphibole- and amphibole-biotite-clinopyroxene quartz-monzodiorites and monzodiorites. The distinction between quartz-diorite and gabbrodiorite is difficult in the field due to their gradual transitions (Fig. 3). The intrusive breccias consisted of mafic round enclaves enveloped by leucocratic and more small-grained acid rocks are often met. This fact along with the complex zoning of their plagioclases and the manifestations of spotted and reverse zoning, porphyroid textures, taxitic strips and the spatial position of the individual bodies in the field are indications of realized essential mixing of contrasted in composition magmas.

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The principal minerals are plagioclase (36-73 %, PlI=An50-45, PlII=An30-25) and amphibole (5-32%, magnesio-hastingsite and tschermakite having Mg# 0.70-0.80). Minor minerals are biotite (1-6%, Mg# 0.65-0.70), quartz (2-16 %), potassium feldspar (low sanidine to interim sanidine with Or80-76, t1=0.72-0.76) and clinopyroxene (0.3-8 %). The fairly rare amphibole-biotite-clinopyroxene variety is more melanocratic (only 30-35% plagioclase An55-40), clinopyroxene (27-34 %), orthopyroxene (0-3%), amphibole (4-10 %, tschermakite), biotite (5-11 %), quartz (2-8 %) and potassium feldspar (9-15 %). Accessory minerals are apatite, magnetite, ilmenite, and zircon. Magnetite is low-Ti and ilmenite is rich in pyrophanitic component.

A P

Q

QM

GdMG

Qmd

MdM

Qd

SG

Fig. 3. Modal nomenclature APQ after Le Maitre et al. (1989) of rock specimens from the Varshilo pluton

Symbols: ▲- aplites, ○ – all other rock species. Fields: Qd – quartz-diorites, Md – monzodiorites, Qmd – quartz-monzodiorites, Gd – granodiorites, MG – monzogranites, QM – quartz-monzonites, M – monzonites

Quartz-diorite unit occurs in the internal parts of the plutonic bodies and

demonstrates sharp intrusive contacts to granodiorites and to gabbrodiorites and gabbro as well. The contacts of this unit to monzodioritic rocks solely are transitional and it is almost impossible to be distinguished in the field. A characteristic feature of these rocks is that they have leucocratic appearance (M=4-20%) with relatively more small-grained texture. Biotite-amphibole- and amphibole- varieties are distinguished. The only one principal mineral is plagioclase (59-78 %, PlI=An53-45, PlII = An42-34). Minor minerals are amphibole (2-17%, magnesio-hornblende, Mg# 0.73-0.68), biotite (Mg# 0.62-0.55), clinopyroxene (0-10%), quartz (6-15 %), K-feldspar (3-4%, low sanidine and interim orthoclase with Or84-80, t1=0.69-0.76). Accessories are apatite, magnetite and titanite.

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1 Table 1

Chemical composition of selected clinopyroxenes and biotites

Mineral Clinopyroxene Тneral Biotite Rock Gabbro Gabbrodiorite Rock Qd QMd Gd

Sample 64/130 64/130 17/37a 6/51b 23/279 33/282 Sample VII X VI Analyses CPx-

1/c CPx-2-r

CPx-3 CPx-4 CPx-5 CPx-6 Analyses Bt-1 Bt-2 Bt-3

SiO2 52,25 53,60 49,83 51,39 51,56 51,76 SiO2 36,77 41,47 34,32 TiO2 - - 0,56 0,26 0,35 0,39 TiO2 4,18 2,82 3,13

Al 2O3 2,10 0,93 4,45 1,78 3,66 2,42 Al 2O3 11,47 10,05 15,39 Fe2O3 2,51 8,94t 8,67t 7,87t 2,43 7,76t Fe2O3 7,23 5,13 6,14

FeO 5,03 - - - 5,48 - FeO 12,96 10,67 12,85 MnO - 0,16 0,11 0,11 0,07 0,08 MnO 0,27 0,43 0,95 MgO 17,18 11,97 13,22 15,83 14,97 16,87 MgO 12,03 13,41 12,55 CaO 20,96 24,39 22,57 22,48 21,94 20,64 CaO 2,44 5,87 2,80

Na2O - - 0,42 0,27 0,26 0,07 Na2O 0,21 0,71 0,66 K2O - - - - - - K2O 7,72 7,70 6,11

Cr2O3 - - 0,175 - 0,013 - H2O+ 4,31 2,24 3,88

Total 100,03 99,99 100,00 99,99 100,00 99,99 Total 100,45 100,50 99,74 Structural formula on 6 Structural formula on 22

Si 1,918 2,032 1,864 1,905 1,893 1,913 Si 5,20 5,60 4,84 IVAl 0,082 - 0,136 0,078 0,051 0,087 Ti 0,44 0,29 0,33 Fe 3+ - - - 0,018 0 0 Al 1,91 1,60 2,56 VIAl 0,009 0,042 0,060 0 0,051 0,019 Fe3+ 0,77 0,52 0,65

Ti 0 0 0,016 0,007 0,010 0,011 Fe2+ 1,53 1,20 1,52 Fe 3+ 0,007 0 0,069 0,082 0,054 0,050 Mn 0,03 0,05 0,11 Fe 2+ 0 0,255 0,113 0,036 0,066 0 Mg 2,54 2,70 2,64

Cr 0,003 0 0,005 0 0 0 Ca 0,37 0,85 0,42 Mg 0,919 0,676 0,737 0,875 0,819 0,920 Na 0,06 0,19 0,18

ΣM1 1,001 0,973 1,000 1,000 1,000 1,000 K 1,39 1,33 1,10 Fe 2+ 0,154 0 0,062 0,084 0,116 0,165 Mg# 0,62 0,69 0,63 Mn 0 0,005 0,003 0,003 0,002 0,003 Mg 0,022 0 0 0 0 0,009 Ca 0,824 0,991 0,904 0,893 0,863 0,818 Na 0 0 0,030 0,020 0,019 0,005 K 0 0 0 0 0 0

ΣM2 1,000 0,996 0,999 1,000 1,000 1,000 ΣМКt 4,000 4,000 4,000 4,000 4,000 4,000 Mg# 0,81 0,72 0,75 0,80 0,77 0,81 Wo 41,5 51,4 47,9 44,9 44,9 41,6 En 47,3 35,1 39,0 43,9 42,7 47,3 Fs 11,2 13,5 13,1 11,2 12,4 11,1

Aug Di Di Aug Aug Aug

З и: 1. я : Aug - , Di – . 2. Mg# = 100 Mg/(Fe2+ + Mg)

Notes: The abbreviations for the pyroxene species are: Aug – augite, Di – diopside. 2. Mg# = 100. Mg/(Fe2++Mg)

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2 T a b l e 2

я Chemical composition of selected amphiboles from Varshilo pluton

Rock Gabbro Quartz-monzodiorite Granodiorite

Sample I/25 1/86 33/289 II/52 VIII 2/BK IV/157 VI/283 IX/BK Analysis Hb-1 Hb-2 Hb-3 Hb-4 Hb-5 Hb-6 Hb-7 Hb-8 Hb-9

SiO2 43,96 43,34 44,17 43,54 43,85 41,25 41,61 42,17 42,63 TiO2 1,85 1,43 1,97 2,03 1,59 1,72 1,84 1,87 1,89

Al 2O3 10,31 10,41 11,13 10,26 9,54 12,69 10,11 11,61 10,40 FeOt 12,63 14,94 8,84 13,37 13,70 14,12 13,64 14,08 14,02 MnO 0,34 0,29 0,11 0,33 0,50 0,31 0,57 0,44 0,58 MgO 14,12 13,13 16,55 12,99 13,42 12,14 13,16 12,83 12,51 CaO 11,82 12,21 11,95 11,79 11,23 12,50 12,17 11,79 13,53

Na2O 1,82 1,63 1,78 1,77 1,40 1,69 1,53 1,22 1,57 K2O 0,68 0,62 0,79 0,90 0,79 0,88 0,94 0,96 0,97

Total 97,53 98,00 97,29 96,98 96,02 97,30 95,57 96,97 98,10 Structural formulae at sum of cations FM = 13

Si 6,393 6,331 6,316 6,433 6,466 6,118 6,260 6,193 6,246 IVAl 1,607 1,669 1,684 1,567 1,534 1,882 1,740 1,807 1,754 VIAl 0,159 0,122 0,190 0,218 0,122 0,334 0,051 0,201 0,040

Ti 0,202 0,157 0,212 0,226 0,176 0,192 0,208 0,207 0,208 Fe3+ 0,719 0,833 0,772 0,487 0,963 0,540 0,722 0,956 0,743 Fe2+ 0,816 0,993 0,286 1,165 0,727 1,211 0,994 0,773 0,975 Mg 3,061 2,859 3,528 2,861 2,950 2,684 2,952 2,809 2,962 Mn 0,042 0,036 0,013 0,043 0,062 0,039 0,073 0,055 0,072

B-Ca 1,842 1,911 1,831 1,866 1,774 1,986 1,962 1,855 1,964 Na-M4 0,158 0,089 0,169 0,134 0,226 0,014 0,038 0,145 0,036

Na-A 0,355 0,373 0,324 0,374 0,174 0,472 0,408 0,202 0,410 K 0,126 0,116 0,144 0,170 0,149 0,166 0,180 0,180 0,181

ΣA 0,481 0,488 0,468 0,543 0,323 0,639 0,588 0,382 0,591 Mg # 0,79 0,74 0,92 0,71 0,80 0,69 0,75 0,78 0,75 Fe3+/

Fe2++Fe3+ 0,47 0,54 0,73 0,29 0,57 0,31 0,42 0,55 0,43

Type Tsc Tsc Tsc MHas Tsc MHas MHas Tsc MHas

З и: 1. Leake et al. (1997): Tsc – , MHas – ; 2. Mg# = Mg/(Mg + Fe2+) (apfu). 3.

13 FM=13 (apfu) AnНОrson, SmТtС (1995), FО2+ FО3+ Spear, Kimball (1984).

Noteя: 1. Nomenclature after Leake et al. (1997): Tsc – tschermakite, MHas – magnesiohastingsite; 2. Mg # = Mg/(Mg + Fe2+) (apfu). 3. The crystallochemical formulae are calculated on the basis of 13 cations FM=13 apfu, after Anderson, Smith (1995) and Fe2+ and Fe3+ are relocated according to Spear, Kimball (1984)

Granodiorite unit is mapped in the central inner parts of the pluton where a bit

larger body having diameter around 3 km was detached. Several smaller granodioritic bodies westwards and eastwards of this body were also distinguished on the map. The mafic index M=5-15%. The structure is massive and the textures are granitic, even-grained or weakly porphyritic. Biotite- and amphibole-biotite varieties are differentiated. The principle minerals are plagioclase (42-58 %, PlI=An54-35, PlII=An25-18), quartz (17-25 %) and K-feldspar (7-25%, low sanidine, Or80-90, t1=0.50-0.65). Minor minerals are

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amphibole (2-18 %, magnesio-hastingsite and tschermakite, Mg# 0.65-0.55) and biotite (2-11 %, Mg# 0.60-0.65). Accessories are rutile, magnetite, titanite, zircon, apatite and pyrite.

3 Table 3

я Chemical composition of magnetites from Varshilo pluton

Rock Gb Qd Gd

Sample I VIII/B V/B III/B IV/B Fe2O3 41,24 59,81 51,30 58,27 50,39

FeO 24,62 28,96 26,27 28,01 42,35 MnO 0,13 0,24 0,24 0,19 0,25 TiO2 3,22 1,79 2,20 2,65 1,11

V ppm 2500 1500 1600 1365 1427 S 0,41 0,08 - - 0,74 8,393 - 8,395 8,394 - Mt90

Ilm 9,1 Mt95,1 Ilm4,0

Mt94,0 Ilm5,9

Mt94,4 Ilm5,6

Mt95,4 Ilm3,9

Ni 282 118 17 24 80 Co 60 55 14 22 42 Cr 845 143 127 - 146 Sc 20 4 - - 3 As 3,4 2,2 - - 3,3 Sb - 0,1 - - 0,5 Zn 47 312 - - 154 Hf - 9,2 - - 8,3 Th - 5,1 - - 9,6 La 1,4 7,9 8,7 Ce 340 50 42 Sm 0,9 1,2 1,9 Eu - - 0,2 Th - - - Yb - 0,4 1,0 Lu - - 0,45

Ni/Co 4,07 2,10 1,24 1,2 1,7

З и: 1. - ppm; 2. я я : Mt - ; Ilm – ; 3. – . 4. я : Gb –

, Qd – , Gd – ; 5. 90 %, я

я ; 6. - INAA , XRF.

Notes: 1. The trace elements in the magnetites are in ppm; 2. Abbreviations for the mole composition (in per cent): Mt – magnetite, Ilm – ilmenite; 3. ao – crystallochemical unit parameter; 4. Abbreviations for the rocks: Gb – gabbro, Qd – quartz-diorite, Gd – granodiorite; 5. The samples are separated out of the artificial heavy concentrates and the mixtures in the monomineral fractions are at least 10 %. This is the reason the crystallochemical formulae not to be calculated. 6. The trace elements are analyzed by the methods: INAA, AA and XRF.

Aplite unit is widespread as veins 1 to 20 cm in thickness cutting all other rock

varieties. Predominantly biotite-amphibole granite-aplites (Fig. 3) are observed. The post-plutonic dykes are intermediate and basic ones. Diorite porphyry,

monzodiorite porphyry and syenite porphyry dykes are determined.

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Pressure estimations during the crystallization of gabbro, quartz-monzodiorite and granodiorite rocks are calculated by Johnson, Rutherford (1989) method on the basis of microprobe analyzed amphibole compositions. Gabbro rocks solidified at pressures between 4.0 and 4.5 kbar. Gabbrodiorite rocks show homogeneous distribution of the estimates around 3.0 to 4.0 kbar. Only the hybrid quartz-monzodiorites have wide range of their estimates – between 3.0 and 5.5 kbar demonstrating that they contain mineral associations in disequilibrium bearing traces of crystallizations at different depths.

Crystallization temperature estimations are executed by Blundy, Holland (1990) method putting in their equations the above-calculated pressures. Using the plagioclase-amphibole equilibriums from gabbro samples the estimated temperatures are between 1000o and 1200oC while estimated temperatures for granodiorites range 800-815oC. Interestingly the crystallization temperature estimations for quartz-monzodiorite samples are again more dispersed in wide range – 650-820oC. This peculiarity suggests probably the influence of magma mixing processes between partial magma batches having different crystallization temperatures.

GEOCHEMISTRY

A selection of new 130 silicate analyses, 90 X-ray fluorescent and neutron-activation analyses, supplemented by 130 atomic-absorption analyses contributed to the geochemical characteristics of the rocks studied. The samples are taken out uniformly from all rock varieties. A representative excerpt of analyzed samples is present in Table 4 and the statistical data of the trace element concentrations arranged in rock units are shown in Table 5.

4 Table 4

я Selected representative chemical analyses from Varshilo pluton in Strandja Mountain

Unit I II III V IV VI VII Rock Pyroxenite Gabbro Gabbrodiorite QMd Quartz-diorite Granodiorite Ap

Sample 39 40 14 15 17 31 65 43 87 48 88 59 62 SiO2 46,90 39,62 44,28 41,93 45,79 49,36 51,04 53,50 59,92 58,30 66,78 67,28 73,62 TiO2 0,81 1,43 0,78 1,19 0,98 0,89 0,62 0,80 0,47 0,51 0,27 0,35 0,25

Al 2O3 6,61 8,45 17,06 18,22 15,65 14,84 19,55 17,43 15,11 17,40 15,48 15,65 13,12 Fe2O3 6,17 9,64 6,15 8,22 8,10 4,71 3,95 3,55 5,78 3,85 2,93 2,30 0,92

FeO 6,90 8,17 5,08 5,37 4,08 5,38 3,90 4,02 2,30 2,13 1,06 1,43 0,49 MnO 0,24 0,16 0,19 0,18 0,16 0,16 0,16 0,18 0,15 0,06 0,12 0,13 0,01 MgO 12,64 11,91 9,39 7,47 5,19 7,33 4,25 4,23 2,66 3,08 0,73 1,32 0,39 CaO 17,37 14,40 13,69 13,52 14,59 10,80 9,97 8,50 6,08 7,58 4,13 4,33 1,43

Na2O 1,02 0,98 1,75 2,00 2,15 2,82 3,65 3,95 4,12 4,48 4,12 4,27 2,77 K2O 0,52 0,63 0,30 0,50 1,11 2,17 1,57 2,45 2,44 1,80 3,68 1,91 6,69 P2O5 0,23 0,03 0,13 0,31 0,18 0,38 0,25 0,08 0,10 0,03 0,11 0,31 0,11 H2O

+ 0,40 1,67 0,65 0,61 1,38 1,91 0,46 0,29 0,81 0,55 0,23 0,93 0,30 H2O

- 0,10 0,36 0,23 0,07 0,20 0,02 0,21 0,24 0,01 0,08 0,21 0,05 0,11 SO3 0,06 0,16 0,08 0,11 0,13 0,05 0,08 0,08 0,05 0,04 0,05 0,05 0,10

Total 99,97 99,61 99,76 99,70 99,69 100,82 99,66 99,30 100,00 99,89 99,90 100,31 100,20 Rb 40 48 22 30 60 72 53 95 91 53 134 60 150 Ba 35 112 40 35 112 100 366 371 597 309 673 293 428 Sr 172 276 535 774 1124 936 776 875 707 692 879 424 411 Zr 29 40 15 21 48 64 10 106 73 59 118 93 72 Th 0,6 0,3 0,8 0,5 3,6 3,8 3,20 4,9 6,7 2,5 4,7 2,3 9,6

11

U 0,7 0,11 0,09 0,09 0,72 0,36 0,82 1,1 3,0 0,5 1,6 0,7 2,5 Hf 0,50 0,50 0,50 0,60 1,60 2,50 1,30 3,6 2,7 1,4 1,4 2,0 3,1 Ta - - - - - 0,10 0,20 0,30 0,3 0,15 0,2 0,2 0,3 Cr 180 33 71 41 87 200 12 37 12 13 5 6 6 Sc 80 48 43 29 27 45 31 19 13 11 2,3 3,4 1,7 As - - 0,6 - 0,7 1,58 0,98 - 2,4 0,10 - - 0,46 Zn - - 20 20 31 41 60 60 44 50 16 120 -

Mo - - - - - - - 4,3 - - 1,1 - - Co 52 58 29 36 34 41 23 23 11 10 7 3,5 2,9 V 373 562 443 333 298 286 258 214 137 175 67 34 40

La 6,70 1,99 2,78 5,80 10,80 21,37 16,80 24,47 15,20 7,00 12,00 8,10 12,00 Ce 13,00 3,0 3,60 12,00 16,00 40,00 27,00 41,00 28,00 12,00 17,00 15,00 17,00 Sm 5,40 2,2 2,40 3,20 5,10 6,30 4,80 7,20 3,70 2,60 2,00 2,10 1,40 Eu 0,87 0,27 0,44 0,99 0,88 1,47 1,25 1,01 0,91 0,86 0,32 0,49 0,22 Tb 1,00 0,5 0,05 1,00 1,00 1,00 0,80 1,20 1,00 0,70 0,40 0,50 0,15 Yb 1,80 0,7 1,10 1,50 1,70 2,10 2,00 1,90 1,80 0,14 0,90 1,40 0,70 Lu 0,10 0,12 0,12 0,04 0,20 0,14 0,17 0,18 0,30 0,14 0,18 0,20 0,20

З и: 1. я : Md – ; Ap – . 2.

: , (1989) - . 3. я ,

; Rb, Na я я; Ba, Sr, Zr, V - я

; Zn Mn я; Cr, REE, Sc, W, As, Mo, Co, Th, U, Hf, Ta NAA я . 4.

- 39 – - ; 40 – - ; 14 – - ; 15 – ; 17 –

; 31 – - ; 65 – ; 48 – ; 87 – ; 43 –

- ; 88 – ; 59 – ; 62 – - - . 5. 130 я . , .

Notes: 1. The abbreviations for the modal nomenclature are: Md – monzodiorite, Ap – aplite. 2.

Sources: Kamenov, Andreev (1984) and unpublished trace-element analyses of the author. 3. Major oxides were obtained by classical wet silicate analysis in the Geochemical Laboratory at Faculty of Geology and Geography, Sofia University. The elements Rb, K and Na were analyzed by flame photometry. The ОХОmОnts BК, Sr, Гr КnН V аОrО КnКХвгОН Лв БRF Тn “EUROTEST” Co, SoПТК. Гn КnН Mn аОrО НОtОrmТnОН by AAA. Cr, REE, Sc, W, As, Mo, Co, Th, U, Hf and Ta were obtained by NAA in the Laboratories of “EUROTEST” Co. 4. BrТОП pОtroРrКpСТМ НОsМrТptТon oП tСО sКmpХОs: 39 – gabbro-pyroxenite, 40 – amphibole-bearing pyroxenite, 14 – olivine-pyroxene gabbro, containing a few amphibole grains, 15 – amphibole gabbro having relics of olivine, 17 – common pyroxene gabbro, 31 – pyroxene-bearing monzogabbrodiorite, 65 – amphibole monzogabbrodiorite, 48 – amphibole quartz-diorite, 87 – biotite-bearing amphibole quartz-diorite, 43 – amphibole monzodiorite, 88 – biotite granodiorite, 59 – amphibole granodiorite porphyry from marginal facies. 5. The representative analyses are selected from the set of 130 unpublished silicate analyses.

The major oxides are grouped into independent fields corresponding to the above-

detached rock units and form typical trends (Fig. 4). The prevailing part of the analyses from the first four rock units are lain close to the boundary line separating the normal and transitional alkaline rocks, while the analyses from the units V and VI fall mostly in the calc-alkaline trend. Only the pyroxenite unit analyses are deviated from these two trends on account of their cumulative origin. Aplite analyses completed the petrochemical evolution in the transitional alkaline trend.

12

5 Table 5

я - ( ppm) Statistical data of trace-element concentrations (in ppm) in rock units

Rock units I II III IV V VI VII Total

number analyses

7 9 12 12 12 10 3 65

Rb X* S**

32,29 13,67

24,55 12,63

60,91 14,75

83,42 16,18

67,92 21,39

96,40 26,80

165 73,05 44,46

Ba X S

90,29 38,48

81,44 41,42

211,18 113,43

526,50 161,20

564,25 143,40

586,50 190,82

428 -

357,26 269,37

Sr X S

174,29 59,70

632,33 137,19

727,27 201,83

753,17 100,23

673,33 89,17

691,50 162,81

411 -

610,41 242,00

Zr X S

41,57 12,84

42,11 46,47

60,27 30,45

93,58 35,56

76,25 17,74

123,40 55,43

72 -

69,46 46,57

La X S

3,80 2,00

6,24 3,88

12,38 4,30

15,13 4,42

12,13 3,83

14,98 5,62

13,17 -

11,43 5,87

Ce X S

7,79 4,63

12,95 7,76

21,54 7,89

26,42 6,97

19,25 5,46

22,50 8,09

19,00 -

19,58 8,87

Sm X S

2,96 1,40

3,75 1,71

3,86 1,52

4,21 1,31

3,11 0,63

2,54 0,83

1,40 -

3,68 2,49

Eu X S

0,45 0,23

0,75 0,37

0,88 0,38

0,95 0,25

0,64 0,26

0,58 0,16

0,37 -

0,70 0,36

Tb X S

0,69 0,34

0,83 0,55

0,84 0,30

0,86 0,37

0,73 0,16

0,48 0,15

0,30 -

0,72 0,37

Yb X S

1,04 0,52

1,49 0,57

1,52 0,39

1,58 0,67

1,46 0,45

1,21 0,34

0,80 -

1,39 0,53

Lu X S

0,19 0,25

0,10 0,06

0,11 0,05

0,17 0,05

0,18 0,12

0,18 0,06

0,17 -

0,14 0,12

Th X S

0,54 0,43

0,98 1,25

2,64 1,01

4,17 1,70

4,35 1,54

5,74 1,77

7,90 -

3,44 2,49

U X S

0,15 0,25

0,18 0,32

0,48 0,36

1,20 0,54

1,73 0,96

1,60 0,82

5,40 -

1,01 0,90

Hf X S

0,66 0,72

0,53 0,52

1,43 0,46

1,97 0,81

1,96 0,69

2,20 0,53

2,40 -

1,58 0,91

Ta X S

0 -

0,03 0,06

0,12 0,06

0,18 0,09

0,21 0,08

0,23 0,08

0,23 -

0,14 0,11

Cr X S

369,4 380,0

63,3 35,4

64,7 58,4

39,7 79,9

10,3 3,6

9,7 4,4

6,5 -

71,7 164,8

Sc X S

60,29 13,55

34,22 13,99

25,18 8,83

17,43 9,07

10,70 4,31

4,12 1,71

1,90 -

21,90 18,78

W X S

0,66 1,18

0,48 0,68

0,25 0,40

2,22 4,54

1,34 1,76

0,86 1,06

1,60 -

1,12 2,59

Zn X S

1,20 2,58

16,44 14,76

48,91 20,14

59,25 28,92

80,33 108,49

53,10 40,52

24,00 -

41,49 58,43

Co X S

43,36 11,45

29,78 9,56

23,91 7,75

17,87 11,21

9,88 2,97

5,97 2,31

2,77 -

19,88 14,10

З и: X* - ; S** - . : I - ; II – ; III – ; IV - -

; V – ; VI - ; VII -

Notes: X* - means; S** - standard deviation. Rock units: I - pyroxenite; II – gabbro; III –gabbrodiorite; IV - quartz-diorite; V - quartz-monzodiorites and monzodiorite; VI - granodiorite; VII – aplites

13

35 40 45 50 55 60 65 70 75 800

1

2

3

4

5

6

7

8

9

10

Gb Pxt

Mgb

Gbd

Md

Qd

Gd

Ap

1

2

3 4

5

6

7

SiO wt%2

Na

O+

K O

wt%

22

Fig. 4. Rock samples from the Varshilo pluton in the TAS-diagramme SiO2 vs. Na2O+K2O

(Bogatikov et al., 1981) Fields: 1 – pyroxenites, 2 – gabbro and monzogabbro, 3 – gabbrodiorites, 4 – monzodiorites and

quartz-monzodiorites, 5 – quartz-diorites, 6 – granodiorites, 7 – aplites

On the SiO2-K2O plot (Peccerillo, Taylor, 1976, extended by Dabovski et al., 1989)

two trends are also outlined (Fig. 5): (i) the first one demonstrates comparatively steep and faster increasing of the potassium alkalinity in the sequence of the rock units I, II, III and IV, accommodated in the following series: tholeiitic, calc-alkaline, high-K calc-alkaline and partially shoshonitic and (ii) the second trend encompasses the rock units V and VI included mainly in the series calc-alkaline and high-K calc-alkaline, characterized by relatively weaker increase of the alkalinity in return for the rapidly increase of the acidity. The analyses from the unit VI cover mainly the shoshonitic series but there are some diversions to the ultra-potassium shoshonitic series. That sort of differences in the trend directions reflected probably the existence of two different type parental magmas interacted between them and formed the hybrid rock unit IV (monzodiorite and quartz-monzodiorite).

Almost all rock analyses from the first two units and the predominant number of analyses from the unit III fall in the outlines of the tholeiitic trend of chemical evolution on the diagrammes FeOt/MgO vs. SiO2 and AFM (here not shown), whereas all analyses from the other units fall in calc-alkaline fields. Such differences in the serial sequences became apparent by the application of method of Factor Analysis executed on the selection of silicate analyses from plutonic rocks (Kamenov, ndreev, 1989).

14

35 40 45 50 55 60 65 70 75 800

1

2

3

4

5

6

7

TH

CA

HKCA

SH

UKSH

1

2

3

4

5 6

7

SiO wt%2

K O

wt%

C

Fig. 5. SiO2 vs. K2O diagramme for rocks from the Varshilo pluton after Peccerillo, Taylor (1976) with the extension by Dabovski et al. (1989)

The fields of the rock units are the same as in fig. 4. Series: TH – tholeiitic, CA – calc-alkaline, HKCA – high-potassium calc-alkaline, SH – shoshonitic, UKSH – ultra-potassium shoshonitic

Chondrite-normalized REE spectra of representative samples are demonstrated in

figures 6, 7 and 8. Pyroxenite unit (Fig. 6, A) and gabbro unit (Fig. 6, B) distributions show the typical for the weakly fractionated basic rocks slight enrichment of the LREE over the HREE, but they differ from the other patterns by the deeper negative Eu-anomaly in pyroxenite. Such peculiarity indicates the participation of plagioclase in the fractionated mineral association. Their cumulative origin is supported by the mutually conjugated patterns between gabbro and pyroxenite chondrite-normalized REE distributions. REE patterns of gabbrodiorite and monzogabbro units (Fig. 6, C) and of the monzodiorite unit (Fig. 7, A) manifest moderate fractionation of the LREE and not so deep negative Eu-anomalies. They differ in between only by the relatively higher normalized values for the whole specter of REE in the richer in alkali oxides monzodiorites. This regularity is broken in the next quartz-diorite unit (Fig. 7, B) where the ratios LREE/HREE are lower taken as a whole; their negative Eu-anomalies are deeper and all the normalized values are lower for the whole REE range. These geochemical differences make most unlikely quartz-diorite to originate from monzodiorite by a common process of evolution from one and only parental magma.

The chondrite-normalized pattern of granodiorite (Fig. 7, C) shows genetic connection with quartz-diorite relative to the REE-distributions and the both patterns are nearly generally similar. Only the relative higher enrichment of the LREE in relation to the HREE and the higher normalized absolute all concentrations are the lightly distinction of the granodiorite analyses. The unit of aplite (Fig. 8) shows REE patterns typical for the residual portions of the crystallization differentiation. The values of REE are depleted

15

during the preceding episodes of magma evolution and the chondrite-normalized distributions are marked by deeper negative Eu-anomalies. The close similarity between the REE patterns of granodiorite and aplite rocks grounds the idea that the last ones are possible leucocratic apophyses of granodiorite magma.

1

10

100

La Ce Pr NdSm Eu Gd Tb Dy Ho Er TmYb Lu

Sa

mp

le/c

ho

nd

rite

1

10

100

La Ce Pr NdSm Eu Gd Tb Dy Ho Er TmYb Lu

Sa

mp

le/c

ho

nd

rite

A

Pyroxenites

B

Gabbro

1

10

100

La Ce Pr NdSmEu Gd Tb Dy Ho Er TmYb Lu

Sam

ple

/chond

rite

Gabbrodiorites and monzogabbro

C

Fig. 6. Chondrite-normalized REE diagrammes for a selection of representative samples of the basic rock units from the pluton (normalizing values after Nakamura, 1974)

16

1

10

100

La Ce Pr NdSmEu Gd Tb Dy Ho Er TmYb Lu

Sam

ple

/ch

ond

rite

1

10

100

La Ce Pr NdSmEu Gd Tb Dy Ho Er TmYb Lu

Sa

mple

/chon

drite

1

10

100

La Ce Pr NdSm Eu Gd Tb Dy Ho Er TmYb Lu

Sam

ple

/chond

rite

Monzodiorites

Quartzdiorites

Granodiorites

A

C

B

1

10

100

La Ce Pr NdSm Eu Gd Tb Dy Ho Er TmYb Lu

Sa

mp

le/c

ho

nd

rite

Aplites

Fig. 7. Chondrite-normalized REE patterns for a selection of representative samples from the intermediate rock units

Fig. 8. Chondrite-normalized REE patterns for a selection of representative samples from the rock unit of aplites

17

MORB-normalized trace element distributions on the same selection of

representative analyses are manifested in figures 9, 10 and 11. All models are typical of an island-arc geodynamic setting showing distinct negative Ta anomalies. The other negative peaks for Zr, Hf and Ti are controlled by the chemistry of the melting magma source. The high LILE/HREE ratios are indicative for the subductional origin of the magmas and probably they are dependant also on the fluid phases influence. The observed systematical variations may be explained by realization of magma mixing between two different types parental magmas – gabbro and quartz-diorite ones. Such process is likely to be effected during the extended fractional crystallization – MFC process. The monzodiorite and quartz-monzonite unit could be a clear expression of such mixing process. On the background of the general similarity in the MORB-distributions some differences in the levels of the normalized values and in the consecutive variations of the ratios BaN/ThN and ZrN/HfN are perceived.

0,1

1

10

100

Sr K2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sa

mp

le/M

OR

B

Pyroxenites

A

0,1

1

10

100

SrK2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sam

ple

/MO

RB

Gabbro

B

0,1

1

10

100

SrK2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sam

ple

/MO

RB

Gabbrodiorites

C

Fig. 9. MORB-normalized patterns (Pearce et al., 1984) for selected samples from the basic rock units of the pluton

18

Mutually conjugated variations in the units showing tholeiitic sequence (Fig. 9) are

in accord with a crystal differentiation process accomplished in the basic parental magma. The sequences of quartz-diorite, granodiorite and aplite (Fig. 10, Fig. 11) indicate also typical geochemical differences in their models, possible result of the fractionation of their parental quartz-diorite magma. Potassium feldspar crystals have been probably involved in such fractionation. The hybrid unit of monzodiorites is distinguished by lack of the negative barium anomaly due to its increased concentrations in the alkalized at the mixing process magma. The positive peaks of Ce and Sm are more expressive in these hybrid rocks and they could be connected with the increased fluid pressure during such a process.

0,1

1

10

100

Sr K2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sam

ple

/MO

RB

Quartzdiorites

0,1

1

10

100

SrK2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sa

mpl

e/M

OR

B

Granodiorites

0,1

1

10

100

Sr K2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sam

ple

/MO

RB

Monzodiorites

Fig. 10. MORB-normalized patterns for selected samples from the intermediate rock units of the pluton

19

0,01

0,1

1

10

100

SrK2ORb Ba Th Ta Nb Ce Zr Hf SmTiO2Yb

Sam

ple

/MO

RB

Aplites

MODELLING OF MAGMATIC EVOLUTION

The regular variations in the petrochemical and geochemical composition of the plutonic rocks from Varshilo Pluton and some of the peculiarities of their rock-forming minerals support the idea of the predominant significance of a process of fractional crystallization realized in two different parental magmas. Such idea was backed with arguments following from the application of methods of Correspondence Factor Analysis, CFA (Kamenov, Andreev, 1989) and from the studies in detail of the rock-forming plutonic feldspars (Kamenov et al., 1989). Signs left from the interactions between both types parental magma are revealed also in the above-stated characteristics of the femic minerals and they resulted almost from most of the geochemical interpretations.

The strong negative correlation between the oxides MgO, CaO, Fe2O3, FeO and SiO2 in the plutonic samples, the way it is in our case, usually is explained as a manifestation of fractionation of femic minerals and basic plagioclase. The parallel variation of V, Ti and Fe is an indication of titanomagnetite incorporation to this fractionating association of minerals. All these suppositions can be verified and qualitative meaning to be attached to them by the application of the mathematical model of simple mixing (Le Maitre et al., 1979a, b). We made use of the version of this method portrayed by the following equation:

n Cin

i = Σ Ci.Xi + Xdif. Cdif j=1

where, У = (1…n-1); Cin, Ci and Cdif are the amounts of oxides in the parental initial magma, in the differentiated partial magma and in the fractionating mineral phases and БТ…Бn are weight proportions of every one of the separating mineral phases in parts out of unit, while Xdif is the proportion of the deduced differentiated partial melt by mathematical calculations. The calculations are accomplished for 10 petrogenetic oxides using the Method of Least Squares in the programme GENMIX (Le Maitre, 1979a,b), adapted by A. Andreev from the geological Institute at Bulgarian Academy of Sciences. The difference between value predicted by the theoretical model and the values actually observed experimentally is expressed by tСО НТstКnМО R=√D2 (root-mean-square deviation - RMSD) as the frequently used measure for the success of the procedure is its value

Fig. 11. MORB-normalized patterns for selected samples from the rock unit of the aplites

20

R<2. Basically, RMSD represents the sample standard deviation of the differences between predicted values and observed values. The very modeling was carried out step by step selecting pairs of analyses of rock samples and acceptable from petrological point of view associations of rock-forming minerals. The same procedure is used to estimate the possibility of simple magma mixing between partially crystallized acid magma and separate inputs of more primitive mafic magma. Only seven out of the numerous attempts are selected to exemplify the probability of principle that both processes of magmatic evolution – fractional crystallization and magma mixing have been realized in this pluton (Table 6). The microbe analyzed real chemical mineral compositions are used in the calculations.

1. Pxt16 = (58 % CPx + 4 % Ol + 2 % TiMt) + 36 % Gb7 2. Gb17 = (24 % CPx + 31 % Pl + 8 % TiMt) + 24 % Gbd11 3. Gbd11 = (24 % CPx + 42 % Pl + 5 % TiMt) + 28 % Md69 4. Qd50 = (11 % Am + 38 % Pl + 3 % Bt + 2 % Mt) + 46 % Gd54 5. Gd54 = (4 % Am + 41 % Pl + 2 % Mt) + 53 % Apl62 6. Md69 = 20 % Gbd82 + 80 % Qd72 7. Md69 = 51 % Gbd32 + 49 % Gd8 The used sample analyses of the rock varieties are marked by the following symbols:

Pxt – pyroxenite; Gb – gabbro; Gbd – gabbrodiorite; Md – monzodiorite; Qd – quartz-diorite; Gd – granodiorite and Apl – aplite, and their number is presented as subscript. The fractionating minerals are abbreviated as follows: Pl – plagioclase; Am – amphibole; CPx – clinopyroxene; Ol – olivine; TiMt – titanomagnetite; Mt – and Bt – biotite.

An illustration of the mathematically deduced chemical trends is the drawing of Fig. 12, where the experimental and observed compositions are put together in the frames of the terms basicity – alkalinity. The basicity measure when the thermodynamic characteristics of the mineral forming reactions are used is the affinity to the proton reduced to the standard atomic formula at temperature of 900oC (Marakushev, 1973). Three compositional trends are revealed by the comparison between the alkalinity, expressed as a sum (Na2O+K2O) in weight per cent with the basicity ΔZ

900 kj:

I. A cumulative trend of solidus evolution of the basic parental magma where pyroxenites and part of the gabbro rocks are distinguished by their basicity levels mainly, while the differences in the alkalinity levels are small.

II. A trend of magma evolution in the basic parental magma where gabbro and gabbrodiorite rock units are distinguished by their alkalinity levels mainly, whereas both units are significantly overlapped by their basicity.

III. A calc-alkaline trend in the acid rock units (quartz-diorite, granodiorite and aplite), where both parameters alkalinity and basicity vary in a co-ordinated manner. A characteristic feature of this trend is its fast

21

decreasing of the basicity and almost equal but a bit weaker increasing of its alkalinity

The different orientation of the both principal magma evolutional trends (II and III) contradicts to the idea of a common magma crystallization differentiation process only from one parental basic composition. The corollary is that the intermediate and acid rock units could not be a direct product of fractional crystallization of basic parental magma. Another one essential conclusion is that the unit of quartz-monzodiorite and monzodiorite rocks (group 4), which on the Fig. 12 is situated just on the intersection between the both expressed trends, could be a hybrid result by mixing of the two principal parental magmas. A part of the samples from the aplitic veins falling in the field of granodiorite unit is evidence that they could be apophyses of the granodiorite magma.

O

3

5

7

9

1

II

III

I

Fig. 12. Results of the mathematical modeling of the magmatic evolution in the pluton in the

diagramme ЛКsТМТtв (∆Г900 after , 1973) vs. alkalinity (Na2O+K2O) Abbreviations for the rock units: Pxt – pyroxenites, Gb – gabbro, Gbd – gabbrodiorites, QMd –

quartz-monzodiorites, Qd – quartz-diorites, Gd – granodiorites, Ap – aplites. Only the numbers of the КnКХвsОs аСТМС КrО sСoаn Тn tКЛХО 5 КrО mКrФОН. TСО НОsТРnКtТon “supОrsМrТpt” Тs Пor tСО МКХМuХКtОН compositions and these without – for the experimental compositions. Magma evolution trends: I – cumulative segregation, II – tholeiitic differentiation, III – calc-alkaline differentiation

The results of the carried out mathematical modeling (Table 6 and Fig. 12)

substantiates additionally the assumed principal processes of magma evolution in the pluton (fractional differentiation and magma mixing between two contrasted types magma). It turns out that such idea does not contradict to the actual petrochemical and mineralogical data. The pyroxenite trend (trial 1) is reproduced successfully out of the cumulative mineral association of clinopyroxene, olivine and titanomagnetite separated from the gabbro Gb7. Gabbro may be represented as a mixture between not fully separated fractionating mineral association, consisted of clinopyroxene, plagioclase and titanomagnetite with gabbrodiorite magma (trial 2) and the trend between the basic rocks is also successfully modeled (trial 3). Granodiorite is worked out through process of

22

fractionation of the mineral association amphibole, biotite, plagioclase and magnetite from the second parental quartz-diorite magma (trial 4) while the aplite residuum is a possible product of the fractionation of amphibole, magnetite and plagioclase but separated from the partial granodiorite magma (trial 5). The hybrid unit of quartz-monzodiorites and monzodiorites is equally likely to be a result of the mixing of nearly 20 % gabbrodiorite magma and 80 % quartz-diorite magma (trial 6), or of 51 % gabbrodiorite magma and 49 % granodiorite partial magma (trial 7). However, the modeling demonstrates that monzodiorite magma could be produced also at a process of fractionation of the gabbrodiorite magma (trial 3) after separation of some fractionate consisted by clinopyroxene, plagioclase and titanomagnetite. Both different interpretations for this eventually polygenetic type rocks in the pluton are supported in the petrographical peculiarities of the rocks – many outcrops of rocks expressing porphyroid textures. The fine-grained groundmass is built up by acid plagioclase, K-feldspar and quartz with subordinated biotite and their modal relationships correspond to granodiorite and granite mineral compositions. Glomeroporphyritic clusters of xenocrysts made up by clinopyroxene, magnetite and basic plagioclase, enveloped by aureoles of amphibole and biotite are observed among the groundmass of these rocks. The mixing is backed with arguments more confidently by the mineralogical features – spotted patchy replacements in the plagioclases, many cases of reverse zoning, along with normal one, multiple gabbrodiorite in composition small-grain enclaves within the quartz-diorites and granodiorites showing fine-grained chilled marginal zones.

One of the basic conclusions out of the carried out here mathematical modeling is that the suggested still by Kamenov, Andreev (1984) mechanisms of magma interaction now are modelled successfully not only by qualitatively but also by quantitatively general mixing calculations.

A suitable method to emphasize the primary importance of the fractionation crystallization in the evolution of Varshilo Pluton is the suggested by Gill (1981) and developed by Cocherie (1986) as a graphic means of juxtaposing the concentrations of the high incompatible elements to the ones of the compatible elements during the magmatic process (Fig. 13). The Low of Rayleigh (1902) for the ideal fractionation crystallization is applied on Fig. 13 where one of the illustrative cases of the Varshilo Pluton is shown (Kamenov, 2003).

HКrФОr’s КnН otСОr РОoМСОmТМКХ НТКРrКmmОs rОЯОКХ tСКt tСО ОХОmОnts СКЯТnР highest compatible behaviour had been Sc, V, Cr and Co while the high incompatible elements during the magma evolution had been Sr, Zr, Ba, Th, U, K, Pb, Ce, La, Hf, Ta and Rb. The relationships of Sc and Th are singled out for illustration here, but analogous results are obtained also at diagrammes of the sort Co – K2O, V – Rb, Cr – Ce etc. Almost linear variation in the frames of the system Sc-Th for all rock varieties, with the exception of gabbro and pyroxenite, conforms well to process of fractional crystallization (materialized in the trends II and III). All diversions out of the general direction at ultramafic pyroxenite and at gabbro analyses logically follow the direction of the cumulative process (trend I). This plot materializes successfully the idea that the pyroxenite is a cumulative product from crystallization of parental gabbrodiorite magma.

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6 Table 6

я GENMIX Results of the modeling the fractional crystallization and magma mixing using the programme GENMIX

Trial 1 2 3 4 5 6 7 Rock Pyroxenite Gabbro Gabbrodiorite Quartz-diorite Granodiorite Monzodiorite Monzodiorite

Sample 36 36I 17 17I 11 11I 50 50I 54 54I 69 69I 69 69I SiO2 47,03 46,98 46,82 46,65 51,80 51,65 61,38 61,26 66,91 66,73 57,68 57,88 57,68 57,61 TiO2 0,76 0,45 1,00 0,80 0,49 0,82 0,44 0,42 0,24 0,26 0,66 0,63 0,66 0,91

Al 2O3 6,38 6,68 16,00 16,32 17,78 18,05 17,45 16,98 16,12 16,21 18,31 17,63 18,31 18,16 Fe2O3 6,38 6,42 8,28 7,86 4,99 4,59 3,42 3,14 2,59 2,19 4,42 4,40 4,42 3,79

FeO 5,54 5,94 4,17 4,99 3,68 4,22 2,32 2,49 0,86 1,39 2,46 2,47 2,46 2,88 MnO 0,22 0,07 0,16 0,09 0,06 0,17 0,15 0,16 0,11 0,04 0,09 0,13 0,09 0,14 MgO 14,80 15,98 5,31 5,90 5,14 5,58 2,75 2,48 1,17 0,71 3,45 3,49 3,45 3,01 CaO 17,64 18,06 14,92 14,80 10,66 10,12 6,57 6,43 4,48 4,55 6,91 7,27 6,91 7,34

Na2O 1,05 0,32 2,20 2,06 3,70 3,24 3,34 4,38 4,11 4,00 3,59 3,59 3,59 3,32 K2O 0,19 0,00 1,13 0,54 1,72 1,54 2,17 2,25 3,39 3,93 2,43 2,43 2,43 2,83

R=√D2 1,10 1,32 1,18 1,24 1,00 0,80 1,13

З : ( ), – ( e ). R=√D2 - я ( я).

Note: The calculated theoretically analyses are marked by the superscript I and the ones without this designation are the experimental

compositions. R=√D2 - the distance between the experimental and calculated compositions.

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D = 5,33Sc

C = 530ppmSc

S

Gdp

Кя

Gb

Pxt

Gbd

Qmd

Qd

Gd

Ap

-

я

Sc

100

0,1

10

1101 Th,ppm100

Fig. 13. Plot logCSc vs. logCTh after Cocherie (1986) for samples from the plutonic rocks of

Varshilo pluton The abbreviations of the rock units are the same as in the fig. 12. The average values of the rock

units are shown with larger symbols. DSc – the calculated bulk partition mineral/liquid coefficient for the element Sc. CsSc – concentration of Sc in the solid source during the melting. The arrows are the directions of the main differentiation mechanisms in the pluton

On the other hand, Fig. 13 demonstrates that gabbro compositions may be presented as various mixtures between cumulates and gabbrodiorite melt (imperfect fractionation). Too indicative for the hybrid character and polygenetic origin of quartz-monzodiorite is the locality of the field made up by the points of its compositions – just in the intersection of the both trends, that of the basic and that of the intermediate and acid compositions. Even the porphyry varieties granodiorites could be interpreted as mixtures between fractionate and partial magma because their site is deviated along the direction of imperfect fractionation.

The plot on Fig. 13 gives a fair chance of determining the initial concentration of the compatible element (Sc in the case) in the solid melting source of the parental basic magma (CS

Sc=5,33), as well as its eventual bulk distribution coefficient (DSc = 5,33). The slope of the linear trend is used (average for the case α = 77 .

The applied ways to study the chemical variations within the rocks from Varshilo Pluton underline the conclusion that they owe their peculiarities to the combine influence of the processes of imperfect or complete cumulative segregation, mixing of different magmas and fractionation, dependant on the change of crystallization conditions in the magma chambers. The role of fluid impacts on the magmas in every one of these processes supplemented the reasons of the magmatic variety in the plutonic rocks. The

25

irregular latemagmatic amphibolization over the rocks is a manifestation of such influences (Fig. 2).

In view of the fact that the probability of realization of process of crystal fractionation does not contradict to the results of modelling with the major rock forming oxides we tried to use the same successful trials of the calculations to model the distribution of the trace elements. It was accepted that the equilibriums are attained only between the surface of the crystallizing phases and the melt. It is fairly admissible for the cases of shallow emplaced magma chambers where the crystallization is relatively fast. Petrological evidences for such not deep level of crystallization in the Varshilo Pluton are adduced in Kamenov et al. (1989). The theoretical deduced concentrations of a given trace element in the rocks during crystallization at fractionation are obtained applying the equation describing the trace element behaviour (Allégre et al., 1977). The calculated weight proportions of the partial magmas are taken out from the trials with the major oxides. The programme RAYFC elaborated by A. Andreev facilitates the calculations and compares the experimental (obtained from the chemical analyses) and modeled mathematically values in the accepted for initial source rock (Cocalc – Coexp). The appropriate partial and bulk distribution coefficients are matched out of the literature, according to the particular conditions in the plutonic evolution.

One of the most important results of such modelling is that only the distribution of the elements Sc, V, Co, Sr, Hf, Cr and Zr agrees well with a process of ideal fractionation while the elements Rb, La, Ce, Sm, Yb, Th and U have been introduced in the rocks in higher amounts than are required for the process of crystallization differentiation. This conclusion is an additional argument of the role of mixing between different parental magmas, as well as of the fluid influences during the course of fractionation. All this means that the magma evolution had been realized in the pluton in open system.

CONCLUSION

The plutonic rocks from Varshilo Pluton reveal mineral and chemical features

determining them as a product of subduction-related magma evolution in island arc geodynamic setting. The rock variety in the pluton is a result of complex interaction between the processes of fractional crystallization and magma mixing of two contrasted in composition parental magmas. The established three trends in the compositional variations of the rocks are explained by the differences in the crystallization conditions of their magmas and probably by their differing sources. The large-scale magnetite crystallization under reducing environment had followed the crystallization of the other rock forming minerals and in this way the basic melt developed a tholeiitic evolution. The cumulative segregation of the heavier femic (ferromagnesian) minerals conditioned the formation of pyroxenites and the non-perfect separation between the liquidus and solidus mineral phases in the magma had been typical for gabbro and gabbrodiorite rock units. The magma evolution at the interim levels had been directed to the calc-alkaline tendency due to the increased oxygen volatility provoked by fluid inputs in the magmatic chambers. Water-bearing mineral phases had been involved in the magma fractionation producing granodiorite and aplite partial melts. Some mineral associations had been in disequilibrium judging from the compositional mineral zoning. Some field, structural and compositional indications for magma mixing processes is also produced by the new geochemical analyses. The quartz-monzodiorite unit is successfully modelled by general

26

mass-balance calculations and its hybrid origin is evidenced. Magma-fluid interactions complemented this scenario as part of the large-ion elements had been introduced into the rocks in this way. The prolonged co-existence of the magmas with metal-enriched fluids turned out to be an important condition for transformation of the mantle distillate into ore-bearing solutions extracted out of the magma and of the wall rocks predominantly iron, copper and zinc.

The hypothetical picture of the magma evolution in the Varshilo Pluton above-stated in this paper could be verified also by isotope methods when such an opportunity is available. That is just what is needed to turn this well examined pluton into a key-case for studying similar petrological processes in the Srednogorie Upper Cretaceous island-arc system in Bulgaria.

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Accepted March 2007

Published 2007 Translated into English 2014

This is author’s trКnsХКtТon oП tСО pКpОr puЛХТsСОН Тn Bulgarian.