YEAR XVIII

5, 2011

6, 2011

GEOLOGY AND MINERAL RESOURCES ISSN 1310 – 2265 PUBLISHING HOUSE GEOLOGY AND MINERAL RESOURCES LTD

THE VOLCANIC COMPLEX MOUNT BOWLES FROM LIVINGSTON ISLAND, ANTARCTICA – A CASE OF BURIAL

TYPE METAMORPHISM

Prof. d-r Borislav K. Kamenov

Abstract. Volcanic rocks metamorphosed at subgreenschist facies are widespread in Livingston Island, Antarctica. They occupy large tracts of Hurd Peninsula and are referred to the Mount Bowles Formation. Despite their wide distribution such rocks are not studied regarding the style and conditions of the low-grade metamorphism. No petrological research of their secondary mineral assemblages is published up to now.

In the first attempt to quantify the pressure-temperature conditions under which these low-grade rocks are recrystallized and applying modern hemographic projections and mineralogical approach, here we state the arguments for the new idea that the metamorphism imprinted in the textures, mineralogy and chemistry of the volcanic rocks is of burial type. The transition from prehnite-pumpellyite to the low levels of greenschist facies is deduced out of the studied mineral parageneses. Geothermometric and geobarometric methods have also been used and the P-T determined estimations are characteristic for rather high heating flow in the region. Uprising of mantle-derived magmas in back-arc environment was supposed to be the reason for such high geothermal gradient in the area. The contribution demonstrates that the subgreenschist facies assemblages show extensive overlapping in the derived P-T space as a consequence of variation in the whole-rock chemistry.

The main conclusion of the investigation is that the burial metamorphism is thus envisaged as an integral part of an “Andean tectonic cycle” – a novel and innovative model for the Mount Bowles Formation.

Introduction

The low grade metamorphism on basaltic and intermediate volcanic rocks is perhaps least understood of any metamorphic process operations in Earth crust. Accordingly, in recent years such rocks are amongst the most studied and provided the greatest advances in understanding the metamorphic evolution. The orogenic belt of the Pacific margin of South America and of Antarctic Peninsula in Antarctica contains various magmatic rocks in different facies. These magmatic rocks are generated in active continental margin settings and they are referred to a wide geochronological range – from Jurassic to Neozoic. The prevailing parts of them are affected by a regional low-grade metamorphism. A key-peculiarity of such metamorphism is the general increasing of metamorphic degree with the aging of the stratigraphical sequences. The metamorphic style of South America examples was determined in different ways, but recently it is referred to the genetic type of burial metamorphism. The Andean examples of burial metamorphic low-grade rocks are well known in the petrological literature (Aguirre et al., 1989;

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Levi et al., 1982, 1989; Offler et al., 1980) and the same holds true in respect to the genetic model developed to account for the burial processes there.

In spite of the fact that most of the volcanic exposures in Western Antarctica and especially in South Shetland Islands are studied intensively from different points of view, no special attention was paid on the metamorphic imprints on them. The volcanic sequences of basaltic to andesitic rocks in the archipelago have just the same petrographical composition and are ejected in the same age interval as in the Andes. The published descriptions of the alterations on the volcanic and dyke rocks outcropped in Livingston Island refer only to the presence of secondary minerals as zeolites, pumpellyite, actinolite, chlorite and epidote, but no well-grounded models on their origin existed.

Fig. 1. Geological sketch of the area of occurrence of Mount Bowles Formation

Mt.B – Mount Bowles, BP – Burdick Peak, WN – Willan Nunatak, MP – Moores Peak, CM – Cerro Mirrador, HP – Hesperides Point, NC – Nunatak dell Castillo, BAB – Bulgarian Antarctic Base “St. Kliment Ohridski”. Inserts: a) South Shetland Islands sketch; b) Livingston Island sketch.

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The main aims of the most of the former investigations on the volcanics have been their stratigraphical position and nomenclature or the tectonic regional process and the geochronological relationships. The investigators in the area of Livingston Island assume that the assemblages of the secondary minerals on the volcanic rocks were result of contact metamorphism (Smellie et al., 1995), or that they were products of hydrothermal local metamorphism (Willan, 1994). Our observations and more detailed investigations on the Mount Bowless Formation (Kamenov, 2011) in the island gave good reason to suggest another idea for the development of the alteration processes. In conformity with the analogical cases in the Andean Cordillera, our new multianalytical approach to the precise mineralogical studies, make us to believe that here low-grade regional burial metamorphism is revealed, probably provoked by some regional thermal event. The examining of the progressive developments of secondary minerals, reaction progress in mafic phyllosilicates and topological variations in the low-grade assemblages by new detailed mineralogical and petrological analysis of the volcanic successions of Mount Bowless Formation is the basis of our attempt to find clear metamorphic demarcation between the different events and to record the metamorphic grades and facies changes. The stated results eventually could contribute to the sound interpretation of the metamorphic history in the area.

A. Primary petrology of the lava-pyroclastic complex Mount Bowles1. General geology

Livingston Island contains the most complete record of the magmatic rock sequences, resulted to the activity of the Antarctic Peninsula Mesozoic-Cenozoic arc. The island hosts several geological units (Fig. 1), best exposed within Hurd Peninsula where the Bulgarian Antarctic base is located. The main rock units in the peninsula are as follows:

Metasedimentary Miers Bluff Formation – MBF (Hobbs, 1968; Smellie et al., 1984, 1985) is built up by alternating turbiditic sandstones, conglomerates and siltstones, deformed and weakly metamorphosed in the low-grade facies. The depositional age of the formation was recently accepted as Late-Cretaceous (Pimpirev et al., 2006).

Volcanic Mount Bowles Formation (Smellie et al., 1984; Kamenov, 2011) assumed to be of Cretaceous age on account of regional correlations (Smellie et al., 1984), or 40Ar/39Ar isochrone dating (Zheng et al., 1996). This formation covers the rocks of MBF uncomformably.

Plutonic complexes of Late Cretaceous (Kamenov, 1997) and of Tertiary age (Kamenov et al., 2005) and dyke sequences (Kamenov, 2008).

Mafic volcanic Inot Point Formation (Pliocene to Recent) consisting of explosive and lava products of alkaline and tholeiitic affinity (Veit, 2002; Kamenov, 2004).

2. Petrology of Mount Bowles Formation

The Mesozoic volcanic rocks from the eastern part of Livingston Island are correlated with the Antarctic Peninsula Volcanic Group (APVG) and according to the lithostratigraphic scheme of Smellie et al. (1995) are referred to the Mount Bowless Formation. The main exposures of these rocks are in Hurd Peninsula and northeastern of it. The predominant parts of the isolated exposures are coarse-grained lapilli’s-tuffs, interbedded with lava flows and sedimentary rocks. The total thickness of the sequence is unknown, but it seems that is at least

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650 m, if the sections around Falls Bay are considered. The indurated character and absence of well-developed bedding on account of pervasive everywhere thick joint set, lead to the impossibility to describe a reliable type section. The most typical sections are the ones west of the Willan Nunatak and in the southern part of the Mount Bowless, where the lava varieties occurred more often. The volcanoclastic rocks predominated southward of Burdick Peak. Only in this outcrop these rocks rest unconformably on the MBF beds. The structural relationships between the isolated exposures are unclear, because of their fragmentary character and their great variety of the bedding orientation.

The exposures situated south of Burdick Peak contain badly sorted massive dark colored volcanoclastic rocks, consisted of course-grained lapilli’s tuffs and breccias. The lava flows there are thin and alternate with green volcanoclastic sandstones, cut by dykes and sills. The sedimentary rock fragments included within the lavas indicate that these volcanic rocks are younger than the Miers Bluff Formation. The wide occurred alteration of the rocks is almost ubiquitous.

The volcanic rocks around Mount Bowless are mainly amygdaloidal lavas and agglomerates. The clasts consist of angular fragments of sandstones, siltstones, arkosic greywackes, shales and plutonic rocks. Most of them show boulder dimensions and are massive without amygdaloidal structures. Sometimes the lavas are subporphyric, the typical phenocrysts of feldspars being very rare. The dark fine-grained matrix has an intergranullar and intersertal textures and rarely is felsitic. Usually the rocks are closely jointed and injected by veinlets of secondary minerals, most often quartz and epidote. Frequently no trace of the original form of the coloured silicate minerals remains. The vitreous constituents are preserved exceptionally rare. In the neighbourhood of Moores Peak some pale coloured monomictic tuffs occurred entirely built up by angular fragments of porous volcanic glass.

Poorly sorted and rarely observed sandstones intercalate with siltstones and shales. They are fine-grained arkosic arenites and greywackes. Feldspar is the predominant mineral in the clasts being around 75 per cent out of the other minerals.

The subvolcanic body at Moores Peak was assumed as coeval with the volcanic rocks of Mount Bowles Formation (Smellie et al., 1995), but radiogeochronological Ar-Ar data (Kamenov et al., 2005) proved that it is to be referred to the Eocene small intrusive bodies, which are apophyses of the large Barnard Point Batholith.

Specimens from the lavas, subvolcanic dykes and sills, as well as from the agglomerates are classified into the following petrographical nomenclatures, according to the modal relationships of their rock forming minerals: 1. pyroxene basalt with rare entirely altered olivine subporphyries; 2. quartz bearing basaltic andesite with scare phenocrysts of clinopyroxene and plagioclase; 3. highly porphyritic clinopyroxene bearing amphibole andesite; 4. quartz bearing andesite with amphibole and clinopyroxene. The textures are varied and similar to the ones in the fragments of the pyroclastic varieties. Applying the chemical classification these rocks are referred to the species basalt, andesite and mugearite (Fig.2). The phenocrystals include andesine-oligoclase, augite and opaque minerals and occasionally apatite. The main rock forming minerals are clinopyroxene and plagioclase but within the quartz bearing varieties amphibole occurred as well. Minor minerals are quartz, potassium feldspars and magnetite. The accessory minerals apatite and magnetite are in association with titanite, anatase and zircon.

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Plagioclase occurs as different in size grains. The plagioclase phenocrystals are partly preserved from the superimposed alterations. The zoning in the central and intermediate zones of the individual grains is difficult to be observed and only the peripheral margins are more acid. The compositional range in the central cores is An55-An48, progressing to An25-An20 in the rims. The presence of anomalous high anorthite values in some of the intermediate zones marks magma mixing events and left traces in the plagioclase composition of the phenocrysts. Many of the plagioclase phenocrystals are overfilled with micro inclusions of brown euhedral amphibole or magnetite. Very often the plagioclase suffered of spotted chloritization and sometimes its grains are replaced by epidote, albite, scapolite, calcite, leucoxene, prehnite and clay minerals. The epidote occurs preferentially in the crystal cores, but occasionally the whole of the plagioclase crystal is replaced.

Clinopyroxene is partially preserved from the alterations. Its crystals are smaller in size in comparison with the plagioclase ones. Usually clinopyroxene forms monomineral clusters and glomeroporphyritic aggregates with feldspar. Chemically analyzed unaltered pyroxene fragments are rich in magnesium augites with Mg# numbers ranging 76 to 68 in the cores and high wolastonite constituents of 47 to 42. The marginal parts of the clinopyroxenes fall also in the range of the augites, but their Mg# numbers are lower. The optical and chemical zoning in the composition of pyroxenes are in correspondence of magma process of fractionation, complicated with some contamination events. The pyroxenes are partially replaced by uralite and chlorite, the development of epidote and calcite being common.

Fig. 2. Classification TAS diagramme SiO2 vs. (Na2O+K2O) for analyses from the volcanic complex Mount

Bowles

Fields: B - basalt, BA – basaltic andesite, TB - trachybasalt, Mdj - mugearite, A - andesite. Symbols for

the samples: ● –Hanna Point; ▀ - Mount Bowles; △ – Willan Nunatak.

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Amphibole is pale-brown in colour and falls in the field of high-silica magnesiohornblende (Leake et al., 1997). The ratio Mg# is lower than in the associated clinopyroxenes. Its average value is 65. The late magmatic amphibole is often replaced by pale-green postmagmatic actinolitic amphibole and by chlorite.

The groundmass is fine-grained intergranular in texture and usually is altered significantly. It contains clinopyroxene, plagioclase and magnetite and sometimes quartz (less than 3-4 %) and amphibole. The last mineral is present by prismatic and partly chloritized crystals with pleochroism in brown and pale beige colours. The ratio Mg# varies between 62 and 52 while the richer in iron varieties are typical of the latest amphiboles. It is possible that part of these amphiboles were of metamorphic origin.

Magnetite is assigned to the titanomagnetite species. It is usually confined to the groundmass and is strongly degraded. The crystals are euhedral and occasionally they have a skeletal form. Often the titanomagnetites have narrow selvages of titanite and epidote. When after ferromagnesian minerals, the pseudomorphs are outlined by finely disseminated vermicular iron ore. Ilmenite also shows some alteration to both leucoxene and titanite.

The secondary minerals are epidote, chlorite, albite, pumpellyite, leucoxene, calcite, prehnite, scapolite, actinolite, and smektite type clay minerals. Quartz veins cut the altered rocks and sometimes copper hydroxides are observed around the veinlets.

3. Primary geochemistry

The available chemical silicate analyses (Table 1) are assigned mainly to the calc-alkaline series and rarely to the tholeiite series. The serial trend corresponds to the ones of the other plutonic representatives in the Hurd Peninsula being medium potassium (Fig. 3). The rocks are Si-saturated, metaluminous, normal calc-alkaline, according to their normative characteristics also.

The amount of the normative quartz is between 1 and 2 % in the basalt and between 5 and 7 % - in the andesite. The differentiation index is between 21 and 37, while the normative anorthite composition is in the span 60-70 in the basalt and 50-70 in the andesite.

All analyzed samples show high ratios LILE/HFSE with the typical for the arc-derived magmas depletion of Nb. The ratios Zr/Y have moderate values, which is a typical peculiarity of the continental arcs. TiO2 and P2O5 correlate positively with SiO2 and with Zr and the ratios Rb/Sr and CeN/YbN are relatively low as a proof of geochemical characteristics of tholeiitic arc magmas. Geochemical evidences determine volcanic island arc setting of the magma origin. Medium potassium calc-alkaline series (Fig. 3) is characteristic for their magmas.

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Table 1 Chemical composition of selected volcanic specimens from Mount Bowles

Sample 407 434 43 44 262 45D 80А 13/BKLocality Hanna point Mount Bowles Willan Nunatak East of

Hurd P. Rock Mdj BA BA BA BA A BA B

SiO2 51.76 52.6 53.84 54.05 53.13 57.49 55.87 47.22TiO2 1.39 1.21 1.29 1.73 1.27 0.96 1.05 1.08

Al2O3 15.92 16.51 17.09 15.35 16.04 17.29 17.04 16.96Fe2O3 11.43* 11.27 9.72 11.35 10.93 8.25 9.11 10.98MnO 0.20 0.18 0.19 0.21 0.18 0.14 0.15 0.18MgO 4.63 4.52 3.59 3.6 4.89 3.45 3.76 9CaO 7.59 8.71 8.73 5.68 5.92 6.91 7.38 9.98

Na2O 3.64 3.18 3.54 4.35 4.67 3.67 3.81 1.83K2O 1.59 0.62 0.32 1.01 0.49 1.29 1.07 0.3P2O5 0.26 0.13 0.23 0.27 0.21 0.19 0.18 0.17LOI 1.56 1.04 1.66 2.2 2.14 0.73 0.89 2.38

Total 99.97 99.96 100.20 99.79 99.89 100.38 100.31 100.09Cr 43 60 31 15 92 20 32 488Ni 27 9 9 11 18 12 16 160

Rb 34 15 10 22 16 42 28 13Sr 434 498 245 214 771 428 453 305Y 36 25 24 32 26 27 24 17

Zr 142 86 101 153 96 148 132 79Nb 6 4 4 7 4 4 3 4Ba 272 115 143 298 218 297 309 67V 309 395 265 416 365 216 256 247

La 10 0 7 7 4 4 13 3Ce 35 19 28 34 21 32 29 28Nd 16 11 14 19 24 28 34 23Th 2 1 3 4 1 6 1 3

Note: The abbreviations for the rock species are: В – basalt, ВА – basaltic andesite, Mdj- mugearite, А – andesite.

Fig. 3. Magmatic series in the samples from Mount Bowles Formation. The symbols are as in Fig. 2

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Fig. 4. Discrimination diagrammes for samples from Mount Bowles Formation. A. Discrimination after Meschede (1986) – volcanic-arc setting; B. Discrimination after Mullen (1983) – island-arc tholeiite setting.

Different discrimination diagrammes suggest tholeiite series of the magmas (Fig. 4). The relationships FeO/MgO vs. SiO2 (Myiashiro, 1974) or TiO2 vs. P2O5 (Mullen, 1983) also put us on the same track. However, the serial assignment is not simple. Some of the trace elements have

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higher concentrations than in the typical arc tholeiite and according to them the volcanic rocks resemble more to calc-alkaline series. Thus, the ratios Na2O/K2O and K/Rb bear resemblance rather to calc-alkaline series than to the tholeiite one. The samples falling on the tholeiite field on Fig. 3 are rare, while the bigger part of them come under the category of the calc-alkaline series. Similar ambiguity is typical also for other outcrops of Cretaceous age of the arc volcanism within the South Shetland Islands. The possible explanation is the nearness to the trench of this arc volcanism, spreading everywhere in the thin leading end of the Antarctic Peninsula arc.

Fig. 5. MORB-normalized spidergrams of volcanic rocks from several typical localities of Mount Bowles Formation.

Note: The rock abbreviations are the same as in Fig. 2.

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The samples from Cape Shirreff, Hannah Point and Siddons Point are quite fresh and their silicate analyses are reliable points of reference for the primary magma composition. The MORB-normalized trace-element diagrammes of these samples (Fig. 5) are very much alike to those of the samples from exposures around Mount Bowless and Moores Peak and hence the alteration did not influence essentially to their geochemical properties. The enrichment in normalized concentrations of the incompatible elements in relation to the compatible ones does not only imply island-arc volcanic setting, but also could be a result of fractionation of plagioclase, pyroxene, olivine, apatite and Ti-bearing phases (ilmenite, rutile etc.). Obviously the rocks were affected by the post volcanic migration of some elements (especially K, Rb and Ba).

4. Volcanological interpretation of the depositional processes

We did not find any proofs (fossils, pillow lavas, hyaloclastites, and turbidities) for the deposition of the rocks of Mount Bowless Formation in marine environment. It is difficult to interpret indisputably also the clastic lithofacies. The thick massive beds of polymictic debris resemble lahar’s flows the most. Sometimes the vitroclastic textures are preserved in the fine-grained matrix. The presence of many angular polymictic clasts mainly of lavas and of juvenile fragments having blocky and non-vesicular shapes is a typical feature of the tephra from freatomagmatic eruptions. In such cases, the coeval eruptive volcanics are clear products of intercalated lavas. The too high proportion of accessories and accidental clasts is also characteristic of the eruptions from maars, despite that usually the sequences in maars depositions are thinner bedded and the possibilities to be preserved are weak. On the other hand, the freatomagmatic deposits are well-bedded, but it is possible that the primary fine-bedded structures have been masked by the pervasive alterations and deformational jointing. The monomictic vitreous vesicular tuffs and the porous fragments with rare flame structures are likely products of ash flows from drier eruptions. As the dip of the beds is steep, we could assume that they were deformed from the nearness of plutonic intrusions.

No well expressed sedimentary structures are observed in the poorly sorted alternating arkosic sandstones. The clasts population is the same as in the sandstones of the Miers Bluff Formation. The difference is in their higher content of feldspars and the poorer of the accessories. The arkosic sandstones of the Mount Bowles Formation may be classified as immature sandstones. Nearly equal sizes of quartz and feldspar grains suggest that they did not suffered prolonged transport before their deposition. Therefore, the arkosic sandstones have been derived probably from a province with uplifted and already exposed complex of the Miers Bluff Formation.

B. Regional metamorphic processes1. Petrography

Almost all volcanic rocks of the Mount Bowless Formation are altered everywhere. Particularly strong is the alteration of the volcanoclastic rocks where the primary textures of the groundmass are completely replaced by the secondary minerals. Usually the plagioclases are

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partially replaced by albite, but frequently they are completely recrystallized to a secondary assemblage comprising sericite, scapolite, calcite, epidote, prehnite, talc and magnetite. Actinolite, biotite and chlorite, replacing the plagioclase crystals occur preferentially along cleavage and twin planes. Pyroxenes are usually having a narrow selvage of acicular amphibole (actinolite and cummingtonite), pumpellyite, chlorite and some carbonates. The groundmass exhibits considerable alteration to chlorite, biotite, epidote, actinolite, smektite type clay minerals, sulfides and oxide minerals, rutile and leucoxene. Patches of quartz are present on some places. The occurrence of biotite and cummingtonite is limited to the exposures around Burdick Peak. The amygdales usually have a very irregular outline and a composed of zeolites, prehnite, epidote, chlorite, quartz, pumpellyite and calcite. The same assemblage of secondary minerals is present in the anastomosing microfractures and in the thin veinlets often cutting the volcanic rocks. The intensively altered areas are accompanied by copper hydroxides around the quartz veins.

The fine-grained character of the recrystallized products in the low-temperature metamorphic rocks is a serious problem when studying the rocks. The recrystallization is often not immediately obvious because of this fine-grained scale of the products. Moreover, it is very difficult to delineate exact boundaries between the superficial and the close-to-the surface processes like weathering, diagenesis and metamorphism. The reason is that the subgreenschist grade mineral assemblages overlap in wide range of pressure-temperature space as a consequence of variation in their whole-rock chemistry. Subgreenschist grade metabasites almost invariably exhibit petrographic evidence of incomplete recrystallization and therefore only partial equilibration of their metamorphic mineral assemblages. In addition, most low-grade rocks display evidence of relic magmatic textures, sedimentary or hydrothermal histories, which precede any superimposed burial metamorphism. The textural variations of the protoliths control the physical properties, as porosity and permeability and consequently their response to subsequent hydrothermal alteration or regional metamorphism. For example the nature of the metamorphic mineral assemblages depends strongly on the primary permeability of basalt flows which is high in scoraceous flow tops and low in massive flow interiors. Whether occurred within the relic porphyry mineral phases, or in the ground mass, in the middle of amygdales, and in the veinlets cutting the rock, the new-formed metamorphic assemblages have different mineralogy.

Our study of the protoliths of the Mount Bowless Formation is founded on an integrated multianalytical approach including electron microprobe, X-ray diffraction (XRD), transmission electron microscopy (TEM) and analysis of electron images (BSE) to support the petrographic evidences for chemical equilibrium amongst coexisting minerals. Some graphic constructions and petrogenetic thermodynamic grids are also used. The final purpose of the applied methods was an estimation of P-T conditions of the metamorphism. Our sampling covered nearly the main compositional heterogeneity existed at the outcrops, hand specimens and thin sections. The specimens taken are from different lava flows with various morphology, pyroclastic tuffs, agglomerates and breccias. The volcanic glass heterogeneity was proved in images BSE.

The alteration of primary plagioclase to albite or Na-zeolites is perhaps the most critical of all observed processes. The Ca and Al that are released help promote crystallization of calcite, mafic layer silicates, and calcsilicates such as prehnite, pumpellyite, epidote and titanite. All these minerals we found in pore spaces, veins and vesicles, or within the primary plagioclase itself, filling the cleavage planes or the cores of the grains. Albitization of primary plagioclase commonly proceeds with minimal disruption of primary textures.

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The replacement of primary clinopyroxene by actinolite and lesser chlorite is well demonstrated in considerable number of thin sections and is here interpreted as a process generally associated with the onset of greenschist conditions of metamorphism. The observed assemblage epidote-albite-chlorite-quartz is a distinguishing mark of such uralitization.

The stability of primary Fe-Ti oxides in the studied titanomagnetite and ilmenite is disturbed and very often they are pseudomorphed by mixtures of low-Ti magnetite and ilmenite which is characteristic for the metabasites under low grade burial metamorphic conditions. In that case the primary exsolution lamellar structures are preserved well, no matter how strong the rocks are affected by the secondary process. It seems that albitization is apparently an important factor in freeing up aluminum for recrystallization of titanite after Fe-Ti oxides. Conversely, the hydrothermal alteration of metabasites under low pH, high fluid/rock conditions may result in leaching of Al and Si. Under these low pH conditions, primary Fe-Ti oxides may be pseudomorphed by blotchy mixtures of low-Ti late magnetite and TiO2 (rutile or anatase), and the alteration phases do not appear to pseudomorph the primary lamellar structures. The last case relationships are not observed in the thin sections studied by us. We did not found secondary rutile or anatase phases which show that the mineral assemblages of the metavolcanics from Mount Bowless Formation suffered low-grade metamorphism but they are not products of a hydrothermal alteration.

2. Mineral composition

The following typomorphic secondary minerals (significant for the analysis of the metamorphic evolution) are chemically analyzed: prehnite, epidote, pumpellyite, chlorite, albite, zeolites, and actinolite (Table 2).

Chlorites are the most abundant secondary minerals in our very low-grade metavolcanics rocks (Appendix 1 also). The standard optical spectrographic methods assisted our study by XRD technique for the unequivocal recognition of mixed-layer chlorite/smectite. The identification was supported also by BSE images on the basis of their textures. Smectite and corrensite were tentatively differentiated from chlorite based on the presence of Ca in the later seen in energy dispersive spectra (EDS). The relative peak heights of Si kα and Ca kα that appear on EDS spectra were used as an indication of the presence of smectite versus chlorite. The content of SiO2

and MgO in the microprobe analyses is essential. Using 28 oxygen (or 56 anionic charges) basis for recalculating the structural formula of chlorites in any trioctahedral smectite-chlorite mixture, values exceeding 6.25/formula unit generally implied the presence of some smectite either as interlayer or crystallites mechanically mixed with chlorite. The presence of only minor minute inclusions of either dioctahedral chlorite or non-layer silicate minerals, especially calcite, can make the Si cation contents of discrete chlorite to exceed the 6.25 value. We did not find pure smektite minerals, but recalculation of the microprobe analyses show that the numbers of noninterlayer cations (NIC) are between 18.9 and 19.9, which is indicative of the fact that chlorite layers are in the interval 50-90 per cent (Fig.6). All analyzed mafic layer silicates document development of the transformations smektite to chlorite. It is probable the pure smektite mineral products to present in the unexposed deep parts of the sections. The initial point of the reaction change of the mineral composition under metamorphic conditions for the investigated by us parts of the section is around 50 per cent chlorite layers (specimen BK/226 with NIC=18.9) and the progress of the reactions is directed to the end component of the clinochlor.

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Табл. 2.

Representative electron microprobes of secondary minerals from the volcanic rocks of Mount Bowles Formation

Epidote Chlorite Prehnite Pumpellyite Zeolite ActinoliteSampl

e4/L 5/L 26 21 76 21 77 76 21 226 227 38 21

SiO2 37.77 37.01 33.44 27.60 28.36 42.56 46.20 37.03 36.13 51.9 60.3 54.28 55.94TiO2 0.18 0.02 0.00 0.01 0.04 0.00 0.00 0.10 0.02 0.0 0.0 0.00 0.04Al2O3 25.00 22.14 16.32 17.15 17.02 19.74 21.25 24.30 24.12 21.8 17.2 1.01 1.09Fe2O3 12.77 14.34 - - - - - - - 0.2 0.3 - -FeO - - 17.27 30.67 24.93 5.07 2.93 6.26 5.99 0.0 0.0 10.19 8.26MgO 37.77 37.01 16.33 11.58 16.89 0.00 0.00 1.86 1.90 0.0 0.0 17.63 18.45MnO 0.18 0.02 0.43 0.36 0.08 0.00 0.21 0.05 0.08 0.1 0.0 0.35 0.24CaO 25.00 22.14 1.18 0.19 0.14 26.49 24.45 22.21 22.79 10.9 9.2 13.08 12.96Na2O - - 0.04 0.06 0.04 0.00 0.00 0.02 0.16 0.0 0.4 0.28 0.28K2O - - 0.13 0.03 0.05 0.00 0.00 0.01 0.02 0.1 0.1 0.04 0.03Total 98.67 96.68 85.13 87.63 87.53 93.86 95.04 91.84 91.22 85.1 87.4 96.86 97.18

Crystallochemical formulaeSi 2.97 3.00 6.87 5.99 5.96 3.07 3.20 6.09 6.00 8.10 9.00 7.81 7.91Ti 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00Al 2.32 2.11 3.96 4.39 4.22 1.67 1.73 4.71 4.72 4.00 3.00 0.17 0.18

Fe3+ 0.73 0.87 - - - - - - - - - - -Fe2+ - - 2.97 5.57 4.38 0.31 0.17 0.86 0.83 0.00 0.00 1.23 0.98Mn 0.00 0.01 0.07 0.07 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.04 0.03Mg 0.01 0.02 5.00 3.74 5.29 0.00 0.00 0.46 0.47 0.00 0.00 3.78 3.89Ca 1.97 1.99 0.26 0.04 0.03 2.04 1.81 3.91 4.06 1.80 1.50 2.02 1.96Na 0.00 0.00 0.01 0.03 0.02 0.00 0.00 0.01 0.05 0.00 0.10 0.08 0.08K 0.00 0.00 0.03 0.01 0.01 0.00 0.00 0.00 0.00 0.10 0.00 0.01 0.01

NIC - - 18.87 19.75 19.88 7.09 6.92 - - - - - -IC - - 0.31 0.08 0.06 - - - - - - - -

XMgO - - 0.63 0.40 0.55 - - 0.35 0.36 - - 0.76 0.80x - - 0.42 0.86 0.93 - - - - - - - -

AF* - - 0.18 0.19 0.18 - - -.080 -1.19 - - -0.41 -0.41Σcat 8.01 8.00 19.18 19.83 19.94 7.06 6.92 16.10 16.10 14.00 13.60 15.54 15.04

Notes : 1. Σcat – sum of cations in the crystallochemical formula; 2. chlorite formula is calculated at 28 negative charges in the clinochlor and 20 positive charges; 3. NIC – number of noninterlayer cations; IC – interlayer cations; 4. XMgO = MgO/(MgO+FeO) mol. %; 5. х – ratio of the chlorite layers in the chlorite/smektite; 6. AF*=100(Al2O3+Fe2O3-0.75CaO-Na2O+0.75TiO2)/(Al2O3+Fe2O3-0.75CaO-Na2O+0.75TiO2+FeO+MgO). 7. Ca in the chlorite structure is an indication of presence of a little amount of smektite; 8. Sample 26 of the chlorites approaches corrensite and all other samples are trioctahedral chlorites with an insignificant quantity of smektite layers; 9. Sample 226 of zeolites is yguawaralite and all of the rest are laumontite; 10. Pumpellyite formulae are calculated on the basis of 24.5 oxygen and 16 cations, while the actinolite samples are calculated on the basis of 23 oxygens.

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Fig. 6. Noninterlayer cations (NIC) vs. AlTotal plot for minerals in the specimens from Mount Bowles

Formation. The reaction progress in specimens from Chilean Andes (Schmidt, 1993; Schmidt & Robinson, 1997) and the zoning there expressed is shown by the overshadowed in colours fields with the indicated assemblages near to them. Thm – thomsonite; Sco – scolecite; Heu – heulandite; Stb – stibnite; Sm – smektite; Lmt – laumontite; Chl – chlorite; Prh – prehnite; Pmp – pumpellyite; Act – actinolite; Ab – albite.

Mafic layer silicates with different optical and textural characteristics occurred sometimes in varied domains as for instance the case is in the specimen BK/224, which was taken from lava with feldspar phenocrystals. The large amygdales in this specimen are filled with radial mafic layer silicates showing contrasting petrographical characteristics in their rims and in the cores of the aggregates. The rims are fine grained dark brown colored, suggestive of Fe-rich and Mg/Al-rich composition, whereas the cores are filled with coarser grained and light-coloured crystals. Microprobes of these different domains show that the total Al varies in the range 4.3-4.7 and the values of NIC are in the interval 19.1-19.7, but the repeated microprobe analyses show compositions overlapped in wide area and the values Mg# for the different domains are in quite limited envelopment – 0.462-0.483. So that, no significant compositional differences of the mafic layer silicates from the different domains are confirmed, in spite of the fact that their petrographical characteristics vary.

Epidote (Table 2, Table I) occurs in changeable amounts in approximately 50 per cent of the thin sections. Observed in vesicles, veins, in recrystallized groundmass or within the primary plagioclase, epidote was identified using standard petrographical methods and was analyzed by microprobes. The replacement of Al by Fe in its formula is in the interval 10-32 %.

Pumpellyite (Table 2, Table IV) was observed in around 40 % of the studied specimens like radial aggregates together with chlorites and zeolites. Usually it is present in the outer rims within the fillings of the amygdale cavities and rarely as irregular replacements on feldspar crystals. EDS analysis is essential for at least tentative identification of pumpellyite. It is readily distinguished from prehnite and epidote by its diagnostic small, but always present Mg peak. The pumpellyite compositions in volcanic rocks from Mount Bowless Formation are grouped in

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narrow interval around Al(0.70):Fe(0.22):Mg(0.08) and only one sample has a composition richer in Mg at Al(0.75):Fe(0.11):Mg(0.14). The structural formula of pumpellyite we used contains 16 cations in four sites distributed as following: W4X2Y5Z6О20+x(OH)8-x, where W contains primarily Са2+; X (Fe2+, Mn, Mg2+) + (Fe3+, Al3+), and Y contains only trivalent cations (Fe3+, Al3+), and Z contains (Si4+, Al3+). The value of X in such procedure ranges from about 0.70 to 1.70, corresponding to 7.3-6.3 hydroxyl ions per formula unit. The proportion of Fe3+ and Fe2+ is commonly adjusted so that all 16 cations are full, but depends also on the value of “x”. The substitutions of R3+ in the X site are compensated by substitutions of O2- for OH- , univalent for divalent cations in the W site, and Al for Si in the Z site.

Prehnite was found in more than 60 % of the specimens examined as filling the cavities but also developed on plagioclases and pyroxenes. The substitutions of Al for Fe are between 3 and 16 %, average value close to 10.5%.

Zeolites are rarely observed, but all microprobe analyzed grains are from Ca-rich phases with insignificantly quantities of Na- and K-zeolites. EDS was also used for distinguishing Na vs. Ca-zeolites. Zeolites occur as relatively coarse, amygdale-filling grains. Laumontite was identified as amygdaloidal fillings and also as pseudomorphs after plagioclase phenocrystals. Yguawaralite was diagnosed in late and very thin veinlets cutting the other secondary minerals.

Amphibole was identified in few specimens and it is referred to actinolite species. The amphibole containing rocks are coarse-grained porphyritic lavas showing large cavities and amygdales. The cavities are filled most often also with fibrous chalcedony and epidote replacing intensively the feldspars. Distribution Mg/Fe coefficient between actinolite and chlorite from a sample with both minerals analyzed is 0.78. This value is unusually low and it seems that is not typical for other representative couples of these minerals occurred in regional low-grade metamorphism setting. For example, the average respective value of KD in prehnite-actinolitic facies of the Welsh Basin (Bevins and Robinson, 1993) is 1.2 and for pumpellyite-actinolite facies rocks from the Western Alps in Switzerland (Coombs et al., 1976) is 1.8.

3. Variations in the mineral facies

Different approaches are applied to estimate the achieved degree of equilibration in the low-grade metamorphic mafic rocks from Mount Bowless Formation. The relationship between chemical composition space and mineral assemblages, the reaction progress and the mineralogical transformations are studied by some petrochemical projections, which are useful means to check the correspondence between the whole rock chemistry, the observed mineral assemblage and the chemistry of the secondary minerals. Several such projections are applicable for our case. One of them is showed in Fig. 7, where the relationships in the system CMASH are illustrated. The projection is from quartz and water apex (present in all our specimens) to the plane ACM, defined by Al2O3, CaO and MgO. The thick point designated B is our basaltic specimen BK/13. The abbreviations for the possible minerals are as follows: Grs – grossular, Prh – prehnite, Czo – zoisite, Lmt – laumontite, Pmp1 – pumpellyite (richer of Al and poorer of Mg), Pmp – pumpellyite, Tr – tremolite, Cln – clinochlor, Gln – glauconite. The presence of chlorite, pumpellyite and amphibole permits us to analyze the paragenetic relationships amongst these minerals and in the system NCMASH by using the epidote projection from Ab+Qtz+Ep+H2O (Beiersdorfer and Day, 1995). The data are illustrated in Fig. 8, where most of the tie-lines of chlorite-pumpellyite are subparallel and therefore their relationships are in equilibrium.

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Fig. 7. Relationships in the system CMASH in projection from quartz and water apex to the plain АСМ. В – specimen 13/BK. Coordinates of Pmp1 (XAl2O3=0.33; XCaO=0.53), Pmp (XAl2O3=0.25; XCaO=0.50), and of Cln (XAl2O3=0.17; XCaO=0), на Prh (XAl2O3=0.33; XCaO=0.67).

Zeolites (mostly laumontite) are intergrown with chlorite and pumpellyite in the cavity fillings, but in a later stage they are observed in veins cutting the amygdales. Within these veins the zeolitic species yugwaralite is diagnosed. Such relationships evidenced that the zeolites are result of a regional metamorphic event of the type of the burial metamorphism. The presence of yugwaralite, associated probably with wairakite (chemically unproved, but with its characteristic optical properties) could be related to some local hydrothermal event. The specimen was taken near to one of the hypoabyssal intrusive bodies in the vicinity of Willan Nunatak. The presence of prehnite, pumpellyite, epidote and actinolite in most of the thin sections documents very likely a higher level of this low-degree metamorphism.

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Fig. 8. Metabasites from Mount Bowles Formation in the epidote projection.

AF*=100/(Al2O3+Fe2O3-0.75CaO-Na2O+0.75TiO2)/

(Al2O3+Fe2O3-0.75CaO-Na2O+0.75TiO2+FeO+MgO); XMgО=Mg/(Mg+Fe). The distribution coefficients for the pairs Pmp-Chl and Act-Chl are put down at the side of the main figure, together with the values of the distribution coefficients for the low-grade metamorphic settings of the Welsh Basin (Bevins and Robinson, 1993) and for the Alps in Switzerland (Coombs et al., 1976). The abbreviations for the minerals are as follows: Prh - prehnite, Pmp - pumpellyite, Chl - chlorite, Act - actinolite. The boundaries diagenesis/anhyzone (Kd = 0.42) and anhyzone/epigenetic zone (Kd =0.52) for Welsh Basin are also noted.

Especially interesting is the presence of amphibole in these rocks. Usually the actinolite occurs in the higher-grade metamorphism in rocks with suitable composition together with prehnite and pumpellyite, but not associated with zeolites. Consequently, these rare actinolite minerals may be products of an anomalous event also because the distribution coefficient Mg/Fe between actinolite and chlorite has values below one (here 0.78), while in the well studied cases in areas of regional subgreenschist facies they are quite higher. Thus for the prehnite-pumpellyite to greenschist facies in Welsh Basin (Bevins and Robinson, 1993) they are between 0.93 and 1.4, average 1.2, whereas in the Alps, in area of pumpellyite-actinolite facies they reach to 1.78 (Coombs et al., 1976). All this could be interpreted that actinolite is unrepresentative for certain regional metamorphic degree and it is logical to suggest that it is a result of alternative thermal effects. Moreover this actinolite in our case is observed only in the neighbourhood of the Eocene in age apophyses of the Barnard Point Batholith, like the hypoabyssal intrusive body in area of Moores Peak (Kamenov et al., 2005). Then the presence of pumpellyite, prehnite and epidote associated with chlorite may be assigned partially to the following zones, according to the phyllosilicates composition: (1) zone 3 (laumontite-chlorite) and (2) zone 4 (prehnite-pumpellyite) of the low-grade metamorphic sequence Keweenawan (typical volcanic group in the state Minnesota, USA, Schmidt, 1993) on account of the single smektite-chlorite composition, as the prevailing part of the chlorite compositions is justifiable to be referred to zone 5 of the same sequence (chlorite-actinolite-albite). The gradual compositional changes of chlorites, which in this plot are correlated mutually, demonstrate that the changes in the lower levels (zeolite facies)

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to the higher parts of the facial units (prehnite-pumpellyite facies) are not accompanied with sharp metamorphic hiatus.

The patterns of the distribution of ratio Mg/Fe between chlorite and pumpellyite (Fig. 8) are interesting with the fact that the tie-lines are not perfectly parallel and the values of the distribution coefficients of this ratio between pumpellyite and chlorite (Pmp-Chl KD Mg-Fe) are too changeable in the wide interval 0.27-0.64. In the classical example of metabasites from Welsh Basin the corresponding values for the diagenesis/anhyzone and anhyzone/epimetamorphic zones are 0.42 and 0.52 (Bevins and Robinson, 1993). The analogous tie-lines from Alps in Switzerland have approximately the same range of these coefficients (0.38-0.44) (Coombs et al., 1976). The assemblage pumpellyite-actinolite-chlorite, which is established in the rocks from the Mount Bowless Formation usually, is referred to pumpellyite-actinolite facies. It is accepted that such assemblage documents some higher pressures rather then the one of the prehnite-pumpellyite facies. It means that such transition postulates pressure increasing in our complex with the time. The presence of pumpellyite in equilibrium with chlorite having high XMgO (>0.54, which is the boundary between chlorite-pumpellyite and chlorite-actinolite assemblages) determines the affiliation of the most strongly metamorphosed rocks of Mount Bowless Formation rather to the pumpellyite-actinolite facies than to the prehnite-pumpellyite facies.

In conclusion, the available mineral data of the metamorphic assemblages are indicative of envelopment of the conditions of metamorphism from prehnite-pumpellyite to the low levels of greenschist facies. Altogether the alterations are mainly within the space of prehnite-pumpellyite facies.

4. Pressure-temperature estimations

Petrogenetic grids are useful tools for illustration the physical and chemical conditions of metamorphism. They are an objective basis for estimation of relative stability of mineral assemblages, permitting a good, but general overview of the stability fields of assemblages that are essential to understanding of metamorphic history. The petrogenetic grids for low-grade metabasites, calibrated with experimental data (Frey et al., 1991) suggest that the assemblage Pmp+Ab+Chl is stable over a wide range of P-T conditions, but in little bit higher baric levels. At slightly lower pressures the assemblage Pmp+Act is stable with chlorite, whereas Prh+Act+Chl are stable at reduced still further low pressures.

The analysis of experiments on the equilibria in the model “basalt” Fe-free system suggests that the most probable boundaries of the estimations of P-T metamorphic conditions for similar to Mount Bowless Formation rocks are quite wide – 200-340oC and 1.2-2.0 kbar. The petrogenetic grid oversimplifies natural assemblages by ignoring Fe as an additional component and that is why we tried to find another and better approach to estimate the maximum temperatures of the metamorphism, using the petrogenetic grid of Powel et al. (1993) for the system NCMASH. This model system is useful especially for the determination of the conditions in subgreenschist to greenschist facies. The upper temperature limit of the pumpellyite-actinolite facies depends on the reaction: pumpellyite+chlorite+quartz → tremolite+epidote+H2O. The calculated equilibria of this reaction using the program TWQ by Berman (1991) gave an almost isothermal curve at temperatures in the interval 275-299oC and for pressures in the range 2.5-5.0 kbar.

An indirect estimation of the pressure may be made also by using the distribution coefficients of the ratio Mg/Fe between actinolite and chlorite, proposed as a potential barometer (Maruyama et al., 1983; Cho and Liou, 1988). Similar discrimination between higher baric and lower baric metamorphic settings has already been done successfully in Alps and in Welsh Basin

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and we show their results in Fig. 9. The distribution of this ratio in the samples from Mount Bowless Formation (Act-ChlMg-Fe) has values of 1.29 and 0.78. The higher value of 1.29 falls between the assemblages of the prehnite-pumpellyite and greenschist facies of the low-baric region of the Welsh Basin (Bevins and Robinson, 1995) in sharp contrast with the value of 1.8 for the higher pressure settings as in the Western Alps (Coombs et al., 1976).

Fig. 9. Distribution coefficients between Mg and Fe of actinolite and chlorite in samples from the Mount Bowles Formation compared to data from metabasites in Welsh Basin (Bevin & Robinson, 1993) and from the western Alps (Coombs et al., 1976).

Further estimation of the baric setting of metamorphism in the pumpellyite-actinolite facies could be established analyzing the petrogenetic grid of the system NCMASH (Powel et al., 1993). The pseudo-invariant point CHEPPAQ (= Chlorite-H2O-Epidote-Prehnite-Pumpellyite-Actinolite-Quartz) in Fig. 10 indicates restricted P-T conditions and separates chlorite-prehnite lower-P bathozone from actinolite-pumpellyite-epidote-chlorite-quartz higher-P bathozone (Powell et al., 1993).

For unit activity for all involved phases this point is localized at around 315oC/3.6 kbar. Using the real activities of the end-members of the assemblage studied with one individual amphibole composition, together with the average activity of prehnite of 0.8 for metabasites (Frey et al., 1991), we got new minimal value of this pseudo-invariant point CHEPPAQ= 260oC/1.1 kbar. The conclusion is that the transition between this mineral assemblage and the other assemblages may be provoked also by downshift of this point because of the variations in the mineral chemistry, reducing the pressure. Therefore, it is very probable P-T conditions between prehnite-pumpellyite and pumpellyite-actinolite facies to overlap.

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Fig. 10. Petrogenetic grid for determination of low-grade metabasites in the system CMASH, projection from

the apex of chlorite, quartz and water. Pseudo invariant point CHEPPAQ (Chlorite, Water, Epidote, Prehnite, Pumpellyite, Actinolite and Quartz) separates the bathozones of high pressure from these of low-pressure. The abbreviations of the minerals are as in Fig. 8.

The empiric analysis of the low-grade subgreenschist facies (prehnite-pumpellyite, prehnite-actinolite, pumpellyite-actinolite and prehnite-epidote ones) indicates that especially for the rocks from Mount Bowless Formation all these facies are really widely overlapped in the P-T space. This demonstrates that not only the P-T conditions are diagnostic for the facial transitions, but similar factor could be also the changes of the ratios Mg/Fe. Thus, there is enough evidence that not always P-T conditions control composition of the metabasic rocks in the transitional zones.

Out of the many examined by us specimens we found only one (BK/21) with the assemblage prehnite+pumpellyite-epidote+actinolite (+Ab, +Chl, +Ttn, +Qtz) giving the rare chance to define the three-phase field on the epidote projection AF* vs. MgO/(MgO – Fig. 11. This sample has a chlorite XMgO ratio of 0.50 that lies very close to the dividing value of 0.54 identified as separating the contrasting assemblages. Actually this specimen has an assemblage that is equivalent to the pseudo-invariant point (CHEPPAQ) in the petrogenetic grid for the ideal system NCMASH (Liu et al., 1987; Frey et al., 1991; Powell et al., 1993).

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Fig. 11. Pumpellyite-chlorite-actinolite relationships in specimen BK/21- metabasites from Mount Bowles Formation.

The boundary XMgO=0.54, dividing the both assemblages is determined in metabasites from South Welsh Basin (Bevins & Robinson, 1993). The red dotted tie-lines are for our specimen BK/21. The mineral abbreviations are as in the Fig. 8.

The uniqueness of this specimen suggests it recrystallized at restricted P-T conditions. According to Berman (1991) this pseudo-invariant point, when the end-member activities are calculated using the TWQ software and the mineral compositions are from the sample under question, should define exactly the P-T conditions, which for the case are 315oC/3.6 kbar. But when the real activities of the analyzed by us minerals are utilized, the derived P-T conditions were lowered to c. 278oC/1.1 kbar (Fig. 12).

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Fig.12. Petrogenetic grid for low-grade metabasites in the system CMASH - projection from the apex of chlorite, quartz and water.

The hatched lines are for the theoretical activity (activity = 1) for all phases and the dense lines are calculated with the real activities of the minerals in specimen BK/21. The calculations are done after the method suggested by Berman (1991). The numbers indicate the following reactions: (1) Pmp+Chl+Qtz → Act+Ep+F; (2) Pmp+Act+Qtz → Prh+Chl+F; (3) Pmp+Qtz →Prh+Ep+Chl+F; (4) Prh+Chl+Qtz →Act+Ep+F. The both vertical dense lines on the X axes represent the average chlorite temperatures of 263о С and 311о С for the subgreenschist facies and for the greenschist facies respectively. The data are from Bevins & Robinson (1993) and from Robinson et al. (2002).

This already more accurate estimation gives a geothermal gradient of c. 60oC km-1. Just this new estimated condition is one of the reasons to link the low-grade metamorphism of the rocks from Mount Bowless Formation to a genetic burial-type origin. The high heat flow, along with a primary depth control (the depth of the volcanic sequence Mount Bowless Formation for the time being is difficult to be estimated because of the fragmentary character of outcrops hidden below snow surface) on the metamorphic grade were two of the main important features of the burial metamorphism (Robinson and Bevins, 2004; Bevins and Robinson, 1988). Applying the thermal models, it is envisaged that an early, syndepositional high heat flow occurs in relation to diapiric rise of mantle material in a marginal back-arc or continental margin setting linked to lithospheric extension.

An important conclusion from the carried out study is that when the real average activities out of many analyses are calculated and not only the theoretical ideal activities of the phases in the system NCMASH are applied, a significant overlapping between the different facies is established. Consequently, the remarkable differences in the deduced thermal gradients could be an artificial result of mineral composition variations, producing shifts of the facial boundaries in the same P-T space. In the light of this conclusion, the changeable low-grade metamorphic facies should be subject of re-estimation working with larger database of microprobe mineral compositions. Obviously, applying modern techniques of petrogenetic grids and new chemographic projections would facilitate the more accurate estimations of P-T metamorphic conditions.

The overall reconstruction of the tectonic setting in low-grade metamorphism revealed over Mount Bowless Formation and the determination of its tectonic style could be completed only when the here stated results are compared to similar investigations on other metavolcanics rocks in the area of South Shetland Islands or in the Antarctic Peninsula. This is a task, which up to now has not been worked out in the geological projects of the other nations and such task requires a very clear idea about the geological and geochronological relationships of all other geological formations within the Livingston Island settings.

Whether the leaps of the metamorphic degree and the changes of the facies are real or imaginary is a problem requiring large new volume of detailed studies and revisions of the existing descriptions of metabasites through more accurate quantitative methods of diagnostic and by adequate to the present-day analyses of the metamorphic assemblages.

Conclusions

1. The volcanic Mount Bowless Formation is composed of silica-saturated metaluminous calc-alkaline magmatic products, assigned mainly to the basaltic andesite with rare deviations to basalts and andesite.

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2. The primary compositional variation of the rocks is a possible result of fractional differentiation.

3. The geodynamic setting, according to the geochemical properties is of volcanic island arc.

4. The petrological analysis of the metamorphic equilibria and of the secondary mineral compositions determines metamorphic conditions characteristic of the prehnite-pumpellyite facies to the lower levels of greenschist facies.

5. The maximum metamorphic conditions for the area under investigation in Hurd Peninsula are estimated as follows: pressure – 1.1 kbar and temperature – 278oC. The deduced geothermal gradient of c. 60oC/km suggests high values of the heat flow, due probably to a diapiric uplift of mantle magmas in back-arc setting.

6. The established significant overlapping of P-T conditions of the both prehnite-pumpellyite and greenschist facies emphasizes the changeable chemistry of the protoliths control on the equilibria.

Acknowledgments: The study is a result of the project “Complex geological, geochemical, and geophysical and ecosystem investigations in the area of the Bulgarian Antarctic Base “St. Kliment Ohridski”, 2005-2010, sponsored by the Bulgarian Ministry of Environment and Waters. Dr D. Dimov and Dr C. Pimpirev provided some of the samples. The author acknowledges the Spanish Antarctic Programme for the transport to Antarctica.

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__________ . . . __________

The paper is published in the Bulgarian magazine GEOLOGY AND MINERAL RESOURCES, book 5, 2-7 and book 6, 32-40 in Bulgarian. Editorial office: 1000 Sofia, “Princess Maria Louisa” Bd, № 22.The translation into English is realized by the author.

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