The main features of composition, structure and origin of garnet pyroxenites and eclogites of Anrakhai metamorphic complex in Southern Kazakhstan

Skoblenko A.V., Tretyakov A.A.

Geological Institute RAS, Moscow, Russia

skoblenkoa@mail.ru

In Southern Kazakhstan metamorphic formations occur within Chu-Yili Mountains [1] and they are traditionally related to Dzhalair-Naiman belt. These rocks form the narrow tectonic zone, extended in north-westward for 100 km. Within the zone metamorphic rocks display different composition and conditions of formation. These rocks are combined into Anrakhai metamorphic complex [5].

The rocks of the complex are overthrusted from south and south-west onto Precambrian formations [2] of Uzunbulak series [7] that are represented by gneissic-granites. Serpentinites, pyroxenites and gabbroids, presenting a part of Early Paleozoic ophiolitic suite [10], are overthrusted from north-east onto the metamorphic formations.

Within Anrakhai complex biotitic paragneisses (Grt + Bt + An₂₀ assemblage) are predominant. Widespread muscovite in these rocks is the mineral of retrograde metamorphism that was formed under conditions of epidote-amphibolite facies [4]. Biotitic paragneisses contain tectonic lenses that are composed of rocks, having mafic and ultramafic composition. The lenses due to their structural features and mineral composition may be subdivided into two types.

The first type of lenses is developed within the south-western part of complex and consists of amphibolites (either garnet or without garnet) and less metamorphosed ultramafic rocks, represented by talcites, talc-antigorite schists.

Retrograde alterations that are occurred in both amphibolites (Czo + Ep + An₉ assemblage) and metamorphosed ultramafic rocks (Tlc + Mag assemblage) correspond to epidote-amphibolite facies [4]. In doing so metamorphic transformations within the first-type lenses and enclosing mica schists likely reflect their relation with one metamorphic event that reaches amphibolite facies in the prograde stage.

The second type of tectonic lenses is developed in north-eastern part of complex and consists of eclogites and garnet pyroxenites that form both individual, isolated bodies and appear as an intercalation within one lens.

Eclogites present the rocks with porphyroblastic texture, the mineral assemblage of prograde stage for which is Grt (Alm₆₃), Cpx (Jd₄₀), Qtz, Rt, making up about 35-40% of the whole rock.

According to chemical composition (SiO₂ = 43-45 wt. %; ΣNa₂O + K₂O = 1-2 wt. %) eclogites belong to ultramafic rocks and correspond to picrobasalts.

Garnet pyroxenites that are rhythmically interchanging with eclogites are represented by differences with porphyroblastic texture and directional structure, the mineral assemblage of prograde stage for which is Grt (Prp₂₂, Alm₄₈), Cpx (Di₈₅), Opx. This assemblage doesn’t contain quartz.

According to chemical composition (SiO₂ = 41-43 wt. %; ΣNa₂O + K₂O = 0.6-1.7 wt. %) garnet pyroxenites also belong to ultramafic rocks and correspond to magnesia basalts.

Petrochemical features of the rocks allow us to classify them as ultrabasic eclogites of type B, in accordance with the classification of Coleman [3].

A close link between eclogites and garnet pyroxenites, appearing in their rhythmic intercalation, suggests mutual metamorphic transformations of the rocks. The results of geothermobarometric investigations (T°C = 830 – 840; P = 18 kbar) [6, 8, 9] indicate the close P – T conditions of formation of eclogites and garnet pyroxenites, that correspond to eclogite facies.

Differences in mineral and chemical compositions in the rocks reflect the principal distinctions in compositions of their protoliths. Information about protoliths can be obtained by means of research of chemical zonation of garnets of these rocks and relict mineral inclusions that are contained inside the garnets.

Garnets from eclogites comprise various mineral inclusions, a location of which finds the certain regularity. Inclusions of An₂₀, Ep, Act-Tr occur in the central part; inclusions of Hbl and Czo are characteristic of the intermediate zone; and finally sparse inclusions of Rt, Qtz, Cpx occur within the peripheral part of garnets. Chemical zonation of porphiroblasts of garnets in eclogites is appeared as well: Fe³⁺-Mn core, Fe²⁺-Ca intermediate part and magnesia periphery. A comparison of distribution of the relict mineral inclusions of prograde stage with chemical zonation of garnets suggests potential protolith for eclogite as plagioclase-bearing igneous rock (gabbroid).

In garnet pyroxenites that are rhythmically interchanging with eclogites relict mineral inclusions of porphiroblasts of garnets are represented by Opx and Cpx, a location of which reveals the regularity, too: inclusions of Cpx and Opx of magmatic stage attend in the central part of grains; inclusions of Cpx of metamorphic stage are characteristic of peripheral zone. In doing so chemical zonation of garnet grains is appeared: Ca - central part, Fe²⁺-Mg peripheral zone and magnesia selvage. This regularity concludes, that protolith for garnet pyroxenites is igneous rock without plagioclase (for instance, websterite). Characteristic feature for garnet pyroxenites is also a presence of large accumulations of apatite and large allocations of rutile, likely presenting the relict minerals of magmatic protolith. Both the form and the dimension of rutile and apatite that are typical accessory minerals of magmatic rocks indicate the plutonic rather than volcanic origin of garnet pyroxenites’ protolith. It indicates the alkaliс (or subalkalic) composition of protolith as well.

Differences between eclogites and garnet pyroxenites are appeared in retrograde alterations. In garnet pyroxenites that were formed on the protolith without plagioclase (websterite) retrograde alterations are expressed only in the replacement of diopside Cpx by amphibole, represented by cummingtonite. In eclogites that were formed on the plagioclase-bearing protolith (gabbroid) retrograde changes are far more diverse: the earliest alterations result the replacement of omphacite Cpx by symplectic aggregate of diopside Cpx and acid plagioclase (Di₇₅ and An₁₂, respectively). This assemblage corresponds to conditions of granulite facies. Further decrease of temperature and pressure leads to formation of amphibole (edenite) – plagioclase (An₂₂) paragenesis. The association meets the amphibolite facies of metamorphism.

Petro-geochemical data of the rocks (increased FeO = 7.7 – 9 wt. %; TiO₂ = 1.9 - 2.8 wt. % and common high content of REE for enrichment LREE relatively HREE ((La/Yb) n = 1.65 – 12.05)) are typical for basalts OIB or continental basalts, whose primary melts were derived by melting of enriched source.

Thus, we may suggest that eclogites and garnet pyroxenites were formed as a result of high-pressure metamorphism of igneous rocks – gabbroids and pyroxenites. In accordance with the rhythmic interchange of the rocks and their petro-geochemical features these rocks likely represented a part of Early Paleozoic layered intrusion. Such layered mafic-ultramafic massifs are widespread within Chu-Yili Mountains of Southern Kazakhstan (Tymlaysky massif).

A comparison of metamorphic transformations in eclogites, garnet pyroxenites and enclosing paragneisses shows, that “amphibolite - gneiss” and “eclogite – pyroxenite” series which were formed under different conditions of metamorphism, are tectonically combined within Anrakhai metamorphic complex.

The investigations were carried out with financial support of RFBR (project 011-05-00924) and Grant of the President of the Russian Federation (MK-187.2011.5).

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A melilite-bearing calcic skarn, the Tazheran massif, Western Baikal area, Russia

Starikova A.E.