2011

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Neo Archean potassium metavolcanics in the Sharyzhalgay granulite complex of PreSayan uplift of Siberian platform

Levitsky I.V, Levitsky V.I.

Institute of Geochemistry, Siberian Branch RAS, Irkutsk, Russia; ilevit@igc.irk.ru

The petrologo-geochemical and isotope-geochronological investigations in the Sharyzhalgay metamorphic complex of Pre-Sayan (Sharyzhalgay) marginal uplift revealed two cycles of granulite facies metamorphism and overlapped ultrametamorphism: Neo Archean (2.56-2.65 Ga) and Paleo Proterozoic (1.85-1.87 Ga). Ne Archean formations are dominated by the differences of average composition interpreted as latites, K-rich andesites and K-rich low-Si andesites. Basic rocks are least spread: these are shoshonite, abskorite, K-rich and Al-rich basalts; in places alkaline picrites. This is low-Mg composition of rocks (in addition to biotite orthogneisses), elevated varying contents of TiO₂, Al₂O₃, P₂O₅, K₂O, Na₂O, Ba, Sr, LREE, Zr and V, moderate and low concentrations of Rb, Pb, Yb and widely varying amount of Y, Cr, Ni, Co.

The Sharyzhalgay metamorphic complex occurs in the Pre-Sayan (Sharyzhalgay) marginal uplift of the Siberian platform. It extends for 300 km northwest of the southern termination of Lake Baikal towards the Oka River. It is traditionally referred to as the granulitic facies formation, actually being two-pyroxene plagioschist (tholeiitic metabasalts) and plagiogneiss (andesites), meta-terrigenous garnet-biotite gneisses, which experienced ultrametamorphic transformations, their products being the plagioclase and kalispar migmatites and granites (Petrova, Levitsky, 1984). The petrologo-geochemical and isotope-geochronological investigations for the past 10 years revealed two cycles of granulite facies metamorphism and overlapped ultrametamorphism: Neo Archean (2.56-2.65 Ga) and Paleo Proterozoic (1.85-1.87 Ga).

In different blocks of the Pre-Sayan uplift each Precambrian cycle is typified by certain rock types with specific geochemical features. The metamorphosed rocks of granulite facies of the Sharyzhalgay complex of Neo Proterozoic cycle in the Irkutny, Zhidoy and Kitoy blocks consist of: biotite-bibyroxene plagioschists (reconstructed as subalkaline and alkaline basalts, mainly potassic), biotite-hypersthene and biotite plagiogneisses (alkaline, subalkaline metaandesites), rarely moderately aluminous biotite-garnet (siltstone, greywackes) and highly aluminous (silty politic argillites) gneisses, metagabbro-anorthosites, calcitic marbles. The metamorphic rocks of Paleo-Proterozoic cycle are dominated by metatholeiitic bipyroxene plagioschists, in minority are meta-terrigenous biotite (metapelites, greywackes), biotite-garnet (metaarkoses) plagiogneisses, quite seldom there are dolomitic marbles and quartzites. The data prove their formation in the trough supra-structure of settings or oceanic islands or island arc dated as 2.3-2.4 Ma. The rocks of Neo-Proterozoic cycle were reported in (Petrova, Levitsky, 1984).

Considering geological relationships, the Neo-Proterozoic part of the complex encompasses bedded and lens-like bodies of biotite-bipyroxene plagioschists and plagiogneisses, biotite and biotite-hypersthene plagiogneisses (enderbites-1); differently-shaped separations of biotitic fine-grained orthogneisses and meta-ultrabasites intercalated with metaterrigenous garnet-biotite and biotite plagiogneisses; thin layers of diopside calciphyres, quartzites and alumina-rich gneisses. Metamorphic rocks incorporate cross-cutting bodies of gabbro, gabbro-anorthosites, micro-gabbro dated as 2649±6 Ma (Salnikova et al., 2007). All these rocks underwent ultrametamorphic transformations with formation of plagioclase and kalispar migmatites and granites, medium- and coarse-grained enderbites (2, 3), charnockites and pegmatites embodying the inclusions of above-described rocks. The age of pegmatites cutting younger metamorphic rocks pointing to the termination of ultrametamorphism is 2562±20 Ma.

The biotite-bipyroxene plagioschists and plagiogneisses, biotite and biotite-hypersthene plagiogneisses, massive fine-grained hypersthene plagiogneisses (enderbites-1), biotite fine-grained orthogneisses occur nearby and probably represent protoliths of the most ancient potassic calc-alkaline and common calc-alkaline series. The Neo-Archean rocks of tholeiitic series occur locally and dominate in the formations of Paleo-Proterozoic cycle.

When conducting reconstructions in deeply metamorphosed complexes the questions arise relative to the isochemical essence metamorphic processes and preservation of protolith composition. We used the samples of unaltered rocks without traces of ultrametamorphic and post-ultrametamorphic transformations. The primary characteristics of protolith substratum are markedly different from geochemical parameters of products of overlapped alterations. Available materials indicate that Ne Archean formations are dominated by the differences of average composition interpreted as latites, K-rich andesites and K-rich low-Si andesites. Basic rocks are least spread: these are shoshonite, abskorite, K-rich and Al-rich basalts; in places alkaline picrites. The rocks are referred to these series according to their geochemical characteristics (Table). This is low-Mg composition of rocks (in addition to biotite orthogneisses), elevated varying contents of TiO₂, Al₂O₃, P₂O₅, K₂O, Na₂O, Ba, Sr, LREE, Zr and V, moderate and low concentrations of Rb, Pb, Yb and widely varying amount of Y, Cr, Ni, Co (see Table).

Table. Average chemical (mass %) and rare-element (ppm) composition of rocks
№	1 (2)	2(3)	3(2)	4 (3)	5 (3)	6(4)	7(3)
SiO₂	50,60	48,42	49,83	54,66	54,60	57,80	56,78
TiO₂	2,43	1,10	1,08	1,17	1,24	0,82	1,37
Al₂O₃	13,92	17,73	13,26	20,12	17,34	15,23	17,01
Fe₂O₃₍_tot)	12,8	12,34	10,36	5,00	10,26	8,99	9,84
MgO	3,21	3,86	8,86	2,04	3,10	4,48	2,32
CaO	6,07	8,11	8,41	4,47	6,67	6,23	5,68
P₂O₅	1,53	0,79	0,88	0,32	0,48	0,64	0,51
K₂O	3,89	2,84	4,25	3,30	1,19	2,03	1,38
Na₂O	4,14	3,29	1,01	5,79	4,24	2,87	4,46
Rb	58	47	184	121	24	62	25
Ba	1900	1266	992	768	436	1636	427
Sr	830	1391	773	440	506	728	367
La	110	174	147	46	74	94	62
Ce	210	350	304	91	142	151	140
Nd	110	143	134	41	67	69	56
Yb	8,0	3,6	1,7	1,3	2,8	4,0	1,5
Y	60,0	144,7	22,5	18,5	47,2	36,1	20,2
Zr	250	331	422	1005	182	220	270
Pb	20	9	13	13	15	9	10
Cr	5	34	447	9	2	220	33
V	150	192	186	70	130	175	110
Ni	22	20	208	29	22	90	25
Co	28	24	39	16	24	23	23
Mg#	33	38	63	36	23	33	19
Note 1-2 – biotite-bipyroxene plagioschists of the Irkutny (1) and Zhidoy (2) blocks of marginal uplift; 3 - 4 – biotite orthogneisses – potassic (3) and sodium-potassic (4); 5-6 – biotite-bipyroxene plagiogneisses; 7– biotite-hypersthene plagiogneisses–enderbites.

When reconstructing the environment it was found that protoliths of metavolcanics had formed at the early stages in the settings of island arcs and oceanic islands, and at later staged these were mature island arcs and active continental margins. Different geodynamic settings of rock emplacement prove differences in the age of volcanism with the following development of metamorphism and Neo Archean collision with formation of Neo Archean composite terrane. The other indicators of collision in the PreSayan uplift are mapped massifs of Neo Archean potassic post-collisional granitoids of the Kitoy ultrametamorphic complex. Exploration indicates formation of mature continental crust in the marginal uplift at the Neo Archean, and most likely, at the Meso Archean stage.

. This study was financially supported by RFBR grant 09-05-00563.

References

Petrova Z.I., Levitsky V.I. Petrology and geochemistry of Baikal granulite complexes. Novosibirsk: Nauka. 1984. 200 p.

Salnikova Е.B., Kotov А.B., Levitsky V.I. et al. The age borders of high-T metamorphism in crystalline complexes of the Sharyzhalgay uplift of the Siberian platform basement: results of U-Pb dating of some zircon grains // Stratigraphy. Geological correlation. Volume 15. № 4. 2007. P. 3-19.