ORIGIN OF LATE PALEOZOIC GRANITOID HETEROGENEITY IN WEST TRANSBAIKALIA

Tsygankov A.A.

Geological Institute SB RAS, Ulan-Ude, Russia, tsygan@gin.bscnet.ru

In recent years, more data have been obtained on close temporal association of various geochemical type granitoids, previously referred to various age magmatic complexes. Such data were first obtained in West Transbaikalia (Yarmolyuk et al., 1997), where nearly complete temporal overlapping the granitoids of the Barguzin and Zaza complexes previously considered Early- and Late Paleozoic formations, respectively, was established. Similar facts were found in the Newfoundland (Whalen et al., 2006), Sinai peninsulas and some other places.

Thus, closely temporal and sometimes subsynchronous formation of various type granitoids within comparatively small areas of lithosphere can be considered as the recognized fact.

The aim of the present report is to show on example of West Transbaikalia (basin of Lower Kurba River) compositional dependence temporally associated, but petrogeochemically heterogeneous granitoids on type of crustal protolith.

Our studies have been carried out in the basin of the Lower Kurba River and adjacent districts of Ulan-Burgassy ridge in the area of about 2 thousand km². The larger part of the mentioned area is composed by Late Paleozoic granitoids that are grouped into the Barguzin (autochthonous gneiss-granites of Zelenogrivsky – 325.3 ± 2.8 Ma, giant porphyry-like Bt granites of Temensky – 318 ± 4 Ma and Bt granites of Goltsovy – 313.3 ± 3 Ma plutons), Zaza (leucogranites of Angyrsky – 303.4 ± 7.3 Ma (Yarmolyuk et al., 1997) and Unegetei – 289.2 ± 3.7 Ma massifs, veins of leucogranites (294.4 ± 1 Ma) in quartz syenites of the northern Khangintui pluton – 302.3 ± 3.7 Ma) and Low Selenga (Burgassy pluton – 287.3 ± 4.1 Ma) intrusive complexes. In addition, the Khasurta quartz syenite-monzonite pluton of age 283.7 ± 5.3 Ma is referred to the Barguzin complex. All data of the isotope ages have been obtained by U-Pb technique on zircons, ion probe SHRIMP-II in St.Petersburg and Pekin (Tsygankov et al., 2007; author's unpublished data).

Thus, the studied area is composed by petrographically various granitoids developed in period from 330 to 280 Ma that completely corresponds to the Late Paleozoic stage of granitoid magmatism in West Transbaikalia (Tsygankov et al., 2007). Time intervals between intrusion and crystallization of separate plutons averagely equal several millions years. In some cases (including uncertainty of age determination), formation of massifs composed by different rocks (leucogranites of Angyrsky (303 Ma) and quartz syenites of Khangintui (302 Ma) plutons) occurred simultaneously.

By chemical composition, the considered granitoids can be divided into two groups: a) pure granites and b) quartz syenites and monzonites. The difference of these group rock chemical composition is most clearly observed in their different basicity that is reflected on their microelement composition as well. The value of agpaitity ratio (Na₂O+K₂O)/Al₂O₃ (mol.) varies from 0.66 in monzonitoids to 0.89 in Zaza leucogranites. In ANK-ACNK diagram, monzonitoids and quartz syenites lie in the field of metaluminous rocks (ASI)<1, Barguzin and Zaza granites vary from metaluminous to moderately peraluminous varieties (ASI from 0.9 to 1.1 and sometimes more).

Monzonites and quartz syenites are characterized by similar REE distribution with moderate LREE enrichment related to MREE and HREE and subhorizontal profile in HREE field. The value of La/Yb_(n) ratio varies from 14.9_(aver) in quartz syenites of the Burgassy massif to 27.0 in those of Khangintui pluton. Distribution of rare earth and rare elements in the Barguzin and Zaza granites is largely similar, however, each massif is differed by its own geochemical peculiarities.

Geological, experimental and isotope-geochemical data are sources of information about composition of salic magma protoliths. The issue related to protolith for the Barguzin granites of autochthonous facies is solved rather simply. These were crystalloschists that hosted gneiss-granites of the Zelenogrivsky massif. Bulk composition of these schists corresponds to average composition of the Phanerozoic greywackes depleted in quartz. Isotope composition of gneiss-granites (Isr = 0.7076, εNd = -12.8, δ¹⁸O of quartz = 12.0 ‰) and comparison with experimental data on dehydration melting of various crust rocks (Patino Douce, 1999; Altherr et al., 2000) are in full agreement with this conclusion. Biotite granites of Goltsovy massif have similar bulk and isotope (Isr = 0.7063, δ¹⁸O of quartz = 11.6 ‰) compositions that permits to suggest similar composition of protolith as well. The Temensky granites are differed by higher melanocratic content, and they occupy an intermediate position between amphbolite and metaterrigenous protoliths in the experimental diagram. The mixed (metaterrigene-basic) character of crustal protolith can be suggested due to the lower δ¹⁸O (quartz) = 8.0 ‰ value being close to mantle ones (with account of heavy isotope accumulation in late products of crystallization, i.e. quartz) as well as intermediate isotope composition Sr (Isr = 0.7061).

The leucocratic granites of the Zaza complex are likely to have been also formed due to melting the terrigene, possibly, more leucocratic protoliths, in spite of distinct petrographical and geochemical differences from the Barguzin granites (macro- and microelement composition). This suggestion agrees to experimental and isotope data on granites of the studied massifs: δ¹⁸O in quartz varies from 10.6 to 11.5 ‰, Isr = 0.7067-0.7072, εNd = -10.8.

For granitoids of higher basicity (monzonites, quartz syenites), the comparison with experimental data allows to suggest orthoamphibolite source of primary magmas, possibly, with some contribution of rocks being of sedimentary origin. The data on isotope composition of Sr – I_Sr = 0.70622-0.70644 (Khasurta and Khangintui massifs) and neodymium (εNd = -4.1 ÷ -6.8) and also mantle values δ¹⁸O = 5.0-5.9 ‰ in titanite from Khasurta monzonites and Khangintui endocontact quartz diorites and 9.3 ‰ in quartz from quartz syenites of the Burgassy pluton.

Thus, the obtained data indicate that various granitoid formation in the basin of the Lower Kurba River and adjacent districts of the Ulan-Burgassy ridge continued for about 50 Ma. In early stage of magmatism (Barguzin granites – 330-310 Ma), the formation of acid magmas was associated with melting the ancient metaterrigenous source of model age near 2.0 Ga. From nearly 305 Ma, melting spread on relatively younger, i.e. amphibolite source of model age ~1.5 Ga as well that is likely related to the extension of magma generation area in vertical direction.

The work has been carried out with financial support of RFBR-Siberia (08-05-98017), RFBR-MNTI (06-05-72007), inegrational projects SB RAS N6.11 and 6.5.

References

Altherr R., Holl F., Hegner E. et al. High-potassium, calc-alkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany) //Lithos. 2000. V.50. P.51-73.

Litvinovsky B.A., Zanvilevich A.N.,Alakshin A.M. et al. (1992) The Angara-Vitim batholith is the largest granitoid pluton. Novosibirsk: Izd-vo UIGGM SB RAS. 141p.

Patino Douce A.E. (1999) What do experiments tell us about the relative contributions of the crust and mantle to the origin ofgranitic magmas? //Understanding Granites: Integrating New and Classical Techniques (Castro A., Fernandez C., Vigneresse J.L. Eds.).Geological Society Special Publications. V.168. P.55-75.

Tsygankov A.A., Matukov D.I., Berezhnaya N.G. et al. (2007) Sources of magmas and stages of Late Paleozoic granitoid formation in West Transbaikalia // Geology and Geophysics. V.48. N1. P.156-180.

Whalen J.B., McNicoll V.J., van Staal C.R., et al. (2006) Spatial, temporal and geochemical characteristics of Silurian collision-zone magmatism, Newfoundland Appalachians: An example a rapidly evolving magmatic system related to slab beak-off //Lithos. V.89. P.377-404.

Yarmolyuk V.V., Budnikov S.V., Kovalenko V.I. et al. (1997) Geochronology and geodynamic position of Angara-Vitim batholith // Petrology. V.5. N5. P.451-466.